Hydroforming without axial feeding
Fig. 3 shows the part obtained by hydroforming without axial feeding. When the internal pressure is 105 MPa, the transition radius is 13.5 mm, which is 7.5 mm larger than the design value. However, the bursting occurred at the transition zone due to excessive thinning.
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Fig. 3.
Bursting in hydroforming without axial feeding.
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In this case, the axial cylinders do not feed any material into the expansion zone, and the expansion of tube is conducted at plane strain state. The forming limit is lower and plastic deformation focuses mainly on the transition zone. Therefore, more material should be pushed into the expansion zone to change the stain state and improve the forming limit.
2.3.2. Hydroforming with axial feeding
Fig. 4 shows the shape of the part obtained with Path 2, by which the total axial displacement is 8 mm and the maximum calibration pressure is 120 MPa. Due to the axial displacement increasing faster than the pressure going up, folding occurs in the workpiece at the right side. Although the subsequent calibration pressure is very high, folding cannot be removed. The reason for the folding is that the material fed by the axial compression cannot be converted into the circumferential expansion. This material, therefore, is accumulated along the axial direction so that the folding is formed.
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Fig. 4.
Folding in hydroforming with unsatisfactory loading path.
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If the internal pressure increases faster than the axial feeding moves, such as Path 3, the axial feeding cannot supply enough material for the circumferential expansion. As a result, the thickness becomes thinner and thinner, the bursting also occurred in transition zone as the calibration pressure is very high. In order to form a sound part with relatively uniform thickness distribution, it is necessary to choose a reasonable loading path to avoid folding, wrinkling and bursting.
Fig. 5 shows a sound part obtained with Path 4. When the axial feeding is 5.5 mm at each end of the tube, two barrels occurred along the axial direction. When the internal pressure is 240 MPa and the axial feeding reaches 16 mm, the sound part was formed, as shown in Fig. 5(b).