Research Status and Development Trend of Wire Arc Additive Manufacturing Technology for Aluminum Alloys

Abstract

It is difficult for traditional aluminum alloy manufacturing technology to meet the requirements of large-scale and high-precision complex shape structural parts. Wire Arc additive manufacturing technology (WAAM) is an innovative production method that presents the unique advantages of high material utilization, a large degree of design freedom, fast prototyping speed, and low cast. As a result, WAAM is suitable for near-net forming of large-scale complex industrial production and has a wide range of applications in aerospace, automobile manufacturing, and marine engineering fields. In order to serve as a reference for the further development of WAAM technology, this paper provides an overview of the current developments in WAAM both from the digital control system and processing parameters in summary of the recent research progress. This work firstly summarized the principle of simulation layering and path planning and discussed the influence of relative technological parameters, such as current, wire feeding speed, welding speed, shielding gas, and so on. It can be seen that both the welding current and wire feeding speed are directly proportional to the heat input while the travel speed is inversely proportional to the heat input. This process regulation is an important means to improve the quality of deposited parts. This paper then summarized various methods including heat input, alloy composition, and heat treatment. The results showed that in the process of WAAM, it is necessary to control the appropriate heat input to achieve minimum heat accumulation and improve the performance of the deposited parts. To obtain higher mechanical properties (tensile strength has been increased by 28%–45%), aluminum matrix composites by WAAM have proved to be an effective method. The corresponding proper heat treatment can also increase the tensile strength of WAAM Al alloy by 104.3%. In addition, mechanical properties are always assessed to evaluate the quality of deposited parts. The mechanical properties including the tensile strength, yield strength, and hardness of the deposited parts under different processing conditions have been summarized to provide a reference for the quality evaluation of the deposition. Examples of industrial products fabricated by WAAM are also introduced. Finally, the application status of WAAM aluminum alloy is summarized and the corresponding future research direction is prospected.

1. Introduction

Aluminum alloys have excellent comprehensive properties such as low density, high specific strength and stiffness, good conductivity, and corrosion resistance [1,2,3,4]. They have been widely applied in the aerospace, automotive manufacturing, and petroleum chemical industries [5,6,7,8,9]. The traditional manufacturing mode of aluminum alloys includes plastic forming, casting, and powder metallurgy. However, these processes all have problems such as low manufacturing efficiency, low design freedom for complex structural components, complex manufacturing processes, and difficulty in improving performance for large high-precision structural components [10,11,12,13]. Additive manufacturing (AM) technology uses a digital platform for layer-by-layer material deposition to achieve a complete three-dimensional component [14], which offers significant advantages in reducing the processing circle, as well as improving deposition rate, and material utilization. AM technology realizes digitization, intellectualization, and parallelization. The characteristics make it a competitive new method for manufacturing complex aluminum alloys components, which further shows great potential for development in the field of manufacturing and processing [15,16,17]. According to the different heat sources, AM technology is mainly divided into laser additive, electron beam additive, and wire arc additive [18,19,20]. Compared with the first two manufacturing technologies, wire-based wire arc additive manufacturing (WAAM) uses an electric or plasma arc as the heat source to melt the metal wire and does not require a closed vacuum chamber. Moreover, the equipment has a series of advantages, such as lower manufacturing cost, higher material utilization, and deposition rates, which can effectively solve the problems of easy oxidation and poor spreading of aluminum powder. Therefore, it has a wide application prospect in the manufacturing of large-sized aluminum alloy structural components [21,22,23,24].

WAAM is a directional energy deposition technology based on the principle of ‘discrete accumulation’ [25], which uses an electric or plasma arc as the heat source to melt the metal wire, and then uses large Computer Numerical Control (CNC) machine tools or industrial robot arms as the moving mechanisms to obtain dense and complex structural components layer by layer from bottom to top according to the predetermined forming path [26,27]. Compared with the additive technology using powder as raw material, WAAM technology has the advantages of high material utilization rate and fast forming speed and it is suitable for processing low-cost and fast near-net molding of large-sized complex structural parts. Therefore, it has become one of the research hotspots in the field of additive manufacturing. WAAM is a technology that combines computer-aided model design and material processing CNC systems. The schematic of the WAAM process is shown in Figure 1, which contains the welding machine, shielding gas, robot, controller, computer, and wire feeder [28]. Specifically, the main components include a digital control system and a basic forming hardware system. The digital control system includes three-dimensional (3D) geometric modeling, data slicing processing, path planning, and process parameter control. The basic forming hardware system includes an arc heat source, a wire feeding system, and a robotic arm or multi-axis CNC machine tool movement system. In the process of WAAM, 3D modeling by Computer-Aided Design (CAD) software first needs to be established, after which 3D slicing is carried out and the forming path is determined. Then, the metal wire is melted to form a deposition layer by arc heating with a welding gun. During this process, the welding gun is moved through the control program to make the deposition layer accumulate and form according to the pre-planned moving path. Nowadays, the WAAM technology is suitable for fabricating a variety of metal parts (titanium, aluminum, magnesium, nickel, etc.) and the components have been successfully used in aerospace, clean energy, and automotive manufacturing fields, such as large aircraft structural components, solar photovoltaic cell brackets, and engine parts.

