Continuous Zone Melting Research Articles

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.

Fabrication of Lotus-Type Porous Iron by Thermal Decomposition Method

Lotus-type porous iron was fabricated by continuous zone melting technique through thermal decomposition of chromium nitride(Cr1.18N). Nitrogen dissolves into the molten iron through thermal decomposition of Cr1.18N. When the molten iron is solidified in one direction, insoluble nitrogen forms the directional gas pores aligned along the solidification direction. The porosity increases with increasing transfer velocity. For most of lotus metals fabricated by pressurized gas method, the porosity does not change with the transfer velocity owing to constant gas solubility in liquid and solid phase. On the other hand, the porosity of lotus metal fabricated by thermal decomposition method depends on the transfer velocity. This difference is attributed to the decomposition behavior of gas compound dependent upon the heating rate.

Investigation of the Mechanical Properties of Lotus-Type Porous Carbon Steel Made by Continuous Zone Melting Technique

The lotus-type porous carbon steel with cylindrical pores has been fabricated by continuous zone melting technique in pressurized mixture of hydrogen and helium gases. In order to investigate the mechanical properties, the tensile and compression tests were carried out. The ultimate tensile strength of the specimen with cylindrical pores parallel to the solidification direction is lower than the estimated value assuming that the strength is decreased in proportion to decreasing cross section area of the specimen, while the yield strength is higher than that estimated. The compressive yield strength is also higher than that estimated. The increase in yield strength is attributed to the precipitation strengthening. The tensile strength is increased by quenching and tempering, while the elongation decreases. Such mechanical properties are discussed in terms of microstructureal analysis. Furthermore, simulations of the mechanical properties by FEM analysis were carried out.

Fabrication, properties, and applications of porous metals with directional pores.

Lotus-type porous metals with aligned long cylindrical pores are fabricated by unidirectional solidification from the melt with a dissolved gas such as hydrogen, nitrogen, or oxygen. The gas atoms can be dissolved into the melt via a pressurized gas atmosphere or thermal decomposition of gaseous compounds. Three types of solidification techniques have been developed: mold casting, continuous zone melting, and continuous casting techniques. The last method is superior from the viewpoint of mass production of lotus metals. The observed anisotropic behaviors of the mechanical properties, sound absorption, and thermal conductivity are inherent to the anisotropic porous structure. In particular, the remarkable anisotropy in the mechanical strength is attributed to the stress concentration around the pores aligned perpendicular to the loading direction. Heat sinks are a promising application of lotus metals due to the high cooling performance with a large heat transfer.

Fabrication of Porous High-Purity Iron with Directional Pores by Continuous Zone Melting Technique

Lotus-type porous iron rods were fabricated by continuous zone melting technique under the pressure of 2.5 MPa of hydrogen, using iron with three different purities: 99.5%, 99.9% and 99.999%. It was found that the pore size in lotus iron with high purity (99.999% pure) is much larger than that in lotus iron with lower purity (99.5% and 99.9% pure), while the porosity in lotus iron with high purity is lower than that in lotus iron with lower purity. The transfer velocity does not affect the porosity in all lotus iron, but may affect the pore size; the pore diameter decreases with increasing transfer velocity. Thus, it is apparent that the purity of iron significantly affects the pore morphology and the impurities in iron serve as the nucleation sites of pores to increase the pore number density and also suppress the growth of pores due to slower diffusion of hydrogen. [doi:10.2320/matertrans.M2010219]

Fabrication of Porous Iron with Directional Pores through Thermal Decomposition of Chromium Nitride

Lotus-type porous iron with long directional pores was fabricated by a continuous zone melting technique through thermal decomposition of chromium nitride Cr1.18N. Nitrogen decomposed from the nitride powders dissolves in the molten iron. Insoluble nitrogen evolves the directional gas pores when the melt is solidified in a direction. The porosity increases with increasing transference velocity, while the pore diameter is almost constant. The porosity change with the transference velocity is attributed to the difference in decomposition rate of chromium nitride. The compound Cr1.18N is composed of CrN and Cr2N, the latter of which is considered to evolve the pores because of the coincidence of heating rate of the continuous zone melting with that for the decomposition of Cr2N.

