Bridging processing and performance in Ti–6Al–4V: WAAM parameters and wear behavior across traditional and additive manufacturing routes

Abstract

Wire-arc additive manufacturing (WAAM) has garnered attention in academia and industry and has huge potential because of its ability to fabricate complex geometry and economic viability compared to other metal additive manufacturing (MAM) processes. Its unparalleled design freedom, ability to manufacture near-net shaped components, less material wastage, ability to produce large components with a variety of materials, and flexibility in influencing material properties by varying the process parameters makes WAAM promising for industry. Ti–6Al–4V is the most prevalent titanium alloy because of its exceptional material properties and extensive application in aerospace industry, the marine sector, and bio-medical implants, among others. However, its use is limited due to its poor wear behavior due to oxidation, delamination, and spalling. Thus, it is crucial to improve the wear resistance of Ti–6Al–4V by surface engineering, manufacturing processes, and post-process treatments. In this review, we aim to provide a brief introduction to popular MAM processes, process parameters of WAAM, and path planning that influence the material properties and explore the wear behavior of the Ti–6Al–4V alloy in different environmental conditions, process parameters, and post-processing treatments. Due to the dearth of literature available on the wear behavior of the Ti–6Al–4V alloy manufactured using WAAM, the wear behavior of Ti–6Al–4V manufactured by L-PBF, SLM, and other conventional methods is reviewed. Additionally, this review paves the path of future exploration of the wear characteristics of the Ti–6Al–4V alloy manufactured by WAAM.

1 Introduction

Additive manufacturing (AM, also referred as 3D printing, additive layer manufacturing, rapid prototyping, and solid freeform fabrication) has emerged as a revolutionary manufacturing technology and challenged traditional fabrication practices. AM enables the fabrication of intricate geometries of components with minimum material wastage and allows mass-customization and enhanced design freedom compared to that with conventional subtractive manufacturing processes (Tofail et al., 2018). AM technologies make use of the latest computer advancements in achieving precision for the fabrication of products (Taghizadeh and Zhu, 2024). In AF, the part is designed first, and then a three-dimensional model is generated using software tools. Thereafter, the 3D-modeled part is sliced into multiple layers having either variable or same thickness depending on the complexity of the part. Various studies have been reported on the slicing algorithm of the three-dimensional (3D) stereo lithography (STL) model (Ding et al., 2016a). Then, the saved file is fed into the AM machine that fabricates the 3D part by adding and joining the material from the bottom to the top in a layer-by-layer manner. Wire, powder, or sheet is mostly used as the raw material, which is consecutively melted by a heat source and solidifies at the designated coordinates provided by the 3D model data (Li et al., 2019). Additive manufacturing can be used to repair and restore damaged components. For cost-effective production of high-value components, the significance of AM increases significantly in material utilization factors (MUFs) of less than 15%.

Currently, there are various types of materials that can be produced using AM. Among various AM technologies, the manufacturing process used for the fabrication of metals has made a huge impact. Metal additive manufacturing (MAM) has revolutionized the manufacturing of metals such as stainless-steel alloys, titanium alloys, polymers, super-alloys, and metal-based composites. Based on the Advanced Standards Transforming Markets (ASTM) International, the additive manufactured processes are categorized into seven categories (refer to Figure 1), namely, a) direct energy deposition (DED), b) powder bed fusion (PBF), c) vat photopolymerization, d) sheet lamination, e) material extrusion, f) material jetting, and g) binder jetting, (ISO/ASTM ISO).

FIGURE 1

ISO/ASTM ISO).

Titanium and its alloys have garnered attention from researchers, academics, and the industrial sector due to their exceptional properties such as high specific strength, high corrosion resistance, the strength-to-weight ratio, low density, and biocompatibility (Tiwari et al., 2025; Vrancken et al., 2014). Ti–6Al–4V, composed of α+β phases, is the most common among different compositions of titanium alloys, and it has exceptional properties, such as superior weld fatigue characteristics, biocompatibility, high strength, and toughness (Jo et al., 2026; Baharzadeh et al., 2025). Ti–6Al–4V has a market share of 75%–85% (Cui et al., 2011) and is also well-adopted in various engineering applications, such as oil and chemical processing industries, heat exchangers, biomedical applications, power sector, aerospace, and marine industry (Li et al., 2025; Gurrappa, 2003; Nguyen et al., 2022; Oryshchenko et al., 2020; Bandekian et al., 2025). Aerospace and shipbuilding are the first and second largest consumers of titanium and its alloys, respectively (Oryshchenko et al., 2020). In the marine industry, titanium and its alloys can be found in steam generators, pipes, valves, pumps, heat exchangers (Gorynin, 1999), electrochemical antifouling system (Wake et al., 2006), and marine fasteners (Been and Faller, 1999), among others.

Titanium sponge, which is the commercially used form of titanium, is extracted from titanium ore and then converted into titanium ingots. Its applicability increases significantly in biomedical devices (such as dental implants and knee replacement) and aerospace and marine equipment (such as hydraulic actuators and last-stage bucket) as these components are exposed to a highly corrosive environment. The fabrication of complex geometries of Ti-6Al-4V components through conventional processes faces numerous challenges because of the limited production flexibility in manufacturing units and the economic viability (Fu et al., 2017). The machinability of Ti–6Al–4V is very poor as it has low thermal conductivity, which leads to high cutting temperatures (Parry et al., 2016; Pramanik et al., 2013). Due to low thermal conductivity, strong adhesion takes place at the interface between the work-piece and the tool, and the temperature increases significantly. The high chemical reactivity with cutting tool materials, low modulus of elasticity, and high strength of Ti–6Al–4V further drastically decrease its machinability (Che-Haron and Jawaid, 2005; Ezugwu and Wang, 1997). However, Ti–6Al–4V is known for its poor wear performance and its susceptibility to fatigue failure pertaining to its low hardness (Li et al., 2025; Beaman et al., 2020). Therefore, various AM technologies are able to provide a viable and suitable method for the fabrication of Ti–6Al–4V components as per the requirement compared to that with the traditional manufacturing processes.

Wear is a very dynamic phenomenon that directly affects the service-life of a component and is dependent mainly on friction along with other factors such as the temperature, working load, surface topography, lubricating conditions, speed, and environment. Wear of the material leads to changes in the dimensions and removal of material due to the surface-to-surface contact during the interaction of the components (Meng et al., 2020). Wear-induced friction is the reason behind the failure of approximately 80% mechanical components, causing an economic loss of more than 4% of the global GDP (Liu et al., 2025).

1.1 Novelty statement

The application of wire-arc additive manufacturing (WAAM) is widespread across many industries. Understanding the wear characteristics of WAAM-fabricated components under various conditions is critical before its application in different contact regimes. Therefore, it is necessary to discuss various process parameters involved in WAAM and its impact on various mechanical properties, surface topology, and wear behavior of the additively manufactured Ti–6Al–4V components. Understanding the wear characteristics of the additively manufactured components may improve the wear performance, reduce failures, increase the service life, and foster the acceptability of AM technologies in the industry.

