The Study Of Mechanical Property And Microstructure Evolution Of Al Alloy Weld In Double-Pulsed MIG Welding

The aluminium structures are widely used in automobile industry for their good mechanical performance and low density. Fusion welding of aluminium alloy is difficult due to the large deformation during the welding process andthe unsatisfactory mechanical property of welded joint.The present study is mainly focused on the fundamental investigation of double-pulsed MIG welding process of 6061-T6 aluminium alloy. The effect of welding wire composition, welding parameters and post welding heat treatment to the microstructure and properties of Al alloy thin wall structures, were systematically investigated.3-D FEM model of T type welding joint was established to simulate the DP-MIG process using the commercial software ABAQUS, the temperature, stress and strain fields were analyzed. With the periodic transformation of strong and weak pulse, the welding temperature and the dimension of molten pool changed regularly, which are benefit for smooth ripple and finer microstructures. The results show that the average welding current shows direction ratio with welding temperature, residual stress of welded joint, difference of current, temperature and stress under strong and weak pulse current. When the strong, weak pulse current difference is 40A, the average welding current is 90A, the maximum temperature of the welding pool exceeds the base metal melting point of about 200 ℃ and the strong and weak pulse temperature with 100℃ difference,. The welding ripple shows excellent appearance and the material presents googd weldability.Based on the simulation results of DP-MIG process of T type weld joint for thin aluminium sheet, four different welding sequences of the aluminium bumper beam have been analyzed. The results manifest that the continuous annular welding process obtains the lowest residual stress due to the homogeneous temperature distribution near the welding seam of energy absorption box. The symmetric welding process based on the single energy absorption box has the smallest deformation due to the counteract of deformation during welding process. The optimum welding sequences has been determined based on numerical simulation, the appearance and quality of the welding satisfies the demand of industrial production.The effects of welding wire compositions on microstructures and mechanical properties of T type aluminium alloy joint during double-pulsed MIG welding process (DP-MIG) were investigated. The results indicate that the strength of welded joint is determined by welding seam and the heat-affected zone. The distributions of hardness in the welded joint zone which processed by various welding wires have the same trend, which have approximately symmetrical distribution away from the welding seam line. The welding seam processed by ER5356 shows a cast structure with fine dendriticcrystal, the microstructure obtained using ER4043 presents cast structure with coarser dendriticcrystal. The microstructures present columnar grain structures in fusion zone, and fine equiaxed grains near the heat affected zone were found when adopting both of the welding wires. When compare to the original microstructure of base metal, the grains in heat affected zone undergone some degree of coarsening.The change of hardness and tensile properties of weld seam zone under different aging time and temperature was analyzed. The results indicate that the welded joint can obtain the best comprehensive properties under the condition of 180℃-8h. The effect of post welding heat treatments on the microstructure evolution has been discussed via analyzing the morphology of tensile fracture, the second phase in welding seam and the characterization of the precipitated phase. It can be noted that a large amount of dislocation in the welding seam zone without heat treatment and no precipitation was found. With the increasing of aging temperature, the dislocation density decreasing, with appearance and grown up of the precipitations at the same time. When the post welding heat treatment condition is 200℃-4h, the strengthen phase Q’with a large size can be observed, which is accounted for the peak tensile strength.