Transition Region Design And Numerical Study In Sectional Multi-Point Forming

Multi-point forming (MPF) is a flexible manufacture technology for three-dimensional surface part of thin sheet metal. Its main idea is dividing the curved surface of the traditional stamping dies into many pairs of matrices of elements, and then the three-dimensional shape of sheet metal is formed by the curved surface of elements (element group forming surface). Relative to the traditional stamping technique, the MPF has saved a lot of time for die designing, die manufacturing and debugging. It has shortened the exploiting cycle and reduced the costs. Now, the MPF technology can be applied extensively in many fields, such as ship shell plates, the skin of high-speed locomotive, building structure, and medical engineering. As a new process of the MPF, sectional MPF process is a concentrated expression of MPF flexible characteristics and can achieve the whole deformation by regional deformation to use small equipment forming large parts.The strainedcondition and deformation of parts are more complex than the whole MPF and traditional die forming and prone to generate local severe plastic deformation in sectional MPF process. The local severe plastic deformation usually induces workharding which is difficult to eliminate and affects the part quality. The reasonable transition region design is the key of sectional MPF process. It can eliminate the local severe plastic deformation between active forming region and free deformation region and advance part quality and accuracy of shape. A transition region design method based on the pattern of punches is put forward. Dynamic explicit finite element software is introduced to simulate the sectional MPF process to study the harmonious results of transition region, the part quality, accuracy of shape and the impacts of sheet metal material, thickness and surface shape on severe plastic deformation of transition region. The main content and results are as follows:1) Transition region design in sectional MPF process A transition region design method based on the pattern of punches is put forward. The basic thought of the method is that divide the transition region curved surface into mesh based on the pattern of punches,make certain the node coordinate of transition region curved surface mesh through homogeneous variation of shape from active forming region to free deformation region, calculate the heights of punches based on the node coordinate values. Three transition region design methods are analysed. Curvature homogeneous transition, slope homogeneous transition and reduced curvature homogeneous transition. Comparative study on the three methods in transition region design of cylinder shows that the curvature homogeneous method is better than the slope homogeneous transition method and the reduced curvature homogeneous transition method is optimal design method because of avoiding solving nonlinear equation.2) Numerical simulation study on the harmonious results of transition region Dynamic explicit finite element software is introduced to simulate the first step in sectional MPF process to study the impacts of sheet metal material, thickness and surface shape on severe plastic deformation of transition region. Metal material and thickness both show the degree of deformation and have the same effect on transition region design. Transition region should be shortened when metal plate is prone to deform, otherwise the transition region should be lengthened. Further, the boundary condition of active forming region and free deformation region correlates with the degree of deformation. Cross and longitudinal Curvature affect the transition region design together and have some mutual effect in one-way sectional MPF process. The value of longitudinal Curvature has little effect on the design.3) Numerical simulation of sectional MPF processThe main things in numerical simulation of sectional MPF process are analysed and the finite element model of sectional MPF process is built to study the effect of virtual forming velocity on the results of sectional MPF. The virtual forming velocity in numerical simulation is 2m/s based on integrated consideration of part quality and computational efficiency. Simulation study on sectional MPF of spherical part shows that better surface quality and accuracy of shape are obtained. Comparative study on the sectional MPF and the whole MPF shows that the sectional MPF has a better surface quality and an extensive applied range under the same condition.