Researches On Connection And Interface Stability Of Ultra-fine-grained Copper And Aluminum Jointed By Friction Stir Welding

Ultrafine grained materials have excellent comprehensive mechanical properties,but their heat input often leads to structure recrystallize and instability in the welding process,resulting in a sharp decline in joint performance,which limits the application of ultrafine grained materials.Friction stir welding(FSW)is a kind of welding technology with low heat input,which is commonly used in the connection of middle-high stacking fault metals such as stainless steel and aluminum alloy.Under the action of a large number of stirring(deformation),dynamic recrystallization occurs in original micro structure,and fine grains are generated,which improves the mechanical properties of the weld.Copper and aluminum connectors are widely used in the industry,and are often used to prepare conductive bars,radiators(sheets).Therefore,ultrafine grained copper(UFG-Cu)and coarse grained 6061 aluminum alloy(CG-Al)are used as the research objects.Through FSW welding,the competitive relationship between UFG-Cu dynamic recrystallization coarsening and CG-Al dynamic recrystallization refinement during thermal mechanical deformation is used to find a welding process to inhibit the grain growth of ultra-fine grained materials.The welding process parameters have been selected:the stirring head speed of 800 rpm and 1000 rpm,the welding speed of 50 mm/min.The micro structure and performance observation of the welded joint are carried by X-ray diffraction analysis,backscattered electron diffraction analysis,Microhardness test,tensile test and electrochemical test.AB AQUS software is used to simulate the temperature distribution in the welding process,and the processing window with stable micro structure of UFG-Cu has been plotted.The main conclusions are as follows,(1)Under the same process conditions,the element diffusion of UFG-Cu/CG-Al joint is more sufficient than that of CG-Cu/CG-Al joint,and the "Hook" groove is smaller,resulting in intermetallic compounds such as Al4Cu9 with high copper.Under the parameters of 800 rpm-50 mm/min,ultrafine grained copper and aluminum can be observed at the UFG-Cu/CG-A1 joint(the grain sizes are both 700 nm),and the grain sizes of the micro structure at the CG-Cu/CG-Al joint are below 1 ?m.Under the parameters of 1000 rpm-50 mm/min,the copper side of the UFG-Cu/CG-Al joint grows up(grain size?20 ?m),and annealing twins can be observed clearly;the average grain size of the aluminum side is?5 ?m,The micro structure of the CG-Cu/CG-Al joint is unevenly distributed.(2)The(?)is eseutablished?The tempratre distribution of copper/aluminum weld during welding is carried out by ABAQUS.The results show that the highest temperature on the copper side of UFG-Cu/CG-Al joint appears a plateau under the welding parameters of 800 rpm-50 mm/min,which indicates that the recrystallization coarsening of UFG-Cu and the recrystallization refinement of CG-A1 reach a dynamic equilibrium under the heat input of FSW,and this phenomenon has not been observed at the CG-Cu/CG-Al joint.(3)By calculation of Z parameters at each point of copper/aluminum weld and EBSD results,lnZ and grain size processing map can be drawed.When lnZ is in the range of 31.4?32(temperature distribution 300??400?),the weld micro structure is still ultrafine grain(grain size?700 nm),which is suitable for pure aluminum and pure copper welding.