Review of Tribological and Wear Behavior of Alloys Fabricated via Directed Energy Deposition Additive Manufacturing

Abstract

Additive manufacturing (AM) is a rapidly evolving technology that enables the fabrication of complex 3D components across a wide range of materials and applications. Among various AM techniques, direct energy deposition (DED) has gained significant attention for its ability to produce metal and alloy components with moderate geometric complexity while maintaining a high deposition rate. This makes DED particularly suitable for real-world applications, including in-situ repair and restoration of metallic parts. Due to the nature of the DED process, components undergo extreme heating and cooling cycles, leading to microstructural evolution, process-induced defects, and variations in properties. While extensive research has explored the microstructure and mechanical properties of DED-fabricated alloys, studies on their surface degradation remain incomplete. Corrosion behavior has been well documented, given its significance in AM alloys; however, their tribological performance remains largely unexplored. This paper provides a comprehensive review of the wear behavior of DED-manufactured alloys, emphasizing the potential of DED technology for producing durable components. Specifically, it examines the wear characteristics of four key material groups—Fe-based, Ni-based, Ti-based, and Cu-based alloys—by summarizing existing studies and analyzing the underlying mechanisms influencing their wear resistance. Finally, the paper identifies research gaps and outlines future directions to advance the understanding of wear performance in DED alloys, paving the way for further innovation in this field.

1. Introduction

Additive manufacturing (AM), as defined by the American Society for Testing and Materials (ASTM F2792-12A) [1], is a process that constructs objects layer by layer based on 3D model data, distinguishing it from conventional subtractive manufacturing techniques. Unlike traditional methods, AM inherently minimizes material waste and has the potential to decouple economic value creation from the environmental footprint of industrial activities. This technology provides three key sustainability benefits: extended product lifespan, enhanced resource efficiency, and the transformation of value chains [2]. By 2027, the energy, automotive, and aerospace industries are projected to account for approximately 52% of the total revenue in metal additive manufacturing [3].

AM technologies have demonstrated their effectiveness in producing 3D metallic components. Among them, powder bed fusion (PBF) and directed energy deposition (DED) are two key AM processes by which fully dense metal parts can be fabricated for a wide range of industrial applications. The method used to deliver the powder or wire feedstock in these processes significantly influences factors such as part complexity, material flexibility, support structures, and the surface roughness of the deposited component [4].

To date, DED technologies have been successfully utilized to produce 3D metallic components from a variety of metals and alloys, including steels [2], superalloys [5,6], titanium alloys [7,8,9], copper alloys [10], and aluminum alloys [9]. The properties and overall quality of DED-manufactured parts are influenced by several key factors: (i) the specific type of DED technology used, such as the heat source and feedstock type; (ii) the build environment, whether inert gas, vacuum, or ambient conditions; (iii) the interactions between the energy beam and the material; (iv) deposition parameters, such as scanning speed, laser power, powder feed rate, hatch spacing, and scanning strategy; and (v) the characteristics of the feedstock material [4].

Furthermore, DED-built components undergo rapid and repeated heating and cooling cycles during layer-by-layer deposition. This thermal cycling can lead to distinct microstructural features, residual stress accumulation, the formation of non-equilibrium phases, directional solidification, solidification cracking, porosity, warpage, and delamination [4].

The failure of additively manufactured alloys is categorized as either bulk or surface failure. Bulk failure mechanisms include fracture, fatigue, creep, deformation, buckling, thermal shock, and stress cracking, while surface-related failures are primarily driven by corrosion and wear [11]. Among these, wear—especially at the interface of rubbing surfaces [12]—is a critical factor in assessing the reliability of AM alloys. It can drastically alter the performance of machining components [13] and has a profound impact on the operational lifespan of mechanical systems, ranging from nanoscale devices to large-scale industrial machinery [14]. Notably, wear has been identified as a major contributor to global energy consumption, accounting for nearly one-third of the world’s energy usage, as significant resources are continuously expended to counteract its effects [15].

The primary causes of wear include a combination of factors, such as suboptimal design, improper or insufficient lubrication, poor manufacturing quality, rough surface finishes, inadequate clearances, contamination by dust or metallic particles, exposure to moisture, chemicals, or extreme temperatures, and the use of inappropriate tooling. Among all these, friction remains one of the most significant contributors [16].

Several interrelated factors, such as material microstructure, surface finish, wear type, and underlying wear mechanisms, influence the wear behavior of AM alloys [11]. The wear behavior of additively manufactured alloys has not been extensively reviewed in the literature [11,17,18], with the exception of a recent study focusing on the impact of rare-earth elements on the wear resistance of AM alloys produced via PBF technology [19]. As a result, a comprehensive overview of the wear resistance of AM metals and alloys remains largely unaddressed.

Figure 1 illustrates the growing number of publications on DED alloys from 2010 to 2024, highlighting the increasing research interest in this field. DED technologies offer numerous advantages, including their compatibility with a wide range of feedstock materials, ability to process multi-material and composite structures, localized control over part characteristics, high deposition rates, and the capability to fabricate large and complex designs while integrating additive and subtractive manufacturing. Additionally, DED facilitates coating and repair within a single machine, supports additive manufacturing on non-horizontal surfaces, utilizes larger and more cost-effective powder particles, and is suitable for in-space printing under zero-gravity conditions [20,21,22]. Given these benefits, this paper aims to provide a comprehensive review of the wear properties of DED-fabricated alloys.

In AM technologies, the repeated reheating inherent to the layer-by-layer deposition process results in heat accumulation and gradual dissipation, effectively producing an in-situ heat treatment. This thermal effect can potentially contribute to improved mechanical properties of the material, including increased tensile strength, hardness, ductility, and fatigue resistance [23]. Beyond these inherent benefits, post-process heat treatment cycles can be employed to further tailor the properties of AM components. By carefully controlling the parameters of these treatments, the performance of AM parts can be optimized to meet the specific requirements of their intended applications [24].

In addition to evaluating the wear behavior of DED-fabricated components, it is equally important to consider the wear of cutting tools used in subsequent machining operations. Tool wear significantly influences the overall efficiency and quality of manufacturing processes. Metal cutting is a cornerstone of modern industrial economies, underpinning key sectors such as the automotive, aerospace, shipbuilding, and electronics industries. While the metal cutting industry itself is relatively small compared to the sectors it supports, it contributes significantly to manufacturing—accounting for approximately 15% of the value of mechanical components [25].

A thorough understanding of the material removal process is essential for effective tool selection and design, as well as for achieving the desired dimensional accuracy and surface finish. Friction encountered during cutting operations directly influences power consumption, tool wear, surface integrity, and overall production costs. Excessive tool wear can result in dimensional inaccuracies, unplanned downtime, and increased operational costs, ultimately reducing productivity [25]. Therefore, investigating and understanding the wear behavior of cutting tools used in surface preparation of DED alloys is vital for optimizing machining strategies and improving the overall efficiency of additive-subtractive hybrid manufacturing systems.

Figure 1. The number of publications on DED-fabricated alloys in the literature [26].

Figure 1. The number of publications on DED-fabricated alloys in the literature [26].

In this context, the paper begins with a brief introduction to DED technologies, followed by an overview of wear mechanisms. It then categorizes the wear behavior of DED alloys based on alloy type. Subsequently, a general assessment of the wear performance of DED alloys is presented to clarify the underlying mechanisms governing wear behavior. Finally, the paper presents key conclusions and highlights existing research gaps.

2. Directed Energy Deposition (DED) Technology

The DED process enables the fabrication of near-net-shape components using either powder or wire feedstock, powered by a high-energy source such as laser, plasma/electric arc, or electron beam [27]. This technology is widely utilized across various industries, including aerospace (for brackets, engine components, and turbine blades), medical (for earplugs, scaffolds, and implants), and automotive (for gears, bumpers, knobs, and gearboxes), owing to its capability to produce metal parts while minimizing the need for assembly. DED processes are commonly classified based on factors such as feedstock type, as illustrated in Figure 2 [28].

Table 1. Comparison of DED processes with various heat sources (Note: 1 represents the lowest magnitude, while 4 indicates the highest) [4].

Heat Source	Build Volume	Detail Resolution	Deposition Rate	Coupling Efficiency	Potential for Contamination
Laser	3	2	2	1	3
Electron beam	4	1	3	4	4
Plasma/electric arc	3	1	3	4	2

provides a comparative analysis of different DED processes based on various heat sources and selection criteria. In this table, build volume refers to the maximum component size that each process can accommodate, while detail resolution indicates the capability to produce fine features. Deposition rate represents the speed at which material is deposited, and coupling efficiency measures how effectively energy is transferred from the source to the substrate. Additionally, the potential for contamination highlightunderscores the risk of introducing impurities, such as dirt, gas, or other contaminants, into the part.

Several commercially available DED technologies include Laser Solid Forming (LSF), Laser Metal Deposition (LMD), Laser Engineered Net Shaping (LENS™), Direct Metal Deposition (DMD), Electron Beam Additive Manufacturing (EBAM^®), Directed Light Fabrication (DLF), and Wire Arc Additive Manufacturing (WAAM). Some DED systems, such as EBAM, DLF, and LENS, operate within a sealed chamber under a controlled atmosphere or vacuum, while others, like WAAM and DMD, utilize an inert gas shroud to mitigate oxidation. In addition, certain DED technologies are capable of depositing multiple materials and fabricating intricate geometries. This process is particularly valuable for filling cracks, retrofitting components, and repairing high-cost metal parts. DED also provides high deposition rates, ranging from 0.5 kg/h in LENS to 10 kg/h in WAAM, with large build volumes [4].

2.1. Wire-Feed Additive Manufacturing Processes

Wire-feed additive manufacturing technologies, such as Laser Wire Welding Additive Manufacturing (LWWAM) and Wire Arc Additive Manufacturing (WAAM), are commonly used to fabricate large and moderately complex structures, including flanges. LWWAM employs a laser as its primary energy source, utilizing metal wire as the feedstock, which can be either cold or preheated based on the process requirements. The key components of this system include the laser source, an automated wire-feed mechanism, a preheating or cooling system, and the metal wire itself. Using a preheated (hot) wire improves energy efficiency by reducing the laser power needed compared to a cold wire [28]. In DED processes, the wire can be introduced in three orientations relative to the build direction—front, side, or rear (see Figure 3). The choice of wire feeding direction, along with its angle and positioning, significantly affects the quality of the final component. Similar to lateral powder feeding, lateral wire feeding imposes constraints on the scanning strategy, thereby limiting process flexibility [29].

Arc-based DED, commonly referred to as Wire + Arc Additive Manufacturing (WAAM), utilizes wire as its feedstock, similar to the filament used in Fused Deposition Modeling (FDM). The WAAM process is classified into three main categories based on the welding technique employed: Gas Metal Arc Welding (GMAW)-based, Gas Tungsten Arc Welding (GTAW)-based, and Plasma Arc Welding (PAW)-based [30].

Both GTAW and GMAW processes are susceptible to defects such as cracks, porosity, and residual stresses, primarily due to melt pool instability, excessive heat input, and spatter formation. To mitigate these challenges, the Cold Metal Transfer (CMT) technique, an advanced variation of GMAW, is actively being explored. CMT offers several advantages, such as reduced spatter, lower heat input, and higher deposition rates, making it a promising solution for enhancing WAAM process efficiency [30]. The characteristics of these techniques are illustrated in Figure 4.

2.2. Laser Powder DED (LP-DED)

Laser Powder Directed Energy Deposition (LP-DED), also known as Laser-Directed Energy Deposition (L-DED), overcomes the size constraints of Laser Powder Bed Fusion (L-PBF) while enabling the fabrication of thin-wall structures as small as 1 mm in thickness. It is important to note, however, that L-PBF is capable of producing even thinner walls and offers superior dimensional accuracy compared to all DED-based technologies [31]. In the LP-DED process, a laser serves as the energy source, melting metal powder that is blown into the melt pool, thereby forming successive deposited beads (as illustrated in Figure 5). The deposition head, equipped with laser optics, is mounted on a gantry or robotic system for precise motion control, allowing the creation of intricate thin-wall features [32].

LP-DED enables the production of freeform structures based on CAD-generated toolpaths, including complex internal features such as channels and flow passages. This capability makes it suitable for various applications, including replacing cast and forged components, cladding and repair operations, primary structural components, and large-diameter thin-wall structures such as heat exchangers with integrated channels [32].

Several key process parameters, including laser power and material flow rate, significantly influence the final product’s quality. The deposition process takes place within an enclosed chamber, where an inert gas directs the powder into the melt pool. As the laser melts the material, the part is built layer by layer through gradual solidification. Additionally, nozzle design plays a critical role in determining surface roughness, making it a crucial factor in achieving high-quality components [28].

3. Wear Mechanisms Overview

Before evaluating the tribological performance of DED-fabricated alloys, it is important first to discuss the primary wear mechanisms. Establishing this foundation enables a clearer understanding of the wear behavior of DED alloys, as discussed in the following section. Moreover, this structured approach minimizes redundancy, ensuring a more concise and engaging discussion.

