Influence of the Laser Heat Treatment on the AA5754-H32 strain path during hydraulic bulge tests

The hydraulic bulge test represents an effective experimental method to characterise sheet metals since the equivalent strains before failure are much larger than those measured during tensile testing and there is nearly no frictional effect on the results. Recently this test has been proposed not only for extracting data concerning the equi-biaxial strain condition, but to determine the forming limit diagram (FLD) in the range of positive minor strains. In the proposed methodology, different strain paths can be obtained by merely using a test blank having two holes with a suitable geometry and position to be tested, without the need of dies with elliptical apertures. However, a carrier sheet is necessary, thus implying results may be affected by friction effects. This paper proposes a new methodology for the determination of the right side of the Forming Limit Curve (FLC), based on the adoption of local heat treatments aimed at determining different strain paths on the blank to be tested while using the classical circular die for bulge tests. In particular, the formability of the alloy AA5754-H32 was investigated; 3D Finite Element simulations were conducted setting different laser strategies and monitoring the resulting strain path. Results revealed that the proposed methodology supports obtaining many additional points in the right side of the FLC, thus being effective and friction free.