Study On Microstructures And Properties Of Laser Welded5754-H32 Aluminum Alloy Joints

Lightweight design is the main trend of vehicle manufacturing. Aluminum alloy is the key material to realize the lightweight design. For car body with 5754-H32 aluminum alloy manufacturing, using conventional MIG and TIG welding method,weld joints always have a large deformation and mechanical properties decrease greatly. Because the heat source of MIG and TIG welding is divergent, their penetration ability is poor with a low energy density at the limited welding speed. To solve these problems, in the study, laser welding method with high energy density,high welding speed and small welding deformation is adopted for 5754-H32 aluminum alloy. In the experiment, the microstructures and properties of two kinds of laser welded joints which are lap-fillet joints and butt joints were studied. The effects of welding parameters on the weld formation, microstructures and mechanical properties of the two kinds of joints were analyzed, and the thermal cracking tendency and the mechanism of weld defects of two kinds of joints were studied.The experimental results show that the depth of penetration and the thickness of the actual weld were more influenced by laser power. The weld width was affected more significantly by the welding speed. In order to obtain the good weld formation,the laser power and welding speed should be matched with each other. The weld microstructures of two joints were all uniform and fine equiaxed grains and there was also a small amount of equiaxed dendrite in butt joints. Fusion zone was composed of compact columnar crystals. HAZ was narrow without obvious coarsening. The main organization was the α-Al solid solution in the weld. For the lap-fillet joints, hardness of the weld was 65.64HV; hardness of the base metal was 62.05 HV, and hardness of the HAZ was 62.02 HV. The maximum tensile shear strength of the lap-fillet joints was 5.2kN, and the the joints fractured at the weld. The weld center was ductilefracture, and the edge of the weld belonged to shear fracture. Under the conditions of the stress amplitude of N=2 ×106, the fatigue strength of lap-fillet joint was 53 MPa,and the fatigue fracture location was in the weld. For the butt joints, hardness of the weld was 62.48HV; hardness of the HAZ was 62.50HV; hardness of the parent material was 63.68 HV. The maximum tensile strength of the butt joint was 274 MPa,and the joints fractured at the weld, which belonged to ductile fracture of micro porous aggregation. In bending test of the butt joints, there were no cracks in the case of 180° bending angle, and the anti-bending deformation ability was good.In the experiment of fish-bone docking method test, the hot cracking tendency of the laser welding joints was not obvious.The study on weld surface depression were found that element loss of Mg was the main factor for the lap-fillet joints. Instability of the molten pool’s behavior was the main factor for butt joints. The research on the porosity defects in the joints was found that oxide film porosity and hydrogen porosity existed in the lap-fillet joints. Except the reason that the oxide film was not cleaned before welding, the oxide film porosity was also related to the reformation of a new oxide film by the intrusion air between the upper and lower plates of the lap-fillet joint. There was only hydrogen porosity in the butt joints. Incompletely melted oxide film in the weld provided a hydrogen source for the formation of hydrogen pores. Too large solubility difference of hydrogen between the solid and liquid aluminum alloys was the key factor to generate the hydrogen porosity.