Experimental artefacts affecting characterization of the evolving interfacial heat transfer coefficient in hot stamping of Al-Si coated 22MnB5 steel

In automotive hot stamping, knowledge of the interfacial heat transfer coefficient (HTC) between the blank and die is crucial for predicting the mechanical properties of the formed part. The HTC is derived from the ratio of the instantaneous heat flux between the blank and the die, and the corresponding difference between the blank and die surface temperatures. In many cases the heat flux and die surface temperatures are inferred from time-resolved subsurface temperature measurements using an inverse heat conduction analysis, while the blank is often modelled as thermally-lumped. This study highlights how seemingly subtle aspects of this experiment, like the positioning and time-constant of the thermocouples, may impact the inferred HTC. Furthermore, nonuniformity of the interfacial pressure appears to lower the target pressure at which the HTC saturates, and diminish the time-averaged HTC with increasing target pressures.