Effect of process parameters on product surface finish and thickness variation in hydro-mechanical deep drawing

Hydro-mechanical deep drawing combines the features of conventional deep drawing and hydroforming. In this process, the punch deforms the blank to its final shape by moving against a controlled pressurized fluid. Hydro-mechanical deep drawing offers several advantages over conventional deep drawing like higher drawability, uniform strain distribution, better surface finish and improved dimensional accuracy. The process parameters such as initial pressure in the chamber (pre-bulging pressure), the peak chamber pressure (cutoff pressure) and oil gap have strong influence on the process. In view of this, the effect of the pre-bulging pressure and the cutoff pressure on thickness distribution and surface finish in hydro-mechanical deep drawing were studied in this paper. Experiments have been carried out on extra-low carbon steel sheets by varying these parameters. It was found out that thickness distribution along cup wall is more uniform for intermediate pre-bulging pressures. At too low pre-bulging pressure necking takes place at punch corner (similar to the conventional deep drawing) and at very high pre-bulging pressure, necking shifts towards wall region. So a region of safe pressure (combination of pre-bulging and cutoff pressure) was found out in which cups of better quality in terms of uniform thickness distribution and better quality surface can be drawn successfully.