Role of interfacial reaction on the mechanical performance of Al/steel dissimilar refill friction stir spot welds

Al/steel welds were successfully fabricated by refill friction stir spot welding. The welding parameters were optimised based on the clamping ring temperature and weld strength. 85.7% of welds achieve a strength which exceeds the American Welding Society requirement when the clamping ring temperature ranges from 230 to 265 degrees C. Cracks are formed under the pin and sleeve in the Al substrate at the Al/steel interface, which are associated with the tool sleeve plunging period and attributed to the alpha-Al + Al-Zn eutectic structure. The interdiffusion between Al and Zn at the steel surface produced an Al-Zn eutectic structure layer at the Al/steel interface, while part of the zinc coating materials is squeezed out of the sleeve periphery, leading to a brazing effect which contributes to weld strength. Nanoscale discontinuous Fe4Al13 and FeAl intermetallics form as a layer localised at the Al-Zn layer/steel substrate interface.