Study On Microstructure And Properties Of Fillet Welding Joint On Dissimilar Aluminum Alloy For Containers

With the rapid development of aluminum alloy processing technology,aluminum alloy as a lightweight material has been widely used in all walks of life.In this paper,the dissimilar aluminum alloy welding of6061 corner post and A356 corner fitting for aluminum containers is taken as the research object,and the pulse MIG welding procedure experiment of T-shaped fillet welded joint was carried out.The effects of welding speed and welding current on the weld formation,microstructure and mechanical properties of the joint were investigated.For the inevitable residual stress and deformation of aluminum alloy welding,we used ABAQUS finite element software to carry out welding simulation,and explore the methods of controlling and eliminating residual stress from the aspects of pre-welding preheating and welding sequence.The research conclusions are as follows:(1)The MIG fillet weld surface quality of 6061 and A356 dissimilar aluminum alloy plate is good.When the welding heat input is small,the depth of fusion is shallow,the bond with the base metal is poor,and many pores appear.The increase of heat input makes the depth of fusion and fillet weld size increase,and the joint shape changes from convex angle to flat and concave angle.The concave fillet joint with proper depth and good shape can be obtained by the welding process of 160 A current and 6mm/s welding speed.The joint consists of weld zone,fusion zone and heat affected zone.The microstructure of the weld edge is columnar crystal growing along the heat dissipation direction,the middle part are dendrites and equiaxed crystal.The partial melting zone in the A356 side is composed of α-Al matrix and very fine Al-Si eutectic.There are many ironrich phases in the eutectic organization.With the increase of welding heat input,the grain size of weld zone and heat affected zone also increases gradually.(2)Increasing welding current with fixed welding speed or decreasing welding speed with constant welding current can improve the heat input of welding,make the heat affected zone wider and reduce the overall microhardness of joint.The line energy of welding is too small,resulting in insufficient fusion between the weld seam and the base metal.The tensile specimen is broken in the A356 side partial melting zone,and the fracture is characterized by brittle fracture and intergranular fracture.When the welding heat input is increased,the weld formation is better and the tensile performance is improved.The maximum tensile force of fillet joint reaches 4104.1N when the welding current is 160 A and welding speed is 6mm/s.The fracture is in the middle of the welding seam,and there are many parabolic dimples in the fracture,which is ductile fracture.(3)Based on ABAQUS finite element software,the 6061/A356 tubeplate welding model and the corner post-corner fitting welding model were established.The welding temperature field and residual stress field distribution were obtained through welding simulation.The experimental results of residual stress and temperature were basically consistent with the finite element simulation.The residual stress is concentrated near the weld seam,which is manifested as the tensile stress along the welding direction,and there is a large compressive stress in the heat-affected zone.The residual stress decreases away from the weld.(4)Preheating and appropriately reducing the welding current can keep the maximum welding temperature unchanged,increase the initial temperature of welding,slow down the heating and cooling rate of the weldment,which can effectively reduce the residual stress and deformation of tube-plate welding.When the welding current is 120 A and the preheating temperature is 150℃,the residual stress is reduced by 40.8%and the deformation is reduced by 20.3% compared with the original procedure.When the welding sequence is changed,the peak value of longitudinal residual stress has little difference,but the stress distribution changes significantly.The residual stress in the middle of the weld is relatively stable,while the stress at the end changes significantly due to the thermal cycle of multiple welds.Welding at a later stage that produces a reverse heating effect in the welding direction of a certain weld can effectively reduce the residual stress at the end of the weld.Considering the residual stress and deformation,the optimal welding sequence is the segmented welding scheme with the right-angle vertex as the welding starting point.