An Overview of the Benefits, Drawbacks and Strategies Used for the Fabrication of 316L Stainless Steel and Inconel 625 Functionally Graded Materials Using Wire Arc Additive Manufacturing

Abstract

Wire arc additive manufacturing (WAAM) is an efficient, low-cost technique for fabricating large-scale metallic components and, in particular, functionally graded materials (FGMs). This review focuses on the fabrication of 316L stainless steel–Inconel 625 FGMs by arc-based WAAM processes, examining Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW) and Plasma Arc Welding (PAW) in terms of their microstructural outcomes, compositional control strategies, residual stress development and mechanical performance. A critical finding emerging from the reviewed literature is that direct compositional interfaces between 316L and Inconel 625 can yield superior tensile strength and ductility and lower residual stresses compared to smooth gradient strategies, owing to the formation of detrimental secondary phases such as δ-phase, Laves phase and MC carbides at intermediate iron–nickel compositions encountered only during graded builds. The potential of Submerged Arc Additive Manufacturing (SAAM) as a future high-deposition-rate alternative for large-scale FGM fabrication is also discussed. Key challenges, including dilution control, Laves phase formation, residual stress management and the corrosion characterization of the graded region, are identified, together with priority research directions for advancing the industrial adoption of arc-based FGM components.

1. Introduction

Functionally graded materials (FGMs) have earned significant attention in both research and industry due to their unique ability to tailor material properties for specific applications [1]. Over recent years, the development of FGMs has progressed rapidly, due to their ability to adjust local properties such as strength, hardness, corrosion resistance, wear resistance, high-temperature stability and weight reduction, depending on their specific application. This adaptability is achieved through controlled gradients in properties like chemical composition and microstructure [2].

FGMs can be categorized into two main types based on how the gradients are structured: continuous and stepwise. In a continuous gradient, the material properties change smoothly over the entire volume, allowing for a gradual transition between different microstructures and compositions. In contrast, the stepwise gradient involves distinct layers with different microstructures and properties, separated by defined interfaces.

Both approaches offer unique advantages, with continuous gradients being suitable for applications requiring seamless transitions in material properties and stepwise gradients for multilayered components with specific, well-defined properties at each layer [3].

The ability to design and adjust local material gradients makes FGMs ideal for use in situations where conventional homogeneous materials fall short. For example, in some applications, it may be desirable for a material to simultaneously fulfil multiple functional requirements, such as being lightweight while also offering superior resistance to high temperatures, corrosive and oxidizing environments, or prolonged neutron irradiation [4].

Until recently, achieving this level of multi-functionality typically required the use of fasteners or welds to join together alloys with different compositions and properties. However, this approach often imposes significant design constraints, as structures must be optimized to minimize stress on the fasteners and accommodate the limited geometries achievable through welding. Despite these efforts, joints made from dissimilar metals remain vulnerable to stress concentration and accelerated degradation, often exhibiting mechanical properties that are far inferior to those of the individual base materials [5,6,7].

Their versatile design possibilities have led to a broad range of potential manufacturing methods and applications. Notable fields of application include energy harvesting, electronics, defense and aerospace, automotive, biomedical engineering, orthopedic implants, and nuclear industries.

Among the many material combinations explored for FGMs, the integration of 316L stainless steel and Inconel 625 has attracted considerable interest. 316L stainless steel is widely used due to its excellent corrosion resistance, good mechanical properties, relatively low cost and ease of processing, making it a common structural material in chemical processing, marine, and biomedical applications. In contrast, Inconel 625 is a nickel-based superalloy known for its outstanding high-temperature strength and superior resistance to oxidation, creep and aggressive corrosive environments.

Combining these two materials in a functionally graded structure allows the advantageous properties of each alloy to be utilized within a single component. For instance, a component may benefit from the structural reliability, good corrosion resistance and relatively low cost of 316L in regions exposed to moderate conditions, while transitioning toward Inconel 625 in areas subjected to higher temperatures, aggressive corrosive environments, or increased mechanical loading. This approach enables a more efficient distribution of material properties throughout the component, optimizing performance while potentially reducing the overall use of expensive nickel-based superalloys. As a result, the combination of 316L stainless steel and Inconel 625 in a functionally graded structure offers a promising pathway to engineer components with locally optimized properties.

Two industrial contexts illustrate the practical relevance of this material pair. In subsea oil and gas systems, transition joints between stainless steel piping and nickel-alloy components in sour service environments are vulnerable to stress corrosion cracking at the dissimilar weld interface; a graded 316L–Inconel 625 FGM would eliminate this discrete joint while allowing the bulk of the structure to be fabricated in lower-cost 316 [8]. In nuclear applications, reactor internal components and pressure boundary structures must withstand simultaneous radiation, high temperature and corrosive primary coolant; a graded transition enables structural efficiency by concentrating Inconel 625 only in the highest-stress, highest-temperature regions while 316L handles the remainder.

2. Methods

The present review was carried out through a comprehensive literature search across the scientific databases Scopus, ScienceDirect and Google Scholar. The keywords used included “wire arc additive manufacturing” (WAAM), “arc additive manufacturing”, “submerged arc additive manufacturing”, “functionally graded materials” (FGM), “bimetallic structures”, “316L Inconel 625 additive manufacturing” and “dissimilar metal welding 316L Inconel”. Various combinations of these terms were applied to ensure thorough coverage of studies related to the fabrication of functionally graded materials using additive manufacturing techniques. The search encompassed publications from 2014 to early 2025, with particular emphasis on studies published from 2019 onwards to ensure alignment with the current state of the art. Articles were considered for inclusion if they reported experimental results on the arc-based deposition of dissimilar or multi-material metallic systems, with priority given to peer-reviewed journal articles, conference proceedings, book chapters and review articles, which were included when they contributed a substantive synthesis of the topic or provided primary data not available elsewhere. Studies focused exclusively on metal FGMs, and all articles on non-arc-based additive manufacturing processes were excluded from the scope of this review. The collected articles were subsequently classified according to the type of arc process employed and the material system investigated, as discussed in the following sections.

3. Basic Knowledge on 316L Stainless Steel and Inconel 625

3.1. Stainless Steel

Austenitic 316L stainless steel, whose chemical composition is displayed in

Table 1. 316L stainless steel chemical composition (wt.%) [11].

C	Mn	Si	S	P	Ni	Cr	Mo	Cu	N
0.016	1.11	0.51	0.001	0.015	10.11	16.32	2.06	0.03	0.032

, is renowned across industry sectors for combining robust corrosion resistance with excellent weldability and mechanical ductility. Its alloy chemistry, typically comprising 16–18 wt.% chromium, 10–14 wt.% nickel and 2–3 wt.% molybdenum, with carbon strictly limited to below 0.03 wt.%, is deliberate. Chromium forms a protective passive oxide film, nickel stabilizes the austenitic structure, and molybdenum enhances resistance to chloride-induced pitting and crevice corrosion. The low carbon content minimizes the risk of chromium carbide (M₂₃C₆) formation during thermal cycles, which in other grades could lead to localized corrosion via sensitization [9,10].

316L finds widespread use in chemical and food processing equipment, pharmaceutical and biomedical devices, marine environments, and potable water systems, precisely because of this combination of corrosion resistance and fabricability. Its weldability is exceptional in comparison to higher carbon grades. Mechanical testing in various studies [11,12,13] indicates that 316L exhibits moderate strength (yield stress often around 200 MPa, tensile strength around 500 MPa) coupled with excellent ductility. Its toughness remains high even at sub-zero temperatures, making it favorable for cryogenic applications as well. However, this alloy has its vulnerabilities. The main risk arises when the material is exposed within the sensitization temperature range (450–900 °C) for sufficient duration, such as during poor welding practices or when slow cooling is imposed after relatively high-temperature heat treatments. Chromium carbides precipitate at grain boundaries, depleting surrounding regions of chromium, which undermines the passive film and enables intergranular corrosion, severely compromising long-term corrosion resistance in welded components [14].

Stabilization by titanium or niobium can mitigate this but may reduce high-temperature strength or raise costs, making 316L a cost-effective compromise with proper weld control. Recent surface analyses have shown that chloride ions can destabilize the passive film by penetrating its outer hydroxide-rich layers, yet pre-passivation treatments can significantly enhance resistance by enriching chromium and molybdenum in the oxide layer [15,16].