Fabrication of lotus-type porous carbon steel via continuous zone melting and its mechanical properties

Lotus-type porous carbon steel (lotus carbon steel) AISI1018 rods with long cylindrical pores aligned in one direction were fabricated using the continuous zone melting technique under nitrogen gas pressure of 2.5 MPa. The porosity decreased with increasing transference velocities of 40–160 μm s −1. Tensile tests of the fabricated lotus-type carbon steel rods were performed. The elongation of lotus carbon steel increased after normalizing at 1200 K. The tensile strength and the Young's modulus decreased with increasing porosity. In contrast, the yield strength of lotus carbon steel did not decrease, even with a porosity of 20%, compared with that of non-porous carbon steel. This superior characteristic is attributed to solid-solution strengthening by solute nitrogen.

Compressive deformation behavior of porous γ-TiAl with directional pores

Lotus-type porous Ti 52Al 48, possessing elongated pores aligned in one direction, was fabricated by a continuous zone melting technique under pressurized hydrogen/helium atmospheres. Compression tests were carried out on specimens with various porosities and with loading directions parallel and perpendicular to the elongated pore direction. The compressive properties of lotus TiAl significantly differ from those of conventional lotus metals with ductile matrix. Deformation and initial cracks are observed around the pores, and they progress around the pores locally. Therefore, the densification of the specimen does not occur. This is due to the brittle matrix, which is sensitive to local large stress concentration. Thus, it is concluded that compressive properties of lotus metals depend not only on macroscopic pore structure but also on the degree of ductility of matrix.

Fabrication of Lotus-Type Porous Ni-(15, 28 and 31) at.% Al Alloys by Unidirectional Solidification in Hydrogen Atmosphere

Lotus-type porous Ni- (15, 28 and 31) at.% Al alloys whose long cylindrical pores are aligned in one direction were fabricated by continuous zone melting technique under high-pressure gas of hydrogen of 2.5 MPa. A part of 5-10 mm in length of the rod in the vicinity of the coil was melted by high frequency induction heating, and was moved downwards by electric motors at a constant velocity of 330 μms-1 to 500 μms-1 for unidirectional solidification. The pores are formed as a result of precipitation from the supersaturated hydrogen gas when the liquid metals dissolved with gas atoms is solidified. The porosity and the pore size decrease with increasing aluminum content. An increase of solidification velocity from 330 μms-1 to 500 μms-1 leads to a decrease of pore diameter and an increase of pore number in the porous Ni-28at%Al.

Fabrication of Lotus-Type Porous Metals by Continuous Zone Melting and Continuous Casting Techniques

Lotus-type porous metals with low thermal conductivity are fabricated by continuous zone melting technique, which possess directional elongated pores. The porous metals have been able to be fabricated through the conventional casting method by utilizing the solubility gap between solid and liquid in pressurized gas atmosphere. However, there is a shortcoming that the pores are coarsened in the part farther from the chill plate in the ingot. In order to overcome such a shortcoming, we developed the continuous zone melting technique and successfully produced the lotus-type porous metals with even low thermal conductivity such as stainless steel and superalloys. Furthermore, from the viewpoint of mass production with low cost, we invented novel ”continuous casting technique”. The molten metals dissolving gas are solidified continuously by passing through the mold cooled with chiller and thus, lotus-type porous metal plate as long as one meter was produced for short time. Sufficient uniformity of the porosity and pore size was obtained in such long porous ingots. This technique is prospective method for commercial mass production.

