Postdoc research fellow, McGill University
To improve the formability of Mg alloy sheet through thermomechanical processing& alloying strategy
Magnesium (Mg) is the lightest structural metal. Its density is about 1/4 that of steel and 2/3 that of aluminum. Mg alloys, therefore, have high specific strength and specific stiffness. This makes them promising candidates for lightweight applications in the automotive industry to improve the fuel efficiency and to reduce the emission of CO2. However, Mg shows low ductility and poor formability at room temperature, owing to the insufficient number of slip systems and the strong crystallographic texture. This impedes the applications of Mg wrought products, especially the sheets. In the present study, high speed rolling (HSR) was employed to roll Mg alloy sheets at a speed of 1000 m/min. A high reduction of 72% was achieved in a single pass at an initial temperature of 100 °C. In comparison, the sheet fractured at a reduction of 37% by traditional low speed rolling. The far better rollability achieved during HSR is attributed to the activation of dynamic recrystallization and more non-basal slip and twinning; this, in turn, leads the weaker textures developed during HSR. The maximum texture weakening was achieved by annealing of the heavily twinned and shear-banded structure produced at the reduction of 30% by HSR. Final grain size and texture intensity can be manipulated by controlling the rolling and annealing parameters. It has been widely reported that texture weakening and grain refinement are efficient ways to improve the formability of Mg alloy sheets during the further forming process. Through this study, a new and economical thermomechanical processing strategy is proposed to improve the formability of rolling of the Mg sheets and forming the components from the sheets. This approach can be applied to form other Mg alloys (rare-earth-containing) sheets to optimize the microstructure and texture, and thus to improve the mechanical properties and formability.
Abstract: The effects of phase content and evolution on mechanical properties of the alloys are investigated by extruding Mg95Y2.5Zn2.5 and Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloys with different original microstructures. The 18R-LPSO phase plays a decisive role in the strength of as-extruded alloys. The spherical W-MgYZn2 phase enhances the deformability and ductility of the alloys. The dynamic recrystallization grains can be observed in all as-extruded alloys. However, the 14H-LPSO clusters and dynamic precipitates only form in the as-cast and T41-treated (solid-solution treatment at 530 °C for 3 h with water cooling) alloys after extrusion. Based on synergistic effects of these phases mentioned above, the T41-treated Mg93.1Y2.5Zn2.5Ti1.6Zr0.3 alloy exhibits good compressive mechanical properties after extrusion.
Pub.: 26 May '17, Pinned: 27 Jun '17
Abstract: MagnesiumAZ31alloy sheets were rolled at a high speed of 1000 m/min(HSR) and a low speed of 15 m/min(LSR) at 100 °Cto a reduction of 30%.The as-rolled microstructures were heavily twinned and shear banded at both speeds. Annealing was performed on the as-rolled sheets at three different temperatures (200, 350and 500 °C) for increasing time intervals. Texture weakening was achieved after annealing of both HSRed and LSRed sheets due to static recrystallization (SRX) on twins and shear bands. This, in turn, greatly improved the ductility of the sheets. It was found that the intensities of the basal texture of the HSRed specimens were lower than those of the LSRed specimens at as-rolled and all annealed conditions. This is related to the higher amount of contraction twins and more <c+a> slips activated during HSR. Correlation of the maximum intensity of the basal texture with softening/recrystallization fraction was found to be involved in two weakening stages. During the first stage, nucleation on twins and shear bands was the dominant mechanism for texture weakening, while growth of the non-basal grains was responsible for the second stage of texture weakening. Interestingly, it was found that the weakest texture was attained after annealing at 500 °C for 5 min in the HSRed specimen, which can be related to thermal activation of more random nucleation sites and preferential grain growth of the non-basal oriented grains.
Pub.: 27 Jul '16, Pinned: 27 Jun '17
Abstract: Magnesium AZ31 alloy sheets were rolled at 100 ℃ at two speeds: (i) 1000 m/min and (ii) 15 m/min. The rolled specimens were annealed at temperatures from 200 ℃ to 500 ℃ for increasing intervals of time. The recrystallization kinetics were analyzed in terms of the Johnson-Mehl-Avrami-Kolmogorov model and found to involve two sequential annealing stages, characterized by two different Avrami exponents. Stage 1 is associated with recrystallization in the high stored energy regions, such as these containing shear bands and twins, while stage 2 concerns the recrystallization of some of the low stored energy regions. Nevertheless, in the specimens subjected to low reductions (<30%) and annealed at 200 ℃ for long time periods, some of the elongated grains remained unrecrystallized. This is attributed to the relative lack of formation of twins in these grains during rolling, i.e. to the very low stored energies in these regions. There was no significant difference in the microstructure evolution and recrystallization kinetics of the materials rolled at the two speeds. This is interpreted in terms of the Zener-Hollomon parameter, similar values of which applied to the two types of rolling and therefore to the microstructures produced.
Pub.: 12 Mar '16, Pinned: 27 Jun '17
Abstract: Superplasticity, a phenomenon of high tensile elongation in polycrystalline materials, is highly effective in fabrication of complex parts by metal forming without any machining. Superplasticity typically occurs only at elevated homologous temperatures, where thermally-activated deformation mechanisms dominate. Here, we report the first observation of room-temperature superplasticity in a magnesium alloy, which challenges the commonly-held view of the poor room-temperature plasticity of magnesium alloys. An ultrafine-grained magnesium-lithium (Mg-8 wt.%Li) alloy produced by severe plastic deformation demonstrated 440% elongation at room temperature (0.35 T m) with a strain-rate sensitivity of 0.37. These unique properties were associated with enhanced grain-boundary sliding, which was approximately 60% of the total elongation. This enhancement originates from fast grain-boundary diffusion caused by the Li segregation along the grain boundaries and the formation of Li-rich interphases. This discovery introduces a new approach for controlling the room-temperature superplasticity by engineering grain-boundary composition and diffusion, which is of importance in metal forming technology without heating.
Pub.: 03 Jun '17, Pinned: 27 Jun '17