A Comprehensive Review of Recent Advancements in 3D-Printed Co-Cr-Based Alloys and Their Applications

Abstract

Co-Cr-based alloys are outstanding materials widely used in applications ranging from engineering to biomedical devices due to their excellent physico-mechanical properties, chemical stability, and biocompatibility. The demand for these alloys is steadily increasing, prompting a shift from conventional fabrication methods, such as casting and subtractive manufacturing, to advanced additive manufacturing (AM) techniques. These modern methods enable the production of complex geometrical shapes with enhanced properties. However, comprehensive information on current trends in 3D printing of Co-Cr-based alloys and their performance in specific applications remains limited. Therefore, the present article addresses this gap by reviewing recent advancements in the AM of Co-Cr-based alloys, offering insights for manufacturers, engineers, and researchers looking to develop optimized products. Key characteristics, including physical, mechanical, tribological, chemical, and biocompatibility properties, are thoroughly discussed, along with their applications, with a focus on potential future developments in this field. The exhaustive outlook of this paper provides a strong basis for future research endeavors in the domain of Co-Cr-alloy part production using AM.

1. Introduction

Nowadays, the additive manufacturing (AM) process is a smart choice for component manufacturers due to its ease of fabrication in obtaining complex geometrical shapes with promising material characteristics, which is not always achievable with conventional manufacturing processes [1]. However, there are several challenges with 3D-printed parts, such as poor density, undesired porosity, weak strength, dimensional inaccuracy, and poor surface finish [2]. Nevertheless, these issues can be addressed in the final printed parts by adjusting the processing parameters, such as energy source, material feed rates, build orientation, and others. Additionally, post-processing methods, like grinding, abrasive finishing, and electropolishing, can further refine parts, particularly in the fabrication of metallic alloys [3]. Researchers are committed to addressing these shortcomings and investigating applications across various industrial sectors. According to the laser-based additive manufacturing process, known as laser additive manufacturing (LAM), the 3D printing of metallic parts can be categorized into the following three major material addition systems: powder-bed, powder-fed, and wire-fed [4,5]. Powder-bed systems include techniques such as laser powder bed fusion (LPBF) and selective laser melting (SLM), also known as laser beam melting (LBM), commonly referred to as the direct metal laser sintering (DMLS) process. On the other hand, the powder-fed approach primarily includes methods like laser metal deposition (LMD) or directed energy deposition (DED) or laser-engineered net shaping (LENS). Additionally, wire-fed methods cover wire arc additive manufacturing (WAAM), wire-laser additive manufacturing (WLAM), and similar technologies [6]. Among this variety of printing technologies, LPBF and DED are found to be widely used techniques for fabricating industrial-grade parts, particularly applied in the aerospace, automotive, and biomedical industries, due to their ability to produce parts with desired densities and microstructural and mechanical characteristics [7]. These processes are governed by a computer-aided design (CAD) model, whereby a laser beam fuses and sinters the designated metal powder, depositing it layer by layer or by a controlled powder-feeding system, thereby forming a solid 3D structure. This provides manufacturers with a rapid production system, along with enough flexibility and design freedom to produce intricate parts via near-net shape manufacturing instead of traditional subtractive manufacturing processes, such as metal removal by machining and undercutting [8]. The most commonly used metal alloys for printing parts via AM technology include steel (ss)-based, titanium (Ti)-based, cobalt (Co)-based, and aluminum (Al)-based alloys. In recent years, Co-based alloys have gained significant attention due to their superior mechanical properties, wear and corrosion resistance, as well as biocompatibility, particularly in the medical industry [9]. Certainly, Co-Cr-based superalloys have become a very common choice for developing orthopedic implants such as hip and knee joints [10]. In addition, the outstanding strength and toughness of these superalloys have potential applications in the energy sector, such as wind turbines and jet engine components, as well as in load-bearing applications in aerospace, among others [11,12]. Furthermore, Co-based alloys exhibit excellent strength, as well as wear and corrosion resistance, even at high temperatures, making them suitable for applications such as valve seats in nuclear power plants, aerospace fuel nozzles, and engine vanes [13]. Figure 1 shows typical applications of Co-Cr-based alloys in various fields.

Generally, Co exhibits a hexagonal closed-packed (HCP) crystal structure at room temperature. However, when the temperature exceeds 400 °C, it transforms into a face-centered cubic (FCC) structure. Co-based alloys may exhibit a dendritic α-FCC structure, alongside an HCP structure at room temperature, depending on the manufacturing process. The metastable FCC structure can exist up to 1500 °C, just below the melting point of Co. When cooled below 400 °C, improper phase transformation can result in the presence of a combination of HCP and FCC structures. This phenomenon is also influenced by the particle size of the alloy system [14]. Most commonly, Cr is added to the Co matrix system to form metal carbides, which enhance strength and corrosion resistance. These carbides precipitate at grain boundaries, preventing dislocation and, thus, improving the overall strength of the alloy [15,16]. In some cases, particularly in biomedical applications, Mo and W are added into the Co-Cr matrix system to achieve optimum mechanical characteristics. Mo influences the formation of metal carbide due to the solid solution strengthening, whereas W helps in improving the bond strength [17,18].

Current manufacturing industries are keen to advance toward computer-aided manufacturing (CAM) technologies for Co-based alloys integrated with the AM process. However, this shift requires a deep understanding of the fabrication process and subsequent performance on modern manufacturing platforms. Based on the available literature on the AM of Co-Cr-based alloys between 2014 and 2024, this article presents a comprehensive review systematically, starting from material fabrication techniques to material performance, followed by applications in different industrial sectors and, furthermore, the challenges associated with future research directions. The article focuses on recent trends in the development of Co-based alloys via the AM process and alloy design aspects. In addition, it provides in-depth insights into how the process parameters influence the key characteristics of Co-Cr-based alloys, including their physical, mechanical, tribo-corrosion, and biocompatibility properties.

2. Current Fabrication Trends in Co-Cr-Based Metallic Alloys

In the AM process, a majority of Co-Cr-based alloys are fabricated by a thermal source, i.e., laser. However, some other advanced manufacturing processes can also be adapted for the 3D printing of metallic alloys without the use of a heat source. The advantage over melt printing is the elimination of some post-processing steps, higher scalability, high-speed printing, and, of course, the absence of the thermal defects such as cracks [19,20]. Therefore, the current trends in fabricating Co-based metallic alloys can be categorized into the following two different segments: thermal-based and non-thermal-based AM processes.

2.1. Thermal-Based AM Technologies

As the name suggests, thermal-based AM process typically includes the melting and deposition of metallic powders or wires with the aid of a heat source. It is found in the literature that Co-Cr-based alloys are mostly fabricated with PBF and DED technologies, although a few are reported with electron beam melting (EBM), sometimes referred to as the electron powder bed fusion (EPBF) process. The details of the Co-Cr-based alloy fabricating process are described hereafter.

2.1.1. PBF Manufactured Co-Cr-Based Alloys

The PBF process exhibits promising physico-mechanical characteristics for Co-Cr-based alloys, particularly in biomedical applications [21]. Co-Cr-based alloys produced by SLM, an LPBF process, demonstrate superior yield strength and ultimate tensile strength compared to their casted counterparts. Additionally, post-processing methods like heat treatment are commonly employed to enhance mechanical properties such as microhardness, making these alloys more robust than casted alloy products [22]. Consequently, the PBF process has become a prevalent method for manufacturing Co-based alloys across various industries. In the SLM technique, the common laser sources are CO₂ and fiber lasers such as Nd:YAG and Yb:YAG. The melting of the layered powder is adjusted by varying the laser source, wavelength, and power. Post-processing heat treatment can alleviate the internal stress generated during the AM process due to the thermal gradient between the powder layers [23]. A systematic process flow of LPBF technology is provided in Figure 2a, and further details can be found in the previous literature [24].

2.1.2. DED Manufactured Co-Cr-Based Alloys

The DED technique offers some advantages over LPBF when producing Co-based metallic alloys, particularly in terms of cost-effectiveness, material deposition speed, adequate powder melting, and maximum material utilization [25]. It is important to note that both the LPBF and DED processes use a laser or electron beam power source to melt the powder and deposit it layer by layer, but the key difference between them lies in their deposition techniques. The DED technique uses powder that flows through a nozzle to deposit material onto the substrate while tracing out the build layer. In contrast, in PBF the powder is pre-deposited within the build chamber and requires the energy source only to melt followed by solidifying each layer [24]. Figure 2b shows a schematic of the DED process and more details can be found elsewhere [26]. Like the conventional casting process, the DED technique also offers fully dense and desirable microstructural characteristics in printed Co-Cr-based alloys. However, a very short interaction time between laser and powder materials may result in severe microstructural defects. On the other hand, rapid cooling promotes the formation of a martensitic ε-HCP phase in the γ-FCC crystal-structured matrix of the Co-Cr alloy system. This phenomenon can sometimes be advantageous, as ε-HCP improves mechanical characteristics such as hardness and, at the same time, abrasive wear-resistant properties compared with wrought alloy [27].

Figure 2. Schematic process flow of the printing technologies: (a) LPBF [24]; (b) DED [26].

Figure 2. Schematic process flow of the printing technologies: (a) LPBF [24]; (b) DED [26].

2.1.3. LENS-Manufactured Co-Cr-Based Alloys

LENS is a solid freeform powder-based DED technique mainly adapted for net shape manufacturing of 3D-printed parts. The fabrication process is similar to DED, primarily utilizing powder-based materials to manufacture or repair highly precise parts composed of metallic or ceramic compounds or mixtures of both, melted exclusively by a laser source [28]. Co-Cr-based alloys with specific applications, such as biomaterials like hydroxyapatite (HA)-based Co-Cr alloy materials, can be efficiently fabricated using the LENS technique. This is also suitable for preparing Co-Cr alloys used in load-bearing applications, yielding tribo-mechanical characteristics equivalent to those of conventionally fabricated parts [29].

2.1.4. EBM-Manufactured Co-Cr-Based Alloys

The EBM process is quite similar to PBF, using an electron beam in a controlled vacuum chamber to melt the powder layer and deposit it. Typically, a tungsten filament is heated to emit accelerated electrons, which are then focused into a beam by two magnetic coils. This beam melts the solid powder through the transmission of kinetic energy [23]. CAD models are used to fabricate printed parts in a very similar way to the LPBF process. A distinguishing feature of the EBM process, compared to other additive manufacturing techniques, is its use of a rapidly scanned high-power electron beam to preheat the powder bed to a specific temperature. This preheating step minimizes thermal distortion, which is considered an advantageous feature over other thermal-based AM process. Additionally, this process enables the production of components with fewer defects, cracks, and reduced residual stress. Due to these benefits, many researchers have concentrated on using the EBM technique to produce printed components, especially for the manufacturing of Co-Cr-Mo (CCM)-based, medical-grade alloys [30].

2.2. Non-Thermal-Based AM Technologies

Recently, non-thermal-based AM processes such as cold spray additive manufacturing (CSAM) has found to be an emerging 3D printing process in which no heat source is required, and, thus, melting of feeding materials is not considered. This process has recently been adapted for the printing of metallic parts in many automation industries. In the CSAM process, solid-state powder materials are deposited using a cold spray technique to produce near-net shape parts and repair damaged parts, especially in the aerospace, defense, and energy sectors [31]. A high-velocity stream of powder, carried by inert gas from a high-pressure nozzle, strikes the surface of the existing powder layer deposited on a substrate. This impact generates localized heating, causing thermal softening of the powders and adiabatic shear instability at the contact zone. As a result, strong bonding occurs between the particles due to the induced plastic deformation, even in the absence of any thermal source. This process deposits the powder layer by layer to form a solid structure [31]. However, these 3D structures present challenges regarding mechanical strength and geometrical accuracy. Research is ongoing to address these issues by making this technology more reliable and user friendly. Figure 3 illustrates how powder materials are deposited onto a substrate through a nozzle using high-pressure and high-velocity inert gas. Sometimes a preheated gas is used to carry out such a printing process [31].

