Effects of fusion zone size on failure modes and static strength of aluminum resistance spot welds

This paper examines the effects of fusion zone size on failure modes, static strength, and energy absorption of aluminum spot welded samples using a combined experimental, statistical, and analytical approach. The main failure modes for aluminum spot welds are nugget pullout and interfacial fracture. First, static strength tests using coupon configurations of lap shear, cross tension, and coach peel were performed on the joint populations with a controlled fusion zone size. Thirty replicate static strength tests were performed for each coupon configuration. The resulting peak load and energy absorption levels associated with each failure mode were studied using statistical models. Next, an analytical model was developed to determine the failure mode of an aluminum resistance spot weld based on limit load analyses. It was found that fusion zone size, sheet thickness, and the level and location of weld porosity/defects are the main factors influencing the cross-tension failure mode of an aluminum spot weld. Two additional spot weld populations with different fusion zone sizes were then fabricated to validate the analytical failure mode model. Static cross-tension tests were again performed, and the experimental observations confirmed the analytically predicted failure modes for each population.