Study On Microstructures And Mechanical Properties Of Resistance Spot Welded Joints Of High Strength Steel And Aluminum Alloy Dissimilar Metals

With the increasingly aggravation of energy crisis and environmental pollution,the automotive industry is facing more and more severe challenges.Vehicle lightweight has become the main choice to save energy and reduce emissions.Adding lightweight materials such as high strength steel and aluminum alloy to form steel/aluminum composite structure are an effective measure to realize vehicle lightweight.However,the weldability of high strength steel and aluminum alloy is deteriorated due to the significant differences of physical and chemical properties between steel and aluminum,and it is very difficult to achieve an effective joining.Therefore,the research on microstructures and mechanical properties of resistance spot welded joints of high strength steel and aluminum alloy dissimilar metals is of theoretical significance and practical value.In this paper,the microstructures and mechanical properties of resistance spot welded joints of high strength steel and aluminum alloy dissimilar metals were studied by using the traditional?without process tape?resistance spot welding method and resistance spot welding method with the process tapes,and the effects of process tapes and welding parameters on the microstructures and mechanical properties of the joints were investigated.The results showed that the high strength steel/aluminum alloy resistance spot welded joints had the characteristics of fusion-brazing,and the joints were mainly composed of nugget zone,interface zone and heat affected zone.The aluminum alloy nugget had the characteristics of epitaxial solidification,and from the fusion line to the middle of the nucleus the grain morphology was in turn cellular crystal,cellular dendrite,dendrite,equiaxed dendrites because of increasing the supercooling degree.During the welding,the inter-diffusion of Fe and Al atoms,and interfacial reaction occurred at the steel/aluminum interface,forming intermetallic compound layers with a double-layer structure,Fe₂Al₅ layer generating at steel side firstly and then needle-like Fe₄Al₁₃growing epitaxially from Fe₂Al₅ grains to the aluminum nugget.The grain coarsened in the aluminum alloy heat-affected zone and it had a refinement trend in the high strength steel heat affected zone.Under the condition of traditional resistance spot welding,the welding parameters had obvious influences on the microstructures and mechanical properties of high strength steel/aluminum alloy joints.The results showed that the joint indentation rate,nugget diameter,intermetallic compound?IMC?layer thickness and tensile shear load increased with the increase of welding current?11kA¹3kA?,and the incomplete joining defects reduced to form a continuous interface layer.It was mainly attributed to the increase of the welding heat input.Further increase of welding current?14kA,15kA?decreased the joint tensile shear load,which is mainly due to the excessive heat input leading to the increase of welding spatter and causing shrinkage defects.With the increase of welding time,the nugget diameter,indentation rate,IMC layer thickness and tensile shear load were all rising because of the welding heat input increasing.As the increase of electrode force led to the decrease of contact resistance and heat input,the tensile shear load decreased with electrode force increasing?2.5kN⁵.5kN?.The nugget diameter,interface IMC and nugget shrinkage defect were the main factors to affect the mechanical properties of steel/aluminum spot welded joints.Under this test condition,the welding current of 13kA,welding time of 300ms and electrode force of 3.0kN were beneficial to improve the microstructures and mechanical properties of the steel/aluminum spot welded joint.The nugget diameter,IMC layer thickness and tensile shear load were7.10mm,2.20?m and 4.10k N,respectively.During the tensile shear testing,the steel/aluminum joints had the interface fracture mode,and the joining region between the brittle IMC layer and aluminum alloy nugget was the weak link in the steel/aluminum spot welded joints.Under the condition of resistance spot welding with process tapes,the process tapes and welding parameters had obvious influences on the microstructures and mechanical properties of high strength steel/aluminum alloy joints.Adding low carbon steel process tape on the high strength steel side and stainless steel process tape on the aluminum alloy side were more conducive for improving the joint quality.With the increase of welding current?11kA¹5kA?or welding time?100ms³00ms?,the indentation rate,nugget diameter,IMC layer thickness and tensile shear load increased.When the welding current was increased to 16kA,the formation of nugget shrinkage resulted in decreasing the joint tensile shear load.The welding current of 15kA,welding time of 300ms and electrode force of 3.0kN were helpful to improve the mechanical properties of the steel/aluminum resistance spot welded joint,the nugget diameter,IMC layer thickness and tensile shear load being 10.36mm,2.82?m,and 4.79kN,respectively.The joint fractured in the button pullout failure mode.Compared with the traditional resistance spot welding,the steel/aluminum resistance spot welding with process tapes had the advantages of improving the welding process stability,increasing the welding heat input,nugget diameter and tensile shear load,and decreasing the welding current and shortening the welding time.