The six most commonly reported wear mechanisms are illustrated in Figure 6. Adhesive wear occurs when high loads, elevated temperatures, or pressures cause asperities on contacting metal surfaces to bond and subsequently fracture. This wear mechanism is common in plastic deformation at the interface of similar materials and is influenced by factors such as the material’s tendency to form solid solutions or intermetallic compounds, as well as surface cleanliness. When the adhesive bond resists sliding, stress builds up, leading to crack formation and propagation until a wear particle detaches. Surfaces with thick oxide films generally exhibit lower adhesive wear due to their protective barrier, whereas cleaner surfaces are more susceptible to bonding and material transfer [33].

Abrasive wear occurs when hard particles or surface asperities displace or remove material from a surface. It can be classified into two types: two-body abrasion, where hard asperities or firmly attached particles directly interact with the surface, and three-body abrasion, where loose particles roll or slide between surfaces, causing material loss. In a single-point contact model, abrasive wear can manifest through ploughing, cutting (typically in ductile materials), or fragmentation (in brittle materials). For metals, the volume of material removed due to abrasive wear is inversely proportional to hardness, as described by the Archard equation. This relationship indicates that harder materials generally exhibit greater resistance to wear [33].

Fatigue wear occurs when repeated cyclic loading leads to the initiation and growth of microcracks on the surface, eventually causing the detachment of wear particles. This progressive weakening of the material surface accelerates wear over time [12].

Fretting wear is a specific form of fatigue wear that arises under high-frequency vibratory motion with a small amplitude, typically ranging from 1 to 200 μm. This wear mechanism involves a series of sequential damage processes, including the breakdown of protective surface films, material adhesion and transfer, oxidation of metal wear particles, and the initiation of microcracks on the surface. Over time, these combined effects contribute to progressive material degradation and surface damage [34].

Erosive wear occurs when particles impact system components—such as feeders and pipeline bends—that have lower hardness than the conveyed particles. This type of wear is driven by particle impingement, with the impact angle playing a crucial role in the wear rate and mechanism [35].

Corrosive wear involves the formation of fine corrosive products on the surface, which contribute to wear particle generation. When this protective layer is disrupted or removed due to sliding or abrasion, a fresh layer forms, continuing a repetitive cycle of corrosion and material removal [12].

Figure 6. The common wear mechanisms and their corresponding micrographs [33,36].

Figure 6. The common wear mechanisms and their corresponding micrographs [33,36].

Here, the wear behavior is primarily influenced by sliding velocity, applied load, and the surface characteristics of the contacting materials [37,38]. Among these, surface topography—particularly surface roughness—plays a critical role. Various roughness parameters offer unique insights into the surface profile. For instance, the average roughness (R_a) provides a general indication of surface height variation but is less effective in capturing subtle changes or waviness. In contrast, the root mean square roughness (R_q) is more responsive to deviations from the mean line. Skewness (R_sk) characterizes the asymmetry of the surface texture, helping to differentiate between surfaces dominated by peaks or valleys. Kurtosis (R_ku), on the other hand, describes the sharpness of the height distribution: values below 3 correspond to flatter profiles, while values above 3 reflect sharper peaks or deeper valleys [38].

Given the significant influence of surface characteristics on the tribological performance of materials, surface texturing has emerged as an effective strategy to reduce friction and wear, enhance load-bearing capacity, and improve fluid film stiffness [39]. Surface texturing involves the deliberate creation of micro-scale features—such as dimples and grooves—on contact surfaces, applicable under both dry and lubricated conditions. These texturing techniques are generally categorized into four groups: material removal, material addition, material displacement, and self-forming methods [40]. In this regard, the surface features of as-printed or surface-modified components produced via DED can substantially affect their tribological behavior. However, this aspect remains insufficiently explored in the current literature. As such, a more detailed investigation into the surface-related tribological performance of DED-fabricated alloys is proposed as a direction for future research and will be discussed further in the final section of this paper.

4. Wear Analysis of DED Alloys

This section examines and compares the wear behavior of DED-fabricated alloys. To ensure clarity, each alloy is discussed in separate subsections, allowing for a more structured analysis of its tribological properties. These subsections explore the influence of processing routes, processing parameters, post-processing methods, and wear test conditions on wear performance. Additionally, existing literature highlights the investigation of tool wear during wear testing of DED alloys, which plays a crucial role in surface post-processing. The number of subsections dedicated to each alloy varied based on the availability of relevant research articles.

Before delving into the discussions on the wear behavior of various alloys fabricated via DED, it is important to note that this section presents detailed descriptions of their individual wear responses. For a more comprehensive understanding, Section 5 presents tables that consolidate key DED process parameters, categorize alloys based on their principal elements, and compare their wear behavior. Readers seeking a broader comparison of the findings across the literature are encouraged to consult Section 5 and its corresponding tables.

4.1. Wear Behavior of DED-Fabricated Ferrous Alloys

4.1.1. Comparison with Conventionally Fabricated Alloy

Comparing the wear behavior of DED Fe-based alloys with their as-cast and wrought counterparts offers valuable insights into how the DED process influences the tribological properties of these materials. In a recent study, the wear behavior of WAAM 304 SS was compared to its wrought counterpart, as shown in Figure 7. It was reported that the wear resistance of the WAAM stainless steel was greatly dependent on the extraction location.

For instance, when compared to wrought stainless steel material, the printed end region (ME) performed slightly better than wrought material. In contrast, the beginning region (MB) showed a slightly lower wear resistance than the wrought material. Thus, it was suggested that the microstructure of the AM part, i.e., varying with the weld path, dictates the wear resistance. It was also observed that the surface immediately below the wear scar underwent significant grain refinement (shown in Figure 8a). The grain refinement observed beneath the wear scar is typical of stainless steel and impedes dislocation movement, thereby increasing the yield strength of the material. Such behavior is consistent with conventional stainless steels subjected to external shear loading. However, it should be noted that this study did not provide any details regarding the dominant wear mechanisms. Furthermore, the influence of different extraction locations on the wear properties of WAAM stainless steel was not thoroughly discussed [41].

In a similar study, the wear performance of WAAM 316 SS was compared to its cast counterpart. Gurol et al. [42] conducted a comparative analysis of WAAM-fabricated and sand-cast 316 stainless steel components. The findings revealed that the wear rate of as-built parts increased regardless of their hardness. Additionally, abrasive groove lines and granular oxide structures, characteristic of abrasive wear, were observed in the W-D0, W-D60, and W-D120 samples, which correspond to dwell times of 0 s, 60 s, and 120 s, respectively—selected to mitigate cumulative heat accumulation during wall deposition, as illustrated in Figure 9. The presence of abrasive wear, alongside oxidative and adhesive wear mechanisms in these samples, contributed to the overall increase in wear rate. Furthermore, the wear rate increased notably with higher wear distance and applied load, confirming that abrasive wear played a significant role in the wear behavior of W-D0, W-D60, and W-D120 samples [42].

4.1.2. Impact of DED Processing on Wear Behavior

The DED process and its respective processing variables can potentially impact the wear resistance of an alloy. The tribological performance of WAAM 316L samples, fabricated using the CMT process, was evaluated under varying heat input conditions (low, medium, and high) and loads (15 N, 20 N, and 25 N). The coefficient of friction (COF) showed a slight increase with increasing load, while both higher heat input and greater applied load led to an increased wear rate. The maximum wear rate was observed in samples processed with high heat input at a 25 N load, whereas the lowest wear rate was recorded for samples with low heat input at 15 N [43].

FESEM analysis of the wear tracks revealed the presence of craters, abrasive grooves, cracks, delamination, and adhered particles. The predominant sliding wear mechanisms included abrasive wear, surface fatigue, partial adhesion, and oxidative wear [43]. XRD analysis identified δ-ferrite and γ-austenite phases, present both before and after wear. However, post-wear analysis detected an α′-martensite peak, indicating a phase transformation during the wear process [43].

The DED process can also be affected by the addition of particles, which act as inoculants, altering the microstructure and, therefore, the material’s properties, including wear behavior. In the L-DED process, particles can be deliberately introduced to refine the microstructure and improve wear resistance. Zhu et al. [44] investigated this approach by adding two types of powders—Ti and TiC—into High Chromium White Iron (HCWI) to improve its wear performance. This modification was deemed necessary because the refined carbide morphology resulting from the rapid cooling in L-DED negatively affects the wear resistance of repaired components [44].

The addition of Ti powder promoted the formation of In-situ TiC, while TiC powder led to a microstructure containing both coarse ex-situ TiC and fine in-situ TiC. However, the introduction of both powders also destabilized the austenitic (γ-Fe) matrix. The wear resistance of these modified alloys was assessed using two different wear tests: a high-stress abrasion test with silica sand (which is softer than M₇C₃ carbides) and a pin-on-disk test using a ruby pin (significantly harder than M₇C₃) [44].

In the high-stress abrasion test (Figure 10), where abrasives were softer than the carbides, wear resistance was influenced by the carbide morphology and matrix structure. The addition of Ti and TiC powders improved wear resistance through different mechanisms. At 6.5% Ti (Ti1), strain-induced martensite formation occurred during wear testing, enhancing wear resistance. At 15% Ti (Ti2), wear resistance also improved compared to the base alloy due to the formation of interconnected TiC, though to a lesser extent than Ti1. Meanwhile, in alloys with TiC powder additions, the presence of coarse ex-situ TiC carbides increased hardness, resulting in lower material loss [44].

In the pin-on-disk test (Figure 10), the γ-Fe matrix proved beneficial for wear resistance, whereas an increased carbide fraction did not necessarily enhance performance. The addition of TiC powder resulted in a higher volume loss compared to L-DED HCWI, as the discrete TiC particles were easily removed when the hard ruby pin indented the softer matrix and the M₇C₃ carbides. Among the tested alloys, Ti1 exhibited the lowest volume loss, whereas Ti2 experienced a volume loss 13.9 times greater than that of L-DED HCWI. This difference was attributed to the presence of a soft α-Fe matrix in Ti2, which lacked the ductility of γ-Fe and its ability to undergo strain-induced martensite transformation during wear [44].

In a subsequent study, Afshari et al. [45] incorporated TiC and TiB₂ nanoparticles as inoculants into WAAM-fabricated PH13-8Mo stainless steel. Then they subjected the alloy to post-printing solutionizing and aging treatments to enhance its wear performance. The non-inoculated component, in its as-printed state, exhibited anisotropic scratch and low-cycle wear behavior, with greater resistance along the scanning direction due to its inherent microstructural anisotropy. In contrast, the TiC/TiB₂-inoculated sample demonstrated isotropic and enhanced scratch and wear resistance, attributed to a combination of grain refinement, the formation of an isotropic structure, and the load-bearing and Orowan strengthening effects of TiC/TiB₂ particles [45].

Although the TiB₂-inoculated sample exhibited more effective grain refinement and higher hardness compared to its TiC-inoculated counterpart, the TiC-inoculated sample displayed superior wear resistance. This advantage was attributed to its optimal balance of hardness and fracture resistance, along with a higher retained austenite content in the microstructure, which underwent strain-induced martensitic transformation during reciprocating wear testing [45].

Under heat-treated conditions, all samples experienced reduced wear loss compared to their as-printed counterparts, primarily due to the strengthening effect of fine β-NiAl particles. However, the heat-treated TiB₂-inoculated sample exhibited poorer wear performance, which was linked to the formation of M₃B₂ at the grain boundaries, leading to spalling pits on the wear surface [45].

Across both as-printed and heat-treated samples, the dominant wear mechanisms included adhesive wear, three-body abrasive wear, and oxidative wear. However, in the heat-treated TiB₂-inoculated condition, fatigue wear was also observed [45]. The findings of this study provide valuable insights into improving the wear resistance of additively manufactured components, particularly for applications such as injection molding dies [45].

The DED process can also be adapted to incorporate cold working during the deposition of each layer. In one study [46], this was achieved by cooling each deposited layer to below 150 °C before applying a pressure of 350 bars to induce deformation. The next layer was then deposited on the previously cold-worked layer, creating a modified microstructure and potentially enhancing material properties. This study examined the isotropy of mechanical and wear properties in two WAAM-fabricated stainless steel 347 walls: an as-deposited (AD) wall and an inter-layer cold-worked (CW) wall. The investigation focused on differences in performance along the vertical and horizontal directions [46].

Wear test results indicated that inter-layer cold working enhanced hardness, thereby improving wear resistance and reducing friction. The coefficient of friction (COF) and wear rates showed minimal dependence on the sample orientation. The similarity in strength, elongation, and hardness values between horizontal and vertical samples contributed to the comparable wear resistance and COF in both directions [46].

