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Experimental decklids for the Cadillac STS sedan were made with Al AA5083 sheet outer panels and Mg AZ31B sheet inner panels using regular-production forming processes and hardware. Joining and coating processes were developed to accommodate the unique properties of Mg. Assembled decklids were evaluated for dimensional accuracy, slam durability, and impact response. The assemblies performed very well in these tests. Explicit and implicit finite element simulations of decklids were conducted, and showed that the Al/Mg decklids have good stiffness and strength characteristics. These results suggest the feasibility of using Mg sheet closure panels from a structural perspective.

Temperature effects on the deformation and fracture of a commercially produced transformation-induced plasticity (TRIP) steel subject to a two-step quenching and partitioning (Q&P) heat treatment are investigated. Strain field evolution at room temperature is quantified in this 980 MPa grade Q&P steel with a stereo digital image correlation (DIC) technique from quasi-static tensile tests of specimens with 0°, 45°, and 90° orientations. Baseline tensile properties along with the variation of the instantaneous hardening index with strain were computed. Variations of the bake-hardening index were explored under simulated paint bake conditions. Tensile properties were measured at selected temperatures between -100°C and 200°C and the TRIP effect was found to be temperature-dependent due to stress-induced martensitic transformation at lower temperatures versus strain-induced transformation at higher temperatures.