3. Recent Progress in Additively Manufactured Co-Cr-Based Alloys

A significant volume of research has been published on Co-Cr-based alloys and their subsequent characteristics analysis in the recent past. Among these, the most recent publications are considered here to summarize the latest progress in this area. The bibliographic search was conducted using scientifically accepted databases, such as Scopus, Web of Science (WoS), and Google Scholar. Articles from 2014 to 2024 are considered, and only peer-reviewed journal papers relevant to the subject are included.

Careful measurement is required to achieve the optimal physical and chemical properties when fabricating Co-Cr-based alloys using powder fusion technology such as the LPBF technique. The key requirements include the proper mechanical alloying (MA) of feedstock powder and ensuring an appropriate size distribution for adequate flowability. Additionally, the particle structure can significantly impact the shape, as well as the size, of the printed parts, subsequently affecting physical properties, such as porosity, density, and overall mechanical strength [32,33]. Recently, Khimich et al. conducted a study on an effective and cost-efficient method of producing Co-Cr-Mo feedstock powder using MA and thermal treatments for use in the LPBF process from raw powders of Co, Cr, and Mo [34]. An isothermal annealing of the powder mixture at 400 °C effectively reduced the Cr₂O₃ impurities and lowered the oxygen content, resulting in a decrease in residual oxygen. Specimens produced with this powder offered a homogenized distribution of different phases and a very lower oxygen concentration, typically up to 5%. The fabricated specimens demonstrated a microhardness of 4300 ± 200 MPa, ductility of 0.85, and a yield strength of 1.4 × 10³ MPa. This method resulted in a 4.5-fold cost reduction for feedstock powder compared to commercially available powder. On the other hand, selecting the composition concentration of various alloying elements is crucial to achieving the optimal results for printed parts. While Co and Cr are the main alloying elements, each containing 60–65% and 24–31%, respectively, other elements such as Mo, W, or both are also included in the range of 3–10% to achieve the desired microstructural and mechanical characteristics [35]. A very recent study has explored how different choices of alloying elements can significantly alter the mechanical properties of Co-Cr alloys fabricated by SLM [36]. A set of three different compositions of Co-Cr-Mo, Co-Cr-Mo-W, and Co-Cr-W alloys were produced, with other alloying elements, as detailed in

Table 1. Composition of different Co-Cr alloys [36].

Sl. No.	Specimen	Composition
		Co	Cr	Mo	W	Si	Mn
1.	Co-Cr-Mo	62.06	29.95	5.85	__	0.82	1.32
2.	Co-Cr-Mo-W	60.31	26.25	6.29	6.38	0.89	__
3.	Co-Cr-W	56.83	31.06	__	9.95	1.4	0.76

. The Co-Cr-Mo-W alloy showed the best dispersion of the alloying elements among all. The highest yield strength (1068.0 ± 41.2 MPa) and ultimate tensile strength (1263.4 ± 10.7 MPa) were achieved with the specimen Co-Cr-Mo-W. The highest elongation of 8.9 ± 1.2% was observed with the Co-Cr-Mo specimen. Therefore, these results suggest that the proper design of alloying compounds has a significant impact on the mechanical characteristics of the final printed parts.

Much of the past research on Co-Cr alloys has focused on their applications in medical devices and human body implants. For instance, with metal–ceramic prostheses, Co-Cr alloys are used via the SLM technique to achieve high dimensional accuracy and a density up to 100% [37]. Recently, the fabrication of such metal frameworks has transitioned from conventional casting processes to advanced manufacturing techniques, such as AM, enabled by computer-based technologies. However, there is little difference in the bond strength between metal–ceramic systems and the subsequent surface morphologies, and the oxidation characteristics are quite similar as well. Nevertheless, the porcelain adherence is significantly improved with parts produced by SLM [38]. However, repeated baking of porcelain produced by SLM could reduce the porcelain bond strength with Co-Cr alloys, particularly in dental prosthesis applications [39]. Another study showed the potential applicability of Co-Cr alloys in dental prostheses produced by SLM [40,41]. In dental restoration, a good marginal fit is essential to avoid microleakage and ensure the longevity of the restoration by preventing periodontal disease. AM-fabricated Co-Cr alloys have demonstrated a good agreement of marginal fit for such applications [42]. In a different study, porous CCM alloys were fabricated using the SLM process to produce metallic bone implants, followed by observation for corrosion behavior [43]. The study was carried out to perform stiffness mismatching between implant and bone. The structure exhibited excellent inherent corrosion resistance, making it highly desirable for use as biomaterial in orthopedic implants. A yield stress of 25.46 MPa was achieved with the printed parts. Additionally, a microhardness of 556 HV and an impact strength of 3.17 ± 0.03 J/cm² were obtained.

Some recent studies have focused on the microstructural features of Co-Cr alloys produced by selective laser melting (SLM) and how these features correlate with the mechanical properties. Due to the high cooling rate (10⁵–10⁶ K/s) in the SLM process, a metastable microstructure is formed, leading to internal defects such as dislocations and several stacking faults (SFs) [44,45]. The athermal nucleation of martensite in CCM alloys is associated with defects like dislocations and SFs. The reduction in surface and strain energy, due to elastic field interactions between Shockley partial dislocations, as well as the martensitic embryo, promotes the nucleation of the martensitic embryo [46,47]. A recent study described how the near-martensitic structure influences tensile properties in CCM alloy fabricated by SLM [48]. The yield strength, ultimate tensile strength, and fracture strain of the as-sintered CCM alloys were found to be approximately 735 MPa, 1211 MPa, and 12.8%, respectively. These values further increased, to a certain extent, to approximately 893 MPa, 1214 MPa, and 13.8%, respectively, when the alloys were annealed. The primary factors contributing to the yield strength are the reinforcement provided by the high density of dislocations along the cellular boundaries and the presence of a martensitic structure. Typical microstructures of SLM-printed CCM alloys are provided in Figure 4, and further details can be found elsewhere [48].

The fabrication of Co-Cr-Ni as a medium-entropy alloy (MEA) via the CSAM process has become a new area of research. Decent mechanical characteristics are achievable with such an MEA at room temperature when produced by the novel CSAM process. A recent study demonstrated a compressive strength and strain elongation of ~1400 MPa and ~70%, respectively, at a low strain rate of 0.001 s⁻¹ when the parts were annealed at 1350 °C. The same values remained at ~1266 MPa and ~34%, respectively, when tested at a higher strain rate of 3800 s⁻¹ [19]. The compressive deformation of the annealed Co-Cr-Ni specimens displayed a certain structural change, including multi-structure deformation twins (DTs), numerous nano twins, a deformation-induced HCP phase, and stacking faults (SFs) within the grains. These deformation mechanisms enhanced the plasticity and work-hardening behavior of the parts. As a result, the annealed samples demonstrated an improved combination of strength and ductility during the dynamic compression test. Figure 5 illustrates the structural changes resulting from the compressive deformation of the as-fabricated and annealed specimens at varying strain rates. Additional details can be found in other sources [19].

A recent study has explored how heat treatment as a post-processing technique can enhance the wear-resistant characteristics of Co-Cr-W coatings prepared by DED on a 316L stainless-steel substrate [49]. The study found that using a combination of solution heat treatment and double-aging processes significantly reduced the wear rates of these coatings by up to 50%. The primary contributors to higher wear rates were hard W/Co carbides present on the surface. The solution heat treatment effectively dissolved these carbides within the Co matrix, while the subsequent aging treatment facilitated further precipitation of the carbide phases. This combined heat-treatment techniques resulted in the optimal hardness and wear-resistance for the DED-fabricated Co-Cr-W coatings.

Thus, the latest studies have shown that the current research trends in Co-Cr-based alloys focus on the effects of the initial feedstock powder on various physical and microstructural changes during component fabrication, followed by analyses of the structure–property correlation of the overall matrix. A good volume of work has been carried out on the development of orthopedic implants using Co-Cr-based alloys and their further property enhancements. Additionally, some research has explored the use of AM processes for the fabrication of MEA and wear-resistant coatings and, furthermore, the effects of post-processing techniques such as heat treatment. Further exploration of the impact of process parameters and different material characteristics is provided hereafter.

4. Impact of Fabricating Parameters on the Printed Co-Cr-Based Alloys

In the context of 3D-printing technology, the fabricating parameters play a major role in achieving the desirable characteristics. The key characteristics, such as density, porosity, microstructure, mechanical strength, electrochemical behavior, and tribological performance, largely depend on these parameters, both during the fabrication process and in the post-processing stages. Thus, their impact can be further categorized into the following two sections: i. influence of the process parameters during fabrication and ii. effect of the post-processing treatments. The details are discussed one by one in the next section.

4.1. Influence of Different Process Parameters

The key process parameters include laser power source, laser energy variation, scan strategy, scan speed, nature of material deposition layer, layer thickness, molten pool, and build orientation. Details on how they impact the characteristics of Co-Cr-based alloys are comprehensively discussed further.

4.1.1. Impact of Laser Power

Thermokinetic parameters, such as temperature gradient, solidification rate, and cooling rate, greatly impact the melt morphologies (e.g., depth and width) and grain morphologies (e.g., crystallographic texture, cellular spacing, and size), as well as phase evolution during the fabrication of Co-Cr-based alloys using DED technology [27]. A variation in laser power is required to alter the thermokinetic parameters. A recent study revealed that laser power variation (350, 400, 450, 500, and 550 W) offers different sets of thermokinetic parameters, which resulted in significant changes in the microstructures and phases of CCM alloys. It was also found that changes in the laser power varied the cellular spacing. Figure 6 shows how cellular structures were varied by different laser powers [27]. The local solidification conditions impacted the cellular dendritic microstructures. The cooling rate decreased with the increase in laser power. Moreover, increasing laser power contributed to the formation of martensitic transformative strains in the grain boundary. In addition, an increasing trend in martensitic lath thickness was observed with the increase in laser power. The major phases were found to be a combination of ε-HCP and γ-FCC in the printed CCM alloy matrix.

4.1.2. Effect of Input Energy Densities

Processing parameters, such as laser power, scanning speed, and layer thickness, have been well studied to understand their impacts on printed parts in terms of the microstructural and mechanical characteristics. Nevertheless, it is important to consider their combined effects, rather than focusing on individual factors. To investigate this, energy densities are introduced to vary all parameters simultaneously, allowing for the examination of their interaction effects and the impacts of absorbing different levels of energy. In DED technology, the line energy, or linear energy density (LED), as a process parameter, plays a pivotal role in controlling the alloy deposition’s width, thickness, and mechanical properties, such as hardness, of printed Co-Cr-based alloys. LED is a function of the laser power and scanning speed and can be represented by Equation (1), as follows [50].

$$\mathrm{L}\mathrm{E}\mathrm{D}={\displaystyle \frac{\mathrm{P}}{\mathrm{v}}},$$

(1)

where P = laser power and v = scanning speed.

