Shrinkage Void Formation in Resistance Spot Welds: Its Effect on Advanced High-Strength-Steel Weld Strength and Failure Modes

Among various defects formed in resistance spot welds of advanced high-strength steels (AHSSs), shrinkage void (cavity) is the least addressed topic. Although cavities are formed in numerous sizes and shapes, their influence on the strength and failure modes is yet to be investigated. The present study examines the effect of the cavity size and position on the peak tensile shear load and failure modes in 780 grade dual-phase steel used in automobile body structures. The size and position of the cavity in the nugget were varied carefully by altering the weld parameters and the sheet thickness. The cavity characteristics and its effect on the weld strength were evaluated from shear tensile tests and fracture surfaces. Results show that the size of the cavity increases with sheet thickness and nugget diameter. Further analyses revealed that the shear strength of the weld is influenced by the size and location of the cavity from the interface. A transition from plug-type failure to interfacial failure was observed with an increase in the cavity size. Consequently, the cavities reduce the load-carrying area of the nugget affecting the weld strength and failure mode. The effective nugget area, which is the actual load-bearing area of the nugget without any cavity inferred from the interfacial fracture surface, would be beneficial in determining the interfacial failures in resistance spot welds. Based on the results, a minimum nugget diameter of 5 root t was proposed to obtain plug-type failures in AHSS spots with the cavity.