Evaluation of the stress vs strain curve using a high temperature bulge test device

Abstract

The increasing amount of hot and warm stamping parts in the automotive industry shows that high temperature forming is critical to reach the emission goal fixed by regulation authorities, due to its contribution in reducing the vehicle’s weight. Hot stamping and warm forming take advantage of the material softening with temperature to reduce the forming forces and springback. The use of different temperature conditions also enables exploring the phase transformation occurring with in-tool quenching in hot stamping boron steels or artificial aging in heat treatable lightweight aluminum alloys. These new forming conditions require either new material characterization methods or the current methods need to be adapted to high temperature conditions. The equibiaxial expansion test allows the evaluation of the mechanical behavior of materials on a large strain range. However, the use of high temperature conditions gives rise to difficulties when using Digital Image Correlation (DIC) systems. This study analyses the possibility of using a scanner laser to obtain the stress vs. strain curve from an equibiaxial test performed with an aluminum alloy of the 6xxx series, at 150°C. The aim is to validate an experimental procedure that can be used to evaluate the stress vs. strain curve from high temperature bulge tests performed with other metallic sheets, including quenchable boron steel.