Welding Interfacial Behavior And Mechanical Properties Of Aluminum/Steel Continuous Drive Friction Welded Joint

The aluminum/steel composite structure combines high specific strength,good electrical and thermal conductivity of aluminum and steel's high strength,good creep resistance and other advantages.It can meet many special application requirements,and is widely used in automotive,metallurgy,energy,aerospace and other fields.However,the formation of brittle Fe-Al intermetallic compounds?IMCs?is unavoidable whether fusion welding,fusion brazing or solid-phase joining is used to weld dissimilar metals of aluminum and steel.Continuous drive friction welding is a high-quality,high-efficiency and environmentally-friendly solid-phase welding method,which can better reduce the generation of IMCs when welding dissimilar metals.In this paper,continuous drive friction welding of 1050 pure alum inum and Q235low carbon steel was studied.The characteristics of the welding interface friction torque in the welding process and the characteristics of energy input at differ ent stages of joint formation were revealed.The mechanical properties of welded joints under different process parameters were studied.The morphological characteristics of the IMCs in different areas of the welding interface and their causes were elucidated.The main research contents and achievements are as follows:Firstly,friction torque is a comprehensive manifestation of friction resist ance during the continuous drive friction welding.By monitoring the real-time voltage and current of main motor stator during the welding,evolutions of friction torque on the joint interface were obtained and the effects of rotation speed and forging pre ssure on the friction torque and heat input of each welding stage were analyzed.The results show that the initial friction stage is the process of heat input accumulation,where“stick”friction heat generation is predominant.When rotation speed is at low level,friction torque curve only has a peak or the initial and final peak features are not obvious,the whole friction process is unstable.With the increasing of rotation speed,friction torque increases,heat input of the initial friction stage,the quasi-steady state stage and the whole process is increased,but heat input in forging stage is slowly reduced due to the decrease of the friction heating power during the forging stage.Final peak torque and heat input in forging stage are positively corre lated with the change of forging pressure.Then,the mechanical properties tests of joints shows when the rotation speed is1500r/min,the friction pressure is 30MPa,the friction shortening is 5mm,and the forging pressure is 80MPa,the joint has the best comprehensive mechanical properties.When the rotation speed is at high level,under the condition that the friction pressure and the friction shortening amount are not changed,it is necessary to appropriately increase the forging pressure to prevent the aluminum side metal from overheating in the welding heat affected zone.For continuous drive friction welding,the mechanical properties of different areas of the joint interface are not evenly distributed,that is,the mechanical properties are different in different areas of the interface radius.The combination of the area near the center of the joint is mainly based on mechanical bonding,and metallurgical bonding only occupies a small part,which makes the tensile strength of this part not high.However,if the thermoplastic metal in other areas of the welding process can flow to the center,then the proportion of metallurgical bonding will be greater than that of mechanical bonding,then the tensile strength of this part of the weld may exceed the aluminum base metal.Finally,the formation of IMCs at the interface was preliminarily analyzed from the thermodynamic point of view.The composition and morphology features of IMCs were analyzed from different observation angles by SEM,EDS and XRD.The differences in heating power between different regions of the welding interface during welding led to the differences in the diffusion behavior of Fe and Al at the interface,which led to the differences in the morphology features of IMCs at the interface.The IMCs in central region of the interface are small-needle-shaped Fe₃Al.The IMCs at a distance of 1/2R from the center are rod-shaped Fe Al.The IMCs at a distance of 2/3 R from the center are square-stick-shaped Fe Al₃.The most thermodynamically stable Fe₂Al₅ is not the final interfacial product,and the most thermodynamically unstable Fe₃Al is one of the final products.This may be closely related to the complex and special thermodynamic coupling phenomena in the interface during welding.Fe₃Al and Fe Al in the interface have better ductility than other Al-rich IMCs.So it is advantageous to use continuous drive friction welding to weld aluminum to steel.