Numerical Simulation And Experimental Research On Multi-Point Sandwich Forming Of Steel Sheet

To significantly reduce the time and cost of tooling development for sheet metal parts, the multi-point sandwich forming (MPSF), a new reconfigurable die forming technology, is being developed for parts with various curvatures made in small batch quantities. This process has been successfully used to manufacture steel panels for a contracting section on a slow speed wind tunnel and panels of a large aluminum sphere. In this process, the die surface can be changed by adjusting the height of the widely spaced pins in the lower die. Elastic polyurethane pads are used as the upper die. To enable parts of good surface quality to be formed, an elastic interpolator and die sheet are placed between workpiece and pins. The die sheet is used to provide a near continuous die surface and can be stored for re-use. The interpolator reduces or eliminates the formation of dimples on the surface of workpiece.This forming technology has been patented recently, and the basic deformation mechanics and influencing factors of MPSF have not yet been investigated. In this study, deformation mechanics and influencing factors are investigated and analyzed using experimentation and numerical simulation for a 2D cylindrical arc form. In addition, workpieces with more complex shapes of ellipsoid, saddle and sphere were manufactured. In order to improve the level of tooling design and determine the limits of target shape of workpiece, the effects of pin density and maximum difference in height between adjacent pins on the quality of a target shape of workpiece are put forward. The strength limitation of pins during forming also is analyzed. Moreover, a calculation method for adjusting the height of pins is introduced for curved surfaces, which takes into account the contact point between pin and die sheet.The commercial FE software ABAQUS has been used to simulate the forming process and springback of the workpiece after unloading. Polyurethane is a kind of hyperelastic material and the Mooney–Rivlin material model is used in simulation. The material coefficients of this model were obtained from uniaxial compression test data. The friction coefficients of rubber to rubber and rubber to steel were found to be 0.29 and 0.2 respectively, in pin on disc tests. Dimples on the formed workpiece are a major defect arising from MPSF. In order to investigate the influence of process factors on dimpling,pressing-in analysis of sandwich structures using a single pin has been under taken. Effects of thickness and hardness of interpolator, thickness and yield strength of metal sheet and the radius of spherical head of pin on the metal sheet with small plastic deformation have been analyzed.Experiments on a cylindrical arc workpiece, formed by MPSF, have been carried out to analyze the influencing process factors. These factors include: thickness of die sheet, thickness and hardness of interpolator, shape of the polyurethane upper die and multi-point lower die, thickness and mechanical properties of the workpiece. Also, the lubricant condition between workpiece and upper die was changed during the experiments. The results show forming force and the shape error of workpiece decrease, when lubrication is applied on the surfaces of a workpiece.The multi-step forming method has been proposed to manufacture double-curved workpieces, represented in this work by a saddle, in which the lowest regions are difficult to form without inducing surface discontinuities, as they do not contact the die and lack the necessary restraining pressure.Springback of sheet metal workpieces will occur after forming which is a factor to cause workpiece shape deviation from the designed target shape, and also the upper die with hyperelastic material is another factor in MPSF. In this work, a die shape optimization design method, using inverse displacement compensation along normal directions on the target surface, is presented. This method can be used to compensate shape errors induced by other factors, besides springback. Furthermore, it can be used not only to predict the compensated die shape, using measured data from a real workpiece, but also to optimize the die shape using FE simulation. Spherical and saddle workpieces were manufactured with the aid of this die compensating method. Research results show that a specified shape can be formed quickly and efficiently by combining the die compensating method and with a reconfigurable multi-point die.