3.2. Inconel 625

Inconel 625 is a nickel-based superalloy, whose chemical composition is displayed in Table 2. It is engineered for demanding applications requiring high strength at both low and elevated temperatures, as well as exceptional corrosion resistance and structural stability under thermal and mechanical stress. Its composition (typically Ni ≥ 58 wt.%; Cr: 20–23 wt.%, Mo: 8–3 10 wt.% and Nb + Ta: 3.15–4.15 wt.%) produces a single-phase austenitic (γ) matrix strengthened by solid-solution and, under certain conditions, precipitation mechanisms, enabling a unique combination of properties [16,17].

Table 2. Inconel 625 chemical composition (wt.%) [18].

C	Ti	Si	Ni	Cr	Mo	Nb+Ta	Fe
0.023	0.3	0.4	63.8	21.7	8.9	3.6	1.3

Table 2. Inconel 625 chemical composition (wt.%) [18].

C	Ti	Si	Ni	Cr	Mo	Nb+Ta	Fe
0.023	0.3	0.4	63.8	21.7	8.9	3.6	1.3

The alloy demonstrates impressive mechanical performance. Annealed Inconel 625 typically exhibits yield strengths around 470 MPa, tensile strengths around 935 MPa and an elongation of roughly 55%, reflecting excellent room-temperature strength and ductility, suitable for structural and pressure-bearing components [19,20]. At elevated temperatures (up to 1000 °C), Inconel 625 maintains superior tensile strength and ductility relative to many structural steels, though it may present intergranular cracking when certain strain rates are imposed.

In terms of corrosion, Inconel 625 excels in marine, chemical processing and high-temperature environments, resisting pitting, crevice and intergranular attack, even in harsh acid or chloride-containing media. This resilience is attributed to its high chromium and molybdenum content plus the inert nickel matrix [20]. Applications span marine seawater components, chemical plant equipment, turbine components, heat exchangers, oil and gas, and aerospace engine parts [19].

Nevertheless, certain thermally induced phases can compromise its performance. Between approximately 923 and 1148 K, the precipitation of γ″ (Ni₃(Nb, Al, Ti)), δ-phase (Ni₃(Nb, Mo)) and carbides or Laves phases may occur, particularly in weld or additive manufacturing contexts [17]. The γ″ and δ phases strengthen the alloy when controlled, but may embrittle it if coarse or improperly distributed. Laves phases and carbides may form in interdendritic zones under rapid solidification or high Nb/Mo content, leading to reduced plasticity. These phases can often be dissolved via solution heat treatments (~1148 K for several hours) to restore ductility or creep resistance [17,19,21].

4. Fabrication of FGMs

While much of the current research on FGMs focuses on combining ceramic and metallic materials, recent advancements in additive manufacturing (AM) have also explored the design of metallic FGMs. Among these, copper–stainless steel stands out for its ability to blend copper’s high thermal conductivity with stainless steel’s exceptional corrosion resistance. This combination is widely employed in fusion reactors, heat exchangers and cookware, where both thermal and corrosion properties are essential [4].

Similarly, aluminum–steel bimetallic components are valued for aluminum’s excellent corrosion resistance. These materials find applications in oxygen regenerators as adapters and in electrolytic refining equipment, where durability under corrosive conditions is essential [4,5].

Nickel–steel combinations, while challenging to fabricate due to the differing properties of the materials, provide outstanding mechanical performance. These combinations exhibit excellent resistance to corrosion, radiation damage and high-temperature creep, making them indispensable for demanding applications such as nuclear reactors and gas turbines [6].

In addition to these combinations, Inconel 718, a nickel-based superalloy, is frequently used with austenitic stainless steel. This combination is extensively used in the aeronautics industry and in some nuclear power plants, where high performance under extreme conditions is required [4,5].

Combinations of different types of steel can provide customized solutions for specific needs. Low-carbon steel can be combined with austenitic stainless steel to achieve a balance of strength and ductility [3]. Other notable pairings include maraging steel with tool steel for enhanced hardness [11], ferritic steel with austenitic steel for thermal stability, and martensitic steel with austenitic stainless steel for improved mechanical performance [7].

Recent advancements further highlight the potential of these material combinations. For example, Ahsan et al. [22] fabricated a bimetallic structure (BMS) using low-carbon steel (LCS) and austenitic stainless steel, followed by heat treatment. This innovative approach resulted in significant improvements in mechanical properties, including a 25% increase in ultimate tensile strength (UTS), a 35% rise in yield strength (YS) and a remarkable 250% improvement in elongation. These enhancements were attributed to the transformation of the ferritic microstructure of the as-deposited LCS into ferrite–bainite, demonstrating the potential of such combinations to meet the demands of modern engineering challenges. These innovations continue to expand the range of possibilities for using FGMs in high-performance applications.

Among the material pairings discussed, the 316L stainless steel–Inconel 625 combination is one of the most extensively studied in the context of WAAM-based FGM fabrication. Its relevance arises not only from the complementary properties of the two alloys but also from the industrial demand for multi-functional components capable of operating across wide ranges of temperature, mechanical loading and corrosive exposure within a single structural element. The following sections examine the arc-based WAAM processes used to fabricate FGMs of this and related material systems, together with the associated metallurgical challenges and deposition strategies that have been developed to address them.

5. Joining of 316L and Inconel 625 and Phase Formation in Direct Joints

Austenitic stainless steel 316L and Inconel 625 are frequently combined in components that must resist simultaneous mechanical and chemical stresses. Their baseline chemistries already foreshadow the challenges of a direct fusion joint: 316L relies on ~16–18% Cr and ~10–14% Ni with low C to maintain a stable austenitic matrix and corrosion resistance, whereas Inconel 625 contains ≥ 58% Ni with ~20–23% Cr and substantial Nb and Mo additions that strengthen the alloy by solid-solution and segregation-controlled precipitation. These distinct chemistries associated with different melting ranges, together with differences in thermal expansion and solidification behavior, create steep gradients when the alloys are fused, encouraging segregation and non-equilibrium phase formation [23].

On the 316L side, the primary metallurgical risk in dissimilar welding is sensitization. For example, chromium-rich carbides can precipitate at austenite grain boundaries during weld thermal cycles, locally depleting Cr and degrading intergranular corrosion resistance. In multi-pass or high-heat input procedures, sigma (σ) phase may also appear, either via the eutectoid decomposition of residual δ ferrite or nucleation in Cr-rich regions, further embrittling the heat-affected zone. These transformations depend on time–temperature exposure, weld chemistry (particularly C and Cr) and local diffusion paths near the fusion boundary [24,25].

On the Inconel 625 side, the solidification of the Ni-rich melt in contact with Fe-rich dilution tends to segregate Nb and Mo into interdendritic liquid. When cooling, these solute-rich pools tend to form Nb-/Mo-rich MC-type carbides and Laves intermetallically; both phases harden and embrittle the mixed fusion zone and are well-documented in Inconel 625 weld deposits and overlays [26]. The propensity for Laves increases with Nb, Mo and even Si content, and it is exacerbated by Fe dilution, which reduces Nb/Mo solubility in γ-Ni and intensifies segregation. In dissimilar joints, this typically manifests as heterogeneous interdendritic structures that elevate hardness and lower toughness at or near the Inconel side of the fusion line [27,28]. Because Fe and Ni are mutually soluble in austenite, much of the mixed fusion zone solidifies as γ with a graded Fe/Ni ratio. Nevertheless, the chemical inhomogeneity left by solidification paths and remelting can produce an “unmixed zone” and local composition spikes where σ, chromium-rich carbides and Laves can nucleate. A recent review of Inconel–austenitic stainless dissimilar welds highlights these recurring features (unmixed zones, elemental segregation, Laves formation, sensitization and micro-fissuring) as the dominant microstructural risks that reduce mechanical and corrosion performance in the welded state [3].

Experimentally, multiple studies on the direct joining of 316L–Inconel 625 (using arc or high power welding sources) confirm these mechanisms. For example, an investigation comparing 316L–Inconel 625 dissimilar welds (produced with different fillers and current modes, using line scans and SEM/EDX mapping) showed strong Nb/Mo segregation on the Inconel 625-rich side and compositional depletion near the stainless interface, with the best toughness and corrosion behavior obtained only when Ni-based consumables limited adverse dilution. Similar reports on Inconel 625 coatings over 316L (including wire arc additive manufacturing (WAAM)/cladding) describe columnar and equiaxed dendrites with interdendritic Nb-/Mo-rich phases that concentrate hardness and reduce impact energy unless controlled process parameters limit segregation [23].

