Formability and performance of 6K21-T4 aluminum automobile panels in VPF under variable blank holder force

Abstract

Since sheet aluminum is of low room temperature formability, defects such as excessive thinning, wrinkling, and fracture may easily occur in pressure forming processes. The main purpose of this paper is to research the formability and performance of aluminum hoods in viscous pressure forming (VPF) under an appropriate control of blank holder force (BHF). A new strategy was proposed to determine the reasonable BHF loading condition. Constant and variable BHF were applied to a hood through a proposed design of multipoint BHF loading system. Hood forming experiments under three typical BHF loading schemes were conducted. Research results showed that when a higher initial constant BHF was followed by a gradual decrease in force, the maximum local thinning of the formed hood was 23.3%. When a lower initial constant BHF was followed by a gradual increase in force (the proposed scheme based on the new strategy), the thinning was 18.5%. The thickness strain, microhardness, and microstructure at typical positions of the formed hood under the proposed BHF loading scheme were also investigated. Research results demonstrated that the proposed strategy is an effective method to determine an appropriate BHF loading scheme to improve the panel’s formability and obtain a high-quality thin-walled aluminum hood in VPF.