Although a substantial body of literature is available on WAAM-fabricated components, there is dearth of literature that provides extensive and systematic discussion on the process parameters and path-planning strategies involved in MAM technologies based on wire feedstock and reports the recent development in the fabrication of Ti–6Al–4V using the WAAM process. The novelty of this study is that it provides extensive discussion on available research and elucidates the impact of the various process parameters and path-planning strategies required in WAAM to improve its wear characteristics, mechanical properties, and build quality of Ti–6Al–4V alloy systems compared to Laser powder bed fusion (L-PBF), selective laser melting (SLM), and other conventional methods.

2 Metal additive manufacturing techniques for Ti–6Al–4V

Additive manufacturing is an innovative technology with increasing popularity and huge potential in addressing manufacturing challenges of complex, customized, and large components. In 2019, the global metal AM market was valued at approximately $774 million and projected to exceed $3.1 billion by 2024 (Schmidt, 2020). Due to an increase in the demand for metal additively manufactured components, various processes have been developed to fulfill the needs of various industrial sectors. Among various fusion-based metal AM technologies, ASTM F29792 is divided into two main categories, namely, DED and powder bed fusion (A.S.T.M, 2012). On the basis of these methods, various technologies have been developed over the years, as shown in Figure 2. Generally, metal-based AM processes can be distinguished by the type of feedstock (wire and powder), heat source (laser, plasma, electron beam, arc, etc.), and chamber requirements (vacuum, inert gas environment, and atmospheric conditions). For AM, ASTM F42/ISO TC261 has approved the standard structure on the basis of various parameters, as shown in Figure 3.

FIGURE 2

FIGURE 3

Council, 2014).

MAM technologies have the potential of working with a large range of materials and tailored alloy systems. However, the option of feedstock is limited to only two, i.e., wire feedstock and powder feedstock. The AM processes that use wire as a feedstock show better process flexibility, reduce the probability of contamination with a higher molten metal deposition rate, and increase the efficiency compared with that of the powder bed processes. Moreover, the wire feedstock is cheaper than the powder feedstock as it requires less processing and refining cost

2.1 Wire-arc additive manufacturing (WAAM)

The WAAM technology is an advanced manufacturing process that belongs to the family of directed energy deposition (DED) in MAM and has garnered attention because of its exceptional capabilities of producing large-sized metallic components, high deposition rates, less manufacturing time, the ability to build near net-shaped components, and cheaper equipment compared to that of other MAM technologies’ equipment (Harati et al., 2024). In WAAM, heat is produced by an electric arc between the metallic work-piece and a consumable wire electrode for the layer-by-layer deposition of molten metal on the substrate. It differs from arc welding in basic functionality. In arc welding, the two or more components are joined together, whereas in WAAM, the material is deposited on the substrate or the layer that is already laid. The wire feed direction is usually perpendicular to the substrate. Important components of the GMAW-based WAAM set-up are the computer interface, robot controller, robotic arm or manipulator, programmable welding power source, and shielding gas as shown in Figure 4.

FIGURE 4

WAAM is generally categorized into three groups on the basis of the heat source, namely, gas metal arc welding (GMAW; Gierth et al., 2020), gas tungsten arc welding (GTAW) (Wang et al., 2011), and plasma arc welding (PAW) (Aiyiti et al., 2006), as described in Figure 5. However, cold metal transfer (CMT)–WAAM is also one of the prominent and energy efficient WAAM technologies available (Farabi et al., 2023; Selvi et al., 2018). The benefit of CMT in WAAM is that it aids in better control during welding and requires less heat input than other processes (Yu et al., 2025). The basic difference of GMAW, PAW, and GTAW is demonstrated in Figure 6. Among other WAAM, GTAW is very economical as it uses the cheaper GTAW as an underlying technology (Wang et al., 2013). However, GMAW has the disadvantage of producing a less stable arc, spattering and producing a high amount of fume.

FIGURE 5

FIGURE 6

The buy-to-fly (BTF) ratios of components manufactured using traditional manufacturing technologies can be very high (refer Figure 7). The buy-to-fly ratio is defined as the ratio of the mass of the starting ingot of raw material to the mass of the finished component. As near-net-shaped parts are produced in WAAM followed by the finishing and heat-treatment processes, the buy-to-fly ratio is substantially improved in WAAM compared to that in traditional subtractive manufacturing. This is because less volume of excess material is to be machined during the post fabrication machining process to bring it into desired shape. This will result in bringing down the manufacturing cost (Williams et al., 2016). Other distinctive features of WAAM are high deposition rates of 50 g/min–130 g/min and low equipment expenditure compared to those of electron beam and laser-based AM processes, which have 2 g/min–10 g/min deposition rates (refer Table 1). The energy efficiencies of WAAM processes are 90% in some conditions, making WAAM an eminent process to fabricate medium-to-large-size components (Xia et al., 2022). As WAAM eliminates the need for processing peripheral powder for recycling, it reduces the health and safety concerns and offers significant reduction in price per kilogram when compared to the feedstock in powder form Cunningham et al. (2018). In addition, due to low cooling rates and higher heat input, WAAM is suitable for most of the commercial materials, which makes it suitable for technology adoption by the industry (Wei et al., 2019).

FIGURE 7

In contrast to other AM processes, the surface finish in the WAAM process is affected by the overlapping of the bead deposits during the formation of layers. Ostensibly, the dimensional accuracy of the fabricated component is poor (± 0.2 mm) as the surface finish is dependent on the bead geometry and the overlapping distance between two beads. In certain cases, the surface quality of the component made using WAAM is not acceptable. Overlapping distance between the two beads is affected by various process parameters (Ding et al., 2015a). In contrast, powder-based processes are known for intricate detailing in small-sized components but have limited application due to their inability to build large parts and the low deposition rate.

2.2 Electron beam additive manufacturing (EBAM)

Among various DED processes, electron beam additive manufacturing (EBAM) is very prominently used for the fabrication of Ti–6Al–4V. In EBAM technology, the raw material (either powder or wire) is fed into the path of the focused electron beam in a high vacuum environment and accumulated layer-by-layer on the bed, as shown in Figure 8. When one layer of the metal has been deposited, the powder bed descends, and the next layer of the metal powder is deposited on the surface (Sonkamble and Phafat, 2023; Shi et al., 2023).

FIGURE 8

Electron beam is popular for high build quality as it works in vacuum environments, prevents oxide formation during printing, and produces contamination-free components. When compared to conventional machining in terms of dimensional accuracy, electron beam additively manufactured components have less dimensional accuracy, which ranges from +0.6 mm to −0.1 mm (Juechter et al., 2018). In addition, electron beam also provides high scanning speed and is suitable for materials that have high reflectivity, refractivity, and conductivity. An electron beam system contains an EB extraction source, electron beam focusing and deflection electrode, and the vacuum system. Emission of the electron can be obtained by using plasma, laser, magnetic field, and thermionic, among others.

Electron beam systems that are used for additive manufacturing were developed on the basis of the feedstock. For powder bed AM, the intent is to keep the bed stable as the loose powder may spread unevenly on the bed. To avoid this, x- and y-axes movement of the bed is not preferred. Instead, the head of the electron beam is made flexible so that it can reach the intended workspace (Negi et al., 2020). In order to understand the performance of EBAM-fabricated components, such as the morphology, porosity, mechanical properties, and microstructure, Ngiam et al. (2026) analyzed various key parameters such as the scan speed, line offset, focus offset, and preheating temperature. The experiment highlighted that increasing the scanning speed or line offset refines the microstructure of the sample and increases the hardness, while low scan speeds and line offset leads to coarse microstructure, which reduces the hardness and tensile strength due to insufficient bonding. However, the tensile strength of the sample is negatively affected by too high scan speed, line offset, and focus offset.

