Development of materials characterisation and modelling methods under hot stamping conditions

Abstract

Hot stamping of sheet steels is used widely in the automotive industry to form safety panel components with complex shapes and high strength. Characterisation and modelling of the formability of materials under hot stamping conditions are essential for the process optimisation and applications. Due to the high austenitic temperature in hot stamping, however, it is difficult to evaluate the formability of the materials under these conditions using existing standard tests such as the Nakajima and Marciniak tests. In this thesis, a novel biaxial testing method has been developed to determine forming limit curves (FLCs) which are the most commonly used tools for evaluating the formability, and fracture forming limit curves (FFLCs). Furthermore, a set of continuum damage mechanics (CDM)-based unified viscoplastic constitutive equations has been formulated to predict both FLCs and FFLCs for the materials under hot stamping conditions. The biaxial testing method has been applied to determine an FLC for AA5754 at room temperature, and the FLC determined has been compared with that obtained using the Nakajima test. The fracture occurred near the centre of the test specimens in all straining conditions used, and the FLCs determined using the two methods were comparable. More importantly, the biaxial testing method has been extended and applied to determine both FLCs and FFLCs for boron steel under hot stamping conditions. The resulting test specimens had a nearly uniform temperature distribution in the gauge area and fractured close to the centre of this area under all test conditions investigated. The CDM-based constitutive equations have been developed from a set of dislocation-based hardening constitutive equations, with two damage variables to model the accumulated damage leading to localised necking and fracture separately. The material constants in the equations have been determined from the curve fitting of the experimental FLCs and FFLCs. Good agreement between the computed and experimental results was observed, indicating the high flexibility of the CDM-based constitutive equations for predicting FLCs and FFLCs under hot stamping conditions.