Study On Microstructure,Properties And Connection Mechanism Of USW Joints Of Mg/Ti And Mg/Cu

As a green metal,magnesium and magnesium alloys can reduce energy consumption and reduce environmental pollution.Titanium and titanium alloys have advantages unmatched by other metals,have higher strength,stronger impact resistance,and can be used for a long time in high temperature and corrosive environments.Copper and copper alloys not only have excellent electrical and thermal conductivity,but also have good corrosion resistance,plasticity and ductility.Connecting magnesium with titanium and copper respectively,can obtain high-strength welded joints and apply them in the field of high-tech based on their excellent properties.Ultrasonic welding process is relatively stable,simple operation,fast welding speed,good forming,so it is an ideal connection method.In this paper,AZ31B magnesium alloy was welded with TC4titanium alloy and T2 copper respectively by USW and studied the interface formation,microstructure and mechanical properties of joints under different welding parameters.In addition,based on the basic principles of physical contact,chemical activation,diffusion and plastic flow,the forming mechanism of the two systems of magnesium/titanium?soft/hard metal?and magnesium/copper?soft/soft metal?was analyzed.The test results showed that,when AZ31B magnesium alloy and TC4 titanium alloy were welded with the welding energy and welding amplitude changing,the joint forming characteristics were relatively flat overall interface,local interface had small fluctuations,could not find defects such as cracks,unfusion.Welding defects appear at the interface when the welding static pressure was 0.2MPa.There were no obvious reaction layer in the interface under the three welding parameters.As the welding energy increased,the thickness of the magnesium/titanium interface diffusion layer gradually increased.The thickness of the interface diffusion layer was 4.0?m with the welding energy of 500J.The interface diffusion layer thickness reached a maximum of4.7?m with the welding energy of 1700J.When magnesium/copper were welded,the interface was relatively straight and there was no obvious reaction layer with welding energy of 500J.The certain thickness of Mg₂Cu was formed at the interface with welding energy of 1100J.The uniform thickness of Mg₂Cu was formed at the interface with welding energy of 1400J.The obvious welding cracks appeared inside the Mg₂Cu at the interface with welding energy of 1700J.When Mg/Ti and Mg/Cu were welded,the shear force of the joints increased first and then decreased with the increasing of welding energy.The Mg/Ti joint with welding energy of 1700J,welding static pressure of 0.4MPa and welding amplitude of95%,the joint shear force reached a maximum of 1832N.When the welding energy continued to increase,the joint would appear welding cracks due to fatigue,thereby reduced the joint performance.When the welding energy was lower than 2000J,the fracture mode of magnesium/titanium joint was interface fracture.The fracture mode of the magnesium/titanium joint was button fracture with the welding energy of 2000J and the joint fracture occured in the magnesium-side diffusion layer area and the magnesium-side non-diffusion layer area.The Mg/Cu joint with welding energy of1700J,welding static pressure of 0.4MPa and welding amplitude of 95%,the joint shear force reached a maximum of 1159N.As the welding energy continued to increase,the thickness of the interface reaction layer also increased.Due to the continuoued shear vibration of the joint,cracks appeared at the brittle and hard reaction layer,thereby reducing the performance of the joint.The fracture mode of the magnesium/copper joint was interface fracture with the welding energy of 1400J.The fracture occured mainly at the brittle intermetallic compound of Mg₂Cu,and part of the fracture occured at the magnesium side base metal near the interface.The USW of magnesium/titanium diffused between the atoms at the interface and the new bond formed between the two metals.The two parts worked together to improve the joint strength of the joint,and the uneven interface maked the two metals interact with each other.The shear strength would increase due to the degree of interlocking.USW of magnesium/copper was mainly due to the formation of a thick intermetallic compound layer at the interface,which maked the interface completely connected as a whole,but this compound belongs to the brittle and hard phase,so the connection strength was greater when the thickness moderate.