A pinboard by
Jing Su

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.


Correlationof static recrystallization and texture weakening of AZ31magnesium alloy sheets subjected to high speed rolling

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