Research On Microstructure And Shear Properties Of 6061 Al/QP1180 Steel Joint By Friction Stir Lap Welding

In recent years,automobile lightweight has attracted more and more attention.The automobile with steel frame and aluminum（Al）body can reduce the quality of the automobile while ensuring the impact performance,and finally realize the lightweight of the automobile.However,there are significant differences in physical and chemical properties between Al and steel.Al/steel dissimilar joints prepared by traditional welding methods have some solidification defects such as cracks and pores.In particular,the high heat input in fusion welding produces a large number of intermetallic compounds（IMCs）,which seriously reduce the joining quality of Al and steel.Friction stir lap welding（FSLW）,as a new solid-state welding technique,has a much lower heat input than fusion welding,and can obtain the Al/steel joints without solidification defects and with better mechanical properties.However,the generation of IMCs in the Al/steel dissimilar FSLW joints is still unavoidable,and the formation mechanism of IMCs and the effect on the joint are still unclear.Based on the above,interlayer materials were introduced into the dissimilar material FSLW of 6061-T6 Al alloy and QP1180 steel,and the joints without and with copper（Cu）or nickel（Ni）interlayer are studied in depth.The main conclusions are as follows:Rotation speed is the main factors influencing the heat input in FSLW.In this paper,the evolution of IMCs in Al/steel dissimilar FSLW joints was studied by changing the rotation speed of stir pin.The results showed that during FSLW,the thickness of the joint IMCs increased with the increase of the rotation speed,and Al Fe,Al₅Fe₂,Al₂Fe₃,Al₃Fe and Al₂Fe were the main IMCs generated on the interface.The shear properties of joints first increased and then decreased with the increase of the IMCs layer thickness.When the rotation speed is 500 rpm,the shear strength of the joint reached 301 N/mm.The joint was fractured at the lap interface due to the generation of IMCs,and the fracture mode was ductile and brittle fracture.The effects of Cu powder,Ni powder and Ni foil on the microstructure and properties of Al/steel dissimilar FSLW joint were studied.The results showed that after adding Cu powder,only Al-Cu IMCs were formed at the interface,indicating that Cu powder prevented the interdiffusion between Al and Fe atoms.The average shear strength of the joint was 183 N/mm,and the fracture occurred at the Al/Cu interface.After adding Ni powder,there are Al-Ni and Al-Fe IMCs existed at the interface,indicating that Ni powder could not completely obstructed the interdiffusion of Al and Fe atoms.The average shear strength of the joint was312 N/mm,and the fracture occurred at the Al alloy on the upper side of the Hook.After adding Ni foil,only Al-Ni IMCs were generated at the interface,indicating that Ni foil completely obstructed the interdiffusion between Al and Fe atoms.The average shear strength of the joint reached 329 N/mm,and the fracture also occurred at the Al alloy on the upper side of the Hook.The formation mechanism of IMCs during Al/steel dissimilar FSLW joints was studied by using the effective formation enthalpy and atomic collision theory.The results showed that during FSLW,the generation sequence of IMCs in Al-Fe joints is Al₅Fe₂,Al₃Fe(Al₁₃Fe₄),Al Fe₃,Al₂Fe and Al Fe.The evolution of IMCs was changed by adding Cu and Ni interlayers.On the one hand,the Cu and Ni interlayer inhibited the formation of Al-Fe IMCs by obstructing the direct contact between Al and Fe.On the other hand,Cu and Ni more easily react with Al and Fe,which prevented the formation of Al-Fe IMCs in terms of phase formation mechanism.In this paper,the effects of IMC on the mechanical properties of Al/steel dissimilar FSLW joint were analyzed by using the atomic mismatch theory.The results showed that the mismatches between Al₃Fe and Al₂Fe and the matrix were the largest in the joint without interlayer,indicating that the bonding strength of Al₂Fe to the matrix are the worst,which is the main reason for the fracture of the joint.The bonding ability of Al-Cu IMCs to steel matrix is lower than that to Al matrix in the joint with Cu powder.The average mismatch between the IMC of the joint with Ni interlayer was lower than that between the Al-Fe phase and the matrix,which improved the shear performance.