Study On The Technology For The Optimization Of Variable Blank Holder Force Based On Adaptive Simulation

Recently, aluminum alloy sheet, high strength steel and other lightweight materials have been applied into automobile body panel stamping more and more popularly, result of which decrease the auto body weight greatly and thus reduce gas consumption and emission. However, lower formability of these lightweight materials comparing with traditional low carbon steel brings tremendous challenge to manufacturing industry. As a simple and effective technical solution, Variable blank holder force (VBHF) could increase the formability of these materials. In-depth research has been conducted by various researchers and engineers, which gains a great deal of achievements. Nevertheless, VBHF technology hasn't been used in practical production resulting from many factors, one of which is VBHF optimization technique, especially simultaneous spatial and time optimal VBHF. Addressing this, systematic research has been carried out in this dissertation. New control objective is adopted based on the definition and analysis of BHF formability window. VBHF optimization model is built on adaptive simulation and PID control strategy, from which simultaneous spatial and time variant VBHF trajectories are achieved. The model is used in VBHF optimization of round cup, rectangular box, stepped box and deck lid. The retrieved trajectories are verified on a multipoint variable BHF hydraulic press and showed good effect to formability improvement.Firstly, considering the drawing limit under VBHF, feasible and infeasible BHF formability windows are defined, based on which a definition of optimal BHF considering major quality parameters is given via analysis of critical wrinkling and cracking control strategies. The predicting model taking anisotropy and wrap angle into account for critical cracking BHF of round cup has been built via mechanics derivation, from which theory BHF formability window has been derived. The definition of optimal BHF is verified by comparing part quality under different combination of critical VBHF trajectories.Secondly, measuring method for detecting and numerating flange wrinkling, sidewall wrinkling, cracking, dent-resistance and spring back has been discussed. The VBHF optimization model is built using adaptive simulation and PID control policy, in which critical wrinkling BHF is applied for start-up and majority stamping stroke and critical cracking BHF is utilized at terminal staging of stamping. Spatial VBHF optimizing is implemented by leveraging multi PID processors according separate binders. Thus, simultaneous time and spatial variant VBHF optimization is achieved.Later, the adaptive simulation using restart analysis module of LS-DYNA is discussed. The VBHF optimization model is used for round-cup, rectangular box and stepped box using aluminum alloy sheet, through which the optimal BHF trajectories were determined for each separated binder during the whole punch stroke. Besides, maximum drawing heights for rectangular and stepped box under constant BHF have been determined using constant BHF optimization design. The experiments on multi-point VBHF press correspond well with optimization simulation and maximum drawing depth is increased effectively, which shows the effectiveness of adaptive simulation VBHF optimization proposed in this study.Finally, the optimization model is used to determine optimal BHF trajectories for deck lid of NumiSheet'05 benchmark. Eleven pieces of Physical geometry drawbeads are used in simulation for better reality accordance. The comparison between forming under eleven optimal VBHF trajectories and constant BHF shows maximum thickness reduction could be decreased as well as better uniform in thickness distribution could be achieved using VBHF forming, which shows VBHF could improve formability of aluminum alloy using in this deck lid.Addressing the challenge of simultaneous spatial and time variant VBHF optimization, PID control strategy and adaptive FEM simulation are used in this dissertation to construct VBHF optimization model which could be applicable for simple experimental parts and actual auto panel. This model enables new optimization solution and design technique for BHF controlling, through which formability of aluminum alloy is effectively improved and technical basis is built for VBHF using in actual manufacturing industry.