Study On Preform Shape Optimization In Forging Process Based On Quasi-equipotential Field And Response Surface Methodology

Forging process is widely used in the field of engineering which is of great advantage of high quality and efficiency. In the forging process, the preform shape has great influence on material flow, filling and forming quality. Therefore, preform design plays an important role in forging process design which means to improve the products quality. In addition, the metal flow in forging is very complex and difficult to control, so it is still a puzzle to choose the best one from multi-preforms of great diversity. The traditional perform design based on "trial and error" could hardly achieve the optimal goal. In addition, most of the preform optimization methods only apply to two-dimensional axisymmetric workpieces, there’s barely a three-dimensional preform optimization method that could be universal.Quasi-equipotential Field Method is a non-numercial optimization algorithm, the fundamental principle of which is that the minimum work paths between the undeformed part and deformed part can be simulated by the equi-potential lines generated between them in electrostatic field, and this method has a broad application prospect in the field of forging perform design.In this paper, the perform design method was further discussed based on Quasi-equipotential Field, with the existing research findings gained by our laboratory, and this method has been applied to the forging processes of complex axisymmetric workpiece and three-dimensional non-axisymmetric workpiece for simulation analysis. Based on the principle of Quasi-equipotential Field Method, a new preform design method-Reverse Fitting Method was proposed to reconstruct the equi-potential line and equi-potential surface as the perform shape. The volume of preform was reduced with non-uniform scales to distribute the materials of every part of forging. Specific research contents are as follows:Based on the principle of Quasi-equipotential Field Method, the distribution of equi-potential field between undeformed shape and deformed shape was analyzed by electrostatic field simulation. On the issue of reconstructing the equi-potential lines and equi-potential surfaces, the data of finite element nodes of the geometry model in electrostatic field was extracted with the Finite Element Method. And similarly to Reverse Engineering Techniques, the certain nodes were fitted into curves and surfaces to restore the equi-potential lines and surfaces.The negative consequences that come with uniform scaling of equi-potential lines for perform design were discussed, based on this, the Non-uniform Scaling Method of equi-potential lines was developed and the scaling factor for every dimension was confirmed. According to the constant volume criterion for plastic deformation, various scaling factors were set for every perform described by equi-potential line and the volume non-uniform scaling process was accomplished for further discussion.Combining Quasi-equipotential Field Method with finite element modeling and engineering optimization algorithm, research on preform multi-objective optimization for large deformation two-stage forging was conducted by the example of mechanical axisymmetric brake drum part. The distribution of equi-potential line between blank and forging part was obtained by electrostatic field simulation method. Then, with the value of equi-potential line and volume ratio of perform to final forging as the design variables, the forging filling ratio as optimization goal, response surface analysis about final forging process of brake drum was carried out. Accordingly, the optimum volume ratio and appropriate range of equi-potential lines for the perform was determined. Finally, with the value of equi-potential line as single factor variable, the deformation uniformity and deforming force as optimum goal, the preform multi-objective optimization design was performed by golden section optimization method. Consequently, flashless final forging with complete die fill and optimum deformation uniformity and deforming force was obtained.Research on multi-stage preform process optimization design for long-axis non-axisymmetric parts was conducted with Quasi-equipotential Field Method. In the case of engine connecting rod forging part, three dimensional distribution of equi-potential field between blank and forging part was obtained by electrostatic field simulation method. Then, the equi-potential surfaces were reconstructed with Reverse Fitting Method. Based on the deformation properties of long-axis forgings, non-uniform scaling of perform described by equi-potential surface was accomplished. Response surface analysis and optimization about connecting rod forging part was performed with the value of equi-potential surface and volume ratio of perform to final forging as the design variables, the forging filling ratio as optimization goal, thus, the optimum pre-forging was determined. Cross-section analysis of the optimum pre-forging was performed to design preformed blank. Finally, advisable preformed blank was designed ensuring flashless pre-forging with complete die fill on the basis of FEM. Final forging with complete die fill and no folding defect was obtained after two-stage preform forging processes.