Therefore, based on the above brief overview of WAAM, the following research is divided into five sections to introduce WAAM technology: WAAM digital control system and the parameters monitoring, WAAM process regulation, Mechanical Properties of WAAM, Application, and future prospects. Section 1 introduces the digital control system based on slicing and path planning. And then summarizes the effects of various process parameters (current, wire feeding speed, travel speed, and auxiliary additive manufacturing) on the properties monitoring of aluminum alloys. Section 2 investigates treatment methods for improving WAAM deposition performance. Section 3 describes the mechanical properties under various manufacturing conditions. Section 4 lists the corresponding properties of various materials under different manufacturing conditions. Section 5 summarizes the application of WAAM aluminum alloys. Section 6 looks forward to WAAM development direction, in order to promote the further application of this technology in the field of aluminum alloys.

2. WAAM Digital Control System and the Parameters Monitoring

2.1. WAAM Digital Control System

The WAAM digital control system includes three-dimensional geometric modeling, data slicing processing, path planning, and process parameter control. Each processing step mentioned above involves numerous manufacturing parameters, and the specific parameter plays a pivotal role in the quality of additive products.

2.1.1. CAD Model Processing and Slicing Algorithm

In the process of WAAM, a CAD model should be established first and then transformed into the discrete element representation model based on the standard triangle language ‘.stl’ format. In this format, the outer surface of the model consists of a series of connected triangular facets, each of which contains three vertices and the normal vector of the plane. In order to ensure the accuracy of the model, the correction algorithm based on the surface is usually used to reduce the error of the model conversion [29]. Subsequently, the model needs to be sliced by layers and the slicing algorithm can be mainly divided into plane slicing and surface slicing. The specific process is to gradually raise the three-dimensional model along the Z-axis and then extract a series of cross-sections. The obtained cross-sections can be divided into equal-thickness slices and adaptive slices according to whether the angle of elevation in the Z-axis direction is the same each time. Equal-thickness slicing exhibits the characteristics of high efficiency, fast speed, and accurate processing. It is mainly applied to simple three-dimensional models. For some complex-shaped components, equal-thickness slicing will form a significant step effect, which reduces the accuracy and thus affects the forming quality. Therefore, self-adaptive slicing is proposed and the elevation distance of each layer needs to be determined according to the change in geometric features between this layer and the previous layer [30]. Figure 2 shows the workflow during the slicing process [31].

The above slicing method based on a single processing direction is prone to cause defects such as cracks, porosity, and warped edges in the production of large-sized depositions due to uneven heating and stress concentration during the processing. Hence, in recent years, many researchers have proposed multi-direction and multi-degree-of-freedom slicing algorithms, which have the advantages of reducing step effects, shortening forming time, eliminating warping defects, and improving deposition quality [32,33]. Some examples of current slicing algorithms are displayed in Figure 3. Gohari et al. [34] proposed a new algorithm for slicing two-and-a-half-dimensional parametric surfaces by using the Adam Bashforth multistep method. The method contains three steps to determine the optimal surface subdivision. The result showed that the accuracy and efficiency of the procedure increased via this method (Figure 3a). Fortunato et al. [35] implemented a novel non-planar slicing algorithm, which can combine the traditional planar layers with non-planar ones. The algorithm was tested on three different 3D models used for aerospace, electronics, and biomedical applications, and thus showed robust results (Figure 3b). Ding et al. [33] reported the concept and implementation of a decomposition–regrouping method for multi-direction slicing of CAD models represented in the ‘.stl’ format (Figure 3c). Ding et al. [36] developed a hybrid slicing method by combining the coupled two-axis tilt with a rotatory system. And the method has also been successfully applied to construct a propeller (Figure 3d).

