Microstructures And Mechanical Properties Of Cu/Al Dissimilar Metal Joints Welded By Ultrasonic Spot Welding

In the context of the implementation of the"Belt and Road"strategy and the establishment of the"Asian Infrastructure Investment Bank"in our country,the construction of infrastructures become a central issue around the world,which causes the significant increases in the use of copper.However,most of the enterprises rely heavily on imported materials due to the shortage of copper resources,which makes the Cu/Al composite structure an important way to solve the shortage of copper resources in our country,and the consequent Cu/Al dissimilar metal connection has become a technical problem to be solved urgently.With the huge difference of physical and chemical properties between copper and aluminum,it is difficult to connect Cu to Al with traditional welding methods.Ultrasonic spot welding is an ideal welding method to connect Cu to Al which has many irreplaceable advantages including less intermetallic compounds,safety,high-efficiency and many more.In this paper,the effects of welding parameters on forming,microstructures and mechanical properties of lap joints formed with T2 pure copper/6082-T6 aluminum alloy by ultrasonic spot welding had been studied.It was found that less intermetallic compounds were generated during ultrasonic spot welding process.Due to the fact that strength of the dissimilar metal joints is closely related to the width of the interface compound layer,post-weld heat treatment?PWHT?had been used to improve the mechanical properties of the joints and the effects of different temperature of PWHT on microstructures and mechanical properties were investigated.Furthermore,the surface of the workpiece was modified by electric spark,Zn-15Al-4Sn-2Mg-1Bi alloy and Zn-5Sn-3Cu-1Bi alloy.In addition,the effects of different surface treatments on the interface formation,microstructures and mechanical properties were studied.Experimental results show that,with the increase of welding energy,amplitude and welding static pressure,the peak temperature at joint interface gradually increased.And the joint formation was gradually improved.In addition,the porosities disappeared gradually,the mechanical properties of the joints were also improved,and the maximum shear force reached 3.72 KN?about 91.7 MPa?.However,when the static pressure increased to some extent?0.5 MPa?,the frictional force at the interface was too large,which would hinder the high-frequency vibration of the indenter,resulting in the lower peak temperature.The porosities could also reduce the mechanical properties of the joints.In addition,when the welding energy was lower than 1700 J,no stable Al-Cu intermetallic compounds was generated at the interface.However,when the welding energy reached 1700 J,there was a layered distribution of Al₂Cu phase generated at the Cu/Al interface.With the further increase of welding energy,the thickness of the Al₂Cu reaction layer did not increase obviously due to the high heating and cooling speed and brief residence time at high temperature.The joints formed with 1400 J were used for PWHT.The results show that,when the joints were subjected to PWHT at 200°C,the discontinuous Al₂Cu phase was formed at the interface and the shear force of the joints increased from 2.08 KN to2.46 KN.When the joints were subjected to PWHT at 300°C,the Al₂Cu reaction layer was formed at the interface.Lamellar distribution deteriorated the mechanical properties of the joints relative to a discontinuous distribution.When the temperature of PWHT reached 400°C,not only the Al₂Cu reaction layer but also the AlCu and Al₄Cu₉ reaction layers were formed at the interface,which further deteriorated the mechanical properties of the joints.When the temperature of PWHT was increased to500°C,diffusion reaction zone occurred out of the welding area,which increased the connection part of Cu/Al dissimilar metal.Although the intermetallic compound layers and cracks appeared in the joints,the shear force of the joints still increased to3.14 KN.When surface treatment was used on the workpiece by electric spark,the surface hardness of the modified workpiece increased which could cause the generation of porosities at the Cu/Al interface,and the mechanical properties of the joints were deteriorated.The shear force decreased to 1.80 KN.Besides,Zn-15Al-4Sn-2Mg-1Bi alloy and Zn-5Sn-3Cu-1Bi alloy were used to modify the surface of copper base metal,and the results showed that the mechanical properties of joints were improved.For Zn-15Al-4Sn-2Mg-1Bi alloy,Al_4.2Cu_3.2Zn_0.7 reaction layer,??-Al+?-Zn?eutectic microstructure and CuZn₅ phase were formed at the interface layer,which greatly reduced the brittleness tendency of the joints.The mechanical properties were improved and the shear force reached 5.09 KN?about 125.5 MPa?.For Zn-5Sn-3Cu-1Bi alloy,Cu-Zn intermetallic compounds and??-Al+?-Zn?microstructure replaced the Al₂Cu phase at the interface.The brittleness of the joints was also reduced and the shear force of the joints reached 6.58 KN?about 162.2MPa?.