다점성형은 3차원 곡판 성형에 있어서 혁신적인 기술이다. 다점성형은 높이 조절이 가능한 펀치 배열을 이용하여 기존의 일체형 금형을 대체할 수 있다. 다점성형장치는 펀치의 적절한 배치를 통하여 3차원 곡면을 쉽고 간단하게 구현할 수 있으므로 이는 제품 설계, 생산, 금형 제작에 소요되는 시간과 경비를 크게 절감시킬 수 있을 뿐만 아니라 변형경로를 변경하고 성형성을 개선시키는데 적용이 가능하다. 그러나 불연속적인 접촉에 의하여 발생하는 표면의 결함과 탄성회복에 의한 형상 오차가 발생할 수 있다. 본 연구에서 다점성형공정에서 금속 판재의 변형거동에 대한 수치적 해석과 실험을 통하여 표면에 결함이 발생하지 않는 최적의 다점성형공정을 설계하였으며 성형성을 개선시켰다.다점성형공정에 대한 적절한 수치적 해석 적용을 위하여 유한요소이론에 대한 연구를 수행하였다. 다점성형공정 해석에는 외연적 유한요소법을 이용하였으며 탄성회복 및 다단 다점성형공정해석에는 외연적-내연적 기법을 사용하였다. 유한요소모델을 구성하기 위한 유한요소, 접촉 및 마찰 문제, 재료 모델, 가상일 원리 등에 대한 연구를 수행하였다. 아울러 주어진 곡면에 대한 펀치 위치를 산출하기 위하여 NURBS를 이용하였다.후판에서 관찰되는 딤플의 형성에 대한 분석 및 해석을 수행하였다. 딤플은 주로 후판에서 발생되는 결함이며 목적 곡률이 커질수록 그리고 판재의 두께가 증가할수록 딤플의 발생이 감소함을 보였다. 알루미늄 판재는 철에 비하여 딤플이 쉽게 발생한다. 이러한 결함을 방지하기 위하여 펀치와 펀치 사이의 불연속적인 성형면을 변형에 의하여 보정해 줄 수 있는 탄성 패드를 사용하였으며 이를 통하여 하중을 분산시키는 효과를 얻었다. 초탄성 재료와 선형 탄성 재료의 두 가지 모델을 적용하였다. 그 결과 두 가지 재료 모두 적은 압축 변형율에 대하여 유사한 거동을 보임을 확인하였으며 이에 따라 선형 탄성 재료를 이용하였다. 탄성 패드를 고려한 수치적 해석 결과로부터 탄성 패드가 성형 불량을 개선시킴 확인하였다. 또한 탄성 패드의 두께가 증가함에 따라 표면결함이 상대적으로 크게 감소되었다. 이로부터 탄성 패드의 탄성 계수가 낮은 경우에는 두꺼운 탄성 패드를 사용하고 상대적으로 얇은 탄성 패드는 강도가 높은 것을 사용하는 것이 적합함을 보였다. 구면에 대한 다점성형 시험에 있어서 탄성패드를 사용한 경우와 사용하지 않은 경우를 고려하였다. 그 결과 탄성패드를 이용한 경우가 결함이 없는 완만한 곡면을 보였으며 이를 통하여 탄성패드가 효과를 가짐을 보였다. 수치적 해석을 기반으로 펀치 끝을 탄성체로 한 경우에 대해 비교한 결과 탄성패드를 이용하는 것이 탄성체 펀치를 사용한 경우보다 더 나은 효과를 나타냄을 보였다.다점성형에서의 탄성회복 해석을 수행하였다. 판재의 두께가 증가할수록, 하였으며 목적 곡률이 감소할수록, 탄성 계수가 증가할수록, 항복강도가 감소할수록 탄성회복량이 감소함을 확인하였다. 탄성회복을 줄이기 위하여 다단 성형기술을 제안하였으며 해석결과로부터 탄성회복량을 줄이고 부재의 응력 분포를 개선시킴을 확인하였다. 탄성회복을 최소화 하기 위한 효과적인 다단 성형 공정 설계를 위하여 여러 가지 변형경로에 따른 일련의 수치적 해석 수행하였다. 그 결과 다단 성형공정은 탄성회복량을 줄임을 보였다. 다단 성형 공정에서는 변형 증분을 줄이면서 성형 횟수를 늘리는 것이 탄성회복량을 줄이는데 효과적임을 확인하였다. 실험결과로부터 탄성회복 해석 결과가 적합함을 확인하였다.분할 성형공정을 제안하였으며 이에 대한 해석을 수행하였다. 국부적인 소성 변형과 형상 오차는 분할성형에서 발생하기 쉬운 성형 불량이다. 이러한 성형불량을 제거하고 성형 정도를 향상시키기 위하여 이전 단계에서 성형된 면의 일부에 해당하는 중첩 구간에 대한 설계와 교차 면적에 대한 연구를 수행하였다. 중첩 구간에 대한 설계에 NURBS와 COONS 방법을 이용하였다. 중첩 면을 고려함으로써 국부적인 과도한 변형을 제거할 수 있으며 성형 결함을 효과적으로 방지할 수 있다. 넓은 중첩 구간은 부드러운 곡면을 형성하고 이전 성형 단계에서의 불완전한 성형구간을 바로잡아 준다. 그러나 지나치게 넓은 부분을 중첩시킬 경우 이어지는 성형 공정을 복잡하게 만드는 불필요한 변형 경로를 가지게 될 수 있다. 중첩 구간을 고려한 경우와 그렇지 못한 경우에 대한 분할 성형 시험을 수행하였다. 이로부터 중첩 구간을 고려한 분할 성형 방법은 성형장치의 성형 가능 면적에 비하여 크고 넓은 판재를 효과적으로 성형할 수 있음을 확인하였다.
Multi-point forming (MPF) is an innovative flexible manufacturing technology for three-dimensional sheet metal forming. It replaces the conventional solid dies with a set of height adjustable discrete punches. Based on its flexibility, a significant amount of money and time to design, manufacture and adjust dies can be saved. The main objective of this research is to design and optimize the MPF process to suppress the surface defects, reduce shape error and improve the forming quality. Both the numerical simulation and the experiments were implemented to study the deformation behavior of the thick metal plate in various MPF processes, and the following conclusions were stated.In this study, the numerical simulation using the finite element fundamental theory was investigated for various MPF processes. The dynamic explicit finite element method was used to simulate the forming process, and the explicit-implicit algorithm coupled finite element analysis method was employed to study the MPF springback and the multi-step MPF process. The reduced integration 8-node hexahedral element with enhanced hourglass control, Penalty method and the Coulomb’s friction law model were selected to build the finite element model. The appropriate material model and virtual forming speed were discussed for various simulation objects. The NURBS method was applied to determine the relative positions of the discrete punches to construct the forming surface.The dimples are the inevitable and typical surface defects in MPF process, which are caused by the discrete contacts between the metal plate and the discrete punches. The dimples of the thick metal plate called surface dimples are different from the envelope dimples which occur on sheet