2.1.2. Path Planning

Path planning is the operation of filling scanning lines between the contours of each layer obtained after slicing the 3D model. The scanning line is the corresponding path of the laser. Different deposit paths will affect the molding speed, precision, surface quality, microstructure, and performance of WAAM components. Determining the appropriate deposition path is a key step in ensuring an efficient and stable arc additive manufacturing process. There are five types of typical WAAM filling paths: unidirectional, reciprocal, contour offset, spiral, and fractal line filling (Figure 4) [37,38,39,40]. The unidirectional path shown in Figure 4a is a relatively simple algorithm and has the advantage of strong adaptability and high forming efficiency. However, it is prone to warping in some areas during the actual processing process, and it is also easy to generate a large number of empty strokes. The reciprocal path shown in Figure 4b can achieve the complete filling of complex contours and has the advantage of high forming ability, less arc-starting, and arc-extinguishing times. Nonetheless, there are many inflection points in the paths, which may cause a large amount of under-deposition or over-deposition during the deposition process. The contour offset path in Figure 4c,f is based on the sliced contour and offsets a certain size from the outside to the inside until the entire area between the inner and outer contours is filled. On the one hand, this path has the characteristics of a continuous path, no empty strokes, and small warping deformation. On the other hand, it is necessary to constantly confirm whether each loop intersects during the offsetting process, which results in increased algorithm complexity and reduced molding efficiency. Moreover, the spiral path in Figure 4d refers to the spiral-shaped scanning of an area with the geometric center point of the profile section as the center. The path is suitable for simple contour filling, but it needs to cross the cavity frequently. As a result, it is not suitable for filling complex 2D contours with holes. Finally, the fractal line path usually divides the sliced plane into several square units and then uses Hilbert curves to continuously fill all the square units, as shown in Figure 4e. This method can effectively reduce residual stress and deformation. However, actually, there exist too many 90-degree turning points in this path, which will lead to defects such as weld beading at these orthogonal points, thus affecting the forming quality [38].

With the widespread application of WAAM technology in aerospace and other fields, traditional single-path planning can no longer meet the requirements for the increasing complexity and performance of the structural components. In order to obtain structural components with good forming quality, a composite path-filling method combining multiple paths should be constructed according to the specific conditions of the parts to be processed, and the filling path should be continuously optimized according to the requirements of non-porous deposition.

Zhao et al. [40] proposed a skeleton contour partitioning hybrid path-planning method that takes full advantage of the great geometric reducibility of the contour offset method and the outstanding flexibility of the reciprocal path method. It eliminates the influences of sharp corners and degradation on forming quality in the contour offset method (Figure 5a). Hu et al. [41] developed a region-based path planning method for robotic curved layer path planning, with which all horizontal welding positions can be maintained at an arbitrary region of the complex surface (Figure 5b). Liu et al. [42] proposed a support-free path planning method based on surface/interior separation and surface segmentation, as shown in Figure 5c. It divided the metal part into surface and interior solid (IS). Moreover, the surface is further separated into the overhanging area (OA) and the no overhanging area (NOA). Path planning is carried out by alternatively depositing the OA and the NOA and IS. Kincaid et al. [43] proposed a new offset deposition path to enable an angled (not vertical) hollow geometry. The 57.2 mm radius, 57.2 mm height, and 45-degree cone geometry are shown in Figure 5d. Diourté et al. [44] presented a continuous three-dimensional path planning strategy, which generated a continuous trajectory in spiral form for closed-loop thin parts. In this innovation path planning, an adaptive wire speed coupled with a constant travel speed allows a modulation of the deposition geometry that ensures a continuous supply of energy and material throughout the manufacturing process (Figure 5e).

2.2. Processing Parameters

According to different types of applied heat energy sources, WAAM technology can be divided into gas metal arc welding (GMAW), gas tungsten arc welding (GTAW), tungsten inserts gas welding (TIG), plasma arc welding (PAW), and cold metal transfer welding (CMT). Among them, the CMT technology, which is based on the improvement of GMAW, can effectively avoid the problem of splashing caused by conventional gas-shielded welding, and thus shows the characteristics of stable arc and no splashing. In recent years, it has become the mainstream method for selecting heat sources for WAAM technology [45,46].

Processing parameters are key factors affecting the forming ability and quality of arc additive aluminum alloy parts. The process parameters of WAAM include the arc current, wire feeding speed (WFS), travel speed (TS), shielding gas, and auxiliary additive manufacturing, which shows a straight consequence on the quality of the deposited components. By altering the above processing parameters, the corresponding layer height, width, cooling rate, and heat input during the additive manufacturing process can be changed, so as to significantly affect the microstructure and properties of the deposition.

2.2.1. Current

Current is the main parameter in the WAAM process, which determines the arc energy and heat input. Moreover, the heat input in WAAM has a substantial effect on the melt pool’s size. By directly affecting the stability of the arc, the formation of the molten pool and the melting effect of the metal wire, the macro morphology, and size of the parts are determined. When the welding current is too low, it will lead to difficulty in starting the arc and also increase the arc instability during the forming process. As a result, the melting rate slows down and the molten pool is more unstable. With the increase in welding current, the heat input also increases, resulting in an increase in the fluidity of the molten metal in the molten pool. Therefore, the layer width of the deposition increases and the layer thickness decreases. However, it should be noted that excessive current may easily cause problems such as overheating and burn-through.

