A Review of the Recent Developments and Challenges in Wire Arc Additive Manufacturing (WAAM) Process

: Wire arc additive manufacturing (WAAM) is an emerging and promising technology for producing medium-to-large-scale metallic components/structures for different industries, i


Introduction
Additive manufacturing (AM), commonly known as 3D printing [1], is the technology used to manufacture parts and objects through layer-by-layer deposition of materials. AM is considered one of the most innovative manufacturing technologies of the 21st century. It integrates the leading world technologies, e.g., digital modeling, machining, material processing, and metallurgical engineering. According to ISO/ASTM 529000:2021, AM processes can be divided into the following categories, as shown in Figure 1. AM offers significant benefits in reducing cycle time and saving raw materials by depositing material layer-by-layer under computer control. Especially, it can generate complicated geometrical features in the parts with high precision and lower dependency on cutting tools, dies, fixtures, and other complex machining processes [2,3].
Powder bed fusion (PBF) and directed energy deposition (DED) have recently attracted more attention among all available AM technologies. Although PBF can produce parts with complex geometries and high accuracy, it also has some drawbacks, including high costs (operational cost, equipment cost, material cost) and the use of metal powder, which can be very unhealthy for humans. Furthermore, a controlled environment is needed where the PBF process takes place to prevent people from inhaling the toxic powder. DED is a promising method for AM of metals to overcome the aforementioned issues. DED is an AM methodology in which focused thermal energy is utilized to melt the material as it is deposited. In DED, the wire or powder material is used as feedstock and a heat source (plasma, electric arc, laser, or electron beam) for material deposition on a substrate surface. Although there is a lower resolution in DED compared to PBF, the material deposition rate is much higher in DED. However, some studies of AM technologies also reported a few drawbacks to DED-based AM methods. These drawbacks include the utilization of large volumes of inert gases in laser-based DED, the high vacuum required in electronbased DED, etc. Furthermore, powder-based DED has lower material deposition efficiency than wire-based DED, as a minor portion of the total powder will not be melted. That is why if the wire is used as a feedstock in DED instead of metal powder, it omits the need for recycling and powder protection systems, and the cost of raw materials is reduced significantly. A comparison of different AM processes is presented below in Table 1. Table 1. Comparison of AM processes [4][5][6][7][8]. The wire arc additive manufacturing (WAAM) process has become a renowned AM process in the recent decade, and it is one of the DED technologies. WAAM has been derived from classical arc welding technology [9][10][11]. In WAAM process, the wire is melted through heating by using an electric arc and then transferred to the molten metal pool. Later, it solidifies at the melt pool boundary and produces the part layer-by-layer [12]. A typical WAAM process is shown in Figure 2 below. WAAM process has the potential to develop structures that can be extended to tens of meters and can fabricate fully dense parts with large dimensions, along with higher manufacturing efficiency and low production costs. It is also suitable for repairing and maintaining damaged parts and components. Generally, the direct cost of metal wire used in WAAM process is roughly 10% of the metal powder having the same weight [12]. A comparison of the material deposition rate in WAAM process with other AM processes is presented below in Table 2. Table 2. Comparison of material deposition rates of major AM processes for steels [13,14].

