Effect of internal pressure and loading path on deformation behavior during low pressure tube hydro-pressing

To overcome the limitations of hydro-pressing process with either linear increasing pressure loading path or constant pressure loading path, a hydro-pressing process with two steps pressure loading path was proposed and an experiment was conducted on hydro-pressing process of DP590 steel tubular part with varied rectangular-section perimeters. The effects of internal pressure and loading path on cross-sectional shape and stress distribution were discussed. An analysis was conducted to give the critical punch stroke of the first step, and an optimal loading path was put forward. The pressure loading path was divided into two steps, the result indicates no buckling happens even no internal pressure adopted in the first step. The wall thickness distribution of the tubular part formed under two steps pressure loading path is more uniform than that with constant pressure loading path. Mechanical analysis proves that the flexural modulus of the tube wall in the earlier stage of deformation is high enough due to the short length of the straight side, which is no need for supporting pressure. Under a reasonable loading path, a rectangular-section tubular part with a compression ratio of 5 % was obtained with lower pressure 13.5 MPa. Its relative corner radius is only 1.5, which exceeds the limiting value of that formed by conventional tube hydroforming.