Microstructure And Mechanical Property Of Friction Stir Lap Welded Al Alloy And Steel Joints

In recent years,with the rapid development of automobile industry,some problems,such as the energy consumption,environmental pollution and traffic security became very serious.Automotive light-weight would be the main development trend of auto industry.Because of the lower density and higher specific strength of non-ferrous,some steels gradually were replaced and were used in the automotive industry.Al alloys are one kind of excellent non-ferrous metals.Now,the joints of aluminum and steel as an important welding structure were applied to the automobile industry.However,the Al alloy and steel exhibit great differences in physical and chemical properties,and Al alloy and steel easily form hard and brittle intermetallic compounds during the conventional fusion welding,therefore,it will produce a serious effects on the mechanical properties of joints.In this paper,friction stir lap welds of 6061 Al alloy and IF steel were successfully achieved,and microstructural characteristics and mechanical properties of lap joints were discussed in detail.Furthermore,the post-weld heat treatment and addition of the nickel interlayer were subjected to improve mechanical properties of welds.Under optimum welding parameters(rotation speed of 1000rpm,welding speed of 50mm/min and the pin inserting depth of 0mm),the friction stir lap welding(FSLW)was conducted.No defect was obtained,and the maximum average failure load reached 3.75 kN.The failure locations were located at the interface region of the lap joints,revealing a clear cleavage fracture model.The microstructure of A1 alloy and IF steel lap joints included three zones,the welded zone of Al alloy,interface zone,and the welded zone of IF steel.In this case,the welded zone of A1 alloy consisted of the heat-affected zone(HAZ),thermo-mechanically affected zone(TMAZ)and nugget zone(NZ).Similarly,the welded zone of IF steel was comprised of HAZ,TMAZ,NZ and base metal of IF steel.Furthermore,the interface layer contained of Al13Fe4 and Al2Fe.The effects of the annealing temperature and time on the joint strengths of the steel-Al lap joints made by FSLW were investigated.With the increase in annealing temperature,the average failure load of the joint increased,and decreased when the temperature exceeded 300℃.As the annealing duration became gradually longer,the average fracture load of the joint increased continuously.When at the annealing temperature of 300 ℃ and holding time of 240 min,the maximum average fracture load of the joint reached 4.10 kN.Under the difference of annealing temperature and duration,the average fracture load of the joints were closely related to the thickness of intermetallic compounds layer.Adding the nickel interlayer could obviously increase mechanical properties of the FSLW joints of Al alloys and steel,and the average fracture load of the joint reached 4.0 kN,the failure was located at the interface.The addition of the nickel interlayer could inhibit the inter-diffusion of A1 and Fe atoms at the interface zone of Al and steel,and produce the Al-Ni intermetallic compound layer instead of Al-Fe intermetallic compounds.