Experimental Study On Ultrasonic-assisted Friction Stir Welding Of Dissimilar Al/Mg Alloys

Aluminum is an element with abundant reserves and wide distribution in the earth's crust,with high specific strength,while magnesium has light weight and good shock absorption.With the advent of the lightweight era of automobiles,aluminum alloys and magnesium alloys will be widely used in the automobile manufacturing industry.The heterogeneous metal components of Al/Mg alloys have brought unprecedented challenges to the connection technology between Al/Mg dissimilar materials as aluminum and magnesium alloys have large differences in physical properties and crystal structure.It's very difficult to weld using the fusion welding method,and inevitably produces a large number of intermetallic compounds,which lead to the joint performance cannot meet the actual needs.Friction stir welding(FSW),as a solid-phase connection method,has unique advantages in welding Al/Mg dissimilar alloys.However,the conventional FSW process still has some shortcomings,such as tool wear,large torque and low welding efficiency etc.Ultrasonic vibration assisted friction stir welding(UVaFSW)technology is a new type of friction stir welding process that exerts ultrasonic vibration to the welding process through the stirring tool.It has shown unique advantages when welding the same aluminum alloy.This paper focuses on the welding of Al/Mg dissimilar alloys through UVaFSW.A comparative experiment between FSW and UVaFSW was carried out with 3 mm thick 6061 aluminum alloy and AZ31 magnesium alloy.The differences of the weld surface,internal forming,intermetallic compounds at the interface,mechanical properties were compared.The experimental results show that the application of ultrasonic vibration through the stirring tool can improve the weld surface formation,promote the mixing of Al/Mg dissimilar materials,reduce or even eliminate the internal defects of the joints,and widen the welding window of Al/Mg dissimilar metals.After applying ultrasonic vibration,under different welding process parameters,the tensile properties and elongation of Al/Mg dissimilar alloy joints are improved,and the fracture position is changed from the Al/Mg bonding interface to the nugget zone and thermo-mechanically affected zone district junction.Since ultrasonic vibration can reduce the distribution of residual stress in the joint,the corrosion resistance of the Al/Mg welded joint is improved.Ultrasonic vibration has a significant effect on the Al/Mg intermetallic compound,which can reduce the distribution of strip-shaped intermetallic compounds in the weld nugget;change the Al3Mg2 and A112Mg17 double-layer intermetallic compounds at the FSW Al/Mg bonding interface to single-layer Al3Mg2 intermetallic compound;and not only has a thinning effect on the intermetallic compound at the Al/Mg bonding interface,but also can play a crushing role.After the Zn coating on the magnesium plate,Zn-containing intermetallic compounds are dispersed in the weld nugget,and the intermetallic compound at the Al/Mg bonding interface disappear and become Al-Zn-Mg transitions.Ultrasonic vibration can promote motion of Zn in the weld gap and help form joint without defects.The experiment found that the application of ultrasonic vibration has little effect on the peak temperature of Al/Mg dissimilar alloy welding.Ultrasound is transmitted to the weld through the tool,which can soften the material near the pin,reduce the yield stress,and effectively reduce the axial pressure and torque of welding process,which improves the service life of the welding machine and the tool.Ultrasonic vibration promotes the mixing of materials,widens the weld area,promotes the flow of material at the bottom of the weld nugget,and makes the original vortex-like flow at the weld nugget bottom changes into a streamlined smooth flow.The change in material flow at this place improves the weld formation and the mechanical properties of the joint.