Jung et al. investigated the effect of line energies (60, 90, 160, 210, and 270 J/mm), by varying the laser power and scanning speed, on the physical and mechanical characteristics of printed CCM alloys by DED technology [25]. A set of single tracks of CCM alloy were deposited on SUS316 steel substrates by varying the line energy. Figure 7 shows a longitudinal and cross-sectional views observed from the optical microscope with respect to different line energies and how they impact the build width, height, and area [25]. The images show that the printed track became wider (W₁) and thicker (H₁) as the line energy increased. At the same time, the thickness (W₂) of the fusion zone at the interface also increased. The melting increased with an increasing line energy, resulting in a larger reaction area (A₂). However, the deposited area (A₁) of the alloy track remained almost constant. Figure 8 shows the hardness variation with an increasing build height while varying the line energies [25]. The hardness decreased from 410 Hv to 250 Hv with an increasing line energy and build height. Also, a decrease in elastic modulus was observed while increasing the line energy. The existence of the ε-Co phase was found to be more prevalent with a higher line energy during the isothermal martensitic phase transformation from γ to ε.

Stellite 21, a Co-Cr-based alloy, offers excellent corrosion- and wear-resistant characteristics when applied as a coating material, mostly on steels, via DED technology [51]. To achieve a good degree of intermixing between the elements of the substrate and coating in the DED process, a high input of energy density is required, and the laser power must be properly controlled. Achieving crack-free deposition of Stellite 21 coating is very challenging in practice [52]. Smoqi et al. investigated the influence of the input energy density on depositing Stellite 21 materials on an Inconel 718 substrate and the effect of localized laser-based preheating [53]. Their study focused particularly on crack formation, microstructural evolution, and the dilution of the depositing materials due to the diffusion of iron and nickel from the Inconel substrate. A range of volumetric energy densities (200–277 J/mm³) and laser-based preheating settings (P_h = 0, 300, 400, and 450 W) were selected to carry out the deposition and to understand their effects. The preheating temperature ranged between 150 and 200 °C. The corresponding depositing laser power (P_d) were 200, 225, 250, and 275 W. The volumetric energy density (VED) can be expressed as the following Equation (2) [54].

$$\mathrm{V}\mathrm{E}\mathrm{D}={\displaystyle \frac{\mathrm{P}}{\mathrm{v}·\mathrm{h}·\mathrm{t}}}$$

(2)

where P = laser power, v = scanning speed, h = hatch space between the adjacent scans, and t = layer thickness.

Cracks in the deposited coating were mitigated using a moderate VED of 200 J/mm³ and preheating the substrate. Although a few cracks were observed on the top surfaces, none were found at the coating-substrate interface. These cracks occurred in the inter-dendritic regions, which contained brittle Cr and Mo phases exposed to residual thermal stress. Preheating the substrate significantly reduced the variation in the thermal gradient, leading to a stable temperature during the deposition process. Figure 9 shows the effects of different preheating settings and depositing laser powers on the coating surfaces, specifically in the aspect of crack propagation.

A recent study investigated the effects of varying laser parameters on the phase, microstructure, and mechanical properties of LPBF-printed Co-Cr alloy while maintaining a constant volumetric energy density of 88 J/mm³ [55]. The researchers adjusted parameters such as laser power and scanning speed to observe their impact on the material’s characteristics. By controlling these variables, the study aimed to optimize the quality and performance of the printed alloy. The details of the process parameters are provided in

Table 2. Details of the volumetric energy density with variable process parameters [55].

Specimen	Laser Power, P (W)	Scanning Speed, v (mm/s)	Layer Thickness, t (mm)	Hatch Spacing, h (mm)	Volumetric Energy Density, VED (J/mm3)
1.	80	375	0.03	0.08	88
2.	160	750
3.	240	1125
4.	320	1504
5.	400	1875

. The retention of the γ-FCC phase was prolific with higher laser power and scanning speed, accompanied with fast cooling rate. A phase transformation from γ-FCC to ε-HCP was observed when cooled from a molten state. Additionally, the presence of both ε-HCP and γ-FCC phases were commonly observed in the alloy system across different process parameters. Figure 10 shows the optical images of different specimens taken from the plane parallel to the building direction [55]. Many micropores were observed with lower laser power (80 W) and scanning speed (375 mm/s), as shown in Figure 10(a1); however, surface defects were found to be fewer with an increase in such parameters (160 W and 750 mm/s), and a refined fish-scale-like distribution of molten pool was observed, as shown in Figure 10(b2). Figure 10(e1) shows many irregular pores with larger sizes when the temperature increased. The significant temperature-induced surface tension gradient, which resulted from high laser power, led to a noticeable mass transfer along the fluid interface, a phenomenon known as Marangoni convection, as shown in Figure 10(e2). This effect contributed to the extended duration of the liquid phase and caused vapor-driven gas bubbles to become trapped, ultimately increasing the porosity of the material. On the other hand, changes were observed in the mechanical performance when the process parameters varied. Figure 10 and Figure 11 refer to the microhardness and stress–strain curve, respectively, related to all of the specimens. The microhardness inside the middle region of the molten pool was almost stable; however, a small variation between 383.86 Hv and 396.28 Hv was observed on the edge side of the molten pool. In addition, the ultimate tensile strength, yield strength, and elongation of the specimens showed significant improvements, increasing from 1079.43 MPa, 670.71 MPa, and 8.3% to 1247.5 MPa, 727.05 MPa, and 22%, respectively, as the laser power and scanning speed were raised from 80 W and 375 mm/s to 160 W and 750 mm/s, as found in Figure 12. With a higher laser power and scanning speed, the formation of large pores due to the Marangoni convection effect led to a reduction in the mechanical strength. The values of the ultimate tensile strength, yield strength, and elongation of the specimens was reduced to 1000.37 MPa, 650.33 MPa, and 9.25%, respectively.

Mahmood et al. developed an analytical model to understand the correlation between grain size and hardness values in relation to the laser volume energy density. The numerical model was validated using LPBF-printed Co-Cr alloys, achieving good agreement between the experimental and simulated results, with deviations ranging from 10 to 15%. The study observed that increasing the laser power led to a reduction in the dendrite grain size. Additionally, a direct relationship was found between the laser volume energy density and hardness, with higher hardness values obtained as the laser volume energy density increased. Further details about the simulation’s conditions and validation methods can be found elsewhere [56].

4.1.3. Effect of Laser Scan Strategy

The crystallographic orientation in individual grains can be controlled by varying the scan strategy while printing CCM by the SLM process. Zhou et al. adopted two different laser scanning techniques, e.g., zigzag and cross-hatching, as shown in Figure 13 [57]. The figure also shows the hatch spacing and point distance considered for the investigation. Columnar grains parallel to each other were observed along the building direction. Moreover, the orientation of these grains corresponded to the (001) and (111) crystallographic planes. This was achieved due to the actions of radial grain growth and a directional moving laser heat spot. However, with the cross-hatching strategy, the scan vector was rotated by 67° on every layer and the crystallographic texture significantly changed.

In addition, optimization of scanning strategy plays an important role in controlling cracks, microstructural growth and texture of the printed parts. By altering the scanning strategy, the geometry of molten pool and the heat flux direction during solidification can be easily changed [58]. A study revealed how alternating hatches with single (scan-I) and double passes (scan-II) could influence the structure–property relationship of SLM-printed CCM alloy [11]. Figure 14 shows the schematic of the two different laser scanning passes adopted for the study. A better densification was achieved with double-scanning passes and no significant surface defects such as cracks or pores were found. This promising density (relative density ~97%) was achieved due to the higher working temperature, which resulted in greater input energy, thereby increasing the size of the molten pool and the amount of melted metal powder, thereby the chance of filling the pores was found more often compared to a single scanning pass. Furthermore, the mechanical strength was significantly improved due to higher densification and grain boundary strengthening [11].

Generally, the surface roughness is found to be higher with SLM-printed specimens compared with traditionally manufactured (casted or milled) specimens [59]. However, the surface roughness can be controlled by considering adequate laser scanning strategy while fabricating Co-Cr alloy via the SLM process. A study unveiled the effect of two different laser scanning strategies on the roughness of Co-Cr alloy fabricated via the SLM process, typically by melting (TM) and adaptive remelting (AR) of the powder bed applied to rescan the outer boundary contour [60]. The schematic is provided in Figure 15. The AR process is involved in the rescanning of 50% of the contour with a laser power of 80 W and a speed of 700 mm/s. The roughness (Ra: 2.3–4.1 μm) was improved up to 45% compared to TM. In addition, an improved microhardness was also achieved with this scanning strategy. More details about this study can be found elsewhere [60].

4.1.4. Scanning Speed Effects

Scanning speed significantly influences the microstructural, mechanical, and wear characteristics of the final printed Co-Cr-based alloys. Zou et al. recently conducted research on the effect of scanning speeds (700, 950, and 1200 mm/s) in the SLM-printed Co-Cr-W alloy [61]. The microstructural defects were found to increase with an increase in the scanning speed, especially at the highest scanning speed (i.e., 1200 mm/s), which showed that an improper diffusion of powder led to several fusion defects. The mechanical strength, such as yield and ultimate strength, showed a decreasing trend with and increase in the scanning speed. In addition, the wear test at room temperature revealed the lowest COF of 0.52 and a wear area of 3672.5 μm² in the specimen fabricated at a scanning speed of 700 mm/s, primarily dominated by an abrasive wear mechanism.

4.1.5. Impact of Interlayer Time and Hatch Spacing

The interlayer time, which is the duration between printing two successive layers, has a considerable impact on the crystallographic structure and microhardness of printed specimens. A shorter interlayer time can effectively increase the fusion temperature at the powder bed. A study demonstrated that the low interlayer time for CCM alloy significantly influenced material deposition on a C35 steel substrate [62]. The layer thickness increased with reduced interlayer time. Additionally, the diluted Fe content was higher at lower interlayer times, resulting in a greater thickness in the deposited material. The microhardness also increased due to the presence of Fe in the deposited layer which, in turn, induced the martensite phase. A cellular microstructure was observed in the deposited layer.

An adequate fabrication strategy is crucial for accurately building printed specimens with desirable pore characteristics. This strategy includes factors such as hatch spacing, layer overlap, and layer thickness [63]. Hatch spacing plays a significant role in controlling pores in printed Co-Cr-based alloys. Reducing the spacing size and increasing the overlap of molten particles can significantly reduce porosity, as shown schematically in Figure 16 [64]. Therefore, adjusting these parameters effectively during the fabrication process is a key approach to minimizing the porosity in printed Co-Cr-based alloys.

4.1.6. Effect of Build Orientation

When considering the process parameters, the build orientation plays a crucial role in controlling the mechanical anisotropy, which affects the performance of the final printed parts [65]. Hitzler et al. conducted a study demonstrating the effects of different build orientations (0°, 30°, 60°, and 90°) on the physico-mechanical characteristics of printed Co-Cr-Mo and Co-Cr-W alloys [66]. The various build orientations examined in the study are shown in Figure 17 for better clarity. Two different types of specimens were considered for measuring the characteristics, i.e., as-built (AB) and heat-treated (HT). The heat-treatments were carried out at a temperature of 800 °C for Co-Cr-Mo and at 1150 °C for Co-Cr-W specimen. The relative densities were found to be ~99.93% and ~99.86% for Co-Cr-Mo and Co-Cr-W alloys, respectively. However, a significant variation in tensile properties was observed with both AB and HT specimens. The tensile properties of the specimens are provided in

Table 3. Tensile properties of printed Co-Cr-Mo and Co-Cr-W alloys at different build orientations [66].

