Solidification microstructures and softening behavior of laser welded 6082-A356 aluminum alloy dissimilar butt joints

In this work, the solidification microstructures and softening behavior of laser-welded dissimilar butt joints between 6082 and A356 aluminum alloys are analyzed. The underlying mechanisms governing microstructural evolution, including grain and phase transformations are revealed. Several novel phenomena in the welding of A356/6XXX series dissimilar aluminum alloys are reported for the first time. It is observed serrated grain boundaries appear within the fusion zone (FZ), some of them transforming into very small equiaxed grains. Meanwhile, finely granular eutectic Si particles are observed along the fine cellular alpha-Al dendrites. Needle-like AlFeMnSi and pi-Al9FeMg3Si5 phases, as well as polygonal platelet-shaped Q-AlCuMgSi phase surrounding Si, are revealed in the FZ. pi-Al9FeMg3Si5 phases are formed in a spray form within the eutectics located in the partially melted zone (PMZ) on the A356 side. Phase transformation remains incomplete in both heat-affected zones (HAZs). Except for beta", beta', and beta phases, B ' phase also exists in the HAZ on the 6082 side. Coarsened beta" precipitates, bloke-like theta-Al2Cu phases, fine Si precipitates, and theta' precipitates are identified within the alpha-Al matrix in the HAZ on the A356 side. The welded samples mainly fail in the HAZ on the A356 side despite its higher hardness compared to the HAZ on the 6082 side. This work is of great significance in researching the non- equilibrium microstructures of the laser-welded dissimilar aluminum alloy joint, including the weld seam and HAZ.