Increasing the applied load from 5 to 10 N led to greater plastic deformation, resulting in higher COF and wear rates for both AD and CW samples. Pin-on-disk test results confirmed that COF and wear rate were largely unaffected by orientation. The primary wear mechanism observed was abrasive wear, with increased load causing more significant plastic deformation within the wear track, leading to higher wear rates and COF values [46].

DED printing with different alloys offers a pathway to tailoring material properties based on specific application requirements. In this context, a single thin-walled structure was fabricated with a graded material design, featuring Super Duplex Stainless Steel (SDSS) 2507 at the bottom and Inconel (IN) 718 at the top. Wear testing was performed under dry, unlubricated conditions using a high-load ball-on-disc tribometer. The results indicated that SDSS 2507 exhibited the lowest wear rate and COF at low loading conditions. Worn surface analysis revealed grooving, delamination, delamination cracks, ploughing grooves, parallel grooves, and particle adherence, confirming adhesion and abrasive wear as the primary wear mechanisms. The highest wear rate was observed in IN 718, while the lowest wear rate was recorded in SDSS. The wear rate at the SDSS 2507–IN 718 interfaces was intermediate, falling between those of SDSS 2507 and IN 718. Furthermore, energy-dispersive spectroscopy (EDS) analysis showed no presence of oxygen elements, indicating the absence of oxidative wear [47].

4.1.3. Wear Behavior of DED-Fabricated Coatings

As discussed in previous sections, arc-DED technology is a promising method for applying metallic coatings to metals and alloys. In a study by Zhang et al. [48], high-manganese (HiMn) and high-manganese medium-chromium (HiMnMeCr) coatings were deposited using arc-DED to examine the impact of Cr addition on wear behavior. Experimental findings revealed that the incorporation of Cr altered the microstructure of WAAM HiMn coatings, transforming them from a biphasic austenite-martensite structure to a single-phase austenite [48].

The addition of Cr extended the running-in period of the wear process and deteriorated the wear resistance of WAAM-HiMnMeCr coating due to the work-hardening effect caused by severe plastic deformation of austenite (Figure 11). Specifically, the presence of martensite in the HiMn coating enhanced the wear resistance of HiMn coating by strengthening the microstructure. In contrast, the HiMnMeCr coating was primarily composed of austenite, a ductile phase with lower frictional resistance, which made it more prone to wear. However, at higher loads, the wear rate of the HiMnMeCr coating decreased due to the work-hardening effect. This suggests that the austenite phase in HiMnMeCr coatings can improve wear resistance by undergoing plastic deformation and work-hardening during frictional contact [48].

At applied loads of 50 N and 100 N, a combination of abrasive and adhesive wear was suggested as the underlying wear mechanism for the HiMn coating (see Figure 12a,b). However, at 120 N, the wear mechanism shifted to predominantly adhesive wear (Figure 12c). In contrast, the HiMnMeCr coatings exhibited abrasive wear as the primary wear mechanism across all tested loads (Figure 12d–f) [48].

4.1.4. Impact of Post-Processing on the Wear Behavior of DED Ferrous Alloys

DED-fabricated alloys often contain internal defects and residual stress, requiring post-processing treatments to enhance their structural integrity before service. Among the most commonly employed methods, hot isostatic pressing (HIP) and heat treatments are particularly effective in reducing residual stress and modifying microstructure, leading to improved mechanical and wear properties where microstructural evolution plays a critical role [49,50].

Afshari et al. [51] investigated the impact of post-fabrication heat treatment on the wear performance of WAAM-fabricated PH 13-8Mo stainless steel. This study examined the wear performance of PH 13-8Mo stainless steel fabricated using WAAM under different post-printing heat treatment cycles and identified an optimal aging condition: solutionizing at 1050 °C for one hour, followed by aging at 500 °C for four hours. This treatment significantly improved hardness, scratch resistance, and wear resistance while reducing the coefficient of friction.

Microstructural analysis revealed that the optimal aging process eliminated detrimental δ-ferrite and promoted precipitation hardening through the formation of nano-scale β-NiAl precipitates. The study also showed that both as-printed and heat-treated samples exhibited similar wear mechanisms. During scratch tests, ploughing was observed, while reciprocating sliding wear tests revealed adhesive, oxidative, and three-body abrasive wear mechanisms. Despite these similarities, the heat-treated samples demonstrated superior wear resistance compared to the as-printed condition. These findings suggest that WAAM-fabricated PH 13-8Mo stainless steel has the potential to serve as a viable alternative to its wrought counterparts for applications that demand high wear resistance. The improvements achieved through post-printing heat treatments further establish WAAM as a promising approach for enhancing the mechanical properties and durability of this material [51].

In another study, a multi-track, multi-layer wire-arc directed energy deposition (WA-DED) process was used to deposit ER420 martensitic stainless steel onto an AISI 4140 substrate. The findings revealed that post-fabrication tempering significantly improved the abrasion resistance of the hardfacing layer, though at the expense of reduced hardness [52].

In the as-deposited state, the top layer exhibited both untempered and tempered martensite regions, resulting in high hardness but reduced toughness. This led to a pronounced abrasion wear rate dominated by a micro-cutting/micro-cracking mechanism. However, post-processing tempering enhanced the toughness, shifting the wear mechanism from micro-cutting/micro-cracking to micro-ploughing, which significantly improved wear resistance [52].

The improved wear resistance in the tempered condition was attributed to the dispersion of ultra-fine carbides with a short mean free path within the high-toughness tempered martensite matrix. This refined microstructure provided strong shielding against abrasion wear while enhancing ductility, toughness, and work-hardening ability [52].

Austenitic stainless steels have also been fabricated using WAAM and subsequently subjected to heat treatment to evaluate their wear behavior. Their performance was assessed under dry sliding conditions at elevated temperatures and compared to that of a conventional 347 stainless steel substrate (Figure 13). While WAAM samples exhibited higher hardness, they also experienced greater wear volume loss and wear rate due to non-uniform oxide formation, which hindered the development of protective glaze layers.

At higher temperatures, oxygen-rich layers formed, sealing wear scratches and reducing wear when supported by the substrate. The formation of Fe₃O₄ acted as a self-lubricant, stabilizing the wear rate, though the 347 substrate demonstrated better overall wear resistance, particularly at higher temperatures (Figure 13). SEM analysis (Figure 14 and Figure 15) showed that with increasing temperature, aluminum peaks decreased while iron peaks increased, confirming the healing effect of Fe₃O₄. WAAM-processed 347 exhibited lower friction and increased aluminum peaks, while Fe₃O₄ aided self-lubrication.

The wear mechanism evolved from adhesive wear at 200 °C to mild oxidative wear at 400–600 °C, where wear debris formed a mechanically mixed composite layer containing Fe₂O₃, Fe₃O₄, and Al₂O₃. At 600 °C, a stable glazed oxide layer developed, contributing to surface hardening and reducing wear. Despite the lower coefficient of friction in WAAM samples, the 347-substrate displayed superior wear resistance due to more effective oxide layer formation, which helped maintain surface integrity during prolonged sliding [20].

L-DED-fabricated ferrous alloys have also been subjected to post-process heat treatments to modify their wear behavior. One study examined the wear resistance of as-built and heat-treated AISI M2 high-speed tool steel fabricated using the DED process, highlighting the impact of heat treatment on its tribological performance. The as-built M2 alloy primarily consisted of both α′-martensite and γ-austenite, along with elongated carbides (WC, MoC, and VC) that formed between dendrite arms due to elemental segregation during solidification. Heat treatment effectively homogenized the microstructure by dissolving these elongated carbides and reprecipitating them as fine secondary carbides, leading to enhanced hardness and tensile strength [54].

The wear behavior was analyzed under varying loads and sliding speeds using ball-on-disk tribology tests with two different counterpart materials: bearing steel and ZrO₂. The results showed that when a bearing steel ball was used, the M2 alloy exhibited negligible wear damage. However, when tested with a ZrO₂ ball, a small but measurable wear weight loss was observed, highlighting the influence of counterface material on the wear performance of heat-treated L-DED M2 steel [54].

Overall, the DED-fabricated M2 alloy demonstrated exceptional wear resistance, outperforming both fully carburized conventional steel and high-wear-resistance steel produced via DED. This superior performance was attributed to the formation of a lubricious tribo-oxide film on the worn surface, which played a crucial role in reducing wear. Additionally, microstructural analysis of the worn surfaces and wear debris revealed that abrasive wear of this tribo-oxide film was the dominant wear mechanism. The wear weight loss of DED-produced M2 was over six times lower than that of fully carburized structural steel and 1.5–4 times lower than that of other high-wear-resistance steels produced by DED [54].

4.1.5. Impact of Wear Test Loading on Wear Behavior of L-DED Ferrous Alloys

Normal load is one of the key parameters influencing wear loss in materials [55]. During wear testing, the applied load is typically varied to better simulate real-world service conditions, where components may experience different loading scenarios. In one study [56], single thin walls of austenitic stainless steel (ASS 308L) were fabricated using ER308L filler wire to assess their wear performance under such conditions. Wear tests were performed under dry (unlubricated) conditions using a high-load pin-on-disc tribometer. AM-ASS processed with lower heat input exhibited a lower COF and reduced wear rate. Analysis of wear debris indicated that martensite was the predominant phase. During wear, austenite transforms into martensite due to the metastability of ASS under plastic deformation. At higher stress levels, plastic deformation of the sliding surfaces becomes more significant. Additionally, as the applied load increases, the ferrite (%) concentration decreases due to the strain-induced transformation of austenite into martensite. As the wear debris undergoes rolling deformation, it hardens, fractures, and forms cylindrical shapes, confirming that adhesive wear is the dominant wear mechanism [56].

In a study by Yadav et al. [57], a functionally graded deposition (FGD) structure of dissimilar steels (SS316LSi and ER70S-6) was fabricated using twin-wire arc additive manufacturing, and its wear properties were examined. This study highlighted the impact of applied load on wear behavior, revealing that adhered material, abrasive wear, grooves, and delamination become more pronounced at higher loads (see Figure 16). Abrasive wear continuously removes the oxide layer, exposing fresh material to oxidation, while the circular motion in sliding wear disrupts passive oxide layers. Adhesive wear and mechanical alloying contribute to the formation of brittle particles and material removal. Additionally, frictional heating raises surface temperatures, accelerating oxidation, particularly of iron, as confirmed by EDS analysis, showing elevated oxygen content on worn surfaces. Consequently, increased applied loads intensify plastic deformation, roughen surfaces, and enhance wear mechanisms such as abrasion and adhesion, leading to accelerated material degradation [57].

In a later study conducted by Yadav [58], 316LSi austenitic SS was fabricated using arc-DED, and its wear behavior was evaluated under different applied loads. The COF for WAAM-processed austenitic SS samples ranged from 0.5410 to 0.5688. The highest wear rate was observed at 30 N, while the lowest occurred at 20 N (Figure 17). This increase in wear at higher loads is attributed to greater compressive stress, which leads to intensified plastic deformation in the structural layers beneath the worn surface [58].

As shown in Figure 18, the worn surfaces exhibited significant plastic deformation and a metallic appearance. Detailed FESEM analysis revealed various wear tracks, including particle adhesion, ploughing grooves, parallel grooves, and delamination, all of which indicate an adhesive wear mechanism characteristic of ductile materials. The presence of oxygen in different regions of the worn surfaces suggests that frictional heat contributes to elevated surface temperatures. This temporary rise in temperature promotes oxidation upon exposure to air, similar to how iron oxidizes at high temperatures [58].

During the wear experiment, the oxide film is continuously removed due to abrasion, exposing fresh material to ambient oxygen, as illustrated in Figure 18. The repeated movement of the WC ball over the samples leads to the ploughing and removal of any protective oxide layers by the opposing material. Moreover, the scattered attachment of worn material suggests that mechanical alloying occurs continuously during adhesive wear, helping to remove brittle fragments from the surface. This cycle persists until the wear test is complete [58].

4.1.6. Tool Wear Behavior

Additively manufactured (AM) parts often undergo post-surface treatments to enhance surface finish for their intended applications. However, during these processes, tool degradation occurs due to tribological interactions at the interface between the tool and the AM component. As a result, tool wear has been the subject of several studies to assess how AM-fabricated parts influence tool longevity. One such study [59] on hybrid additive and subtractive manufacturing (HASM) examined the wear resistance of a TiAlN-coated tool at various cutting speeds while milling DED 316L stainless steel under dry and thermal conditions. HASM aims to optimize manufacturing by reducing the number of clamping steps and energy consumption [59]. The findings revealed that tool wear was more pronounced in thermal milling compared to room-temperature milling. However, high-temperature milling without cooling offered several advantages, including improved machining accuracy, enhanced bonding strength, reduced clamping steps, and lower energy consumption [59].

