Thermal Control and Uncertainty Evaluation for Characterising Aluminium Formability Under Hot Stamping Conditions

Material formability under hot stamping conditions can be characterised by using an innovative Gleeble-based biaxial testing method with cruciform samples. However, due to the temperature gradient in the cruciform samples, the determined material forming limits have significant uncertainties, therefore compromising the accuracy of the formability testing method. In this paper, 2D remote measurements of the temperature field over the gauge area of cruciform samples are presented. A manganese-doped fluorogermanate phosphor - Mg4FGeO5.5:Mn, mixed with a flame-resistant chemical binder is sprayed over cruciform samples, machined from aluminium alloy 6082, forming a thin paint that is later optically interrogated. This resulting coating emits temperature-dependent luminescence (peak spectral emission shift) when excited by UV light that is later exploited for thermometry purposes. The region of the gauge area of the cruciform sample is defined based on the measured temperature uniformity. The effect of temperature gradients in the gauge area on the determined forming limit curve is evaluated at elevated temperatures. Accurate online determination of the instantaneous temperature field during deformation will allow for improved process temperature control, leading to determination of characterised material formability with a lower uncertainty.