Different Alloys and Their Build Orientations	Young’s Modulus, E (GPa)	Yield Strength, RP0.2 (MPa)	Elongation at Failure, At (%)	Ultimate Tensile Strength, Rm (MPa)
Co-Cr-Mo 0°_AB Co-Cr-Mo 0°_HT	98.38 ± 17.01 156.48 ± 19.50	702 ± 15.4 819 ± 29.1	5.7 ± 1.04 13.3 ± 2.32	923 ± 32.4 1097 ± 21.6
Co-Cr-Mo 30°_AB Co-Cr-Mo 30°_HT	105.32 ± 19.39 149.73 ± 6.29	783 ± 23.7 1002 ± 41.1	6.7 ± 1.94 8.3 ± 0.81	1102 ± 45.2 1262 ± 14.1
Co-Cr-Mo 60°_AB Co-Cr-Mo 60°_HT	112.11 ± 58.51 105.67 ± 13.18	696 ± 34.3 808 ± 37.8	7.5 ± 2.10 9.4 ± 1.32	1012 ± 24.0 1054 ± 20.9
Co-Cr-Mo 90°_AB Co-Cr-Mo 90°_HT	100.21 ± 6.95 164.54 ± 15.10	674 ± 9.0 757 ± 7.2	14.8 ± 1.62 16.7 ± 1.51	1033 ± 12.4 1052 ± 6.3
Co-Cr-W 0°_AB Co-Cr-W 0°_HT	183.44 ± 15.58 216.32 ± 21.99	917 ± 9.9 655 ± 26.6	11.1 ± 1.14 15.0 ± 1.54	1263 ± 8.6 1111 ± 8.9
Co-Cr-W 30°_AB Co-Cr-W 30°_HT	147.50 ± 20.57 186.96 ± 7.90	965 ± 5.9 651 ± 4.9	10.4 ± 1.31 15.5 ± 1.04	1272 ± 10.3 1127 ± 12.3
Co-Cr-W 60°_AB Co-Cr-W 60°_HT	167.17 ± 26.23 214.51 ± 29.25	845 ±11.1 669 ± 20.0	17.1 ± 1.35 18.0 ± 2.90	1247 ± 6.1 1162 ± 13.4
Co-Cr-W 90°_AB Co-Cr-W 90°_HT	138.55 ± 6.93 202.35 ± 22.08	755 ± 8.7 658 ± 7.1	24.3 ± 0.70 16.9 ± 1.51	1188 ± 6.3 1108 ± 10.9

[66]. The tensile properties were improved with heat-treated Co-Cr-Mo alloy specimens, and a similar trend was also observed with heat-treated Co-Cr-W alloy specimens, except at an inclination of 90°. The specimens exhibited a defect-free microstructure after the heat-treatment as well. The microhardness values were found to be almost the same in both the specimens printed in different inclinations; however, a difference was observed after heat-treatment in both the alloys (∼435 HV for Co-Cr-Mo and ∼360 HV for Co-Cr-W).

4.1.7. Impact of Process Parameters with Melt-Pool Dynamics

During the SLM process, parameters such as the laser power and scanning speed control the temperature distribution and grain structure of the melt pool, which in turn influence the mechanical properties of the final printed parts. Thus, finding the correlation between the process parameters and molten pool dynamics is important to determine the characteristics of the printed parts. Yin et al. performed a study on how the process parameters affect the molten pool dynamics during the SLM for Co-Cr alloy [67]. While increasing the laser power from 80 W to 200 W, the width and depth of the molten pool increased from 63 μm to 118.3 μm and 18 μm to 46.8 μm, respectively. On the other hand, when the scanning speed was increased from 400 mm/s to 1450 mm/s, both the width and depth of the molten pool decreased from 137.06 μm to 70.1 μm and 58.75 μm to 24.9 μm, respectively. In addition, a similar trend was also observed with the temperature gradient. The temperature gradient increased with an increase in the laser power and decreased with an increase in the scanning speed. The microhardness at the overlapping molten pool boundaries showed an intriguing correlation with the process parameters. As the laser power increased, the microhardness steadily rose, revealing a direct proportionality. However, when the scanning speed continuously increased, the behavior became more complex—initially, the microhardness followed an upward trajectory, only to later diminish as the scanning speed reached higher thresholds. The defects between the molten pool were found to be greater with a higher scanning speed. The study concluded that at a laser power of 160 W, the molten pool achieved optimal fluidity and sufficient time for overlapping with adjacent molten pools. Therefore, this led to an enhanced bonding between layers, promoting better material properties in the final printed Co-Cr alloys as well.

4.2. Effect of Post-Processing Treatments

Unsatisfactory level of surface integrity and inadequate dimensional/form accuracy are the inherent limitations experienced by AM components. As a consequence, the direct adoption of additively printed Co-Cr alloys in the human body or in other industrial applications can adversely affect part functionality and, hence, are not recommended. Post-processing stages are inevitable after 3D printing to ensure the desired functionality of the component at the application level. The research studies corresponding to various post-processing operations carried out on AM Co-Cr alloys are elaborated in the following sections. The post-processing variants can be categorized into surface treatment and heat treatment methods.

4.2.1. Surface Treatment

Laser Polishing

Laser polishing is considered to be the efficient surface treatment method for refining and smoothening 3D-printed metallic surfaces. The process involves levelling of roughness peaks and valleys by localized melting of the surface using a laser beam [68]. The possibility of finishing the AM components in the same setup using the printing laser source itself enables high dimensional and geometrical accuracy after laser polishing. Investigations into the laser polishing of additively printed Co-Cr alloys are clearly evident in the past literature. Initial studies by Gora et al. [69] showed that laser polishing can enhance the surface quality of AM Co-Cr parts by ~85% to 95% depending upon the initial roughness of the part. Later Richter et al. [70] conducted an in-depth investigation of the laser polishing of powder bed fusion Co-Cr alloy to explore the effect of process parameters. The study reported a drastic roughness reduction (Sa) to ~1 µm from an as-printed roughness of ~11.4 µm. Figure 18a demonstrates the laser polished Co-Cr surface free from powder particles and other surface irregularities relative to the as-printed surface (Figure 18b). Further, the laser power was found to be the predominant factor influencing the geometry of the met pool and polishing efficacy as compared to other parameters, like scanning velocity, hatch spacing, and beam spot diameter. The study also confirmed that the final surface finish achieved post laser polishing is dependent on the initial surface roughness of the AM specimen.

To investigate the adaptability of laser polishing to complex/intricate geometries, Yung et al. applied the technique on Co-Cr surfaces fabricated by SLM [71]. The proposed method involved a layered polishing approach, where the workpiece geometry was divided into small-sized discrete layers. For each layer, a specific relationship was established between the laser focus position and the surface area to be finished, ensuring a consistent defocusing distance throughout the process for complex surfaces. Despite a 93% improvement in surface quality, the surface hardness of laser polished surface was elevated by ~8%, thereby highlighting the favorable surface treatment potential of the operation. Considering the significance of AM Co-Cr alloys in implant applications, Wang et al. investigated the impact of laser polishing on corrosion performance, a critical factor in terms of biocompatibility [72]. The outcomes of a potentiodynamic polarization test revealed an enhancement of the corrosion resistance of the SLM Co-Cr alloys after laser polishing. Moreover, the study stated that a high magnitude of the laser power at lower object distances can favor the evolution of complex oxide films, thereby contributing excellent corrosion resistance. In addition, the extent of the corrosion behavior was found to be strongly dependent on the surface morphology achieved after laser polishing. As shown in Figure 19, the difference in the defocusing distance yields two distinct surface morphologies on the LPBF Co-Cr alloy. The hatching boundaries (Figure 19a) and smooth re-melting stripes (Figure 19b) exhibited distinct oxide levels. Consequently, the chances of pitting corrosion were highly evident in the former case relative to the latter.

Further, an in-depth investigation along the same direction revealed the impact of the defocusing distance on the final surface finish of Co-Cr alloys [73]. As visible in Figure 20a, smaller defocusing distances (df) yield finer surface finishes, whereas the surface deteriorates as the df approaches the focus distance. Similarly, optimal values exist for the scanning velocity and hatch distance beyond which the surface roughness increases (Figure 20b,c). The minimal surface finish obtained by the Co-Cr alloy at optimal settings is Sa = ~0.45 µm. As the surface finish reflects the wettability characteristics of the sample, an analogous trend in the contact angle’s variation can also be observed relative to the surface finish variation at different process parameter variations (Figure 20).

Electropolishing

Electropolishing can be adopted for AM components whenever a high-quality surface finish is of prime importance in an application. Demir et al. applied electrochemical polishing to SLM Co-Cr stents to assess the finishing feasibility of the process [74]. The polishing was executed for 3 min at a current density of 2.1 A/cm² with the assistance of a stainless-steel cathode and an electrolyte comprising H₂SO₄ (45 vol.%), H₃PO₄ (50 vol.%), and H₂O (5 vol.%). Electropolishing effectively eliminated the loosely adhered powder particles and unwanted melt material from the strut surface thereby contributing to an ~85% reduction in the surface roughness. However, a slight reduction in the strut width was experienced after the electropolishing process. In a similar study, electrochemical polishing (ECP) was carried out on as-printed LPBF Co-Cr stents, and a comparison was made relative to commercial stents (Make: MULTI-LINK VISION) [75]. The as-printed stents featured an uneven surface with the presence of powder particles that had a size of ~10 µm. However, the ECP process eliminated such irregularities by leaving behind a few pits on the strut surface, which can be attributed to an overreaction of the non-uniform chemical composition of the material with the electrolytic acids. The ECP process reduced the as-built roughness (Ra) from ~4.53 µm to ~1.55 µm, which was nearly as close to the roughness associated with a commercial stent (~1.09 µm). The surface morphologies corresponding to the as-printed, electro-polished, and commercial stents are shown in Figure 21.

Finazzi et al. also investigated electropolishing of cardiovascular Co-Cr stents printed using LPBF [76]. As shown in Figure 22a,b, the process could successfully remove the sintered particles by maintaining the cell shape, however, at the cost of the strut size thickness. Stents built at higher peak power exhibited a larger strut size on account of the larger melt pool resulting from the high energy input. Nevertheless, the variation in strut sizes tended to fade after the polishing process. The surface roughness associated with the printed stent (Ra = ~8.4 µm) was reduced to ~2.2 µm, thereby contributing to an ~80% improvement in the surface finish. However, the study concluded that the combination of other finishing operations and the adoption of different etching solutions are important to meet the surface finish requirements of stents in biomedical industries (Ra = ~0.5 µm).

Mechanical Surface Treatments

Mechanical surface treatments involve the utilization of mechanical forces to smoothen or modify the metallic AM surface. Takeyama et al. investigated the potential of mechanical treatments, such as manual polishing, barrel finishing, and shot peening, on AM clasp specimens (Figure 23a) built using Co-Cr powder [77]. With the assistance of a silicon point and Robinson brush, manual polishing was performed on the specimen followed by a buffing operation for 10 min. Although the process yielded a scratch-free mirror finished surface (Figure 23b) of ~11 nm (Sa), the method was very much labor intensive. Similarly, barrel finishing using Al₂O₃ abrasives contributed a surface (Figure 23c) characterized by irregular depressions involving cracks with a finish of ~38.4 nm. On the contrary, a shot peening operation with the aid of ~50 µm zirconia powder jet (0.3 MPa) ensured a final finish of ~41.9 nm by leaving shallow wave-like depressions (Figure 23d) on the Co-Cr surface. However, the study demonstrated that the electropolishing method can excel in performance relative to the aforementioned mechanical treatments.

Abrasive fluidized bed (AFB) finishing is another variant tested on 3D-printed Co-Cr parts [78]. The method can efficiently improve the surface finish by removing partially melted powder agglomerates. Longer treatment times and higher rotational speeds result in smoother surface morphologies when angular steel grit is used as the abrasive media in the AFB finishing process. Despite the surface quality enhancement, the process also retards the crack initiation tendency, indicating an elevated fatigue performance of the samples after post-processing.

4.2.2. Heat Treatment

Heat treatment (HT) operations are inevitable for eliminating any microstructural inhomogeneity and thermal residual stress existing in a metal AM part. To date, various HT procedures have been tested by researchers on Co-Cr alloys to formulate a processing window considering industrial applications.

