Study On Microstructure And Properties Of Dissimilar Friction Stir Welded QP980 Steel To 6061 Aluminum Alloy

In recent years,the lightweight manufacturing technology of automobiles has become a hot research topic,while the steel/aluminum body structure is one of the effective solutions.In the actual application process of automobile industry,the welding of steel and aluminum is essential in order to obtain both technical and economic advantages.However,due to the great difference in physical properties between steel and aluminum alloy,a large number of intermetallic compounds are easily formed in traditional fusion welding.The joint has low mechanical properties.In view of this technical problem,this paper puts forward the solution of friction stir welding(FSW)technology.The defect-free and and excellent mechanical properties of steel/aluminum dissimilar metal welding joints were successfully obtained.In this paper,the quenching-partition 980 steel and 6061-T6 aluminum alloy are studied by the method of theoretical calculation and experiment.the microstructure,grain boundary characteristics,phase distribution,dislocation density and texture in different regions of steel/aluminum dissimilar joints were characterized,and the microhardness and tensile properties of the joints were tested.Meanwhile,the evolution mechanism of intermetallic compounds and its damage to the joint during welding were studied.The main conclusions of this paper are as follows:The offset of the stir tool is the key factor affecting the tensile properties of the joint during FSW.The excellent mechanical properties can be obtained when the stir tool is offset to the aluminum side of 1.3mm.After FSW,the grains in aluminum side stir zone and steel side stir zone are significantly refined due to crushing and dynamic recrystallization.During the FSW process,the Mg₂Si phase precipitates under the influence of thermal cycle in different regions of the aluminum side,and the retained austenite in different regions of the steel side is much lower than that of the base metal and transformed into more martensite.Compared with the base metal,the dislocation density decreased significantly and the pole density increased of the aluminum side,while the dislocation density increased and the pole density decreased of the steel side.The grain size of the steel side is significantly smaller than that of the aluminum side at the interface.The morphology and thickness of the intermetallic compound layer formed at the interface vary with the offset of the stir tool,and the thickness ranges from2?m to 3?m.The essence of the intermetallic compound formation during the FSW process of dissimilar steel/aluminum joint is the element diffusion reaction.Four kinds of intermetallic compounds dominated by Al-Fe phase are formed at the interface,and the order of formation is Al₃Fe,Al₂Fe,FeSi and Al₁₁Mn₄.With the decrease of dislocation density after FSW,the average microhardness of the stir zone on the aluminum side of the joint is lower than that of the base metal,while the average microhardness of the stir zone on the steel side is increased because of grain refinement.In the process of tensile testing,under the influence of brittle intermetallic compounds,the joint breaks at the bonding interface.The fracture mode is ductile-brittle mixed fracture,and the tensile strength is up to 70.1%of the 6061-T6 aluminum alloy base metal.The interface bonding strength of steel/aluminum joint is the core factor affecting its mechanical properties.When the offset increases,the number of intermetallic compounds at the joint interface is relatively low,the layer thickness decreases and is relatively smooth and uniform,so the mechanical properties are excellent.