Investigations of interfacial heat transfer efficiency in HFQ® process of high strength aluminum alloy

Abstract

Hot form quench (HFQ®) is a novel hot stamping process to deform complex-shaped parts applied in automobile industry, and interfacial heat transfer coefficient (IHTC) play an important role inHFQ® process. In this research, the average IHTC values were compared under different forming conditions during die quenching based on the calculation and finite element (FE) simulation results. Beside, transmission electron microscope (TEM), differential scanning calorimeter (DSC) and tensile tests were performed to investigate the relationship among IHTC, microstuructures and mechanical properties. The results show that the experimental results agree well with those achieved by FE simulation. The value of IHTC for AA7075 increased with the increased contact pressure during HFQ® process. In contrast, the increasing temperature of forming die has a negative impact on IHTC. The critical quenching rate (Qc), inducing the formation of a large number of equilibrium phases η, is between 14.3 °C s-1-18.5 °C s-1, and the corresponding IHTC range is 2468W m2 k-2656W m2 k. With regard to the condition with cooling rate lower thanQc, the formation of a large amount of η is recognized as the reason for the strength decrease.