6. The Use of Interlayers in Joining 316L to Inconel 625: Approaches, Outcomes and Literature Examples

To mitigate the segregation, sensitization and residual-stress problems of direct fusion welding, a widely adopted strategy is to insert an interlayer, on one or both sides, before completing the joint. For the 316L–Inconel 625 dissimilar pair, the most common interlayers are Ni-based fillers (ex: ERNiCr-3/82, ERNiCrMo 3/625) or Ni-Fe alloys deposited as thin buffer layers. Due to nickel’s high solubility in both stainless steel and Inconel 625, it forms a smooth chemical bridge, while a Ni-rich buffer adjacent to 316L prevents carbon transfer and therefore suppresses sensitization at the stainless steel interface [29]. Controlled studies support these benefits. In the dissimilar GTAW of 316L to Inconel 625, deposits made with ERNiCr 3/ERNiCrMo-3 fillers exhibit more suitable transition zones, reduced Laves networks and improved impact energy relative to joints made with stainless fillers or heavy Fe dilution. A study comparing ERNiCr-3, 316L and a hybrid “twisted” filler demonstrated that the Ni-based filler provides superior toughness and corrosion resistance, effectively minimizing Fe dilution in the Inconel 625-enriched region [23].

Graded strategies extend this idea: a thin Ni or Ni-Fe buffer is deposited onto the stainless side before final joining to Inconel. Studies on buffer-layered buttering (ERNiCr-3/ENiCrFe-3) show that introducing a Ni-bearing intermediate layer reduces hard intermetallic content at the interface, smooths hardness profiles and raises bend/impact performance, particularly for multi-pass welds typical of thick sections. Reports focused on nuclear and pressure-boundary applications highlight that the buffer shifts the most critical microstructural reactions away from the 316L HAZ and lowers residual stress concentration at the dissimilar interface [30].

Recent work in cladding and additive manufacturing contexts reaches similar conclusions: across these configurations, the interlayer functions as both a chemical and thermomechanical buffer, reducing galvanic severity and accommodating a mismatch in the coefficient of thermal expansion (CTE) [27,31]. The synthesis of the literature suggests three design principles for interlayers in 316L–Inconel 625 joints. First, place a Ni-rich butter directly on 316L to prevent carbon-assisted sensitization and to maintain passive-film integrity in the stainless heat-affected zone (HAZ). Second, limit Fe dilution into the final Inconel-rich passes to minimize interdendritic Laves/MC precipitation, often requiring low-dilution parameters or graded sequences (316L → Ni or Ni-Fe buffer → 625). Third, qualify the joint with microchemical mapping, hardness profiling, impact/bend tests and corrosion measurements to verify that the interlayer achieved the intended microstructural control. Together, these practices consistently outperform direct joints in terms of toughness and localized corrosion resistance, while also enabling an economical use of expensive Ni filler by confining it to the most critical regions. Importantly, recent studies have shown that even Fe-rich interlayers can produce successful results, especially when introduced intentionally as graded transitions. For instance, in a hybrid additive manufacturing study that deposited ER70S6 carbon steel (a generally Fe-rich filler) and Inconel 625 sequentially, strong bonds were achieved without the formation of brittle intermetallic phases at the interface. The interfacial microstructure remained clean and the interface exhibited a shear strength around 452 MPa, with fracture behavior showing a mix of ductile dimples and cleavage rather than brittle failure. This outcome indicates that, when proper processes, such as WAAM combined with Laser-Directed Energy Deposition (LDED), are used, Fe-rich interlayers can serve as effective transitional layers between steel and nickel super alloys [32].

These results show that Fe-based interlayers, not just Ni-based ones, can work under appropriate operating conditions, particularly when additive manufacturing techniques allow tight control over dilution, thermal cycles and microstructure. The key is minimizing harmful phases like Laves, carbides and σ-phase and ensuring a clean metallurgical bond.

7. Methods Used in the Fabrication of FGMs

It is well-established in the welding industry that achieving a strong metallurgical bond between dissimilar alloys is often a challenging task. This difficulty can be understood by considering the fundamental principles of physical metallurgy. Alloy design typically involves carefully balancing the proportions of various elements and optimizing thermomechanical processing conditions to produce a material with consistent microstructure and properties [33]. Even small deviations from the intended composition and/or imposed thermal history can, at best, slightly alter the material’s performance and, at worst, compromise its most critical properties (such as ductility, strength or corrosion resistance, for example) by introducing undesired phases. Therefore, introducing a joint that features intermediate compositions or microstructures not aligned with any certified alloy is a difficult task.

These challenges are further intensified by the sharp interfaces that inevitably arise when joining alloys with significantly different mechanical and thermophysical properties. In general, a successful dissimilar alloy joint must be at least as strong as the weaker of the two base materials. However, sudden gradient shifts between the alloys can become preferential locations for residual stress concentrations during both fabrication and service, which can lead to premature failures that would not occur in either base material alone. Sudden changes in composition can also create chemical potential gradients, driving the migration of alloying elements and impurities across the joint. This movement can aggravate existing failure mechanisms, and, in some cases, mismatched properties, such as thermal conductivity or melting temperature, can even prevent certain joining processes from being successfully applied [33].

The welding industry has also developed various techniques for material grading, particularly to address challenges in dissimilar metal joints, which often involve sharp interfaces. A wide range of welding and joining processes are used for fabricating dissimilar metal joints, including arc welding, laser and electron beam welding, friction stir welding, brazing, diffusion bonding, and explosive welding. While substantial compositional grading is not always necessary for dissimilar metal joints, it is common practice to use dissimilar filler metals and interlayers to address issues such as solubility, thermal expansion mismatch, dilution and other incompatibilities between the two alloys. True graded joints, however, have only been successfully demonstrated in traditional welding techniques on a limited number of occasions [29,34].

Since the concept of FGMs was first introduced, a variety of processing methods have been developed to produce them. Among the methods employed, chemical and physical vapor deposition (CVD/PVD) as well as plasma spraying have been particularly useful for depositing functionally graded thermal barrier coatings (TBCs). Similarly, laser cladding processes provide greater versatility, enabling the deposition of dense surface coatings that can enhance wear and corrosion resistance on existing components. However, these processes have relatively slow deposition rates, making them unsuitable for manufacturing bulk components, only being used in surface treatments [35].

In contrast, powder metallurgy offers a more viable option for the bulk production of FGMs. This method involves mixing, stacking and pressing powders to achieve the desired spatial distribution of materials, which can then be consolidated using techniques such as centrifugal forces, hot isostatic pressing, or solid-state sintering. While powder metallurgy allows the production of bulk components, it has limitations, including constraints on the size and shape of produced parts and potential issues with porosity, depending on the powder properties and the consolidation method used. Additionally, sintering parameters may vary considerably across different materials, complicating the fabrication of multi-material parts [35].

8. Wire Arc Additive Manufacturing Techniques Used in the Fabrication of Functionally Graded Materials

WAAM has emerged as a promising technique for fabricating FGMs due to its high deposition rate, flexibility in feedstock materials and ability to deposit multiple wires during a single build [36]. In WAAM systems, material gradation can be achieved by sequentially switching filler wires or by simultaneously feeding multiple wires and adjusting their feed rates. This capability allows the gradual transition between dissimilar materials, enabling the production of multi-material structures with tailored properties. The high heat input inherent to arc-based processes also promotes metallurgical bonding between layers, which is essential for achieving strong interfaces in graded structures [36,37]. The following subsections focus specifically on the arc-based WAAM processes that have been applied to the fabrication of 316L stainless steel-Inconel 625 FGMs and closely related dissimilar systems, reviewing the process characteristics, reported microstructural outcomes and mechanical performance data that are directly relevant to this material pair.

WAAM can be further categorized based on the welding process used, including Gas Metal Arc Welding (GMAW), Gas Tungsten Arc Welding (GTAW), Plasma Arc Welding (PAW) and Submerged Arc Welding (SAW).

8.1. GMAW and GTAW-Based WAAM for FGMs

Gas Metal Arc Welding (GMAW) and Gas Tungsten Arc Welding (GTAW) are two of the most widely employed arc welding processes in wire arc additive manufacturing (WAAM) and, by extension, in the layer-by-layer fabrication of functionally graded materials (FGMs). Despite sharing the same fundamental operating principle (the use of an electric arc to melt metallic feedstock), they differ substantially in electrode type, heat input controllability, deposition rate and arc stability, each making them preferable under different manufacturing conditions, [38,39].

