To establish the solid-state recycling technology of remarkably inflammable magnesium alloys, Bulk Mechanical Alloying (BMA) process based on the cyclic plastic deformation has been applied to demonstration experiment. Even starting from large machined chips of AZ91D magnesium alloys as input materials, microstructures of original AZ91D can be refined in the solid state via BMA. In particular, the fine intermetallics Mg₁₇Al₁₂ are distributed uniformly in the matrix. BMAed AZ91D shows high hardness more than threefold increase from the starting material. Much lower energy consumption is necessary for this solid state recycling compared to the conventional casting process. BMA is one of the suitable solid-state recycling technologies to be morphology-free from starting materials.

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Magnesium and its alloys have been widely used in various fields because they are the lightest metal in the structural materials and they possess many excellent properties such as high specific strength, low density, good and economic processability by using cast technology and high recycling potential. The recent progress in rolling technology enables the production of magnesium alloy plates. In this study, we tried to ultrasonically weld the magnesium plate of AZ31B, and examined the effect of the surface treatment on the ultrasonic weldability. Four kinds of surface treatments were adopted: (I) Wet polishing, (II) Alkaline cleaning, (III) Pickling with CrO₃ solution and (IV) Pickling with CrO₃ plus H₂SO₄ solution were adopted. The following results were obtained in this study. The outer surface of the magnesium cleaned in alkaline solution is covered with Mg(OH)₂ thick film, resulting in the deterioration of the ultrasonic weldability. The magnesium alloy of AZ31B contains inevitably some contents of Mn₆Al₇ intermetallic compounds, and these compounds appearing on the faying surface deteriorate the ultrasonic weldability. The surface treatment by pickling in aqueous solution consisting of CrO₃ and H₂SO₄ is effective to remove the Mg(OH)₂ film and the Mn₆Al₇ intermetallic compound and leads to increase the tensile strength of the ultrasonic weld. Heating the specimen to 100–200°C during ultrasonic welding can increase the tensile strength of the weld approximately 2.5 times as high as that of the weld performed at a room temperature.

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Weldability of Friction Stir Welding (FSW) of AZ91D magnesium alloy thixomolded sheet with 2 mm in thickness was evaluated by changing joining parameters of tool rotating speed and specimen travel speed. Square butt joint with good quality was obtained at the optimum FSW conditions of 50 mm/min of travel speed with 1240 to 1750 rpm of tool rotating speed by using a tool with 12 mm of shoulder diameter, 3 mm of pin diameter and 2 mm of pin length. Higher travel speed or lower rotating speed made a lack of bonding in the joint. Stir zone in a central part of the joint consisted of very fine recrystallized structure with 2∼5 μm in mean grain size. Hardness of stir zone increased slightly with decreasing mean grain size. Transverse tensile test of FSW joint resulted in the fracture at base metal. The tensile strength of stir zone showed 330∼360 MPa, much higher than that of base metal, about 250 MPa.

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In this study, in order to obtain the high deformability of magnesium alloy sheets, effects of the combination of rolling and heat treatments on mechanical properties and crystallographic textures in AZ31 magnesium alloy sheets were examined. The main results were as follows, (1) At all rolling conditions, the best mechanical properties were obtained at the 523 K annealing temperature. (2) As final rolling reduction in hot rolling was higher, the mechanical properties became good, and the anisotropy strengthen. (3) The final rolling process influenced large effect for the crystallographic texture. (4) The recrystallization textures at 523 K showed {0001} <1010> orientation in the hot rolling sheets and showed {0001} <1120> orientation in the commercial and cold rolling sheets. (5) By selecting the annealing condition, the crystallographic texture of {0001} oriented to rolling direction gradiently for the rolled sheet surface, therefore, AZ31 magnesium sheet exhibited the good properties for the deformation.

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The effects of billet homogenization and extrusion conditions on microstructure and mechanical properties of AZ31B magnesium alloy extrusions have investigated. Mg–Al and Mg–Al–Zn compounds observed on grain boundaries of as cast billet have disappeared by the homogenization. Then, the elongation of the extrusion increases by the homogenization. As the extrusion ratio increases, the elongation of the extrusion also increases by grain refining. As the billet temperature is lower and the extrusion speed is smaller, the grain size of the extrusion becomes smaller. As the result, the elongation of the extrusion increases.