Takaichi et al. studied the role of the HT operation on the microstructural aspects and mechanical characteristics of LPBF Co-Cr alloy [79]. From the microstructural analysis, the melt pool boundaries were clearly visible in the as-built Co-Cr specimens built at various orientations (0°, 45°, and 90°), as shown in Figure 24a–c. However, such traces of a melt pool vanish with the HT operation, as shown in Figure 24d–j. The grain boundaries started to evolve in the samples upon HT under lower temperature conditions (750 °C), showing a transition from equiaxed to columnar grains in the 0° and 90° specimens, respectively. Nonetheless, HT at higher temperatures (1150 °C) transformed the grain structure into an equiaxed mode for all specimens, regardless of the build orientation. As-printed Co-Cr was identified as a single phase that was rich in Co and Cr, without precipitates. Similarly, the fine cellular dendritic structure observed in the as-built Co-Cr specimens tended to fade upon heat treatment and almost disappear at higher HT temperatures. Moreover, the precipitation induced upon HT at lower temperatures (750 °C) started diminishing with a further increase in the HT temperatures. It is worth noting that recrystallization through HT improved the microstructural homogeneity and mitigated the anisotropic mechanical properties. However, weak anisotropy persisted post-recrystallization due to variations in the surface processing, which are influenced by the build orientation.

In another study, the impact of solution HT was assessed for the mechanical properties of CCM alloys produced by the LPBF process [80]. The ultimate tensile strength (UTS) and yield strength (YS) of as-printed CCM specimens were found to be nearly twice relative to the casted counterparts. However, the strength characteristics were reduced after the solution treatment with a rise in the ductility owing to the increase in grain size. Further, the microstructural analysis revealed the formation of carbides at the grain boundaries after the solution treatment operation.

Ko et al. also reported a similar trend in mechanical strength after the HT procedure on LPBF CCM alloy [35]. However, the UTS and hardness were higher in contrast to the as-printed specimens when HT was conducted at lower temperatures (750 °C). The UTS and hardness continued to decrease as the HT temperature increased, with the lowest value at 1150 °C. On the contrary, the ductility tended to rise with an increase in the HT temperatures. The fracture surfaces provided in Figure 25 also confirm the trend observed in the mechanical properties pre- and post-HT. Both the casted and as-printed specimens exhibited cleavage patterns, dimples, and cracks that were blended together. HT at 750 °C resulted in the evolution of large cleavage steps, cleavage facets, and cracks after the tensile test. In contrast, specimens heat treated at 950 °C and 1150 °C were characterized by the presence of dimples, implying the transition from brittle to ductile fractures. Furthermore, the study demonstrated that heat treatment at 750 °C significantly reduced Type I residual stresses. However, residual stresses of Type II and Type III were still present even after HT at 1150 °C.

The effect of an aging treatment on additively printed CCM alloys was investigated to ascertain the phase changes and precipitation behavior in detail [81]. Aging for a shorter duration reduced the microhardness of the samples in comparison with as-printed specimens by virtue of the disappearance of honeycomb precipitates. Contrarily, aging for a longer time promoted martensitic transformation and induced evolution of the M23C6 in the form of granular precipitates, thereby increasing the microhardness of the CCM specimens. The microhardness reached the highest level when the aging was conducted for 10 h at 900 °C. The aging operation for longer durations also transformed the mixture of γ and ε phases present in the as-printed specimens to a nearly pure ε phase, as illustrated in Figure 26.

To a greater extent, Razavi et al. analyzed the effect of hot isostatic pressing (HIP) treatment on the part properties of AM Co-Cr alloys. The HIP operation conducted at 1020 bar and 1150 °C significantly reduced the internal porosity of as-printed samples (~1.25%) to ~0.54%. Moreover, the ductility of the specimens increased by ~329% (on account of γ-phase prevalence), with notable enhancement in fatigue behavior. An enhancement in fatigue strength by ~135% was achieved for AM Co-Cr specimens after the HIP process. In short, the HT operation can be used for tailoring the microstructure and mechanical properties of AM Co-Cr alloys in order to ensure the desired functional performance of the parts.

4.3. Critical Facts and Scope for Further Development

Both pre-processing and post-processing parameters play crucial roles in determining the overall performance of printed Co-Cr-based alloys. The influence of process parameters, such as laser power and energy density, are significant, as a higher laser power leads to grain refinement, increased martensitic lath thickness, and modifications in cellular spacing. Additionally, line energy and volumetric energy density impact hardness, porosity, and phase evolution. The scanning strategy also plays an essential role in 3D printing, where cross-hatching strategies help reduce cracks and improve densification to approximately 97%. Rescanning, also known as adaptive remelting, enhances surface roughness and microhardness in Co-Cr alloys. Furthermore, the build orientation significantly affects the properties of printed components, with orientations at 0°, 30°, 60°, and 90° influencing the anisotropy, tensile strength, and microstructure. Higher scanning speeds tend to increase fusion defects, which negatively impact mechanical strength and wear resistance. Moreover, the laser power and scanning speed affect the fluidity of the molten material, thereby influencing bonding, porosity, and stress–strain behavior in the final product.

Post-processing parameters, particularly surface treatments, play a critical role in determining the final characteristics of printed parts. Laser polishing can reduce roughness by 85–95%, improving both corrosion resistance and biocompatibility. Similarly, electropolishing decreases surface roughness by 80–85%, making Co-Cr alloys more suitable for biomedical applications, such as stents and implants. Mechanical treatments such as shot peening and barrel finishing enhance fatigue performance and surface properties, further improving the durability of printed components. Thermal treatments also provide substantial benefits; heat treatment in the range of 750–1150 °C eliminates anisotropy, improves grain structure, and enhances ductility while reducing residual stress. Aging treatment, particularly when conducted for 10 h at 900 °C, promotes martensitic transformation and increases microhardness. Additionally, HIP significantly improves material properties by reducing porosity by 57% and increasing fatigue resistance by 135%.

Despite the advancements in optimizing process parameters, further improvements can be made to enhance material characteristics. Multi-factorial optimization, rather than analyzing individual parameters like laser power and scanning speed separately, is necessary to achieve a more comprehensive understanding of their combined effects. Cracks and residual stresses remain a challenge, and further control can be achieved by optimizing the heating and cooling cycles. While preheating techniques have proven to reduce crack formation, their long-term effects on fatigue behavior require further investigation. Additionally, studying the impact of interlayer cooling rates and controlled thermal gradients on microstructure stability could lead to significant improvements in printed Co-Cr alloys. Beyond process optimization, advancements in post-processing techniques also hold great potential for improving material properties. Hybrid post-processing methods, such as combining electropolishing with laser polishing, could result in superior surface finishes and enhanced wear resistance. Further research is required to explore the role of chemical etching and plasma-assisted polishing in biomedical applications, given that a significant proportion of Co-Cr-based materials are used in medical implants. By refining both process and post-processing parameters, the overall performance of additively manufactured Co-Cr alloys can be further improved, making them more suitable for high-performance applications in biomedical, aerospace, and industrial sectors.

5. Key Characteristics of Additively Manufactured Co-Cr-Based Alloys

Additively manufactured Co-Cr-based alloys possess a wide range of desirable characteristics, making them highly suitable for industrial applications. These alloys offer excellent microstructural integrity and physical properties, such as required density combined with high strength. Additionally, they are widely used in wear and corrosion-resistant applications. Due to their superior biocompatibility, Co-Cr-based alloys are also employed in manufacturing biomedical instruments and orthopedic devices. In this section, a step-by-step discussion will be provided on the key characteristics of these printed Co-Cr-based alloys across various printing technologies.

5.1. Density and Porosity

The sintering temperature significantly influences the physical characteristics of 3D-printed CCM alloys. A sintering temperature of 1380 °C for 2 h yielding can produce a near fully dense part, with a relative density of 99.10%, when printing by binder jet technology [82]. However, a large variation in linear shrinkage is noticed, mainly from 1240 °C to 1380 °C, which is 2% and 19%, respectively. Thus, it is recommended to scale up the dimension of complex parts while printing by binder jet technology. According to a recent study conducted by Khademitab et al., the relative densities of the CCM alloys were approximately 53% and 64% when sintered at 1240 °C and 1320 °C, respectively [82]. Nevertheless, the relative density drastically increased when increasing the sintering temperature up to 1380 °C. At a low sintering temperature, the linear densification behavior might lead to the creation of interconnected pores. Moreover, at a higher sintering temperature, the remelting of the surface and the filling in of the pores can reduce the porosity, thereby increasing the relative density of the final printed parts.

The density of CCM alloys varies when incorporating other alloying elements such as Ti. In some implant materials, the porosity is intentionally created to improve osseointegration and to reduce the overall density without compromising the load-carrying capacity [83]. However, the SLM process used to fabricate such alloys may lead to many unwanted characteristics, such as higher thermal distortion, low material deposition, and high energy consumption. Therefore, to overcome these limitations, Jhavar et al. adopted a relatively new technique called the micro-plasma-based additive manufacturing (MPBAM) process and used it for depositing a material on the same substrate. More details about this technique and its advantages can be found elsewhere [84]. Kumar et al. adopted this MPBAM technique for printing Co-Cr-Mo-Ti alloys with variations in Ti (2–6 wt.%) elements on the base material of CCM alloys, especially for fabricating knee implant materials [85]. They reported that the optimum porosity was achieved at 4 wt.% of Ti, while maintaining other desirable properties, such as the absence of cracks and the microhardness, with smaller pore sizes, as shown in Figure 27. The bulk density was decreased with the increase in the Ti element, since Ti has a lower density (4.5 g/cm³) compared with Co-Cr-Mo alloys (8.29 g/cm³). The average porosity of the alloy with 4 wt.% Ti showed a porosity of 3.16%. The formation of pores occurs since Ti has a lower thermal conductivity compared with CCM alloy, leading to shrinkage porosity during the solidification of the molten pool. As a result, the porosity increases with the increase in the Ti concentration within the CCM alloy [86].

5.2. Microstructural Characteristics

In CCM alloys, the Co has two phases, i.e., γ-Co phase (FCC) and ε-Co phase (HCP), as already discussed previously. However, it is observed from the phase diagram that the ε-Co phase was thermodynamically stable at below ~900 °C [87]. Moreover, a single thermodynamically unstable γ-Co phase was observed during the SLM of the CCM due to rapid cooling. This phase transformation of γ-Co to ε-Co phase takes place under high temperature, and the precipitation of the second phase refers an isothermal phase transformation [88]. This isothermal phase transformation significantly influences the microstructural characteristics of the printed CCM alloys. Therefore, it is of great importance to control this precipitation. A study conducted using grain boundary engineering (GBE) showed that it can manipulate the Laves phase’s (a group of intermetallic compounds having atomic ratios between 1.05 and 1.67) precipitation and isothermal phase transformation in CCM alloy [89]. The density of precipitates in the isothermally aged CCM alloy was found to be as low as <0.5%, as shown in Figure 28a–d at different magnifications. The precipitates formed at random boundaries and later grew into the grain interior. Furthermore, the TEM analysis revealed that the Laves phases, with an island-like shape, were primarily found at the grain boundaries, as illustrated in Figure 28e–g. Additionally, the precipitates near the twin boundaries were smaller compared to those at random boundaries. This suggests that a high number of twin boundaries in the GBE CCM alloys did not favor the formation of Laves phases. Furthermore, the density of lamellar ε-Co phases in the CCM alloy after the isothermal aging treatment was also low. There was a significant effect of the aging heat treatment on the microstructures of the CCM alloys produced by SLM. The hexagonal ε-Co phase, which developed from the γ-Co matrix during the SLM process, became more pronounced after aging at moderate temperatures (650–950 °C). This resulted in improved nanohardness and enhanced scratch resistance. However, heating at higher temperatures (1150 °C) led to a uniform recrystallized microstructure, eliminating residual stress and the ε phase, which in turn caused a sharp decline in hardness and scratch resistance [90]. On the other hand, while comparing the microstructures of SLM-printed Co-Cr alloy with a casted one, the characteristics of the SLM were found to be better. The microstructures of the SLM Co-Cr-based alloys, characterized by fine grains and uniformly distributed second-phase particles, were superior to those of the casted specimens. In the SLM specimens, the γ-Co phase with an FCC structure was predominant (72 vol%), whereas the casted specimens contained a higher proportion of the ε-Co phase with an HCP structure (67 vol%) [91].

