Reduction of Friction and Calibration Pressure by Section Preform during Hydroforming of Tubular Automotive Structural Components

In the past decade, hydroformed structural components have been widely used in industrial areas in North American and Europe and there is a large potential market in Asia with the development of automotive industry. An extraordinary feature of tube hydroforming is that a hollow component with a three dimension axis and variable closed-sections can be integrally formed, so that the number of component parts is minimized, weight is reduced and stiffness of the part is increased. The section shapes in many automotive hollow components are complex and it is usually difficult or impossible to hydroform these typical sections directly from circular section tube, so that satisfactory preforms are necessary. In this paper, a method to reduce the friction and calibration pressure through a petal-like cross-section shape preform is presented and its mechanism is analyzed. Preforms for rectangular, trapezoidal and polygonal sections used in hollow components are investigated and satisfactory preforms are obtained. Taking an engine cradle as example, satisfactory preform section shapes have been designed, by which the thickness distribution was controlled and the flash generated from the die-parting surface in the final forming was avoided. So a carefully selected preform process can contribute significantly to the reduction of friction and calibration pressure and it is a dominant factor for successful hydroforming.