Papers by Keyword: AZ31 Mg Alloy

Abstract: The influence of initial texture on the formation of primary twin system of AZ31 Magnesium rolled plate was investigated in this work. Uniaxial compression tests were carried out on samples cut along the rolling direction (RD) and normal direction (ND) of rolled AZ31 Mg plate at various temperatures (RT, 150, 200, 250, 300, 350, 400, 450°C) with the fixed strain rate (0.01s-1). The results showed that the primary twinning system of AZ31 Mg alloy (c-axis extension twin) occurred actively in the RD compression specimens, which promoted homogeneous deformation as compared to the ND compression specimens. The effect of temperature on the formation of deformation twins was also investigated, and slip/twin transition temperature was found to be 250°C.

Abstract: In order to investigate the effect of the as-cast microstructure on the hot working behavior of an AZ31 magnesium alloy, specimens were cast in copper moulds with and without water cooling. A series of compression tests were performed at a temperature of 350 °C, a strain rate of 0.01 s-1, and at strains up to 1.0. It was found that as-cast microstructure is very sensitive to the solidification conditions, which leads to a significant difference in flow behavior and dynamic recrystallization (DRX) characteristics. It appears that more uniform and refined as-cast grain size promotes dynamic recrystallization and reduces the flow stress. It is also possible that second phases (>1+m in size) contribute to DRX by acting as nuclei.

Abstract: Optimum variable strain rate forming paths based on two multiscale deformation-based stability criteria are developed. The first criterion is based on Hart’s linear stability analysis while in the second criterion; we introduce a modified one dimensional nonlinear long wavelength analysis introduced by Hutchinson and Neale [7] based on the well known 2-D Marciniak-Kuczynski criterion. The stability criteria are calibrated for the AZ31 Mg alloy at 400 °C yielding two different variable strain rate forming paths. These paths show that the nonlinear wavelength analysis is more sensitive to strain rate sensitivity and results in larger attainable uniform strains than Hart’s approach especially at low strain rates. This result is demonstrated through finite element simulations of a deep rectangular box using pressure profiles derived from the two variable strain rate forming paths. The FE results clearly illustrate that Hart’s approach underestimates the amount of uniform deformation and therefore prolongs the forming time to prevent failure compared to the nonlinear analysis.

Abstract: Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 )m) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Abstract: Accurate constitutive modeling of superplastic deformation is critical for successful simulation and optimization of superplastic forming. The selection of the forming pressure profiles in gas blow forming of superplastic materials is still based on trial and error due to the limited predictive capabilities of current models describing superplastic deformation. In general, these models are based on uniaxial loading condition, assuming isotropic behavior. In this work, we examine the biaxial bulge forming of AZ31 magnesium alloy using pressure profiles derived from different available analytical models. The results clearly show the need for more accurate description of superplastic deformation to improve predictive capabilities. In addition, a pressure profile that is generated using FE in conjunction with a microstructure-based constitutive model is used and the results indicate better ability to predict the height of the bulged sheet.

Abstract: Molten Mg alloys burn rapidly in air if not protected. In order to handle molten Mg safely, the molten metal should be protected from oxidation by covering the surface with flux or protective gases. Sulfur hexafluoride (SF6) gas is widely used for Mg alloy as a cover gas and has proved to be a successful inhibitor. However, the use of SF6 gas is limited because of its high cost and its significant impact on global warming potential (GWP). Therefore, SF6 gas is being replaced by alternative protection gases such as tetrafluoro ethane (HFC-134a) and 3M NovecTM 612. These gases are less expensive and have lower GWP, compared with SF6 gas. Recent studies show that there has been another attempt by adding CaO into Mg alloys to manufacture CaO added AZ31Mg alloy in terms of increasing ignition temperature and reducing protective cover gas amount during melting and casting. This paper discusses effect of CaO on ignition and oxidation behaviors of AZ31 Mg alloy as well as on surface roughness and mechanical properties of AZ31 Mg alloy extruded bars.

Abstract: Ion implantation was applied to modify the surface properties of magnesium alloy. About 75 keV titanium ions with a dose of 5×1017 ions/ cm-2 and about 35 keV nitrogen ions with a dose of 3×1017 ions/ cm-2 were implanted into AZ31 magnesium alloy, respectively. AFM and FESEM were used to examine the surface morphology of the treated samples. In contrast to the smooth surface treated by N ion implantation, the Ti-treated surface was severely damaged. AES analysis reveals that both kinds of ions implanted in AZ31 magnesium alloy took on a quasi-Gaussian distribution in the near surface region. In the corrosion test, the N-implanted sample showed a better corrosion resistance in 3.5wt.% NaCl solution than bare AZ31 sample, whereas the Ti-implanted sample accelerated the corrosion behavior of AZ31 in this solution

Abstract: The strip casting of AZ31 were carried out by a Ø250×150mm of vertical twin roll caster at different casting speeds (9-15m/min) and in different casting temperatures (630-660°C). The solidification microstructure of the strips was examined. The experiment results showed that the crystal grain size of the casting was smaller than that of conventional ingot, and decreases while the casting speed is raised, or the casting temperature decrease. The casting temperature strongly affected the dendrite structure that changed into sphere-like when the casting temperature was lower. The appropriate casting temperature for AZ31 magnesium alloy is 640°C, nearby its liquidus temperature.

Abstract: Based on the two-stage forming technology, the casting AZ31 Magnesium alloy bar was forged into cylindrical straight inner gear between the temperature 250°C-400°C. At 250°C, the teeth of the inner gear are almost formed. But there are some cyclic cracks on the surface of the sample. When improving the temperature above 300°C, the surface quality of the sample has greatly improved. According to the result of this experiment, the best temperature range for forging AZ31 magnesium gear is 280°C to 380°C.The forming load gradually reduced with the temperature improved. At 250°C, the forming load is 93t. At the 400°C, the forming load reduces to 80t.The initial grain size of AZ31 magnesium alloy bar is 22μm. The microstructure evolution during the warm deformation was observed by optical microscopy (OM). It is demonstrated that the grain refinement happened during the deformation process.

Abstract: Magnesium alloy AZ31, which processed by conventional rolling or extrusion, has high anisotropy of mechanical properties in its strength and elongation at room temperature. We compared the influence of differential speed rolling with conventional rolling process on microstructure and mechanical properties of commercial AZ31 sheet. Commercial AZ31 alloy sheets were processed with conventional and differential speed rolled with thickness reduction ratio of 30% at a various temperature. The elongation of AZ31 alloy, warm-rolled by differential speed rolling is larger than rolled by conventional rolling. Besides, grain size and distribution on microstructure of the conventional rolling were coarsely(~30μm) and inhomogeneously but, that those of the differential speed rolling were fine(~13μm) and homogeneously.