Depending on the volume, high-pressure spherical titanium tanks are produced by either machining or high-pressure machining. These conventional processes are highly time-consuming and material-intensive (Lee et al., 2020). The experimental data for the fabrication of Ti–6Al–4V spherical tanks using EBAM show up to 75% reduction in waste generation, decreasing the production time by 80% and significantly reducing the production cost by 55% (Additive Manufacturing of Titanium Parts). Wire-feed additive manufacturing technologies have high future prospects in the fabrication of meter-sized metallic components having moderate levels of geometrical complexity as powder-based processes are not suitable for manufacturing large components (Osipovich et al., 2023).

2.3 Laser powder bed fusion (L-PBF)

In powder bed fusion, the feedstock is first spread over the bed using a roller. Thereafter, the powder is selectively melted using the heat source across the powder bed. Various types of the heat source (such as laser power, electron beam, plasma, and ultra-sonication, among others) can be used to melt the feedstock in powder bed fusion-based additive manufacturing techniques. Technologies that use powder as a raw material are able to achieve thin layer thickness between 20 and 50 µm (Monteiro et al., 2022) and quick heating and cooling rates approaching 10³ and 10⁸ Ks^-1, leading to a lower deposition rate due to the low melt volume of metal.

L-PBF is one of the AM techniques in which the metal powder is melted using high-power laser and deposited in a layer-by-layer manner, and the two consecutive layers are joined according to the order provided by the computer-generated CAD file until the part is near-net shape (Hooper, 2018). Usually, the metal powder used in L-PBF are atomized spherical powders that facilitate excellent flowability and high-quality near-net shaped components specifically for porous structures, such as human prosthetics replicating bone structure (Lu et al., 2025; Hejripour et al., 2020). In the beginning, the melted metal is deposited on the baseplate, and then the next layer of metal is applied on the deposited surface (King et al., 2015). Powder-based L-PBF has an edge over other AM technologies as it does not require any support or scaffold structures for support, as shown in Figure 9. The overhanging part of the component is supported by the unfused powder material around the component. Since no scaffolds or support structures are required in this method, it allows the fabrication of complex components coupled with high accuracy and high finishing (Meng and Du, 2024; Ullah et al., 2025).

FIGURE 9

LPBF has limitations on fabrication of larger-sized components due to physical space constraints, using only single-build material, slower build rate due to heat dissipation, and high porosity. Improper heat dissipation may lead to thermal distortions and may be the reason behind defects such as warping (Cooper et al., 2018). Due to the rapid heating and cooling cycles, LPBF-fabricated components have high residual stresses, which can lead to formation hot cracking (Lu et al., 2025). Although lasers as an energy source are quite popular due to its precision, they have very poor energy efficiency (Unocic and Dupont). Residual stress is also a very common phenomenon in parts fabricated using LPBF. Fathin et al. found that residual stress can be reduced in parts manufactured using LPBF by using heat-treatment methods (Jamhari et al., 2023). In addition, the porosity and dimensional accuracy of the large parts fabricated using LBPF is also influenced by the continuous and pulsed energy input for various materials (Demir et al., 2017). The powder used in the L-PBF process also influences the mechanical properties of the components, as depicted by Dong et al. (2025). The study demonstrated that a sample printed using the unprocessed powder of Ti–6Al–4V has comparatively lower ultimate tensile strength of 1,280 MPa, yield strength of 1,137 MPa, and the measured elongation rate of 4.5% compared to that of samples processed using cold plastic deformation (CPD) Ti–6Al–4V, having the ultimate tensile strength of 1,500 MPa, yield strength of 1,320 MPa, and measured elongation rate of 6%.

Reichardt et al. demonstrated the joining of Ti–6Al–4V and SS304L using laser metal deposition by transitioning from titanium alloy to austenitic stainless steel. In study, the deposition of titanium alloy was followed by vanadium in 25% increments. After 75% V and 25% Ti–6Al–4V, 25% SS304L was introduced, replacing the titanium alloy. At the juncture of introduction of SS304L, the authors observed the crack and delamination in the layer due to the formation of brittle sigma phase in high V-content regions. When the fabrication was carried out in a reversed manner, wherein the procedure started with vanadium and SS304l and the titanium was introduced at a later stage, a similar trend of cracking and delamination was observed. This transition fails pertaining to the brittle Fe–C–Cr sigma phase formation at intermediate compositions (Reichardt et al., 2016).

In summary, different MAM technologies are available in the industry for the fabrication of components. However, in most of the cases, the equipment and its maintenance cost along with the requirement of trained manpower to operate these complicated is a huge investment in itself. WAAM provides an economical solution as it uses cheaper GMAW, PAW, or GTAW equipment for manufacturing large near-net-shaped metal components. WAAM can fabricate materials using most of the commercially available materials, such as steel and its alloys (including SS316L) and titanium and its alloys (including Ti–6Al–4V), among others.

3 Effects of the process parameters on parts fabricated using WAAM

The geometric accuracy of the deposited part is dependent on the various process parameters, as shown in Figure 10. The selection of various process parameters has a significant impact in joining applications and plays a role in minimizing the distortions and post-processing finishing allowance and achieving higher resolution, stable (spatter free) operation, and high resolution in WAAM. The process parameters, building sequences, and welding disturbances can have a huge impact on the process stability and repeatability. Traditionally, experienced technologists used to set the process parameters, such as the deposition path, wire-feed rate, and travel speed, as per the desired shape, material, and energy source. As an automated process planning strategy that can set the process parameters is not available, the commercialization of the WAAM is a huge challenge. In addition, residual stresses, process optimization, and defects in the finished components are some of the major issues encountered for the automation of WAAM process planning (Pan et al., 2018). For the automation of WAAM, the process monitoring and close loop control of the process parameters is an essential element. There are several challenges for manufacturing reliable components using WAAM, which can be classified into three categories, namely, manufacturing accuracy, quality assurance, and automation systems (Xia et al., 2020).

FIGURE 10

3.1 Deposition rate and wire feed speed

The wire diameter plays a vital role in the performance and final product quality as it has a direct impact on the deposition rate and bead shape in the WAAM process. Higher deposition rate of WAAM processes is one of its major advantages over other AM processes (Sequeira Almeida and Williams). A higher deposition rate is suitable for the production of large components while reducing the manufacturing time and overall cost. However, if the process parameters are not optimized, it leads to the deposition of extra material than actually required, which will lead to higher BTF ratios and additional economic implications as it will require additional machining of the component. Unoptimized process parameters will also lead to high deposition rates, which will affect the fidelity of the component. To ensure that the BTF ratio less than 1.5, it is further proposed to maintain a medium deposition rate, i.e., 1 kg/h for titanium and its alloys (Williams et al., 2016). The deposition rate can be increased for a given energy input by increasing the diameter of the feed wire and wire feed speed. Alternatively, the energy input may also be varied for different feed wire diameters and wire feed speed to achieve the desirable deposition rate based on the requirement. In the arc-welding process, a small part of the energy input is used in the melting the feed wire since a significant amount of energy is dissipated in the conduction losses and into the surroundings (Dupont and Marder, 1995).

