Numerical Study On The Typical Process Parameters In Multi-Point Stretch-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. Applying the MPF to craft skin stretching has realized rapid and flexible forming which is suitable for the diversification, varieties and large size.The thickness of elastic cushion and the amount of elements have much effect on simulation results. To suppress the dimple defect the elastic cushion is adopted, which is the blanket between punches and blank, to distribute the pressure on the blank by its own large deformation filling the interspaces among elements during the forming course. However, the thickness of elastic cushion has great impact on not only dimples but also the accuracy of the part. Multi-point die acts on the sheet metal to accomplish the forming through the elastic cushion. Different amount of the elements will also change the effective area between the sheet metal and the multi-point die. To study the influence on the part owing to the two parameters, dynamic explicit finite element software is introduced to simulate the multi-point stretch-forming. The main content and results are as follows:(1) The numerical simulation of effect of the elastic cushion on suppressing dimple defect and the accuracy of shapeThe finite element model of multi-point stretching for air craft skin is built to study the cause of the dimpling and the effect of elastic cushion on suppressing dimpling defect. Through changing the thickness of elastic cushion in multi-point stretch-forming for paraboloid part and saddle part,the effect of thickness of the cushion on dimples is studied. The results indicate that applying cushion is effective for suppressing the dimples. When the thickness is 2mm, dimples can not be suppressed obviously. When the thickness is 5mm, there are still some slight dimples. the dimples can be eliminated thoroughly when the thickness is 8mm.In the meantime, the shape of part with different thickness elastic cushion are compared with the objective part .The results show that the accuracy of the shape gets lower with the increasing thickness of elastic cushion. Therefore it is necessary to adjust the elements considering the deformation of the elastic cushion.(2) The study on the effect of the amount of elements on the stress distribution and accuracy of the partThe simulation results of the paraboloid part and saddle part indicate that the amount of the elements has effect on the shape error .Using fewer elements will increase the shape error, so using more elements could improve the accuracy of the shape. The amount of elements has little influence on the maximum stress and minimum stress of the part, but when there are not enough elements the dimpling will lead to hyperdispersion of the stress on the part.(3) The study on the effect of the amount of elements on the stress distribution and accuracy of the partIn multi-point stretch-forming,resistance angle influences the shape accuracy of part. In the range permitted by stretching, the shape accuracy of part can be improved by enlarging the resistance angle to make the blank coating the die closely.(4) The numerical simulation research on multi-point stretch-forming of actual partAllowing for the big size and complex shape of the craft skin in practice, the finite element model is set up by choosing suitable process parameters and loading manner according to the shape characters of different the craft skin. The simulation results show that multi-point stretch-forming can be successfully applied to the craft skin in practice.