Takuya Ide

Fabrication of Porous Aluminum with Directional Pores through Continuous Casting Technique

Lotus-type porous aluminum with slender directional pores is fabricated via a continuous casting technique in pressurized hydrogen or a mixed gas containing hydrogen and argon. The influence of solidification conditions such as hydrogen partial pressure, solidification velocity, temperature gradient, and melt temperature on the porosity and pore size is investigated. The porosity and pore size increase upon increasing the hydrogen partial pressure or the melt temperature, whereas the porosity and pore size decrease upon increasing the solidification velocity or the temperature gradient. Furthermore, the mechanism of pore formation in lotus aluminum is examined based on the results of an improved model of hydrogen mass balance in the solidification front, which was originally proposed by Yamamura et al. The results from the present model agree with the experimental results. We conclude that the diffusion of hydrogen rejected in the solidified aluminum near the solid/liquid interface is the most important factor for pore formation because the difference in hydrogen solubility between solid and liquid aluminum is very small.

Heat Transfer and Pressure Drop of Lotus-Type Porous Metals

Metal foams are of interest for heat transfer applications because of their high surface-to-volume ratio and high convective heat transfer coefficients. However, conventional open-cell foams have high pressure drop and low net thermal conductivity in the direction normal to a heated surface due to the fully random structure. This paper examines heat transfer elements made by stacking thin layers of lotus metal which have many small pores aligned in the flow direction. The reduction in randomness reduces the pressure drop and increases the thermal conduction compared to conventional metal foams. Experimental results are presented for the heat transfer performance of two types of lotus metal fins, one with a deterministic pattern of machined holes and one with a random hole pattern made by a continuous casting technique. The layer spacing, the hole diameter, the porosity, and the flow Reynolds number were all varied. The measurements show that spacing between fin layers and the relative alignment of pores in successive fins can have a substantial effect on the heat transfer performance.

Fabrication of lotus-type porous copper through thermal decomposition of titanium hydride

Lotus-type porous copper was fabricated by unidirectional solidification through thermal decomposition of titanium hydride. Effects of additive method and additive amount of titanium hydride on pore formation were investigated. The porosity of lotus copper depends on additive method and additive amount of titanium hydride. The pore formation effectively occurs in the method that titanium hydride decomposes in molten copper. For all the additive methods of titanium hydride, the porosity increases and pore diameter does not change with increasing additive amount of titanium hydride. While, for adding large amount of titanium hydride, the porosity became constant. This is because hydrogen solubility in liquid phase does not change owing to bubbling of hydrogen gas.

Effect of Transfer Velocity on Porosity of Lotus-Type Porous Aluminum Fabricated by Continuous Casting Technique

Lotus-type porous aluminum was fabricated by continuous casting technique in mixture gas of hydrogen and argon at various transfer velocities in order to understand formation process of pores. The porosity and pore diameter decrease with increasing transfer velocity. The transfer velocity dependence of the porosity in lotus aluminum is different from that in other lotus metals such as stainless steel and copper. It is considered that the difference is attributed to lower solubility in aluminum than that in other metals.

Microstructure and Deformation Behavior of Lamellar Ti-Rich TiAl Crystal with Lotus-Type Aligned Pores

A porous Ti-48.0at.%Al (Ti-rich TiAl) crystal, in which lotus-type long cylindrical pores were aligned and (γ/α2) two-phase lamellar structure was simultaneously developed, was fabricated by floating zone method under the pressure of hydrogen and helium mixed gas. Plastic deformation behavior and microstructure of the Ti-rich TiAl crystal with lotus-type aligned pores were investigated by focusing on the elongated pore direction. The as-grown and annealed crystals show a well-developed lamellar structure and no texture accompanied by 52% porosity and a mean pore diameter of 380 μm. Yield stress strongly depends on the loading direction against the elongated pore. When loading directions are parallel and perpendicular to the pore direction, yield stresses obey K=1 and 2.5, respectively, in equation of σ=σ0(1-p)K, where σ is the yield stress with pores, σ0 is the yield stress without pores and p is porosity. This reflects macroscopically homogeneous and locally heterogeneous plastic deformation between pores, respectively.