The process parameters have a significant impact on the flow of molten materials, heat transfer, and the solidification behavior of CCM alloys. These factors also influence the morphology, grain size, and texture of the microstructures. To accurately predict these characteristics in printed parts, a finite-volume-based computational fluid dynamics (CFD) approach would be a suitable method. Further details about the model simulation can be found elsewhere [92]. According to the study, the CCM alloys exhibited carbide precipitation and an α to ε phase transformation during equilibrium solidification, which influenced the resulting microstructure. In SLM-fabricated CCM alloys, α-FCC was commonly observed. This is likely due to the rapid solidification process in powder bed AM, where the grain size and morphology established during solidification become key features of the final microstructure. The microstructures were composed of a combination of equiaxed, mixed, and columnar grains, presented in varying proportions at different depths within the melt pool.

5.3. Surface Finish and Wettability

The surface finish of Co-Cr-based components depends to a large degree on the manufacturing process, whether it is subtractive or additive manufactured, and this is further modified by considering some post-processing techniques [93]. When printed Co-Cr-based alloys are employed in critical areas, such as medical applications, the surface roughness and wettability play pivotal roles in accurate fittings, as well as their desired functionality. Rough surfaces of printed specimens do not satisfy these specific requirements and, thus, often require further processing such as laser polishing [94]. Generally, laser polishing technology is adopted to achieve a good surface finish and to manipulate the wettability of the printed parts. With a laser defocusing distance of +6 mm, a surface quality of ≤1 μm can be achieved in Co-Cr alloys [73]. To prevent oxidation, argon can be considered as a shielding gas with a flow rate of 6 l/min. The contact angle can be increased with an increase in the surface roughness and can be controlled by varying the laser defocusing distance while preparing a hydrophobic surface. In general, the surface quality and wettability depend on the laser polishing parameters.

Printed porous scaffolds, made of Co-Cr alloys, are extremely useful in biomedical applications with complex geometries. This is due to enhancement of the osseointegration and reduction in stress shielding effects [95]. However, the adherence of loose residual particles on the surface may create issues while these parts are implanted in a human body. Hooreweder et al. investigated the effect of chemical etching on the surface roughness of Co-Cr scaffolds produced using the SLM process. They employed a mixture of hydrochloric acid (HCl) and hydrogen peroxide (H₂O₂) as etching agents, which effectively altered the surface characteristics of the scaffolds [96]. The surface roughness was significantly influenced by the concentration of the etchants. An increase in HCl (~27 vol.%) and H₂O₂ (~8 vol.%) concentrations resulted in a reduction in roughness. This effect was attributed to the removal of loose residual particles from the surface during the chemical etching process, making the treated surface more suitable for implantation.

5.4. Mechanical Characteristics

Co-based printed alloys are widely utilized in critical applications where mechanical properties like strength, hardness, and toughness play a crucial role. In some instances, improper or partial melting of the metal powder can lead to heterogeneous microstructures, poor surface characteristics, porosities, and residual thermal stress [97]. This can result in a suboptimal mechanical performance. Therefore, achieving the optimal characteristics requires careful selection of the process parameters and laser density. The hardness of CCM alloys is strongly influenced by the formation of metal carbides during the AM process [98]. A homogeneous microstructure with cellular dendrites can provide a desirable hardness of 460 HV, which exceeds values found with traditional manufacturing processes [99]. Barucca et al. also demonstrated how the hardness was affected by the distribution of ε-Co (HCP) lamellae within a γ-Co (FCC) phase [100]. The high density of the ε-lamellae resulted from a large nucleation of ε-embryos due to lattice defects formed during rapid cooling. Carbides within the alloy grains likely formed during solidification. The hardness was notably higher than parts produced by other methods because the ε-lamellae grew on the {111}_γ planes, which restricted the dislocation movement in the γ (FCC) phase. Additionally, slip within the ε-lamellae was prevented by the intersection with other ε-lamellae or FCC regions, further contributing to the increased hardness.

When CCM alloys are used in knee joint implants, they are subjected to repeated loading and require high fatigue crack resistance. Saraiva et al. conducted a study on the effect of cyclic loading on CCM alloys and found that the results aligned well with Co-28Cr-6Mo feedstock powder when fabricated using the LPBF process [101]. Furthermore, they thoroughly investigated how the FCC to HCP phase transformation impacted crack propagation under cyclic loading. The initial linear increase in plastic strain was linked to the volume change during the FCC to HCP phase transformation, along with the rising volume fraction of the HCP phase. Slip incompatibility at the FCC/HCP interfaces led to localized plastic strain, encouraging crack initiation. The crack then propagated along these interfaces, forming a zig-zag path, which slowed the crack growth rate and, thus, provided good fatigue crack resistance. The yield stress and ultimate tensile strength were found to be 1035 MPa and 1450 MPa, respectively, as shown by the stress–strain curve shown in Figure 29 (red line). The strain hardening rate (SHR), represented by the green line, experienced a sharp decline from the yield point up to a strain of about 0.036, after which it gradually decreased at a slower rate. This change indicates the initiation of the phase transformation or the activation of other softening or hardening mechanisms.

Co-Cr-W alloy is suitable for many industrial applications due to its promising strength and toughness at elevated temperatures. However, a high thermal gradient can lead to cyclic thermal shock and, as a consequence, thermal fatigue (TF) may be observed [102,103]. To prevent such TF failures and oxidation, a number of researchers have changed the feedstock powder concentration and, at the same time, added pure Al and Ni powders to commercial Co-Cr-W powders [104,105]. Yang et al. found that incorporating Al (1 wt.%) and varying the Ni content (0–25 wt.%) in a Co-Cr-W alloy system, produced through DED, significantly influenced the carbide formation and oxidation processes. This, in turn, enhanced both the tensile properties and TF resistance of the alloy [105]. The 1Al15Ni alloy composition demonstrated the highest elongation of approximately 11%, with an impressive ultimate tensile strength of 1365 MPa. This alloy exhibited superior resistance to the initiation and propagation of TF cracks. The addition of Al and Ni promoted the formation of a stable, continuous oxide layer, preventing rapid oxidation of carbides, reducing the TF crack propagation. Furthermore, these alloying elements played a role in stabilizing the γ-Co matrix and impeding the γ/ε strain-induced martensitic transformation.

Materials designed to offer control over their bulk properties while introducing new or functional characteristics are referred to as metamaterials. When these metamaterial architectures are developed for biomedical applications, they are termed meta-biomaterials [106,107]. Recently, Wanniarachchi et al. introduced a new category of meta-biomaterials called auxetic meta-biomaterials, which exhibit a negative Poisson’s ratio (−υ) [108]. These materials demonstrate exceptional strain behavior, making them ideal for load-bearing applications, such as bone scaffolds. Their study involved printing CCM alloys with varying porosities suited for such scaffolds. They considered five distinct auxetic unit cell (UC) designs and their corresponding scaffold designs, as illustrated in Figure 30. The Poisson’s ratio for these architectures ranged from −0.1 to −0.24, with porosities between 73% and 82%. Tensile tests on the bulk material processed via LPBF revealed a Young’s modulus of 194.23 GPa, a yield strength of 975.6 MPa, and an ultimate tensile strength of 1169.81 MPa. Meanwhile, compressive tests on the auxetic meta-biomaterials showed an elastic modulus and compressive strength ranging from 1.13 to 1.66 GPa and 32 to 56 MPa, respectively.

Three dimensionally printed lattice structures made of Co-Cr alloys have promising strength and deformation resistance when specially applied for orthopedic implants [109,110,111]. Liverani et al. conducted a study of the stiffness behavior of LPBF-printed porous lattice structures by compressive test and found a stiffness of nearly 108 kN/mm [111]. In the lattice unit structure, plastic deformation was initiated with the development of a “high stress plane”, where localized deformation occurred perpendicular to the applied stress. The maximum deformation was concentrated within an elliptical region connecting adjacent units, likely due to local variations in the cross-sectional area. This structural integrity of Co-Cr alloys suggests the suitability for use in mechanical load-bearing implant applications.

5.5. Tribological Behavior

In addition to the physical and mechanical properties of Co-Cr-based alloys, their tribological behavior is another key factor in determining their suitability for applications where wear resistance is critical, such as dental prosthetics and medical implants. Wear behavior in these applications is of particular concern because unwanted wear can release particles or ions, leading to potential toxicity within the human body [112]. Notably, Co-Cr alloys are renowned for their resilience in a range of operational environments, including dry, wet, and corrosive conditions, making them highly adaptable for diverse applications [113]. Recent advancements in additive manufacturing have shown promise in further optimizing the friction and wear properties of these alloys, attracting significant research attention. This section delves into the current understanding of the tribological characteristics of Co-Cr alloys, with a focus on how additive manufacturing techniques may enhance their performance and broaden their applications in demanding environments where the conventional manufacturing process is not considered.

5.5.1. Tribological Characteristics of Co-Cr-Mo Alloys

Printed CCM alloys may not achieve a good surface quality; therefore, this can lead to undesired surface wear at the contacting interfaces during sliding. Thus, further post-processing of the surface is required. Ultrasonic nanocrystal surface modification (UNSM) is an established post-process technique that is generally adopted to improve the surface roughness, microstructural defects, surface residual stress, and hardness characteristics of AM parts so that the metallic surface can offer good tribological characteristics [114,115]. Amanov et al. conducted research on the post-processing of printed CCM alloys by USNM at 500 °C to improve the tribological behavior, such as the coefficient of friction (COF) and specific wear rate (k), under certain conditions; details about the processing technique can be found elsewhere [116]. The roughness of the as-printed specimen by SLM was ~8.484 μm, and this value was significantly diminished to ~1.044 μm after the UNSM treatment. In addition, the hardness also improved by ~44.3% after this treatment. The COF and k values decreased to 0.0498 and 1.24 × 10⁻¹² mm³/N·m, respectively, from 0.0959 and 5.47 × 10⁻¹² mm³/N·m, as found in the as-printed specimens. A combination of adhesive and abrasive wear modes were observed on the worn surfaces of the UNSM-treated CCM alloy specimens.

In articular implants, a combination of CCM and ultra-high molecular weight polyethylene (UHMWPE) are used, particularly for knee and hip joints [117]. A study showed the effect of cavities on the biotribological behavior of the CCM-UHMWPE combination in bovine serum, a body fluid analogue [118]. Higher porosity (~19.7%) in the SLM-printed CCM as a counter surface can significantly impact the wear behavior of UHMWPE. Due to the increasing porosity and the sharp edges, the metallic CCM surface becomes very abrasive compared with softer UHMWPE, leading to debris (polyethylene) formation. Therefore, it is recommended to reduce surface cavities by reducing the porosity of the CCM counterpart, which provides the UHMWPE with an adhesive wear that is acceptable form for implant applications. However, third-body particles can fill cavities and act debris collectors during wet sliding tests. In a different study, calcium phosphate in the form of hydroxyapatite (HA) was added to raw CCM alloy powder followed by printing with the help of the LENS process and testing for biotribological behavior in body fluids (Dulbecco’s Modified Eagle Medium + hyaluronic acid) [119]. The addition of 3 wt.% HA showed potential tribofilm (dark region) formation when wear testes were carried in a reciprocating tribo setup against Si₃N₄ balls in body fluid, and the as- and worn surface microstructures are illustrated in Figure 31. The lowest COF and k values were approximately 0.15 and 1.5 × 10⁻⁶ mm³/N·m, respectively, indicating a promising match for manufacturing tribo-pairs for implant applications.

