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Sheet Metal Forming: Spring-back of hydro mechanical deep drawn parts

Active hydro mechanical forming (AHMF) has been developed in order to meet the demand of the automotive industry for economical production of sheet metal parts with more individuality in small lot sizes. Conventional deep drawing of automobile parts which have large outer surface areas (such as roofs, doors and hoods) leaves them with a very small dent resistance. This is caused by the low deformation degree in the middle of the part. This low component stiffness has a negative effect on the crash resistance of vehicles. By using the AHMF technology a consistent plastic strain distribution can be brought into the part and therefore its stability will be improved. Within the design of conventional deep drawing tools the spring-back of a part is well understood because of the long experience with the process and its influence on the stress distribution inside the deformed part. In AHMF, as with any new process, the spring-back phenomenon is still under investigation. There are a considerable number of parameters to control in order to regulate the stress distribution and the shell thickness, which leads to more spring-back possibilities. This paper presents a simulation of spring-back in AHMF to find an efficient method of tool design and to generate the optimum process parameters, in the prototyping or later production. The FEM simulation is based firstly on the validation of the results of the AHFM process given by the mathematical calculation with the computer generated model. Secondly, spring-back simulation is introduced to see the influence of pressure curves and the blank holder forces or the parameters of the pre-bulging step. The quality of the FEM simulation is verified using practical applications in the automotive industry.