An tube hydroforming processes, tube is expanded by a high-pressure medium (usually water) until it fills a die cavity. Since the volume of tube material (the product of its cross section multiplied by its length) remains constant, the tube wall thins to accommodate the expansion. Often axial feeding is used to reduce wall thinning, but this is not always possible. In such cases, other efforts are made to reduce wall thinning to produce a hydroformed component with nearly uniform wall thickness along its length. Research conducted at the Engineering Research Center for Net Shape Manufacturing (ERC/NSM) examines the use of extruded aluminum tubes that vary in thickness both circumferentially and axially for these purposes. The hydraulic bulge test was used to investigate the wall thinning that occurs during hydroforming. In this test, the tube is sealed at both ends and pressurized until it bursts. The tube is formed freely at the midsection (middle of the bulge). Figure 1 (left) shows the deformation stages in the hydraulic bulge test, and the right side of Figure 1 shows that maximum thinning occurs at the center of the bulge.
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