Formation And Microstructure Evolution Of Zn-added Friction Stir Lap Welding Of Aluminum And Magnesium Dissimilar Alloys

Due to the difference in physical and chemical properties,a large amount of hard and brittle intermetallic compounds(IMCs)are formed during welding process of Al and Mg dissimilar alloys,resulting in the serious reduction of mechanical properties.Friction stir welding with one or more assisted processes has enormous research value for regulating the type,amount and morphology of IMCs.The Zn-added friction stir lap welding(Zn-added FSLW)was employed in this study.Moreover,the quality of joint has been further improved by the optimization of welding process parameters and the Zn-added ultrasound assisted FSLW(Zn-added UaFSLW)process.7075-T6 Al and AZ31 B Mg alloys were selected as the base materials in this study,and the pure Zn foils with the thicknesses of 0.02,0.5,0.1,0.2 and 0.3 mm were employed as the interlayer.The results state that the interlocking was improved,and the effective lap width(ELW)and the boundary length between the stir zone(SZ)and the thermo-mechanically affected zone in the lower plate were increased by the Zn interlayer.Meanwhile,the fine Mg-Zn IMCs particles were formed and dispersed in the SZ of the Zn-added FSLW joint replacing the continuously distributed Al-Mg IMCs in the conventional joint,significantly improving the tensile shear load of the joint.Taguchi optimization method was used in this study,and the largest tensile shear load at the optimal parameters combination of 1000 rpm-50 mm/min-0.05 mm reached 8.68 kN.Besides the optimization of welding process parameters,the improvement of the joint quality was also realized by the ultrasound assisted process.On the basis of adding Zn interlayer,the mechanical and cavitation effects of ultrasound significantly affected the joint formation and Mg-Zn IMCs morphology.Zn-added UaFSLW joint had the bigger ELW and SZ area,the more complicated mechanical interlocking and the finer Mg-Zn IMCs compared to the Zn-added FSLW joint.For the Zn-added UaFSLW,the tensile shear load of the joint increased with the increase of the ultrasound power in an appropriate range.