Numerical Simulation Of Process Mode And Deformation Defects On Multi-Point Forming

Multi point forming (MPF) is a flexible manufacturing technology for threedimensional surface part of thin sheet metal. MPF utilizes a series of adjustablediscrete elements arranged regularly to form intricacy curve surface, and achieves3 D sheet figuration. By computer controlling the displacement and speed ofelements the equipment of MPF can be adjusted figuration rapidly. So contrast totraditional die, which means one die equipment only uses one figuration, MPF cansave a lot of time and cost of studying and manufacturing die, and one MPFequipment can realize several uses, also its merits conclude forming without die,optimizing distortion path and forming big workpiece with small equipment. NowMPF has been applied to auto panel, medical engineering (titanium sheet forming),architectural decorative, also applied to ship hull plate, pressure vessels, plane bodycover and city sculpture etc. Recently MPF has been applied to bend and distortionworkpieces of nation gymnasium (nest engineering) for Beijing Olympic Gamesundertook in 2008 year.Dimples and wrinkles often appear in MPF, and they influence the beauty andapplication, also restrict the more application and popularization of MPF. Currentlyby constructing FEA mold CAE numerical simulation can well forecast all kinds ofdefects in the course of forming. So CAE is powerful tool and instrument to avoidand decrease bugs in sheet forming, and improves forming quality and efficiency.Also besides forecasting bugs in the course of forming by CAE, it can simulatenew forming measures and technology molds. Thereby CAE can save time and cost,and it is assistant means for MPF.FEA software LS DYNA has been applied to this paper to simulate two kindsof typical technologies mold —Multi point die(MPD) formingand Multi pointpress(MPF) forming, and contrast to the capability for restraining dimples,wrinkles, springback and crack, also simulates flexible blank holder equipment ofMPF. This paper contrasts to forming capability of rigid blank holder(RBH) andflexible blank holder(FBH). Consequently these works can afford crediblefoundation for MPF technology affirm and restrain bugs, at the same timepromoting MPF perfection and development.The main study content and conclusion of this paper as followed:1. FEA numerical simulation discussion and establish FEA moldBased on the theory of continuous media mechanics, using center differentiamethod dynamical equation will be explicit, and studied contact and friction mold,also establish FEA mold of Multi point die forming and Multi point press forming,besides several questions of establish rigid and flexible blank holder FEA mold hasbeen discussed, for example elements of FEA selection, boundary conditionconfirmability, influence to results by virtual forming speed and material moldselection etc.2. Multi point die and Multi point press forming mold have been applied tosimulate dimples and validated by experimentBy stretching experiment to confirm the constitutive equation of L2Y2 and08Al material, it is a basement of simulating sheet. Contrast to different thickness,material and forming different distortion quantity, the simulation of two kinds oftechnologies influencing dimple birth and develop and restrainability of dimple hasbeen studied, thus dimple can be show by stress and strain and depth of globe andsaddle. We can conclude that Multi point press can restrain dimple defect at best, atthe same time several sheet metal material has been studied the influence to reducedimple, at last discuss the effect of elastic medium reduce dimples, and studies theforming error.3. Simulation study and experiment validated to wrinkle by Multi point die andMulti point press and traditional die forming.Contrast to birth and develop of wrinkle by MPD and MPP forming, andcombine traditional die, by contrasting the three technologies forming globeworkpiece, the capability of reducing wrinkle has been studied by the threetechnologies forming. With illumination photo, depth strain and outline of globewe can confirm that MPP has advantage to reduce wrinkle than the othertechnologies. For more certain the merits of MPP, by forming saddle workpiece inMPD forming and MPF forming, the workpiece thickness is 0.9mm and curvatureradius in both directions is 100mm, simulation study and experiment validatedshowed the workpiece formed by MPP has no wrinkle, and workpiece curvesurface smooth. At last built two molds (wrinkle height, wrinkle width) to judge thesize of wrinkle, only confirm wrinkle molds can we judge the size of wrinkleaccurately.4. Simulate sheet springback by MPD and MPP formingBy symmetry springback mold in this paper, sheet formed with explicitarithmetic, and sheet unload springback with implicit arithmetic. In the software ofLS DYNA the integral point in the thickness direction is 9. First calculate globeforming by MPD forming without blank holder, and sheet formed with 2mm thickblank than 1mm springback value is small, also sheet formed by MPP springbackvalue is more small using the same sheet, when 2mm thickness sheet formed thesheet formed by MPP springback value is about 0. Second calculate sheet formingwith blank holder, and we increase distortion quantity, also contrast to springbackand stress release, so it can provide basement for MPF springback compensating.5. Simulation study and experiment validated to crack by Multi point die andMulti point pressFLD has been studied for confirmed crack, and the transmit force section ofsheet crack curve has been studied for stress and strain following forming.AlsoFLD has been studied for crack, and several conditions have been gived to avoidcrack, with these conditions perfect workpiece photos presented too. By discussingthe relationship of BHF and crack we summarized that when BHF is big,workpiece will crack;when BHF is small, workpiece will wrinkle. Flexible blankholder forming has been put forword to avoid crack, besides forming limit andforming quality has been analyzed in flexible blank holder forming, also sheetfluied in blank holder studied, in evidence, flexible blank holder forming can wellsettle the contradiction relationship of wrinkle and crack.6. Simulation study and experiment validated by rigid and flexible blank holder,and discussed all kinds of factor to influence blank holder flexible and forming.By manufacturing flexible blank holder, simulation study and experimentvalidated show the two kinds of blank holders advantage and disadvantage usingthe MPF equipment with the function of blank holder. When the blank holder forceis small, sheet formed by rigid blank holder wrinkle;when the blank holder forceis big, sheet formed by rigid blank holder crack. With the same condition sheetformed by flexible blank holder is good. Also sheet will be saved with flexibleblank holder, when saddle sheet size is 480×400mm, and curvature radius is400mm in both directions, flexible blank holder forming can save 17.48% sheetarea, so with the smaller sheet also can form workpiece as well as formed by rigidblank holder. By contrasting saddle sheet outline it can be true that flexible blankholder can save sheet, and by contrasting 08Al material with the thickness of0.5mm, the influence of blank holder force to transition section height has beenstudied by using 16 96KN blank holder force. At the same time depth strain andforming force has been studied by using the two kinds of blank holders forming.So flexible blank holder is perfect method, and it has abroad foreground.At last, the thickness of blank holder influence to transition section andforming force in the course of flexible blank holder forming, and blank holderforce influence to sheet flexible also studied in this paper, also illumination photo,thickness and stress are studied to confirm fit blank holder force by calculatingblank holder force 16～64KN, at the same time the thickness of blank holderinfluence to sheet flexible and the least blank holder force engendered no wrinklein the course of flexible blank holder. By forming saddle workpiece with curvatureradius 300mm in both directions with flexible blank holder, also this workpiece didnot formed by rigid blank holder. So this example shows that flexible blank holdercan increase figuration quantity greatly. Sheet material influence to sheet flexibleand forming force are also studied in this paper.