Furthermore, a pulse waveform current can be obtained by periodically changing the magnitude and direction of the current. This pulse current can stir the molten pool to make the molten pool oscillate, which is further beneficial in promoting dendrite fragmentation and refining the grain size. When the pulse current is at its peak current value, the heat input is high, which makes the wire quickly melt into the molten pool. At the same time, the base current value is small, which rapidly reduces the heat input after droplet transfer, thus facilitating the rapid cooling of the molten pool. Pan et al. [47] compared the effects of pulse current and non-pulse current modes on GMAW arc additive Al-Mg alloy. The results showed that in the pulse mode, the arc heat source is more concentrated and the penetrating capability is stronger. Therefore, the obtained component shows a smaller grain size, lower porosity, and higher tensile strength. Wang et al. [48] analyzed the influence of CMT, CMT + Pulse (C + P), and CMT + Advanced (C + A) arc models on the forming quality of the Al-Mg alloy parts manufactured by WAAM. The results showed that the arc in C + A mode was more stable and the heat input was lower compared with the other two modes. The corresponding grain morphologies via EBSD inverse pole Figs (IPFs) in Figure 6 show that the grain distribution of the sample in C + A mode is more uniform and the grain refinement effect is more obvious. As a result, the comprehensive mechanical properties are the best.

2.2.2. Wire Feeding Speed (WFS)

WFS directly affects the supply of welding wire, which determines the shape and size of the molten pool. WFS is directly proportional to the heat input. By adjusting the WFS, the macroscopic morphology and heat input of the deposited part can be controlled [49]. With the increase in wire feeding speed, the heat input per unit length of the metal wire decreases, which leads to a lower temperature of the molten pool and a higher cooling rate. Hence, the fluidity of the molten pool metal liquid decreases. As a consequence, the width of the deposition decreases while the thickness increases [50].

2.2.3. Travel Speed (TS)

The TS has an obvious influence on the quality of the WAAM depositions. As the TS increases, on the one hand, the forming time is shortened while the heat input is also reduced. The experimental result of the WAAM 2219 Al alloy part by Zhou et al. [51] in Figure 7c shows that the grain size of the alloy is more refined. On the other hand, the increasing TS will also reduce the stirring effect of the arc on the molten pool surface, and consequently, the surface of the deposited structure will be rough and both the height and width of each layer reduce (Figure 7a). In addition, the higher TS can also reduce the diameter of the pores, as shown in Figure 7b, thereby improving the strength of the component. When the TS is too low, the fusion between the forming layers will be enhanced and the boundaries between the deposited layers will become blurred. As a result, the side-forming accuracy of the deposition is reduced [51].

2.2.4. Shielding Gas

In order to avoid the contamination of the welding pool during the WAAM process, sufficient shielding gas is required to protect the region surrounding the hot molten material. The introduction of shielding gas can effectively prevent the possibility of shortcomings such as porosity, inclusions, and weld cracking, and, importantly, also avoids the formation of detrimental oxides and nitrides. The shielding gas in WAAM mainly includes Ar, He, CO₂, N₂, etc. Moreover, inert gases such as Ar and He are often used as shielding gases in WAAM aluminum alloy. When a certain amount of O₂ is added to it, the fluidity of the molten pool can be further enhanced and the surface tension of the molten metal can be reduced. Therefore, the TS is accelerated. Silva et al. [52] evaluated the effect of O₂ content in Ar-based shielding gases over the arc cathodic emission behavior in wire + arc WAAM of thin aluminum walls and its consequences over layer formation. The results showed that when the O₂ content is too low, there is no significant effect on the arc. Increasing the O₂ content can solve the problem of arc wandering, but excessive oxidation will occur when the O₂ content is too high (Figure 8). In addition, it is necessary to maintain a suitable gas flow rate during the additive process. When the flow rate is too high, the arc penetration will be reduced. As a consequence, porosity can be observed in the WAAM parts due to the mixing of large turbulent flows.

2.2.5. Auxiliary Additive Manufacturing

In addition to changing the above processing parameters, the researchers further improved the WAAM system by applying external energy fields on the basis of the original system, such as adding ultrasonic assistance, alternating magnetic fields, and interlayer cold working Through the assistance of external energy fields, deposition with higher mechanical properties and fewer defects can be achieved.

