Development of an Experimental Friction Testing Platform and a Finite Element Simulation for Hot Stamping

Abstract

In hot stamping, the material formability, thermal interaction, and metallurgical microstructure are coupled. Because of this complex forming mechanism, forming defects and product strength are difficult to predict. Therefore, accomplishing the process design and performing strength analysis by using finite element simulations is more efficient. To analyze forming characteristics and facilitate finite element analysis, material properties of boron steel at increased temperatures and other crucial process parameters such as the friction coefficient and interfacial heat transfer coefficient, which affect sheet thickness reduction and the die quenching rate considerably, must be determined first. In this study, an experimental platform was developed to measure and derive the coefficients of friction at the interface between the sheet metal and the die surface. To ensure that the designed platform was sufficiently strong to sustain both normal and pulling forces in friction tests, finite element simulations were performed. An optimal structural design was then determined and the experimental platform was constructed accordingly. Friction experiments at various increased temperatures and contact forces were conducted using non-coated 15B22 boron steel sheets as test specimens. The compression mold was made of SKD61 steel, which is commonly used as the die material in hot stamping. In each friction test, the pulling force exerted on the sheet specimen was recorded and the friction coefficient was calculated. The experimental results showed that the coefficients of friction ranged from 0.50 to 0.53 for various conditions. The friction coefficient was also shown to be more dependent on the forming temperature than on the contact force. A finite element simulation was then constructed using the experimental data and material properties of boron steel, which were calculated using the JMatPro software. The finite element simulations for hot stamping an automotive door beam, including transportation, hot forming, and die quenching analyses, were then performed. The validation of the finite element results for the hot stamped production part confirmed the importance and accuracy of the developed experimental platform for friction tests.