Papers by Author: Yoshihito Kawamura

Abstract: Magnesium alloys are known for its light weight. Due to the desirable properties,magnesium is required in the fields such as transportation from the point of significant energy savings.Magnesium alloys are also being expected as an alternative for the next generation materials in fieldsof variety. At present, casting and thixomolding process are the main manufacturing methods formagnesium alloy parts. The major problem for forging of magnesium alloy is the lack of basicprocess data, such as strength and ductility.Mg96Zn2Y2 is a high strength magnesium alloy material newly developed by Dr. Y. Kawamurain Japan. It has attracted much attention recently. The name of this alloy is called as 'Kumadai Goukin'.'Kumadai Goukin' of high strength magnesium alloy material is expected as forged parts of theautomobile and materials for aerospace applications. It is necessary to clarify forging processcharacteristics of this material.The purpose of the present study is to evaluate the processing property of Mg96Zn2Y2 material.In this study, cylinder upsetting tests were performed under various deformation temperatures andspeeds for 'Kumadai Goukin'. Furthermore, the dependence of flow stress and ductility of thesematerials to processing temperature and speed was evaluated. We performed microstructureobservation to examine mechanism. We were analyzed that ductility of this material had improved.

Abstract: Mg₉₆Zn₂Y₂ is widely studied due to its high tensile yield strength and elongation. These excellent properties were considered to relate to the nanostructure of the alloy such as fine-grained α-Mg matrix and bent long period stacking ordered (LPSO) structure. In order to extend the application of this Mg alloy, development of the welding technique is indispensable. In the light of the reliability and productivity, resistance spot welding is an attractive method to join this Mg alloy. In this study, we joined the Mg alloys by the resistance spot welding with cover plates. Mechanical properties and structures of the nugget of the joints were investigated. In the case of the joints welded with cover plates, the welded region of the Mg alloy sheets were covered with steel plates to supply adequate heat for welding. Joints of extruded Mg₉₆Zn₂Y₂ were obtained at relatively low welding current. The tensile strength of Mg₉₆Zn₂Y₂ alloy was higher than that of the commercial Mg alloy AZ31B joints. Mg₉₆Zn₂Y₂ base alloy had extended LPSO phase along extruded direction in α-Mg grains. At the nugget, net-like secondary phase was produced in the Mg grains. The relationship between the microstructure in the joints and mechanical properties were discussed.

Abstract: Recently, Mg₉₆Zn₂Y₂ alloy was widely studied due to its excellent mechanical properties. These properties were considered to relate to the nanostructure of the alloy such as α-Mg matrix and bent long period stacking ordered (LPSO) structure. In this study, as the first step to the mechanical property control, we applied various spot welding methods to the Mg alloy to control the structure of the joints. The specimen used was an extruded Mg₉₆Zn₂Y₂ alloy. The sheets were cut from this alloy rod for resistance spot welding. Three kinds of spot welding methods were applied to join the Mg alloy, such as spot welding with cover plates, long-time spot welding, and water-cooled spot welding. In the base metal, secondary phases extended along extruded direction at approximately 30μm intervals were produced in the α-Mg matrix. The secondary phase contained 14H-type LPSO structure. In the nugget of the joint welded with cover plates, a secondary phase was produced like a net in the α-Mg grain which was approximately 30μm in diameter. Both 14H-type and 18R-type LPSO were observed in the secondary phase. In the nugget produced by the long-time spot welding, a secondary phase tended to segregate to the grain boundary of the α-Mg grains. The structure of LPSO was 18R-type. In the water-cooled welding, strong directionality of the secondary phase texture was observed around the edge of the nugget. These results showed that the nugget nanostructure of Mg₉₆Zn₂Y₂ dramatically varied depending on the above adopted methods of spot welding.

Abstract: Microstructural factors that govern the plastic deformation of the long-period stacking ordered (LPSO) phase were clarified. The decrease in length of the long-axis for the plate-like shape of LPSO-phase grains increases the yield stress of the alloy in which basal slip is predominant in deformation. On the other hand, the yield stress tended to increase as the thickness of the plate-like shapes of the grains decreased for the alloy in which the formation of deformation kinks carried the strain.

Abstract: In this study, long period stacking ordered structure (LPSO) type magnesium alloys have been developed. They consist of LPSO phase and α-Mg phase and have excellent mechanical properties at both ambient and elevated temperatures. In the present study, the influence of electromagnetic stir (EMS) on the structure of LPSO type Mg₉₆Zn₂Y₂ (at. %) billets of φ200 mm in diameter was investigated in order to establish the large-scale casting technology of LPSO type Mg alloy. EMS refined the grain size of α-Mg up to approximately 1/3.5, and reduced the size difference between the surface and the center of the billets. By applying EMS, it was found possible to provide large-scale casting billets with fine and homogeneous microstructure.

Abstract: Metallic glass or amorphous alloys have been gaining popularity due to their low density, high strength, high fracture toughness, good corrosion and good wear resistance. Especially, bulk metallic glasses (BMGs), which are made in bulk form with a thickness of ~10mm at slow cooling rates of the order of 1~100K/s, have been noted as an industrial application. The welding to other materials becomes very important. Explosive welding of most popular Zr41.2Ti13.8Cu10Ni12.5Be22.5 bulk metallic glass to crystalline pure Cu plate is investigated in this paper and the BMG was found to retain the amorphous structure. The sound bonding with other materials is expected to push forward the application of BMGs for industrial usage.

Abstract: The purpose of the present study is to evaluate the processing property of Mg96Zn2Y2 of casting material. Mg96Zn2Y2 is a high strength magnesium alloy material newly developed by Mr. Kawamura in Japan. The name of this alloy is called as 'Kumadai Goukin'. For this purpose, cylinder upsetting tests were performed. In addition, the marketing community of AZ31 magnesium alloy collates the information with the new material. We examined and compared the deformation resistance and ductility of the two materials. Furthermore, the dependence of deformation resistance and ductility of these materials to processing temperature and speed was evaluated [1].

Abstract: Mg96Zn2Y2 is a high strength magnesium alloy material developed by Mr. Kawamura in Japan. The name of this alloy is called as 'Kumadai Goukin'. The purpose of the present study is to evaluate the processing property of Mg96Zn2Y2 material. For this purpose, cylinder upsetting tests were performed. In addition, the marketing community of AZ31 magnesium alloy collates the information with the new material. We examined and compared the deformation resistance and ductility of the two materials. Furthermore, the dependence of deformation resistance and ductility of these materials to processing temperature and speed was evaluated [1],[2].