Straight Tube Hydroforming of Dual Phase (DP780) Steel Tubes With End‐Feed

Dual phase (DP780) 76.2 mm (3”) diameter steel tubes were hydroformed with various levels of end‐feed (EF). The results of this study showed that when zero EF was applied during hydroforming, the average burst pressure was 70 MPa (10,075 psi) and the corner‐fill expansion (CFE), which is a measure of formability, was 6.4 mm. When an EF force of approximately 50% of the material yield strength was applied during hydroforming, the tube was able to support an internal pressure of 151.7 MPa (22,000 psi) without failure and achieved a CFE of 11.5 mm. Finite element (FE) models of the hydroforming process accurately predicted the CFE of the tubes for the various EF cases tested. Upper and lower bound strain‐based forming limit curves (εFLC) were determined from free‐expansion burst test data. These curves were then used to derive the upper and lower bound extended stress‐based forming limit curves (XSFLC), which were in turn used to predict the necking (failure) pressure in the FE models. For the two cases where burst was achieved in the experiments, the upper and lower XSFLC failure criteria curves bound the measured burst pressures. Also, two different friction coefficients were used in the FE models to evaluate their effect on predicting the failure pressure, CFE and end‐feed displacement.