For AM components, tool wear occurred in two distinct stages due to the uneven surface and milling-induced vibrations. In the initial stage, stable tool wear resulted in a relatively smooth surface with minimal residual chips. As milling progressed, residual chips began to adhere to the surface, significantly degrading surface quality. In the later stages of tool wear, large residual chip bulges created noticeable differences between peak and valley heights on the surface [59]. In this context, lower milling speeds led to better surface roughness, and a quadruple-edged milling cutter demonstrated superior wear resistance compared to a double-edged cutter. While the study provided detailed insights into surface roughness and microscopic analysis of wear, it did not explicitly identify the specific wear mechanisms (e.g., adhesive or abrasive wear) responsible for tool degradation [59].

4.2. Wear Behavior of DED-Fabricated Nickel-Base Alloys

This section examines the wear response of nickel-based alloys fabricated through the DED process. To provide a clear and systematic analysis, each study is discussed in separate subsections, highlighting the influence of various fabrication methods, process parameters, post-processing techniques, and wear test conditions on tribological performance.

4.2.1. Comparison of Wrought and WAAM-Fabricated Ni-Based Alloys

In an investigation, Alloy 825 was WAAM-printed, and its wear behavior was compared to its wrought counterparts. All specimens produced through WAAM underwent a standardized dry sliding wear test to assess their wear characteristics. Factors such as load, sliding velocity, and sliding distance were evaluated for both the wrought and WAAM alloys. The findings indicated that wear was minimized at lower loads combined with higher sliding velocities, while the most significant wear occurred under higher loads and lower sliding velocities. Additionally, the study examined the wear mechanisms and debris associated with both the lowest and highest wear scenarios. The shift from minimal to maximal wear rates in both alloy types was linked to a transition from abrasive wear mechanisms to adhesive and delamination wear [60].

The worn surfaces of the wrought nickel-based alloy revealed that conditions promoting lower wear resulted in smoother surfaces with protective oxide layers. In contrast, higher wear conditions led to the formation of deep pits, ploughing, and increased material loss attributed to adhesion and delamination. The analysis of the worn surfaces of the WAAM nickel-based alloy showed that under the lowest wear conditions, narrow grooves and signs of abrasive wear with some adhesion were present. Conversely, under the most severe wear conditions, severe delamination was evident, characterized by wider grooves and greater material loss due to the absence of a protective oxide layer. Wear debris analysis indicated that lower wear conditions produced small, ribbon-like particles, while higher wear conditions resulted in larger, lump-shaped and flake-like debris, confirming the presence of adhesive and delamination wear mechanisms in both the wrought and WAAM alloys [60].

4.2.2. Comparison Between Powder-Fed and Wire-Fed DED Processes of Nickel Alloys

A study examined the tribological behavior of NiTi alloy samples produced using laser wire and laser powder DED (LW-DED and LP-DED) technologies across a range of temperatures, from room temperature to 200 °C. In LP-DED samples, a combination of adhesive and abrasive wear mechanisms, along with Fe transfer from the ball to the flat sample at lower temperatures, contributed to a reduction in wear volume loss. As a result, the LP-DED samples exhibited significantly lower wear volume loss—33.7% less compared to the LW-DED samples. Conversely, in LW-DED samples, abrasive wear was the dominant mechanism at lower temperatures, as indicated by the absence of Fe in EDS signals, leading to a higher wear volume loss. The superior wear resistance of LP-DED samples can be attributed to a synergistic effect of stabilized austenite at room temperature and increased hardness, as confirmed by DSC, XRD, and microhardness analyses [61].

Despite differences in wear mechanisms between LP-DED and LW-DED samples at lower temperatures, no significant distinctions were observed at higher temperatures. In both cases, Fe transfer from the ball increased, indicating a transition toward an adhesive wear mechanism. Additionally, wear volume and depth exhibited a decreasing trend with increasing temperature, reaching their lowest values at 200 °C [61].

The dominant wear mechanism in steel balls reciprocating against LP-DED NiTi samples remained abrasive across all tested temperatures. However, for LW-DED samples, the ball wear mechanism transitioned from adhesive at lower temperatures to abrasive at higher temperatures due to the tribological interactions [61].

4.2.3. Impact of DED Processing on the Wear Behavior of Nickel-Based Alloys

Heterogeneous microstructure designs have garnered significant interest due to their ability to achieve an optimal balance between two conflicting properties, while remaining cost-effective and scalable for industrial applications. WAAM technology, like laser-DED, has the capability to create spatially heterogeneous microstructures with significant microstructural variations in metals by adjusting process parameters. This approach has been utilized to fabricate both homogeneous and heterogeneous NiTi alloys, enabling a comparative analysis of their wear behavior. Specifically, homogeneous and heterogeneous NiTi thin-walled components were fabricated using WAAM technology by adjusting the heat input. The homogeneous WAAM NiTi component demonstrated superior wear resistance, with the coefficient of friction decreasing from 0.760 to 0.715 as the deposition height increased. The primary wear mechanisms in homogeneous NiTi samples were adhesive wear and oxidative wear, whereas heterogeneous NiTi samples were predominantly affected by adhesive wear (Figure 19) [62].

SEM analysis of wear scratches in both WAAM-fabricated NiTi alloys (Figure 19) revealed the presence of furrows, spalling, and adhesion, with oxidation mainly occurring in the upper regions due to rapid cooling rates. The homogeneous NiTi alloy exhibited material buildup on both sides of the scratches and underwent a combination of oxidative and adhesive wear. In contrast, the heterogeneous NiTi alloy displayed more pronounced scratches, with adhesive wear being the dominant mechanism [62].

Another study aimed to enhance the abrasive wear resistance of EN-8 steel for applications in farm machinery components. To achieve this, Inconel 625 was successfully cladded onto EN-8 steel using the WAAM-CMT process, with the addition of TiC powder at varying concentrations (10%, 20%, and 30%). All developed clads exhibited superior abrasive wear resistance compared to the base EN-8 steel, primarily due to their higher microhardness values. During abrasive wear testing, smaller particles, when carried by a medium sand flow rate under higher loads, caused more damage to the clads due to their irregular shapes and ability to penetrate the surface more effectively. In two-body abrasion testing, the Inc625 + 30TiC clad demonstrated the lowest mass loss rate [63].

Kishor et al. [64] conducted an in-depth study to understand variations in the local microstructure and texture of components fabricated using WAAM. These variations directly influence the mechanical and tribological properties of the manufactured parts. Deposition patterns and process parameters play a crucial role in shaping the microstructure, thereby affecting the overall performance and wear resistance of the builds.

The influence of extraction location (i.e., top, middle, and bottom) on the wear properties of WAAM Inconel 625 has also been investigated [64]. It was revealed that the COF varied across different regions of the build (Figure 20). The top region exhibited the lowest COF due to Cube-related textures, which enhanced wear resistance (

Table 2. Variation in tribological properties across different regions of WAAM-Inconel 625 [48].

Location	COF	Maximum Penetration Depth (nm)	Wear Volume (10−5 × μm3)	Dominant Texture *
Top	0.337	0.015	2.52	Cube, Cube-RD, and Cube-ND
Middle	0.456	0.050	8.42	Brass, Goss, Cu, S
Bottom	0.323	0.016	2.69	Cube-ND, Cube- RD, Brass

* Cube {001}<100>, Cube-ND {001}<310>, Cube-RD{013}<100>, Goss {011}<100>, and E{111}<110>, Brass {110}<112>, Copper {112}<111> and S {123}<634>, Cu {112}<111>.

). In contrast, the middle region showed the highest COF and the greatest material loss, influenced by Brass {110}<112>, Goss {011}<100>, Cu {112}<111>, and S {123}<634> textures. The bottom region had a slightly lower COF than the middle, displaying moderate wear resistance due to a combination of Cube and Brass textures [64].

These variations in COF and wear behavior were attributed to differences in crystallographic orientations. The dominance of the {101} family of planes in the top region provided higher strength and improved wear resistance, while the prevalence of the {001} family of planes in the middle region contributed to increased material loss [64].

In a prospective study, WAAM technology was employed to fabricate composite NiTi/Nb layered heterostructured materials (Figure 21a), and their wear properties were subsequently analyzed (Figure 21b). The thickness ratios of the NiTi and Nb layers were 1:1 (LHS1) and 2:1 (LHS2), respectively. Under a 20 N load, the wear rate of the NiTi layer in the LHS samples was lower than that of the Nb layer. In fact, the LHS-NiTi surface exhibited superior wear resistance compared to the Nb layer. When subjected to friction against an Al₂O₃ ball, the Nb layer underwent plastic deformation, forming furrows and accumulating abrasive particles. The continuous friction-induced heat further promoted oxidation and increased wear. In contrast, the NiTi layer, being harder and superelastic, experienced less deformation and lower wear rates, though minor furrows formed due to debris transfer [65].

A comparison between LHS1 and LHS2 revealed that LHS2 demonstrated superior wear resistance, attributed to its gradient heterogeneous microstructure, which facilitated varied recoverable deformation. On the other hand, LHS1, with its higher Nb content and uniform eutectic structure, experienced more severe wear due to its lower ability to accommodate deformation [65].

In summary, the primary wear mechanisms for the LHS-Nb surface were abrasive, adhesive, and oxidative wear, while adhesive wear was dominant on the LHS-NiTi surface. LHS2 exhibited superior cross-sectional wear resistance compared to LHS1 due to the formation of a gradient heterogeneous eutectic structure in the NiTi region, which contributed to improved mechanical stability under sliding wear conditions [65].

4.2.4. Effect of Post-Processing on the Wear Performance of DED-Processed Nickel Alloys

Boronizing is a thermo-chemical surface treatment that involves the diffusion of boron atoms into a metal’s surface, creating a hard, wear-resistant metal boride layer. This technique greatly improves the material’s hardness and resistance to wear, often extending the lifespan of components by two to five times compared to traditional heat treatments [66].

In this context, a study by Gunen et al. [67] applied a simultaneous heat treatment and boronizing process (980 °C for 1 h) to Inconel 625 parts fabricated using the GMAW-based DED process. This single-step treatment aimed to enhance wear performance, addressing common challenges in DED-processed superalloys, such as low surface hardness, high heat input, and severe elemental segregation [67].

The results showed that the boride layer formed on the surface was primarily composed of the Ni₂B phase, along with smaller amounts of CrB₂, MoB₂, and Nb₃B₂ hard boride phases. In addition, the high affinity of boron for oxygen facilitated the formation of finely dispersed hard oxides, leading to a lower friction coefficient at both room temperature and 500 °C compared to non-boronized samples [67].

Wear tests revealed that arc-DED-manufactured and boronized Inconel 625 exhibited a remarkable 58.17-fold improvement in wear resistance at room temperature and a 3.86-fold improvement at 500 °C compared to non-boronized samples. The wear mechanisms also varied with temperature and treatment. For non-boronized samples, abrasive and delamination wear dominated at room temperature, transitioning to oxidation-assisted adhesion and plastic deformation at 500 °C. In contrast, boronized samples primarily exhibited micro-cracks and oxidative wear at room temperature, while at 500 °C, the wear mechanism shifted to oxidation-assisted polishing [51].

Analyzing machining parameters and their influence on material properties is crucial for optimizing manufacturing strategies and improving process efficiency. Nickel-based superalloys, such as IN718, are particularly challenging to machine due to their high strength and wear resistance. A study on WAAM-fabricated IN718 investigated a hybrid additive manufacturing (AM) approach that combined AM with post-processing techniques to enhance surface quality [68]. The research specifically examined how the heat treatment cycle of IN718 affects tool wear behavior. A semi-finishing TiAlN insert was used as the cutting tool, featuring a rake angle of 6° and a clearance angle of 0° [68].

Overall, adhesive and abrasive wear were identified as the primary wear mechanisms affecting the tool during the dry turning of both as-printed and heat-treated IN718. Specifically, the as-printed IN718 (AP IN718) exhibited higher adhesive and abrasive wear on the tool, which was attributed to its greater ductility and lower mechanical strength. These characteristics resulted from the rapid thermal cycling during the L-DED process, which inhibited the formation of strengthening phases [68].

Conversely, the complete heat treatment cycle of IN718—including homogenization, solution treatment, and double aging—led to the dissolution of detrimental Laves phases, the formation of strengthening γ′ and γ″ phases, and a transition from a dendritic to an equiaxed microstructure. However, during high-speed machining, heat-treated IN718 caused crater formation on the tool’s rake face in the initial stages, leading to accelerated wear and premature tool failure (Figure 22) [68].

Another study highlights the critical role of post-DED surface finishing processes, such as polishing and grinding, in enhancing the surface quality of components produced through the DED method. Historically, LP-DED-fabricated parts were deemed unsuitable for aerospace applications due to their inherent surface roughness and related limitations. However, these challenges can be addressed using mechanical surface finishing techniques, which significantly improve the mechanical performance of the components. Various advanced surface treatment methods, including ultrasonic shot peening (USP), shot peening (SP), high-pressure torsion (HPT), ultrasonic impact treatment (UIT), laser shock peening (LSP), surface mechanical attrition treatment (SMAT), and ultrasonic nanocrystal surface modification (UNSM), have shown promise in enhancing mechanical properties. These improvements are primarily attributed to reduced surface roughness, the development of a hardened surface layer with nano-grains, and the mitigation of microstructural defects [69,70,71,72].