Wang et al. (2021) analyzed the different combinations of feed wire diameters and wire feed speed while varying the energy input for each wire type on plasma transferred arc (PTA) based WAAM or PAW–WAAM to obtain the optimized combination for a high deposition rate and study the effect of energy input on the deposition rate. It is reported that the wire size does have an impact on the deposition rate, and with an increase in the feed wire size, the deposition rate also increases. However, the larger feed wire diameter is not always efficient as it requires more energy. Moreover, melting of the feed wire is also dependent on the wire feed speed as higher wire feed speed leads to partial melting of the wire feedstock. The study reported that for specific energy input, the wire feed speed reduces with an increase in the feed wire diameter for proper melting of the feedstock.

For cold metal transfer (CMT)–WAAM, microstructural analysis was conducted by Nie et al. (2018) for different deposition conditions. The increase in deposition rate leads to a decrease in microhardness, which is observed due to the softening effects due to the increase in excessive heat input in the heat-affected zone. The results also show reduction in the ultimate tensile strength, which is an outcome of a higher deposition rate. Moreover, Farabi et al. (2023) analyzed the microstructure and mechanical properties of Ti–6Al–4V and showed that in high deposition rate conditions, mechanical anisotropy can be removed within the material and improved mechanical properties of the material can be achieved, which makes CMT better than other WAAM processes.

3.2 Welding torch speed

Welding torch speed is a crucial parameter in building parts as it has a huge impact on the weld quality because it influences the microstructure, porosity, grain structure, surface quality, material deposition rate, and mechanical properties of the fabricated component. It is noted that higher welding torch speed leads to deeper penetration of the molten metal. The arc is generated directly above the weld pool at the shower welding torch speed. This leads to the deposition of the molten metal from the feed wire directly into the weld pool instead of reaching to the base material due to surface tension, as seen in Figure 11. In contrast, the arc is generated at the leading edge of the weld pool at faster welding torch speed, due to which the molten metal directly goes to the base of the material (GMAW). Thus, it is important to optimize the welding torch speed by trading-off with other welding parameters such as the wire feed speed and deposition rate in order to minimize the flaws.

FIGURE 11

Ueyama et al. (2005) studied the various parameters and their impact on bead formation at high welding torch speed on tandem GMAW in order to reduce or eliminate the commonly occurring flaws such as undercut and humping beads. It is observed that at lower welding torch speeds, less molten metal reaches to the base of the weld pool. This causes flaws such as porosity and undercut due to the lack of penetration. In two-wire configurations, the maximum welding torch speed of 2 m/min–3 m/min was obtained, which is 60%–150% higher than the speed of a single-wire pulsed GMAW.

Under the varying welding speeds, Xian et al. (2023) analyzed the microstructure and anisotropic properties of Ti–6Al–4V fabricated using WAAM and followed by hammer peening treatment on each for three times. The results showed that the weld pool shape, i.e., the width and height of the bead, varies with changes in welding speed. Smaller temperature gradients and higher solidification rates were also noted in higher welding speed conditions. Furthermore, anisotropy increases within the material immediately with increasing welding torch speed. Hence, to control the grain structure and reduce the anisotropic properties, lower welding torch speed is proposed as the optimum welding parameter. The result of experiments conducted by Terrenoir et al. (2023) depicts that the ultimate tensile strength and failure strain is mainly affected by changing the welding torch speed, while the wire feed speed has almost negligible impact on it. The analysis shows the correlation between the welding torch speed and wire feed speed and its impact on the mechanical properties of the SS316L.

Furthermore, the welding torch speed is a critical factor in the determination of heat input (HI) used fabrication, which is calculated by using Equation 1 (Kumar et al., 2024):where HI represents the heat input (J/mm), η represents the coefficient of process efficiency, U (V) represents voltage, I (A) represents current, and travel speed (TS) (mm/min) represents the travel speed.

3.3 Slicing

The 3D CAD model is segmented into a sequence of 2D sliced layers perpendicular to the orientation of the product. The layer thickness can be pre-defined or adaptive depending upon the product complexity and product quality requirement. However, the requirement of the support structure is negligible or very minimal in WAAM, but the slicing direction of a model is a very necessary parameter in AM as it determines the quality and topology of the final product. Depending on the complexity of the product and the time required to build the product, the layer thickness can be varied in the slicing process, i.e., the thickness of the layers may not necessarily be the same for every layer. If the thickness of each layer is high, the build time can be reduced by compromising on the geometrical accuracy of the product. The thin layer tends to provide improved geometrical accuracy than high layer thickness, but it increases the manufacturing time (Sun et al., 2007).

In 1987, the STL file was developed for building a component using the SLA machine. Since then, the .STL file has gained popularity and became the most-used interface between CAD software and AM systems for slicing the 3D graphic model (Jacobs, 1992; Jacobs, 1995). Apart from STL file slicing, there are other slicing approaches such as direct slicing developed by Jamieson and Hacker (1995) and adaptive slicing developed by Suh and Wozny. Many flaws of the .STL file slicing have been resolved by the direct slicing approach. However, the adaptive slicing approach provides more accuracy and reduces the build time than the direct slicing approach.

3.4 Path planning strategies

Development of the lucid path planning strategy for the fabrication of components based on the complexity of the geometry is among one of the crucial requirements in WAAM. Path planning is essential to accumulate the optimum material deposit for varying layers according to the geometry, and it is required to produce good-quality products. Path planning strategies are well reported, such as raster paths or parallel line, contour paths or equidistant method, zig–zag path, MAT, and MPP. However, every path-planning strategy is suitable for a particular type of geometry. For automation in WAAM, it is challenging to develop a method that can use the information from the CAD file, and generate and adapt the changes in geometry in a complex part.

The raster path approach or parallel line method deposits a number of layers in the unidirectional paths to build the component. Due to the same build direction, frequent turning of the deposition head is required, which leads to the geometrical accuracy of the build component. The components fabricated using the raster path approach faces warpage and anisotropic problems (Jin et al., 2013). In addition, the discontinuous path demands the deposition process to be robust enough to start and stop frequently. As the WAAM is an arc-based system, it needs a certain time to establish the stable arc so that the deposited material corresponds to the steady-state bead behavior according to the bead modeling. Thus, frequently starting and stopping deposition is not desirable. Moreover, the raster path approach may be used for thick work-pieces but not in thin-walled structures. As the paths always cross, in the thin-walled structures, the crossing paths will produce peaks where the beads overlap at the crossing points.

The contour path planning approach or equidistant method solves the warpage and anisotropic problems. Unlike in the parallel line method, the contour planning approach deposits the material in a continuous path from the starting point to the end point. The motion from rest at the starting point accelerates up to specified limit, changes along the profile, and decelerates to the end point, where it comes to the stop position. This leads to improved surface quality when compared with the product built using the raster path approach. As the layers are identical in the build direction, the flaws will accumulate on top of every subsequent layer, which will lead to poor mechanical properties of the build component (Yang et al., 2002). One such flaw is the generation of voids within the deposited layers, which has a huge impact on the quality, especially on the thin-walled structures. Ideally, there should not be crossing points between the paths. To avoid overlapping of beads due to crossing paths, it is essential to formulate an approach in which no path crossing takes place.