Type of AM Process SLM Process EBM Process LMD Process WAAM Process
Material deposition rate (g/h) 40-100 100-300 150-2400 500-10,000 WAAM process can be divided into three major categories depending on the type of arc-based welding technology used to melt the feed wire and generate a 3D object/structure layer by layer [15][16][17][18]. These three different types include gas tungsten arc welding (GTAW), plasma arc welding (PAW), and gas metal arc welding (GMAW), as shown in Figure 3. In GTAW-based WAAM, an electric arc is generated between a non-consumable tungsten electrode and the substrate material for heat generation, as shown in Figure 4a below. A filler wire is fed from the side, which melts due to the arc's heat and deposits on the substrate's surface to get the desired geometry (width, height, cross-section, etc.) and mechanical properties. The PAW process is similar to GTAW in that a non-consumable electrode is used in both methods. However, PAW differs from GTAW in welding torch construction and is more efficient than GTAW because the plasma arc is constricted in a nozzle. In PAW, an arc is generated between the tungsten electrode and the water-cooled nozzle, as shown below in Figure 4b. The inert gas flowing through the arcing zone of the torch gets ionized, i.e., the gas turns into a plasma state. This plasma jet is then transferred through a small orifice to the substrate. This plasma jet turns the filler wire into molten form due to the high heat. In addition, shielding gas is used to protect the molten pool from contamination. But the initial cost of PAW is very high compared to GMAW and GTAW. GMAW is widely used for WAAM process nowadays compared to other WAAM processes due to its higher material deposition rate, high material utilization, and shorter lead time. In GMAW, the wire works as a consumable electrode, as shown in Figure 4c below. The arc generated between the wire and the workpiece melts the wire material and deposits the molten material on the substrate's surface. GMAW consists of metal inert gas (MIG) and metal active gas (MAG). In MIG, a non-reactive (inert) gas, i.e., helium or argon, is used. MIG is mainly used for non-ferrous metals. MAG uses carbon dioxide or a mixture of inert gases, i.e., a trimix of Ar + CO 2 + O 2, for shielding purposes. MAG is primarily used for ferrous metals. The material deposition rate of the GMAW process is higher as compared to other WAAM processes, and this comparison is presented below in Table 3. Table 3. Comparison of the material deposition rates of different WAAM processes [3,19,20].

Type of AM Process GTAW PAW GMAW
Material deposition rate (kg/h) 1-2 2-4 7.8 GMAW has four metal transfer modes: short-circuiting, globular, spray, and pulsed spray. Each metal transfer method has distinctive characteristics related to the welding current, equipment, and electrode type [21]. In the conventional GMAW welding process, during standard short-circuiting transfer (dip transfer) mode, when the welding electrode comes in contact with molten pool and immerses in it, short-circuiting occurs, and an enormous current flows through it. Thus, it results in the abrupt melting of the electrode wire, so droplet transfer happens via the surface tension of the molten metal pool. The arc re-ignites, and the hot process after the completion of the metal transfer process continues. A significant improvement in WAAM was achieved when the CMT (cold metal transfer) welding process was introduced. CMT is an improved version of GMAW and is based on the short-circuit transfer mode. The Austrian company "Fronius" developed the CMT method, a renowned method for generating meager heat input and high arc stability using an innovative wire feeding setup with a digital control system [22]. When the wire electrode tip comes in contact with the molten metal pool, the "robacter drive" servomotor reverses the filler wire via digital process control. Hence, it results in causing the wire to withdraw, producing droplet cutting, while the welding current quickly decreases to a near-zero value [23]. Thus, it causes lower heat input and less spatter during the material deposition. CMT can be divided into four types, i.e., CMT-conventional, CMT-advanced, CMT-pulse, and CMT-advanced pulse. Each mode has its unique features and characteristics.

General Steps Involved in WAAM Process
The main steps involved in WAAM process, as reported by literature [2,24], are presented below in Figure 5.

Introduction to Different WAAM Process Parameters
The process parameters used in WAAM process vary according to the type of WAAM process and the wire material. In WAAM process, material deposition is based on melting a wire using an arc as a heat source for fabricating or repairing a component. During material deposition, the inter-pass temperature between layers increases due to the heat accumulation as further layers are added. The process parameters are crucial for producing a good-quality part, i.e., a defect-free structure with excellent mechanical properties. These process parameters include welding current, voltage, torch travel speed, wire feed speed, interlayer temperature, interlayer dwell time, shielding gas composition and flow rate, torch and substrate angle, electrode diameter, heat input, and contact tip-to-work distance (CTWD). These parameters affect the properties of WAAM product differently, and a suitable combination of these parameters is needed for specific objectives, i.e., high mechanical strength, lower surface roughness, etc.

