Kenji Murakami

Anisotropic mechanical properties of porous copper fabricated by unidirectional solidification

Porous copper whose long cylindrical pores are aligned in one direction has been fabricated by unidirectional solidification of the melt in a mixture gas of hydrogen and argon. The porosity depends on the melting temperature of copper melt, while the orientation of the pores is controlled by the freezing direction. The anisotropy in the uniaxial tensile behavior of the porous copper is examined. The ultimate tensile strength and the yield strength of the porous copper with the cylindrical pores orientated parallel to the tensile direction decrease linearly with increasing the porosity. For the porous copper whose pore axes are perpendicular to the tensile direction, the ultimate tensile strength decreases significantly with increasing the porosity at low porosity, while the yield strength reaches a maximum at a low porosity and then decreases with increasing the porosity. These results are explained in terms of the stress concentration induced in the vicinity of the pores during tensile deformation.

Fabrication of porous copper by unidirectional solidification under hydrogen and its properties

Abstract Lotus-structured porous copper was fabricated by unidirectional solidification in the mixture gas of hydrogen and argon at the high pressure up to 2 MPa. The pore size and porosity were controlled by the melting temperature and the mixture ratio and pressure of the gases, while the pore growth direction was controlled by the freezing direction. The pore diameter ranges from 5 μm to 5 mm and maximum porosity is about 70%. The porous copper with aligned long pores displays superior mechanical properties, which is different from conventional porous materials with random pores such as sintered materials, foamed materials, cell-structured materials etc. In this paper the fabrication and the result of the mechanical property and the internal friction measurement are described.

Evaluation of porosity in porous copper fabricated by unidirectional solidification under pressurized hydrogen

Lotus-type porous copper with elongated pores has been fabricated by a unidirectional solidification method in a mixture of hydrogen and argon gases with high pressure. The porosity and the pore morphology in the Lotus-type porous copper are significantly affected by the partial pressures of hydrogen and argon. The porosity has been evaluated on the basis of the idea that the gas pores are formed by the insoluble hydrogen during solidification of the liquid copper dissolving hydrogen. The evaluation gives the porosity as a function of the partial pressures of hydrogen and argon. The evaluated values are in good agreement with the experimental results.

Fluid flow in interdendritic space in cubic alloys

Abstract The permeability of interdendritic space was measured using unidirectionally frozen organic borneol-paraffin alloys whose columnar dendrite structure was the same as that of metallic alloys of f.c.c. and b.c.c. An aqueous solution containing a small amount of red ink and 25 vol.% surface active agent was made to flow in the interdendritic space in the directions parallel to and normal to columnar dendrites. In both flow cases, the permeability was approximately proportional to the cube of the fraction liquid. For flow normal to columnar dendrites, the permeability increased with primary and secondary arm spacings. When the flow direction was parallel to columnar dendrites, the permeability increased with increasing primary arm spacing but it decreased with increasing secondary arm spacing.

Fluid flow in the mushy zone composed of granular grains

Abstract The permeability of the mushy zone consisting of granular grains was measured as a function of fraction liquid. The grain diameter was about 180 μm. The permeability was approximately proportional to the cube of fraction liquid within the range of fraction liquid of 0.27–0.48. Based on Kozeny—Carmans equation, the permeabilities were estimated at some fractions liquid from the measured specific surface area of the granular grains and were in fairly good agreement with the measured ones.

Rapid solidification and self-annealing of FeCSi alloys by low pressure plasma spraying

Abstract Powders of Fe-2.20wt.%C-0.85wt.%Si and Fe-4.16wt.%C-0.87wt.%Si alloys were plasma sprayed on a steel substrate in a low pressure chamber. The deposite layers formed on non-water-cooled substrate are composed of stable phases of ferrite and cementite. When the substrate is water cooled, the main constituents of the deposit layers are retained austenite and a metastable h.c.p. ϵ phase on the free-surface side for the respective alloys, and self-annealing occurs to some extent on the substrate side. On spraying, molten powders solidify at a cooling rate higher than 10 6 K s −1 in the case of a water-cooled substrate, and the cooling rate is larger than that attained in flame spraying. Such a high cooling rate in low pressure plasma spraying is attributed to the fact that the flattened particles which come from the molten powders to produce a deposit layer are thin and that the heat resistance at the interface between the flattenned particle and the underlying deposit layer is small because of the reduced amounts of pores and oxides at the interface.