Ding et al. (2015b) proposed utilizing the medial axis transformation (MAT)-based path planning based on the skeleton of the given geometry to generate the offset path curves beginning the filling from the inside toward the outside boundary. This method uses the divide-and-conquer approach proposed by Lee (1982), which generates the Voronoi diagrams consisting of simple polygons. The generated trimmed path covers the entire sliced layer by generating a close loop path. The excess amount of material is deposited outside the boundary to prevent the development of gaps. In post-processing, the excess material is removed using subtractive manufacturing equipment.

Modular path planning (MPP) leverages the feature-centric design approach and incorporates it into the conventional layer-by-layer path-planning strategy. In MPP, segmentation of path design is the core fundamental principle. The layers are segmented into manageable discrete sections called sections to generate a unique path for each section using multiple path-planning strategies to avoid abrupt width variation, which can lead to defects after the deposition of multiple layers. In addition, MPP introduces zoning of the path, which is identified by the color. The parameters of each zone are specified by the user post tool path planning. In the end, a single layer path is generated by combining different path strategies for each section and integrating each zone having specific deposition rates into each section of the individual layers (Michel et al., 2019).

The microstructures are also influenced by the printing strategies. Palmeira Belotti et al. (2024) studied the microstructure and properties of stainless steel influenced by the printing strategies, namely, layer-wise weaving, bidirectional zig–zag scanning paths, and layer-wise unidirectional strategy. The layer-wise unidirectional strategy shows good surface finish compared to the bidirectional zig–zag strategy, which has shown irregularity on the side surface. Meanwhile, the surface finishing of the weaving strategy is superior to that of both the layer-wise unidirectional and bidirectional zig–zag strategies. In addition, the variability in ductility behavior is observed in both layer-wise unidirectional and weaving strategies due to its dependence on the microstructure orientation. Conversely, better isotropic mechanical behavior and less heterogeneous local microstructure were observed in the bidirectional zig–zag strategy.

3.5 Bead modeling

Several studies have been conducted to understand the bead geometry related issues and its effect on deposition accuracy. Researchers have developed bead geometry prediction models to understand path planning and slicing algorithms while appropriating layer thickness and step-over distance.

Suryakumar et al. (2011) studied hybrid layered manufacturing (HLM), which is an arc-welding-based AM process. The correlation between the bead process parameters (such as feed wire diameter, torch speed, feed wire speed, and the distance between consecutive beads) and the single bead geometry for pulsed synergic type in GMAW deposition was established in the study. The study was conducted on beads assuming that bead geometry is a parabola, which was validated with the successive single-bead experiments. A similar study was conducted on multi-bead deposition. The overflowing behavior is shown in Figure 12. In multi-bead deposition, the overlapping models were proposed considering duplication of the material and step-over increment (i.e., the center distance between two beads). It is assumed that between the valley of the overlapping bead and the area of the overlap or the area of the valley, the area of overlap will be the same, and a flat surface will be generated. Depending on the step-over increment, the overlapping volume overflows, thus forming the fillet of concave, flat, or convex shape. The analysis recommended the maximum possible torch speed, minimum possible wire speed, and optimum center distance between two beads (step-over increment) as 0.667w (where w is the width of a bead).

FIGURE 12

Ding et al. (2015a) proposed a tangent overlapping model (TOM) and conducted experiments based on the proposed model. The proposed model was evaluated against the conventional flat-top overlapping model (FOM). The author observed that the conventional model produces unstable deposits with unacceptable variable height in multi-bead deposits. With the help of experimental data, it was demonstrated that the proposed TOM made better approximations than the traditional FOM, which underestimates the optimal step-over distance. For stable overlapping for multi-bead deposits, the proposed TOM proposes the optimum center distance between two beads as 0.728 w.

Step-over distances vary widely across different AM processes. In WAAM, the step-over distance typically ranges from 3 to 8 mm depending on factors such as the material selection and feed wire diameter; in contrast, for powder-based AM step-over, the distance ranges from 0.1 to 1 mm (Ding et al., 2016c).

The wire-arm AM offers a huge option to customize the product on the basis of the requirement of the industry. This section discussed how various process parameters associated with WAAM technology have different effects on the surface finish, surface roughness, mechanical properties, microstructure, and dimensional accuracy. In order to get the superior build quality of the component, it is necessary to optimize these parameters considering the economic constraints.

4 Wear behavior and mechanisms of Ti–6Al–4V

Wear is a very dynamic process that occurs in every material that is in contact with other material during its functionality. Wear is dependent on several factors such as the load, temperature, sliding speed, lubrication, and working environment, among others. Although titanium and its alloys have a low density of the order 4.5 g/cm³, with good mechanical and thermal properties, the application of titanium and its alloys for structural purposes is limited due to their poor wear behavior. Some of the work is to understand the wear behavior and wear-resistant properties of Ti–6Al–4V so that it can be effectively used in biomedical, aerospace, and other applications.

Molinari et al. (1997) carried out experimental work on cold-rolled Ti–6Al–4V alloy against itself on disc-on-disc apparatus to analyze the dry sliding wear mechanism under the influence of varying load and sliding speed. In the experimental conditions, the occurrence of both oxidative wear and delamination wear is reported. The phenomenon of oxidative wear at low sliding speed is observed due to the formation of surface oxide layers pertaining to low activation energy, which was first identified by Wilson et al. (1980). The surface oxide layer provides poor surface protection and enhances the mechanical instability of the subsequent layers caused by the plastic deformation. It is also discussed that higher sliding speed tends to increase the surface temperature, which induces thermal softening behavior of Ti–6Al–4V because of the decreasing yield point and increases the wear. Thus, at higher sliding speed, there is a continuous increase in delamination wear of the material. The point of transition from oxidative wear to delamination wear offers the maximum wear resistance, as shown in Figure 13. It is also reported that the phenomenon of plastic shearing of the substrate will increase with the increase in load that is completely unaffected by the sliding speed.

FIGURE 13

Straffelini and Molinari (1999) studied the wear characteristics of Ti–6Al–4V on a disc made up of Ti–6Al–4V and AISI M2 steel at sliding speed and applied normal load ranging between 0.3 and 0.8 m/s and 50 N–200 N, respectively. Ti–6Al–4V alloy shows distinct mechanisms depending upon the sliding speed and normal load against the counter-face material. Oxidative wear is the dominant mechanism for both counter-faces at lower speeds ranging between 0.3 and 0.5 m/s. Due to AISI M2 steel’s abrasive behavior because of its high hardness, accelerated abrasive wear of Ti–6Al–4V alloy is reported. At higher speeds, AISI M2 steel effectively dissipates heat, reducing the thermal softening, thus decreasing the wear rate. The formation of tribo-oxide layers at lower speeds on the surface of rotating Ti–6Al–4V during out-of-contact period leads to oxidative wear of the stationary Ti–6Al–4V member. This behavior of Ti–6Al–4V is akin to oxidative wear at low sliding speeds in steel (Stachowiak and Batchelor, 2025). In the interactions of Ti–6Al–4V alloy against itself, the result shows that as the sliding speed increases, the wear rate first decreases and reaches a minimum. With further increase in the sliding speed, the wear rate rapidly increases, which is predominantly metallic wear. This marked increase in thermal softening, plastic deformation, and delamination of Ti–6Al–4V is attributed to strain hardening originating due to intermixing with small oxide fragments and lower thermal conductivity of the Ti–6Al–4V alloy. The decrease in oxidative wear at higher sliding speeds can also be attributed to the reduction in oxidative wear intensity due to the decreased formation of oxide layers at high speeds (refer Table 2).