Gas Metal Arc Welding, commonly referred to as MIG (Metal Inert Gas) or MAG (Metal Active Gas) welding, depending on the shielding gas employed, uses a continuously fed consumable wire electrode that simultaneously serves as the electrical conductor and the filler material. An electric arc is established between the wire tip and the workpiece, and the wire is replenished from a spool at a controlled, constant feed rate. A coaxial flow of shielding gas (typically argon, helium, argon/CO₂ mixtures or other active gas blends) protects the molten weld pool from atmospheric contamination by oxygen, nitrogen and hydrogen. One of GMAW’s defining characteristics is its metal transfer mode, which governs the mechanism by which molten droplets detach from the wire and are deposited into the pool. In the short-circuit transfer mode, the wire tip contacts the pool at high frequency, producing low heat input and minimal spatter, which is suitable for thin-section materials and all-positional welding. In spray transfer, fine and axially directed droplets are propelled at high rates, producing minimal spatter and high deposition efficiency, requiring argon content above 80% and elevated current levels. Pulsed spray transfer is an advanced variant that employs a pulsed power source alternating between peak and background current levels, delivering the stability of spray transfer at reduced average heat input, which is particularly valuable in WAAM for limiting inter-pass thermal accumulation during long multi-layer builds [38,39].

A notable GMAW variant extensively studied for WAAM is Cold Metal Transfer (CMT), which uses a mechanically assisted wire retraction mechanism during droplet detachment to reduce heat input and virtually eliminate spatter. CMT and its pulsed variants have demonstrated superior porosity control and improved microstructural homogeneity compared to conventional GMAW in multi-material WAAM builds, making them particularly relevant for FGM fabrication [40].

In contrast, Gas Tungsten Arc Welding uses a non-consumable tungsten electrode to generate the arc, while the filler material is introduced separately via an automated wire feeder in WAAM configurations. Because the electrode does not melt into the pool, heat input and filler addition are independently controlled: the welding current governs the energy transferred to the workpiece, while wire feed rate and travel speed determine the volume of material deposited per unit length. Shielding is provided by high-purity argon or helium, ensuring an excellent oxidation protection that is critical for reactive alloys such as titanium and nickel-based superalloys. This decoupling between heat source and material delivery is GTAW’s principal advantage in FGM applications, as it allows the fine and independent adjustment of the melt pool size, penetration depth and dilution between dissimilar materials at each transition layer [38,39].

Gas Metal Arc Welding is one of the most widely used processes for WAAM due to its high deposition rate and relatively stable arc characteristics. GMAW-based WAAM is particularly suitable for fabricating FGMs because dual-wire feeding systems can easily adjust the relative contribution of different materials during deposition, enabling programmable compositional gradients within a single deposited wall [38,39].

For example, Sasikumar et al. [41] fabricated a functionally graded structure using stainless steel 316L and the nickel-based superalloy Inconel 625 through a GMAW-based WAAM process. The study reported a defect-free bimetallic interface with good metallurgical bonding between the two materials. Microstructural analysis revealed dendritic structures near the interface and uniform elemental distribution confirmed by energy-dispersive spectroscopy (EDS). The authors concluded that the process allowed the successful fabrication of graded stainless steel–nickel alloy structures with promising mechanical properties.

Another example is a triple-material FGM fabricated using GMAW–WAAM, combining ER316L stainless steel, duplex steel ER2205 and Inconel 718 [42]. The study demonstrated that gradual transitions between the alloys resulted in improved tensile strength and microhardness along the build direction. Microstructural examination showed columnar grains in the stainless steel region and the formation of carbides and Laves phases near the Inconel region. The Laves phase formation in this system is attributed to the segregation of Nb and Mo to interdendritic sites during non-equilibrium solidification, a phenomenon that is particularly pronounced in Inconel alloys mixed with iron-containing steels and which can impair ductility and corrosion resistance if present in excessive fractions [43].

On the other hand, GTAW provides greater control over heat input and arc stability compared to GMAW, making it advantageous for applications requiring a precise control of the melt pool and dilution between materials. Although GTAW typically has lower deposition rates, it enables the improved control of compositional gradients and reduced spatter during the additive process [38,39].

A study on twin-wire arc additive manufacturing using a GTAW power source fabricated a 316L stainless steel to Inconel 625 FGM, evaluating both a direct interface strategy and a smooth compositional gradient strategy. Both approaches produced defect-free builds free from cracks, porosity and fissuring. Synchrotron X-ray diffraction analysis revealed that the smooth-gradient strategy promoted the formation of secondary phases absent in the direct interface build, specifically, δ-phase (Ni₃Nb), σ-phase and MC carbides in between the regions with 60–80 wt.% Inconel, arising due to the elevated iron content destabilizing the Inconel matrix and driving Nb and Mo segregation to interdendritic regions. Neutron diffraction measurements further revealed that the smooth-gradient FGM developed significantly higher residual stresses compared to the direct interface FGM, a result that challenges the widely held assumption that smooth gradients are inherently preferable for stress mitigation. In terms of mechanical performance, the direct interface FGM achieved higher ultimate tensile strength and elongation at fracture than the smooth-gradient build, with fractographic analysis confirming predominantly ductile fracture in the direct interface build, while the smooth gradient exhibited a mixed ductile/quasi-cleavage failure mode attributed to the presence of TCP phases at the fracture locus [44].

In a smooth-gradient build, the wire feed ratio is adjusted incrementally across multiple deposition layers, meaning that the deposit traverses the 60–80 wt.% Inconel 625 compositional range over an extended number of passes. Each subsequent layer reheats the previously deposited material, maintaining intermediate compositions within the 700–900 °C precipitation temperature range for Laves, δ (Ni₃Nb), σ and MC carbides for a prolonged cumulative thermal exposure [45]. The elevated Fe content in this range reduces Nb and Mo solubility in the γ-Ni matrix without preventing their segregation to interdendritic sites [29], creating thermodynamic and kinetic conditions favorable to the heterogeneous nucleation of these detrimental phases. In contrast, a direct interface strategy transitions abruptly from 100% 316L to 100% Inconel 625 wire within a single or very few passes, minimizing the time spent at intermediate compositions and thus restricting both the driving force and the kinetic window for nucleation and coarsening of these phases.

A comparative study conducted also examined the differences between GTAW and GMAW processes for WAAM deposition using steel and stainless-steel wires. The results showed that GTAW provided higher process stability and better control of the melt pool, although GMAW achieved smoother walls and higher deposition efficiency [46]. This trade-off is consistently echoed in the broader WAAM literature, and the selection between the two processes is ultimately governed by the balance between productivity and metallurgical control required for a given application. For large structural components, where build time and material cost are dominant considerations, GMAW’s higher deposition rate and lower environmental footprint make it the preferred choice. For precision components requiring the tight control of compositional gradients or involving reactive and high-value alloys, GTAW’s superior arc stability and independent heat and filler control justify the lower throughput [47].

It should be noted that the conclusion favoring direct interfaces over smooth gradient strategies is most strongly supported by GTAW-based twin-wire WAAM studies and by the optimized gradient work of Zhu et al. [48]. For PAW–WAAM, the available evidence is indirect, drawn from analogous bimetallic systems rather than from 316L–Inconel 625 builds specifically. For GMAW, the studies reviewed report broadly consistent findings but do not systematically compare interface strategies under controlled and varied parameter conditions. The generality of this conclusion across all arc processes, heat input regimes and deposition sequences therefore remains to be confirmed experimentally and should be treated as a well-supported hypothesis rather than a universally established principle [48]. Regardless of the arc process employed, several metallurgical challenges are common to WAAM-fabricated FGMs. The repeated thermal cycles during deposition can significantly affect microstructural evolution, grain growth and phase transformations [36,37]. Controlling inter-pass temperature through idle time management or active cooling strategies is therefore essential for ensuring dimensional and microstructural repeatability across the build height. In addition, the directional solidification driven by layer-by-layer deposition produces columnar grains oriented along the build direction, resulting in mechanical property anisotropy. Post-processing techniques such as inter-pass rolling or ultrasonic impact treatment can promote grain refinement and mitigate this anisotropy.

8.2. PAW-Based WAAM for FGMs

Plasma Arc Welding (PAW) is another WAAM variant characterized by a highly concentrated plasma arc and higher energy density compared to Gas Tungsten Arc Welding (GTAW). In PAW, the arc is constricted by forcing the ionized shielding gas through a small-diameter copper nozzle surrounding the non-consumable tungsten electrode, concentrating the arc energy into a significantly smaller area than is achievable with the open arc of GTAW. The result is an arc energy density approximately three times greater than that of GTAW, producing deeper penetration, higher welding speeds and improved arc stability [49]. Two gases are used in PAW: a plasma gas (typically argon or argon/hydrogen mixtures) that is ionized to form the plasma column and a separate shielding gas delivered coaxially around the nozzle to protect the melt pool from oxidation. Like GTAW, the electrode is non-consumable and filler material is introduced separately via an automated wire feeder [50]. These characteristics result in deeper penetration, improved arc stability and a better control of the melt pool, which can be beneficial for producing FGMs with strong metallurgical bonding and refined microstructures. Ding et al. reported that arc-based additive manufacturing processes, including plasma arc-based deposition, are capable of producing dense metallic structures with relatively refined microstructures due to the concentrated heat source and stable arc behavior during the deposition process [51].