In this context, post-surface treatment techniques such as ultrasonic nanocrystal surface modification (UNSM) have been investigated for their ability to enhance the dry fretting wear resistance of LP-DED Inconel 718. To isolate the effects of UNSM, surface roughness was intentionally maintained at a similar level between polished and UNSM-treated samples, ensuring it did not influence the fretting wear performance [73]. Fretting wear behavior was evaluated using a ball-on-disk configuration at 24 °C and 60 °C. The results showed that the polished samples exhibited enhanced fretting wear resistance at both temperatures, with higher temperatures further improving wear resistance (Figure 23). This improvement was primarily attributed to the nanostructured surface layer formed by the UNSM treatment, which helped delay or even prevent crack initiation and propagation compared to untreated samples [73]. Specifically, a key factor in this enhanced wear performance was the introduction of high compressive residual stress (see Figure 24), which played a crucial role in preventing microcrack formation. Additionally, the removal of pores from the top and near-surface regions after UNSM treatment further contributed to the improved fretting wear resistance of L-DED Inconel 718 [73].

4.2.5. Finite Element Modeling of Post-Surface Treatment in Ni-Based L-DED

This study utilizes the finite element method (FEM) to investigate the effect of ultrasonic nanocrystal surface modification (UNSM) on the wear behavior of Inconel 718. A fretting wear model is developed using an energy-based FEM approach, incorporating the Johnson–Cook constitutive model and linear isotropic hardening to simulate plastic deformation. Additionally, the influences of the coefficient of friction and wear coefficient, modified by UNSM, are analyzed separately to better understand their role in wear mechanisms [74]. The simulation results exhibited remarkable consistency with experimental data, showing a wear loss discrepancy of less than 2%. Findings indicated that as the normal load increased, the enhancement in wear resistance provided by UNSM gradually diminished. In addition, UNSM proved effective in reducing shear stress distribution, thereby minimizing the progressive wear depth, as predicted by the energy-based wear model [74].

4.3. Wear Behavior of DED-Fabricated Titanium-Based Alloys

This section explores the wear performance of titanium-based alloys processed via different DED techniques. Given the widespread use of titanium alloys in aerospace and biomedical applications, understanding their tribological properties is essential for optimizing their performance under service conditions. The subsections discuss the impact of fabrication methods, microstructural variations, temperature effects, and reinforcement strategies on wear behavior.

4.3.1. Comparison of L-DED and L-PBF-Fabricated Ti Alloys

Comparing the wear behavior of titanium alloys processed via L-DED and L-PBF provides insights into how different DED techniques influence tribological properties. In a study by Kang et al. [75], the sliding wear performance of L-DED Ti-6Al-4V was evaluated against L-PBF and forged counterparts, highlighting the role of microstructural differences in wear resistance. Both L-PBF and L-DED samples exhibited nonequilibrium microstructures with lath/acicular α-Ti due to their high cooling rates [75]. When evaluating hardness at multiple scales, the L-DED sample showed the highest microhardness, likely due to the presence of high-hardness β-phase in both L-DED and L-PBF processed samples. However, at the nanoscale, the forged Ti6Al4V exhibited the highest hardness (approximately 4.7 GPa). This discrepancy arises because nano-indentation is highly localized, often measuring only a single phase, whereas microhardness testing—especially in fine-grained structures—captures multiple phase boundaries [75].

In terms of wear performance, the forged Ti6Al4V sample demonstrated a lower wear rate than both L-DED and L-PBF samples (Figure 25a). This was attributed to the presence of high strain in certain interface regions between α grains, which were located far from the plastic deformation zones in the L-DED sample. It was revealed that the L-PBF sample experienced less surface plastic deformation compared to the L-DED and forged samples. As shown in Figure 25b–c, the maximum groove depth in the L-PBF sample (~47 μm) was significantly smaller than that in the L-DED and forged samples (~70 μm), which was due to the lower ductility of the L-PBF material. Regarding wear mechanisms, the L-DED and forged samples exhibited a combination of adhesive, abrasive, and oxidative wear, whereas the L-PBF sample primarily underwent adhesive and abrasive wear [75].

Multi-wire arc additive manufacturing (MWAAM) utilizes dual or multiple wires as feedstock, enabling in-situ alloy synthesis by adjusting wire feed rates. It can also enhance deposition efficiency by simultaneously feeding multiple wires of the same alloy at identical speeds. While MWAAM offers greater deposition efficiency and material utilization than conventional WAAM, its complex and sensitive droplet transition affects both forming accuracy and metallurgical quality [76]. In a relevant study, MWAAM and WAAM were used to fabricate TC11 titanium alloy, resulting in thin walls with a microstructure of α- and β-Ti. It was revealed that the microstructure of MWAAM TC11 is finer compared to the WAAM counterpart (Figure 26). Besides, selective grain growth orientation was absent in both WAAM and MWAAM samples, likely because of the intricate variations in heat flow direction induced by the Marangoni thermal convection effect within the melt pool. More importantly, wear tests revealed that TC11 samples produced using MWAAM exhibited lower wear rates (Figure 27), with abrasive, adhesive, and oxidative wear being the predominant wear mechanisms [77].

Typically, the heat generated by the friction pair during reciprocating sliding leads to the formation of an oxide layer on the sample surface. As the wear process progresses, fragments of this oxide layer start to spall off, resulting in oxidative wear. In this context, the sample surface, due to its higher oxidation rate, tends to form an oxide layer that protects the surface. However, this protective layer is gradually worn away during the cyclic loading of the friction counterpart. If the rate of oxide layer depletion surpasses its formation rate, the oxidized debris produced by friction can intensify oxidative wear, reducing friction performance. Conversely, if the consumption rate is slower than the formation rate, the oxide layer can regenerate in time to shield the coating surface before significant surface wear occurs, thereby enhancing friction performance. In the case of sample 1 (WAAM sample), the oxide layer’s depletion rate was higher, leading to accelerated wear due to oxidized debris, which aligns with the observation of substantial wear debris on the sample surface [77].

4.3.2. Impact of DED Processing on Titanium Alloys

In this section, the effects of DED processing on titanium alloys are explored, with a focus on microstructural evolution, mechanical properties, and reinforcement strategies. One such approach involves the incorporation of tungsten carbide particles to enhance the performance of titanium alloys. A tungsten carbide particle-reinforced Ti-6Al-3Nb-2Zr-1Mo alloy coating was successfully fabricated using WAAM with simultaneous wire-powder feeding. This titanium matrix composite coating exhibited significantly enhanced wear resistance compared to the substrate material, achieving an 86.96% reduction in the average wear rate. The composite coating undergoes multiple wear mechanisms during dry sliding friction, including adhesive wear, abrasive wear, three-body wear, and oxidation wear, which collectively contribute to its superior performance and durability [70].

During the sliding friction process, material transfer from the titanium alloy matrix to the particle surface leads to the formation of a thick oxide layer containing Ti elements on the tungsten carbide particles. Additionally, the wear debris surrounding the tungsten carbide particles consists of Fe-rich oxides. As friction progresses, the oxide layer thickens until the particles can no longer support it, causing large pieces of oxide film to peel off onto the substrate surface [70].

Tungsten carbide particles play a crucial role in hindering the flow of oxides. Under repeated extrusion by the friction ball, oxide products may adhere to the wear surface, forming a dense protective layer around the particles. Grain refinement increases the number of grain boundaries, offering more diffusion paths for oxygen and promoting oxide film formation. This oxide layer acts as a lubricant, reducing shear forces and lowering the COF of the coating [70].

Under reciprocating friction, fatigue cracks initiate and propagate within some tungsten carbide particles, leading to their local breakage. The fractured tungsten carbide fragments become new abrasive debris, contributing to three-body wear. An interesting observation is the accumulation of wear debris around tungsten carbide particles (Figure 28). These hard, broken particles cause the matrix near them to develop deep craters, which, in turn, trap wear debris and slow down abrasive wear [70].

In another investigation, the impact of operating temperature on the wear behavior of WAAM-fabricated Ti alloy was examined. The TA15 Titanium alloy is extensively used in aircraft engines for load-bearing structural components subjected to sliding wear at varying operational temperatures. The study revealed that both the COF and wear volume underwent multi-stage changes as the temperature increased. Initially, the crack morphology consisted of randomly distributed pores, but as the temperature rose, these transformed into delamination cracks parallel to the sliding direction (Figure 29). This shift was attributed to increased thermal stress and the brittleness of the oxide layer [78].

At high temperatures, the classic oxidative wear model failed to accurately describe wear behavior. To address this, a modified model incorporating dissipated energy and a delamination coefficient was introduced, improving the prediction of TA15 alloy’s wear volume under high-temperature conditions [78].

4.4. Wear Behavior of DED-Fabricated Copper-Based Alloys

Comparison with Conventionally Fabricated Alloys.

The tribological performance and wear resistance of copper (Cu) alloys fabricated through DED play a crucial role in determining their durability and reliability. Copper alloys are widely utilized in applications that demand high thermal and electrical conductivity, such as heat exchangers, electrical contacts, and aerospace components. This section examines the tribological properties of DED-processed copper alloys in comparison to their wrought and as-cast counterparts, providing insights into their suitability for demanding industrial applications.

In this context, an investigation was conducted using L-DED to fabricate CuSn10 on a steel substrate, with its wear behavior evaluated under dry conditions. The friction coefficient of the DED specimens (0.52–0.58) was comparable to that of wrought CuSn10 (0.55), with minimal variation in wear rate (Figure 30). This slight reduction in wear rate was attributed to the higher microhardness of the DED specimens. No debonding between the CuSn10 deposition and the steel substrate was observed during wear testing. Additionally, increasing the laser power significantly reduced the presence of lack-of-fusion pores. The wear mechanism was primarily abrasive, characterized by microploughing between the CuSn10 and the steel counterpart (Figure 31). Both DED and wrought specimens exhibited a smooth surface, though the distribution of wear debris was higher on the wrought specimen.

In a subsequent study, the wear performance of L-DED CuSn12Ni2 bronze was assessed using a pin-on-disc test. The results indicated that the wear test did not cause debonding at the interface. Besides, the measured friction coefficient, ranging from 0.55 to 0.6, was found to be comparable to that of wrought bronze [80].

To assess the viability of additive manufacturing for copper-based alloys, it is essential to compare the properties of WAAM-fabricated materials with their conventionally processed counterparts. Nickel aluminum bronze (NAB), a widely used Cu-based alloy, has been successfully fabricated using WAAM, and its wear behavior has been evaluated against that of its as-cast counterpart. In the as-cast NAB alloy (Figure 32a,b), the presence of large, hard κ phases led to the formation of abrasive particles during friction. As these hard particles interacted with the substrate, cutting and squeezing effects occurred, resulting in abrasive wear. With continued friction, the generated heat softened the material, while micro-convex bodies created furrows that caused significant material loss, further increasing wear (Figure 32c). Consequently, the wear surface of the as-cast NAB alloy exhibited severe deformation, characterized by collapsed abrasive chips and delamination. EDS analysis revealed the presence of carbon (C) and oxygen (O) on the sample surface (Figure 32d). The adsorption of C was attributed to molecular diffusion, while the presence of O indicated mild oxidation of the material [81].

Compared to the as-cast NAB alloy, the WAAM-NAB alloy in Figure 32e–h exhibited a smoother wear surface with significantly reduced wear depth and volume. Analysis of the friction coefficient curves, wear depth, and volume measurements confirmed that the WAAM-NAB alloy demonstrated superior wear resistance [81].

4.5. Wear Behavior of DED-Fabricated Tantalum-Based Alloys

Tantalum (Ta) is a durable refractory metal with excellent mechanical, thermal, and electrical properties. Its biocompatibility, high melting point, and corrosion resistance make it valuable in medical, aerospace, military, and nuclear industries. A key feature of tantalum is its self-healing oxide layer (Ta₂O₅), which provides exceptional protection against harsh environments, even in low-oxygen conditions. This natural resistance makes tantalum a preferred choice for coatings to enhance the durability of other metals.

In a study, tantalum-based powder was used with L-DED, a technology that has also been employed for applying metallic coatings to improve the performance of metals and alloys. Xie et al. [82] successfully deposited a crack-free Ta–Zr alloy powder layer (Ta:Zr = 7:3 by weight) onto a Ti6Al4V substrate using L-DED. The coating primarily consisted of rectangular BCC α-Ta grains, ranging from 10 to 20 nm in size, surrounded by the TiZr phase, with the Al₃Zr₄ phase concentrated at grain boundaries [82].

