Analysis of hydroforming of noncircular boxes

Abstract

The success of the hydroforming process without any defect depends greatly on the fluid pressure vs. punch stroke relationship. Departure from a proper curve results in necking at the punch shoulder or wrinkling at the flange and the deformed sheet has uneven distribution of thickness. The present study is concerned with hydroforming of noncircular boxes in which he product has a uniform distribution of sheet thickness. For the analysis of hydroforming of generally shaped boxes, a kinematically admissible velocity field is proposed with some appropriate assumptions. From the proposed velocity field, the fluid pressure vs. punch stroke relationship so as to render a uniform thickness and the deformed configuration are determined by minimizing the total power consumption with respect to some chosen parameters. In computation the deforming region is divided into the finite number of small elements and the work-hardening effect is considered for each elemental region. The analysis is made in three stage according to the shape of boxes. i) Hydroforming of regular polygonal boxes. ii) Hydroforming of longitudinally curved boxes with regular polygonal cross-section. iii) Hydroforming of arbitrarily shaped boxes; as examples, shaped boxes of which cross-section changes gradually from an ellipse to a circle and of which cross-section changes gradually from a trocoidal shape to a circle. The effect of various process parameters including blank shape and size, work-hardening exponent, frictional coefficient on the pressure curve is discussed. In order to confirm the validity of the proposed velocity field, a CNC hydroforming press has been developed and experiments have been carried out with cold-rolled steel sheets using the developed machine. The fluid pressure is controlled by the PI-control algorithm using the theoretically computed fluid pressure vs. punch stroke relationship. The validity of the assumption of uniform thickness in the analysis for hydroforming and th...