Research comparing PAW-based WAAM and GMAW-based WAAM for 316L stainless steel deposition revealed differences in microstructure and mechanical properties between the two processes. The study observed an austenitic matrix with δ-ferrite distributed in dendritic morphologies, with the PAW-fabricated samples exhibiting higher microhardness values due to the more concentrated heat source and refined solidification structure. The retention of δ-ferrite in WAAM-deposited 316L has been identified in several studies as a positive outcome, since δ-ferrite improves resistance to hot cracking and contributes to the overall strength of the deposit [52]. Similarly, Williams et al. investigated different arc-based additive manufacturing technologies and highlighted that plasma arc systems provide improved melt pool stability and deeper penetration compared to GTAW-based processes, which can enhance interlayer bonding during the layer-by-layer deposition process [53].

The use of PAW in multi-material and graded structures has also been investigated for stainless steel systems combining alloys with distinct phase stability. A study fabricated a multi-material layered structure by depositing alternating layers of AISI 316LSi austenitic stainless steel and AISI 430 ferritic stainless steel onto an AISI 316L substrate using a plasma wire arc additive manufacturing process. Comprehensive microstructural characterization revealed good weldability between the two deposited steels and between the deposit and the substrate. A graded duplex (α–γ) structure was observed at the layer interfaces, with micro- and macro-hardness measurements evidencing a harder phase at the interface compared to the individual base materials, consistent with the presence of a mixed ferritic–austenitic microstructure [54]. The study confirmed that PAW–WAAM is particularly advantageous for depositing different wire materials due to the stability of the molten pool and arc, which allows the implementation of various wire mixture strategies without process interruption.

The interfacial challenges associated with PAW-based WAAM FGMs are further illustrated by work on bimetallic structures combining Inconel 740H superalloy and P91 steel. Microstructural characterization revealed a large gradient zone with coarse grains at the interface, and intergranular cracks were observed along the gradient zone. Non-equilibrium solidification simulations identified two root causes of cracking: the sudden change in the volumetric coefficient of thermal expansion due to MC carbide formation and the development of local thermal residual strains [55]. The authors concluded that introducing graded intermediate layers between the two deposits, rather than a direct interface, would be beneficial for achieving defect-free bimetallic builds. This finding reinforces the importance of deposition strategy selection in multi-material WAAM, a consideration that is equally relevant across GMAW, GTAW and PAW processes.

The thermal cycles inherent to arc-based additive manufacturing processes also play a significant role in microstructural development. Studies on the arc-based deposition of metallic materials have shown that repeated heating and cooling during the manufacturing process can lead to directional grain growth along the build direction, resulting in columnar dendritic microstructures. These thermal effects can influence mechanical properties and residual stress development, highlighting the importance of controlling process parameters such as heat input, travel speed and interlayer temperature during plasma arc additive manufacturing processes [49]. In PAW–WAAM, heat accumulation during large builds is a particularly relevant concern. As successive layers are deposited and the build height increases, the thermal dissipation rate decreases, causing a progressive rise in inter-pass temperature that can lead to coarser grain structures, increased oxide formation risk and greater susceptibility to distortion and residual stress buildup in the upper layers of the build [56].

Despite these advantages, several challenges remain when applying PAW in additive manufacturing. The high energy density of the plasma arc may lead to excessive heat accumulation during large builds, which can result in distortion, residual stresses and microstructural coarsening in the heat-affected zones. Furthermore, maintaining process stability requires careful control of parameters such as plasma gas flow rate, welding current and deposition speed. PAW systems are also inherently more mechanically complex than GMAW or GTAW setups, requiring dual gas circuits, a plasma orifice nozzle and pilot arc ignition systems, which increase capital cost and maintenance requirements. Nevertheless, PAW-based WAAM continues to attract increasing research interest due to its ability to produce dense metallic structures with good metallurgical integrity and relatively refined microstructures, making it a promising technique for the fabrication of functionally graded and multi-material components [51,56].

Table 3. Comparison of GMAW, GTAW and PAW processes for WAAM-based FGM fabrication.

Parameter	GMAW	GTAW	PAW
Electrode type	Consumable wire	Non-consumable tungsten	Non-consumable tungsten
Heat input control	Moderate	High (independent of filler)	Very high (constricted arc)
Arc stability	Good	Very high	Highest
Deposition rate	~3–4 kg/h (highest)	Lower than GMAW	Intermediate
Spatter	Moderate to high	Minimal	Minimal
Surface finish	Smoother walls	Good	Good
Notable variants	CMT, pulsed-GMAW	PC-GTAW	3DPMD (powder)
Typical FGM systems	SS/Inconel, SS/duplex	SS/Inconel, Ni superalloys	SS austenitic/ferritic, Inconel/steel

provides a comparative overview of the key process parameters and typical FGM systems for which each arc variant has been applied.

8.3. Challenges in WAAM Fabrication of FGMs

Despite the significant advantages of WAAM for FGM production, several challenges remain. One of the main issues is controlling dilution between dissimilar materials to avoid the formation of brittle intermetallic phases [36]. As discussed in Section 5, differences in melting temperature, thermal expansion coefficients and solidification behavior can lead to residual stresses and cracking within the graded region. The repeated thermal cycles during deposition can significantly affect microstructural evolution, grain growth and phase transformations. In some cases, these thermal cycles can promote homogenization and improved bonding, while in others they may lead to grain coarsening or the segregation of alloying elements [36,37].

A particularly important metallurgical challenge in the 316L–Inconel 625 system is the formation of Laves phase in the Inconel-rich regions of the graded structure. Laves phase precipitates at interdendritic sites due to the segregation of Nb and Mo during solidification, a process that is intensified by iron dilution from the 316L side, which reduces the solubility of these elements in the γ-Ni matrix. Although Laves phase can be partially dissolved by post-deposition solution heat treatments at temperatures above approximately 1148 K, such treatments may simultaneously coarsen grain structures or alter the δ-ferrite content in the 316L-rich layers, making parameter selection for heat treatment a trade-off that requires careful consideration [3,57].

Residual stress development is another key challenge that is intrinsic to the layer-by-layer deposition strategy of WAAM. Thermal gradients generated during deposition induce residual stresses that can cause distortion, cracking or premature failure under service loading. Neutron diffraction measurements on 316L-Inconel 625 T-WAAM FGMs have shown that residual stresses can reach values as high as 468 MPa in the build direction for smooth-gradient interfaces, exceeding those of direct interface builds by a factor of more than two. Managing inter-pass temperature through controlled dwell times, forced cooling or active feedback systems is therefore essential for limiting stress accumulation across the build height [44].

The directional solidification driven by layer-by-layer deposition also produces columnar grains oriented preferentially along the build direction, resulting in mechanical property anisotropy. In situ rolling applied between deposition passes has been shown to effectively refine the columnar grain structure into an equiaxed morphology, reduce dislocation density gradients and improve the isotropy of tensile and fatigue properties in WAAM 316L [58]. Post-deposition heat treatment at temperatures above 1000 °C further improves the strength–ductility balance of rolled WAAM components, although heat treatment at intermediate temperatures (approximately 650 °C) can promote σ-phase formation from δ-ferrite, increasing hardness while reducing ductility [24]. The corrosion behavior of WAAM-fabricated FGMs also warrants specific attention. Research on the PAW– and GMAW–WAAM of 316L stainless steel has shown that differences in heat input between processes can significantly influence the δ-ferrite content and distribution, which in turn affects the electrochemical response of the deposit in corrosive media [25]. In graded 316L–Inconel 625 structures, the intermediate compositional regions are of particular concern, as galvanic coupling between Fe-rich and Ni-rich zones may create preferential sites for localized corrosion, sensitization or pitting in service environments involving chloride media or elevated temperatures [20,52,59]. Studies on microstructure and corrosion behavior of wire arc additively manufactured graded steel combinations have confirmed that the graded region exhibits distinct electrochemical activity compared to the monolithic end members, underlining the importance of corrosion characterization as part of the qualification of WAAM-fabricated FGMs.

