Microstructures and mechanical properties of Al-Mg2Si-Si alloys resistance spot welded with Al-Si interlayers

This study investigates Al-Mg2Si-Si alloy joints produced via resistance spot welding (RSW) with and without the use of Al-Si interlayers. The joint microstructures produced both with and without Al-Si interlayers consist of three zones: the base material (BM), the heat affected zone (HAZ) and the weld nugget (WN). The WN is oval, and increases in the welding current increase the WN size in both welding methods. The joints produced via RSW with an Al-Si interlayer have larger WNs than those produced without an interlayer under the same welding current conditions. Relationships between the welding current and the WN size are established by Gaussian fitting and Lorentz fitting. A comparison of the two welding approaches shows that a successful weld can be obtained at a low welding current when an Al-Si interlayer is used, whereas a larger welding current is required to obtain the same result without the use of an Al-Si interlayer. The transition layer (interface between the HAZ and the WN) in the joints welded without an interlayer is a single eutectic phase structure; however, in the joints welded with an interlayer, the transition layer generates an equiaxed-columnar crystal zone. The formation of the equiaxed-columnar crystal zone and the change in composition of the WN (according to the results of differential scanning calorimetry tests) improve the tensile-shear loads (TSLs). The equations of the peak TSLs as a function of the welding current and WN size are established by Gaussian fitting.