Wire Feed Speed
Yildiz et al. [25] studied the effect of the WFS (wire feed speed) to TS (torch speed) ratio on the weld bead geometry and mechanical properties. This study showed that the WFS/TS ratio is the most crucial parameter in WAAM process for controlling heat input. Heat input is linearly correlated with the characteristic weld bead geometry, including the weld bead height, width, penetration, penetration area, and reinforcement area, for the analyzed parameter range. An inverse relationship was observed between heat input and hardness in single weld beads. Wang et al. [26] studied the effect of wire feed speed and other parameters on the weld bead geometry. This study showed that as the wire feed speed increases in GTAW-based WAAM of 4043 Al-alloy, the weld bead height increases while its width decreases. It was also reported that the arc energy required to melt the wire increases as the wire feed speed increases. Thus, it may lead to a lack of fusion and a decrease in the width of the bead. Different features of a typical weld bead are presented below in Figure 6.

Torch Travel Speed
Dinovitzer et al. [27] examined the effect of torch travel speed and other parameters on microstructure and bead geometry in WAAM of Hastelloy X alloy. In this study, statistical tools, i.e., Taguchi and ANOVA (analysis of variance), were used to determine the effect of process parameters (wire feed, travel speed, current, and argon flow rate) on the bead's microstructure and geometry (bead shape and size, roughness, melt through depth, and oxidation level). This research revealed that current and travel speed significantly affect the weld bead's microstructure and geometry. It was revealed that torch travel speed is inversely related to the heat input in WAAM process. In WAAM process, WAAM component's microstructure depends on the heat input; hence, the part's microstructure also relies on the torch's travel speed. Furthermore, the effect of torch travel speed on the width of the weld bead was significant compared to its impact on the bead height because the travel speed affects the angle of contact (wetting angle).

Heat Input
Heat input is very critical in WAAM process. Multiple fusion and solidification cycles produce irregular heat propagation in the as-manufactured part. This non-uniform heat input accumulation and distribution defines the part's formation (shape and volume of the bead), mechanical properties, and microstructure obtained via WAAM process [28,29]. Heat input in WAAM process affects internal defect formation, phase transformation, residual stresses, and distortion [30]. Heat input can be calculated as follows from Equation (1) [31]: where: Heat input control is vital for alloys subjected to liquation cracking. As with increasing heat input, the quantity of liquated material increases, hence the chances of cracking. As WAAM process involves partial melting of the previous layer/bead, improper control of heat input will assist in partial liquation at the fusion zone [32].
Moreover, an increase in heat input will affect the cooling rates of WAAM components and, hence, the part's microstructure. As microstructure affects the part's mechanical properties, heat input indirectly affects the mechanical properties of WAAM component. Sometimes, residual stresses also arise in the part produced through WAAM process due to high heat input; in fact, that part experiences thermal stress with the deposition of each weld bead. Thus, appropriate welding technology, interlayer dwell time, and path strategy are needed to mitigate this effect.

Substrate Temperature
The substrate's pre-heat temperature defines the shape of the deposited material. As the pre-heat temperature increases, the deposited bead's width increases, whereas its height decreases [32,33]. Thus, pre-heating the substrate leads to smooth material deposition and, in return, also improves the surface deformation of the metal deposited.

Interlayer Temperature
The inter-layer temperature is crucial for obtaining the desired shape of the end product in the multi-layer WAAM process. Proper inter-layer temperature is essential for obtaining a smooth and stable surface while depositing material. Hence, properly controlling the workpiece temperature will improve its surface finish, reduce residual stress, and improve its mechanical properties [34,35]. Therefore, adequate interlayer cooling time (dwell time) is needed to get the desired interlayer temperature. This time should be chosen based on the trade-off between desired product features and production time. The idle waiting time will increase for lower inter-layer temperatures, resulting in a loss of useful production time [36].

Electrode Extension or Electrode Stick-Out
Electrode extension, or electrode stick-out, is the distance between the end part of the contact tip and the wire end, as shown in Figure 7 below. Electrical resistance increases with an increase in electrode extension. Thus, it influences the melting of the wire, and in return, it affects the weld bead geometry. Extended electrode extension causes the pre-heating of the electrode wire. If the electrode extension is insufficient, the electrode will not get sufficiently pre-heated. Increasing the electrode extension while keeping the arc length constant enlarges the electrical resistance. More material has to be deposited due to an increment in the electrode extension but with low heat energy provided by the arc produced due to a rise in electrical resistance. In return, it will result in slight penetration and steep-crowned weld bead geometry.

