Weld morphology, microstructure evolution, and mechanical properties of laser beam welding of wire arc additive manufactured Al-Cu substrate

Laser welding is a feasible process for joining the additive manufactured (AMed) parts to meet the manufacturing demand of specific large-scale components. Microstructure and property evolution as well as weldability of the laser welding of wire arc additive manufactured (WAAMed) Al-Cu alloy are investigated. Results indicate that the WAAMed Al-Cu alloy has excellent laser weldability and the joint is nearly free of defect. The width of the heat-affected-zone (HAZ) and equiaxed crystal zone (EQZ) as well as the grain size of the fusion zone (FZ) will increase with higher laser power and heat input. The joint with laser power of 3500 W has the highest tensile strength and elongation, reaching 203.48 MPa and 4.13%. The result from electron backscatter diffraction (EBSD) test indicates that the texture intensity will affect tensile properties. The tensile strength and elongation of the laser welded WAAM sample perpendicular to the deposition direction are higher than that parallel to the deposition direction. The microhardness value of HAZ is higher than that of FZ and BM due to 163 HV. The feasibility of laser welding of WAAMed samples is validated, and process parameters are found for hybrid manufacturing of large-scale components.


Introduction
Additive manufacturing (AM) technology is a rapid fabrication method. A layer-by-layer deposition approach is used to create a part from the digital 3D model [1,2]. The mechanical performances of AMed aluminum alloy parts have been on par with cast and wrought as well as the AM technology can directly produce the workpiece, avoiding mold and cast defects [3,4]. Thus, AM technology is widely used in the aerospace industry [5][6][7][8]. Cold metal transfer (CMT) technology is an additive manufacturing method using the electric arc as the heat source in wire arc additive manufacturing (WAAM) that has the advantages of low heat input and no metal splashing [9,10]. Therefore, this method can ensure better molding quality of largescale components [11,12].
Unfortunately, the development and application of WAAMed large-scale components are facing challenges. The reason is a severe restriction on the size of the shielding gas envelope [13,14]. AM technology cannot meet the manufacturing demand of specific large-scale aluminum alloy components in the aerospace industry [15,16]. Therefore, this paper will propose a method that using laser weld smaller WAAMed parts to form large-scale components without size limit [17][18][19][20]. Laser welding is a green technology with high-quality, efficient and flexible [21]. The material surface absorbs high-power laser as well as metal is melted and solidified [22][23][24]. The laser welding technology has advantages including the energy utilization rate, higher processing efficiency, lower residual stress, and slight distortion [25,26]. Thus, laser welding technology suits highquality and high-precision welding in aerospace equipment [27,28].
Recently, Yang et al. [15] utilized the laser welding method to weld thin plates cut from SLMed 304 stainless steel blocks, and studied the joint morphology, microstructure, and tensile properties. The results indicated that the microstructure of laser-welded joints consists of a coarser dendrite structure, and the tensile properties of the welded joint decreased by 27.6% compared with the base metal. The finer dendrites and higher tensile strength could be obtained by welding along the deposition direction. Hanchen et al. [18] utilized laser to weld molding of the SLMed TC4 titanium alloy with titanium alloy forging. They studied the welding joint microstructure, tensile properties, and fatigue life. The results showed that the fatigue life of welded joints is far lower than those of SLMed TC4 and traditionally wrought annealed TC4. Nahmany et al. [17] studied the microstructure and mechanical properties of SLMed AlSi10Mg prepared by electron beam welding. The results showed that increasing heat input is beneficial in reducing joint porosity and improving mechanical properties. The joint region's yield strength and tensile strength were similar to the base metal.
But, relevant studies are still limited. Few investigations have been attempted to research laser-welded WAAMed 2319 aluminum alloy joints. 2319 aluminum alloy belongs to Al-Cu alloy and has been widely used in the aerospace field because of its advantages of good mechanical properties, lower density, and higher strength [29,30]. Under the condition of meeting the strength requirements, aluminum alloy can reduce aerospace equipment's weight and fuel consumption [31]. The components are required to have large size and complex structures, suggesting that the investigation on laser welding of WAAMed 2319 aluminum alloy is indispensable.
In the current study, the feasibility of the technology is explored by laser-weld 3-mm-thick samples cut from WAAMed 2319 aluminum alloy. In addition, the welding properties, microstructures, and mechanical properties of the welded joints under the different laser powers and welding types are evaluated. This experiment is significant in exploring the welding mode of laser-welded WAAMed 2319 aluminum alloy parts and solving the problem that AM technology difficultly produces large-scale components.

