Forming limits in biaxial stretching of aluminium sheets and foils

Abstract

The formability of AA 8014 aluminium alloy (1.4% Fe and 0.4% Mn) in sheet and foil gauges has been characterized in terms of limit strains in forming-limit diagrams. Attention was focussed first on two main industrial conditions of the material. These were representative of the materials that could be processed by the commercial batch-annealing process (slow heating, long time) or by a continuous annealing process (fast heating, short time). Both regimes developed good combinations of strength and formability, giving proof-stress levels of up to 100 MPa while displaying forming limits in biaxial stretching comparable to those of commercial purity aluminium. The excellent formability of the material is associated with favourable combinations of strain hardening and strain-rate hardening. It appears that strain hardening is the dominant factor when the material is fast annealed, while rate sensitivity is more important with the slowly-heated conditions. Although it is believed that the effect of an initial defect of a given size on strain localization becomes progressively greater as the sheet thickness is reduced, the AA 8014 in thin gauges showed high formability, as indicated by limit strains and limiting bulge heights. This implies that by proper thermo-mechanical treatments the present material could be one of the best choices for those applications in which formability and strength are of prime concern. These applications range from 7 μm foil, household and packaging foil to highly formable 100 μm fine stock for automotive radiators, and to ordinary sheet gauges required for severe press-forming operations.