The formability of Mg alloy sheet in the as-hot rolled condition depends on the microstructure developed during hot rolling. In general, the formability of Mg alloys is improved by finer grain sizes. 'Microalloying' levels of calcium (Ca), strontium (Sr), and cerium (Ce) have been found to refine the as-cast structure, but there is no information as to whether this effect will be reflected in the as-hot worked structure and formability. Thus, in this work, the effects of microalloying levels of calcium (Ca), strontium (Sr), and cerium (Ce) on the microstructures (from as-cast to as-hot rolled) and subsequent hot deformation behavior of AZ31, nominally 3% Al, 1% Zn, and 0.3%Mn, were systematically investigated. / To include the effect of solidification rate these alloys were cast in different moulds (preheated steel mould, Cu-mould, and water cooled Cu-mould). One-hit compression testing at temperatures between 250°C ∼ 400 °C, strain rates of 0.001, 0.01, 0.1 s-1 and strains from 0.2 up to 1.0, was performed to investigate the basic hot compression behavior, while two-hit compression testing was conducted to determine the static softening behavior. Hot rolling of the microalloyed AZ31 alloys was then carried out to study the effects of microalloying on as-hot rolled structure under two sets of rolling schedules. To investigate the formability of these microalloyed sheets, tensile tests were completed over a temperature range between ambient and 450°C, at strain rates between 0.1 and 0.0003 s-1. / Results show that Ca and Sr act to refine the as cast grain size and the second phases, consistently promoting fine and uniform as-hot rolled grain structure. With regard to grain refinement, calcium has the strongest effect, whereas Ce is most effective for second phase refinement. In addition, microalloying retards grain growth during hot tensile testing. Multiple alloying presents a combined and complementary effect. / A refined and uniform grain structure combined with well dispersed and thermally stable second phases significantly improves the hot formability of AZ31 sheets by promoting dynamic recrystallization (DRX) in the matrix, resisting grain coarsening, and retarding the development of cavitation and necking. Under the superplastic condition of 450°C and 0.0003 s -1, the elongation was improved by 17% with Ca only, 26% with Ca and Ce, 51% with Ca and Sr, and 59% with Ca, Sr and Ce.

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Automobile manufacturers are interested in lightweight materials, including magnesium, to increase vehicle fuel economy, improve performance and reduce emissions. In this work the deformation behavior of as-cast and rolled magnesium AZ31 alloy has been studied. In as-cast material, it was found that reheating at 400°C and above for 60 minutes increased the homogeneity of the as-cast structure and gave rise to repeatable deformation. At compression temperatures above 300°C dynamic recrystallization occurred; below 200°C, there was significant twinning. Annealing completely recrystallized the structure deformed below 200°C, but did not change the dynamically recrystallized structure. AZ31 alloy was also rolled at temperatures of 350, 400 and 450°C and rolling speeds of 20 and 50 rpm for 15 and 30% reduction in thickness to produce sheet. Before rolling, the alloy was preheated for I and 10 hours at the rolling temperatures. The sheets were then tensile tested at 300, 400 and 450°C with strain rates of 0.1, 0.01 and 0.001s-1. The flow curves and microstructures indicated that the tensile deformation mechanism changed with processing conditions. Two deformation mechanisms were present in the magnesium sheet depending on the strain rate and grain size. At slow strain rates and small grain size, the active deformation mechanism was grain boundary sliding. As grain sizes increased there was also a component of dislocation creep. At the fast strain rate, the deformation mechanism, regardless of grain size, was dislocation creep. At a true strain rate of 0.001s-1, it was found that rolling at 350°C with 30% reduction per pass yielded the finest microstructure and subsequently, the best hot deformation characteristics. At a true strain rate of 0.1s-1, rolling at 450°C with 30% reduction per pass yielded a coarser, more recrystallized microstructure with best hot deformation characteristics.

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Magnesium alloys have a great potential in automotive industries, compared to steel and aluminium (Al), Magnesium (Mg) is much lighter and this weight reduction improves fuel efficiency and lowers green gas emission. Due to its hexagonal crystal structure, magnesium has poor ductility at room temperature. Magnesium's ductility improves significantly above about 200°C due to thermal activation of additional slip systems. This has lead to efforts to form auto-body panels with commercial AZ31 magnesium sheet at elevated temperatures. In this work, various AZ31 magnesium alloy materials were used to investigate the influence of deformation conditions on texture and microstructure. Moreover, it is to define the correlation between formability and different deformation mechanisms. / It was observed that only basal slip and twinning contributed to room temperature deformation. As deformation temperature increased, an increase in ductility in Mg contributed to dynamic recrystallization occurring readily at elevated temperatures (≥300°C). Even coarse grain material experienced significant tensile elongation due grain refinement. Depending on temperature and strain rate, different deformation mechanisms were activated and lead to different failure modes (moderate necking, cavity, strong necking). More specifically, deformation at elevated temperature in the low-strain-rate regime with stress exponent n about 2-3 and activation energy close to grain-boundary diffusion of Mg (Q = 92 kJ/mol) is characteristic of GBS. Deformation at elevated temperature in the high strain rate regime showed that the stress exponent increased to a value close to 5 and that the activation energy was consistent with the one for Mg self-diffusion (135 kJ/mol) and for diffusion of Al in Mg (143 kJ/mol). This was indicative of a dislocation creep deformation mechanism. Plus the six-fold symmetric patterns of the {1 100} and {1120} pole figures and the splitting of basal plane distribution are another indication of slip mechanism or of dislocation creep mechanism. / The optimum deformation behavior for AZ31 sheet was found to be for the material with fine grain microstructure. The highest elongation of 265% was obtained with the material having initial grain size of 8 mum. In addition, strain-rate sensitivity, which is a good indication of material's ductility, also was the highest in material with 8 mum grain size. As a common trend, the strain-rate sensitivity increased with decreasing strain rate, increasing temperature and decreasing grain size. / In terms of drawability of AZ31 sheet, the deformation controlled by GBS resulted in a fair drawability/formability property with r-value about 1 whereas a deformation mechanism controlled by dislocation creep showed a good drawability with r-value above 1.5. Due to activation of additional slip systems (non-basal <a> and <c+a>), the thinning of the sheet was prevented, in particular at deformation conditions of 450°C with 0.1s-1 where r-value was highest. This deformation condition might suggest good forming process parameters, especially for deep drawing, for the commercial AZ31 sheet under investigation. A preliminary study of Forming Limit Diagram for AZ31 sheet was performed by the Limit Dome Height test method at 300°C. The FLD0 of AZ31 was found to be 67%; the part depth of biaxial forming was 1.86 in; and the maximum LDH varied from 2.4 to 2.6 in.

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