Ultrasonic vibration is a surface strengthening process, which mainly uses ultrasonic vibration energy to produce large plastic deformation on the metal surface and further make the coarse columnar crystals in the molten pool break up. Hence, the nucleation rate increases, and the grain size decreases. In addition, ultrasonic treatment can also effectively reduce the porosity and increase the dislocation density, which further forms the dislocation walls and cells. Consequently, the surface residual stress is reduced and the hardness and strength are both increased [53,54]. Figure 9 summarizes the examples of ultrasonic treatment WAAM aluminum alloys, including ultrasonic impact treatment (Figure 9a) [53], WAAM-ultrasonic nanocrystal surface modification (UNSM)-treated (Figure 9b) [55], and ultrasonically assisted (UA) (Figure 9c) [54]. The results showed that the comprehensive properties of the WAAM aluminum alloys can be improved effectively by appropriately applying the external energy.

During the WAAM process, the application of external magnetic fields can effectively improve the arc shape, droplet transition, and molten pool flow state, thereby regulating the microstructure of the deposition. The external alternating magnetic field exerts Lorentz force on the charged particles in the arc and droplet. Then, the particles undergo complex movements in reciprocation, which generates the stirring effect on the melt pool and further changes the direction of heat transfer and maximum temperature gradient. As a consequence, the direction of grain growth is deflected by the accelerated heat diffusion, which inhibits the growth of columnar crystals and enhances the effect of grain refinement [56]. Zhao et al. [57] studied the effects of electromagnetic fields on heat transfer and molten pool flow. The results showed that moderate electromagnetic field flux density not only compresses plasma and reduces the average temperature of molten pool metal liquid, but also increases the peak velocity of plasma and expands the high-speed plasma region. In addition, electromagnetic fields can accelerate the heat dissipation of the molten pool, making it narrower and shallower. And with the increase in magnetic flux density, the change trend becomes more obvious.

As a cold working process, on the one hand, interlayer rolling can significantly reduce deformation and residual stress. On the other hand, the mechanical properties of the alloy can be improved by a combination of refining the alloy grains, increasing the dislocation density, and reducing the grain orientation difference. In order to further eliminate porosity in the arc additive manufacturing of 2319 aluminum alloy, Gu et al. [58] used interlayer rolling technology. The results showed that the pores in the aluminum alloy after interlayer rolling treatment were compressed into flat spherical shapes, and the number of pores was also significantly reduced. In addition, as another commonly used cold working method, interlayer hammering also has a certain impact on the microstructure and properties of WAAM aluminum alloy, to which researchers have paid increasing attention. For example, Zhou [59] and Fang [60] et al. studied the impact of interlayer hammering on 5B06 and 2319 aluminum alloys, respectively. The results showed that with the increase in interlayer deformation, the porosity in the alloy decreased and both the tensile strength and yield strength significantly increased. However, the main strengthening mechanism of the hammered samples was high-density dislocation strengthening caused by plastic deformation, so the elongation of the hammered samples was decreased. Sun et al. [61] designed an interlayer rapid cooling device for WAAM. The results showed that the proportion of small-angle grain boundaries in the IRC-WAAM (Interlayer Rapid Cooling-WAAM) component was 10.893% greater than that in the WAAM component, and the proportion of small grains was greater. Figure 10 summarizes the examples of the interlayer cold working process, including interlayer rolling technology (Figure 10a) [58,62], interlayer rapid cooling process (Figure 10b) [61], and interlayer hammering process (Figure 10c) [59].

In summary, both the welding current and wire feeding speed are directly proportional to the heat input while the travel speed is inversely proportional to the heat input. Moreover, the introduction of shielding gas can effectively prevent the possibility of shortcomings such as porosity, inclusions, and weld cracking, and, importantly, also avoids the formation of detrimental oxides and nitrides. In addition to changing the above processing parameters, external energy fields, such as adding ultrasonic assistance, alternating magnetic fields, and interlayer cold working, were often applied on the basis of the original system to further improve the WAAM system. Through the assistance of external energy fields, deposition with higher mechanical properties and fewer defects can be achieved.

3. WAAM Process Regulation

WAAM process has great advantages in manufacturing large-size, high-precision, and complex geometric shapes of aluminum alloy structural parts. However, due to the high thermal conductivity and high expansion coefficient of aluminum alloys, non-equilibrium solidification and residual stress during the WAAM process may lead to problems of deformation, non-uniform shrinkage, porosity, cracks, uneven microstructure, and poor mechanical properties. These defects greatly limit the further engineering application of aluminum alloy WAAM technology. Optimizing the microstructure through controlling the heat input, alloy composition, and heat treatment to minimize processing defects is the key to the WAAM manufacturing process.

3.1. Heat Input

Heat input is an important process parameter that determines the surface quality, microstructure, defect formation, and mechanical properties of WAAM deposited layers. The heat input is directly proportional to current and voltage, and inversely proportional to TS. Affected by these variables, the specific expression of heat input is as follows:

$$Q=\tau {\displaystyle \frac{IU}{v}}$$

(1)

where Q is the heat input (J/m), τ is the power coefficient, I is the welding current (A), U is the welding voltage (V), and v is the welding speed (m/s).

