Weld microstructure and mechanical properties in ultrasonic enhanced friction stir welding of Al alloy to Mg alloy

A preliminary investigation was carried out into the application of angularly exerted ultrasonic vibrations in friction stir welding for the joining of AA 6061-T4 alloy to AZ31B at different tool rotation speeds. The variations in the welding process and weld properties due to the applied acoustic field were investigated. The process temperature was increased, the material flow path was widened and mechanical interlocking features at weld interfaces were improved in the presence of ultrasonics. Morphology and distribution of intermetallic compounds were influenced by the added vibrations at all rotation speeds. Formation of intermetallic layers at the weld interfaces was driven by heat input. Composition of the intermetallic compound was roughly unaffected but the layer thickness was reduced by the additional acoustic field. The ultrasonic enhanced improvement in weld mechanical properties was significant at very low rotation speeds but less substantial at higher rotation speeds.