Determination of the interfacial heat transfer coefficient in the hot stamping of AA7075

Abstract

Abstract. The interfacial heat transfer coefficient (IHTC) is a key parameter in hot stamping processes, in which a hot blank is formed and quenched by cold dies simultaneously. The IHTC should therefore be identified and used in FE models to improve the accuracy of simulation results of hot stamping processes. In this work, a hot stamping simulator was designed and assembled in a Gleeble 3800 thermo-mechanical testing system and a FE model was built in PAM-STAMP to determine the IHTC value between a hot aluminium alloy 7075 blank and cold dies. The IHTC was determined at different contact pressures under both dry and lubricated (Omega-35) conditions. In addition, a model to calculate the IHTC value at different contact pressures and area densities of lubricant was developed for the hot stamping process.

Determination of the interfacial heat transfer coefficient for a hot aluminium stamping process

“…This influence could be verified by the study of Liu et al (2015) in which the peak IHTC value for AA7075 under dry conditions when using mild steel tools, 12.3 kW/m 2 K, lies between the peak values obtained when using H13 and cast iron tools, and the thermal conductivity of mild steel is in between both H13 and cast iron.…”

“…In order to retain the full mechanical strength of the material during ageing processes that follow forming, the quenching rate during the hot stamping process must be above the critical cooling rate such that no solute elements precipitate out as coarse particles (Milkereit et al, 2009). The interfacial heat transfer coefficient, an important thermophysical parameter in hot stamping processes, such as HFQ, should therefore be identified not only to retain the full mechanical strength of formed components by achieving the critical quenching rates for different aluminium alloys (Liu et al, 2015), but also to optimise the production rate by controlling the quenching process. Furthermore, the critical quenching rates and the corresponding critical contact pressures are beneficial for guiding the tool design, in order to avoid insufficient quenching in particular regions, e.g.…”

“…In order to identify the IHTC under specific experimental conditions, the measured temperature evolutions were compared with those inversely calculated with Beck's non-linear estimation method (Caron et al, 2014), or with those predicted by formulations using a 1D closed form method (Bai et al, 2012). Additionally, FE simulations (run in ABAQUS (Ji et al, 2014), PAM-STAMP (Liu et al, 2015) or DEFORM-2D (Yukawa et al, 2014)) were applied to obtain simulated temperature evolutions to fit the experimental curves to identify the corresponding IHTC values.…”

Numerical modeling of the thermal contact in metal forming processes

“…The IHTC also plays a crucial role in hot/warm sheet metal forming processes, since the heat exchanges on the interface of the sheet are important for the temperature control and, consequently, the mechanical properties of the final part [21,29,33]. In fact, the numerical modeling of the interfacial heat transfer requires the prior determination of the IHTC [27]. Thus, the accurate evaluation of this parameter is essential for the thermal and mechanical analysis of any system.…”

A buckling model for flange wrinkling in hot deep drawing aluminium alloys with macro-textured tool surfaces

“…The temperatures were then used to calculate the stress-strain values during deformation.Table 4gives the relevant material and thermal properties used in temperature evolution modelling. The interfacial heat transfer coefficient is a function of the contact pressure and gap in the hot stamping process, which is obtained using the method developed specifically for the process of hot stamping aluminium alloys[31].…”