Most of automotive hollow components such as chassis parts and space frames have various rectangular and/or similar shape sections adapting to different loading to reduce weight [1] and [2]. An extraordinary feature of tube hydroforming is that a hollow component with variable closed-sections can be integrally formed in one process step so that the number of parts and weight is reduced and stiffness of the part is improved [3] and [4]. With global R&D activities [5], [6] and [7], tube hydroforming has also been recognized as an important technology for manufacturing lightweight structural parts in automotive industry. The applications of the hydroformed structural parts have grown to considerable scale in North American and Europe and there is a large potential market in Asia [8] and [9].
The pressure to form a transition radius of a rectangular section is inversely proportional to its radius value. If there is a very small radius in the component, a very high pressure is necessary to obtain the designed component shape. As a result, a heavy hydraulic press for closing die and axial cylinders with large tonnages are needed and the die structure must be strengthened so that the investment for equipment is higher and the die life is shorter. Therefore, how to reduce the pressure to form small transition radii in a hollow component is very important in tube hydroforming [10] and [11].
In this paper, a comprehensive investigation has been conducted into hydroforming of stainless steel tube in the square section die. The effect of loading path on the failures and thickness distribution is discussed and the reasons are analyzed for bursting and folding. A method is proposed for forming small corner by lower pressure through a petal-like perform section shape. The results were used as guidelines for hydroforming some automotive part prototypes.