Materials Science Forum Vols. 546-549

Abstract: In the present study, warm hydroforming of cell phone cases with magnesium alloy AZ31 sheets was investigated. Fine-grained magnesium alloy sheets were prepared by cross rolling. And the tensile tests were first conducted in order to determine the proper forming temperature. The results showed that the most suitable temperature range appears to be 150-200°C. At last, the magnesium alloy cell phone cases characterized with the small round radius of all edges were formed successfully at 170°C with the low punch velocity and the maximum pressure not less than 5MPa.

Abstract: A conventional extrusion has been carried out to induce significant grain refinement in Mg-Zn-Y-Zr alloy. The results showed that good superplasticity have obtained in this extruded alloy. The simple and rapid processing route may allow it to be put into successfully practical use in industry. The effects of temperature and strain rate on superplastic deformation of the extruded alloy were studied. The optimum condition with the elongation of more than 450% was found to be at 450°C and 3.3×10-4s-1. Jump tests were carried out at 300-450°C and 8.3×10-5 ~ 1.7×10-2s-1. The activation energy for superplastic deformation at 300-450°C is 106kJ/mol and the stress exponent is about 2.8. The superplasticity observed in this studied condition may be attributed to mechanisms of dislocation creep mainly within large grains and grain boundary sliding (GBS) of small grains.

Abstract: In this paper, the fatigue crack propagation (FCP) behaviors of the forged Mg-Zn-Y-Zr alloy with different tempers were investigated. At the stage 1 of the FCP, the
K-da/dN curves were distinguished from each other. However, at the stage 2 of the FCP, the
K-da/dN curves were almost the same. SEM fracture analysis indicated that at the stage 2 of FCP, the fracture characteristics of the forged and T5 samples were similar, with many deep dimples on the fracture surfaces, whereas the fracture surfaces of the T4 and T6 samples were covered with lots of lamellar cleavage planes. SEM observations also indicated that the microstructures were distinguished from each other. For the forged and T5 samples, many big bulk w-Mg3Y2Zn3 phases distributed in the Mg matrix. However, for the T4 and T6 samples, due to the solid solution treatment, w-Mg3Y2Zn3 phases were almost completely decomposed and many coarse rod-like  1 precipitates appeared in the interior of the grains. These firmly confirmed that the variation of the microstructures had great influence on the fatigue crack propagation (FCP) characteristics of the Mg-Zn-Y-Zr alloy.

Abstract: Texture evolution in Mg-13wt%Li-X alloy cold-rolled from 1.35 mm to 0.34 mm thickness was investigated, by obtaining pole figures and orientation distribution functions (ODFs). Punching tests were conducted to reveal the effect of texture nature on formability. It was found that: (1) the textures of the as-received sheet are characterized by α fiber texture, a γ fiber texture and a cubic texture in both cold-rolled and annealed conditions; (2) with thickness reduction though rolling, the intensity of the γ fiber texture continuously increases and finally the γ fiber texture connects into {111} tube texture, the texture of <11 0> orientation flows towards {223}<11 0> along α fiber, the cubic texture of {001}<100> turns into {035}<100>, while some grains concentrate at {011}<41 1> orientation; (3) good punching behavior of the cold-rolled sheet corresponds to the appearance of a well-developed γ fiber texture.

Abstract: The properties and microstructures after different annealing processes of an as-extruded ME magnesium alloy were investigated. The results showed the better properties with the tensile strength of 205 MPa and the elongation of 19.8%, respectively, could be obtained after annealing at 623 K for 4h and air-cooled, meaning that elongation value increased 40% than that of as-extruded sheets while the strength remained almost same. The annealing hold time corresponding with the higher elongation would shorten with the increase of annealing temperature. There is legible necking on broken tensile specimen. The behavior of fracture showed obvious characteristic of ductile rupture, with many tearing edges and cavities.

