Sheet Hydroforming Pre-bulging Numerical Model Improvement

Sheet hydroforming has gained increasing interest during the last years, especially as application in the manufacturing of some components for automotive, aerospace, and electrical appliances[1,2]. Many parameters influence the process of sheet hydroforming, one of them is the pre-bulging[3]. Different studies have been also done to determine the optimal forming parameters through FEA[4,5]. In the case of sheet hydromechanical forming process the blank is first placed on the lower die (a fluid chamber combined with draw ring) and then, after sealing the blank between blank holder and draw ring, punch progresses to deform the blank[6]. Pressure of the fluid chamber is also increased simultaneously with the punch progression[7]. In this paper, the pre-bulging effect on active hydromechanical deep drawing process has been investigated experimentally and numerically. Pre-bulging includes two parameters: pre-bulging height and pre-bulging pressure, which influence the forming process significantly[3]. Numerical simulations and experimental tests were carried out for a given shape to investigate the pre-bulging effect on the maximum hydroforming depth. During this activity, the authors have verified that the low numerical – experimental accuracy detected it was caused also by the simulation of the pre-bulging phase. The authors have analyzed the problem to define a correct procedure to simulate the pre-bulging phase. From this point of view, nine different levels of pre-bulging (taking into account the level equal to zero also) have been tested to experimentally calculate the Thickness Percentage Reduction (TPR) at the maximum pre-bulging height. For each level, the experiment has been conducted two times for a total number of eighteen experiments. The experimental TPR values have been compared with the numerical ones reaching a good accuracy only in the case of pre-bulging height greater than forty millimeters. The experimental activity has given a valid contribution to improve the simulation models reliability and to obtain useful information on the process itself. The effects of pre-bulging on the process performance are also discussed