Numerical Simulation And Process Research Of Friction Stir Welding Of Dissimilar Aluminum Alloys

Friction stir welding(FSW),as a new type of solid-state welding technology,has the great advantage in welding of aeronautical aluminum alloy.In this paper,buttwelding 7075-T6 and 2A12-T6 dissimilar aluminous sheets with the thickness of 6 mm was carried out by changing three parameters of rotating speed,welding speed and material in advancing side.With the combination of numerical simulation and experimental research,the effect of process parameters on the plastic flow behavior of FSW joints of 7075-2A12 dissimilar aluminum alloy was investigated.The influence of 7075-2A12 dissimilar aluminous FSW joints on forming,microstructure and mechanical properties was investigated by means of metallographic observation,phase analysis,mechanical property testing,and fracture obvervation.The plastic flow analysis of joint materials in welding process of 7075-2A12 dissimilar aluminum alloys shows that flow field of joints obtained by numerical simulation used of in the FSW welding process,the plastical material of joint flows from the advancing side to the retreating side surrounded by the frictiopn stir tool,so that the plastical material migrates to the rear of the frictiopn stir tool to form the weld seam.When the welding speed is 40mm/min,increasing the rotational speed appropriately can improve the ability of migration to the rear of the frictiopn stir tool,the ability of migration in welding seam peaks near 800 r/min.As the welding speed increses,the ability of migration and filling to the rear of the frictiopn stir tool for the weld material weakens.It is conducive to improve the ability of migration and filling to the rear of the frictiopn stir tool for the weld material when 7075 aluminum alloy is placed on the advancing side.The forming analysis of 7075-2A12 dissimilar aluminous FSW joints shows that the trend of appearing groove on surface of joints decreases,the raised degree of overlap at the back of joints lowers,the distance of adjacent overflowing belts lengthens,with the rotational speed increased,but the rotational speed is too high to lower the raised degree of overlap at the back of joints.And the tunel-like hole will appear,when the rotational speed is too high.With the welding speed increased,the trend of appearing peel on surface of joints lowers,the trend of appearing groove on surface of joints increases,the trend of appearing the tunel-like hole incrase.However,the welding speed is too high,which leads to increasing the appearing trend of flash and overflowing belt.It is is conducive to lower the trend of appearing peel on surface of joints,when 2A12 aluminum alloy is placed on the advancing side.Changing from 2A12 aluminum alloy placed on the retreating side to 2A12 aluminum alloy placed on the advancing side,the trend of appearing groove on surface of joints increases,the raised degree of overlap at the back of joints increase.The microstructure and mechanical properties of 7075-2A12 dissimilar aluminous FSW joints show that the T-phase and S-phase in base materials are broken into fine small particles in the weld,the microstructure of the welding nugget zone(NZ)transforms from rolled crystals to fine equiaxed crystals,with the frictiopn stir tool stiring and breaking.With the rotational speed increased,the grain size of the welding nugget and heat-affected zone increases gradually,resulting in the widening of the softened zone,the reduction of the hardness value of NZ,the decrease of the toughness of NZ,the decrease of strength and plasticity for joints.With the welding speed increased,the grain size of the welding nugget and heat-affected zone decreases,the hardness softening zone on the joint side becomes narrower,the hardness value in the weld zone increases,the toughness of NZ increases,the strength and plasticity of joints increases.That 7075 aluminum alloy is placed on the retreating side facilitates the grain refinement of joints,resulting in a better joint mechanical properties of the 7075 aluminum alloy placed on the receding side.The tensile fractures of joints are mostly located in the heat-affected zone(HAZ)of the 2A12 aluminum alloy.And The track of tensile failure shows the 450 shear fracture.The second phase(Mg5Si6)has an important influence on the fracture process of joints.The uneven distribution of the second phase in the direction of the thickness of sheets leads to the layered phenomenon of the joint tensile fracture.The tensile fracture of joints belongs to the tough-brittle mixed fracture form.In summary,the joint whose the rotational speed is 700r/min,the welding speed is 40mm/min,and the 7075 aluminum alloy is placed on advancing side is beautifully formed with less internal defects and good mechanical properties.Under this process,the tensile strength of the joint reaches 370 MPa,the elongation after fracture reaches 9.8%,and the impact energy of the joint weld zone reaches 14.98J.