Materials Science Forum Vols. 475-479

Abstract: In the present work the deformation behavior of magnesium alloys with different lithium content has been investigated in terms of microstructures, crystal parameters, textures and mechanical properties. The lattice constants of a phase (hcp) determined by XRD varied with its lithium content and at the same time there are some changes in the texture composition. The plasticity would remarkably increase while b phase (bcc) becomes dominant in the microstructure of alloys.

Abstract: Friction stir welding (FSW) is a new solid-state plastically joining process developed in the beginning of the 1990’s. The influence of the stirrer’s shape and process parameter on joint quality was studied. The investigation result shows that the microstructure and mechanical property of the joint welded with truncated cone and screw welding pin is better than the joint welded with cylindrical welding pin. When welded with proper stirrer and parameter, maximum joint tensile strength can reach 98% of the parent material. And microhardness of friction stir welded MB3 was considered with the grain size distribution in the weld zone.

Abstract: Effects of rolling conditions on warm deep drawability of cast magnesium alloy that were hot rolled after roll strip casting were investigated to ascertain the feasibility of twin-roll strip casting process of AZ31B magnesium alloy. Hot rolling and heat treatment conditions were changed to examine which conditions were appropriate for producing AZ31B wrought magnesium alloys after strip casting process. Microscopic observation of the crystals of the manufactured wrought magnesium alloys was performed. It has been found that a limiting drawing ratio of 2.7 was possible in a warm deep drawing test of the cast magnesium alloy sheets after being hot rolled.

Abstract: In order to improve poor formability in magnesium alloy, grain refinement has been attempted on AZ91E alloy by a thermo-mechanical treatment. Specimens were firstly cold-rolled at 10 %, then solution heat treated at 673 K for 86.4 ks, and hot-rolled at 573 K with about 5 % for four passes, or hot-rolled at 20 % with one pass. The rolled specimens were finally heat treated at 473 to 673 K for 3.6 to 36 ks. Microstructures in the starting material characterized by grain boundary precipitates and aluminum rich regions with about 180 µm in grain size were changed into fine grained microstructures with about 10 to 30 µm in diameter, in which precipitates of Mg17Al12 were uniformly distributed. Although the specimen was prepared by rolling, the (0001) texture was not so remarkable.

Abstract: Super light and high performance Mg2Si/Mg composites, which had excellent mechanical properties, were developed via the combination of solid-state synthesis and hot extrusion process. In this study, cold compacting (CP) and repeated plastic working (RPW) were firstly carried out for the mixture of Mg-Si powders, and the refinement of both Mg grains and dispersoids. Each specimen was evaluated by observation of microstructure and tensile test. As a result, it was understood that Mg2Si dispersoids were refined and dispersed into Mg matrix, and were flowed along extrusion direction. And their mechanical properties were higher than the conventional die casting alloys. Also the effect of RPW as the improvement of properties and the decrease of synthesis temperature were confirmed.

Abstract: Using commercial AZ31 magnesium alloy sheets, we produced a foamable preform sheet containing titanium hydride (TiH2) powder through diffusion-bonding and hot-rolling of four cycles. Heating the preform sheets in Ar atmosphere, we obtained closed-cell magnesium alloy foams with various porosities. The foamed specimen at 883 K showed the maximum porosities of 77%.

Abstract: A new technique has been developed for improving corrosion resistance on magnesium alloys. Specimens of AZ31 magnesium alloy were dipped into molten salt of NaBF4 at 723 K for various times, and then cooled, rinsed with water, and dried in air. Corrosion resistance in the surface treated specimens was evaluated by salt immersion test using 1 % NaCl solution as a time for occurring filiform corrosion. On an un-treated AZ31 alloy, the time for starting the filiform corrosion was about 1.2 ks, while on the surface treated specimen, the time was prolonged into about 1300 ks. Moreover, the surface treated specimen showed corrosion resistance in low pH solutions, such as 1 % HNO3 and HCl solutions.

Abstract: Magnesium alloys have replaced resins as a material for the components of electronic products such as cell phone and notebook personal computer mainly, because of their lightness and rigidity. Thin walls, a complicated shape, and high appearance quality are all needed in the external parts. Die-casting and injection molding are the main method of manufacturing magnesium alloy parts. The optimal cast conditions and mold design have been investigated in order that a few defects such as surface cracks and mold cavities in casting parts would be reduced. Instead of cast, plastic forming technologies such as warm drawing and hot forging have been developed to form thinner walls and less defects. Plastic formability of magnesium alloy in hot working is dependent on a grain size of material. The material with fine grains has advantage of being formed at high strain rate. The characteristics of forming processes of magnesium parts for Japanese home electric appliances are compared in the viewpoint of quality, cost, and productivity.

Abstract: In this study, it refined by the vacuum distilling refining and the billet was produced for the obtained 99.99% or more of purity magnesium deposit by compression forming, extrusion process was performed and the corrosion resistance of extrusions and the tensile characteristic which were acquired were investigated. The specimen showed good corrosion resistance rather than commercial AZ91D magnesium alloy die-castings. The specimen has high ductility and it is good, and compared with 99.9% of purity magnesium.

Abstract: Semi-solid processing is recognized as an attractive alternative method for the near net-shape production of engineering components. Although there has been a significant progress in semi-solid processing of aluminum alloys, very limited information is available on semi-solid processing of magnesium alloys, except for the thixomolding process. Continuous casting process has been utilized to produce the billets with the desirable cross-section at a reduced production cost for many metals, such as steel, copper and aluminum alloys. It has also been commercially utilized to produce the aluminum billets with non-dendritic microstructure for subsequent thixocasting process. However, continuous casting of magnesium billets for semi-solid processing has not yet been commercialized due to the difficulties involved in casting of magnesium alloys. In the present study, a continuous casting process has been developed for the production of the cylindrical billets of magnesium alloys for the subsequent thixocasting process. In order to obtain the desired non-dendritic microstructure with an excellent degree of homogeneity both in microstructure and composition, an electromagnetic stirring system has been utilized. A continuous casting process has been proven to be an efficient way to produce the high quality billets of magnesium alloys for semi-solid processing. A prototype air conditioner cover was produced using the continuously cast billets of AZ91 alloy.