Arc Length
The arc length is the distance between the welding wire tip and the substrate, as shown in Figure 7 above. The roughness of the deposited layer can be reduced by efficient control of the arc length [33].

Defects in WAAM Process
Common defects in WAAM process include residual stresses and distortion, cracks, porosity, and humping. The process parameters that can cause such defects include wire feed speed to torch speed ratio, heat input, contact tip-to-work distance, and gas flow rate. Since in GMAW-based WAAM, the electric current is acting directly on the feedstock [37], it has more chances of problems, i.e., spattering, porosity, and excessive heating, than in GTAW-based and PAW-based WAAM.

Distortion and Residual Stresses
WAAM technology is still in the development phase, i.e., technological and process improvement. Residual stresses sometimes arise in parts produced via WAAM process due to high heat input, as shown in Figure 8. Since the component is generated via layer-bylayer deposition, WAAM generated component experiences alternating heating and cooling cycles, resulting in residual stresses and strain. If the residual stresses are high enough, they significantly affect the mechanical properties of the part produced via WAAM process. It distorts the part if the residual stresses are lower than the ultimate tensile strength (UTS) but higher than the part's yield strength (YS). If it exceeds the UTS of the part, then cracks occur in the part [38]. This problem can be resolved by selecting appropriate process parameters.

Porosity
One of the most common defects in WAAM aluminium alloys is porosity, as shown in Figure 9. This defect reduces the density of the part produced, and as a result, it affects the mechanical strength of the part. This defect occurs mainly due to the moisture, grease, etc., on the wire surface [31,40] or substrate. It may also arise from the entrapment of shielding or other gases in the surrounding environment. Thus, to avoid such problems, clean and smooth wire, appropriate shielding gas, a suitable mixture of gases, and a suitable flow rate of shielding gas should be used [41,42]. Interlayer cold working can also reduce this defect in the case of aluminium alloys [43]. This defect can also be reduced by increasing the cooling time of the melt pool as much as possible so that the entrapped gases get enough time to escape [44].

Humping Defect
Humping is a common defect in WAAM process, mainly due to inappropriate WFS/TS ratio use. It affects further material deposition in the process. This defect appears in the form of humps and valleys, as presented below in Figure 10. It can also be defined as the sequence of repeated undulations of the weld bead [46]. This problem can be resolved by using an appropriate WFS/TS ratio [47]. Gratzke et al. [48] used Rayleigh's instability theory and presented a theoretical concept for avoiding humping defects. In this study, the width-to-length ratio of the molten weld pool was the dominant quantity for controlling humping defects. In order to avoid humping, the width-to-length ratio of the molten weld pool has to be above a certain critical value.

Material Overflow
Material overflow occurs in WAAM products, sometimes due to excessive heat accumulation. This defect occurs as a consequence of the high wettability of the molten weld pool when the interlayer dwell time (5 s) is too short, as shown in Figure 11a below. It may result in collapse, enhancing the machining allowance [49]. This problem can be avoided by using an appropriate interlayer dwell time (120 s). Thus, the next layer should be deposited when the previous layer reaches the proper temperature, i.e., by utilizing the reasonable dwell time, as shown in Figure 11b.

