Buckling in low pressure tube hydroforming

Tube hydroforming is an innovative forming process in which the tube is pressurized by a fluid medium and formed into a complex shape. There are two types, low and high pressure hydroforming. In the high pressure process, the tube is expanded by an internal pressure to fill the die cavity. In the low pressure approach a constant pressure is maintained inside the tube which is crushed to shape by the action of a punch or an upper die movement. It is known that in low pressure hydroforming the required pressure and die closing force are much lower compared to the high pressure process. Implementation of advanced high strength steel tubes in tube hydroforming is a promising way to lower weight by reducing the material thickness. Using high strength materials increases buckling tendency in low pressure tube hydroforming. In the current study, a method using a plastic energy principle is proposed for estimating the minimum pressure required for the low pressure hydroforming of a buckle free component. The present investigation addresses the side wall of the tube as a vertical column pinned at each end. The proposed model shows that the method can predict the minimum pressure required with sufficient accuracy. The model also reveals that the minimum pressure required depends on the yield stress of the tube material, the tube material thickness and the straight length of the tube section that is in contact with the die. Applying sensitivity analysis it is determined that the required pressure is strongly affected by the yield stress of the tube material.