Fabrication and properties of Lotus-type porous nickel-free stainless steel for biomedical applications

Lotus-type porous Fe–25 wt.%Cr, Fe–23 wt.%Cr–2 wt.%Mo alloys and type AISI 446 stainless steel were fabricated by continuous zone melting technique in pressurized hydrogen and helium gas. The porosity of the samples varied in the range 44–48% and the mean pore size values obtained (145–374 μm) are in the biomedical field desired range. The fabricated Lotus-type porous nickel-free stainless steel was nitrided at high temperature up to the nitrogen concentration of 1.0 wt.% and this amount resulted to be sufficient for maintaining almost single-phase austenitic structure at room temperature. The combination of very small magnetic susceptibility, light weight, mechanical properties close to the human cortical bone, together with the known good enough corrosion resistance of high nitrogen nickel-free stainless steel, makes this Lotus-type porous Fe–Cr–N alloys very attractive for bone implant applications.

Three-dimensional image-based modeling of lotus-type porous carbon steel and simulation of its mechanical behavior by finite element method

Lotus-type porous carbon steel with cylindrical pores was fabricated by the continuous zone melting method in pressurized hydrogen gas. A three-dimensional (3D) reconstruction of the microstructure of the lotus-type porous carbon steel was made by using an image-based modeling to simulate the mechanical behavior. The stress–strain behavior of the 3D models of the lotus-type porous carbon steel was predicted by using finite element method (FEM) and was compared with the experimental results. Until the strain reaches 0.05, the compressive stress of the predicted model reaches the almost same value of the experimental result. The tensile stress also reaches approximately 0.9 times value of the experimental result.

ロータス型ポーラス鉄のレーザ溶接

A lotus-type porous iron (AISI 1018) that is fabricated by unidirectional solidification using the continuous zone melting technique in a nitrogen atmosphere under a pressure of 2.5 MPa, was welded by a Nd: YAG laser. The melting property of lotus-type porous iron was investigated to evaluate its melting characteristics at different laser powers and welding speeds. The weld bead surface of the lotus-type porous iron was rough with pits and dents irrespective of the pore growth direction. The remarkable effect of the pore growth direction on the penetration depth of the weld bead was not observed. This was due to the unstable weld bead formation caused by the relatively large-sized pores and the blowing of the remaining gas from the closed pores as well as the smaller anisotropy of the thermal diffusivity as compared to the copper and magnesium cases.

Fabrication and Properties of Porous Materials with Directional Elongated Pores

Lotus-type porous materials whose long cylindrical pores are aligned in one direction were fabricated by unidirectional solidification from the melt dissolving gas. The pores were evolved by supersaturation of gas atoms when the liquid was solidified. Although such porous metals with high thermal conductivity were produced by casting process, the process could not produce porous metals with low thermal conductivity, which possess uniform pore size and porosity. In order to obtain uniform pore size and porosity in metals with low thermal conductivity, we invented a new “continuous zone melting technique”. Using this technique various metals, alloys, intermetallic compounds and semiconductors were fabricated. Mechanical properties of tensile strength on lotus-type porous metals are described; lotus-type porous iron fabricated using a pressurized nitrogen gas instead of hydrogen exhibits superior strength. The lotus materials can be applied to various functional materials. Some examples are also shown in this paper.

Fabrication of lotus-type porous carbon steel by the continuous zone melting method and its mechanical properties

A lotus-type porous carbon steel with cylindrical pores was fabricated by the continuous zone melting method in a pressurized mixture of hydrogen and helium gases. The porosity increases with increasing partial pressure of the hydrogen gas, while the pore diameter remains almost constant, independent of the pressure. The ultimate tensile strength of the specimen with cylindrical pores parallel to the tensile direction is lower than the estimated value, assuming that the strength is decreased in proportion to the decrease of the cross-sectional area of the specimen, while the yield strength is higher than the estimated value. The compressive yield strength is also higher than the estimated value. The increase in yield strength is considered to be due to precipitation strengthening. The tensile strength is increased by quenching and tempering, while the elongation decreases. Such mechanical properties are discussed in terms of the microstructural analysis.