WAAM process
In this experiment, 2319 aluminum alloy metal wire with 1.2 mm diameter is used as feedstock, and 2219 aluminum alloy is used as substrate with 200 mm × 200 mm × 15 mm. The chemical composition mass fraction of filler metal wire and substrate are listed in Table 1. The surface of the substrate is cleaned with anhydrous ethanol and acetone. A deposited sample of 2319 aluminum alloy is manufactured with the WAAM system, consisting of a welding power source (CMT, Fronius A-4600), welding wire feeder, KUKA robot, shielding gas device (Ar gas), and welding torch, as shown in Fig. 1.
In the work, the 2219 aluminum alloy substrate is fixed on the workbench. The shape of the block is designed to be two cuboids, and the specific size of each cuboid is 180 × 60 × 110 mm, as shown in Fig. 2a. The scanning rate is 0.035 m/s, the electrode extension is 10 mm, and the shielding gas flow is 25 L/min. The welding current is 140 A, and the welding voltage is 15 V. CMT mode is adopted in the WAAM process, with the interlayer cooling time to 20 s. Finally, the block will be cut by wire-cut electrical discharge machining to prepare for laser welding, as shown in Fig. 2b.

Laser welding process
The structure (60 mm × 45 mm × 80 mm) is further cut preparing for welding, as shown in Fig. 2c. The deposition direction of WAAM is from bottom to top, and the block is cut into top and bottom parts. The sample (60 mm × 45 mm × 3 mm) on the top part is cut into a metal sheet that is perpendicular to the deposition direction, and the sample (40 mm × 45 mm × 3 mm) on the bottom part is cut into a metal sheet parallel to the direction of deposit. The samples on the top part are welded in different laser powers, as shown in Fig. 2d. The samples on the bottom part are welded in different welding types at 0° or 90°, as shown in Fig. 2e. Clean the metal sheet with 150# sandpaper and wipe it with anhydrous ethanol. Table 2 shows the welding parameters of the six samples. The samples are jointed using a welding system consisting of a fiber laser system, watercooling system, KUKA robot, shielding gas device, and laser focusing system, as shown in Fig. 1. The protected gas flow is 25 L/min, and the welding speed is 4 m/min.

Characterization of microstructure and properties
After welding, the metallographic specimens perpendicular to the welding type are cut with a specific size of 10 mm × 4 mm × 3 mm. The sample's surface is polished with diamond grinding paste until the surface is bright without scratches under the microscope. The metallographic surface is corroded in Keller reagent (1.5 mL HCl, 1.0 mL HF, 2.5 mL HNO 3 with 95 mL H 2 O) for 10 ~ 12 s. The macroscopic morphology and microstructures are observed with an optical microscope (OM). The scanning electron microscope (SEM) scans the metallographic sample and tensile fracture. Electron backscatter diffraction (EBSD) technology is utilized to analyze the crystal orientation of the metallographic samples with a step size of 2.5 μm.
Six tensile test pieces are cut from each group with the weld as the center. The electronic universal testing machine tests the tensile properties at the strain rate of 0.5 mm/min at room temperature. The deformation and tensile strength of the sample are recorded. The data of three repeated tests are averaged as the test results. Microhardness tests of all the joints are performed with a load of 200 g and a holding time of 15 s using the HXS-1000AY microhardness tester. Figure 3 shows the surface morphologies of six laserwelding joints. The 3-mm 2319 aluminum alloy sample has been fully welded. When the laser power is 3000 W and 4000 W, a few defects on the weld surfaces are characterized by severe welding spatters and uneven welding ripples, as shown in Fig. 3a, c. When the laser power is 3500 W, a significant phenomenon is found that the spatter defects and uniformity of surfaces are optimized, as shown in Fig. 3b. So this is the reason why the samples under the different welding types are welded with 3500 W  [32]. The results show that the welded joints of laser-welded WAAMed 2319 aluminum alloy have small deformation, good surface quality, and complete penetration.

Macroscopic morphology
The macroscopic morphology of the weld can indirectly reflect the shape of the molten pool in the welding process. So to study the influence of the laser power and welding type, molten pool size are analyzed. The laser-welded joints' upper, middle, and lower width are measured, as shown in Fig. 4a. The upper width of the welded joint is longer than the middle and lower width. Meanwhile, the base metal (BM), heat affected zone (HAZ), and fusion zone (FZ) can be identified clearly. The interior of the welded joint slightly sags because of the molten pool gravity [22]. Figure 4b and Table 3 show the weld cross-section width. When the laser power increases from 3000 to 4000 W, the joints' width increases from 3.14 to 3.72 in the upper area, from 1.70 to 2.48 in the middle area, and from 1.73 to 2.56 in the lower area. But the joints' width of laser welding under the different welding types does not have a noticeable change. So the joints' width is not seriously affected by welding types [22]. The WAAMed 2319 aluminum alloy structure could be successfully welded in various laser powers and welding types. The result means that using laser welding to joint 2319 WAAM parts is significant in solving the problem that AM technology cannot produce large-scale components.

