The Study Of Microstructure And Property Of Electron Beam Welded Copper-high Entropy Alloy Joints

With the development of science and technology,the application of copper and steel composite structure is more common.But the direct weld of Cu/steel can produce liquid phase separation or weld defects because Cu and Fe mutual solubility in the solid state is very small,which lead to the decreasing of mechanical properties of the joint.In recent years,the researchers found a new system of alloy-high entropy alloy,this alloy has many excellent properties.What's more,due to its high entropy of mixing,high entropy alloys have a certain solid solubility to the new component,and a simple FCC or BCC structure can be formed in a certain range.In this paper,the high entropy alloy CoCrFeMnNi was used as the welding material,and the electron beam welding of Cu/CoCrFeMnNi high entropy alloy is studied.Firstly,the microstructure and mechanical properties of CoCrFeMnNi Cux(x=0,0.2,0.4,0.6,0.8,1,1.2)high entropy alloy with different Cu content is studied.The results show that the CoCrFeMnNi Cux high entropy alloy is a simple FCC solid solution.The high entropy alloys have good plasticity.After compression experiment,all the alloy columns become drum shape,without fracture.This results pave the way for Cu/CoCrFeMnNi high entropy alloy electron beam welding.Then the effects of beam offset and welding speed on the microstructure and bonding strength of Cu/CoCrFeMnNi high entropy alloy electron beam welded joints is studies.The main results is as follows:(1)The beam offset affects the fusion ratio of molten pool.With the offset of the electron beam from the copper side to the high entropy alloy,the content of the high entropy alloy in the weld pool increases,while the content of Cu decreases.When the beam offset is-0.2mm and 0mm,the Cu penetrating crack appear on the side of the high entropy alloy.This is because the content of Cu in the molten pool is higher,there are rich Cu phase at the interface of high entropy alloy.The welding process can produce complex internal stress,which leads to the diffusion of rich Cu phase to the high entropy alloy base along the grain boundaries of the high entropy alloy.The microstructure morphology of welding zone on the side of the high entropy alloy is typical columnar crystal,but there is a transition zone of copper on the copper side.The content of copper in the molten pool has a great influence on the microstructure morphology of welding zone.The welding zone in the all joints form FCC solid solution phase,does not produce brittle intermetallic compound.The tensile fracture of all joints occur on the copper side.When the beam offset is 0.6mm,the tensile strength reaches the maximum value of 225 MPa.The influence of the beamoffset on the tensile strength is small,but it has a great influence on the strain.This phenomenon is attributed to softening of Cu side.The microhardness of the weld zone of all joints changes gently.(2)The welding speed have influence on the heat input.The larger the welding speed is,the smaller the heat input is.The bottom of two base metals is not fused.The smaller the welding speed is,the larger the heat input is.The bottom of two base metals is melted,but the defect in the weld zone is serious.But there are pits in the upper welding zone.The component distribution of the welding zone is extremely uneven in the different welding speed,and the content of Cu in welding zone close to high entropy alloy base is less.The microstructure morphology of welding zone on the side of the high entropy alloy is typical columnar crystal,while there is a transition zone of rich copper on the copper side due to Hysteresis diffusion effect of high entropy alloy.The grain of welding zone is columnar crystal.The average grain size is affected by the welding speed.After tensile test,when the welding speed is1200mm/min,the bottom is not fused,and the fracture occur through the upper welding zone.When welding speed is 800mm/min and 1000mm/min,the fracture occur in the copper side.At 800mm/min,the heat input is large,its strain is relatively small due to softening of Cu side.But the effect of welding speed on the tensile strength is very small under different welding speeds.The microhardness of weld zone and high entropy alloy is approximately equal,and the change trend is flat in the welding zone.