Abstract: The effects of homogenization on the formability for hot deformation of ZM21 wrought magnesium alloys was investigated. The alloys were produced by semi-continuous casting. Experiments including homogenizing annealing, micro-hardness testing and hot compress testing on Gleeble 1500D thermal simulator were carried out. The suggested homogenization parameters from the experimental results for ZM21 alloy is 400
C×12h.

Abstract: Conventional production process of magnesium alloy strips involves many steps, including multi-hot rolling and multi-heat treatment, which is time-consuming and needs high energy cost. By use of thin strip casting, magnesium alloy cast strip with a thickness of less than 8mm can be produced and then rolled to the strip of the needed thickness, which is a short, energy saving and high effective process. In this paper, development of magnesium alloy strips prepared by single and twin roll casting has been summarized. Existing problems have been discussed and its development trends pointed out. Magnesium alloy strips prepared by single roll casting are suitable for Mg-based functional materials but not for structural materials because of low quality or high cost. Horizontal twin roll casting of magnesium alloy, based on aluminium alloy thin strip casting, has been industrialized in Australia and is close to industrialization in China. Vertical twin roll casting of magnesium alloy, based on steel thin strip casting, can reach a speed of more than 30m/min and higher productivity, but the process stability is lower and its research and development is just beginning. Horizontal twin roll casting should be developed for the thicker cast strip, more than 4mm thick, while vertical twin roll casting for the thinner, less than 4mm.

Abstract: The simulation rolling and actual rolling of AZ31 strip with a thickness of 3mm prepared by a vertical twin roll caster (Ø250×150mm) have been made by both Gleeble-1500D thermal simulation testing machine and Ø170×300mm rolling-mill. The influence of various rolling parameters such as temperature (250°C, 300°C, 350°C and 400°C) and reduction (40%, 50% and 60%) on the microstructure of the rolled AZ31B thin strip have been investigated by means of metallographic observation and grain size measurment. The results indicate that when the temperature increases, the grain size of the rolled strip increases and the uniformity decreases. When the reduction is 50% the grain size is 10~30μm and the grains are fine and uniform. When the reduction is 40% or 60%, the grain size is 5~180μm and the grains are coarse in some areas. An optimized rolling process, 350°C×50% is suggested for actual rolling and a uniform microstructure of rolled AZ31 strip is obtained.

Abstract: In the present paper, nine kinds of alloys with different contents of zinc and zirconium were cast and the influence of zinc and zirconium individually and jointly on the microstructure of the ZK alloys were investigated. The results indicated that the shape and amount of the eutectic were closely related to the content of Zn. With the increase of Zn content, the eutectic was prone to have fish-bone-like shape. When the eutectic grew up to contact with each other, then network eutectic formed; Zr could intensively refine not only the grain size but also the eutectic, but when Zr>0.6%, compound containing Zr or pure Zr granules would appear. That the increase of the Zn must go with the increase of the Zr proved the combined influence of the two alloy elements.

Abstract: The hot compression deformation behavior of AZ61B magnesium alloy has been investigated by using a Gleeble-1500D thermal simulator. The samples were compressed to a reduction of 50% at two temperatures (623 and 673K) with different strain rates (0.01, 0.1 and 1s-1). The relationships between flow stress and deformation temperature were analyzed, and the deformation activation energy and stress exponent were calculated based on the experimental results. With aid of the optical microscopy, scanning electron microscopy (SEM) and electron back scattered diffraction (EBSD) techniques, the microstructure and micro-texture of the deformed samples were characterized. The effects of the temperature and strain rates on the hot compression behavior of the AZ61B magnesium alloy have been investigated by detail analyses of the flow stress and microstructural characteristics of the deformed samples. For the 50% compressed samples, dynamic recrystallization occurred during the hot compression. The orientations of the dynamic recrystallized grains with equiaxed shape were investigated by EBSD technique. The relationships among the flow stress, dynamic recovery and recrystallization have been discussed by considering both the temperature and strain rate effects.