Microstructure
The microstructure morphology of WAAMed 2319 aluminum alloy material under 5000 × is observed, as shown   Figure 5 points 1, 2, and 3 are the image of the grain boundary, grain interior, and second phase, and the EDS point scan is performed on the marked area [33,34]. reduced. The result indicates that the Al element is more distributed in the matrix and less distributed at grain boundaries, but the Cu element is the opposite.
Cu is the primary strengthening element in 2319 aluminum alloy. The segregation of Cu occurs at grain boundaries. The second phase particle must tend to aggregate and grow up in the grain interior that they are binary eutectic phases of α(Al) and Al 2 Cu [14]. The size increase in the second phase becomes more evident with higher heat input. Therefore, due to the particularity of the  WAAMed 2319 aluminum alloy, it is a higher challenge to explore the laser-welded WAAMed 2319 aluminum alloy to optimize the welding process, control the precipitation of the second phase, and prevent the generation of coarse grains.
The metallographic of the weld joints under the different laser powers (No.1, No.2, and No.3) is observed in the yellow area, as shown in Fig. 4a. The heat distribution from the welding line center to both ends is not uniform in the laser welding process. The temperature gradient and growth rate are different, so various forms of grain morphology are generated, as shown in Fig. 6a-c. The microstructure of the joint at the edge of the weld is the BM, HAZ, equiaxed crystal zone (EQZ), and FZ from right to left. The structure and morphology of HAZ are changed significantly compared with the BM. The grain coarsening and uneven morphology of the microstructure occurred. Figure 6a 1 -c 1 shows that the fusion line separates from HAZ to FZ. There are small equiaxed crystal bands between the HAZ and FZ. When the liquid molten pool starts to solidify, the heterogeneous nucleation process is more accessible than the spontaneous nucleation process for the liquid metals with similar chemical compositions and lattice types. Therefore, the liquid metal nucleates and grows in the area near the fusion line [35,36]. Because the laser welding speed and the supercooling degree of fluid metal increase, the nucleation rate near the fusion line increases. Finally, the nucleation of liquid metal can be promoted and larger grains can be broken due to the stirring action of laser oscillation. Therefore, fine and dense equiaxed grains appear near the fusion line region.
At the same time, it is found that there are apparent columnar dendrites in the FZ. They grow toward the grain interior perpendicular to the fusion line with different microstructure sizes and irregular shapes. Because the temperature gradient of the liquid metal molten pool near the fusion line is high and the cooling speed is fast, the temperature gradient is the main driving force for the nucleation of fine grains. Therefore, grains begin to grow in the reverse direction of heat conduction and advance perpendicular to the fusion line toward the center of the weld. The growth of other grain orientations is significantly inhibited by preferential growth, eventually forming typical columnar grains.
The welding center has noticeable columnar and fine equiaxed grains, as shown in Fig. 6a 2 -c 2 . The main reason is that laser welding has higher heating and cooling rates. Meanwhile, the columnar grains grow to a certain extent along the fusion line toward the center of the weld [37]. The liquid molten pool will lose the heat dissipation direction with the change of heat conduction direction. The growth of grain is inhibited eventually [38]. To sum up, the base metal is melted to form a metal pool and then cooled to form the as-cast structure under the action of a higher-energy laser.
The boundary of the FZ is the columnar crystal perpendicular to the EQZ, while the center contains columnar crystal and fine equiaxed crystal. It is found that the width of HAZ and EQZ increased with the higher laser power, as shown in Fig. 6. The width of the HAZ increases from 205 to 270 μm. The width of the EQZ increases from 16 to 37 μm. Because with the higher heat input, the residence time of the weld metal in the high-temperature zone becomes longer. The energy transfer from the FZ to the BM becomes larger, providing more driving force for grain boundary migration. Therefore, to control the width of HAZ and EQZ, process parameters with little heat input should be selected as far as possible to ensure weld forming.
