Study On Interface Behavior Of Aluminum-steel Inertial Friction Welding Joint Based On Auxiliary Heat Source

For the dissimilar metals friction welding,the heat source temperature is usually required not to exceed the melting point of low melting point metal.In order to realize the low temperature welding of aluminum steel,a friction welding technology based on auxiliary heat source was proposed in this paper to ensure the welding quality,In this welding method a preheat measure was taken by adding an auxiliary induction heating system on the steel side before inertia friction welding,and temperature difference of Al-steel and the peak torque in the early stage of friction welding were reduced.At the same time,the micro-structure of the joint interface can be improved by the thermal effect and magnetic effect of the induction heat source and the thermomechanical effect of friction welding.Based on the presented technology the interface behavior of Ni-coated and non-Ni-coated steel joints at auxiliary heating temperature of 300°C,400°C,500°C and 600°C were studied by means of SEM,EDS and XRD.In addition,some heat treatments were carried out on the welded joints that obtained under the 500°C auxiliary heating temperature and the characteristic changes of welding interface micro-structure during heat treatment were also analyzed.Subsequently,it was investigated in detail that he growth mechanism of intermetallic compound in low temperature inertia friction welding joint between aluminum and high strength steel.The results indicate that with the increase of the auxiliary heating temperature from300°C to 600°C,the diffusion depth of Al and Fe and the thickness of the IMCs at the interface are increased also.In the meantime the homogeneity of the IMCs and the tensile strength are improved correspondingly.The interface compounds of non-Ni-coated steel joints are mainly composed of FeAl and FeAl₂ phases at 300℃,while at 400℃-600℃a new phase of Fe₂Al₅ is created.However some tough compounds such as Al Ni,Al₃Ni and Al₃Ni₂ are produced at interface with nickel-coated joints.The nickel plating can effectively prevent the diffusion of Al elements to the steel side and prevent the formation of Al-Fe intermetallic brittle compounds.Compared with the non-Ni-coate joint,the distribution uniformity and the thickness of interface compound layer,and the strength of the joint are improved obviously.The results of heat preservation at 300°C for different time after welding show that:the thickness of compound layer increases gradually from 1h to 4h with the extension of holding time while it increases little at the holding time of 1H and 2h.With the extend of holding time,the thickness of compound layer increases obviously.Al Ni₃phase will be generated at the interface of Ni-coate joints and Fe₃Al phase will be generated at non-Ni-coate joints during the holding process.At the same time,the FeAl₂ phase in the welding interface are decomposed into FeAl and Fe₂Al₅ phase.The tensile strength of the joint shows an upward trend during the extending the holding time.The strength of the non-Ni-coate joint is changed obviously when holding time is at the point of 1h and 2h,however it increases evidently after 3 hours.The joint strength will be 124.6 MPa（non-Ni-coate）and125.1 MPa（Ni-coate）when the holding time reach at 4h.The results of calculate that according to the thermal and kinetic theory of Fe-Al and the formation of Fe-Al intermetallic compound show that:the Fe₂Al₅ phase is the main component of the interface compound of the joint,and there is no Fe₃Al phase in the interface of aluminum-steel as-welded.Finally,the growth model of IMCs in the welding process of aluminum-steel and the post-weld heat treatment process is established on the basis of the experiment data and records.As well as the formation rule of IMCs is expounded according to the thermal and dynamic relationship between Fe-Al,which provides reliable theoretical supports for the inertia friction welding auxiliary heat source technology of aluminum and steel.