In the 21st centuries, press forming in automotive and electronic field is one of the most efficient and fundament manufacturing process for mass production. And in the 2000s due to environmental concerns and safety regulations in the automotive industry, the development of strong and lightweight cars has been a hot issue in the last decade. One solution for this purpose would be to use high-strength steel (HSS) and advanced high-strength steel (AHSS). These materials can make the car lighter while maintaining the crash resistance of the vehicle. In developing lighter car with HSS, AHSS and new process, sheet metal forming simulation can be widely used and essential for reducing lead time, saving cost and increasing surface quality. And also design shape and surface quality of outer panel is more and more important parameter for high quality car in the market.
During stamping process the air trapped between blank sheet and upper/lower die cavity, especially outer panel, can be highly compressed and ultimately affect blank deformation. To prevent this problem, air vent holes are drilled into die tool based on export experience and know-how. CAE can be also used for analyzing the air behavior in die cavity during stamping process, incorporating both elasto-plastic behavior of blank sheet and the fluid dynamic behavior of air. This study presents sheet metal forming simulation combined simultaneously with simulation of air behavior in the die cavity. There can be 3 approaches in modeling of air behavior. One is simple assumption of the bulk modulus having a uniform pressure depending on volume change. The next is the use of the ideal gas law having uniform pressure and temperature in air domain. The third is FPM(Finite point method) having non-constant variables in air domain. The approach enables direct coupling of the mechanical behavior of sheet metal and the fluid behavior of air in sheet metal forming simulation, and its result provides the first-hand idea for the location, size and number of the vent holes. In order to use the modeling of ideal gas law and FPM to industrial case, parametric study was performed to save cpu calculation time. That is the reason why the air is compressible and tool moving velocity can affect pressure of trapped air. In general, real tool velocity is about 0.5~1m/s and simulation velocity is 10m/s. Therefore, the target of tool moving velocity for air behavior by the modeling of ideal gas law and FPM was 10m/s like conventional forming and suitable method for equivalent pressure under real tool moving velocity was investigated. And the suitable method for saving cpu calculation time was applied to forming simulation of fuel filler door for correct understanding of the process.
HSS and AHSS have more resistance force in the die structure compared with conventional steel due to their higher yield and tensile strength and thus, these materials have a greater effect on die deformation during the sheet metal forming process. As a result, die deformation can affect the blank sheet’s drawn pattern, strain, and stress as well as springback. This study presents a sheet metal forming simulation that considers die deformation. Typically the explicit method is suitable for sheet-metal forming analysis, and the implicit method is suitable for die structural analysis. Therefore, the best approach to sheet-metal forming simulation considering die deformation is to use both the explicit and implicit methods. To provide a smooth connection between the explicit and implicit methods, a semi-coupling process proposed in this study was used. The simulation process by semi-coupling was compared with conventional simulation methods. Our results indicate that the sheet metal forming simulation with die deformation consideration provides useful information on the die structure as well as formability and springback. For this study commercial S/W, PAM-STAMPTM and PAM-CRASHTM, is used.

Key Words : Sheet Metal Forming Simulation, Die cavity, Bulk modulus, Ideal gas law, FPM(Finite Point Method), Die deformation, Explicit method, Implicit method, Springback, Semi-coupling process