WAAM is a layer-by-layer additive manufacturing process, which undergoes multiple uneven heating and cooling cycles during the deposition. As the thickness of the deposited layer increases, the heat dissipation method gradually shifts from thermal conduction between the deposited layer and the substrate to heat dissipation to the surrounding space, resulting in a decrease in heat dissipation efficiency and the accumulation of heat in the deposited part. Therefore, excessive heat input may lead to the formation of coarse grain structures and defects such as residual stress, porosity, and cracks in the deposited part. Conversely, under the condition of slightly lower heat input, the surface of the molten pool is more uniform without overflow or collapse. In this state, due to the fast cooling rate and high nucleation rate of molten metal, the grains do not have time to grow, and the time for porosity formation, aggregation, and growth is also relatively short. As a result, it is easy to obtain the additive parts with finer grains and fewer porosity defects. However, when the heat input is too low, the fluidity of the molten pool will be reduced, and the liquid phase cannot promptly replenish the cavities caused by solidification, resulting in the formation of incomplete fusion defects between the deposited layers. Therefore, in the process of WAAM, it is necessary to control the appropriate heat input to achieve minimum heat accumulation and improve the performance of the deposited part.

As mentioned above, the heat input is mainly related to the current and TS, so it is necessary to regulate heat input by effectively changing the parameters of the current and TS during the additive manufacturing process. In addition, heat input can also be regulated by controlling the interlayer temperature. In the continuous WAAM process, if the interlayer temperature is too high, the heat of the previously deposited layer cannot be dispersed and dissipated in time, which will easily lead to ‘mixed layers’ and ‘collapse’ of the deposited layer. Currently, the common method of controlling the interlayer temperature is to regulate the residence time between the deposited layer and the subsequent deposited layer. Properly extending the interlayer residence time can effectively reduce the residual tensile stress inside the deposited layer. Generally, in order to achieve deposits with finer grains and fewer porosity defects, the active cooling devices (water tanks, protective gas nozzle, cooling fixtures, high-power insulated gate bipolar transistor, etc.) can be used to reduce the heat input during the WAAM process [63,64,65]. Recently, a new thermal management method named near immersion active cooling (NIAC) was proposed and Silva et al. [65] and explored the effect of NIAC in the WAAM ER5356 aluminum alloy process. The corresponding experimental rig is shown in Figure 11. The result shows that this technique is efficient in mitigating heat accumulation.

3.2. Alloy Composition

The mechanical properties of pure aluminum are poor. Accordingly, adding Cu, Mg, Si, and other alloying elements to form different aluminum alloys can significantly improve the comprehensive performance. The previous studies show that the molten pool of 6XXX (Al-Mg-Si) and 7XXX (Al-Zn-Mg-Cu) aluminum alloy is unstable during the deposition process, which limits the application of the aluminum alloys to a certain extent. However, 2XXX (Al-Cu), 4XXX (Al-Si), and 5XXX (Al-Mg) aluminum alloys with good weldability are widely used in the field of WAAM. In order to further improve the comprehensive performance of aluminum alloys, arc additive structural parts of aluminum matrix composites can be prepared by adding nano-reinforced particles to aluminum alloy [66]. For example, Fu et al. [67] prepared nano-TiC-reinforced aluminum matrix composites via the WAAM method. The results showed that the composite exhibited excellent comprehensive performance (tensile strength of 435 ± 10 MPa, elongation of 7.8 ± 0.8%). Chen et al. [68] prepared different sizes of La₂O₃ particles (ordinary-sized, micron-sized, and nano-sized) reinforced by aluminum matrix composites using WAAM technology. The results show that nano-sized La₂O₃ reinforced particles can effectively improve the mechanical properties of the alloy deposit. Figure 12 summarizes the examples of aluminum alloy-based deposits with particle reinforcement, including 2219 aluminum alloy reinforced by TiC particle (Figure 12a–c) [69], 7075 aluminum alloy reinforced by TiC particle (Figure 12d–f) [67], and 4043 aluminum alloy reinforced by La₂O₃ particle (Figure 12g–k) [68]. It can be seen in Figure 11 that the addition of appropriate reinforcement can effectively refine the grains of aluminum alloys. Furthermore, the result also reveals that the reinforcement amount and size are the two factors affecting the microstructure and mechanical properties of the deposition.

