Study On Laser Tailor Welding Of Aluminum Alloy Sheet And The Formability Of TWB

With the development of the world economy, energy conservation and environmental protection become more and more important. As one popular product which complies with the trend in development of automobile industry, tailor weld blank (TWB) is one of advanced manufacturing technology in transportation industry for its merits such as material saving, weight reduction, cost reduction, and it can increase the security etc. Therefore, the study on technology and formability of the TWB of high performance and light weight materials used in cars plays a significant role to enhance the international competitiveness of the automotive industry in our country, and at the same time, it will creates certain economic benefits and social benefits. In view of this, the TWB of 6061 aluminum alloy with the thickness of 1mm is realized by analyzing the weldability, selecting the welding technology and optimizing the welding parameters in this paper, and then the further study on the formability of the TWB is carried out.Argon tungsten-arc welding (TIG) and laser welding (LBW) were adopted to achieve the joint of the TWB of 6061 aluminum alloy with the thickness of 1mm, and the microstructure and microhardness of the welded joint were tested. The results were shown that fine equiaxial crystal and tree crystal were in the welded seam of the TWB jointed by TIG, and near the fusion line, there were columnar crystals grown in the direction to welding beam with black precipitation phase along the grain boundary. The welded seam of the TWB jointed by LBW contained large quantity tiny equiaxial crystal and the heat-affected-zone (HAZ) was not obvious, the columnar crystals and cellular dendrite near the bond line were grown in the orientation of radiation. The hardness of the whole TIG welded joint was lower than that of the base metal, and the lowest value was in the center of welded beam. While the hardness of the LBW welded joint was as much as that of the base metal, and only fell slightly in the fusion area.Through the comparison and analysis for the two groups test results of the welded joints, laser TWB of 6061 aluminum alloy was selected for further testing, including tensile test, fracture surface scanning, cupping test and simulated the cupping process applying the software of DYNAFORM.The tensile test results of laser TWB indicated that fracture most located in the narrow HAZ that deviate from welded beam, the tensile strength and yield strength were higher than that of base metal, but the strain hardening index and the rate of elongation were lower than that of base metal. It is illustrated that the plastic deformation of the whole TWB was affected by the welded beam, and the forming property is slightly lower than base metal.A large number of equiaxial dimples with small and even size were homogeneous distributed on the tensile fracture surface, which was an indication that the specimens failed with ductile fracture characteristics under the action of tensile loading. The shallow dimples and specimens fractured without obvious plastic flow explained that the plasticity of TWB was inferior to base metal.As the cupping test results of laser TWB shown: cracking generally took place in the narrow HAZ that deviate from welded beam for the crystal grains became rough and resulted in the reduction of the plasticity and toughness with the effect of the thermal cycling. The cupping index of TWB was slightly lower than that of base metal, which indicated that the bulging formability of the TWB was worse compared to base metal. And the forming limit diagram (FLD) displayed that the forming limit of laser TWB of 6061 aluminum alloy was in theoretical ultimate zone of safety.During the numerical simulation process of cupping test with DYNAFORM software, only the location of the welded beam was considered but ignored the type of it, and the results showed that: simulation cracking of the base metal was easily occurred in the lower band of the cupping head, and the thickness varied evenly without discontinuity, simulation results were basically the same as the practical tests. The simulation cracking of the TWB was easily occurred at the welded seam below the cupping head with the influence of the welded seam, and the simulation results were slightly differ from the practical tests that possibly related to the set of the welded seam. The thickness varied relative evenly for the welded seam was narrow, and the simulation cupping index of the TWB was slightly lower than that of base metal, it indicated that the bulging formability of the TWB was worse than base metal.