Growth direction of columnar crystals solidified in flowing melt

Abstract Aluminum-copper alloys containing 0.5–8.8 wt% Cu were unidirectionally solidified. Columnar crystals were grown in flowing melts at flow rates ranging from 4 to 90 cm/s, and the deflection angle of the crystal growth direction in the upstream direction was measured as functions of flow rate and copper content. The columnar crystals originated from nuclei or solid particles with the most suitable crystallographic orientation for growing in flowing melt. The growth direction of the columnar dendrites was nearly equal to that of the columnar grains comprising the columnar dendrites. The deflection angle for the columnar grains increased only slightly with an increase in flow rate or copper content within the ranges investigated, while the inclination angle of the growth direction of columnar dendrites from the [100] direction increased with an increase in flow rate and with a decrease in copper content.

Composite deposits based on titanium aluminide produced by plasma spraying

Abstract A Ti–Al–N composite powder is produced by ball-milling of elemental titanium and aluminum powders in a nitrogen atmosphere, and is plasma-sprayed in an Ar gas atmosphere, yielding titanium aluminide-based deposits. The main constituent of the as-sprayed deposit formed on a directly water cooled substrate is Ti 3 Al(α 2 ), and subsequent heat-treatment of the deposit results in the formation of TiAl (γ), Ti 2 N and Ti 2 AlC. There is little difference in hardness between the as-sprayed deposit and the deposits heat-treated at temperatures up to 1473 K. The constituents of the as-sprayed deposit formed on an indirectly water-cooled substrate are α 2 ,γ and Ti 2 N. This as-sprayed deposit is harder than the deposits formed on a directly water-cooled substrate both in the as-sprayed condition and in the heat-treated conditions.

Effect of capillary pressure on interdendritic liquid flow

Abstract The top free surface of a mushy zone composed of columnar dendrites is exposed to air and the interdendritic liquid is made to flow down out of the specimen by applying pressure difference across the specimen. The liquid flow rate is measured as functions of applied pressure head, fraction liquid and primary and secondary arm spacings of columnar dendrites lying parallel to the flow direction. At a pressure head smaller than a critical value, the flow rate is two orders of magnitude smaller than that in the case where the top of the mushy zone is covered with bulk liquid. Above the critical pressure head, the flow rate increases rapidly with increasing pressure head. The results can be explained in terms of the capillary pressure, acting at the free surface of the interdendritic liquid, as a resistance to liquid flow. It is suggested that the capillary pressure is one of the most important factors in the formation of porosity in castings when the free surface of the riser solidifies to form a mushy zone.

Microstructure and mechanical properties of rapidly solidified deposited layers of FeCCr alloys produced by low pressure plasma spraying

Abstract Six kinds of FeCCr alloy powder containing 1.5−3.8wt.% C and 10.7−39.1wt.% Cr were plasma sprayed on steel substrates in a low pressure chamber and rapidly solidified deposited layers were produced. When the substrates were water cooled during spraying, the constituents of the deposited layers are predominantly retained austenite, which is supersaturated with alloying elements, in alloys with low carbon and low chromium contents, and are mainly a metastable phase and metastable cementite in alloys containing high carbon and high chromium. The layer deposited on substrates which had not been water cooled are composed of the stable phases of ferrite and M 7 C 3 and/or M 23 C 6 ( M ≡ Fe , Cr ). These deposited layers have a lower hardness but a higher tensile strength compared with the layers which were deposited on water-cooled substrates. This is because, in the case of non-water-cooled substrates, the temperature rise in the layers makes the adhesion between flattened particles stronger and also produces fewer pores.