Papers by Author: Yuichiro Murakami

Abstract: Al-Si alloy is widely used as a casting alloy. The α-Al phase in the semi-solid state has low Si content in the Al-Si alloy. Then by separation of these α-Al phases from semi-solid Al-Si alloy, refining of aluminum can be possible. But, in near eutectic Al-Si alloy, few primary α-Al phases can be crystallized. If the fraction ratio of the α-Al phase can be increased, near eutectic Al-Si alloy can refine, and this method can be used for recycling. In this study, the effect of electromagnetic stirring (EMS) on the microstructure, especially the amount of the α-Al phase particles was investigated. A rotational magnetic field was applied to JIS ADC12 alloy which has near eutectic content during slow cooling from the liquid state to the solid-state, by using a three-phase AC coil. By applying EMS at solidification, the shape of the α-Al phase became particle shape from dendrite shape, and the amount of α-Al phase particles was increased. Moreover, by applying unidirectional intermittent EMS, the volume fraction of α-Al phase particles was decreased with increasing intermittent applying time. In ADC12 alloy, the primary α-Al phases can be crystallized only 10% generally, but it could be obtained over 40% by applying EMS. This means that the semi-solid slurry of near eutectic alloy with over 40% of fraction solid can be obtained by applying EMS.

Abstract: The preparation of a semi-solid slurry with fine and spherical solid particles dispersed in the liquid phase is a key technology for the semi-solid process. In this study, we have developed the slurry preparation method by applying mechanical vibration for JIS AC4CH (equivalent to A356 in the ASTM standard) and ADC12 (equivalent to A383 in the ASTM standard) aluminum alloys. In this method, the mechanical vibration was applied to the aluminum alloys during cooling from a liquid state to semi-solid state. The shape of the particle was varied by controlling vibration conditions. Therefore, we investigated the effect of conditions on mechanical vibrations. There are four parameters which are frequency, acceleration amplitude, velocity amplitude and displacement amplitude in the mechanical vibration. Among these parameters, the velocity amplitude had highest effects on the shape of solid particles. By properly controlling of the frequency and the velocity amplitude, the slurry with fine and spherical solid particles could be obtained. Moreover, the fraction solid of slurry could be controlled by varied initial liquid temperature. Especially, ADC12 aluminum alloy has low liquid-solid coexistence temperature range, so it is not considered to be suited for the semi-solid process. However, by using this mechanical vibration method, the semi-solid slurry could be obtained stably. This method is expected to be effective for the semi-solid high-pressure die-casting process.

Abstract: We have developed new type semi-solid injection process for magnesium alloy. This process does not require to use any cover gases and the special magnesium billet such as thixo-billet. In this study, plate specimens were produced by injecting the semi-solid billet with different fraction solid. The microstructure observation, detection of casting defects by an X-ray computed tomography scanner, and tensile test were carried out. With increasing fraction solid, the size and shape of α-Mg solid particles became smaller and more spherical. In the condition of low fraction solid or forming in liquid state, the casting defects were located in the center of the specimen at the thickness direction. Additionally, the volume fraction of the casting defect decreased with increasing fraction solid. Moreover, the casting defects can be reduced by preventing solidifying and clogging of the top of the nozzle. Then, the specimen which has few casting defects could be obtained by injecting the slurry of fraction solid 0.5. However, the tensile strength and yield strength were highest in fraction solid 0.4. It is contemplated that the composition of the solid solution component element in the matrix was increased in fraction solid of 50%, therefore the matrix became brittle.

Abstract: We have developed new type semi-solid injection process, that is, runner-less injection process which can obtain high material yield of about 90% for magnesium alloy. In this process, alloy billets are heated to the semi-solid temperature in the injection cylinder and are injected into a permanent mold. In order to investigate the effects of volume fraction solid and injection speed on microstructure and mechanical properties of AZ91D magnesium alloy injected into the permanent mold, semi-solid forming testing machine which has the same system as a runner-less injection machine, has been made on an experimental basis. The magnesium billet precisely controlled at given temperature has been injected into a permanent mold with two kinds (slow and high) of speed and plate-like specimens with each fraction solid have been fabricated. Microstructure has been observed by optical microscopy and X-ray computerized tomography (CT) scanner. Mechanical properties have been measured by tensile test. The effects of volume fraction solid of the alloy slurry and injection speed on mechanical properties have been clarified.

Abstract: We have been trying to find new oxide compounds with large thermal expansion coefficients and low thermal conductivities by means of a material calculation technique. Among thousands of compounds in the databases, we found that there were some materials with low thermal conductivities and large thermal expansion coefficients in the group of strontium-niobium oxides. For example, Sr4Nb2O9 has a thermal expansion coefficient of 14.5
10-6 / and thermal conductivity of 1.0 W/mK, although a slight amount of other phases appear during long-term annealing. These thermal properties are better than those of yttria-stabilized zirconia, which is the standard material for thermal barrier coatings. To prevent the precipitation of other phases, we prepared the solid solutions, Sr4Nb2-xMxO9. In this study, the thermal conductivities and thermal expansion coefficients of these solid solutions were measured, and their thermal stabilities were evaluated by long-term annealing.

Abstract: A new technology relating to crystal orientation and structure alignment has emerged by the development of superconducting technologies. Now, a high magnetic field covering a rather large space is available even in small-scale laboratories. Under this circumstance it has been found that the crystal orientation in materials can be controlled by imposition of the high magnetic field. This principle due to a magnetization force can be applied not only to magnetic materials but also to non-magnetic materials with asymmetric unit cells. In this paper, three novel processes for the crystal orientation of ceramics and metals are described.

Abstract: The controlled development of texture microstructure in ceramics is one effective way to improve their properties, such as electrical, mechanical properties and biocompatibility. A bioceramics with oriented crystal structure has attracted great interest. In bone reparations, Hydroxyapatite (HAp)-based biomaterials were frequently used. And HAp is the main mineral constituent of the hard tissue of human bodies, which occurs with a hexagonal crystal. A HAp crystal turns out to have different surface properties in a- (or b-) plane and c-plane. In this regard, to get highly oriented HAp is very important before using HAp as a biomaterial. And the crystal orientated HAp is useful not only as biomaterials but also as protein absorbents. In this research, two different kinds of HAp-based biomaterial with oriented structure (HAp bioceramics and HAp-coated titanium composite) were studied.