Study Of Interface Microstructure And Mechanical Properties Of Friction Stir Lap Welded 6061 Aluminum Alloy/Q235 Steel Joints

Aluminum/steel composite structures are widely used on car bodies in order to reduce weight and cost.However,it is difficult to effectively connect these two metals due to the huge difference in physical properties between aluminum and steel.Friction stir welding(FSW)is a new type of solid phase welding technology,which has great advantages in realizing dissimilar metal welding.Therefore,this paper conducts friction stir lap welding(FSLW)tests on 2 mm thick 6061 aluminum alloy and Q235 steel,and analyzes the influence of welding process parameters on the structure and mechanical properties of the joint.Finally,we use numerical simulation software to analyze the temperature distribution and material flowing during the welding process,First of all,in the FSLW of 6061 aluminum alloy and Q235 steel,the microstructure and mechanical properties of the joints under different parameters were studied.Due to the heat input and the mechanical action of the tool,after different welding thermal cycles and plastic deformation,each area in the joint had different microstructure.Due to the diffusion of atoms at the interface,an intermetallic compound(Fe Al3)was formed,and its thickness increases with the increase of the rotation speed or the decrease of the welding speed.Due to the action of the pin,the broken steel mainly exists in three forms: the "hook" on both sides of the stirring zone(SZ)on the interface,the fine steel particles dispersed in the aluminum matrix,and the composite layered structure at the interface.The mechanical performance test of the joint showed that the hardness curve of the aluminum side showed a “W”-shaped distribution as a whole,and the heat-affected zone(HAZ)is the position where the joint performance was weakest,and its hardness value was the lowest.As the rotation speed increases or the welding speed decreases,the thickness of the brittle intermetallic compound(IMC)gradually increases,and the mechanical properties of the joint showed a downward trend.Finally,it was found that when the rotation speed was 600 rpm and the welding speed was 90 mm/min,the mechanical properties of the joint were the best,and the fracture location was in SZ.Four different types of double-pass welded joints were obtained by changing the direction and welding position of the second pass,named after A-A joint,A-R joint,R-A joint and R-R joint.During the second pass of welding,as the first pass goes through the thermal cycle again,the grains grew up,the low-angle grain boundary ratio decreased,the dislocation density decreased,and the performance of this area decreased.This structural difference leads to the hardness of the SZ in the second pass was significantly higher than that in the first pass.At the interface of the two passes,an IMC layer with similar thickness and the same composition was formed,and the main phase was Fe Al3.In the shear tensile test,aluminum/steel joints have three fracture modes: interface fracture,fracture in the SZ,and fracture in the HAZ.The presence of hard and brittle IMCs caused the single-pass welded joint to fracture at the interface;the fracture position of the R-A joint and the R-R joint was located in the HAZ,where the grains are coarse and belong to the weakest position in the joint;the A-A joint and the A-R joint are in the HAZ.The fracture in the SZ of the first pass,because the "hook" was prone to stress concentration,initiation of crack sources,and changes in the structure of the area,the mechanical properties of the fracture were weaker than the R-A joints and R-R joints fractured in the HAZ.The numerical simulation results showed that most of the heat input was provided by the frictional heat of the shoulder.In the temperature field,the hightemperature concentrated area appeared around the shoulder and diffused to the surrounding ring.The highest temperature reached 496°C,which was about 76% of the melting point of the aluminum,so the workpiece would not melt during welding.The temperature and viscosity distribution of the weld cross-section was "bowlshaped".The temperature of the cross-section was asymmetrically distributed with respect to the center line.The isotherm presents the characteristics of wide top and narrow bottom.The temperature on the advancing side was higher than that on the retreating side.Around the SZ,the temperature in the front of tool was lower than that in the back,which can provide certain theoretical guidance for obtaining highquality joints.