Research On Formability Of 6000 Series Aluminum Alloy For Panel Under Viscous Pressure Forming

The need to reduce vehicle weight has increased the interest in using light weight materials, such as aluminum alloy, in automotive structures. But the part quality of aluminum alloy is restricted by its inferior formability at room temperature and serious springback. In this paper, the forming of aluminum alloy panel by viscous pressure forming (VPF) is presented. With the characteristic of viscous medium, the formability of aluminum alloy sheet metal is improved and the spring back is restrained, to promote the application of aluminum alloy sheet metal in automobile.Firstly, the formability and mechanical properties of 6k21-T4 aluminum alloy sheet metal is evaluated by viscous pressure bulging (VPB) test. The results show that the rolling direction has little effect on the limit thickness reduction; the planar anisotropy of sheet metal is weakened by VPB method. According to the limit strain obtained under different strain path using VPB test, the forming limit diagram (FLD) of 6k21-T4 is give out.Secondly, in sheet metal forming, the forming limit is generally determined by the onset of localized necking, however in the forming processes of 6k21-T4, fracture occurs without any obvious necking phenomenon. In the present study, therefore, ductile fracture criterion of Cockroft-Latham is applied to predict the forming limit of 6k21-T4. The material constant is determined by the combination of VPB test and finite element analysis (FEA). From the histories of stress and strain calculated by the FEM, the forming limits of bulging specimens under different strain paths and the double-curved panels are predicted by means of the ductile fracture criterion. The predicted results are in good agreement with the experimental observation. Formability window of the double-curved panel is also given out.The double-curved panel is investigated as having the typical characters of automotive panel. The effect of viscous medium on the plastic flow of the aluminum alloy is analyzed by FEM. The results show that the metal flow towards the difficult deformation zone is promoted by viscous media. The effects of the process parameters, including blank holder pressure (BHP), drawbead and panel depth etc. on the formability of aluminum alloy double-curved panel are also analyzed by the combination of experiment and FEM.Springback is one of the key factors to influence the quality of aluminum alloy panels. The effect of stress distribution on the springback of aluminum alloy double-curved panel is evaluated by FEM, and the effect of panel depth and drawbead on the springback is also investigated by the combination of experiment and FEM.Based on FEA of VPF forming and springback processes of aluminum alloy double-curved panels, its static dent resistance is also researched by FEM. The effect of VPF process parameters on the stiffness and static dent resistance of aluminum alloy double-curved panel are discussed. According to the numerical results, the dent tests of the panels formed by VPF with different process parameters are carried out, and the influences of different process parameters on the stiffness and static dent resistance are obtained.Finally, based on the investigation of VPF of the aluminum alloy double-curved panel, the VPF test of aluminum alloy 6k21-T4 scall-down specimens of a car outer door panel with complex shape is carried out. The character of metal plastic flow and corresponding thickness reduction is researched. The stiffness and static dent resistance at different measure locations and under different restriction conditions are compared based on the static denting experiments. The experimental results are in favour of the application of VPF in the manuafacture field of the aluminum alloy automotive panel.