Research On Formability And Damage Mechanism Of Deep-drawing Sheet Metal

The research on deep-drawing sheet metal formability is crucial to promote the domestic products of sheet metal, reduce the production costs and improve the quality of vehicle. Optimizing the material determination, lubrication condition and stamping process using strain analysis method for vehicle parts, proposing a new method for forming classification of parts and another new method for assessing the press forming levels of deep-drawing sheet metal, developing a damage model corresponding to the metallurgical characteristics of IF steel have been done in this dissertation.The silk screening technique has been developed to meet the process requirements for applying grid of high quality to vehicle parts. This technique has many advantages such as high accurate grid, good adhesion, low cost and high efficiency, and be very suitable to print grid for vehicle parts.The forming limit of sheet metal measured by standard test has been compared with the computer-simulated forming limit. It is shown that the results are very close and the computer-simulated forming limit can be used in engineering. The strain data on the deformed parts have been measured and the forming limit diagrams examined using strain analysis method. Then the critical portions and major forming patterns of part in the press forming have been determined and the forming results of sheet metal evaluated. Accordingly, the material determination, lubrication condition and stamping process can be optimized and validation results show that the optimal selections are appropriate for mass production. Based on strain analysis and material optimization for forty-three different vehicle parts, the material determination data of these parts have been established.The shape and forming pattern of the complicated vehicle parts have been studied and a new method for forming classification of parts has been proposed according to the shape characteristics and major forming patterns of critical portions in the press forming. The vehicle parts are divided into three categories, i.e., drawing type, plain strain type and drawing-bulging type by this method in order to giveadvice for forming.Based on the analysis of forming characteristics of the complicated vehicle parts and the actual formability of sheet metals, a new method assessing the press forming levels of deep-drawing sheet metal has been advanced. The press forming falls into seven levels within the range of plain strain FLD. In addition, the press forming levels have also been classified by using value n, r and sheet gauge according to the statistics relationship between value n and r.The microstructure in different forming stages have been observed and the damage mechanism has been analyzed, then a new model of damage for IF steel has been developed. The results illustrate that the model is corresponding to the metallurgical characteristics and can be used to explain the reason of good plasticity of IF steel. Hill's quadratic yield criterion has been employed to deduce damage evolution equations of IF steel in large plastic deformation and anisotropic plasticity. By measure of density during forming procedure, the damage variables have been calculated and compared with the results based on the damage model, which show a good coincidence.