When tribological pairs operate under sliding conditions, they typically experience four primary wear mechanisms, as follows: adhesive, abrasive, surface fatigue, and tribochemical wear [120,121]. Adhesive wear involves material transfer from a softer to a harder surface due to localized bonding and separation, while abrasive wear occurs as a harder material plows into a softer one, causing surface grooving. Surface fatigue wear results from repeated loading and unloading, leading to delamination, mostly common in rotating tribo-pairs under dry sliding [122,123,124]. Tribochemical wear, on the other hand, is driven by oxidation and corrosion reactions in wet environments [125]. This combination of wear modes is especially relevant in load-bearing implants, where complex wear behaviors emerge under extreme wet conditions like those found in body fluids. A recent study examined the tribocorrosion behavior of LPBF-printed CCM alloys in simulated body fluids, highlighting the critical importance of tribological studies in optimizing 3D-printed Co-Cr-based alloys for biomedical applications [126]. CCM alloys were tested for tribological behavior in different wet solutions of NaCl, ethanesulfonic acid, bovine serum albumin (BSA), and fibrinogen (Fbn) by maintaining a pH level of 7.3, which is generally common for human body fluids. When combined, BSA and Fbn’s notably low COF (~0.20) was achieved due to their lubrication effect. Furthermore, they also minimized the wear track width and tribocorrosion of the CCM alloys. The wear track area showed significantly more oxidation than the areas outside it. In a reference solution without proteins, strong oxidation of Mo within the wear track indicated a potential drop in pH levels. Additionally, no differences were observed in the tribocorrosion behavior across various build orientations of the CCM alloys. The combined use of BSA and Fbn offered the most effective lubrication and minimized tribocorrosive wear compared to BSA alone.

5.5.2. Tribological Characteristics of Co-Cr-W Alloys

Stellite or Co-Cr-W (CCW) alloys are highly promising materials widely used in wear-resistant applications in aerospace and automotive sectors [127]. Research has shown that SLM-printed CCW alloy exhibits different wear characteristics when subjected to varying scanning rates (700–1200 mm/s) [61]. The lowest wear rate, with a COF of 0.60, was achieved at a scanning rate of 700 mm/s for specimens printed in the side direction (along the x-axis). At room temperature, the predominant wear mechanism was abrasive, with the cellular microstructure effectively enhancing the wear resistance. Yang et al. examined the tribological performance of stellite alloy enhanced with Al and Ni additives, using the direct laser energy deposition method [26]. The study found that adding 2 wt.% Al to the commercial CCW powder, mixed by ball mill, resulted in the lowest COF (~0.32) and minimal wear loss (~0.01 mg). In contrast, the addition of 2 wt.% Al with 25 wt.% Ni did not yield favorable results. The improved oxidation facilitated by the Al addition contributed to the formation of a lubricious film, which effectively reduced both the COF and wear of the alloy during dry sliding tests against a GCr15 counterpart. In a similar study, Yang et al. investigated the tribological performance of stellite alloy enhanced with Al and Ni additives and observed that the formation of a mechanically mixed layer consisting nano-oxides on the surface significantly impacted the wear resistance [128]. A higher Ni content (>15 wt.%) decreased wear resistance and increased the volume loss by up to 40% compared to CCW alloys, as it resulted in a brittle mixed layer during dry sliding against GCr15 steel balls. Interestingly, the addition of Al was effective in reducing the volume loss by 50% compared with the CCW alloys due to the formation of a ductile layer and enhanced oxidation. The COFs were found to be lower (0.30–0.35) for CoCrW and CoCrWAl2 but increased to 0.62 with CoCrWNi25Al2. Another recent study reported the tribological behavior of direct laser energy deposited Cr-Cr-Ni-W alloy on a steel substrate with and without an aging treatment at 750 °C–950 °C [129]. It was observed that the as-printed specimens showed a combination of adhesive and abrasive wear mechanisms, which was transformed to mostly abrasive when aging took place. This was due to oxidation after aging and, subsequently, the formation of a harder oxide layer that led to plough effects, such as abrasion marks, especially after treatment at 950 °C [130]. The k values were in the range of (2.39 − 5.6) × 10⁻⁵ mm³/N·m.

5.6. Corrosion Performance

Co-Cr-based alloys are well regarded for their corrosion resistance across a range of wet environments [131]. However, the corrosion resistance of these alloys is influenced by specific process parameters, such as build orientation, and may sometimes require surface quality enhancement through post-processing [132,133]. Atapour et al. conducted a study examining the impact of build orientation on the corrosion resistance of CCM alloys fabricated using the LPBF process [134]. CCM alloys were printed in both XY (perpendicular) and XZ (parallel) orientations, then abraded by SiC, and subsequently evaluated for corrosion performance using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization (PDP). Corrosion tests were performed in phosphate-buffered saline (PBS, pH 7.4), citric acid (CA, pH 2.4), and a mixture of both (PBS + CA, pH 7.4). The results indicated that the abraded specimens exhibited superior corrosion resistance compared with as-received specimens. Furthermore, the corrosion resistance was the lowest in the PBS + CA solution than in other media. Notably, specimens printed in the XZ orientation showed enhanced corrosion resistance relative to those printed in the XY orientation, which is attributed to an improved surface quality. The polarization curves, shown in Figure 32, displayed no signs of localized corrosion. The same specimens were tested in sodium chloride (NaCl) + 2-(N-morpholino)-ethanesulfonic acid (MES) and NaCl + MES + BSA + Fbn (pH 7.3) media. It was observed that BSA + Fbn increased the Co and Cr, as well as Mo, metal release and increased corrosion compared with NaCl + MES [126]. Furthermore, the passive current density and corrosion were found to be higher for the XZ-build-oriented specimens tested in each media.

Narayanan et al. identified a unique corrosion mechanism for CCM alloys produced via the LENS process, characterized by the dissolution of Cr/Mo-depleted regions and the emergence of crack-like features at junctions between different cell types during electrochemical polarization tests in Hank’s balanced salt solution (HBSS) [135]. The corroded surfaces displayed the formation of micro-galvanic cells and crack-like areas, which acted as sites for localized alloy dissolution. Concurrently, micro-segregation of Cr and Mo to the cell boundaries was observed. This behavior is undesirable for implants, as prolonged exposure to physiological conditions can lead to the release of metallic ions, potentially causing cytotoxic effects. To mitigate such issues, further investigation into reducing ion segregation through process parameter adjustments and appropriate post-processing techniques is needed. Figure 33 illustrates these corrosion mechanisms in CCM alloys after cyclic polarization (CP) up to +2 V. As shown in Figure 33a, the microstructure remained largely intact, with visible dissolution of cell interiors. Potential reversal after high polarization appeared to cause the removal or complete dissolution of some carbides, leaving “holes” on the surface as depicted in Figure 33b. The micrograph in Figure 33c reveals “crack”-like features on the polarized alloy’s surface. As indicated in Figure 33d–f, localized dissolution followed these cracks and areas where carbides had been removed, suggesting that selective cell dissolution was accompanied by localized dissolution at microstructural boundaries.

Another study investigating the corrosion characteristics of printed CCM alloys in NaCl and phosphate-buffered solution (PBS), with and without albumin, revealed that corrosion resistance after 15 h of immersion followed the order PBS > NaCl > NaCl + albumin > PBS + albumin [136]. Figure 34 illustrates the polarization curves of CCM alloys in these solutions. In the cathodic range, the presence of albumin led to a slight reduction in the current density in both solutions, functioning as a cathodic inhibitor [137]. The transition from a cathodic to anodic current was observed within the potential range of −750 to −650 mV_Ag/AgCl for albumin-containing solutions and between −250 and −150 mV_Ag/AgCl for solutions without albumin. Comparatively, solutions without albumin showed that phosphate ions contributed to the passivation of the metal surface. The key parameters, including the corrosion potential (E_corr), corrosion current density (i_corr), and breakdown potential (E_bp), from the polarization curves are provided in

Table 4. Impact of albumin on the electrochemical response of printed CCM alloys in NaCl and PBS solutions [136].

Parameter	Solution Tested
	NaCl	NaCl + Albumin	PBS	PBS + Albumin
Ecorr (mVAg/AgCl)	−164 ± 12	−674 ± 14	−201 ± 5	−700 ± 6
Icorr (μA/cm2)	0.56 ± 0.17	2.28 ± 0.92	0.33 ± 0.08	4.11 ± 0.83
Ebp (mVAg/AgCl)	414 ± 4	546 ± 6	539 ± 4	558 ± 5

.

Wei et al. examined CCM alloys fabricated using the EBM process, focusing on the microhardness and structural consistency of vertically printed samples [30]. The results showed that the lower region had higher hardness compared to the upper region, attributed to an increase in the area fraction of the hard M₂₃X₆ phase from 5.26% at the top to 8.73% at the bottom. Interestingly, when tested in 0.9% NaCl solution (physiological saline), the electrochemical behavior remained consistent regardless of the specimen’s location. All samples displayed passive regions without signs of localized corrosion, with minimal variation in the passive current density and transpassive behavior in the polarization curves. This uniform corrosion resistance was likely due to the high Cr content (approximately 28%) in the CCM alloy, making it less influenced by microstructural factors, such as matrix phase, precipitates, and grain boundary distribution. Another study demonstrated that enhancing the corrosion resistance of SLM-printed CCM alloys can be achieved through the application of a Cr oxide layer using a hydrogen peroxide (H₂O₂) treatment during the plasma electrolytic polishing (PEP) process [138]. This approach effectively manipulates the oxidation process of Co-Cr alloys. The results of the electrochemical impedance spectroscopy (EIS) indicated that the primary factor contributing to the improved corrosion resistance was an increase in the charge transfer resistance when tested in 3.5 wt.% NaCl solution. On the other hand, the corrosion resistance of as-printed CCW alloys produced via the SLM process and tested in a 0.9 wt.% NaCl solution showed significant improvement when heat-treated at 1150 °C followed by water quenching, in contrast to conventional furnace cooling [139].

5.7. Biocompatibility

Co-Cr-based alloys are recognized for their biocompatibility, leading to their wide used in orthopedic implants and skeletal structures. However, such alloys lack osteogenic capabilities when compared with other metallic alloys, such as Ti-based ones [140]. Several studies have been carried out to improve these characteristics by adopting a range of processing techniques, such as acid etching to modify the surface topography and bone-morphogenic protein peptide immobilization, but they still require further development [141,142]. Mazumder et al. proposed a comparatively new but innovative technique to provide nanoscale surface modifications via electrochemical etching (e.g., biocorrosion) to enhance the biocompatibility of DED-printed CCM alloys [143]. This treatment increased the roughness of the printed surface, thus increasing the surface area and enhancing the adhesion of the cultured mammalian cells to the substrate. Hence, this is expected to improve the desired biocompatibility of CCM alloys by increasing the mammalian cell growth and spread. Another study reported the cell culture, as well as growth, of MG63 cells on an LPBF-printed surface and found promising results in terms of the adhesion compared with polished surfaces [134]. With lamellipodia and filopodia, there was a tendency of the cells to attach to the surface of the specimens over a longer period time. On the other hand, to improve the wear characteristics of LENS-printed CCM alloy, especially when employed in hip, knee, and spinal applications as rubbing elements, 2 wt.% calcium phosphate (CaP) was found useful for forming a protective tribo-layer when provided on the CCM surface by laser surface melting (LSM) [144]. The wear characteristics improved by five times, which significantly improved the material’s performance as a bio-tribological element and, thus, enhanced the biocompatibility of the alloys. The in vitro cell interaction provided no evidence of a cytotoxic effect.