Fabrication of Porous Metals with Directional Pores Through Solidification of Gas-dissolved Melt

Porous metals with long cylindrical pores aligned in one direction were fabricated by unidirectional solidification using pressurized gas (hydrogen) method (PGM) and thermal decomposition method (TDM). The pores are evolved from insoluble gas when the molten metal dissolving the gas is solidified. In the conventional PGM, the hydrogen pressurized in a high-pressure chamber is used as the dissolving gas. However, the use of high-pressure hydrogen is not desirable because of inflammable and explosive gas, in particular, for scaling up to mass production of lotus metals. In order to overcome this shortcoming, the thermal decomposition method was developed as an alternative simple fabrication method. Gas-forming compounds were added into the molten metal to fabricate lotus metals. The porosity and pore size were controlled by the amount of gas-forming compounds. TDM was applied to fabricate porous copper and aluminium

Fabrication of lotus-type porous NiAl and Ni3Al Intermetallic compounds

Lotus-type porous NiAl and Ni3Al intermetallic compounds, possessing cylindrical pores aligned in the direction parallel to the solidification direction, were fabricated by using a unidirectional solidification technique in a pressurized hydrogen atmosphere of 2.5MPa. The porosity of lotus NiAl is 24.2 %, and the porosity of lotus Ni3Al is 3.2%; the porosity of the porous NiAl is larger than that of Ni3Al. This is because the solubility gap of hydrogen between liquid and solid phases of NiAl is larger than that of Ni3Al.

Fabrication, Properties and Applications of Porous Metals with Directional Pores

Porous metals with long cylindrical pores aligned in one direction are fabricated by unidirectional solidification through pressurized gas method (PGM) and thermal decomposition method of gas compounds (TDM). The pores are evolved from insoluble gas when the molten metal dissolving the gas is solidified in the dissolving gas (PGM) or inert gas (TDM). Three fabrication techniques, mold casting, continuous zone melting and continuous casting techniques, are adopted. The latter two techniques can control the solidification velocity and the last one possesses a merit for mass production of lotus metals. The porosity, pore diameter and its length are able to be controlled by the solidification velocity and ambient gas pressure, while the pore direction can be controlled by the solidification direction. Anisotropy in the elastic and mechanical properties is resulted from anisotropic pore morphology. The anisotropic behaviors of tensile, compressive and fatigue strength are explained in terms of the dependence of stress concentration on the pore orientation. The anisotropic properties of thermal, electrical conduction and magnetization are also found, which are attributed to the scattering of heat flux, electric current and magnetic flux with anisotropic pores, respectively. Several applications of the porous metals to manufacturing products are investigated. The unidirectional pores can be utilized for high performance of heat sinks for electronic devices of cars and computers. Thus, the porous metals are expected to be used for various manufacturing products.

Heat Transfer Performance of Lotus-Type Porous Metals

Metal foams are of interest for heat transfer applications because of their high surface-to-volume ratio and high convective heat transfer coefficients. However, conventional open-cell foams have high pressure drop and low net thermal conductivity in the direction normal to a heated surface due to the fully random structure. This paper examines porous metals made by stacking thin layers of lotus metal which have many small pores aligned in the flow direction. The reduction in randomness reduces the pressure drop and increases the thermal conduction compared to conventional metal foams. Experimental results are presented for the heat transfer performance of two types of lotus metal fins, one with a deterministic pattern of machined holes and one with a random hole pattern made by a continuous casting technique. The layer spacing, the hole diameter, the porosity and the flow Reynolds number were all varied. The measurements show that spacing between fin layers and the relative alignment of pores in successive fins can have a substantial effect on the heat transfer performance.Copyright

Improvement of strength of lotus-type porous aluminum through ECAE process

Lotus-type porous aluminum with cylindrical pores oriented in one direction was deformed by Equal Channel Angular Extrusion (ECAE) through a 150° die with sequential 180° rotations, and the pore morphology and Vickers hardness after the extrusion were investigated. The Vickers hardness increases with increasing number of passes in the extrusions both parallel and perpendicular to the pore direction, accompanied by the decrease of porosity. The densification occurs more easily in the perpendicular extrusions than in the parallel extrusions, and the large deformation by the densification gives rise to the large increase in the Vickers hardness for the perpendicular extrusions.