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 aluminum alloy thin-walled components. It is still very critical to prevent the wrinkling defect through blank-holding. The effects of draw bead and blank-holder force on the wrinkling behaviors in gradient ultra-low temperature forming for a 7075 aluminum alloy hemispherical shell were thus experimentally studied. The wrinkling controlling mechanism was revealed through stress and strain analyzing. The required blank-holder force to prevent wrinkling can be reduced significantly through the setting of draw bead. The blank-holder force of 120 kN can be used to form the sound hemispherical specimen with 200 mm in diameter, being 31.5% of that without draw bead. This benefits from the decreasing hoop compressive stress in unsupported region, which reduced to 165 MPa from 516 MPa. The hemispherical shell formed in gradient ultra-low temperature forming was twice higher than that at room temperature. And the thickness deviation is only 7.2%. Moreover, the reason for the enhanced formability was revealed by microstructure observation. The uniform distributed high-density dislocation at the ultra-low temperature significantly improves the strain-hardening ability so that the deformation can be withstood and transferred. This can avoid the splitting of the unsupported region while setting the draw bead to prevent wrinkling. It is very beneficial to greatly reduce the tonnage requirement, especially for the large-sized thin-walled components, which expands the application range of gradient ultra-low temperature forming.