Wear test results revealed that the coating’s mass loss was approximately 60 times lower than that of the substrate at room temperature (Figure 33a,b). This exceptional wear resistance was attributed to the coating’s high microhardness, resulting from its fine-grained structures and the presence of intermetallic compounds (TiZr and Al₃Zr₄). However, the COF for the Ti6Al4V substrate was found to be lower than that of the L-DED Ta-Zr coating (Figure 33c)—an outcome that appears counterintuitive [82].

As illustrated in Figure 34, this phenomenon is explained by the higher mass loss observed in the rolled Ti6Al4V substrate, which is attributed to its lower hardness. The substrate’s worn surface exhibited significant peel-offs and TiO₂ debris, contributing to the reduced COF. In contrast, the finer grain structure and the presence of TiZr and Al₃Zr₄ in the coating resulted in hardness comparable to that of the GCr15 counterpart material. This caused increased wear on the counterpart’s surface, generating iron filings. Therefore, while the coating exhibited a higher friction coefficient than the substrate, its overall wear rate remained significantly lower [82].

5. A General Overview of DED Processes and Wear Characteristics of DED-Fabricated Alloys

The processing parameters for L-DED and WAAM-fabricated alloys, as reviewed in the previous section, are summarized in

Table 3. L-DED processing parameters of alloys used for tribological analysis in the literature.

Alloy	Laser Power (W)	Shielding Gas	Travel Speed (cm min−1)	Layer Thickness (mm)	Scanning Strategy Rotation (Degree)	Ref.
AISI M2	900	Ar	85	0.25	90	[54]
316L SS	1000	N.A.	42	0.6	90	[59]
high chromium white irons	2250	Ar	100	1	90 (meander)	[44]
CuSn10	600–1100	N.A.	50–75	N.A.	N.A.	[79]
CuSn12Ni2 bronze	2000	Ar	80	N.A.	90	[80]
IN718	1600	N.A.	90	0.8	N.A.	[73]
Ti6Al4V	2300	N.A.	1.5	0.6	N.A. (Zigzag)	[75]
Ta-Zr	3000	Ar	30	3	N.A.	[82]

and

Table 4. WAAM processing parameters of alloys used for tribological analysis in the literature.

Alloy	Shielding Gas	Current (A)	Voltage (V)	Travel Speed (cm min−1)	Layer Thickness (mm)	Wire Feed Rate (cm/min)	Ref.
316L	Ar	110–150	13.2–14.8	35	N.A.	290–420	[43]
ER 316L	97.5% Argon + 2.5% CO2	110	14	45	N.A.	N.A.	[42]
SS316LSi & ER70S-6	Ar	135	16	31	N.A.	N.A.	[57]
SS316LSi	Ar	138	16	33.4	N.A.	N.A.	[58]
304 SS	90% He + 7.5% Ar + 2.5% CO2	N.A.	22	15.2	N.A.	543.5	[41]
ER347	98% Argon + 2% CO2	140	16.2	40	N.A.	451	[53]
ER420 martensitic stainless steel	Ar	N.A.	N.A.	N.A.	2	N.A.	[52]
SDSS 2507	Ar	210	N.A.	0.69	N.A.	N.A.	[47]
PH 13-8Mo	90% He + 7.5% Ar + 2.5% CO2	135	28	24	N.A.	408	[51]
PH 13-8Mo	90% He, 7.5% Ar, and 2.5% CO2	135	28	24	N.A.	408	[45]
high-manganese steel	20% CO2 + 80% Ar	300	30	N.A.	2	N.A.	[48]
IN625	97.5% Ar + 2.5% CO2	150	15.8	50	0.65	N.A.	[67]
IN625	Ar	155	16.5	10	N.A.	550	[63]
IN625	Ar + CO2	150	22	N.A.	2–3	450	[64]
IN718	Ar	190	N.A.	0.89	N.A.	N.A.	[47]
Alloy 825	Ar	N.A.	28	25	2.7	800	[60]
Nitinol 55	Ar	N.A.	N.A.	N.A.	N.A.	44.2	[61]
NiTi	Ar	200–220	-	30	N.A.	80	[62]
NiTi/Nb composite	Ar	NiTi: 100	NiTi: N.A.	NiTi: 30	N.A.	N.A.	[65]
		Nb: 152	Nb: 13.7	Nb: 45
TC11	Ar	120	14–14.5	34–38	N.A.	100	[77]
TA15 alloy	Ar	140.4	13.5	N.A.	2	200	[78]
Ti-6Al-3Nb-2Zr-1Mo alloy	Ar	180	N.A.	12	N.A.	120	[70]
NAB	Ar	60	8.7	10	N.A.	N.A.	[81]

. A comparison of these tables reveals that wear studies on WAAM-produced alloys are considerably more common than those on L-DED-manufactured counterparts. This difference is primarily attributed to the unique advantages of the WAAM process, such as its high deposition rates and the relatively lower cost of wire feedstock compared to powder-based methods, where the chamber size for 3D printing is restricted. These factors make WAAM particularly well-suited for the efficient production of large components [2,83,84]. Additionally, this trend may also stem from the fact that WAAM technology is still in its early stages of development compared to other metal additive manufacturing techniques, prompting increased research efforts to explore its full potential [83].

Table 3 and Table 4 also indicate that most wear studies on DED alloys have focused on Fe-based alloys. This emphasis can be attributed to the distinctive advantages of ferrous alloys, particularly their well-balanced properties. These include excellent corrosion resistance, superior mechanical performance (such as ductility, toughness, strength, and wear resistance), cost-effectiveness, and the ability to develop a wide range of microstructures—from ultra-hard martensitic phases to versatile multiphase compounds. Additionally, their unique functionalities, such as ferromagnetism, further enhance their appeal. As a result, Fe-based alloys are widely utilized in critical industries, including biomedical, marine, petrochemical, and aerospace applications [2,27].

As shown in Table 4, Ni-based superalloys rank second to ferrous alloys in tribological studies on DED-fabricated materials. This is primarily because these alloys play a vital role in industries such as defense and aerospace, where their exceptional mechanical properties and resistance to creep, corrosion, and wear across a range of temperatures are highly valued. However, traditional manufacturing methods like casting and forming pose challenges due to the complex composition of these alloys, often requiring annealing to enhance ductility, which can compromise other material properties. Moreover, the demand for intricate geometries in these applications leads to significant material waste and high production costs. AM overcomes these challenges by enabling greater design complexity, minimizing material waste, and reducing production times without additional costs [85].

Table 3 and Table 4 also reveal that research on the wear behavior of Ti-based and Cu-based alloys processed through DED remains limited. In particular, the challenges associated with fabricating L-DED Cu-based alloys stem from their high thermal and electrical conductivity. When copper and its alloys are additively manufactured using near-infrared laser beam approaches, excessive heat dissipation and laser light reflection can hinder the successful fabrication of 3D components. Therefore, exploring alternative laser wavelengths may offer advantages for laser-based additive manufacturing of these metals [86,87].

In the context of laser additive manufacturing of Ti-based alloys, several challenges continue to hinder their widespread industrial adoption. The primary barriers include [88]: (i) high production costs and a limited selection of alloy compositions, (ii) the formation of columnar microstructures, strong texture, and microstructural inhomogeneity, (iii) poor ductility and low work-hardening capacity, (iv) anisotropic and inconsistent mechanical properties, and (v) high residual stress leading to suboptimal fatigue performance. These limitations may explain the relatively limited focus on wear studies for DED-fabricated Ti alloys.

A review of Table 3 shows that in the L-DED process, laser power typically ranges from 600 to 3000 W, with a printing travel speed of 30–100 cm/min. Additionally, a 90-degree rotation in the scanning strategy is commonly used, and argon is the most frequently employed shielding gas for this process.

A quick review of Table 4 indicates that argon is the predominant shielding gas for WAAM processing, though some studies have also utilized a gas mixture containing Ar, CO₂, and He. The current for the WAAM process typically ranges from 110 to 300 A, with voltage varying between 8.7 and 30 V. The travel speed generally falls within 10 to 50 cm/min, and the wire feed rate ranges from 0.8 to 5.4 m/min.

A summary of all the studies discussed in the previous section is compiled in

Table 5. A summary of wear behavior in DED-manufactured alloys.

