Ultrasonically assisted resistance spot welding: A comprehensive review

Ultrasonically assisted resistance spot welding (UARSW) provides a powerful route for reshaping interfacial metallurgy in both similar and dissimilar-metal systems. For similar metal joints, high-frequency ultrasonic activation successfully breaks up surface oxide films, stabilises early current paths, and refines the weld grains by driving a columnar-to-equiaxed transition. This mechanical action effectively removes typical fusion defects, such as porosity and micro-cracking, in aluminium alloys and high-strength steels. Across Al–Ti, Al–Fe and Ti–Fe configurations, ultrasound modifies transient thermodynamic conditions, enhances liquid–solid wetting, suppresses brittle intermetallic compound (IMC) growth and stabilises fusion-zone evolution under reduced electrical heat input. In Al–Ti joints, acoustic cavitation fragments coarse Al 3 Ti phases and promotes wavy interfacial interlocking. In Al–Fe systems, high-frequency shear stresses act as a kinetic brake that limits Fe–Al IMC thickness to safe levels. In Ti–Fe welding, ultrasound enables non-equilibrium crystallisation and intense melt agitation within the dual-fusion nugget, preventing eutectic-driven expulsion and macro-segregation. These mechanisms collectively improve joint strength, ductility and failure stability across all three systems. Despite these advances, industrial scalability remains constrained by severe electrode degradation, high acoustic-impedance barriers in dual-fusion systems and the absence of real-time adaptive control of transient acoustic fields. Current numerical models struggle to capture the coupled solid-state plastic flow, acoustic-driven melt hydrodynamics and rapid electrical-contact evolution that govern weld formation. Future progress will rely on smart frequency-tuning power supplies, multi-sensor fusion for real-time defect detection, wear-resistant electrode materials and multi-stage ultrasonic sequencing. Developing multi-scale numerical frameworks that bridge atomic-scale diffusion with molten-pool fluid dynamics will be essential for predictive control of IMC morphology and for transitioning UARSW from laboratory demonstrations to robust industrial implementation.