Development of a novel biaxial testing system for formability evaluation of sheet metals under hot stamping conditions

Abstract

Hot stamping and cold die quenching has been developed in forming complex shaped structural components of metals. This study is the first attempt to develop unified viscoplastic damage constitutive equations for the prediction of formability of metals under hot stamping conditions. In order to achieve the aim of this study, test facilities and methods need to be established to obtain experimental formability data of metals under hot stamping conditions. The research work is concerned with four aspects: thermo-mechanical properties of an alloy under hot stamping conditions, feasibility study of a novel biaxial testing system for hot stamping applications, formability tests by cruciform specimens under hot stamping conditions, and developed material models for formability evaluation and prediction. Hot tensile tests were performed at various temperatures and strain rates after heating and cooling processes to study the thermo-mechanical properties of AA6082 under hot stamping conditions. An error analysis of the proposed strain measurement method was carried out using an FE model coupled with thermo-electrical and thermo-mechanical conditions. The viscoplastic deformation behaviour of AA6082 was analysed in terms of temperature and strain rate dependence based on the experimental results. A viscoplastic damage constitutive model was developed to describe the thermo-mechanical response of the metal, material constants in which were calibrated from the hot tensile test results. A novel biaxial testing system was developed, patented and used for formability tests of AA6082 under hot stamping conditions after the feasibility study of this new testing method. Three heating and cooling strategies were proposed to investigate the temperature and strain distributions in a type of cruciform specimen. The dimensions of cruciform specimens adopted for the determination of forming limit under various strain paths were designed and optimised based on the selective heating and cooling method. Formability tests of AA6082 were conducted at various temperatures and strain rates after the heating and cooling processes. Two unified multi-axial viscoplastic constitutive models were developed and determined from the formability test results of AA6082 for the prediction of forming limit of alloys under hot stamping conditions. This research, for the first time, enabled formability data to be generated and forming limits to be predicted under hot stamping conditions. The technique has been verified for a particular aluminium alloy and can be applied to other metals under hot stamping conditions.