Development of a New Biaxial Testing System for Generating Forming Limit Diagrams for Sheet Metals Under Hot Stamping Conditions

Abstract

Conventional experimental approaches used to generate forming limit diagrams (FLDs) for sheet metals at different linear strain paths are not applicable to hot stamping and cold die quenching processes because cooling occurs prior to deformation and consistent values of heating rate, cooling rate, deformation temperature and strain rate are not easy to obtain. A novel biaxial testing system for use in a Gleeble testing machine has been adopted to generate forming limits of sheet metals, including aluminium alloys, magnesium alloys and boron steel, under practical hot stamping conditions in which heating and cooling occur. For example, the soaking temperature is about 900°C and the deformation temperature range is 550-850°C for boron steel [1] and the soaking temperature is about 535°C and the deformation temperature range is 370-510°C for AA6082 [2]. Resistance heating and air cooling were introduced in this pioneering system and the thermal analysis of different heating and cooling strategies was investigated based on a type of cruciform specimen. FE models with a UAMP subroutine were used to predict temperature fields on a specimen in ABAQUS 6.12. Digital image correlation (DIC) system was used to record strain fields of a specimen by capturing images throughout the deformation history and its postprocessing software ARAMIS was used to determine forming limits according to ISO standards embedded in the software. Heating and cooling strategies were determined after the analysis. Preliminary results of forming limit curves at the designated temperatures are presented in order to verify the feasibility of this new method.

Formability evaluation for sheet metals under hot stamping conditions by a novel biaxial testing system and a new materials model

“…A similar temperature history at each location of thermocouples can be seen from previous study [1]. A higher temperature within the central gauge region of a cruciform specimen is beneficial for inducing failure to start in this area.…”

“…A novel planar biaxial testing system was proposed before and was improved and used in this work for formability tests of aluminium alloy 6082 at various temperatures, strain rates and strain paths after heating, soaking and rapid cooling processes. The key dimensions and features of cruciform specimens adopted for the determination of forming limit under various strain paths were developed, optimised and verified based on the previous designs and the determined heating and cooling method [1]. The digital image correlation (DIC) system was adopted to record strain fields of a specimen throughout the deformation history.…”

The comparison of two continuum damage mechanics-based material models for formability prediction of AA6082 under hot stamping conditions

“…A novel in-plane biaxial testing system had been developed to obtain FLDs of alloys for hot stamping applications in this study [1]. A biaxial testing apparatus had been invented and patented for using on the Gleeble thermo-mechanical testing machine to enable biaxial tensile testing.…”

“…It is very difficult to obtain FLDs of metals under hot stamping and cold die quenching conditions by using conventional methods, since the cooling which occurs prior to deformation and constant deformation temperature, strain rate and strain path are not easy to obtain. A novel formability testing system have been proposed and developed to determine formability of metals experimentally for hot sheet stamping applications [1].…”

“…It is very difficult to obtain FLDs of metals under hot stamping and cold die quenching conditions by using conventional methods, since the cooling which occurs prior to deformation and constant deformation temperature, strain rate and strain path are not easy to obtain. A novel formability testing system have been proposed and developed to determine formability of metals experimentally for hot sheet stamping applications [1].Experimentally determination [2] of the FLD of a material is time-consuming and costly, which restricts the number of tests to be conducted. Although various analytical and numerical models have been developed for formability prediction, most of them are applicable to only ambient conditions.…”