To clarify the grain-refining mechanisms of a commercial AZ91E magnesium alloy by superheat treatment, the molten alloy was quenched by using two chilled copper blocks. The superheat treatment was performed at 1123 K for 900 s, then cooled at 2.5 K/s. The quenching temperatures were 1023, 973, 923 and 873 K. In the superheat-treated samples particle-like substances as nucleants could be observed in the center of dendrite crystals. The samples quenched at 1023 K reveals that the particle-like-substances consisted of Al, C and O. However, different types of substances were observed in the sample quenched at 973 K. They were consisted of Al, Mn, Fe, Si, C and O elements. It is concluded that the compound consisting Al, C and O is obviously heterogeneous nucleation substance in the superheat-treated AZ91E magnesium alloy.

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Grain refinement of cast magnesium alloys containing Al is attained by adding carbon materials. This technique is well established. However, it is important problem that C₂Cl₆ as grain refiner generates dioxin matters. Therefore, it is impossible to use grain refiners containing chloride. This study was performed to develop a new grain refiner containing C element instead of harmful C₂Cl₆. Carbon is an important element for grain refining of the commercial AZ91 magnesium alloy. Pure carbon (99.9%) powder with high-purity argon gas carrier was blown into the molten AZ91 magnesium alloy. Fine grains were obtained by addition of the carbon powder. Foreign substances as nucleants could be observed in the center of each grain. The result of EPMA analysis indicated that the foreign substance was consisted of Al, C and O.

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Precipitation in Mg–Li–Zn ternary alloys containing 4 to 13%Li and 4 to 5%Zn (in mass%) with α or β single phases, or with (α + β) dual phases was investigated using a micro Vickers hardness measurements and transmission electron microscopy. Age hardening in the α phase alloy was found to occur, which was attributed to the precipitation of the stable θ (MgLiZn) phase with the following orientation relationships; [1010]_α//[110]_θ, (0001)_α//(111)_θ. In the (α + β) phase alloy, the precipitation of the α phase together with the metastable θ' (MgLi₂ Zn) phases occurred at grain boundaries between the α and β and also β and β grains. The orientation relationship between the α and θ' was as follows; (0001)_α//(011)_θ', [0110]_α//[111]_θ'. Age hardening in the β alloy was caused by the precipitation of the θ' phase and over-aging was attributed to the precipitation of the α and θ phases.

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In this study, the suitable alloys for semi-solid forming are searched considering the advantage of carrying out fabrication of near-net shaped products at a lower temperature than die-casting. Firstly Mg–20%Al alloy as selected as a model alloy for preliminary evaluation of fluidity. Furthermore, in order to improve mechanical properties, Mg–20%Al alloy was chosen as a base alloy, and alloys containing Zn and Ca with reduced Al content were also investigated. These investigated alloys show that solid fraction becomes about 57 to 65% just above the melting temperatures of eutectic regions, and solid particles become spherical due to the introduced compressive strain. Besides, eutectic regions, which are rapidly cooled from the semi-solid temperatures during press forming exhibit fine microstructure. The fluidity of Mg–20%Al alloy and alloys containing Zn is excellent, while the alloys containing Ca are extremely bad. It is possible to harden all of the investigated alloys by aging. However, heat-treated high Al containing alloys show low tensile strength and elongation because a network of the eutectic compound, Mg₁₇Al₁₂ is retained along the grain boundaries even after solution heat treatment, and this becomes the source of crack initiation and propagation. On the other hand, in the alloys having reduced Al content and containing Zn or Ca, the tensile properties increase due to the changes of the morphologies or kinds of eutectic compounds.

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Facing machinability tests were performed on the magnesium alloy castings AZ91 by measuring cutting resistance, shear angle, surface roughness and chip forming. Face cutting done from periphery to center of specimen with cemented carbide alloy K20 in which has various side rake angle. Regardless of the different side rake angles, both cutting forces and machined surface of the alloy were independent of cutting speed. The cutting resistances increased with increasing both feed rate and depth of cut, and decreased with increasing of side rake angle. The feed rate influenced a little to the machined surface, and it was superior to that of theoretical valves on the basis of feed marks and tool configuration. The orderly feed marks were observed on the machined surface, however, in the case of both high feed and large side rake angle, the burr and tear were observed on the machined surface. The size of the chips decreased with decreasing cutting speed and increasing feed rate. The thickness of the chips at start of cutting became thinner than that at end. The cutting ratio showed large value at the outer part of specimen under high cutting speed. The shear angle decreased with decreasing cutting speed. The AZ91 magnesium alloy castings had excellent facing machinability in high speed heavy cutting. The facing machinability of AZ91 magnesium alloy castings was excellent and exhibited good performance in high speed heavy cutting.

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Laser cladding using Al–30%Si alloy powder was applied to AZ91D alloy in order to improve its wear resistance. Area fractions of Mg₂Si and Mg₁₇Al₁₂ compounds with relatively high hardness in the cladding layer with a width of 4 mm and a depth of 1.5 mm increase with an increase in powder feeding rate, resulting in high hardness of the cladding layer. Further increase in the powder feeding rate leads to crystallization of aluminum solid solution with low hardness as the matrix phase of the cladding layer together with further increase in the area fraction of Mg₂Si compound. Therefore, the hardness of the obtained cladding layer becomes lower than that of the layer consisted of Mg₁₇Al₁₂ + Mg₂Si compounds. With the increase in an area fraction of Mg₂Si compound, the wear depth of the cladding layer decreases, but that of Cr-plated pin used as the counter material increases. The wear depth of the layer consisting of large area fraction of Mg₂Si compound and aluminum solid solution is almost same as that of the Cr-plated pin, and this indicates good wear resistance. However, the presence of a large amount of brittle Mg₁₇Al₁₂ compound causes flaking of Mg₂Si compound during wear test and, consequently, leads to ternary abrasive wear, resulting in large wear depth.

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