The relation of the volume of oxide layer ( produced, which is linked to the increase in wear rate with oxide growth, can be expressed by the following Equation 2 (Sullivan and Hodgson, 1988):where is the oxidation growth constant, is the time between contacts at any given area (, where is length of wear track, d is the mean diameter of wear track, and is the sliding speed), is the fraction of oxide, is the density of oxide, and is the real contact area. Arrhenius-type dependency on the surface temperature can be seen in the oxidation growth constant. The sliding speed is inversely related to the volume of the formation of the oxide layer. This indicated that at the equilibrium rate of growth, the removal rate of the oxidation layer is the same as the formation of the oxidation layer, which can be attributed to reduction in the oxidation wear and is similar to that of the results of the experiments.

Hsu et al. pointed out that in the low sliding speed and ambient temperature, the wear process predominately depends on the contact surface topology as it impacts the plastic deformation and accumulation of wear debris on the surface. At higher temperatures and sliding speeds, the wear rate becomes the function of the contact surface temperature (Hsu et al., 1997).

Alam and Haseeb (2002) analyzed the wear response of Ti–6Al–4V and Ti–24Al–11Nb alloys on pin-on-disc apparatus by sliding it against the hardened steel as the counter body in the dry condition. It was observed that the wear resistance of Ti–6Al–4V was 48 times lower than that of Ti–24A–11Nb under normal load of 45 N. At higher sliding speed and under high load, the reason for low wear resistance is the severe delamination of Ti–6Al–4V, which is in-line with the study conducted by Molinari et al. (1997). In addition, sliding marks with deep grooves depicted the plowing, plastic deformation, and smearing in both alloys, but the Ti–24Al–11Nb alloy has shown resistance to plastic deformation compared to Ti–6Al–4V. Oxide debris has been seen in both alloys, which were very brittle in the experimental conditions, which is consistent with the study conducted by Molinari et al. (1997). This behavior of titanium alloy can be explained by the study by Welsch and Kahveci (1988). Their analysis found that in Ti–6Al–4V, formation of oxide layers are made up of TiO₂ (titanium dioxide) and Al₂O₃ (aluminum oxide). Both these oxides are mutually insoluble in nature, which leads to inhomogeneous and porous oxide scale formation, thus offering bad protection against further oxidation and leading to severe wear. However, in Ti–24Al–11Nb alloy, a dense scale of Ti–Nb-oxide is formed, which makes the oxide layer more stable and protective against wear or plastic deformation. Thus, it shows that the addition of larger aluminum concentrations and niobium leads to lower wear rate of Ti–24Al–11Nb.

Kaundal et al. (2026) analyzed the wear behavior of Ti–6Al–4V fabricated using WAAM. In their study, the sample was prepared by using a wire of 1.2-mm diameter and discharge of shielding gas maintained at 20 L/min while varying the process parameters such as the welding speed, current, and voltage, which generate different heat input. The wear test was performed using Taguchi L9 array and varying two input variables for the wear testing rig, namely, applied load and sliding speed, and one variable was used in the fabrication of the sample, i.e., heat input. Higher heat input leads to the formation of fine α′-martensitic phase, enhancing intermolecular bonding between the layers and promoting improved hardness and abrasive wear resistance, which satisfies the Archard’s formulae expressed in Equation 3 (Hanief and Charoo, 2021; Rai et al., 1999):where V (mm³) represents the wear volume, K denotes the dimensionless constant known as wear coefficient or Archard’s wear coefficient, F (N) denotes the applied normal load, L (mm) represents the sliding distance, and H (HV) represents hardness of the softer material in the contact pair. During the run-in period, two-body abrasion wear was profound, pertaining to the initial surface roughness causing material loss. After the initial wear, wear of the sample stabilized after the removal of irregularities on the surface. Before transitioning into the constant wear rate phase, the wear rate gradually increases due to abrasive wear linked to asperities created during the run-in period. Reduced wear rate is mainly attributed to high heat input during fabrication, which leads to increase in surface hardness and fine microstructure. Tribo-oxide layer formation enhances the wear rate at lower sliding speed as heat dissipation is high, while higher sliding speeds lead to rapturing of the protective tribo-oxide layer, thus promoting high wear rate.

A comparison study of the tribological characteristics of Ti–6Al–4V fabricated using forging, laser directed energy deposition (L-DED), and L-PBF was conducted by Kang et al. (2022). Non-equilibrium features were observed along with lath/acicular α-Ti phase in the samples fabricated using L-DED and L-PBF samples, which is caused by the higher cooling rate in L-DED and L-PBF methods. This microstructure behavior agrees with the existing literature (Baharzadeh et al., 2025). The L-DED- and L-PBF-processed samples show higher wear rates than the wrought sample. In wrought samples and L-DED samples, wear mechanism appears to be combination of abrasive wear, adhesive wear, and oxidation wear. Meanwhile, the L-PBF samples exhibit only abrasive wear and adhesive wear mechanisms. However, the microhardness of L-DED samples is higher than that of L-PBF samples, followed by the wrought samples, which is mainly due to the presence of high hardness β phase. Appearance of higher intensity of β phase is observed in L-DED samples with respect to L-PBF samples. In contrast, nanoindentation hardness of the wrought samples is greater than that of the L-PBF and L-DED samples.

Wear characteristics of the SS316L specimen fabricated using CMT-based WAAM shows similar trends of wear rate. High applied load and increased heat input enhances the wear rate, while the coefficient of friction (COF) increases with slight increase in applied load across various heat inputs. In contrast to findings by Kaundal et al. (2026),Lone et al. (2025) concluded that the hardness of the sample decreased with increase in the heat input. For SS3165L fabricated using WAAM, an earlier tribological study shows that the highest factor contributing to wear is abrasive wear, along with adhesive wear, fatigue wear, and oxidative wear (Kumar et al., 2024).

Dong et al. (1997) utilized thermal oxidation (TO) treatments on Ti–6Al–4V at temperatures up to 850 °C and conducted a series of experiments to explore the effects of TO on the friction and wear characteristics of titanium alloy. This study shows that the careful TO treatment of Ti alloy will enhance the formation of a dense and smooth oxide layer with improved adhesion to the surface and significantly reduced porosity. The results show that the coefficient of friction was reduced from 0.7 to 0.3 post the TO treatment of Ti–6Al–4V. The improved hardness of Ti–6Al–4V can be imputed to the dense oxide on the surface. The wear-resistance performance of TO-treated Ti–6Al–4V is not only enhanced from that of untreated Ti–6Al–4V but also increased by more than 10 times of that of hardened 704M40 steel. It further elaborates the reduction of E:H by 50%, which increases the wear resistance of the alloy under lubricated conditions by limiting the plastic deformation mainly due to the presence of the oxide sheet on the surface. The oxide layer on Ti–6Al–4V post the TO treatment of Ti alloy led to outperforming the electrochemical corrosion resistance shown by untreated Ti–6Al–4V in a 3% NaCl solution. When the sliding speed is varied, Li et al. observed that the wear behavior of Ti–6Al–4V is also influenced. At lower speed, the wear mechanism is a combination of delamination and oxidation wear, but metallic delamination wear mechanism is seen at higher speeds, such as at 2.68 m/s, and with further increase in speed of 4 m/s, oxidative wear mechanism is observed. This change in wear behavior at higher speeds is attributed to the formation and collapse of oxide layers (Li et al., 2015).