Crystal orientation is beneficial for studying the relationship between microstructure and mechanical properties of laser-welded WAAMed 2319 aluminum alloy components.  It is found that there are equiaxed grains at the junction of HAZ and FZ, which is consistent with the microstructure observed by an optical microscope. It can also be seen from IPF that red, green, and blue represent grain orientation of (001), (101), and (111), and the crystal orientations of different grain colors are also different. In general, the color distribution of welded samples is relatively uniform, indicating that the preferred orientation of grain growth is weak, which is (001), (101), and (111) mixed orientation [14,30]. The grain size of FZ is calculated by EBSD statistics, as shown in Fig. 7d-f. It is found that the average grain size of 3000 W, 3500 W, and 4000 W laser powers are 15.5 μm, 15.8 μm, and 17.9 μm. To sum up, the grain size increases with higher laser power [39]. The Schmidt factor represents the degree of difficulty in starting a sliding system, as shown in Fig. 7g-i. Schmidt factor is smaller, the slip system starting and the material deforming is more difficult. The slip system {111} < 110 > of aluminum is selected. The redder the material color, the easier it is to slip and deform. The material color redder is more likely to slip and deform. The average number of Schmidt factors for 3000 W, 3500 W, and 4000 W is 0.466, 0.468, and 0.465. Therefore, the welding sample with 3500 W laser power has a higher tendency which is easier to slip deformation and has a strong deformation ability [39,40].
To further determine the strength of texture, the pole diagrams of welded samples (No. 1,No.2,and No.3) are analyzed, as shown in Fig. 8. Sample coordinates are defined by the X 0, Y 0, and Z 0 directions. In these polar maps, axis X 0 is horizontal, while axis Y 0 is vertical. Each polar map's intensity is reached at 2.61, 2.98, and 4.59. The polar diagram is mainly blue-green. In Fig. 8a, b, although there are the area of red high-density, the intensity is low [41][42][43].
Thus, the selected region has uniform crystal orientation and no obvious preferred orientation. In Fig. 8c, the color pairs of the polar map are apparent, and the density and intensity differ significantly. The texture intensity of 3000 W and 3500 W is weaker than that of 4000 W, indicating that their grain orientation is more random. Due to the high energy density of the laser welding process, high thermal stress values are generated. Therefore, the texture intensity of 4000 W is the strongest. Different textures lead to the anisotropy of mechanical properties. Thus, the texture intensity will affect tensile properties [43].
The metallographic of the weld joints under the different welding types (No.4,No.5,and No.6) is observed in the yellow area, as shown in Fig. 4a. The width of the HAZ is 196 μm, 209 μm, and 210 μm, separately. The width of the EQZ is 18 μm, 20 μm, and 18 μm, separately, as shown in Fig. 9. Although the width of weld HAZ and EQZ exist to change, they do not significantly change compared with different laser powers. When the laser power is the same, the width of the HAZ and the EQZ will not be affected by the welding type.
When the welding types are different, the microstructure of the weld pool is affected, as shown in Fig. 9a 2 -c 2 . When the welding type of the BM on the left and right sides is "0° and 90°," grain growth on one side will be parallel to the direction of heat dissipation. In contrast, grain growth on the other side will be perpendicular to the direction of heat dissipation. The microstructure of the FZ will have a slight difference. Meanwhile, more pores are produced near the fusion line.
The distribution of elements in the HAZ is tested with EDS quantitative mapping, including Al, Cu, Si, Mg, Zn, Fe, Ti, and V elements, as shown in Fig. 10. By comparing Fig. 10a with d-i, it is shown that Si, Mg, Zn, Fe, Ti, and V are evenly distributed without particular segregation phenomenon. The comparison between Fig. 10a, b shows that the Al element is evenly distributed inside the grain in the HAZ but sparsely distributed at the grain boundary [44]. The comparison between Fig. 10a, c shows that the Cu element is distributed along the grain boundary in the HAZ and less in the grain interior because the cooling process occurs in eutectic transition. To sum up, the Al and Cu elements are distributed inside the grain boundary in the HAZ. The Si, Mg, Zn, Fe, Ti, and V elements are evenly distributed in the grain interior and boundary.
The EDS line scan tests are conducted along with the line from the FZ to the HAZ. Figure 10j is the 1000 × image  Figure 10k is the content of elements, including Al, Cu, Si, Mg, Zn, Fe, Ti, and V elements. It is observed that the elements' content does not change significantly. Therefore, the distribution of elements is uniform between the FZ and the HAZ junction. Figure 11 shows the tensile properties and elongation of all samples. The welding tensile strengths and elongations of the sample perpendicular to the deposition direction (top sample) are higher than the sample parallel to the deposition direction (bottom sample). The top samples have no apparent anisotropy. The internal bottom samples leave the fusion line produced by WAAM and distribute along the AM direction, as shown in Fig. 2e.

