Development of a general interfacial heat transfer coefficient model to characterise the critical processing parameters in hot and warm aluminium stamping processes

Abstract

Different hot and warm stamping technologies with particular processing parameters were applied to deform aluminium alloy sheets to satisfy desired requirements, of which the post-form strength of formed components is one of the most important criteria. In order to save experimental efforts, the present research described an efficient method to determine the critical processing parameters, i.e. the integration of the finite element (FE) simulated temperature evolutions with the continuous cooling precipitation (CCP) diagrams of the aluminium alloys. Through the optimisation of the processing parameters, the temperature evolutions and CCP diagrams do not intersect, indicating that the post-form strength of the aluminium alloys could be fully retained after proper artificial ageing processes. Therefore, a precise FE simulation of the temperature evolution is of great importance to this method, which requires the implementation of an accurate interfacial heat transfer coefficient (IHTC) as a decisive boundary condition. A general aluminium alloy-independent model with one set of fixed model constants was therefore developed to predict the IHTC evolutions as a function of contact pressure, surface roughness, initial blank temperature, initial blank thickness, tool material, coating material and lubricant material. Subsequently, the predicted IHTCs for 6082 and 7075 aluminium alloys were used to simulate their temperature evolutions, which were then integrated with their CCP diagrams to identify the critical processing parameters in hot/warm stamping processes and thus meet the desired post-form strength of the 6082 and 7075 aluminium alloys. The developed IHTC model and determined critical processing parameters were then experimentally verified by the fast alloy stamping (FAST) of the dissimilar aluminium alloys.