Influence Of Additional Ultrasound On Interface Behavior And Mechanical Properties Of Friction Stir Welded Aluminum/steel Joint

With the increasing proportion of new energy vehicles in the market,the difficulty of lightweight body structure has become the biggest obstacle to the sustainable development of China’s automotive industry.Aluminum/steel composite structures have become a key solution to meet the lightweight requirements of vehicle bodies due to their ability to fully leverage the advantages of the two alloys.However,due to the significant differences in thermal physical and chemical properties between aluminum alloys and steel,it is difficult to obtain high-quality and reliable aluminum/steel heterojunction joints using traditional welding methods.Friction stir welding(FSW)shows great advantages of solid phase welding in the welding of the same and different alloys,but there are still problems such as low mechanical properties of joints and too thick interface brittle intermetallic compound layers(IMCLs)in the welding process of aluminum/steel.Ultrasonic vibration enhanced friction stir welding(UVe FSW)has a certain effect on improving the welding process and joint quality when welding aluminum/copper,aluminum/magnesium and other dissimilar alloys.However,there is limited research on the impact of ultrasonic vibration on the tensile strength of aluminum/steel joints and the formation and evolution of interface IMCLs.This project conducted FSW and UVe FSW docking tests on 3mm thick 2024-T3 aluminum alloy and Q235 carbon structural steel,and compared the differences in weld forming quality,mechanical properties,joint fracture type,and interface IMCLs.The experimental results show that ultrasonic vibration promotes the plastic flow of materials and refines the size of steel particles in the weld nugget zone,avoiding the generation of internal hole defects in the weld seam and expanding the welding process window;When the rotational speed of the stirring head is 550 rpm and the welding speed is 60 mm/min,the tensile strength and elongation of the UVe FSW joint are 12.28% and31.18% higher than those of the FSW joint,respectively;The fracture mode of the joint changed from brittle fracture of conventional FSW to ductile brittle mixed fracture..In the layered tensile test of the joint,it was found that ultrasonic vibration increased the tensile strength of each layer and changed the fracture position of the middle layer.Ultrasonic vibration improves the degree of recrystallization of aluminum alloy grains in the weld nugget zone,refines the grain size,makes the plastic deformation of the material in the weld nugget zone more uniform,improves the strength of the weld nugget zone,and transfers the fracture position from the conventional FSW weld nugget zone to the thermal affected zone;In the middle layer of the joint,ultrasonic vibration significantly reduces the thickness of IMCLs,eliminates the stacked structure,and forms a micro mechanical interlocking structure where IMCLs are inserted into the steel,improving the interface bonding strength and transferring the fracture position of the middle layer from the interface to the weld nugget zone;The lower layers of the joint are all fractured at the interface,but ultrasonic vibration promotes the extension of the hook like structure of the lower layer towards the bottom of the weld,enhances its mechanical interlocking with the aluminum alloy,and improves the tensile strength of the joint.The instantaneous state of the material on the horizontal section near the keyhole was obtained using the "emergency stop" method,in order to study the evolution process of IMCLs at different welding stages.It was found that ultrasonic vibration reduced the gap between the base metal before welding and promoted the tight flow of aluminum/steel;Ultrasonic vibration improves the mixing degree of the base material,making the initial IMCLs generated in the influence zone of the stirring pin much thicker than conventional FSW;In the influence zone of the shoulder,as the distance from the keyhole increases,the IMCLs of FSW gradually thicken and are mostly composed of Al rich phases.However,in UVe FSW,the Al rich phase IMCLs near the aluminum alloy side are largely broken and flow into the weld nugget zone,and the IMCLs gradually become thinner and mainly consist of Fe rich phases;When the interface separates from the shoulder range,the IMCLs in the forming area are basically stable,and the thickness of IMCLs in UVe FSW is smaller than that in FSW.In order to reveal the influence of ultrasound on the evolution of IMCLs,the welding thermal cycle and weld material flow were studied by temperature measurement,interface characteristics analysis and industrial CT testing.It was found that ultrasonic vibration reduced the welding temperature of aluminum/steel dissimilar metals,inhibited the formation of Al/Fe supersaturated solid solution and the transformation to IMCLs;Ultrasonic vibration promotes the plastic flow of materials,reducing welding loads such as axial force and forward resistance on the stirring head,and inhibiting the growth of interface IMCLs.The lower temperature and faster material flow in UVe FSW result in a growth rate of IMCLs that is lower than the failure rate caused by strong plastic deformation,resulting in a gradual thinning of IMCLs.