Arc additive manufacturing has the advantages such as high stacking rate, high material utilization rate, low equipment cost, and ability to manufacture large components. Aluminum alloy parts with complex structures can be made by wire arc additive manufacturing (WAAM). What is more, WAAM can directly produce a complete component, which greatly simplifies the processing process. However, the aluminum alloy component made by WAAM has the defects of poor forming quality, many defects, and low mechanical properties, while the laser induced arc composite additive (LIACD) technology can effectively improve the forming quality and mechanical properties. Recently, most of research on WAAM of aluminum alloys is still focused on single-channel multi-layer thin-walled parts, but there are few researches on multi-channel and multi-layer arc additive manufacturing. Path strategy is the first step of WAAM. In the case of multi-layer and multi-channel, the heat dissipation conditions of different stacking paths are different, resulting in changes in the microstructures and properties of specimens and thus influencing the application fields. The LIACD manufacturing technology is based on arc and supplemented by a laser, which can further improve the forming quality and mechanical properties of arc additive manufacturing. In this paper, the laser induced MIG composite additive manufacturing technology is used to fabricate 2319 aluminum alloys. The effects of two deposition paths, namely unidirectional linear deposition and crisscross deposition, on the microstructures and properties of 2319 aluminum alloy blocks are studied.
In this study, a low power pulsed laser-MIG composite heat source is used for resurfacing welding of ER2319 welding wires on the 2219 aluminum alloy substrate. The protective gas used in this experiment is Ar with a mass fraction of 99.99% and a flow rate of 20 L/min. The welding current is 140 A, the average laser power is 300 W, the scanning speed is 450 mm/min, the wire feeding speed automatically matches the welding current, the inter-channel overlap rate is 25%, and the wire dry elongation is 11 mm. The interlayer cooling time is 60 s after each layer is stacked, and the inter-channel cooling time is 30 s after each layer is stacked. Two paths of unidirectional linear deposition and crisscross deposition are selected to make the aluminum alloy blocks. The two groups of samples are wire cut. The crisscross sections of these samples are first grinded with sandpaper and then polished and etched successively. The microstructures and fracture morphologies are observed under optical microscope and scanning electron microscope, respectively. The mechanical properties of these two groups of specimens are tested by the micro-hardness tester and the universal tensile test machine. The microstructures and fractography of the two groups of samples are analyzed by energy disperse spectroscopy (EDS).
As for the macroscopic forming quality, as shown in Fig. 5(a), for the unidirectional linear deposition, there exists a uniform fish scale shape in its cross section and obvious wave lines between layers. Aa shown in Fig. 5(b), for the crisscross deposition, there exist fish scales and strips alternating in cross section. In terms of microstructures (Figs. 6, 7, 8 and 10), the grains for the unidirectional linear deposition are smaller, and the growth direction of the columnar crystals is consistent. The pores are mainly distributed in the transition region between deposition layers, and the eutectic structures at the grain boundary present a chain distribution. The grains for the crisscross deposition are relatively thick and the distribution of columnar crystals is disorderly. The eutectic structures at the grain boundary show two forms of chain and bone, and the distribution of stomatal defects is wide. Because of the addition of a pulsed laser, a layer of fine equiaxed crystal region is generated between layers, and the distribution of the equiaxed crystal region for the unidirectional linear deposition is continuous, while the cross distribution is discontinuous. The difference in mechanical property for two paths can be seen from the cloud maps of hardness distribution (Fig. 11). The overall hardness distribution for two paths shows a trend of first high, then low, and finally high from bottom to top, and the porosity has a great influence on hardness. This phenomenon is related to the change of grain morphology at different positions of the sample. There is a large softening zone in the crisscross, indicating that the porosity defect has a great influence on the hardness distribution.
Observed from the macroscopic morphology, the grain size of the microstructures for the unidirectional linear deposition is smaller and the defects are less than those for the crisscross deposition. In terms of performance, the average hardness for the unidirectional linear deposition is 97.9 HV, and that for the crisscross deposition is 89.2 HV. The strength and plasticity for the unidirectional linear deposition specimens are anisotropic, while those for the crisscross deposition specimens are isotropic. The tensile strength for the unidirectional linear deposition specimens sampled along the X-axis is 233.58 MPa, and the elongation is 6.34%. The tensile strength sampled along the Y-axis is 275.52 MPa, and the elongation is 11.12%. The ultimate tensile strength for the crisscross deposited specimens sampled in the XY plane is 251.33 MPa, and the elongation is 7.68%. From the tensile property diagram (Fig. 12) and the SEM fractography for the tensile test specimens (Fig. 13), it is found that the crisscross plasticity is poor, and the large holes appear at the fracture, which influences the performance. The results show that the overall microstructure and properties of the aluminum alloy blocks in the Y-axis direction produced by laser induced MIG additive manufacturing unidirectional linear deposition are higher than those obtained by crisscross deposition.
