Impression Technologies (United Kingdom)

During a process development trial, two identical structural automotive parts were formed from a rolled sheet of commercial 6082 Al alloy by Hot Form Quench (HFQ). The rapid transitional contact between the sheet and the die set during die closing imposed a range of complex cooling histories on different locations in the parts. After in-die quenching, one HFQ part was naturally aged at room temperature, while the other was peak-aged at 185 °C for 7 h. The microstructure was compared at two extreme locations; the flange, where die contact is instantaneous leading to immediate rapid cooling, and the sidewall, where the material cools more slowly in air before rapid quenching. After HFQ, some small quench induced grain boundary precipitates were observed in the sidewall specimen but not in the flange. However, subsequent artificial ageing led to a convergence of the microstructures. Consequently, hardness was found to be constant at both locations, and no loss of hardening potential was observed in either case compared with T6 sheet. Samples of the as-received material were subjected to selected quench-hold-quench-age experiments to mimic the HFQ microstructures. Tensile tests showed that the ductility and failure mechanism is insensitive to the small differences in grain boundary microstructure as observed in the HFQ component.

Recently developed elevated temperature metal forming technologies improve material formability and address the springback issues of cold forming. However, the behaviour of alloys at elevated temperatures is viscoplastic and therefore considerably more complex than under cold forming conditions. This complex behaviour creates a barrier for industrial designers when designing for elevated temperature processes, and therefore leads to these processes often being overlooked. To overcome this barrier, a new method is proposed here to determine simpler strain hardening responses that are equivalent to the viscoplastic responses of alloys at elevated temperatures. The equivalent hardening responses are expressed by single material hardening curves and their hardening exponents are taken as parameters to approximate sheet metal viscoplasticity. This method therefore makes it possible to develop early-stage design guidelines that consider different materials and stamping processes. The method was successfully applied to two viscoplastic alloys under hot stamping conditions to determine equivalent hardening responses. The feasibility of the method has been assessed through comparing finite element simulations using the determined equivalent material models with ones using viscoplastic models. The result showed that the thinning distributions obtained were consistent in both cases, providing evidence that the method can be applied to a range of component designs.•The proposed equivalent material models are simpler than existing viscoplastic material models and can be derived directly from experimental stress-strain data.•Creating design guidelines from equivalent hardening exponents enables a straightforward way to compare between cold and hot stamping capabilities. This comparison makes it possible to make manufacturing process and material selection decisions quickly and effectively at the onset of a design process.•Design guidelines enabled by the proposed method are critical resources to encourage the uptake of new elevated temperature metal forming technologies that facilitate lightweighting and improved environmental efficiency.