| With the introduction of the concept of"carbon peak,carbon neutral"and the emphasis on energy conservation and environmental protection,lightweight materials had broad application prospects in the fields of automobiles,ships,aerospace,and other fields.Aluminum alloy and titanium alloy were important lightweight materials.Aluminum alloy had low density,high strength,and good plasticity.Titanium alloy had high strength,heat resistance,and corrosion resistance.In recent years,the connection technology of aluminum alloy and titanium alloy dissimilar metals had received widespread attention.Resistance spot welding had the advantages of low cost,high efficiency,and easy to achieve automatic operation.In order to make aluminum alloy and titanium alloy complement each other and make the best use of their advantages,it was of great significance to study resistance spot welding of aluminum alloy/titanium alloy dissimilar metals.In this paper,6061 aluminum alloy and TC4 titanium alloy were used as research objects to systematically study the macro morphology,microstructure,and mechanical properties of aluminum alloy/titanium alloy spot welded joints.The aluminum alloy/titanium alloy spot welded joint is mainly composed of the titanium alloy solid phase transformation zone,the aluminum alloy/titanium alloy interface zone,and the aluminum alloy nugget zone,presenting typical characteristics of fusion brazing.The solid phase transformation zone of titanium alloy was mainly composed of acicular martensite with different sizes.There were discontinuous and uneven intermetallic compounds in the interface region,mainly Ti Al3 and Ti2Al5 phases.The aluminum alloy nugget zone was mainly composed of columnar crystal zone（CGZ）,equiaxed crystal zone（EGZ）,and partially melted zone（PMZ）.The hardness distribution of aluminum alloy/titanium alloy spot welded joints was uneven,with an average hardness of 75 HV on the aluminum alloy side and 354 HV on the titanium alloy side.Under tensile shear load,there were two fracture modes in spot welded joints:button fracture and interface fracture.Resistance spot welding process parameters（electrode pressure,welding time,welding current）had a certain impact on the macro morphology,cross sectional morphology,mechanical properties,indentation rate,nugget diameter,and interfacial element diffusion layer of aluminum alloy/titanium alloy spot welded joints.The indentation rate,nugget diameter,and interface element diffusion layer width of aluminum alloy/titanium alloy spot welded joints increased with the increase of welding current and welding time.With the increase of welding time and welding current,the tensile shear force that the spot welded joints could withstand first increased and then decreased,with the maximum tensile shear force reached 7.02 k N.With the increase of electrode pressure,the width of interfacial element diffusion layer and indentation rate of spot welded joints showed an upward trend,while the nugget diameter showed a downward trend.With the increase of electrode pressure,the tensile shear force first increased and then decreased,reached a maximum value at2.5 k N.Research showed that adding Al Si12 foil as an intermediate layer between aluminum alloy and titanium alloy could improve the mechanical properties of spot welded joints.The mechanical properties of aluminum alloy/titanium alloy spot welded joints with Al Si12 foil first increased and then decreased with the welding current,with a maximum value of 5.63 k N.Compared with traditional aluminum alloy/titanium alloy spot welded joints,the mechanical properties were not improved,mainly due to excessive interface splashing and poor surface formation.The mechanical properties of aluminum alloy/titanium alloy spot welded joints added with Al Si12 foil first increased and then decreased with the welding current.The maximum tensile shear force could reach 8.02 k N.Compared with traditional aluminum alloy/titanium alloy spot welded joints,the mechanical properties were improved by 14.25%.The fracture mode of aluminum alloy/titanium alloy spot welded joints added with Al Si12 foil after stretching was mainly interfacial fracture.The fracture positions were mainly divided into two types.When the welding current was lower than 7 k A,the fracture was occured between the aluminum alloy and Al Si12 foil.When the welding current exceeded 7 k A,the fracture was occured in Al Si12 foil or between Al Si12 foil and titanium alloy. |