Inhomogeneous Microstructure
WAAM products usually experience complex thermal cycles. The part's microstructure, produced via WAAM process, undergoes significant changes due to different heating and cooling rates. The heat input during WAAM process significantly influences nucleation, thermal gradient, and grain growth rate [50]. This complex thermal history during material deposition results in a blended microstructure, i.e., coarse and fine. Thus, the process parameters directly impact the microstructure and strength due to multiple welding layers and related temperature cycles. Therefore, it is desirable to have a fine microstructure since it contributes to the strength of WAAM part/component. Different in-process and post-process operations for removing anisotropy in the part's microstructure are conducted depending on the applications. Different microstructures were obtained in WAAM of Ti6Al4V using different processing conditions, as shown below in Figure 12. It can be seen that the grain size and microstructure are not notably different at low interpass temperatures in regions b and c, which could be possible due to similar heat dissipation behavior. However, the microstructure becomes more uniform in region c, and the grain size increases at higher interpass temperatures due to slow cooling rates. Furthermore, when CO 2 cooling was used, a more uniform microstructure was obtained due to high cooling rates in regions b and c. The microstructure consists of an acicular α phase along with refined grains.

Controlling Mechanical Properties of WAAM Products
The mechanical properties of the WAAM generated component depend on the microstructure, which is reliant on the heat input. Thus, controlling the part's microstructure via heat input control is a critical challenge. Furthermore, the heat input in the process will vary when different metal arc modes are used while keeping the wire feed constant [52]. Stinson et al. [53] compared the weld bead properties and geometries obtained via two different GMAW-based processes, i.e., MIG and CMT. A mild steel welding wire, ER70S-6, was used for depositing metal on EN3B substrates. This study showed that heat input significantly affects the hardness of the deposited material, as shown below in Figure 13. A decrease in heat input may be favorable for the mechanical properties and microstructure of WAAM component; however, a trade-off between heat input and production time is crucial [30]. Heat input should be selected to produce WAAM components with the desired microstructure without affecting the production time. Kumar & Anandakrishnan [54] used the Taguchi method to investigate the effect of process parameters' on the layer geometry of Inconel 825 deposited via MIG welding. Process input parameters, i.e., wire feed, welding speed, and voltage, were selected to analyze their effect on the weld bead s width and to find the best parameters for obtaining the desired results. After statistical analysis, confirmation tests were conducted for the bead s width using the parameter combinations found via statistical analysis. The error in the predicted values and experimental results was around 5%. Sarathchandra et al. [55] evaluated the process parameter's effect on bead characteristics in WAAM of SS304. This study used ANOVA and RSM (response surface methodology) for parameter estimation. Multiple regression models were generated for relating bead quality with process parameters and showed that the optimum process parameters for the desired bead quality are; current = 116.33 A, weld speed = 0.46 m/min, and standoff distance = 5.11 mm. Critical process parameters, their impact on quality features, and their required level in the GMAW-based WAAM process are presented below in Table 4.

Path Strategy Single Bead Multiple Passes
Lehmann et al. [56] studied two tool paths for material deposition, i.e., unidirectional and bidirectional material deposition strategies, as shown in Figure 14a,b, respectively. The welding torch nozzle traverses these two paths while fabricating a single-track wall, where the electric arc is turned on or off. The two material deposition strategies differ in that the material is deposited only from the start to the end position in the unidirectional material deposition strategy. In contrast, during the bi-directional material deposition strategy, the material layer was deposited when the torch moved in each direction. This research showed that when fabricating a single-track wall using a unidirectional material deposition strategy, the material accumulated on the starting side and formed a bulging area on the starting side of the wall. At the same time, it caused a reduction in material deposition at the end of the wall. This problem can be overcome by controlling the WFS and TS [56]. However, it was resolved easily via a bidirectional material deposition strategy, shown in Figure 14b, because the extra initial material flow was balanced on both ends.

Multiple Beads, Multiple Passes
Suryakumar et al. [19] developed a second-degree regression model based on the experimental data obtained. Bead geometry was defined as a function of the TS and WFS. It was assumed that the bead's geometry is similar to a parabola, and this supposition was extended to a multi-bead material deposition model. Since additional material was not considered in the overlapping zone between two adjacent beads, the results were not reliable and accurate. The suggested model was then improved by introducing a fillet between two consecutive weld beads, as shown in Figure 15 below. Figure 15. Overlapping parabolic bead model with fillet between two consecutive beads, according to [19].
Later, Ding et al. [57] used parabola, cosine, and arc functions to study and present an accurate single-bead model. The experimental data-based curve fitting showed that the parabola and cosine models could represent the single weld bead optimally. This research study further suggested that the tangent overlapping model (TOM) can be used for higher accuracy and efficient material deposition in the multi-bead WAAM process compared to the traditional flat-top overlapping model (FOM). The TOM model is based on the idea of critical valley areas and overlapping areas. The TOM model presented that for multi-bead deposition, the optimal center distance (d opt ) between two beads is d opt = 0.738w (w = width of weld bead), as shown in Figure 16. The material yield (volume of part remaining/volume deposited) was 84.1%, using d opt = 0.738w.

