Development of an interfacial heat transfer coefficient model for the hot and warm aluminium stamping processes under different initial blank temperature conditions

Different initial blank temperatures, or forming temperatures, are applied in hot and warm aluminium stamping processes depending on the component being formed and the desired post-form properties. An initial blank temperature dependent interfacial heat transfer coefficient is therefore an essential boundary condition in such non-isothermal forming processes for obtaining precise temperature fields from the formed components in finite element simulations. This precision is crucial for the optimisation of the processing window and tool design, to subsequently achieve the desired cooling rates and post-form strength of the aluminium components being formed. The IHTC between a 7075 aluminium alloy and tungsten carbide cobalt coated cast-iron was found to approximately increase linearly with increasing initial blank temperature in the present research. The effect of the initial blank temperature was identified as a combination of two mechanisms; the effects of material strength and thermal conductivity. The IHTC was found to decrease with increasing strength of the blank when the contact pressure was lower than a threshold value, while a larger thermal conductivity of the aluminium blank increased the IHTC values. Furthermore, an initial blank temperature-dependent model was developed to predict the IHTC at different initial blank temperatures, which was subsequently verified by the results of non-isothermal forming tests.