Materials Science Forum Vols. 488-489

Abstract: The hot compression simulation of an experimental Mg-Zn-Nd alloy at different temperatures is studied by the Gleebe-1500 equipment. The deformation is performed with the strain rates 0.1s-1, 0.01s-1 and 0.002s-1. The plastic deformation behavior is analyzed at different temperatures and the deformation activation energy is calculated. The microstructures of experimental alloy during the deformation process are observed. The results show that the working hardening, dynamic recovery and dynamic recrystallization (DRX) operate under different temperatures and strain rate. The DRX starts when the temperature is over 473K and the DRX grain size after hot deformation is only 5~10μm. So the refined grains improve both the tensile strength and elongation of alloys at room temperature. The precipitate phase Mg12Nd impede the movement of dislocations, it benefit to the mechanical properties and grain refining of magnesium alloy.

Abstract: In this paper, the flow stress curves during thermal deformation of AZ61B magnesium alloy have been measured by isothermal compression tests on Gleeble-1500D thermal simulation instrument, and effects of deformation temperature and strain rate on flow stress were analyzed. The thermal deformation activation energy of AZ61B alloy could be work out at 185.64 KJ/mol by means of empiric dynamic equation of elevated temperature deformation. The softening mechanism of AZ61B alloy under the testing condition of thermal deformation has been identified to be dynamic recrystallization.

Abstract: In this study, the effect of thermomechanical treatment (TMT) on the aging and mechanical behavior of Mg-Al-Zn alloys has been investigated. Three Mg-Al alloys AZ31, AZ61, and AZ91 were mold cast into a block under a CO2 and SF6 gas mixture atmosphere. The cast specimens were hot-swaged to a rod after homogenizing treatment at 400°C for 4 hours. The prestrain was applied by cold swaging up to 10% RA after solution treatment. From the aging curves obtained at 150°C, it was identified that the hardness of TMT-processed AZ31 and AZ61 did not increase during aging, while that of AZ91 remarkably increased. The tensile strength and elongation of the TMT-processed AZ31 and AZ61 remarkably increased depending on the amount of applied prestrains, indicating the introduction of dislocations prior to aging significantly improved the mechanical properties.

Abstract: Superplastic forming characteristics of AZ31 Mg alloy recycled by solid-state process were investigated. Cylindrical scraps and machined chips were recycled by hot extrusion and hot rolling in air. As a result of tensile test, elongation to failure of the specimens recycled from small and large cylindrical scraps was almost the same as that of the rolled specimen from a virgin ingot. On the other hand, the specimen from machined chips had much lower elongation at elevated temperatures. The oxygen concentration of the specimen recycled from machined chips showed about 60 times higher value compared with that of the other specimens. In superplastic forming tests, the specimen from small and large cylindrical scraps exhibited excellent formability similar to the rolled specimen from a virgin ingot. However, the specimen from machined chips showed poor formability. Thus, oxide contamination adversely affected the formability of recycled Mg alloy.

Abstract: The square-cylindrical cup of AZ31 Mg alloy with a depth of 50mm and a width of 50 mm was successfully deep drawn from a rolled sheet with a thickness of 0.5 mm using a hydraulic press machine with a capacity of 60 tons. The blank was heated between the die and the blank holder kept in a temperature of 300 oC .The forming time was about 3 seconds. The fine-grained microstructure was observed at the corner with a sever strain. The grain refinement caused by the dynamic recrystallization contributed to generate high strain-rate superplasticity.

Abstract: The superplasticity and microstructure of magnesium alloy ZK40 processed by ECAP for 1 and 4 passes were investigated. Tensile experiments at 523 K showed that the elongation of ZK40 was improved with the increasing passes. The elongation-to-failure of ZK40 processed by ECAP for 4 passes tested at 523 K and strain rate of 1×10-4s-1 was 660%. The microstructural analyses explained these results.

Abstract: Microstructure evolution and superplastic behaviors of ZK40 magnesium alloy were investigated in the temperature range of 473 ~ 623 K. Twinning occurred significantly after being processed by equal channel angular pressing (ECAP) for one pass through the die, the mean grain size was 5.6 µm. Finer grains can be obtained after further processing through ECAP, the average grain size of the alloy processed by ECAP for three passes was as low as 0.8 µm; this alloy exhibited low temperature superplasticity at 473 ~ 523 K, elongations at an initial strain rate of 1×10-3 s-1 were 260 % at 473 K and 612 % at 523 K, respectively. The incompatibility between fine and coarse grains was thought to be unfavorable to the improvement of superplasticity. Low temperature superplasticity of the as-ECAP ZK40 alloy can be attributed to the relatively finer grain size and the homogeneity of microstructure.