3.3. Heat Treatment

WAAM aluminum alloy has the characteristics of rapid melting and solidification under the action of an arc heat source. As a result, the obtained additive structural parts are mainly non-equilibrium structures, accompanied by grain orientation growth, coarse grain, and composition segregation. The microstructure will directly affect the mechanical properties of the alloy. In addition, with the reciprocating thermal cycle of the moving arc heat source during the WAAM process, the deposited parts accumulate heat layer by layer. This uneven temperature field is prone to generating residual stresses in the alloy. Heat treatment can homogenize the microstructure in the component, refine the grain, and reduce the residual stress to achieve the effect of improving the comprehensive performance. Therefore, heat treatment is an important method to optimize the comprehensive performance of WAAM aluminum alloy forming parts [70].

The precipitation strengthening phase in aluminum alloys is mainly affected by the synergistic effects of solution parameters (temperature, time) and aging parameters (temperature, time). Scholars have conducted a series of explorations on the heat treatment process of WAAM aluminum alloy. Arana et al. [71] prepared WAAM 2319 aluminum alloy with a porosity of less than 1% and then studied the effect of aging treatment parameters on the properties. The results showed that low aging temperature and short aging time heat treatment cannot guarantee high mechanical properties and low anisotropy. On the contrary, when the aging heat treatment is 190 °C × 26 h, the tensile strength and yield strength of the alloy reached 452 MPa and 324 MPa, respectively, and the elongation was 8%. Gu et al. [72] studied the strengthening effect and mechanism of interlayer rolling and post-deposition T6 heat treatment on CMT forming parts of 2319 aluminum alloy. The results showed that compared with interlayer rolling, post-deposition heat treatment provides a more significant effect on improving the strength of aluminum alloy structural parts. Zhou et al. [73] prepared the as-deposited (AD) and T6 heat-treated WAAM 205A aluminum alloy and further comparatively studied the corresponding microstructure and mechanical properties of the specimens. As shown in Figure 13, it can be seen that the precipitates and mechanical properties are rather different. T6 heat treatment can effectively improve the strength of the alloy, and the ultimate tensile strength reached 510.2 MPa, which is 104.3% higher than the AD state. In addition, the fracture mode is also changed from intergranular fracture to trans-granular fracture after T6 heat treatment.

In summary, in the process of WAAM, it is necessary to control the appropriate heat input to achieve minimum heat accumulation and improve the performance of the deposited parts. To obtain higher mechanical properties (tensile strength was increased by 28%–45%), aluminum matrix composites by WAAM proved to be an effective method. The corresponding proper heat treatment can also increase the tensile strength of WAAM Al alloy by 104.3%.

4. Mechanical Properties of the WAAM Deposition

Tensile strength, yield strength, and hardness of WAAM deposited parts have always been important indexes to evaluate the mechanical properties of various research results. The selection of different materials, heat sources, and processing parameters can lead to different mechanical properties of the deposition. Furthermore, the application of post-treatment can significantly improve the mechanical properties.

Table 1. The summary of different materials under various manufacturing conditions.

Aluminum Alloy	Heat Source	Parameter	Processing Method	Average Grain Size (μm)	Tensile Strength (MPa)	Yield Strength (MPa)	Hardness (HV)	Forming Quality	Ref.
Al-Zn-Mg-Cu	CMT	WFS: 6.5 m/min TS: 0.54 m/min	T6 Heat treatment	6	618 (Ultimate)	542	/	Composed of fine equiaxed grains, eutectic structures were precipitated.	[74]
Al-Zn-Mg	MIG	TS: 0.2–0.35 m/min Current: 97–112 A	/	/	299 (Ultimate)	188	112	The weld bead smoother, with a few pores on weld bead.	[75]
4043	TIG	WFS: 1.0 m/min TS: 0.25 m/min	Laser-arc hybrid additive manufacturing	/	151.91	69.71	49.97	Finer grains, the microstructure morphology in different zones is different.	[76]
2219 + TiC particles	TIG	WFS: 2.0 m/min TS: 0.2 m/min	/	/	384	/	/	The surface roughness of the specimens increased, eliminated the slender columnar grains, and refined the Grains.	[68]
6061	CMT	WFS: 6.0 m/min TS: 0.36 m/min	Friction stir processing	5	257 (Ultimate)	142	99.71	Significant microstructure refinement and porosity reduction.	[77]
2319 + 5087	GTAW	WFS: 2.4 m/min (2319) 1.05 m/min (5087) TS: 0.3 m/min	T4 Heat treatment + T6 Heat treatment	/	458 (T4, Ultimate) 470 (T6, Ultimate)	310 (T4) 374 (T6)	138 (T4) 146 (T6)	Obvious dendrite morphology disappeared, layer-distributing characteristics of the phases became obvious.	[78]
7075 + TiC particles	GTAW	WFS: 3.0 m/min TS: 0.24 m/min	T6 Heat treatment	/	Significant improvement	Significant improvement	193	Finer grains, the adiabatic shear band is first generated with an increase in the strain rate.	[66]
7075 + TiC particles	GTAW	WFS: 3.0 m/min TS: 0.24 m/min	/	15.2	435	310	/	Uneven microstructural features and grain boundary segregation were eliminated	[69]
5087	CMT	WFS: 6.0 m/min TS: 0.6 m/min	Interlayer rolling	/	344 (Ultimate)	240	107.2	Primary coarse grain structures were found to become greatly refined with an evident rolling texture after deformation.	[79]
2319	CMT	WFS: 4.0 m/min TS: 0.48 m/min	Low- frequency vibration	16	266.1 (Ultimate)	120.6	/	Refines the grain size, and reduces the texture density.	[80]
5356 + 7A48	CMT	WFS: 10.1 m/min TS: 0.6 m/min	Hot rolling	51.6	392.3	280.2	72 (5356) 160 (7A48)	Heterogeneous plate with lamella structure and without any noticeable crack defects.	[81]
2319	CMT	WFS: 5.6 m/min TS: 1.8 m/min	Friction stir processing	4.98	289.6 (Ultimate)	162.9	88	Ultrafine grains, equiaxed grains, columnar grains, gradient microstructure.	[82]

