Fabrication, properties and application of porous metals with directional pores

This paper reviews the recent development of fabrication methods, various properties of porous metals with directional pores and its applications. This porous metals are fabricated by unidirectional solidification in pressurized gas atmosphere such as hydrogen, nitrogen and oxygen. The pores are evolved from insoluble gas when the melt metal dissolving the gas is solidified. The nucleation and growth mechanism of the directional pores in metals are discussed in comparison with a model experiment of carbon dioxide pores in ice. Three fabrication techniques, mold casting, continuous zone melting and continuous casting techniques, are introduced. The latter two techniques can control the solidification velocity and the last one possesses a merit for mass production. The porosity and pore size are able to be controlled by solidification velocity and ambient gas pressure, while the pore direction can be controlled by solidification direction. Not only metals and alloys but also intermetallic compounds, semiconductors and ceramics can be produced by this method. Anisotropy in the mechanical and physical properties is resulted from anisotropic pore morphology. The experimental results on the anisotropy in the elastic property and electrical conductivity are consistent with those calculated with an effective-mean-field theory. The anisotropic behaviors of tensile, compressive and fatigue strength are explained in terms of the dependence of stress concentration on the pore orientation. This porous metals exhibit good sound absorption and vibration-damping properties. Several possible applications are in progress for heat sink, golf putter, biomaterials and so on.