Wrinkling control and microstructure of 7075 aluminum alloy hemispherical shell in gradient ultra-low temperature forming

Gradient ultra-low temperature forming is a novel process for improving the forming limit of thin-walled aluminum alloy components. However, it is critical to prevent wrinkling defects through blank holding. It is not clear whether the gradient temperature can meet the deformation requirements for preventing wrinkling defects with draw beads and blank holding. The effects of a draw bead and the blank-holder force on the wrinkling behavior in gradient ultra-low temperature forming of a 7075 aluminum alloy hemispherical shell were investigated experimentally in this study. The wrinkle control mechanism was revealed through stress and strain analyses. The blank-holder force required to prevent wrinkling could be reduced significantly by setting a draw bead. A blank- holder force of 120 kN could be used to form a sound hemispherical specimen with a diameter of 200 mm, which was 31.5% of the force without the draw bead. This benefited from the reduced hoop compressive stress in the unsupported region, which decreased to 165 MPa from 516 MPa. The hemispherical shell formed using gradient ultra- low temperature forming was twice as high as that at room temperature. The deviation in thickness was only 7.2%. Moreover, microstructural observations revealed the reason for this enhanced formability. The uniformly distributed high-density dislocations at ultra-low temperatures significantly improved the strain-hardening ability, thus allowing the deformation to be withstood and transferred. This could prevent splitting of the unsupported region when using a draw bead to prevent wrinkling. This is beneficial for significantly reducing the tonnage requirements, particularly for large-sized thin-walled components, which can expand the application range of gradient ultra-low temperature forming.