Acta Metall Sin  2018, Vol. 54 Issue (5): 727-741    DOI: 10.11900/0412.1961.2018.00027
 Special Issue for the Solidification of Metallic Materials Current Issue | Archive | Adv Search |
Directionally Solidified Porous Metals: A Review
Yanxiang LI1,2(), Xiaobang LIU1
1 School of Materials Science and Engineering, Tsinghua University, Beijing 100084, China
2 Key Laboratory for Advanced Materials Processing Technology, Ministry of Education, Tsinghua University, Beijing 100084, China
Abstract

This paper reviews the recent development of porous metals with directional pores, from the aspects of the solidification principle, fabrication method, properties and applications. This kind of porous metals is fabricated by a directional solidification process in pressurized gas atmosphere, utilizing a metal/gas eutectic reaction (Gasar). By controlling solidification direction, not only lotus-type porous structure but also radial-type porous structure can be produced. The coupled growth of solid/gas phases is discussed by applying a solution procedure similar to that in the classical Jackson-Hunt eutectic growth model. The working window considering hydrogen escape and the formation of directional solidification porous structure has been given. Three fabrication techniques including mold casting, continuous casting techniques and Bridgman-type directional solidification method are introduced. Two new progresses about the fabrication of directionally solidified porous structure are described in details: porous alloy with uniform directional pores and high-porosity directionally solidified porous aluminum. Since directionally solidified porous metals exhibit peculiar physical and mechanical properties such as light-weight, air and water permeability, and anisotropy of thermal and mechanical properties, they are suitable for applications in heat sinks, filters, biomaterials and so on.

 ZTFLH: TG249
Fund: Supported by National Natural Science Foundation of China (No.51371104)
 Fig.1  Cu-rich portion of Cu-H phase diagram under a hydrogen pressure of 0.1 MPa (T—temperature; XH—composition of hydrogen; Tm—melting temperature; Pext—external pressure; TE—eutectic temperature; XH,E—eutectic composition of hydrogen; XH,S—composition of hydrogen in solid phase)[18] Fig.2  Temperature dependence of hydrogen solubility in solid and liquid metals at 0.1 MPa hydrogen pressure[45] Fig.3  A schematic diagram for metal-gas eutectic unidirectional solidification and the corresponding coordinate system for solving the solute field (L—liquid phase; G—gas phase; S—solid phase; v—solidification velocity; rG—radius of the gas pore; rS—one half of the interpore spacing)[17] Fig.4  Comparison between predicted porosities and experimental values on Mg-H system at different hydrogen (a) and argon (b) gas pressures (Ptotal—total gas pressure;$PH2$—hydrogen pressure; PAr—argon pressure)[34] Fig.5  A comparison between experimental results and theoretical values of pore diameter and interpore spacing at different total gas pressures (v=0.4 mm/s, T=1023 K)[17] Fig.6  The working window of the partial pressure ratio and superheat (ΔT′) for the formation of lotus-type porous magnesium (L1—critical line for the formation of lotus-type porous magnesium; L2—critical line for the hydrogen escaping; L3—optimized parameter)[50] Fig.7  A schematic diagram for the effect of argon partial pressure on metal-hydrogen phase diagram[50] Fig.8  Typical structures of radial porous Mg/Cu[51](a) three-dimensional structure of radial po-rous Mg(b) cross section of radial porous Mg(c) cross section of radial porous Cu Fig.9  Three apparatus schematics for fabrication of directionally solidified porous metals by mold casting method(a) common device (1—graphite stopper; 2—graphite crucible; 3—heating coil; 4—high pressure chamber; 5—molten metal; 6—mold; 7—copper chiller) (b) 90° rotation device (c) 180° rotation device Fig.10  A schematic drawing of Bridgman directional solidification apparatus[37] (a) melting and temperature holding (b) unidirectional solidification Fig.11  Cross sections parallel (upper row) and perpendicular (lower row) to solidification direction of directionally solidified porous Cu-Mn alloy[61] Fig.12  Relation between porosity and solidification velocity in directionally solidified porous metals Fig.13  Experimental results of heat transfer coefficient of directionally solidified porous copper heat sink under different heat power densities (ΔP—pressure drop; U—flow rate)[97]