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Impact tensile properties of three different extruded magnesium alloys (AZ31B–F, AZ61A–F and ZK60A–T5) are determined using the split Hopkinson bar. Tensile stress-strain data up to fracture in the extrusion direction at strain rates of nearly 1000/s are presented and compared with those at quasi-static and medium strain rates obtained in an Instron testing machine. The total energy absorbed in fracturing the standard U-notch specimens is measured by the Charpy impact tests. The effect of strain rate on the flow stress, the elongation (or strain at fracture) , and the absorbed energy is examined in detail. It is demonstrated that the tensile strength of extruded magnesium alloys increases with increasing strain rate, and the strain rate dependence of the elongation and the absorbed energy varies, depending on the magnesium alloys tested. In addition, it is shown that there is no correlation between the Charpy impact values and the absorbed energies at high rates of strain.

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In this study, repetitive ECAE processing was applied to Mg–Li–Zn alloys with dual phases consisting of α (Mg solid solution) and β (Li solid solution) phases in order to improve their tensile properties and to achieve low temperature superplasticity by refining both phases and spheroidizing α phase. Such a microstructural change in α. phase is reached by 2–pass ECAE processing at 293 K for LZ101 alloy and 323 K for LZ81 and LZ91 alloys, and subsequent annealing for 1 h at 623 K for LZ81 and LZ101 alloys and 673 K for LZ91 alloy. 4–pass ECAE processing at 293 K for LZ101 alloy and 323 K for LZ81 and LZ91 alloys and subsequent annealing at 398 K for all of the alloys results in grain refinement of β phase. As a result the specimens tested at 293 K show better tensile properties than as-rolled ones. The elongation of as-ECAE-processed specimens tested at 373 K reaches approximately 400% under a relatively low strain rate of about 10⁻⁴ s⁻¹ together with low flow stresses, and the specimens have the relatively high strain rate sensitivity exponents of about 0.4. Microstructures of the specimens after tensile test at 373 K reveal that dynamic recrystallization and precipitation occurs in both phases. These results suggest that an increase in grain boundary area through the fine recrystallized grains and precipitation of β phase on the recrystallized grain boundaries of α phase contribute to grain boundary sliding, resulting in the occurrence of superplasticity at 373 K, which is lower than T_m/2.

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In this study, ECAE (equal-channel angular extrusion) processing was applied to Mg–Al–Zn alloys and subsequently secondary processing by hot rolling was carried out in order to investigate possible changes in properties of the ECAE processed alloys. In 4-pass ECAE processed specimens of a Mg–3%Al–3%Zn alloy, ductility improves remarkably compared to that of as-cast samples because the microstructures of the ECAE processed alloy have fine and equiaxed grains due to dynamic recrystallization during ECAE processing which was carried out at 523 K and because their basal plane orients towards 45° to the extrusion direction. On the other hand, many <1012> {1011} twins which cross at an angle of 60° to the rolling direction are observed in the microstructure of 4–pass–ECAE + rolled specimens. In this specimen, although the basal plane orients parallel to the rolled surface, tensile strength and 0.2% proof strength are improved significantly and good elongation is retained because twinning and basal slip in twins operate during tensile deformation. This result shows that ECAE processed materials exhibit good formability and it is possible to obtain superior mechanical properties by subjecting ECAE processed samples to secondary processing such as rolling and forging without heat treatment.

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AZ31 magnesium alloy was friction welded under various welding conditions. The microstructures near the weld interface were observed with the optical microscope and the transmission electron microscope. The grains were refined at the weld interface due to severe deformation above recrystallization temperature during the friction welding. The mean grain size was 2–3 μm at the weld interface. The refined grains were obtained both by the dynamic recrystallization during the upset process and by the static recrystallizaiton during the cooling process. The grain grew with increasing friction time, and was larger at the central region than at the periphery. Shear bands and twins were induced in the region 2 mm and 11 mm distance from the interface, respectively. The nucleation for recrystallization occurred at the shear bands. The strength of joint was nearly equal to that of the base alloy.

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