Research And Application On The Key Technologies Of High Strength Steel Drawing Die Structure Analysis

To develop lighter, safer and cheaper vehicles, High-Strength Steel (HSS) is more and more widely used in automotive industry. The load on die structure for HSS stamping increases sharply, die failure becomes key issue, die tryout is hard and HSS stamping is more sensitive to process variations.In the improved stamping die design process, the die structure analysis procedure is added into the current design process, and common parameter variations are considered when the sheet metal forming and the die structure analysis numerical simulation are performed. The strength and stiffness of the die are verified, die life is predicted and further the die structure can be optimized before the real die construction.A method for die structure FEA analysis based on sheet metal forming numerical simulation is proposed. The interaction force between the forming blank and the die faces are calculated based on the simulation result of the HSS formability, and then are applied onto the die faces to serve as load boundary conditions for the following die structure analysis. By separating the blank forming simulation process and the die structure analysis process, the method enormously reduces the computational cost and significantly simplifies the model complexity, thus makes the structure analysis of shape-complex and size-large die feasible within the current hardware and software infrastructures. The key technology to realize the proposed method—load mapping algorithm, is deduced systematically, which is used to map the load gotten from the sheet metal numerical simulation to the die structure analysis FEM model.It is founded that the contact forces between the forming blank and the die faces gotten in sheet metal forming numerical simulation contain severe fluctuation. The fluctuation is introduced by FEM. To reduce the amplitude of the fluctuation by FEM technology is computational costly, and theoretically, the fluctuation cannot be removed completely only by FEM technology. Filtering technology is introduced to deal with this problem. Sliding points filtering algorithm in time domain and FFT analysis algorithm in frequency domain are discussed, and the filtering effects of these algorithms are compared. It is found that FFT low-pass filter is effective and parameter robust to deal with this problem. Using the filtered load, the precision of the die structure analysis results increases and the FEM computing time decreases markedly. So, Filtering technology is added to the proposed die structure analysis procedure to further perfect the proposed die structure analysis method.Based on the aforementioned theories, a die structure analysis toolkit is developed and a dynamic stamping die stress/strain data acquisition system is developed. The toolkit is applied to analyze the stress distribution on a die for a DP600 hyperbolic bottomed box drawing and to simulate the unbalanced load absorption by the wear plates set on the die for a DP600 step-like bottomed box drawing, which are both verified by experiments using the developed dynamic stamping die stress/strain data acquisition system. The toolkit is successfully applied in finding the reason for the failure of a real stamping die used in a certain stamping plant, which verifies the proposed methods'effectiveness for its industrial applications.Blank holding force (BHF) and blank holding gap (BHG) are two modes usually used in complex sheet forming process to control formability. There are two common classes of stamping parts in automotive industrial. The formability of the two classes of part under BHF and BHG are different and the forces applied on the die faces are different too. The characteristic of their formability and die stress under BHF and BHG are studied, which disclosures the reason why HSS stamping is sensitive to forming equipments.During stamping process, the material properties, the process design parameters and the production environments inevitably have variation and noisy factors, which will possibly affect the sheet metal formability and the deformation of the die structure. The effect related with initial blank thickness, blank holder force, blank positioning error and die alignment error variations to a DP600 hyperbolic bottomed cup drawing die's forming loads, deformation and stress is studied numerically. The influence level of these variations to the die's forming loads, deformation and stress is disclosed.These findings can guide die design, die tryout and process control for HSS stamping with increased forming load and decreased sheet metal formability.