Numerical Simulation And Forming Performance Comparison On The Process Of Multi-Point Forming And Die Forming For Sheet Metal

Multi-point forming (MPF) is an advanced flexible manufacturing technique for sheet metal forming. In MPF, the conventional solid die is replaced by a series of discrete elements, so the desired shape of the product is formed by the envelope surface of the "element group". The position and movement of each element is controlled independently in real time by the computer. Adjusting the relative positions of the elements, one can easily shape the desired surface according to the design, and then the speedy and flexible sheet metal forming is realized. Therefore, comparing with the conventional die forming method, through the use of MPF technique a significant amount of money and time to design, manufacture and adjust solid dies will be saved. The MPF technology can be applied extensively in many fields, such as vehicles covered pieces, aircraft coving, hull panels, pressure vessels, architectural panels, medical alloyed-rehabilitation of various shapes and so on.In the process of multi-point forming and die forming, the sheet metal is prone to produce wrinkling and tearing failures. The numerical simulation on the process of multi-point forming and die forming and comparison of forming performance of sheet metal between the two forming styles with the change of every parameter can make further understanding of MPF, predict the potential defects, choose the proper technical parameters to restrain or eliminate forming defects, and consequently improve the forming quality of the parts and efficiency of manufacture.There has been lots of numerical simulation study on the multi-point forming for sheet metal, including the analysis on the optimum path, simulations on theforming force, the impact of cushion, high flexible blank-holder and so on. But so far there has not been systemic comparison on the forming performance of sheet metal between multi-point forming and die forming. Based on a dynamic explicit finite element software—LS-DYNA, the numerical simulation of multi-point forming and die forming for sheet metal is conducted, including the creation of finite element model, the choice of material model, the establishment of boundary conditions and the treatment of contact friction and so on. The forming performance under the two forming styles with the changes of technical parameters and material parameters are analyzed and studied in details in this paper.The main contents and results are as follows:1) The choice of affecting factors on forming defectsIn stamping process, many factors affect the sheet metal forming quality, but the affection degree varies from small to big. By seizing and studying the impact of these main factors, technical parameters and material parameters can be selected more accurately. Material parameters of sheet thickness, yield strength, enhanced index and anisotropy coefficient and technical parameters of blank-holder force (BHF) and coefficient of friction are selected to be the affecting factors of multi-point forming and die forming. Considering that geometric parameters are easily to be adjusted in the multi-point forming, the radius of curvature is specially selected to be one of the main affecting factors. Moreover, the intervals of affecting factors largely determine the extent of the impact on forming quality, so only by the reasonable intervals can the real infection of the affecting factors in the practical stamping process be reflected. In this paper, the intervals of affecting factors are decided reasonably based on the actual experience, the real status and so on.2) The comparative study of sheet metal formperformance with two different forming methods about the impact of technical parametersBecause the sheet metal is prone to produce wrinkling and tearing failures, blank-holder should be taken both to the multi-point forming and die forming process. And due to the character of the element shape in MPF, a kind of cushion is introduced in order to prevent the dimple defects. The results showed: When thesame BHF is adopted, the formed blank is more easily tend to wrinkle under multi-point forming style than die forming style;In order to avoid the potential wrinkling defects the BHF for multi-point forming should be managed bigger than it for die-forming;Whether the multi-point forming or the die forming, the trends of wrinkling are both increased when BHF is reduced;With the increase of BHF, the tearing trends of both are increased, and the restraint effect on tearing of multi-point forming is better than that of die forming.The effect of coefficient of friction is quite obvious during the process of multi-point forming. With the increase of coefficient of friction, the tearing trends of formed blank becomes larger;The formed blank is prone to wrinkle while the coefficient of friction is too small, so a suitable lubrication condition should be set to get a proper coefficient of friction to make a stable and top-quality sheet metal forming realized. While under the die forming style, the changes about forming quality of formed blank is not evident within the coefficient of friction interval.Whether the multi-point forming or the die forming, the changes of radius of curvature make a great influence in the wrinkling defects;With the reducing of the curvature radius, the wrinkling defects become more serious;the no-wrinkling limited curvature radius of multi-point forming is a little bigger than that of die forming.3) The comparative study of sheet metal formperformance with two different forming methods about the impact of material parametersThe changes of the sheet thickness impact the enactment of technical parameters largely to the multi-point forming and die forming. The BHF should be decreased while the blank thickness is reduced, or else the tearing defect will easy to occur;In addition, the thinner the sheet is, the interval of the technical parameter to be selected becomes more limited, and the forming robustness is lower, i.e., the techniques become more exigent;Contrarily, the thicker the sheet thickness is, the higher forming robustness the formed blank is of.The yield strength plays an unimportant role in both wrinkling and tearing of sheet metal forming, but it plays an important role in the forming force and formingstress;When the required strength is meet as premises, the forming force of multi-point forming can be largely decreased when a minor yield strength sheet metal is used.Anisotropy coefficient is very important to the formed quality of sheet metal, but the infection to the forming force can be ignored;The trends of wrinkling under the multi-point forming style is severer than which under the die forming style;The choose of bigger value of anisotropy coefficient can obviously improve the distribution equality of formed blank with MPF.Enhanced index impact the forming quality in evidence;With the increase of enhanced index, the distribution of formed blank becomes more uniform;Whether the multi-point forming or the die forming, the forming force and the maximum stress are both change inconspicuously.