Effect of tool coatings on the interfacial heat transfer coefficient in hot stamping of aluminium alloys under variable contact pressure conditions

The interfacial heat transfer coefficient (IHTC) is an essential parameter in hot stamping for determining the optimal process parameters necessary to achieve the critical cooling rate and thus the desired post-form strength in a formed component. Aluminium chromium nitride, chromium nitride and titanium nitride are currently being widely studied in the hot stamping industry as tool coatings due to their excellent wear performance, although their effects on the IHTC have yet to be determined. In practical hot stamping processes, the contact pressure may also change over extremely short periods of time, leading to abrupt changes in the IHTC and consequently the temperature evolutions of the workpiece. These abrupt changes in contact pressure and their effects on the IHTC have yet to be studied either. In this paper, to address those challenges, the IHTCs between an AA7075 aluminium alloy and different coated tools with a 3-stage evolutionary variable contact pressure were quantitatively determined, and a mechanism-based model developed to accurately predict their values. (C) 2019 Elsevier Ltd. All rights reserved.