metal. In this research, the surface defects of the thick metal plate in MPF are studied for the first time. Following the analysis of the deformation features of the surface dimples and envelope dimples, the numerical simulation analysis for thick metal plate MPF were carried on. It is shown that the visible dimples and the thickness non-uniform change were the main forming defects when the forming mode in which the metal plate contacted with the punch groups directly was adopted. And these forming defects decreased with the thickness of the metal plate increased and the objective curvature radius enlarged. The aluminum plate is more easily to produce the forming defects than the steel plate. The elastic cushion was proposed to suppress these surface defects, because it can fill the interspaces of the discrete punches and increases the interface area to make the metal plate stressed uniformly. Both the hyperelastic material model and linear elastic material model were studied to describe the deformation behavior of the elastic cushion in MPF process. It is shown that the deformation behavior of the elastic cushions modeled as hyperelastic material and the linear elastic material are similar only when the compression ratio is small. Based on the numerical simulation of the MPF with elastic cushions, it is shown that the elastic cushion can suppress the forming defects obviously. And the surface defects deceased with the thickness and elastic modulus of elastic cushion increased. In addition, the thickness of elastic cushion has much more influence on MPF forming quality than the elastic modulus. For the thin elastic cushion, increasing its elastic modulus will enhance its action effect remarkably. Based on the high elastic modulus, the relative thin cushion also can work well. However, the thick elastic cushion is always the optimal, since even its elastic modulus is small, it also can work better than the thin one with the high elastic modulus. Experiments for the spherical surface MPF with and without elastic cushion were implemented, and the workpiece formed with elastic cushion is smooth and without defects. It verified that the elastic cushion is effective and the numerical simulation is dependable for the study of the MPF process with elastic cushion. Finally the cover elastic cushion was compared with the sheet elastic cushion based on the numerical simulation. It’s shown that the sheet elastic cushion works better than the cover one.Unloading springback is an inevitable phenomenon for metal plate MPF just like for conventional metal forming method. The influences factors of MPF springback which include metal plate thickness, deformation amount and material properties were studied to investigate the MPF springback tendency. It is shown that springback tendency of the complicated curved surface is less sensitive to thickness and deformation amount than that of the simple curved surface. It is indicated that the springback of MPF decreased with the increasing of the plate thickness, with the reducing of the objective curvature radius, with the increasing of the elastic modulus and with the reducing of the yield stress. The multi-step MPF technology was proposed to reduce MPF springback for the first time, since it can realize changing deformation path economically and conveniently. The simulation results verified that the multi-step MPF can reduce MPF springback obviously and improve the workpiece stress distribution state. The effect of deformation path of multi-step MPF on minimizing MPF springback was researched through simulating a series of multi-step MPF processes with different deformation paths. It is concluded that the multi-step MPF processes both with and without equidifferent curvature deformation path can reduce springback observably, but the multi-step MPF with equidifferent curvature deformation path is more effectively than that without equidifferent curvature deformation