Appropriate Shielding Gas
In WAAM process, shielding gas type and flow rate can affect the product's quality. The leading role of the shielding gas is to protect the molten metal pool from reacting with atmospheric gases and prevent the quality of the weld bead from degradation. In addition, it affects the overall quality of the weld bead, i.e., arc stability, preventing surface oxidation, porosity, and slag formation [58,59]. Each gas or gas mixture has unique characteristics that may affect the overall quality of the weld bead and its mechanical properties. Therefore, special attention should be given to shielding gas selection before starting a WAAM process.
The shielding gas selection depends on the material of the wire used in WAAM process and the quality requirements of WAAM component [31]. In some cases, inert gas is used alone for shielding purposes, but sometimes different types of gases are mixed in a fixed proportion to obtain a good-quality weld bead with less spatter. A summary of the different shielding gases used by various researchers in WAAM process is presented below in Table 5.  Hybrid WAAM uses additional machining and WAAM processes to get the desired geometry. It reduces the chances of material overflow and reduces the excessive machining at the end of the WAAM process. Hybrid WAAM can also improve mechanical properties, i.e., through grain refining. Furthermore, it can also be used to reduce defects in WAAM-generated components. A detailed overview of hybrid WAAM processes is shown below in Table 6. Table 6. Types of hybrid WAAM processes.

Operation
Wire Type Welding Type

Effects of Hybrid WAAM References
In-process milling Aluminium alloy ER4043 CMT (Pulse) (a) For milling thickness (t) 0.4-1.2 mm, surface roughness, and machining allowance were reduced by 22.9% and 31.6%, respectively (b) When t is increased to 1.6 mm, the surface texture becomes rough and poor [145] Interlayer hot forging AISI316L MIG (a) Improvement in yield and ultimate tensile strength (b) Elongation to fracture and the number of pores is reduced [146] In-process rolling ER70S-6 CMT (a) Reduction in residual stresses and distortion and slotted roller was more effective (b) Improvement in grain refinement [93] Depending on the application, different profile rollers can be used for in-process rolling in WAAM process. A typical hybrid WAAM process is shown in Figure 17 below. The significant advantage of the rolling process is that plastic deformation over the entire weld cross-section can be induced instead of only at the surface. It diminishes the residual stress induced during welding more adequately [93].

Additional Steps for Improving Material Properties
In WAAM process, additional in-process operations are utilized to meet the product quality requirements and obtain the desired features. It includes restraining material overflow, lower surface roughness, less waviness, and a homogeneous part s microstructure. A brief overview of the different in-process operations used in WAAM is presented below in Table 7.

Post-Process Operations Milling (Mandatory)
The assembled part must be entirely reworked by milling operations to ensure quality and accuracy requirements. For this purpose, the well-known technology of CNC milling with multi-axial machining is used. The parameters and tools for milling are selected depending on the material properties of WAAM product. For this purpose, the cutting values known from the conventional areas are adapted. A typical part with and without post-process milling is shown below in Figure 18. Table 7. In-process operations in WAAM process for improving material properties.

