Dissimilar metal joining of aluminum to steel by ultrasonic plus resistance spot welding - Microstructure and mechanical properties

Highlights: • Developed new dissimilar metal joining method, ultrasonic + resistance spot welding • Produced sound joints of 1-mm-thick AA6061-T6 to 0.9-mm-thick low carbon steel • Less than 1.5-μm-thick intermetallic layer formed at aluminum to steel interface • Peak tensile shear strength of 3.2 kN with button pull-out failure mode • Temperature in dissimilar joint predicted by multi-physics finite element model -- Abstract: A new joining method, i.e. ultrasonic plus resistance spot welding, is developed for dissimilar metal joining of aluminum alloy (Al) to steel. In this method, a thin Al insert is first joined to a steel sheet using the solid-state ultrasonic spot welding (USW). Next, the Al insert side of the steel sheet is welded to an Al sheet by the standard resistance spot welding (RSW). The U + RSW method is used to join 1-mm-thick AA6061-T6 to 0.9-mm-thick AISI 1008 steel with 0.4-mm-thick AA6061-T6 as the insert. The final Al/steel weld shows a brazing feature with liquid aluminum wetting and spreading on the solid steel surface. At welding current of 16.5 kA, a less than 1.5-μm-thick layer of intermetallics is observed at the Al insert/steel interface, corresponding to a high joint strength of 3.2 kN and a nugget pull-out failure mode. The formation of such a thin layer of intermetallics is attributed to the metallurgical bond formed at Al to steel interface by USW, which in turn reduces the electrical resistance and temperature at this interface during subsequent RSW. The effect of USW and RSW parameters on the interfacial microstructure, nugget size, joint strength and failure mode is further investigated.