Numerical and experimental study of the hot cracking phenomena in 6061/7075 dissimilar aluminum alloy resistance spot welding

Abstract

In this study, a CALPHAD-coupled finite element model was established to reveal the formation mechanism of hot cracks (including liquation and solidification cracks) in the dissimilar 6061/7075 resistance spot welding joints. The partially melting zone (PMZ) and the fusion zone solidified under tension during the cooling stage. When the composition of the local area is susceptible to hot tearing, crack initiation would take place. This can be revealed by the CALPHAD-coupled finite element model. At the 6061/nugget interface, the PMZ was torn apart under the thermal strain to form cracks which are perpendicular to the nugget border (Type I liquation cracks). At the 7075/nugget interface, the PMZ was torn apart by the solidification contraction of the nugget ((fS)nugget>(fS)7075) to form cracks along the tangent line of the nugget border (Type II liquation cracks). The calculation of the ∣dT/d(fS)1/2∣ index shows that the solidification cracking susceptibility of the 6061/7075 nugget is fairly high. The metallographic and energy dispersive spectrometer analysis shows that the Type I liquation cracks were filled by the liquid within the nugget while the Type II liquation cracks and solidification cracks created a local no-bonding area. The Type I liquation cracks did not affect the mechanical properties of the joints while the Type II liquation cracks deteriorated the mechanical properties of the joints.