Evaluation of capabilities of ultrasonic energy on failure load and corrosion resistance of friction stir lap welded Al–steel joints

The lap joints of 6061-T6 aluminum alloy/Q235 steel were prepared by conventional friction stir welding and ultrasonic vibration enhanced friction stir welding. Firstly, the effect of ultrasonic vibration on the microstructure and properties of joints was analyzed, and then the optimization mechanism of ultrasonic vibration on failure load and corrosion resistance of joints was elucidated. The results indicated that ultrasonic vibration could significantly improve the quality of weld formation, expand the width of interface zone, change the shape of the “hook” structure, form a macro mechanical interlock at the joint interface, and effectively refine the grain structure on the steel side. In terms of the microstructure of the joints, additional ultrasonic energy can change the microstructure characteristics at the interface, and can improve the distribution characteristics of precipitates in the weld nugget zone, significantly reducing the number of intermetallic compounds generated across the interface of the retreating side and forming micro mechanical interlock on the advancing side of the interface. After ultrasonic treatment, the failure load of the aluminum/steel lap joint increased by 20.4%, and the corrosion resistance improved by 1.304 times.