Study On The Microstructure And Properties Of Bobbin Tool Friction Stir Welding Of Copper And Copper Alloy

Copper and copper alloys have excellent thermal conductivity,electrical conductivity,corrosion resistance and high strength,etc.,and are widely used in electronic information,rail transport,aerospace,energy and power,ship and marine engineering industries.At the same time,copper and copper alloys have physical properties such as high heat capacity and fast thermal conductivity,which makes the welded joints obtained by brazing and fusion welding methods prone to weld defects or reduced electrical properties,thus affecting the use of welded joints.As a hot research topic in the field of welding in recent years,common FSW is a solid-state joining technology that can solve the above mentioned weldability problems of copper and copper alloys.However,the FSW technique requires a rigid backing plate to support the weld,which restricts the welding of special profiles with hollow structures,and also leaves a keyhole at the end of the weld,in addition to the tendency to produce unfused defects at the root of the joint.Bobbin tool friction stir welding（BT-FSW）is a derivative technology of FSW,using a stirring tool with an additional lower shoulder compared to FSW,which solves the problems arising from FSW welding.At present,the research related to BT-FSW technology for copper and copper alloys has not been reported in the literature,therefore,the research on BT-FSW for copper and copper alloys is of great theoretical significance and engineering application value for the BT-FSW technology in the field of joining copper and copper alloys and dissimilar materials.In this paper,a comprehensive and in-depth study was carried out to investigate the distribution and variation of temperature field,the relationship between microstructure and properties,the influence of welding parameters on the process;the relationship between microstructure and properties of BT-FSW joints of T2 pure copper/H62 brass dissimilar materials,the interfacial forming mechanism,and the flow law of plastic materials in the BT-FSW process of T2 pure copper and H62 brass.In addition,finite element analysis of the thermal process of T2 pure copper BT-FSW was carried out.The results show that the temperature changes at each temperature measurement point of the T2 pure copper BT-FSW process have the same trend,comparing with FSW,the heating rate is more slow,and the distribution of the temperature field is asymmetric,and the peak temperature of the advancing side（AS）is higher than that of the retreating side（RS）.The microstructures of the weld-nugget zone（WNZ）in the BT-FSW joints of T2 pure copper exhibit fine recrystallisation grains,and both the AS heat-affected zone（AS-HAZ）and the RS heat-affected zone（RS-HAZ）are coarse and large grains,and AS exhibits elongated and curved microstructure characteristics.A"strip-like"microstructure consisting of"coarse grain bands"and"fine grain bands"was observed near the stirring needle region.After BT-FSW,the texture type of WNZ changed to recrystallisation texture,while HAZ mainly showed the deflection texture of BM.The dynamic recrystallisation types of WNZ were continuous dynamic recrystallisation（CDRX）,discontinuous dynamic recrystallisation（DDRX）and geometric dynamic recrystallisation（GDRX）.The AS in the welded joint is the weakest region,where the fracture in tensile test occurs.The electrical conductivity of the joint increases and the corrosion resistance decreases compared to BM.Changes in welding parameters mainly affect the heat input of the T2 pure copper BT-FSW and the strain rate to which the material is subjected.The variation and distribution of the temperature field of the BT-FSW process for H62brass was similar to that of T2 pure copper,with the peak temperature of AS being higher than that of RS.The WNZ of the welded joints had fine equiaxed recrystallised grains,with theβ’phase diffusely distributed in theα-phase or along the grain boundaries.DDRX,CDRX,and GDRX occurred in the WNZ,which resulted in a decrease in the percentage of the small-angle grain boundaries.A portion of theβ’phase in the HAZ was dissolved,and another part of theβ’phase was diffusely distributed in theα-phase in a reticulated manner.The mechanical properties of the welded joints were improved compared to those of BM,and the strengthening mechanisms were fine grain strengthening,orowan strengthening and solid solution strengthening.The electrical conductivity and corrosion resistance of the joints decreased compared to BM.The change in welding parameters had a large effect on the microstructure and properties of the welded joints,especially with the increase in welding speed,tunnel-type welding defects appeared in the joints.The microhardness of the joints was related to the grain size,dislocation density and the content and distribution of theβ’phase as the welding parameters were changed.The trend of change in yield strength of the joints was not significant when the rotational speed was increased,however,high tensile strength and elongation were observed at rotational speeds of 1000 and 1200 rpm.When the welding speed increases,the yield strength,tensile strength and elongation of the joints have a tendency to increase and then decrease.The electrochemical corrosion resistance of the WNZ region of the joints showed a decreasing trend with increasing rotational speed or welding speed,and this phenomenon was mainly related to dezincification corrosion.When T2 pure copper placed at AS,well formed welded joints are obtained,and the cross-section of the joints has an obvious"dumbbell"morphology.The H62 brass in WNZ undergoes the organisation of CDRX,DDRX and GDRX,and obtains fine equiaxed grains.The T2 pure copper undergoes the phenomena of DDRX and GDRX,and the grain recrystallisation has an abnormal growth with coarse grains.The microstructure of the two materials in WNZ is divided into two kinds,when T2 pure copper and H62 brass show"river-like"combination of microstructure,the combination mechanism is metallurgical and mechanical interlocking combination;when the two present"block-like"mutual distribution of microstructure,mainly relying on the interface of atomic diffusion metallurgical bonding.The interface obtained through metallurgical bonding produces a new intermetallic compound Cu₅Zn₈.The microhardness of the joint is closely related to the microstructure.The yield strength values of the joints were between the two parent materials,and the tensile strength and elongation were lower than those of the two parent materials.Fracture occurs at the AS where the stress is concentrated,and the tensile fracture mode is mixed type fracture.The electrochemical corrosion resistance of the joints is between the two base metal.Based on the BT-FSW process heat generation model and heat conduction model,combined with the physical and chemical properties of the material,finite element analysis was used to numerically simulate the temperature field of T2 pure copper BT-FSW.The results show that the temperature isotherms of BT-FSW joints are elliptical in shape,and the change of tool rotation speed has a greater influence on the peak temperature,while the change of welding speed affects the shape of temperature field isotherm distribution.The peak temperature of T2 pure copper BT-FSW obtained by numerical simulation is in good agreement with the experimental results.