Operation
Wire Type Welding Type

Effect of In-Process Operation References
Interlayer cooling Thermoelectric cooling Aluminium alloy 2325 Tandem-GMAW (a) Better control of molten pool shape (b) Reduction in the interlayer delay time [147] Air jet cooling ER70S-6 MIG (a) Increase in the height of the deposited material wall between 0.5-2.0 mm (b) Decrease in deposition efficiency by up to 10% [148] Ultrasonic peening Ti-6Al-4V CMT (a) Improvement in tensile strength and anisotropy in tensile strength was reduced to 0.8% from 6% [128] In-process wire heating Ti-6.5Al-3.5Mo-1.5Zr-0.3Si GTAW (a) With an increase in the wire heating, the differences in tensile strength and elongation rate in the scanning and deposition direction disappeared by obtaining equiaxed grains [132]  Other Non-Mandatory Post-Process Operations (Optional) Different post-process operations are utilized over time to improve the quality of WAAM generated components. Heat treatment is one of the post-process operations that is sometimes used to improve the mechanical strength of the part and reduce the residual stresses induced during the WAAM process. Post-process heat treatment improves grain refinement, especially in the case of Inconel and aluminium alloy [150]. Generally, materials with high carbon contents are recommended for post-process heat treatment [43]. Selection of the heat treatment process is crucial as it may increase the chances of cracking in some materials during mechanical loading due to existing thermal stresses in part using inaccurate heat treatment. Thus, post-process heat treatment should be selected according to the type of material and application of WAAM component [150].

Process Monitoring
Although WAAM is an exemplary process for producing large parts and components, the process's control and stability are crucial due to the arc-based material deposition process and long cycle time. Real-time monitoring of the material deposition process and feedback on the deposition state are vital. Defects produced in WAAM process are usually associated with different types of signal generation, i.e., acoustic, optical, thermal, etc. The quality of WAAM products can be ensured by accurately sensing these signals. A detailed overview of different methods used for monitoring various features of WAAM process is presented below in Table 8.  [154] Optical based monitoring Deviations in wire-feeding position [155] Layer height [156,157] Bead geometry [112,[158][159][160] Arc length [161][162][163] Defect detection (porosity) [44] Melt pool size [164] Thermal based monitoring Interlayer temperatures [165,166] Geometry of melt pool [167] Strain fields evaluation [168] Defects detection [169] Spectroscopy based monitoring Defects detection [170,171] Layer Width [172] Electrical based monitoring Forming quality [63] Deposition height [173]

Single-Wire WAAM Process
In the single-wire WAAM process, only a single wire is fed into the molten metal pool for deposition. Multiple studies have been conducted for depositing different types of metals/alloys, including copper and its alloys, aluminium alloys, steel, titanium alloys, and magnesium alloys. The filler material is primarily available as a spool and controlled through a wire feeder. A detailed summary of metals/alloys and different types of energy sources used in WAAM process for depositing these metals/alloys is presented below in Table 9.

Multi-Wires WAAM Process
In the multi-material WAAM process, more than one wire is fed into the molten metal pool for deposition. Different industries are competing to achieve high functional integrity and a reduction in the weight of WAAM-based components due to their economic and environmental advantages. Hybrid materials are usually deposited to improve the features of WAAM-based components, i.e., hardness, toughness, tensile strength, and weight. It is stated in the literature [199] that one of the main challenges in the processing of hybrid materials in additive manufacturing is cracking in the transition zones of materials as a consequence of brittle phases, which are in turn caused by the blending of different materials or by differences in thermal expansion coefficients. However, Hauser et al. [200] showed that properties in components produced through the multi-wire WAAM process using two different aluminium alloy wires are limited by individual material properties and not by the transition zone of the material. A detailed summary of multi-wires and various types of energy sources used in WAAM process is presented below in Table 10. GTAW [126] Titanium alloys Ti-6Al-4V (Ti-64) PAW, GTAW, GTAW-P [184,[187][188][189][190][191] Ti-6.5Al-3.5Mo-1.5Zr-0.3Si GTAW [132] Ti-3Al-8V-6Cr-4Mo-4Zr (Beta-C) GTAW [133] Ti-64 & Ti-5Al-5V-5Mo-3Cr (Ti-5553) GTAW [192] Magnesium (Mg) alloys AZ80M GTAW-P, GTAW [134,193]

