Study On Ultrasonic Spot Welding Process And Interface Properties Of Mg/Al Dissimilar Metals

Magnesium and aluminum alloys are used in the automotive industry to meet the requirements of lightweight development of automobiles.Due to the poor welding performance and brittle intermetallic compound formation,it is difficult for Mg/Al dissimilar metals to weld by conventional fusion welding method.Ultrasonic metal welding as a solid-state bonding technology is widely used in lightweight metal welding,which also has unique advantages for welding Mg and Al dissimilar alloys.In this paper,the mechanical properties,microstructure and interfacial reaction behavior of ultrasonic spot welded joints of AZ31B/6061 dissimilar alloys with and without the addition of an intermediate interlayer were studied by universal tensile testing machine,scanning electron microscope?SEM?and electron backscatter diffraction?EBSD?.Taguchi method was used to optimize the welding parameters.The optimum welding parameters of Mg/Al were P=1400 W,t=700 ms and F=0.5 MPa.The welding energy input is controlled by changing the welding time.The results show that the tensile shear force and peeling force increase first and then decrease along with the welding energy.When the welding energy is 980 J,the maximum peeling force of the joint is 107 kN,and the failure mode of the weld was local Al alloy nugget pull-out.When the welding energy is 1540 J,a brittle and hard intermetallic compound?IMC?layer with thickness of²0?m composed of Al₁₂Mg₁₇ and Al₃Mg₂ phases is formed at the weld interface,which deteriorates the mechanical properties of the joint.The temperature of weld was measured by thermocouple.It was found that the interface temperature increased with the increase of welding energy,and the range of variation was 36.6%⁵5.2%.Under the action of high strain rate plastic deformation and temperature rise,the grain recrystallization occurs near the weld interface,which refines the grain and changes the texture orientation,and accelerates the element diffusion and promotes the growth of the interface IMC layer.The addition of Zn,Cu and Ag foil interlayer in the interface has a good effect on preventing the formation of Mg-Al IMC,especially the addition of Zn foil can effectively improve the mechanical properties of the joint.However,the addition of the Cu foil and the Ag foil causes the joint to have a weak interface of Mg/Cu and Al/Cu,resulting in a low joint bond strength.The formation of Mg-Al IMC can also be effectively prevented by using supersonic cold sprayed Zn coating as interlayer,but the performance of the joint is determined by the strength and thickness of Zn coating.Therefore,obtaining high quality Zn coating is the key to improve the mechanical properties of Mg/Al joint.