AM Technology	Alloy	Test (Condition)	Comment	Ref.
WAAM	Inconel 625	ball-on-disc wear (dry)	Simultaneous boronizing and heat treatment enhanced the wear resistance of WAAM IN625. (WR * = 1.3–73.3 × 10−5 mm3/Nm; COF * = 0.57–0.81).	[67]
WAAM	Inconel 625	nano-indetation and nano-scratch tests (dry)	Wear resistance in WAAM-fabricated Inconel 625 varied across different regions, with the top region exhibiting the lowest COF due to Cube-related textures, while the middle region showed the highest wear and material loss due to Brass, Goss, Cu, and S textures, influenced by differences in crystallographic orientations. (WV * = 2.5–8.4 × 10−5 μm3; COF = 0.32–0.46).	[64]
Hybrid WAAM	Inconel 625	Dry sand/rubber wheel apparatus + pin-on-disc (dry)	The addition of TiC to Inconel 625 clads via the WAAM process significantly improved the abrasive wear resistance of EN-8 steel, with the Inc625 + 30TiC clad exhibiting the least mass loss due to the presence of tough TiC particles, which reduced penetration while increasing surface roughness and friction, resulting in a mixed ductile-brittle wear mechanism. (MSR * = 0.002–0.008 mg/Nm; COF = 0.1–0.2).	[63]
L-DED	Inconel 718	Machining test (dry)	Heat treatment of L-DED IN718 resulted in the formation of strengthening phases, which rapidly degraded the tool’s surface, whereas the inferior mechanical properties of as-built IN718 did not cause such deterioration. (WR = N.A.; COF = N.A.).	[68]
L-DED	Inconel 718	Ball-on-disc (dry)	The high compressive residual stress induced by surface-post treatment enhanced the wear resistance of L-DED IN718 compared to its untreated counterpart. (WR = 10–28 × 10−7 mm3/Nm; COF = 0.4–1.5).	[73]
L-DED	Inconel 718	cylinder-on-plane (dry)	As a post-surface treatment, UNSM * enhanced wear resistance. The finite element results exhibited exceptional consistency in wear loss predictions, with an error margin of less than 2%. (WR = 2.9–16.8 × 10−3 mm3/m; COF = 0.3–0.48).	[74]
WAAM	Alloy 825	pin-on-disc (dry)	The wear characteristics of both wrought and WAAM alloys were influenced by load and sliding velocity, with lower wear occurring at high velocities and low loads, while higher wear was associated with low velocities and high loads, leading to different wear mechanisms such as abrasive, adhesive, and delamination wear. (WR = 2.9–16.8 × 10−3 mm3/m; COF = 0.30–0.48).	[60]
WAAM	NiTi	Low speed friction (dry)	The homogeneous WAAM NiTi component exhibited superior wear resistance with a decreasing COF, primarily undergoing oxidative and adhesive wear, while the heterogeneous NiTi alloy experienced more pronounced scratches dominated by adhesive wear. (WR = N.A.; COF = 0.71–0.76).	[62]
LW-DED & LP-DED	Nitinol 55	ball-on-flat (dry)	At lower temperatures, LP-DED samples exhibited improved friction and wear performance, primarily due to their higher hardness and the presence of a stable NiTi B2 austenite phase in the microstructure. However, as the temperature increased, both LP-DED and LW-DED samples displayed similar wear mechanisms, indicating a convergence in their tribological behavior under elevated thermal conditions. (WV * = 0.8–5.8 × 108 μm3; COF = 0.50–0.70).	[61]
WAAM	NiTi/Nb composite	ball-on-flat (dry)	The wear response of NiTi/Nb composite changes as a function of Nb content. The higher Nb content caused lower wear resistance. (WR = 2.1–3.3 × 10−4 mm3/Nm; COF = 0.70–0.81).	[65]
WAAM	SDSS 2507 & Inconel 718	ball-on-disc (dry)	The maximum wear-rate was for IN 718 while the minimum wear-rate was for SDSS 2507. The wear-rate of the SDSS 2507–IN 718 interfaces falls between the wear-rates of SDSS 2507 and IN 718. (WR = 0.2–2.6 × 10−5 mm3/m; COF = 0.50–0.63).	[47]
L-DED	316L SS	Machining test (dry)	Tool wear was more severe in thermal milling than in room-temperature milling. Besides, lower milling speeds resulted in improved surface roughness, while a quadruple-edged milling cutter exhibited greater wear resistance than a double-edged cutter. (WR = N.A.; COF = N.A.).	[59]
WAAM	316L	Ball on plate (dry)	The wear resistance of WAAM SS316L samples was influenced by heat input and load, with higher values increasing wear rate and COF, while wear mechanisms included abrasive wear, surface fatigue, adhesion, and oxidative wear, accompanied by a phase transformation from γ-austenite to α′-martensite. (WR = 1.2–3.5 × 10−3 mm3/m; COF = 0.17–0.31).	[43]
WAAM	ER 316L	ball-on-flat (dry)	This study demonstrates that the WAAM process can produce SS316 parts with mechanical, wear, and corrosion properties comparable to or better than conventional as-cast parts. (WR = 2.5–18.5 × 10−4 mm3/m; COF = 0.45–0.60).	[42]
WAAM	SS316LSi	ball-on-flat (dry)	Increasing applied load led to higher wear rate. Adhesive wear was the dominant wear mechanism. (WR = 2.5–4.0 × 10−3 mm3/Nm; COF = 0.54–0.57).	[58]
Twin-WAAM	SS316LSi & ER70S-6)	Ball-on-disc (dry)	Printed material has higher wear resistance, but the wear resistance decreased with increasing load. (WR = 2.0–2.8 × 10−3 mm3/Nm; COF = 0.44–0.49).	[57]
WAAM	ER347	Pin-on-disc (dry)	The wear mechanism transitioned from adhesive wear at 200 °C to mild oxidative wear at 400–600 °C, where a mechanically mixed composite layer formed, stabilizing the wear rate. (WR = 0.5–3.0 × 10−3 mm3/m; COF = 0.40–0.47).	[53]
WAAM	SS308L	pin-on-disc (dry)	The wear resistance of AM-fabricated ASS 308L is influenced by heat input, with lower heat input reducing the COF and wear rate, while severe stresses promote plastic deformation, inducing austenite-to-martensite transformation and leading to adhesive wear as hardened debris fractures and rolls into cylindrical shapes. (WR = 0.02–0.12 mm3/m; COF = 0.46–0.66).	[56]
WAAM	304 SS	reciprocating wear test (dry)	Extraction location of WAAM 304 SS and the corresponding microstructure significantly influence the wear behavior. (WR = 0.5–2.5 × 10−5 mm3/Nm; COF = N.A.).	[41]
WA-DED	ER420	Dry sand/rubber wheel apparatus (dry)	Post-tempering alters the wear mechanism from micro-cutting and micro-cracking to micro-ploughing, significantly enhancing wear resistance compared to the as-printed sample. (Mass loss = 0.5–1.6 g; COF = N.A.).	[52]
WAAM	PH 13–8Mo	Scratch tests & Dry sliding (dry)	WAAM-fabricated PH 13–8Mo stainless steel, after optimal heat treatment at 500 °C for four hours, exhibits improved hardness, wear resistance, and lower friction due to the elimination of δ-ferrite and the formation of β-NiAl precipitates, making it a viable alternative to wrought counterparts for wear-resistant applications. (WR = 0.8–2.8 × 10−6 mm3/Nm; COF = 0.48–0.57).	[51]
WAAM	PH 13-8Mo	Scratch tests & Dry sliding (dry)	This study reveals that TiC/TiB2 inoculation enhances wear resistance in additively manufactured components by promoting grain refinement and strengthening effects, with TiC-inoculated samples demonstrating superior wear performance due to an optimal balance of hardness and fracture resistance. (WV = 0.23–0.62 mm3; COF = 0.46–0.57).	[45]
L-DED	high chromium white irons	high stress abrasion test (dry) and pin-on-disk wear (dry)	When exposed to softer abrasives, wear resistance is primarily influenced by carbide morphology and matrix microstructure. However, when subjected to harder abrasives, the γ-Fe matrix plays a crucial role in enhancing wear resistance. (WV = 3–29 × 10−2 mm3; COF = N.A.).	[44]
WAAM	high-manganese steel	ball-on-flat (dry)	Cr addition deteriorated the wear performance of arc-DED high-manganese steel. (WR = 1.4–7.5 × 10−8 mm3/Nm; COF = 0.55–0.75).	[48]
L-DED	AISI M2	ball-on-disk (dry)	M2 tool steel alloy fabricated via L-DED showed superior wear resistance to conventional counterparts due to its capability to produce tribo-oxide film. (Mass Loss = 0.1–23.0 mg; COF = 0.50–0.75).	[54]
WAAM MWAAM	TC11	linear reciprocating (dry)	Multi-WAAM refines the microstructure and thus, the wear resistance is enhanced compared to WAAM counterpart. (WR = 0.7–1.7 × 10−4 mm3/Nm; COF = 0.39–0.46).	[77]
L-DED	Ti6Al4V	Pin-on-disc (dry)	The wear resistance of L-DED alloy was inferior to L-PBF and forged counterparts due to high strain in some interface regions between α grains which were far from plastic deformation area. (WR = 3.8–4.7 × 10−5 g/Nm; COF = N.A.).	[75]
WAAM	TA15	ball-on-plate (dry)	Introduces a new model to predict oxidation wear at higher temperatures, addressing the limitations of the previous model. (WV = 0.1–2.8 × 107 μm3; COF = 0.50–0.67).	[78]
WAAM (GTAW)	Ti-6Al-3Nb-2Zr-1Mo alloy	ball-on-plate (dry)	The composite coating exhibits complex wear mechanisms during dry sliding friction, including adhesive wear, abrasive wear, three-body wear, and oxidation wear. (WR = 3.5–27.1 × 10−5 mm3/Nm; COF = 0.30–0.40).	[70]
L-DED	CuSn10	Pin-on-disc (dry)	L-DED CuSn10 exhibited wear resistance comparable to that of its wrought counterpart. (WR = 1.8–3.1 × 10−5 g/m; COF = 0.50–0.55).	[79]
WAAM	NAB	friction wear (dry)	The WAAM-NAB alloy exhibited superior wear resistance compared to the as-cast NAB alloy, with reduced wear depth and volume, a flatter wear surface, and less severe deformation. (WV = 7.0–11.3 × 107 μm3; COF = 0.25–0.39).	[81]
L-DED	Ta-Zr	block-ring wear (dry)	The fine-grained structure of the Ta-Zr coating, along with the presence of intermetallic compounds, enhanced its hardness, thereby significantly improving its wear resistance. (Mass Loss = 7.3–450.4 mg; COF = 0.30–0.37).	[82]

* WR: wear rate, WV: wear volume, MSR: mass loss rate, COF: coefficient of friction, WAAM: wire arc additive manufacturing, MWAAM: multi-wire arc additive manufacturing, L-DED: laser-directed energy deposition, UNSM: ultrasonic nanocrystal surface modification.

, highlighting various strategies explored to enhance the wear resistance of Ni-based superalloys. For WAAM-fabricated Inconel 625, boronizing [67] and TiC addition [63] have been found to improve wear resistance, while the texture variations in different regions influence its wear performance [64]. In the case of L-DED Inconel 718, both heat treatment [68] and surface post-treatment [73,74] have proven effective in enhancing wear resistance. For WAAM-built Alloy 825 [60], wear behavior was observed to depend on applied load and velocity. Regarding NiTi alloys, improved homogeneity in WAAM-processed NiTi contributed to enhanced wear performance [62]. Additionally, laser powder-DED was found to be more effective than laser wire-DED in improving the wear resistance of NiTi [61]. However, in WAAM NiTi/Nb composites, a higher Nb content was associated with reduced wear resistance [65].

For ferrous alloys summarized in Table 5, studies have shown that the wear resistance of WAAM-fabricated 316L stainless steel is comparable to its as-cast counterpart [42]. However, higher heat input [43] and increased applied loading during wear testing [57,58] have been found to negatively impact wear performance. For other grades of austenitic stainless steels produced via WAAM, factors such as wear test temperature [53], heat input [56], and extraction location [41] have also been identified as influential in wear properties. In the case of WAAM martensitic stainless steels, post-heat treatment [51,52] has been shown to enhance wear resistance, similar to the beneficial effects of TiC/TiB₂ additions [45]. For other ferrous alloys, L-DED tool steel exhibited superior wear resistance compared to its conventionally manufactured counterpart [54]. However, Cr addition in WAAM high-Mn steel [48] and the presence of an unfavorable microstructure in L-DED high-Cr white iron [44] resulted in reduced wear resistance.

For Ti-based alloys, Multi-WAAM [77] and L-PBF [75] technologies have been shown to be more effective in enhancing wear resistance compared to WAAM and L-DED, respectively. For Cu-based alloys, DED-fabricated variants exhibited superior wear resistance compared to their conventionally produced counterparts [79,81]. Lastly, in the case of L-DED Ta-Zr alloys, grain refinement induced by the DED process contributed to improved wear resistance [82].

Finally, to gain a more comprehensive understanding of the tribological performance of alloys fabricated via DED, it is valuable to compare the coefficient of friction (COF) values alongside degradation rates, typically reported in terms of wear rate, volume loss, or mass loss. However, it is important to note that the lack of standardization in how these metrics are reported across studies presents challenges for direct comparisons. Nevertheless, the data compiled in Table 5 indicate several clear trends.

DED-fabricated Inconel alloys generally exhibit superior wear resistance compared to DED NiTi alloys, making them more suitable for applications requiring high wear durability. Furthermore, DED-Inconel alloys outperform DED austenitic stainless steels such as 304, 308L, 316L(Si), and 347 in terms of wear resistance. However, DED-processed precipitation-hardenable (PH) martensitic stainless steels—benefiting from the formation of a hard martensitic matrix—demonstrate even greater wear resistance than Inconel alloys, positioning them as ideal candidates for extremely demanding wear applications.

Interestingly, the COF values reported for DED Inconel alloys tend to be lower than those of DED ferrous alloys, which may be advantageous depending on the application requirements. In contrast, DED-fabricated Ti and Cu alloys exhibit slightly lower wear resistance than both Inconel and ferrous alloys, possibly explaining the comparatively limited focus on their tribological performance. However, it is essential to highlight that tribocorrosion—degradation resulting from the combined action of wear and corrosion [89]—is a particularly critical concern for titanium alloys [90,91].

Although Ti alloys show moderate wear resistance, their use in biomedical applications remains critical. In such contexts, excessively high mechanical properties can be detrimental due to stress shielding, where a mismatch in Young’s modulus between the implant and surrounding bone leads to uneven stress distribution and impedes the healing process. Therefore, despite their lower wear resistance, DED-fabricated Ti alloys merit continued investigation for biomedical use [92].

6. Mechanisms Influencing Wear Behavior of DED-Fabricated Alloys

6.1. Effect of Hardness on Wear Resistance of DED Alloys

Previous studies have highlighted that hardness plays a crucial role in enhancing material wear resistance. A significant contribution by Archard explores the influence of contact conditions, including contact area, mechanical wear, and conductance [93]. In his work, wear rates for various material pairs were determined using pin-on-ring tests, leading to the development of a model for material loss. The theoretical framework expressed in Equation (1) relies on an asperity contact model, which assumes that wear particles are hemispherical and have the same radius, a, as the asperities. The wear rate, W, defined as the volume loss per sliding distance, was found to be dependent on both the radius and the applied load, P [93]:

$$W=K{\displaystyle \frac{P}{3a}}$$

(1)

The proportionality factor, K, varies depending on the dominant wear mechanism, categorized as ‘light’ or ‘heavy’ wear. It can be demonstrated that the asperity size, a, is influenced by the surface hardness, H, in such a way that Equation (1) can be rewritten as follows [93]:

$$W=K{\displaystyle \frac{P}{H}}$$

(2)

where K has the same meaning as K but takes a different numerical value [93]. Accurately determining wear rates requires thorough testing and a clear understanding of the underlying wear mechanisms. The reported range of wear rates in the literature spans several orders of magnitude, from 10⁻⁹ to 10⁻³ [93]. It has been observed that for adhesive wear, the factor K typically falls between 10⁻⁴ and 10⁻³, whereas for abrasive wear, K is generally around 10⁻¹ [12].

During wear testing, the frictional force depends on the force needed to plastically deform the surface. When surfaces are harder, they resist deeper indentation by the ball or pin tip during wear. As a result, the force required for plastic deformation is reduced, which significantly enhances wear resistance [12].

The literature consistently indicates that higher hardness in DED alloys contributes to improved wear resistance. For DED Ni-based alloys, the wear resistance of L-DED IN718 was enhanced by high compressive residual stress introduced through post-surface treatments, in contrast to its untreated counterpart [73]. For WAAM Alloy 825, increased friction force under high wear conditions raised the surface temperature, softening the material and increasing the wear rate. However, due to its higher hardness from the deposition process, the WAAM alloy exhibited better friction resistance compared to the wrought alloy [74].

In WAAM IN625, a high proportion of recrystallized grains, especially Cube, Cube-ND, and Cube-RD textures, improved hardness, elastic modulus, and tribological properties by resisting deformation and reducing adhesion [64]. When boronized, WAAM IN625 samples showed significantly better wear resistance than their non-boronized counterparts, owing to the formation of a boride layer that was 10 times harder than the base material [67]. For TiC-reinforced WAAM Inconel 625 clads, abrasive wear tests demonstrated that all clads outperformed the steel substrate in terms of wear resistance, primarily due to their higher microhardness. The addition of TiC particles, particularly at higher concentrations, further enhanced wear resistance by strengthening the substrate and preventing the penetration of the disc and debris into the surface [63].

