Transition in Interfacial Failure Mechanism of Resistance Spot Welds during Tensile–Shear Loading: Role of Fusion Zone Hardness

The failure of resistance spot welds through the fusion zone along the sheet/sheet interface (i.e., interfacial failure) is critical for automotive crashworthiness. This paper investigates the effect of fusion zone hardness on the interfacial failure behavior of resistance spot welds during the tensile–shear test. AISI 1040 medium carbon steel, producing a high level of hardness mismatch during resistance spot welding, was selected as the base metal. By ex situ tempering heat treatment, various levels of fusion zone hardness are achieved in the welds with constant fusion zone size. It is shown that the interfacial failure of the spot welds is a competition between ductile shear failure and rapid crack propagation. It is found that there is a critical fusion zone hardness beyond which the interfacial failure mechanism transitions from ductile shear failure to rapid crack propagation. In welds with high fusion zone hardness, the mechanism of interfacial failure is rapid crack growth, and fusion zone fracture toughness is the governing factor for the interfacial failure load. Conversely, in welds with low FZ hardness, the mechanism of interfacial failure is a ductile shear failure, and fusion zone hardness would be the governing factor for the interfacial failure load.