8.4. WAAM Studies Using Inconel 625 and 316L FGMs

Several studies have focused on the fabrication of stainless steel 316L–Inconel 625 FGMs using different WAAM strategies, investigating aspects such as compositional control, microstructural evolution, residual stress development and the resulting mechanical properties of the graded structures.

An important study investigated the fabrication of 316L/Inconel 625 FGMs using twin-wire arc additive manufacturing (T-WAAM) [44]. In this work, different deposition strategies were evaluated, including direct interfaces and smooth compositional transitions. The results indicated that graded transitions between the two materials significantly influenced residual stresses and mechanical performance. Direct interfaces showed higher tensile strength and elongation, whereas smoother transitions helped to reduce residual stresses and improve compositional uniformity along the graded region. The study highlighted the potential of multi-wire WAAM systems for controlled compositional gradients in multi-material components. The residual stress profiles obtained for both samples are shown in Figure 1.

More recently, research has also explored optimized compositional gradients between SS316L and Inconel 625 in WAAM-fabricated FGMs [48]. This study demonstrated that adjusting the wire feed rates of both alloys allows precise control of the compositional transition within the graded region. The study reported that certain compositional ranges (around 20 wt.% Inconel 625) resulted in reduced mechanical properties and increased cracking susceptibility. However, increasing the Inconel content in the transition zone improved microhardness and tensile properties while eliminating cracks. These findings highlight the importance of carefully designing the gradient path in multi-material WAAM structures. Photographs of these mechanisms and the observed cracks are shown in Figure 2.

In addition to two material FGMs, some researchers have also explored multi-material graded structures involving carbon steel, stainless steel 316L and Inconel 625 fabricated via WAAM [60]. In this approach, stainless steel acts as an intermediate layer to improve metallurgical compatibility between carbon steel and the nickel-based superalloy. Microstructural analysis showed the presence of δ-ferrite within the austenitic matrix of the 316L layers, while the Inconel 625 region contained Nb-rich Laves phases in interdendritic areas due to its high niobium content. Mechanical testing revealed gradual variations in hardness across the graded structure, demonstrating the feasibility of producing multi-material structures with continuous property transitions using WAAM.

Another study evaluated the microstructure and mechanical properties of FGMs in the as-built condition and after heat treatment [61]. The results indicated that, consistent with previous findings, the 316L stainless steel within the final structure exhibited lower ductility compared with monolithic 316L structures. In contrast to other reports, however, the FGMs displayed a strongly bonded interface, as evidenced by high hardness values. This behavior was attributed to grain growth in the epitaxial interface layer relative to the grains of the previously deposited layer. Heat treatment resulted in a reduction of approximately 16–20% in δ-ferrite content, leading to increased hardness and improved mechanical properties. Furthermore, longer heat treatment durations promoted greater grain misorientation, particularly in regions close to the bimetallic interface [61].

The influence of deposition sequence on the resulting microstructure and mechanical properties has also been investigated [62]. Results showed that the deposition sequence had no significant effect on the microstructure of either 316L stainless steel or Inconel 625. However, the relative amounts of δ-ferrite and Laves phases in the final structure were affected. Due to the different melting points of the two alloys, the 316L–Inconel 625 interface appeared significantly more distinct, whereas the Inconel 625–316L interface exhibited greater elemental mixing, which promoted the formation of a larger fraction of Laves phases. Cracks were observed exclusively at the Inconel 625–316L interface, likely associated with alloying element diffusion and Laves phase formation. Hardness values remained within the typical ranges for both alloys. Mechanical testing revealed ductile fracture in regions corresponding to 316L stainless steel and more brittle fracture behavior in areas corresponding to Inconel 625, with fracture occurring predominantly near the Inconel 625–316L interface where cracks had previously been detected [62].

Xiaoyan et al. further investigated the microstructural evolution and mechanical properties of FGMs composed of 316L stainless steel and Inconel 625 produced by Dual-Wire Plasma wire arc additive manufacturing [63]. A functionally graded structure with a 50 wt.% stainless steel to Inconel 625 ratio was successfully fabricated. The resulting microstructure consisted predominantly of columnar and equiaxed dendrites, with interdendritic spacing increasing along the build direction. A gradual transition from FCC to BCC phases was observed and grain growth occurred preferentially along the ⟨001⟩ direction. Within the compositional gradient, three distinct compositions were identified as a result of remelting effects during deposition. Energy-dispersive spectroscopy (EDS) analysis revealed the presence of secondary phases in the transition regions, while areas containing 100 wt.% Inconel showed the formation of Laves phases. Hardness values decreased with increasing 316L content and increased with higher fractions of Inconel.

A different approach was adopted by Liu Wei et al., who examined the influence of build orientation by fabricating specimens deposited both vertically and horizontally [64]. In general, the interfaces were found to be defect-free, differing primarily in terms of dilution. Horizontally deposited specimens exhibited lower dilution and a more heterogeneous, wavy interface morphology. Similar to most previously reported studies, Laves phases were identified in the Inconel regions, while the stainless steel consisted predominantly of austenite with δ-ferrite located along the grain boundaries. EDS analysis revealed a non-uniform diffusion of Ni within the stainless steel layer, resulting in abrupt compositional changes in the transition regions accompanied by segregation of Nb, Mo and C at grain boundaries. These microstructural discontinuities likely contributed to the inferior tensile properties observed in vertically built specimens compared with horizontally deposited samples.

Similar findings were reported by Motwani et al. [40], who also investigated bimetallic structures composed of Inconel 625 and 316L stainless steel produced using wire arc additive manufacturing. Their results indicated that, when the final structure was free of cracks, fracture during mechanical testing occurred predominantly on the 316L stainless steel side and was typically ductile in nature. In contrast, when fracture occurred in the Inconel 625 region, elemental segregation and the presence of secondary phases were consistently observed, resulting in a more brittle fracture behavior.

Overall, the reviewed studies demonstrate that 316L–Inconel 625 is a metallurgically compatible material pair for WAAM-based FGM fabrication, with good interfacial bonding and austenitic phase continuity achievable across the graded region under appropriate deposition conditions. However, the persistent formation of Laves phases, the composition-dependent instability window identified in the 60–80 wt.% Inconel range, the sensitivity of residual stress levels to gradient strategy, and the influence of deposition sequence and build orientation on crack susceptibility collectively indicate that this system presents significant metallurgical challenges that are not yet fully resolved. Characterizing the system as broadly suitable for WAAM-based FGM fabrication would therefore overstate the current state of readiness; a more accurate assessment is that it represents a promising but demanding material combination whose successful implementation requires the careful, application-specific optimization of deposition strategy, compositional path, thermal management and post-processing route. A synthesis of key mechanistic interactions in WAAM 316L-Inconel 625 FGM fabrication is represented bellow in

Table 4. Synthesis of key mechanistic interactions in WAAM 316L–Inconel 625 FGM fabrication.

Mechanism	Governing Variables	Effect on Microstructure	Effect on Mechanical Performance	Design Implication
Fe dilution into IN625	Wire feed ratio, travel speed, inter-pass temp.	Reduces Nb/Mo solubility in γ-Ni → promotes Laves + MC carbides at interdendritic sites	Increases hardness locally; reduces ductility and impact toughness near IN625-rich region	Minimize dwell in 20–40 wt.% IN625 range; avoid slow cooling through 700–900 °C
Smooth vs. direct interface	Gradient strategy (step size)	Smooth gradient: δ-phase (Ni3Nb), σ-phase and MC carbides form in 60–80 wt.% IN625 range; absent in direct interface builds	Direct interface: higher UTS and elongation; smooth gradient: mixed ductile/quasi-cleavage fracture	Direct interface preferred for structural performance; smooth gradient not inherently superior for stress
Residual stress	Gradient strategy, CTE mismatch, inter-pass temperature	Thermal gradients build up tensile stresses along build direction; smooth gradient reaches ≥468 MPa vs. lower values in direct interface	Exceeding yield stress locally can cause distortion or cracking under service loading	Control inter-pass temperature; favor direct interface or minimize gradient length
Deposition sequence (316L → IN625 vs IN625 → 316L)	Order of material deposition	IN625-on-316L interface shows higher elemental mixing and larger Laves fraction than 316L-on-IN625	Cracks observed exclusively at IN625 → 316L interface due to Laves concentration	Prefer 316L deposited first (bottom), IN625 last (top) for crack-free builds
Build orientation	Vertical vs. horizontal deposition	Horizontal builds: lower dilution, wavy interface; vertical builds: more uniform composition but inferior tensile properties	Vertically built specimens show inferior tensile properties vs. horizontally deposited equivalents	Horizontal deposition preferred where geometry permits
Post- processing (HT/rolling)	Solution HT temperature, rolling force	HT > 1000 °C: reduces δ-ferrite, improves ductility; HT ~650 °C: promotes σ-phase from δ-ferrite, raises hardness	In situ rolling refines columnar grains, reduces anisotropy, improves fatigue properties	Use HT > 1000 °C; in situ rolling between passes for property isotropy

.