WAAM of Copper-Based Materials
Copper is a highly conductive non-ferrous metal with a reddish appearance. Apart from that, copper has medium strength, good formability, and excellent corrosion resistance. Its thermal and electrical conductivity is only second to silver's. For some applications, the properties of pure copper may not be adequate. Hence, a small amount of an alloying element is added for this purpose, leading to the development of many copper alloys. Adding small alloying elements can significantly improve pure copper's properties, i.e., strength, machinability, and softening temperature [203]. It should be noted that the properties of the alloy depend on the metal added to it. For example, the addition of nickel results in higher corrosion resistance, and due to this property, CuNi alloys are used in marine structures. Similarly, its properties differ when aluminium is added, resulting in increased strength. However, the addition of impurities may affect the conductivity of the copper. A list of the typical copper-based filler wires and rods used for welding and their composition is presented in Table 11. Table 11. Chemistry of copper-based filler wires and rods, according to [204][205][206]. Most copper-based alloys are arc weldable, but this weldability is lower than that of steel. The main challenge with such types of materials is their high thermal conductivity. Furthermore, applications of these copper-based filler wires are given below in Table 12. Table 12. Features and typical applications of copper-based filler metals, according to [204][205][206]. The high reflectivity, thermal conductivity, and surface oxide formation complicate the welding of pure copper and its alloys. During WAAM process of such materials, rapid heat dissipation is observed, resulting in high residual stresses and distortion. A detailed overview of WAAM for copper-based materials is presented below in Table 13. To study the feasibility of obtaining a Cu-Al alloy using two different wires 2.

AWS
Investigating alloy composition, mechanical properties, and effect of heat treatment on alloy homogenization [208] CuSi3Mn1

1.
Study of the microstructure developed during pulsed and short-circuiting metal transfer modes 2.
To study the feasibility of creating large components via WAAM process 2.
To investigate the location effect on mechanical properties and microstructure of the NAB alloy deposited structure [139] 1.
To study the effect of interlayer temperature on mechanical properties and microstructure with and without ultrasonic vibrations [197] ERCuAl-A2 & ER-120S-G 1.
To investigate the fabrication of Cu6.6%Al-3.2%Si alloy using twin wires and to standardize the process parameters 2.
To study the mechanical properties of the deposited alloy [141] 1.
To develop a customized alloy by feeding two different wires simultaneously into the weld pool 2.
To investigate the mechanical properties of the deposited alloy by varying Si and Al contents in the weld pool by changing the wire feed [142] CuAl8

Conclusions and Future Research Directions
WAAM-based processes have the strong potential to dominate the AM industry in the near future due to their superior features compared to other methods. These properties include a shorter lead time, low material waste, and low energy consumption. A comprehensive review of the different aspects related to WAAM process has been presented in this paper. This paper includes various challenges faced in WAAM process, including in-process and post-process operations, process monitoring, shielding gases, and materials used in WAAM process. The major highlights of this review paper and future research directions related to WAAM process can be summarized as follows: a.
WAAM is a promising technology for fabricating parts/components with complex geometries. It has been explored for metal alloys such as steel, aluminium, etc. However, it should be explored further for new advanced materials such as functionally graded materials; b.
Material deposition in WAAM process is generally accompanied by different types of defects and challenges, i.e., material overflow, poor weld bead quality, humping defects, etc. These problems can be addressed via suitable parameter selection and an optimal material deposition strategy; c.
In-process operations, i.e., rolling, forging, etc., can enhance the microstructure and mechanical properties of simple WAAM products. However, for complex geometries, suitable in-process operations need to be developed. Furthermore, post-process operations are also crucial for enhancing microstructure and mechanical properties; d.
Defect generation is associated with different signals, i.e., optical, thermal, etc. These signals can be sensed accurately by employing an advanced signal detection system to ensure and improve the quality of WAAM products. Furthermore, non-destructive testing techniques should be utilized to assess the in-service performance and life of WAAM products; e.
It was found that the correlation between process parameters and part quality has not yet been sufficiently explored. It should be further investigated using the specific test methodology, and the relationship should be described as a model. Hence, a WAAM process that meets quality requirements can be developed in the planning phase.