Thermal Contact Resistance Modeling in AA7075 Hot Stamping

Abstract

Hot stamping and die quenching (HS/DQ) process of AA7075 aluminum alloy is one of attractive forming techniques for producing high strength automotive structural components to encounter their poor formability at room temperature. In this technique, quenching rate of this alloy is very crucial as it affects precipitation kinetics after artificial ageing of part formed, which in turn determines the final in-service mechanical properties and corrosion performance of part. Thermal contact resistance (TCR) between two solid surfaces is the main parameter that controls heat transfer between hot AA7075 sheet and cold steel dies, and thus affects quenching rate of part formed. Therefore, the final properties of automotive parts produced by hot stamping is indirectly influenced by TCR. The common methods of determining TCR in HS/DQ are often impracticable as they require thermocouples to be inserted into complex-shaped stamping dies, punches and thin aluminum sheet (blank) to be formed. A potential mechanistic approach for determining TCR could be an attractive alternative due to its avoidance of embedded thermocouples into the tooling and blank. The mechanistic method emphasizes on physical mechanisms (roughness etc.) governing interfacial heat transfer between cold forming tools and hot blank. The proposed work focuses on utilizing the mechanistic method to predict TCR between multiple cylindrical asperities on a nominally flat (and heated) AA7075 blank surface and a rigid, flat, asperity-free (and cold) steel die surface. The asperities were considered to deform elastoplastically, increasing contact area. Subsequently, TCR correlation as a function of temperature, contact load, and contact area was formulated. To validate the mechanistic model, a series of surface asperity flattening experiments using thermocouple-embedded AA7075 blank and polished stainless steel planar dies were carried out. Good agreement between mechanistic model predictions and experimental results in term of contact area and TCR as a function of contact load were observed.