Electrochemical Behaviour of Lotus-Type Porous SUS304L and SUS316L Stainless Steels

Lotus-type porous metals are expected to be used in various applications such as lightweight structural materials and biomedical materials. Lotus-type porous stainless steel is particularly promising as a structural material because stainless steel has useful properties such as high corrosion resistance, high workability, low cost and so on. However, there is a possibility that dissolved hydrogen or the microstructure of lotustype porous stainless steel affects its corrosion behaviour. In this study, the electrochemical corrosion behaviour of lotus-type porous SUS304L and SUS316L stainless steels fabricated by the continuous zone melting technique under pressurized hydrogen was investigated using a potentiodynamic polarization in 0.1-kmol/m 3 sulphuric acid solution. The current density of lotus-type porous SUS304L was higher than that of nonporous SUS304L at around � 100 mV (� 100 mV peak), while it was similar to that of nonporous stainless steel in the passive and transpassive regions. The specific current peak observed for the porous SUS304L at around � 100 mV disappears when the pores are filled with epoxy resin or the specimens are dehydrogenated. Thus, it is concluded that the � 100 mV peak is attributed to the dissolved hydrogen at pore surface. In the case of lotus-type porous SUS316L, corrosion behaviour is similar to that of SUS304L. [doi:10.2320/matertrans.47.2229]

Compressive properties of lotus-type porous stainless steel

Lotus-type porous stainless steel (SUS304L), possessing cylindrical pores aligned in one direction, was fabricated by means of a continuous zone melting technique undera pressurized hydrogen/helium atmosphere. Compression tests were carried out on the resultant lotus stainless steel not only in the directions parallel and perpendicular tothe elongated-pore direction but also in other directions to reveal its anisotropic compressive behavior. The macroscopic deformation modes depend on porosity and the angle between the elongated-pore direction and compression direction, which is a unique characteristic resulting from its anisotropic porous structure. The yield stress in the pore direction decreases almost linearly with increasing porosity, while that in the perpendicular direction decreases more rapidly. The yield stress in the direction of θ from the elongated-pore direction decreases monotonically with increase in θ. The yield behavior of lotus stainless steel was described using micromechanical mean-field theory.

Fabrication of Lotus-Type Porous Ni3Al with and without Boron

Ni3Al with and without boron was melted and unidirectionally solidified in pressurized hydrogen atmosphere by using a continuous zone melting method. For Ni3Al without boron, cylindrical pores elongated in the direction parallel to the solidification direction, are formed in Ni3Al matrix. On the other hand, no pore is formed in Ni3Al with 0.23 mol% B solidified under the same condition. This result suggests that the solubility gap of hydrogen between liquid and solid Ni3Al with boron is extremaly small.

Laser Welding of Lotus-Type Porous Iron

A lotus-type porous iron (AISI 1018) that is fabricated by unidirectional solidification using the continuous zone melting technique in a nitrogen atmosphere under a pressure of 2.5 MPa, was welded by a Nd:YAG laser. The melting property of this was investigated to evaluate its melting characteristics at different laser powers and welding speeds. The weld bead surface of the lotus-type porous iron was rough with pits and dents irrespective of the pore growth direction. The remarkable effect of the pore growth direction on the penetration depth of the weld bead was not observed. This was due to the unstable weld bead formation caused by the relatively large-sized pores and the blowing of the remaining gas from the closed pores as well as the smaller anisotropy of the thermal diffusivity as compared to the copper and magnesium cases.

Fabrication of lotus-type porous carbon steel by unidirectional solidification in nitrogen atmosphere

Abstract Lotus-type porous carbon steel AISI 1018 was fabricated by the continuous zone melting technique in pressurized nitrogen atmosphere. Cylindrical pores parallel to the solidification direction were observed in the porous carbon steel. The pore diameter and porosity can be controlled by the pressure of nitrogen. The nitrogen concentration in matrix of porous carbon steel fabricated under nitrogen pressure 2.5 MPa was 0.12 wt.%. The micro Vickers hardness increases with nitrogen pressure, which is attributed to the solid-solution hardening.