Abstract
Due to their high specific strength, EN AW-7xxx aluminium alloys are promising materials for reducing the weight of automotive structural parts. However, their formability at room temperature is poor due to pronounced natural ageing. Therefore, we investigated hot stamping and W-temper forming for EN AW-7075 and a modified variant of EN AW-7021. For hot stamping of the modified EN AW-7021, a low-temperature stabilisation heat treatment (pre-aging at 80 °C for 1 h) was incorporated into the process chain design to inhibit natural ageing after forming. The process chains were compared with respect to dimensional accuracy, mechanical properties, microstructure, precipitation status (assessed by differential scanning calorimetry) and crashworthiness. It was found that hot stamping is suitable to form failure-free parts with good dimensional accuracy for both alloys while W-temper forming suffers from springback. Within a time-span of 21 days after forming, hardness values of hot stamped and stabilised parts did not increase significantly. Compared to non-stabilised parts, stabilised parts also showed significantly improved folding behaviour in quasi-static compression testing and absorbed approximately 15% more energy.
Highlights
Most modern mainstream cars have considerably increased in weight compared to their predecessors
It was found that hot stamping is suitable to form failure-free parts with good dimensional accuracy for both alloys while W-temper forming suffers from springback
EN AW-7075 and modified EN AW-7021 aluminium sheets. It highlights the importance of considering the entire process chain for achieving good material properties with respect to both processability and crashworthiness
Summary
Most modern mainstream cars have considerably increased in weight compared to their predecessors. For aluminium sheet metal parts, mostly the medium to high strength alloys like the EN AW-5xxx and the EN AW-6xxx series are used so far This preference is mainly due to their better formability at room temperature compared to the Metals 2019, 9, 305; doi:10.3390/met9030305 www.mdpi.com/journal/metals. Even after applying a subsequent post-forming heat treatment simulating paint baking, the initial strength values could not be regained. For the AW-7075 alloy, yield strength values that were only were observed after the stabilisation (pre-ageing) heat treatment and a simulated paint bake process. For the modified EN AW-7021, a stabilisation heat treatment after hot stamping was investigated because it is known that stable material properties can improve reproducibility of self-pierce riveting [10] Another rationale behind stabilising was to study its effects on crashworthiness. Test methods used were tensile testing, hardness testing, optical microscopy, differential scanning calorimetry (DSC) and quasi-static compression testing of formed and joined top-hat profiles
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