Four loading paths, i.e. the relationship of internal pressure and axial displacement, used in this experiment are shown in Fig. 2, indicated as Path 1, Path 2, Path 3, and Path 4. These paths can be divided into two groups: one is hydroforming without axial feeding, as shown by Path 1. It means that the axial cylinders do not feed materials into the expansion zone, the axial forces are used only for sealing and the tube is formed by free bulging. The other, including Path 2, Path 3 and Path 4, is that the axial cylinders push additional material into the expansion zone. In this case, the tube is formed under the simultaneous action of the internal pressure and the axial feeding.