Forming limit diagrams of tailor rolled blank in hot stamping and its verification

Abstract

The purpose of this study is to predict the rolling-orientation and temperature dependent forming limit diagrams (FLDs) of tailor rolled blank (TRB) in hot stamping process. The rolling orientation is defined by the angle of major loading direction and rolling direction and influences to hot formability of TRB. When forming take places at elevated temperature, the formability of TRB is greatly affected not only by the rolling orientation but also by inhomogeneous forming temperature. In this study, TRB of thickness ratio 1.43(1.5/1.05 mm) was employed to evaluate the formability at elevated temperature using stretch forming test which was performed according to various deformation modes. The experimental FLDs were determined by the second derivative Bragard method which was based on the analysis of strain distribution near failure. In order to obtain the rolling-orientation and temperature dependent FLDs, the experimental FLDs were used to establish interpolating equation using major loading direction, rolling direction and forming temperature. The determined equation was applied to predict failure by comparison of calculated major strain with the value of FE-simulation. The rolling-orientation and temperature dependent FLDs were also applied to FE-simulation for hot stamping of rectangular cup drawing to predict limit drawing height (LDH). The predicted LDH was verified by comparison with experimentally measured one. As a result, hot formability of TRB can be precisely predicted by the suggested FLDs compared to conventional FLDs considering thickness of TRB.