Friction Behavior And Radial Distribution Of Microstructure And Mechanical Properties Of The Joint Prepared By Rotary Friction Welding

Rotary friction welding(RFW),an efficient and environmentally-friendly solidstate welding method,has extensive application prospect,especially in the aviation and aerospace fields that require high reliability joint,resulting from the lack of systematic research on corresponding forming theory and the distribution of microstructure and mechanical properties along the radial direction of joint.As a result,this would result in the absence of design criterion and optimum operating windows.On accout of above limitation,in this research,the friction behaviors and the effect of welding parameters on these friction behaviors in the RFW process would be systematically investigated in depth utilizing rotary friction welding experiments and materials possessing different physical properties(high-temperature yield strength).Also,the investigation of radial distribution of microstructure and mechanical properties in the joint would be conducted.Finally,on basis of above RFW theory,the forming process of hybrid joint would be investigated,which would lay theoretical and technological foundation of broad application of RFW in the field calling for sound and high reliability joint.Firstly,based on the RFW experiment of alloy possessing different hightemperature yield strength,the impact of high-temperature yield strength and the ratio of heat liberation in the first phase to the whole process on the friction behavior would be revealed.When the contribution of heat liberation in first phase is high,approximately above 20%,and high-temperature yield strength is low,the variation of welding parameters has significant impact on physical quantities including welding time,total heat generation by friction,axial shortening rate and heat pattern.By contrast,when the contribution of heat liberation in the first pahse is low,approximately below10% and high-temperature yield is high,the variation of welding parameters has little impact on the corresponding physical quantities.It is worthy of noting that,in the RFW process of GH4169,two torque peaks appear caused by the insufficient heat generation in the first phase,the dissoluteion of precipitated phase in the initial friction stage and the low density of dislocation at that time.Secondly,by the comparison of the radial distribution of microstructure in the joint welded under different parameters,the difference in such distribution has been revealed and such difference varies with different welding parameters.For AA6061 joint,the grain size in the mid-radius and center of the joint is relatively finer than that in the periphery zone.The fraction of low-angle grain boundary(LAGB)and the texture are different along the radial direction of the joint and the difference would be lessened with the increasing friction pressure.To be specific,under low friction pressure,the fraction of LAGB in the center of the joint would be nearly equal to that of base material and this zone has underwent full recrystallization by the continuous dynamic recrystallization(CDRX)and discontinuous dynamic recrystallization(DDRX).Also,the type of texuture in this zone is cubic texture {001}<100>.By comparison,in the mid-radius and periphery of the joint,the fraction of LAGB is far more than that of base material thereby indicating the existence of dynamic recovery(DRV)apart from CDRX and DDRX.The texture is cubic texture {001}<100> in the mid-radius zone while that is cubic texture {001}<100> and rotate cubic texture {001}<110> in the periphery zone of the joint.When the friction pressure is high,the fraction of LAGB in the joint is more than that of base material and CDRX,DDRX and DRV appear in whole joint.Besides,the texture is cubic texture {001}<100> in the whole joint and the corresponding intensity has little difference.For the RFW process of GH4169,there is a difference in the dislocation evolution between the center and periphery zone of the joint,i.e.,the multiplication of dislocation and the degree of dynamic recrystallization.Then,by means of radial distribution of mechanical properties of RFW joint,the welding parameters has strong impact on its distribution.In detail,rotation speed has significant impact on the hardness profile and radial distribution of tensile strength of AA6061 joint.At 900 rpm,the radial distribution of mechanical properties of the joint is even.For GH2132,under low friction pressure,the hardness profile in the periphery zone of the joint presents U-shaped,while that in the mid-radius and center zone of the joint presents W-shaped;under high friction pressure,the hardness of the whole joint increases and the hardness profiles along the radial direction change,namely the Ushaped hardness profiles in the periphery and mid-radius zone and W-shaped hardness profile in the center.Also,rotation speed has effect on the radially-distributed tensile strength and elongation at break.At 900 rpm,the difference in elongation at break along the radial direction of joint is large and there is difference in the evolution of local strain along the radial direction.After heat treatment,the radially-distributed mechanical properties of the joint would be mitigated.Finally,above results would be applied to RFW of TC4/SUS321 and the effect of rotation speed on radially-distributed microstructure and the mechanical properties of the whole joint.The profile of equilibrium temperature vs rotation speed presents Vshaped with the minimum value at 600 rpm.The temperature along the radial direction decrease from the center to the periphery zone.And the rotation speed mainly affects the morphologic distribution of IMCs rather than their thickness and the morphologic distribution changes from dispersive pattern to laminar pattern with the increasing rotation speed.The dispersive-distributed intermetallics would promote the tensile strength of this hybrid joint.All these results would facilitate the application of RFW in aviation and aerospace fields.