path. It’s also shown that the springback reduced with the deformation amount of each forming step of multi-step MPF process decreased. Finally the relevant experiments were implemented and the results validated that the multi-step MPF can reduce springback obviously, and the multi-step MPF with the equidifferent curvature deformation path is much effective to minimize the MPF springback.In the sectional MPF, the workpiece is unseparated and formed in a section by section way, so its deformation behavior is more complicated than that in the entire MPF. In the sectional forming process, the local severe plastic deformation occurs between the forced deforming region and the undeformed region, which is not easy to remove in the subsequent forming process and causes the surface defects. At the same time the deformation in the subsequent forming process will change the shape of the formed part in the previous forming process, therefore the forming quality and accuracy of the whole workpiece can not satisfy the forming requirement. The transition surface was used to avoid the local severe plastic deformation, which should satisfy G2 continuity. The NUBRS and COONS methods were introduced to construct the transition surface for sectional MPF. Through the simulation of the metal plate sectional MPF with transition surface, it is validated that the transition surface can eliminate undesired local severe plastic deformation and prevents the forming defects effectively. The method to design the appropriate transition surface for the thick metal plate sectional MPF was studied for the first time in this research. Based on a series of simulations of the sectional MPF processes, it is shown that the sharp transition surface can not eliminate the local severe deformation effectively, and the superfluous transition surface will cause the redundant deformation which makes the forming complicated in the subsequent forming process. The wide transition surface results in the smooth formed shape and the more deformation amount in the previous forming process which is favorable to the subsequent forming since the transition region is the performing area of the subsequent forming. The overlap area is set to reduce the effect of the deformation of the transition region in the subsequent forming process to the effective forming region formed in the previous forming step. It is shown that the forming accuracy increased with the increasing of the overlap area. The influencing factors included the metal plate thickness, deformation amount and material properties were studied based on the numerical simulations of sectional MPF processes. It is shown that the thick metal plate with the material which is hard to form and the large deformation amount needs large transition surface to prevent the forming defects in sectional MPF process. The experiments of sectional MPF with and without the transition surfaces were carried out. It is validated that the sectional MPF with transition surface and overlap area is the effective forming technology for the large and super large metal plate forming.Finally, several future research works in this field are recommended. Firstly, the deformation of the elastic cushion is not uniform, thus the shape of the formed workpiece will deviate from the objective forming shape in the MPF with the thick elastic cushion. Therefore the study on the shape compensation through adjusting the punches relative positions should be needed. Secondly, the excessive forming steps may cause the work hardening, so the optimum deformation path for the complex workpiece should be studied. Thirdly, the research on the sectional MPF in both the length and the width direction of the metal plate should be carried out. Finally, in this research just the MPF for the regular shape surface was studied, so the investigations about the deformation behavior of the complex curved surface in MPF should be considered.