The friction and wear characteristics are also dependent on the interface temperature of the surfaces of the tribo-pairs. Increase in the interface temperature softens the surfaces and changes the microstructure, which can cause changes such as plastic deformation, phase transformation, and recrystallization, among others. Qiu et al. studied the friction and wear behavior of Ti–6Al–4V pin specimen against a GCr15 steel disc in dry sliding and high-speed pin-on-disc apparatus and discussed the effect of increasing interface temperature. The formation of TiO, TiO₂, and V₂O₃ is established from analyzing the debris of worn-out surfaces using XRD. At lower speed, TiO was mainly present, and TiO₂ and V₂O₃ oxides were mainly predominant at higher speeds. It is observed that the maximum contact temperature is found corresponding to the maximum contact pressures. The formation of the oxide layer increases with the increase in the friction temperature. With the increase in temperature up to 800 ^°C, the coefficient of friction increases, and then, the co-efficient of friction decreases suddenly. However, with the increase in temperature, no decrease in wear was noted, which was due to the phase transformation occurring from the β-phase of body-centered cubic (BCC) to α-phase of hexagonal-close-packed (HCP) and thermal softening of the material. The results depict changes in the Ti–6Al–4V microstructure due to the variation of friction temperatures and its effects on the tribological behavior. Furthermore, the poor wear resistance of Ti–6Al–4V alloy stemmed from the development of a loosened and fragile oxide layer (Ming et al., 2006). Magaziner et al. analyzed the wear characteristics of Ti–6Al–4V subjected to reciprocating sliding on dry and lubricated surface conditions. They reported that adhesion wear followed by abrasive wear is a prominent wear mechanism prevailing under the loading conditions; however, the effect of tribo-oxides on the wear behavior was not taken into consideration (Magaziner et al., 2009).

Mao et al. (2013) elucidated that Ti–6Al–4V does not possess poor wear resistance in every condition. The experiment was conducted on pin-on-disk apparatus wherein Ti–6Al–4V (40HRC) was used as a pin against GCr15 steel (50HRC). The experimental data depict that tribo-oxides and thermal oxides play different roles under various conditions to influence the wear behavior of Ti–6Al–4V. It was clearly depicted in the experiments that at temperatures between 400 °C and 500 °C, Ti–6Al–4V showed higher wear resistance when compared to the wear resistance at 25 °C–200 °C, as shown in Figure 14. Mao et al. attributed this behavior to the appearance of the tribo-oxide layers, which in turn improves the hardness of tribo-layers at higher temperatures, than the substrate is attributed to better wear behavior of Ti–6Al–4V. This result is consistent with the analysis of Dong et al. (1997). At temperatures ranging between 400 °C and 500 °C at lower loading conditions, adhesive and abrasive wear prevailed. Meanwhile, at normal load 100 N–200 N, the tribo-layer provided higher wear resistance to Ti–6Al–4V than the substrate, and oxidative wear dominated the delamination wear.

FIGURE 14

Mao et al., 2013).

The ball burnishing process was used by Revankar et al. (2017) to improve the wear behavior of Ti–6Al–4V alloy under dry conditions, with constant sliding speed of 2.68 m/s and normal applied load of 10 N for 180 s on pin-on-disc tribometer. It is also reported that ball burnishing helps in improving surface roughness and decreasing the coefficient of friction and specific wear rate. Similar trends were also pointed out by Li et al. (2015) during their analysis of Ti–6Al–4V under varying sliding speeds.

Balla et al. (2014) studied the wear behavior using the ball-on-disk test rig, wherein the hardened chrome steel is used as a ball against a laser-melted and untreated Ti–6Al–4V disc that is is used as counterpart for the experiment. It was found that the hardness of the laser-melted Ti–6Al–4V alloy specimens is higher than that of the untreated specimens. This is attributed to the decrease of the β-phase and the formation of acicular in the laser-melted sample (Biswas et al., 2007). The experimental results depict the lower wear resistance of untreated specimens compared to that of the laser-treated specimens. The variation of wear resistance in the treated specimens has been observed with respect to changes in laser power, and the number of passes is influenced by the increase in hardness and formation of passive oxide layers.

Joshy and Kuriachen (2025) studied the wear characteristics of Ti–6Al–4V at elevated temperature fabricated using L-PBF technology. The study was conducted for two build directions (vertical build direction and horizontal build direction), and different heat treatments were conducted on the Ti–6Al–4V specimen, such as annealing at 700 ^°C (AT), solution heat treatment at 1,090 ^°C for 1 h (SHT), and cryogenic soaking to −196 ^°C for 48 h. The wear experiments of the Ti–6Al–4V specimen against EN31 hardened steel as a counter-body were conducted on a pin-on-disc tribometer. Up to 0.9 m/s, the specific wear rate decreases with increase in the sliding speed, and beyond that, the specific wear rate increases with increase in the sliding speed. Opposite trends were observed with the coefficient of friction (CoF), where the coefficient of friction increases till 0.9 m/s, then obtains the lowest CoF at 1.5 m/s, and is followed by stabilizing after covering the sliding distance of 200 m. With an increase in sliding speed from 0.6 m/s to 0.9 m/s, the wear mechanism shifts from oxidative and abrasive wear to predominantly oxidative wear. Abrasion and plowing wear can be observed at 1.2 m/s, and oxidative wear is suppressed. At 1.5 m/s, due to the increase in sliding peed, the flash temperature also increases, which enhances the frictional heating of the surfaces and melting of the oxide layers accumulated on the surface, which causes severe oxidative wear in the specimen. Increase in the flash temperature aids the plastic deformation of asperities at higher sliding velocities. However, further increase in sliding speed to 1.8 m/s causes mainly delamination wear along with abrasive wear, which can be attributed to thermal softening of the material. It is also noted that the co-efficient of friction for constant load and constant sliding speed conditions generally decreases with increase in temperature up to 500 ^°C and then markedly increases at 600 ^°C, which largely indicates the third-body interactions and the presence of wear debris particles on the surface. In contrast, the wear rates first decrease from room temperature to 200 ^°C, then increase up to 500 ^°C, and then markedly decrease by more than 50% at 600 ^°C. Furthermore, the vertically built orientations specimens show higher wear rates than the horizontally built specimens at 500 third-body interactions. Meanwhile, the co-efficient of friction of the vertically built specimens treated with annealing and non-heat-treated is slightly lower than that of the horizontally built specimens. However, the specimens that are built horizontally and have undergone solution heat treatment and cryogenic soaking showed marked increase in the co-efficient of friction as that of the vertically built specimens that underwent the same treatment.

The friction and wear characteristics of electron beam melting (EBM)-fabricated Ti–6Al–4V alloy was analyzed for abrasion wear on a dry sand–rubber wheel set-up by Herrera et al. (2023). Compared to conventionally manufactured Ti–6Al–4V alloy and under the same experimental conditions, poor abrasive wear resistance behavior of the specimens fabricated using EBM was noted. However, there is no significant difference in the Vickers micro-hardness, and macro-hardness was observed in the specimen fabricated by casting and EBM, respectively.

