Design Of Blank Holders In Magnesium Alloy Forming Based On Level Set Theory And Metamodels

Because of the rich resource of magnesium alloy materials and excellent properties of its products,magnesium alloys have great application potential in industry.Due to the poor plasticity of magnesium alloy at room temperature,the formability of magnesium alloy can be usually improved by heating,and non-isothermal stamping is an effective warm forming technology for magnesium alloys.Improving the prediction accuracy of a metamodel and improving the quality of magnesium alloy non-isothermal stamping are the focus in this research.The main work is as follows:An improved Particle Swarm Optimization(PSO)is proposed.In the proposed algorithm,the learning factors and the weight coefficient are updated adaptively during iterations.The initial population is generated based on the Latin hypercube design,and the proposed algorithm can improve the distribution uniformity of the initial population and the ability of searching for global optimal solutions.Based on the maximum expected improvement criterion(EI),an improved adaptive sampling criterion is proposed.Compared with the EI criterion,the proposed criterion has stronger searching ability in the local maximum region and the accuracy of the kriging model based on the improved criterion is higher.Based on the proposed criterion,a parallel infilling strategy is proposed.Multiple new points are generated in the proposed strategy,this can make the kriging model converge faster and the sampling period is shorter.At the same time,the improved PSO is employed to optimize the kriging model,improving the accuracy of the optimal solution.Based on the finite element model of a fender part in NUMISHEET 2002,the drawbead restraining forces are taken as variables to construct the kriging model between the forces and the forming quality.The kriging model is optimized based on the proposed parallel strategy and the improved PSO,and the optimal drawbead restraining forces are quickly obtained,eliminating the crack and improving the forming quality of the forming part.Based on the level set theory,a kriging-based level set structure optimization method based on the parallel infilling strategy is proposed.A cross-shaped part with magnesium alloy in NUMISHEET 2011 is researched and the average thinning rate of the forming part thickness is taken as optimization objective.The proposed method is applied to the structure optimization of the blank holder in non-isothermal stamping.The design domain of the blank holder is determined firstly,and a discretized method is used to disperse the design domain.Some nodes are taken as variables and every node is assigned a random level set value between [-1,1] based on the Latin hypercube design.The first kriging model between the node and the level set value is constructed,and a blank holder structure for the kriging model can be determined according to a threshold value.The forming quality can be obtained according to the corresponding finite element model.Then,the level set on the node is taken as the variable to construct the second kriging model between the level set and forming quality.Based on the parallel infilling strategy,the second kriging model is updated and the best level set values are obtained by the improved PSO.Finally,the optimal blank holder structure is determined,and the results show that based on the optimized blank holder,the stress distribution and the thinning rate of the forming part are improved,and the quality of the forming part with AZ31 B magnesium alloy in non-isothermal stamping has been effectively improved.