Determination of the interfacial heat transfer coefficient in hot stamping of aluminium alloys

Abstract

The characteristic properties of aluminium alloys, e.g. their high strength-weight ratio, high thermal conductance, excellent corrosion resistance and good recyclability, render them ideal materials to reduce air pollution and improve the fuel economy of vehicles. However, their low formability at room temperature limits their application in industry. In recent years, hot stamping was developed as a promising technology to form sheet metal components from aluminium alloys at elevated temperatures to increase their formability. The interfacial heat transfer coefficient (IHTC), an essential thermophysical parameter in hot stamping processes, should therefore be identified not only to retain the full mechanical strength of the formed components by achieving the critical quenching rates for different aluminium alloys, but also to optimise the production rate by controlling the quenching process. The present research aims to determine the IHTC values for 7075 and 6082 aluminium alloys under different experimental conditions. A dedicated IHTC test facility, IHTC-mate, was developed to precisely measure the temperature evolutions of the specimens and thus accurately determine their IHTC values with high stability and repeatability. Subsequently, the effects of the contact pressure, tool material, coating material, specimen thickness, lubricant and initial blank temperature on the IHTC were identified. It was found that the IHTC increased logarithmically with increasing contact pressure. In addition, the applications of tools, coatings and lubricants with higher thermal conductivities, as well as specimens with larger thickness and higher initial blank temperature could raise the IHTC values. Furthermore, a mechanism-based IHTC model was developed to predict the IHTC evolutions as a function of those influential factors, and enable the interaction between the IHTC evolutions and lubricant layer thickness diminution with sliding distance at different contact pressures and sliding speeds. Hemispherical dome and B-pillar forming tests were conducted to form 7075 and 6082 aluminium alloys. The good agreements between the experimental and simulated temperature evolutions of the components being formed validated the determined IHTC results and developed IHTC model. Consequently, the temperature evolutions and cooling rates of the components being formed in hot stamping processes could be predicted. Furthermore, the processing window and tool design could be optimised to achieve the critical cooling rates and thus retain the full mechanical strength of the formed components.