Effect of material inhomogeneity on fracture modes in aluminium–steel resistance spot welds

Abstract Resistance spot welding (RSW) is attractive for joining dissimilar materials, especially, aluminium to steel in automotive body. The direct joining of aluminium to steel forms an intermetallic compound (IMC) layer at their interface that dominates mechanical behaviour of the joint. A new formula was developed that considers material inhomogeneities such as the different mechanical properties in the weld such as base metal, heat affected zone (HAZ) and the weld nugget to accurately calculate the minimum weld nugget diameter required to enable pull‐out fracture. The shear strengths of weld regions such as the HAZ and IMC were directly measured and used as inputs to this new formula. The new formula was validated using experimental measurements from six combinations of aluminium–steel welds in comparison with analogous aluminium–aluminium welds. The new derivation was able to accurately predict fracture modes for all material combinations.