Comparison of Bake Hardening Effects on AHSSs and Extruded Aluminum Alloys Applied in BEV Reinforcement Structures 2024-01-2240

At the dawn of battery electric vehicles (BEVs), protection of automotive battery systems as well as passengers, especially from severe side impact, has become one of the latest and most challenging topics in the BEV crashworthiness designs. Accordingly, two material-selection concepts are being justified by the automotive industry: either heavy-gauge extruded aluminum alloys or light-gauge advanced high-strength steels (AHSSs) shall be the optimal materials to fabricate the reinforcement structures to satisfy both the safety and lightweight requirements. In the meantime, such a justification also motivated an ongoing C-STAR^TM (Cliffs Steel Tube as Reinforcement) Protection project, in which a series of modularized steel tube assemblies, were demonstrated to be more cost-efficient, sustainable, design-flexible, and manufacturable than the equivalent extruded aluminum alloy beams as BEV reinforcement structures. Tangent to this comparative study, the present work shed some light on the bake hardening (BH) effects during a paint-baking cycle, which was a necessary processing procedure for a body-in-white (BIW), on some representative AHSSs and extruded aluminum alloys via various coupon-level mechanical experiments under precise in-situ strain/displacement and temperature control conditions at multiple strain rates and stress states. The corresponding material mechanisms were also reviewed and explained. Eventually, the test results revealed some tremendously distinct changes induced by the BH effects on the two types of metallic materials: the baking-induced Cottrell atmosphere could effectively enhance the strength without weakening the local ductility of the target AHSS, while the baking-induced precipitation slightly hardened the selected aluminum alloy yet lowered its fracture limit. Such a distinction further indicated the advantages of the AHSSs in this application. The ultimate objective of this work was to provide relative references for future finite element simulations and BEV structural designs.