With the wide application of deformed and cast aluminum alloy structural parts in automotive lightweight manufacturing, the microstructure and properties of welded joints of dissimilar aluminum alloys under various thicknesses, heat treatment states, forms of welded joints, welding methods, and welding process conditions have attracted increasing attention from scholars. However, due to the different thermal physical parameters of dissimilar aluminum alloys, the fluidity of the weld pool and the content and distribution of elements in various overlapping forms differ, leading to issues such as high porosity, segregation, and severe thermal cracking in the laser overlapped joints of A356/6082 dissimilar aluminum alloys. Therefore, this study investigates the macroscopic morphology, microstructure, and pore distribution and formation mechanism of A356/6082 welded joints with various overlapping forms in detail.

In this study, mechanical polishing, cleaning, and clamping are performed before welding. During the welding process, high-purity argon gas protects the upper surface of the molten pool, and the flow and keyhole behavior of the molten pool are monitored and analyzed using an Acuteye high-speed image V4.1 system. After the welding test, X-ray nondestructive testing, optical microscope (OM), and scanning electron microscope (SEM) are used to analyze the pore defects, microstructure, and precipitates in the weld. In addition, a tensile shear test of the welded joint is conducted on a universal test machine by removing the residual height of the weld surface and adding a shim at the position of the holding end of the tensile shear sample.

Under the same laser line energy, compared to the overlapped joint when A356 is at the upper side and 6082 is at the lower side, there is a larger area of incomplete welding on the back of the overlapped joint when A356 is at the lower side and 6082 is at the upper side, the weld depth and width are reduced, and there are larger pores in the cross section (Fig. 3). When A356 is at the upper side and 6082 is at the lower side, the maximum pore diameter, average pore diameter, and porosity inside the weld are smaller than those when A356 is at the lower side and 6082 is at the upper side. With the same laser overlapping form, the maximum pore diameter, average pore diameter, and porosity in the weld increase with increasing laser power (Figs. 4 and 5). Due to the narrow overflow channel, long overflow path, and high cooling rate, bubbles generated in the lower area of the weld cross-section of the 6082/A356 laser overlapped joint can easily remain at the bottom of the cross-section to form pores. Bubbles generated in the middle of the weld cross-section are more likely to move to the transition position with lower energy due to the thermal diffusion of the residual air at this position. In the middle and upper parts of the weld cross-section, due to the solidification of the equiaxed crystal region on the upper side, bubbles generated in this region or other bubbles spilling into this position have sufficient time to overflow the weld surface (Fig. 9). The average tensile shear strength of the 6082/A356 overlapped joint is 70 MPa, while that of the A356/6082 overlapped joint is 125 MPa, representing 56% of the strength of the A356 base metal. The 6082/A356 overlapped joint strips at the transition position between the upper and lower plates, whereas the A356/6082 overlapped joint breaks at the 6082 side weld. The fracture source of the A356/6082 overlapped joint originates from a pore defect in the weld, expands rapidly in the side weld of 6082, and finally breaks through the heat-affected zone at the bottom of 6082 (Fig. 13).

The form of the laser overlapping significantly affects the weld depth-to-width ratio, porosity, pore size, pore distribution, and bubble overflow. Air intrusion, evaporation of Mg, violent flow of the molten pool, and instability of the keyhole are the main causes of pore formation. Meanwhile, the temperature difference of the laser heat source in the depth and width directions, the difference in the thermal properties of the materials, and the difference in the bubble overflow path are the main reasons for bubble retention in the weld metal. The depth-to-width ratio and pore size of the laser overlapped joint are important factors in determining the mechanical properties of welded joints.