lists the corresponding mechanical properties of various materials under different manufacturing conditions.

From Table 1, it can be seen that the mechanical properties of 2319 aluminum alloy show slight differences under different parameters [80,82], but the corresponding 2319 + 5087 composite can significantly enhance the mechanical properties [78]. In addition, the result also shows that fine-grain microstructures can be obtained through friction stir processing [77,82], while the composite deposits have larger grain sizes [69,81].

5. Application

WAAM has become a promising manufacturing process for large-sized complex structural parts due to its advantages of high molding efficiency, low cost, and high flexibility. It has been successfully applied to the preparation of various industrial products and has been widely used in aerospace, shipbuilding, construction, and other fields. For example, Lu Bingheng’s et al. [83] produced the world’s first 10 m high-strength aluminum alloy connecting ring for heavy-lift carrier rockets via the WAAM method (Figure 14a). Pan et al. [84] prepared the thin-walled aircraft parts with different thicknesses and multiple intersections by WAAM aluminum alloy (Figure 14b). Vishnukumar et al. [85] successfully repaired the corroded surface of AA5052 aluminum alloy structural component via depositing ER4043 aluminum alloy on the AA5052 substrate. The MX3D company used WAAM to produce the world’s first bridge and FeAl alloy impeller for petroleum applications [86,87]. The research team of Cranfield University used WAAM to manufacture a 6 m long lightweight high-strength aluminum alloy wing beam [88]. The results showed that this method saved a significant amount of time and cost compared to traditional manufacturing methods.

6. Future Prospects

At present, WAAM aluminum alloy has been widely used in aerospace, automotive, and other fields. The current most primary study aspect is to improve the forming ability and forming quality of the deposition. This article comprehensively summarizes the influence of the forming process and various process parameters on the microstructure and properties of the deposited parts. Based on the problems of deformation, uneven shrinkage, porosity, cracks, uneven microstructure, and low mechanical properties of the WAAM aluminum alloy deposited parts, the corresponding regulation principles and research progress of parameters such as heat input, alloy composition, and heat treatment are described and analyzed. Furthermore, the application status of WAAM aluminum alloy is also summarized. In order to further expand its application fields, in-depth research should be carried out in the following three aspects in the future:

(1)

Combining the numerical simulation with WAAM experiments. Numerical simulation technology is necessary to be used in simulating the entire process of metal materials moving, softening, melting, flowing, solidifying, and accumulating under the action of heat sources. Then, the effects of process parameters on the morphology, microstructure, and stress of the deposited parts should be deeply analyzed. Finally, the defects and microstructural evolution principles in the forming process should also be explained from the perspectives of the temperature field, flow field, and stress field in the WAAM process. Furthermore, the internal relationship among path planning, processing parameters, and alloy composition are explored, so as to guide parameter regulation and optimization in the actual experimental process.

(2)

Developing special wire materials for aluminum alloy arc additive manufacturing. 6XXX and 7XXX series aluminum alloys are attractive materials applied in the aerospace industry. However, due to the difficulty in manufacturing and processing special wire materials for high-strength aluminum alloys, there are still few research reports on WAAM for high-strength aluminum alloys. In addition, in the face of the characteristics of different regional performance requirements during the service process of complex components, it is urgent to develop and select different-composition metal wire materials to improve the overall performance and service life of structural components.

(3)

To realize the additive manufacturing technology of aluminum alloy suitable for complex service environments. It is urgent to develop in situ repair and rapid manufacturing technology for damaged parts in extreme environments, such as ocean carriers, underwater, underground, and space, so as to effectively solve the problem of difficulty in replacing the damaged parts.