9. Submerged Arc Additive Manufacturing: Principles, Current Applications and Potential for Future FGM Fabrication

Submerged arc additive manufacturing (SAAM) is an innovative process that combines the precision and flexibility of wire arc additive manufacturing (WAAM) with the well-established principles of submerged arc welding (SAW). This method has emerged as a notable solution for fabricating large-scale, high-strength components, particularly for industries that demand exceptional structural reliability and cost-efficiency. By using submerged arc processes, SAAM achieves efficient, high-quality material deposition, making it an attractive choice for manufacturing large-scale components with minimal material waste [65,66].

9.1. Principles of Submerged Arc Welding

Submerged Arc Welding (SAW) is a welding process that falls under the category of arc welding techniques and can be performed either automatically or semi-automatically. In this process, an electric arc is maintained between a bare consumable electrode and the workpiece within a protective inert gas atmosphere. SAW is known for its high material deposition rate and is particularly well-suited for horizontal welding positions [67].

What sets this process apart from other welding methods is that the arc is completely submerged within a granular flux, making it invisible during operation. This feature not only reduces heat loss but also allows the process to achieve a thermal efficiency of over 90%. To operate, SAW requires essential components such as an automatic wire feed mechanism, a constant voltage power source and digital controls, which need a steady and uniform power supply. The automatic wire feed system ensures a continuous supply of filler wire, while the granular flux is dispensed from a hopper in a controlled manner, providing a protective atmosphere for the weld zone, improving welding efficiency and effectiveness, and also providing alloying elements to the weld pool [67]. A schematic of a Submerged Arc Welding setup is shown in Figure 3.

Welded joints produced by SAW are characterized by high strength and ductility, with low hydrogen and nitrogen content. This process is widely used in applications requiring butt weld configurations or fillet welds with grooves. Its versatility makes it suitable for industries such as automobile manufacturing, railroad equipment production, large engineering structures, shipbuilding, pressure vessel fabrication, pipe welding and storage tank construction.

The quality of welds and the HAZ in SAW are influenced by various welding parameters, including welding current, voltage, wire feed rate, flux type and overall welding conditions. Adjustments to these parameters can significantly impact weld quality and performance, making careful control essential for optimal results [35,67].

Efforts to enhance the productivity of the conventional Submerged Arc Welding (SAW) process have focused on increasing both welding current and welding speed. However, these approaches come with limitations. Higher welding currents often lead to issues such as undercutting, humped weld beads and severe joint distortion. On the other hand, increased welding speeds can result in defects like centerline cracking and incomplete penetration [35,37].

An alternative solution is the use of the multi-wire SAW process, which offers a significant improvement in welding productivity, especially for joining medium- to high-thickness plates. However, the introduction of multiple wires considerably increases the complexity of process parameters, making it challenging to determine optimal welding conditions [37]. A thorough quantitative understanding of how process variables affect weld quality is both essential and difficult to achieve for the effective application of SAW and multi-wire SAW processes.

To date, extensive research has been conducted to examine the impact of SAW process parameters on weld quality using various experimental methods [35,67,68]. Although similar investigations for the multi-wire SAW process are still emerging, they are gaining momentum as researchers work to optimize this advanced welding technique.

Welding current, voltage, polarity, welding speed, wire electrode diameter, electrode extension and the inclination of the electrode relative to the weld seam play a critical role in determining the electrode and flux melting rates, weld bead dimensions and overall joint properties in the SAW process [35]. Robinson et al. [69] investigated the influence of welding current and polarity on the melting rates of the electrode and flux in a single wire SAW process. Their findings revealed that increasing the welding current resulted in higher melting rates for both the electrode and flux. Among the different polarities, the electrode melting rate was found to be highest in DCEN (Direct Current Electrode Negative), followed by AC (Alternating Current) and DCEP (Direct Current Electrode Positive) for the same current. Conversely, the flux melting rate was highest in AC polarity, followed by DCEN and DCEP at higher current levels. The researchers attributed this to a higher surface temperature at the electrode tip in DCEN, which led to the formation of larger droplets and a faster melting rate compared to DCEP. Additionally, reducing the arc voltage increased the electrode melting rate but decreased the flux melting rate. Lower arc voltage shortened the arc length, increasing the electrode extension and resistive heating, which enhanced the electrode melting rate. However, the shorter arc length reduced the flux melting rate.

9.2. Submerged Arc Additive Manufacturing

SAAM is particularly significant in industries such as aerospace, oil and gas, marine engineering, and energy, where the demand for scalable and reliable manufacturing solutions continues to grow. The submerged arc process used in SAAM offers inherent thermal control, which reduces residual stresses and distortion while maintaining even microstructural properties [66,70]. These benefits, along with high deposition rates and material versatility, make SAAM an innovative method in advanced manufacturing and its ability to solve common problems in additive manufacturing makes it an attractive focus for research and industrial use.

The process mechanism of SAAM integrates layer-by-layer material deposition with in situ intrinsic heat treatment, including multi-layer penetration normalizing, inter-critical annealing and tempering. This combination ensures microstructural uniformity while minimizing residual stresses, which is critical for maintaining the mechanical integrity of the fabricated components [66]. Unlike other AM methods, such as WAAM, SAAM achieves much higher deposition rates and enhanced thermal efficiency, which result in consistent layer quality and reduced post-processing requirements [71]. Moreover, the process’s adaptability to a variety of materials, particularly high-strength low-alloy (HSLA) steels, expands its utility across various industrial applications [66].

High-strength low-alloy steels, such as X80-grade pipeline steel, are the primary materials used in SAAM due to their toughness and fatigue resistance [66]. These steels are crucial in structural applications, including pipelines for natural gas transportation, where mechanical reliability is crucial. The microstructures of SAAM-fabricated components are predominantly composed of polygonal ferrites, which contribute to an advantageous combination of strength and toughness. This is achieved through controlled thermal cycling in the process, which also reduces anisotropy and ensures isotropic mechanical properties [65,66]. The uniformity in tensile strength between vertical and horizontal directions highlights SAAM’s ability to overcome directional weaknesses that are common in other AM methods. Additionally, the process significantly enhances fatigue resistance by controlling crack initiation and propagation through refined microstructural evolution.

SAAM’s applications are vast and impactful. As referenced before, in the oil and gas industry, it has played a key role in manufacturing high-performance pipelines using X80-grade steel. These pipelines exhibit superior toughness and fatigue resistance, making them ideal for cyclic loading conditions caused by pressure fluctuations. The cost-efficiency of SAAM further enhances its appeal, as it minimizes material waste and reduces production lead times. In aerospace and marine engineering, SAAM enables the manufacture of lightweight, structurally robust components such as propellers, pressure vessels and landing gear assemblies. The near-net-shape fabrication capability of SAAM aligns with critical industry requirements for material efficiency and fast production [66]. Beyond these traditional applications, SAAM is becoming popular in emerging fields such as military, nuclear energy and automotive manufacturing, where its ability to produce topologically optimized components from high-value materials offers a competitive advantage.

However, despite its advantages, SAAM faces several limitations that can delay its broader adoption, particularly when it comes to fabricating complex geometries and managing thermal issues during deposition.

One of the most significant challenges SAAM faces is its ability to handle complex geometries. While the process is highly effective for large, simple structures, it struggles with the fine details and sharp transitions required in intricate, complex designs [66,71]. The layer-by-layer deposition method used in SAAM, while efficient for large-scale parts, does not lend itself easily to the precision needed for manufacturing smaller or more detailed features. Achieving the fine geometrical control required for parts with intricate internal features or thin walls becomes particularly difficult, as the process is optimized for higher deposition rates rather than fine resolution.

