| Abstract Scope |
This presentation provides an overview of material characterization and model development studies performed on a texture-modified rare earth magnesium alloy sheet (ZEK100). Wrought magnesium alloys are attractive for automotive industry applications due to their low density and high specific strength. However, commercial magnesium alloys, such as AZ31B sheet usually have poor formability at room temperature due to limited activity of slip systems. Additionally, due to the twinning deformation mechanism activated in specific loading directions, magnesium alloys exhibit an asymmetric stress-strain response in uniaxial tension and compression tests. The formability of magnesium alloys can be improved by deforming at elevated temperatures; however, warm forming of AZ31B requires a more complex heated tooling setup which increases the cost of the forming operation. Alternatively, the formability can be improved by the addition of rare-earth elements such as Ce, Nd, Y and Gd, for example, which have been shown to weaken the basal texture. Constitutive, formability and fracture characterization of both AZ31B and ZEK100 sheet is presented, considering both room and elevated temperature conditions over a wide range of strain rate. The mechanical behavior can be related back to the initial crystallographic texture in light of known deformation mechanisms operating at different orientations and strain rates. Extensive tensile and compressive constitutive characterization experiments were performed on both alloys, including characterization of anisotropy with strain and material strain rate- and temperature-sensitivity. Forming limit characterization was also performed at elevated temperatures using in situ digital image correlation (DIC) strain measurement. The ZEK100 alloy exhibits significantly higher formability at temperatures below 250°C, whereas the two alloys have similar formability in the 250-300°C range. Yield criteria capturing the evolving anisotropy and asymmetry of magnesium sheet alloys are proposed to model the complex behavior of magnesium alloys at room and elevated temperatures. At room temperature, the material behavior of both alloys is highly anisotropic and asymmetric; however, the degree of asymmetry and anisotropy is diminished at elevated temperature. The proposed material model is validated against several laboratory-scale experiments: 3-point bending, limiting dome height (LDH) and limiting draw ratio (LDR) experiments. Full-scale forming trials are performed considering prototype door inner and roof outer tooling. AZ31B and ZEK100 blanks were formed with initial elevated temperatures, but with room temperature tooling. The AZ31B blanks failed during forming whereas the ZEK100 blanks were successfully drawn for temperatures above 250°C. Recent constitutive models suitable for warm forming conditions using commercial forming software (Autoform) and are shown to provide predictions in accord with the forming trials. |