LP-DED NiTi samples showed improved friction and wear performance at lower temperatures due to their higher hardness and stable NiTi B2 austenite phase. At higher temperatures, both LP-DED and LW-DED NiTi samples exhibited similar wear mechanisms [61]. For WAAM NiTi, the upper region demonstrated superior wear resistance compared to the lower and middle regions due to higher microhardness, better structural properties, and a higher content of the B19′ phase [62]. In the case of arc-DED NiTi/Nb composites, increased NiTi content contributed to enhanced wear performance due to its greater hardness and superelasticity [65].

The enhancement of wear resistance in DED ferrous alloys due to increased hardness has also been reported. For WAAM 347 SS, interlayer cold working increased the hardness, improved wear resistance, and reduced the COF [46]. In contrast, in WAAM 308L, the highest wear rates were observed at elevated heat inputs, resulting in reduced hardness and increased plastic deformation [56]. For WAAM 316L, wear rates increased with rising thermal heat input due to the formation of coarser grain structures, which lowered hardness according to the Hall–Petch relationship. Moreover, wear resistance varied within the deposited walls, with the bottom sections exhibiting lower wear rates due to finer grain structures, while the top sections experienced higher wear due to grain coarsening and reduced hardness [43].

The as-cast PH 13-8Mo SS sample showed higher wear rates compared to the heat-treated as-cast sample under identical conditions, likely because of its lower hardness and coarser grains, which facilitated plastic deformation and smoother surfaces [51]. Heat treatment improved the wear resistance of WAAM PH 13-8Mo, with the sample aged at 500 °C performing the best, as both wear rate and friction coefficient decreased with increasing hardness, influenced by microstructural features [51]. In addition, the incorporation of TiC particles into High Chromium White Iron (HCWI) increased hardness due to the presence of ex-situ TiC carbides, thereby enhancing wear resistance [44]. Additionally, it is important to note that low hardness in WAAM IN718 resulted in greater plastic deformation in high heat input specimens (upper zones of the wall), whereas lower heat input led to reduced plastic deformation in WAAM SDSS 2507 due to its higher hardness [47].

For WAAM-built WC-reinforced Ti-6Al-3Nb-2Zr-1Mo composite coatings, wear resistance improved significantly, with the average wear rate reduced by 86.96% compared to the substrate. The WC particles help support external loads, while the increased hardness reduces plastic deformation. During sliding friction, the high-hardness WC particles make initial contact with the friction pair, enhancing load-bearing capacity and protecting the Ti alloy matrix, thereby reducing wear and extending service life [70].

Similarly, L-DED deposition of a Ta-Zr coating on Ti6Al4V alloy significantly enhanced wear resistance, mainly due to the higher hardness of the coating, resulting from its fine-grained microstructure and the presence of intermetallic compounds such as TiZr and Al₃Zr₄ [82]. For L-DED CuSn10, the increased hardness contributed to better wear resistance compared to its wrought counterpart [79].

6.2. Effect of Metallurgical Factors on Wear Resistance of DED Alloys

Higher hardness in alloys does not always directly lead to improved wear resistance, as microstructural characteristics also play a critical role in influencing wear behavior. As mentioned earlier, ferrous alloys can undergo heat treatment and phase transformations, which enhance their mechanical properties and wear resistance [2]. For WAAM 308L SS, wear resistance varied along the deposition direction, suggesting that microstructural differences, influenced by the weld path, affect wear performance [41]. Similarly, in WAAM-produced 316 SS, the wear rate decreased with increased dwell time on the top layers due to a decrease in heat accumulation, which reduced the amount of delta ferrite and increased grain size [42].

WAAM 347 SS exhibited a higher wear rate than its 347 SS substrate due to its non-homogeneous composition, which impeded the formation and uniform distribution of oxide layers at high temperatures [53]. For WAAM 420 martensitic SS, the tempered hardfacing variant demonstrated superior wear resistance compared to its as-deposited counterpart, despite having lower hardness. This improvement was attributed to a shift in the dominant wear mechanism from micro-cutting and micro-cracking to micro-ploughing. The dispersion of ultra-fine carbides in a tough matrix enhanced the alloy’s resistance to abrasion while improving its ductility, toughness, and work-hardening ability [52].

In the case of TiC/TiB₂-inoculated PH13-8Mo SS, although TiB₂ inoculation provided more efficient grain refinement and higher hardness, the TiC-inoculated sample exhibited the best wear resistance. This was due to its superior balance of hardness and fracture resistance, along with a higher retained austenite content, which transformed into martensite during wear testing [45]. Additionally, despite having the lowest hardness, the 600 °C aged PH13-8Mo sample outperformed the as-printed version due to strain-induced austenite transformation. Overall, heat-treated WAAM PH13-8Mo exhibited slightly superior wear performance compared to its wrought counterpart, thanks to its finer lath martensitic structure [51]. It has also been reported that strain-induced martensite formation, caused by the addition of Ti to High Chromium White Iron (HCWI), enhances wear resistance [44].

For DED Ni-based alloys, a multi-step heat treatment cycle in L-DED IN718 was found to modify the microstructure, enhancing mechanical performance through the formation of γ′ and γ″ phases and the transition from a dendritic to an equiaxed structure. As a result, the improved mechanical properties resulting from heat treatment led to significant wear-induced degradation of the TiAlN tool, whereas the as-built L-DED IN718 did not cause immediate tool failure [68]. In the case of L-DED IN718, the removal of pores from the top and near-surface regions following UNSM treatment—though not directly related to microstructure—further contributed to improved fretting wear resistance [73].

For WAAM IN625, variations in the COF, scratch width, and depth across different build regions were influenced by microstructural and textural differences. A high fraction of recrystallized grains, particularly Cube, Cube-ND, and Cube-RD textures, improved hardness, elastic modulus, and tribological properties by resisting deformation and reducing adhesion [64]. Similarly, for WAAM NiTi, the upper region exhibited the highest wear resistance due to the presence of the B19′ phase, which enhanced stress distribution during phase transformation. This reduction in stress concentration contributed to improved wear performance under frictional conditions [62].

For L-DED Ti6Al4V, its inferior wear resistance compared to L-PBF and forged counterparts was attributed to the presence of high strain in specific interface regions between α grains, which were located outside the primary plastic deformation zones in the L-DED sample. These strain concentrations likely contributed to increased susceptibility to wear-related degradation [75].

6.3. Effect of Surface Oxide Film Formation on the Wear Resistance of DED Alloys

During dry wear, oxygen adsorption occurs at grain boundaries on the alloy surface, where higher chemical activity and defects facilitate oxide formation. These oxides accumulate into a continuous film, altering the wear mechanism to one of abrasive wear, reducing friction, and minimizing wear loss by limiting direct metal contact [12]. Similarly, in L-DED M2 tool steel, the formation of a tribo-oxide film on its surface was found to enhance wear resistance compared to fully carburized conventional steel and high-wear-resistance steel produced via DED [54]. In the case of boronized WAAM IN625, boron’s strong affinity for oxygen led to the development of a surface oxide layer, lowering the COF and thereby reducing wear damage [67].

The WAAM-built WC-reinforced Ti-6Al-3Nb-2Zr-1Mo composite coating significantly improves wear resistance, reducing the average wear rate by 86.96% compared to the substrate. The formation of an oxide lubricating layer further enhances its wear resistance. As oxidation increases with temperature, the high hardness of WC particles elevates sliding wear temperatures, accelerating oxidation and promoting protective oxide formation [70].

For WAAM TA15, at temperatures below 1000 °C, the COF initially rises before stabilizing due to the formation of a tribo-layer. As the temperature increases, the COF decreases, reaching its lowest value at 600 °C, where a dense TiO₂-based oxide layer forms, acting as a lubricant. However, above 600 °C, the oxide layer begins to delaminate, increasing surface roughness and inhomogeneity, which raises the COF [78].

In the case of WAAM and MWAAM TC11, it has been observed that wear under cyclic loading degrades the protective oxide layer on the sample surface. If the oxide layer depletes faster than it regenerates, oxidized debris accumulates, increasing wear and reducing friction performance. Conversely, if regeneration outpaces depletion, the oxide layer protects the surface and enhances friction performance. In the WAAM sample, rapid depletion led to increased wear debris and greater wear rates [77].

7. Conclusions and Future Perspectives

7.1. Conclusions

This paper provides a comprehensive review, discussion, and interpretation of the existing literature on the wear characteristics of DED-fabricated metals and alloys. The key conclusions drawn from this study are as follows:

• The arc-DED process, commonly known as WAAM, remains the predominant DED technology for fabricating various alloys, as evidenced by the relatively fewer studies on the Laser-DED technique. This preference is largely due to the lower cost of wire feedstock and the higher deposition rate of WAAM. Additionally, the limited chamber size in L-DED imposes restrictions on its applications, making WAAM more favorable for large-scale manufacturing.
• Research on DED alloys for wear-resistant applications primarily focuses on four main alloy systems: Fe-based, Ni-based, Ti-based, and Cu-based. This emphasis arises from the widespread use of these materials—particularly Fe-based and Ni-based alloys—in their as-cast and wrought forms for conventional wear-resistant applications. Moreover, these alloys often exhibit phase transformation capabilities within their microstructure, which can be triggered either through post-heat treatment cycles or friction-induced heating during the wear process.
• Overall, DED-manufactured alloys typically demonstrate superior or at least comparable wear resistance to their as-cast or wrought counterparts. To further enhance their wear performance, researchers commonly modify the DED process through approaches such as ceramic particle reinforcement and heat input optimization. Furthermore, post-processing techniques—including heat treatment, boronizing, and surface post-treatment—are widely employed to further improve wear resistance.
• Three key mechanisms contribute to the enhanced wear resistance of DED alloys: (a) The rapid thermal cycling inherent to the DED process often leads to increased hardness, which minimizes plastic deformation during wear, thereby improving wear resistance. (b) Various metallurgical aspects play a crucial role in enhancing wear performance. These include microstructural evolution induced by post-heat treatments, phase transformations triggered by friction-induced heating, and the elimination of porosity through surface post-treatments—all of which contribute to improved wear resistance. (c) The DED process has been observed to increase surface reactivity with oxygen, promoting the formation of an oxide film. This oxide layer can act as a protective solid lubricant, reducing friction and subsequently lowering the wear rate of DED alloys.

7.2. Future Perspectives

This paper provides a comprehensive review of the wear behavior of DED-fabricated alloys. However, several research gaps have been identified that warrant further investigation. Addressing these gaps in future studies will not only enhance the existing body of knowledge but also provide a systematic framework for applying these findings to real-world applications. The key research gaps are as follows:

• While the influence of processing parameters on the microstructure and mechanical properties of DED alloys has been extensively studied [83,94], their effect on the wear resistance remains underexplored. This review highlights that only a limited number of studies have examined the impact of heat input and ceramic particle addition on wear resistance. To bridge this gap, more comprehensive investigations—particularly on alloys known for their high wear resistance in as-cast or wrought forms—are strongly recommended.
• Most existing studies primarily investigate the wear response of DED alloys under dry conditions. However, in real-world applications, these components often operate in lubricated environments [95,96]. Investigating their wear behavior under lubricated conditions, comparing it with dry conditions, and examining the role of tribofilm formation could provide valuable insights into their real-world performance. Such research would be crucial for optimizing DED alloys for wear-critical applications.
• With the increasing adoption of marine systems and the growing demand for seawater lubrication, the performance of critical marine alloys used in propulsion systems, bearings, and mechanical seals is receiving heightened attention. Additionally, the expansion of marine autonomous systems, renewable energy technologies, and aquaculture highlights the need for a deeper understanding of the tribocorrosion behavior of marine alloys [97]. Therefore, it is strongly recommended to investigate the tribocorrosion performance of DED alloys, particularly those whose as-cast or wrought counterparts are widely used in marine applications, such as nickel-aluminum bronze (NAB) alloys [22,98].
• One of the key advantages of DED technology is its ability to deposit clad layers on substrate surfaces to enhance properties or repair damages [29,99]. This capability can be leveraged to fabricate wear-resistant clads on alloys that are inherently more susceptible to wear, thereby expanding their service life. To maximize the effectiveness of such coatings, future research should focus on optimizing process parameters for depositing wear-resistant alloy claddings and systematically evaluating their wear behavior under relevant operating conditions.
• When surface texturing is approached as a design tool that can be strategically engineered to optimize system performance, it paves the way for significant innovation. Numerous studies have demonstrated that surface features can be designed to introduce directionality, guide fluid flow, customize the real area of contact, and regulate flow rates to enhance functions such as heat transfer—an example being torque transmission plates [100]. Despite this potential, the influence of surface post-treatments on the wear behavior of DED alloys remains largely underexplored in current literature. In particular, modifying the surface texture of the outermost layer and adjusting surface roughness based on specific end-use requirements could provide valuable insights into broadening the functional applications of DED-fabricated components.