Ultrasonic-assisted resistance spot welding of dissimilar AA5754/TA2 joints: process modelling, interfacial metallurgy, and geometric reliability thresholds

The structural integrity of dissimilar aluminium/titanium joints is governed by a complex competition between interfacial metallurgical bonding and the geometric morphology of the weld. This study characterises the failure evolution of AA5754/TA2 resistance spot welds, establishing quantitative criteria for joint reliability. Rather than process optimisation, the focus is placed on the transition mechanisms between brittle interfacial shearing and ductile button pull-out. Detailed fractographic and cross-sectional analyses demonstrate that the interfacial reaction layer, primarily TiAl 3 , must be maintained within a mean critical thickness of 3.7 μm to prevent premature interfacial cracking. However, fulfilling the metallurgical requirement is insufficient for ensuring structural reliability; the failure path is ultimately dictated by the local stress concentration at the weld periphery. A definitive geometric criterion is proposed: a 75% residual thickness threshold of the aluminium substrate is required to divert the crack propagation path into the base metal, thereby facilitating a desirable pull-out failure mode. High-fidelity predictive models ( R 2 >0.87) for these critical geometric features are provided, offering a robust failure-assessment framework for lightweight multi-material structures.