Research On Microstructures And Properties Of Friction Stir Welded Lap Joints Of Aluminum-copper Dissimilar Materials

Copper has been widely used in electrical,energy and chemical industries because of its excellent thermal conductivity,electrical conductivity and corrosion resistance.Aluminum alloys are rich in content,light in weight and well in formability,so the aluminum alloys have good application prospects.With the development of modern industry,the greening and light weighting of industrial equipment has become an inevitable trend.The aluminum-copper composite structures not only have the advantages of both,but also can reduce their weight and cost.So it is valuable to research the aluminum-copper composite structures.However,it is difficult to obtain sound dissimilar aluminum-copper joints by traditional fusion welding because of the differences in physical and chemical properties of aluminum and copper.Friction stir welding is a new and green solid phase welding technology,and during the welding process,the base metals did not melt,but were welded in the plastic state.Therefore,friction stir welding can effectively reduce the formation of welding defects,and has been widely used in the joining of aluminum-copper dissimilar materials.In this paper,5083 aluminum alloys plates with the thickness of 2mm and T2 pure copper plates with the thickness of 3mm were placed on the top layer respectively for friction stir lap welding.The surface formability,microstructures,phase's distribution,mechanical properties and electrochemical corrosion resistance of the lap joints were studied.What's more,the influences of welding process parameters on the microstructures and properties of the lap joints were analyzed.Moreover,the tests of introducing zinc intermediate layer were carried out,and the effects of zinc intermediate layer on the microstructures and properties of lap joints were investigated.As a result,test references were provided for the friction stir lap welding of aluminum-copper dissimilar materials.Results show that it can obtain sound dissimilar lap joints whether aluminum alloys or copper was on top layer at reasonable welding parameters in the tests.And the interface was a combination of mechanical occlusion and metallurgical bonding.During the welding process,base metals underwent intense plastic flow.As a result,Al-Cu hybrid structures such as HOOK structures,self-locking structures and layer structures were generated.What's more,these structures had different effects on the properties of lap joints.Dynamic recrystallization and grains refinement occurred in stir zone,so the microhardness increased,and the maximum microhardness was 266HV.Lap joints of Cu side can be clearly divided into nugget zone,thermal mechanically affected zone,heat affected zone and base metals.However,the partitions of aluminum alloys side were not obvious.The microhardness of lap joints on both sides was distributed in a“W”shape,and the lowest microhardness was located at transition zone at advancing side of the aluminum alloys plates.Therefore,a part of lap joints with aluminum alloys on the top layer were broken there in shear tensile test.However,other lap joints'interface peeled due to welding defects.At the parameters of 900rpm and 60mm/min,lap joints had the best shear tensile resistance and the maximum shear tensile force was4.05kN.The fracture mechanism of lap joints with Cu on the top layer was mainly related to the HOOK structures and interface defects.And at the parameters of 900rpm and 100mm/min,lap joints had the best shear tensile resistance and the maximum shear tensile force was 3.48kN.In addition,the XRD results show that brittle and hard intermetallic compounds such as Al₂Cu,AlCu₄,and Al4Cu₉ were generated at the interface,and they reduced the mechanical properties of lap joints.Moreover,the generation of defects or Al-Cu intermetallic compounds decreased the electrochemical corrosion resistance of the interface.However,the introduction of a zinc intermediate layer can increase the fluidity of the materials and promote more compact and continuous interactive structures.Also zinc can react with base metals to form compounds such as Al_0.4Zn_0.6,CuZn and Cu₅Zn₈,which can improve interface bonding properties,shear tensile resistance and electrochemical corrosion resistance.