In addition to the challenges of fabricating complex shapes, SAAM also counts with significant thermal limitations. The inherent thermal gradients in the process, due to the high heat input from the submerged arc welding technique, can lead to excessive heat accumulation in the workpiece. This thermal buildup can cause several issues, including dimensional inaccuracies and distortion, which are especially problematic when working with geometrically complex parts [71]. The uneven cooling rates across different sections of the component can also induce residual stresses, leading to warping or cracking, particularly in parts with large cross-sectional variations or thin-walled structures. These thermal issues make it difficult to achieve the dimensional precision required for parts that need tight tolerances or uniform mechanical properties across all directions. As a result, additional post-processing steps such as machining or heat treatment are often required to correct these distortions, adding both time and cost to the manufacturing process.

Another limitation tied to both complex geometries and thermal issues is the impact of residual stresses on the mechanical properties of SAAM-fabricated components. The thermal gradients can induce internal stresses that affect the overall structural integrity of the part, leading to reduced fatigue resistance or even premature failure under cyclic loading conditions [70,71]. While SAAM’s layer-by-layer deposition method offers some degree of heat treatment during deposition, the process does not provide the same level of thermal control as some other advanced manufacturing methods, making it more challenging to manage these residual stresses effectively.

Material limitations further complicate the use of SAAM for complex geometries. Although SAAM is highly effective with materials like HSLA steels, its material compatibility is still somewhat limited. The process works well with these steels due to their toughness and fatigue resistance, which are essential for applications such as pipeline fabrication and structural components [66]. However, the ability to use lightweight alloys like aluminum or titanium, which are often necessary for industries such as aerospace, automotive, or marine engineering, remains a challenge. These materials present difficulties due to their different thermal properties, which can aggravate issues related to heat accumulation and distortion. As such, SAAM’s current material capabilities limit its adoption in industries requiring lightweight, high-performance components that are often needed for complex geometries [70].

In addition to material limitations, achieving high-resolution surface finishes and fine tolerances for complex parts typically requires extensive post-processing. SAAM’s surface quality is generally not as refined as that produced by other additive methods or traditional machining processes, especially when working with intricate geometries. This means that many parts fabricated with SAAM need to undergo additional steps such as grinding, polishing, or machining to meet the required surface finish and dimensional accuracy. While SAAM’s primary advantage lies in its speed and efficiency for large-scale parts, the need for post-processing reduces the overall cost-efficiency for smaller, more detailed components, further limiting its applicability for some high-precision applications.

9.3. Potential of SAAM for FGM Fabrication

Although the majority of SAAM research to date has focused on homogeneous HSLA steel structures, the process offers characteristics that are inherently advantageous for multi-material and functionally graded applications [66]. The high and stable heat input of the submerged arc process promotes deep penetration and thorough interlayer fusion, which is beneficial for achieving strong metallurgical bonding at the interfaces between dissimilar alloys. The flux-shielded environment further reduces the risk of oxidation and hydrogen contamination of the melt pool during deposition, which is a relevant consideration when depositing nickel-based alloys such as Inconel 625 that are sensitive to interstitial contamination [67].

The intrinsic heat treatment effect inherent to SAAM, which combines normalization, inter-critical annealing and tempering within the same deposition cycle, could, in principle, be leveraged to tailor the microstructure of the graded region between 316L and Inconel 625 [67,68]. By controlling the heat input per layer and the inter-pass temperature, it may be possible to promote the partial dissolution of Laves phases and carbides that typically form in the intermediate compositional zones, thereby improving the mechanical integrity of the graded structure without requiring a separate post-processing heat treatment step [71]. Furthermore, the high deposition rates achievable with SAAM (significantly exceeding those of GMAW or GTAW-based WAAM) would be particularly advantageous for fabricating large FGM structures such as bimetallic pressure vessels, pipe transition joints or structural components for nuclear and offshore applications, where both material volume and mechanical performance are critical requirements.

Nevertheless, several limitations currently constrain the direct application of SAAM to FGM fabrication. The high heat input that characterizes the process, while beneficial for interlayer bonding, makes the fine control of the compositional gradient more challenging: each deposited layer tends to remelt and dilute a substantial portion of the underlying layer, which can blur the intended compositional transition and make it difficult to achieve sharp or precisely defined gradients [70]. The process is also currently limited in its material compatibility, being well-established for low-carbon and HSLA steels but not yet demonstrated for nickel-based superalloys such as Inconel 625, which have different flux–alloy interactions and solidification characteristics compared to steels. Research into suitable flux formulations and process parameter windows for high-nickel alloys in the SAAM context therefore represents an important and currently open area of investigation. Additionally, the geometric constraints of the SAW process (limited to near-flat or slightly inclined deposition orientations) impose restrictions on the range of FGM component geometries that can be fabricated without auxiliary tooling or substrate manipulation [35].

In summary, the applicability of SAAM to dissimilar and functionally graded structures remains a future research direction rather than a demonstrated capability. The existing SAAM literature is almost exclusively focused on homogeneous low-carbon and HSLA steel builds, and no experimental study reporting the direct fabrication of a 316L–Inconel 625 or analogous nickel–steel FGM by submerged arc additive deposition has been identified in the reviewed literature. The discussion of SAAM’s potential in this context is therefore necessarily prospective and extrapolative, drawing on the known process characteristics of SAW and reasoning by analogy from the challenges already established for GMAW and GTAW-based WAAM of the same material system. Dedicated experimental studies are needed before SAAM can be meaningfully positioned alongside GMAW, GTAW and PAW as a viable route for 316L–Inconel 625 FGM fabrication.

Several material-specific challenges must be considered before SAAM can be evaluated as a viable route for 316L–Inconel 625 FGM fabrication. Standard SAW flux formulations are developed and qualified for low-carbon and HSLA steels; their chemical interaction with the high Ni, Cr, Nb and Mo content characteristic of Inconel 625 is not established, and there is a credible risk of preferential oxidation of Nb and Mo (both critical strengthening elements [20]) or of unwanted alloying from flux decomposition products such as S, P or Si. SAAM’s characteristically higher heat input and slower cooling rates relative to GMAW- or GTAW-based WAAM are also thermodynamically and kinetically unfavorable for Nb/Mo-bearing alloys: slower interdendritic solidification increases elemental segregation and provides greater time for the nucleation of the Laves phase, whereas the dissolution of pre-existing Laves would require sustained temperatures above approximately 1150 °C [17] that are difficult to maintain uniformly across a large build. Furthermore, SAAM’s deep arc penetration and high per-layer dilution risk blurring the compositional transitions that define an FGM, potentially collapsing a designed multi-step gradient into a broader, less controlled mixed zone. Addressing these open questions experimentally would be a necessary prerequisite before SAAM can be meaningfully evaluated as a route for 316L–Inconel 625 FGM fabrication at scale.

10. Conclusions

The reviewed literature collectively demonstrates that arc-based WAAM is technically capable of producing 316L stainless steel–Inconel 625 FGMs with metallurgically sound interfaces and austenitic phase continuity across the graded region. Among the arc processes examined, GMAW offers the highest deposition efficiency and is well-suited to dual-wire gradient control, while GTAW provides the superior independent regulation of heat input and dilution at the cost of lower throughput. PAW occupies an intermediate position, offering improved arc constriction and melt pool stability relative to both, and CMT–GMAW represents the most heat-input-efficient variant for thermally sensitive multi-material builds.

The central and counterintuitive finding synthesized from the reviewed experimental studies is that direct compositional interfaces between 316L and Inconel 625 consistently outperform smooth gradient strategies in tensile strength, ductility and residual stress levels. This result is mechanistically explained by the existence of a composition-dependent instability window at 60–80 wt.% Inconel 625, where Fe dilution simultaneously destabilizes the Inconel matrix and promotes the co-precipitation of δ, σ, Laves and MC carbides, phases that are absent or present only in minor fractions in direct interface builds. This finding challenges the widely held assumption that smoother compositional transitions are inherently preferable for graded structures and has direct implications for gradient path design in industrial FGM fabrication.

Three key challenges remain unresolved and define the priority research agenda for this material system: the development of quantitative gradient path design rules that specify which compositional trajectories avoid the instability window under given arc process and parameter conditions; the systematic characterization of the corrosion behavior of the graded region, which has received comparatively little attention relative to microstructural and mechanical studies; and the optimization of post-processing routes, particularly heat treatment and in situ rolling, for residual stress relief without reactivating secondary phase precipitation.

Relative to alternative metal additive manufacturing routes, arc-based WAAM offers a compelling combination of high deposition rate, near-100% material utilization and scalability to large structural components that LPBF and LWDED cannot currently match at equivalent build volumes and at the cost of lower gradient spatial resolution and higher per-layer heat input. SAAM presents a future opportunity for very large-scale FGM fabrication but requires dedicated experimental validation for high-nickel alloys before its suitability can be assessed.