Huang et al. (2021) treated the Ti–6Al–4V alloy fabricated using SLM in liquid nitrogen at −196 ^°C for 2 h (CT02) and 72 h (CT72) to study the hardness and tribological behavior of the alloy. The wear test was conducted on a ball-on-disc tribometer having Al₂O₃ as the ball and against a Ti–6Al–4V disc. The results show that cryogenic treatment of the Ti–6Al–4V has very little effect on the hardness of the alloy compared to that of the untreated alloy. The Ti–6Al–4V alloy treated with cryogenic fluid for 72 h shows less co-efficient of friction as the spalling pits density and prevention of microcracks due to the formation α + β-phase decomposition. However, untreated and CT02 shows high co-efficient of friction, which implies the presence of spalling pits and wear debris on the wear surface.

The wear behavior of the material prominently depends on the performance of the surface layer; the engineered or modified surface can provide better wear performance and enhance the longevity of the metal. In bio-medical applications, Ti–6Al–4V is widely used as a load-bearing and non-load-bearing implant. Granchi et al. reported that one of main reasons for the implant failure is due to osteolysis, and it is derived from the wear debris of the implant, which leads to abrasive wear (Granchi et al., 2006). The surface engineering of Ti–6Al–4V may enhance the wear-resistant properties to prevent or minimize the implant failure due to osteolysis and metal sensitivity caused by electrochemical dissolution and wear of Ti–6Al–4V. The surface modification technology has been widely analyzed, including thermal oxidation (Dong et al., 1997), ion implantation (Li et al., 2021), plasma nitriding (Balasubramanian et al., 2021), laser nitriding (Morton et al., 2020; Li et al., 2020), laser alloying (Zhao et al., 2023; Zhao et al., 2021), electron beam nitriding (Li et al., 2023), diamond-like carbon coating (Ghio et al., 2023), TiN/CrN coating (Hsu et al., 2023), ball burnishing process (Revankar et al., 2017), plasma electrolytic oxidation (PEO) process (Garcia-Cabezón et al., 2021), and laser processed compositionally gradient coatings (Lu et al., 2018). Among the various methods to protect Ti–6Al–4V from wear surface modification techniques, thermal oxidation has been proven to be the most effective method to significantly improve wear resistance properties.

Pramanik et al. (2013) analyzed the tool wear during the milling operation performed on WAAM-fabricated Ti–6Al–4V. Flank wear, notch wear, and scour wear were noticed in the tool, specifically on the cutting edge. The surface waviness affects the forces on the tool, which vary the cutting force in a tool radial direction, and the resulting average force is small. The smaller resulting average force reduces the tool wear when milling the wavy surface of the titanium alloy and has no significant impact on the final surface quality of the fabricated component. This shows that the surface finish of the fabricated component influences the tool wear.

In addition, various studies indicated that post-processing methods, such as heat treatment and rolling, among others, may enhance the mechanical properties, corrosion resistance, and microstructural modifications. Due to the high rate of cooling for SLM processes, the microstructure of as-printed Ti–6Al–4V specimen exhibits long columnar β grains consisting of fine acicular α′ martensite, which leads to reduced ductility and toughness of sample (Thijs et al., 2010; Song et al., 2012). The optimum combination of lamellar and globular α-phase grains in the microstructure is required fFor achieving excellent fatigue properties (Lütjering and Williams, 2003). In study conducted by Pan et al., Figure 15b represents equiaxed crystals and basket-weave microstructures in micro-rolled Ti–6Al–4V alloys fabricated using plasma-arc additive manufacturing (PAAM), yielding higher tensile strength and elongation than that of unrolled specimens, while Figure 15a represents the coarse grain structure (Pan et al., 2022). To induce globular grains, the plastic deformation route may be followed; however, plastic deformation after fabricating near-net shaped component negates the purpose of using AM technology. According to Jiang et al. (2018), with the thermos-mechanical methods of plastic deformation and aging treatment, the alloy has superior mechanical properties and ultrafine duplex microstructure. This ultrafine equiaxed microstructure is derived from combined effect of globularization of deformed α-phase and decomposition of the martensitic phase.

FIGURE 15

However, post-processing heat treatment does not always lead to enhanced mechanical properties or better microstructure behavior. Transforming lath/acicular α-phase grains into equiaxed α-phase grains within basket-weave microstructure using two-phased field annealing treatment leads to coarsening of grains, which leads to decreased strength while improving its ductility significantly (Sabban et al., 2019). On-step annealing at 800 °C temperature leads to transformation of the α + β microstructure from α′ martensite, which leads to enhancement of ductility by 18% and a marked reduction in yield strength (Cao et al., 2018).

On the other hand, there are several limitations of WAAM-based fabrication of Ti–6Al–4V alloy. High heat input and higher cooling rates lead to porosity, increased concentration of dislocations, deformations due to residual thermal stresses, and poor wear performance. Even though WAAM-fabricated components are near-net-shaped, they do require surface finishing to achieve the desired shape, which leads to the loss of valuable material and time and additional equipment/machinery and manpower. Higher cooling rates lead to a metastable microstructure, which is very frequently observed in parts fabricated using metal AM technologies. Fabricating the Ti–6Al–4V part using WAAM and achieve the desired porosity of bone structure or implants for biomedical applications still remains a huge challenge for WAAM; meanwhile, such applications can be achieved using L-PBF and EBM (Haase et al., 2023). Apart from being expensive, Ti–6Al–4V is also under scrutiny for its applicability in biomedical applications pertaining to chronic health issues posed by it. Degradation of Ti–6Al–4V releases ions of Al and V, which is linked to nerve damage, Parkinson’s disease, and Alzheimer’s disease (Toledano-Serrabona et al., 2021; Yick et al., 2023).

Since the last 30 years, a lot of research work has been reported on understanding the wear mechanism of Ti–6Al–4V alloy against various metals fabricated or surface engineered through different technologies. The most common reported wear mechanisms involved in the wear of Ti–6Al–4V alloy are oxidative wear, delamination wear, and abrasive wear (refer Table 3), which are dependent on various factors, such as sliding speed, normal applied load, temperatures, and surface roughness, among others. In addition, various specific surface engineering techniques are used to improve the wear-resistant behavior of Ti–6Al–4V.

5 Conclusion and future scope

In this study, the WAAM technology was analyzed due to its rising demand in both academia and industry as it offers lower economic cost, unparalleled freedom of design, fabrication of complex geometry, compatibility with a wide range of material, and the ability to manufacture large components, among others. Various process parameters of WAAM were discussed in order to achieve good surface finish, improved build quality, and superior mechanical properties, among others, so that industry can adopt the technology for a wider range of operations.

The current study concludes that WAAM process can be used in manufacturing components with a wide range of materials that have properties that are equivalent or in some cases better than the conventionally manufactured components. Moreover, mechanical properties, corrosion resistance, and wear resistance can be further enhanced by using post-processing methods. In addition, fabrication of functionally graded material can also be achieved using process optimization and controlling the process parameters. However, a comprehensive approach for developing the WAAM as a single-stop technological solution from product designing and planning to the final product is still absent. The WAAM technologies will become prevalent in manufacturing processes when the cost of fabrication of the components will subsequently reduce to reach the point of breaking even or even expected to generate more profit margins in the larger production systems.

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