Tensile properties and microhardness
The area around the fusion line easily produces pores with poor binding force. The pore is the weak position of mechanical properties of WAAM components. Therefore, the physical properties of the bottom samples are generally lower than the top samples [45][46][47]. Table 4 is the tensile properties' value of six group samples. The tensile strength of the welded joints reaches 193.06 MPa, 203.48 MPa, and 191.60 MPa. The elongation of the welded joints reaches 3.60%, 4.13%, and 2.87% under the different laser powers. It can be seen that the mechanical property of welded joints with the laser power of 3500 W is better than others with the laser powers of 3000 W and 4000 W. Meanwhile, the previous conclusion that the Schmidt factor of 3500 W is bigger than 3000 W and 4000 W also indicates that the 3500 W laser power welding sample has a higher plastic deformation capacity. The 4000 W laser power is more elevated. The existence of porosity will reduce the effective joint area of welded joints, and the mechanical properties will decline. The 3000 W heat input is lower, so the joint is poor fusion and poor tensile strength.
The tensile strength of the welded joints reaches 170.33 MPa, 180.77 MPa, and 151.88 MPa. The elongation of the welded joints reaches 1.70%, 2.40%, and 1.62% under the different welding types. It can be seen that the mechanical properties of the "0° and 0°" samples are better than other samples because the tensile direction of the "0° and 0°" sample is perpendicular to the base metal's fusion line. The fracture crack is perpendicular to the fusion line when the sample is stretched. It is almost impossible to fracture along the fusion line, so the overall binding force of the sample is relatively strong. On the contrary, due to the tensile direction being parallel to the parent metal fusion line, the mechanical properties of the "90° and 90°" sample are the lowest among the three data sets. For the "0° and 90°" welding joint, the fracture position occurs on the "90°" base metal but "0°." Because the "0°" base metal tensile strength is higher than the "90°" base metal, so the physical performance of "0° and 90°" is between "0° and 0°" and "90° and 90°." Therefore, the anisotropy of "0°" and "90°" is the primary influence factor on the tensile properties of the welding joint of laser-welded WAAMed 2319 aluminum alloy under the different welding types.
The fracture position of Fig. 12a-d is at the FZ. After the welding joint is heated, cooled, and placed at room temperature, the second phase dissolves in a solid solution. The solid solution and aging treatment are realized. The grain is refined, and the strength increases. The fracture surface is relatively unfair with sharp edges and corners, and many dimples indicate that it has apparent ductile fracture characteristics.
The fracture position of Fig. 12e, f is at the BM. It is observed that there are many small and deep dimples in the fracture area and there are many cleavage planes formed by tearing fibers around the dimples. It shows that the fracture mode of the joint is ductile-brittle mixed fracture mode, which eventually reduces the overall plasticity of the welded joint. So the tensile properties of the "90° and 90°" and "0° and 90°" samples welding joints are the lowest, as shown in Fig. 11.
The microhardness of welded joints is tested, as shown in Fig. 13. The whole curve has an M shape. The microhardness of HAZ is harder than FZ and BM, with the highest point reaching 163 HV. Overall trends of hardness variation are similar. Another, there are some mutation points with low hardness values in the FZ zone

Conclusions
The microstructure and property evolution of the laserwelded WAAMed 2319 aluminum alloy joint are   Textures lead to the anisotropy of mechanical properties. Therefore, the texture intensity will affect tensile properties. On the other hand, the increase of porosity will reduce the effective joint area of welded joints, leading to a decline in the mechanical properties of the welded joint. 4. The tensile deformation uniformity of welded joints under the different welding types is related to the crystal orientation of the base metal. "0° and 0" has the higher tensile strength and elongation, reaching 180.77 MPa, 2.40%. Due to the same laser power, the HAZ and EQZ zone have a similar width. Anisotropy plays a significant role in tensile properties. The sample fracture mainly occurs on the "90°" base metal but "0°." Because the "90°" base metal has plenty of fusion lines, it is easy to produce pores with poor binding force. 5. The microhardness value of HAZ is harder than FZ and BM due to 163 HV. Overall trends of hardness variation are similar. FZ zone is affected by pores, so the hardness value fluctuates up and down. 6. This study has demonstrated that it is possible to laserweld WAAMed 2319 aluminum alloy and provide solutions that AM technology difficultly produces large-scale components.

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Conflict of interest
The authors declare no competing interests.