Nozomu Kawabe

Properties of AZ31 and AZ91 Sheets Made by Twin Roll Casting

A feasibility study has been made to produce the Mg alloy sheet of high tensile strength and formability by strip casting and an optimized rolling process. 4mm thickness AZ31 and AZ91 strips were cast successfully. While the microstructure of the cast AZ31 strip consisted of columnar zones, the cast AZ91 strip consisted of fine, equiaxed dendrite and fine precipitations. All samples were rolled to 0.5 mm thickness using an optimized finish-rolling schedule, and their grain size was around 4μm. The ultimate tensile strength (UTS), yield strength (YS) (0.2% offset) and elongation (EL) of the rolled AZ31 sheet were higher than those of commercial AZ31B sheet, and the UTS and YS of the rolled AZ91 sheet were 30% higher than commercial AZ31B sheet. The press formability of the rolled AZ31 and AZ91 sheet was equal or superior to commercial AZ31B sheet.

Superplasticity in a Chip-Consolidated Mg 97 Zn 1 Y 2 Alloy with LPSO Phase

Novel Mg97Zn1Y2 alloy of equiaxed fine grains with long-period stacking ordered (LPSO) phase was prepared by consolidation of chips machined from a twin-roll-cast alloy. The chip-consolidated Mg97Zn1Y2 alloy exhibited superplasticity at a wide initial strain-rate ranging from 1 × 10−3 to 3 × 10−1 s−1 at temperatures of 623 and 673 K. The maximum elongation was approximately 560% at 3 × 10−2 s−1 at 673 K. The strain rate sensitivity index (m-values) was shown to be greater than 0.3 in this study; this value seems to be large enough to ensure uniform elongation during superplastic deformation. SEM/EBSD measurements revealed that the chip-consolidated Mg97Zn1Y2 alloy was composed of mixed equiaxed fine-grains of α-Mg and LPSO phases of a mean size of ~1 μm. The grain boundary sliding in the equiaxed fine-grains appears to account for most of the mechanism of superplastic deformation in this alloy.