5.8. Summary and Future Prospects—The Roadmap Ahead

This paper encompasses the key findings on the state-of-the-art characteristics of printed Co-Cr-based alloys. Physical properties such as density and porosity are influenced by fabrication technologies, each with advantages and limitations. Binder jet technology achieves near full density (~99.1%) at 1380 °C but causes linear shrinkage, while SLM reduces porosity at high temperatures but introduces thermal distortions and high energy consumption. Micro-plasma-based additive manufacturing (MPBAM) is used to control thermal effects and optimize porosity, particularly in biomedical implants. The microstructure also plays a crucial role in mechanical properties, where Co-Cr alloys exhibit FCC (γ-Co) and HCP (ε-Co) phases, affecting strength and toughness. Rapid cooling in additive manufacturing results in γ-Co dominance, necessitating post-processing treatments for phase optimization, while grain boundary engineering (GBE) helps control Laves phase precipitation, enhancing hardness and strength. Surface finish modifications through post-processing improve functional performance, with laser polishing reducing the surface roughness to ≤1 µm, which enhances the corrosion resistance, and chemical etching (HCl + H₂O₂) removes residual particles for safer biomedical applications. The mechanical properties of printed Co-Cr alloys are further influenced by phase transformations, where SLM-printed components exhibit superior hardness (460 HV) and fatigue resistance, and Co-Cr-W alloys modified with Al and Ni additives improve the thermal fatigue resistance and mechanical strength. Tribological characteristics are enhanced through post-processing techniques, particularly ultrasonic nanocrystal surface modification (UNSM) at 500 °C, which improves wear resistance and offers an ultra-low COF at 0.0498. Although the porosity in SLM-printed alloys can increase abrasive wear in joint implants, in bio-tribological applications, hydroxyapatite (HA) reinforcement in Co-Cr alloys forms smooth tribo-layers, thereby significantly reducing wear rates. Corrosion performance is influenced by building orientation and post-processing treatments, while biocompatibility remains a critical characteristic for medical applications. Electrochemical etching enhances mammalian cell adhesion, improving biocompatibility, and calcium phosphate (CaP) coatings provide additional wear resistance by forming protective tribo-layers in implants.

Future research may focus on improving the characteristics of printed Co-Cr alloys, particularly in controlling residual stress and surface defects such as cracks and voids during fabrication using SLM and DED technologies. Advanced thermal control strategies should be explored to enhance material integrity, while promising reinforcements like graphene, TiC, and SiC could significantly improve tribo-mechanical properties, requiring further efforts for efficient manufacturing. Detailed tribo-testing under lubricated conditions is necessary for real-life implant applications, and hybrid surface modifications, such as micro-/nano-coatings combined with laser polishing, could further improve wear characteristics, which extend the tribological exploration of Co-Cr-based alloys. Additionally, plasma-assisted polishing and electrochemical deposition should be investigated for the biomedical field, since this is the most common area of application. Additionally, high-temperature oxidation studies remain an unexplored avenue for Co-Cr alloys in aerospace applications.

6. Cutting-Edge Applications of 3D-Printed Co-Cr-Based Alloys

Co-Cr-based alloys have been extensively adopted in many applications owing to their biocompatibility and excellent mechanical properties. As conventional fabrication of Co-Cr alloy through casting, cutting, etc., raises challenges due to high melting point, high hardness, and lower ductility, the AM of Co-Cr alloys becomes increasingly important in modern scenarios. Moreover, AM facilitates the accurate control of various process parameters to ensure the desired metallurgical and mechanical properties in Co-Cr alloys. Thus, the AM of Co-Cr-based alloys finds applications in many industrial sectors as discussed below.

6.1. Biomedical Applications

The significance of the AM process is clearly apparent from the ever-increasing requirement of complex and patient-specific customized CoCr implants in the biomedical sector. Co-Cr alloys built by AM are broadly adopted in prosthodontics, cardiovascular implants, and orthopedic implants in view of their excellent corrosion resistance, biocompatibility, and mechanical strength.

6.1.1. Prosthodontics

AM technology has gained huge momentum in prosthetic dentistry. AM can be employed for the fabrication of Co-Cr metal posts in dentistry (Figure 35) as they exhibit internal adaptation similar to those posts fabricated using milling and traditional casting [145]. Past research studies have also determined that PBF Co-Cr dental crowns express better marginal fit than traditionally cast crowns, indicating the potential of AM Co-Cr alloys in dentistry [40]. Further, the mechanical properties of laser-melted Co-Cr alloys such as the elastic modulus and yield strength are superior compared with casted and milled samples [146]. This can be attributed to the relatively finer and homogenous microstructure resulting from localized melting and faster solidification in laser-melted samples. Consequently, restricted metal release, excellent corrosion resistance, and reduced cell proliferation will be experienced while adopting PBF Co-Cr alloys in prosthodontics applications [147].

6.1.2. Cardiovascular Stents

Three-dimensionally printed stents are in the early phase of their evolution and will soon transform as effective solution over existing stent fabrication methods. Due to superior mechanical properties, Co-Cr stents can be manufactured with struts as thin as 60–80 µm [149], thereby providing greater flexibility than SS stents, resulting in reduced rates of stent thrombosis. Moreover, the higher density of Co-Cr alloys, relative to SS, enhances their radio-opacity. This improves the visibility of Co-Cr stents during placement in blood vessels, reducing the likelihood of complications during percutaneous coronary interventions (PCIs). The design rules proposed by Finnazi et al. [76] for fabricating expandable AM Co-Cr stents are promising in this regard. Less than 1 mm in length is recommended for strut overhangs with a spacing of more than 0.3 mm, as per the design rule. The stents produced by following the rules were able to withstand expansion without any evidence of cracks or failure, as shown in Figure 36. Likewise, Omar et al. introduced a fabrication window for future stent manufacturing by evaluating seven distinct designs of Co-Cr stents printed using LPBF. The stents achieved an average density of ~92 to 97% after the LPBF process.

6.1.3. Orthopedic Implants

Printed Co-Cr alloys also have potential scope in orthopedic implantology, especially in hip and knee implants [150]. Brogini et al. confirmed from his studies that printed Co-Cr alloys could favor good osseointegration and provide a suitable atmosphere for bone regeneration [151]. Further, Co-Cr alloys printed along the build direction have better strength and elongation properties over the non-build direction and are preferred for load-bearing orthopedic implants [152]. Figure 37 shows the prototype of a typical CCM alloy femoral knee implant manufactured by EBM followed by HIP in accordance with the ASTM-F75 standard. Figure 37 also describes the potential of the AM process in the production of customized porous structures in implants with the provision of a varying inner cellular mesh structure so as to control bone ingrowth and stress-shielding effects.

6.2. Engineering Applications

Despite biomedical applications, AM CoCr alloys find practical importance in many other engineering applications due to their excellent mechanical characteristics and high-temperature strength.

6.2.1. Aerospace, Oil, Gas, and Power Generation

The thermal stability and high-temperature strength associated with Co-Cr based alloys make them suitable in aerospace applications. Kennametal Inc. was the first manufacturer to produce AM powder of Stellite 21 meeting the qualifications for LPBF process [153]. Stellite 21 typically comprises a Co-Cr-Mo alloy matrix with dispersed hard carbides. The company claims that the parts built using the introduced powder can offer the high corrosion resistance and wear resistant properties demanded in the oil and gas and power generation industries. Further, a complex valve cage printed using company’s Stellite 6 powder for a combined cycle power plant was found to contribute outstanding wear characteristics in various trials, as shown in Figure 38 [154]. Since 2015, LPBF fuel nozzles built by GE Aerospace using Co-Cr powder have been successfully implemented in Boeing 737 MAX, A320neo, A321neo, Airbus A220, and COMAC C919 airliners [155]. In comparison with previous engine generations, an ~15% improvement in fuel efficiency was achieved using the LPBF Co-Cr fuel nozzles.

6.2.2. Remanufacturing and Repair

Stellite is a commonly used alloy in LMD process, specifically for the remanufacture or repair of components like turbine blades and rotating shafts [156,157]. One study by Diaz et al. demonstrated that laser powder welding with Co-based alloy can be utilized in the repair of steam circuit parts needed in thermal power plants [158]. Similarly, a comprehensive study conducted by Foster et al. also reported that LMD of Stellite 21 is an appropriate strategy for the repairing of hot-forging dies and tools [159]. This can be attributed to the fact that Stellite can guarantee sufficient compatibility in forging and machining for remanufacturing operations even after repair.

7. Future Perspectives and Challenges Involved

The AM of Co-Cr-based alloys presents transformative potential across the biomedical, aerospace, and energy industries. Future developments should emphasize on advanced material design, for instance, exploring novel alloy compositions and metamaterials like auxetic meta-biomaterials to enhance mechanical and biocompatibility traits. In addition, process optimization needs to be further polished to achieve cost-effective solutions. Fine-tuning of parameters such as laser energy density, scanning speed, and build orientation are required to achieve superior microstructural and mechanical properties while minimizing defects. Further investigations should be carried out on more post-processing innovations such as advanced heat treatments, chemical etching, and surface modification techniques to further improve the wear, corrosion resistance, and surface finish. Last but not the least, developing cost-effective feedstock powder production and energy-efficient printing technologies to facilitate widespread adoption must be taken into consideration for higher scalability of the Co-Cr-based alloys. Figure 39 gives an overview on the research progress and future directions.

Despite significant progress, critical challenges remain in Co-Cr-based 3D-printed components, particularly in microstructural control, tribo-corrosion resistance, scalability, and regulatory approval for biomedical applications. Uniform phase transformation and minimizing porosity while maintaining mechanical strength are ongoing concerns related to microstructural defects. Wear resistance in wet and corrosive environments, especially for implants, presents difficulties in achieving reliable corrosion behavior. Scaling production from laboratory settings to industrial levels is another hurdle, as ensuring consistent part quality during mass production remains a significant challenge for Co-Cr-based components. For biomedical use, adherence to stringent safety and biocompatibility standards adds another layer of complexity.

8. Concluding Remarks

This review of recent advancements in Co-Cr-based alloys was conducted based on the existing literature, primarily sourced from the Web of Science (WoS) database and the Elsevier digital library published within the last decade. This article highlights the significant advancements in the AM of Co-Cr-based alloys, particularly their adoption in biomedical, aerospace, energy, and various industrial sectors. These alloys exhibit excellent mechanical, tribological and corrosion resistance along with biocompatibility when processed via modern AM techniques, such as LPBF, DED, and EBM. However, achieving defect-free structures with optimal physical and mechanical properties remains a challenge. Innovations in alloy composition, process parameter optimization, and post-processing treatments hold the key to addressing these limitations. Post-processing techniques play a crucial role in improving the properties of Co-Cr-based alloys manufactured via additive methods. Processes like heat treatment and surface finishing significantly enhance the mechanical strength, microstructure, corrosion resistance, and overall performance. These steps are essential for optimizing Co-Cr alloys for demanding applications, particularly in the biomedical field. Future research should focus on improving biocompatibility, wear resistance, and cost efficiency while ensuring scalability, as well as compliance with industrial standards. Thus, the comprehensive outlook and detailed analysis on Co-Cr additive fabrication presented in this paper offer valuable insights for engineers, researchers, and manufacturers, propelling the development of advanced Co-Cr-based alloys for next-generation applications.