Directional solidification of metal-gas eutectic and fabrication of regular porous metals Directional solidification of metal-gas eutectic and fabrication of regular porous metals

Directional solidification of metal-gas eutectic and fabrication of regular porous metals

  • 期刊名字:中国铸造
  • 文件大小:361kb
  • 论文作者:Yuan LIU,Huawei ZHANG,Xiang CH
  • 作者单位:Key Laboratory for Advanced Manufacturing by Materials Processing Technology
  • 更新时间:2020-09-15
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论文简介

Vol 2 No. 3, Aug. 2005CHINA FOUNDRYDirectional solidification of metal-gas eutecticand fabrication of regular porous metals*Yuan liu, Huawei ZHANG, Xiang Chen, Y anxiang LI(Key Laboratory for Advanced Manufacturing by Materials Processing Technology, Dept of Mechanical EngineeringTsinghua University, Beijing 100084, P.R. China)Abstract: Directional solidification of metal-gas eutectic (Gasar) is a novel process for making regular porous metalsThis process is based on a solid-gas eutectic reaction involving a gaseous medium and a metal or a ceramic phase, andallows an easy control of the porosity, such as its pore size, pore orientation and morphology in a wide range by properlydjusting its melting and solidification conditions. The latest progress and our research work in this field are reviewed inKeywords: metal-gas eutectic; unidirectional solidification; Gasar process; porous metalsCLC number: TG2499Document: AArticle ID:16726421(2005)03-0184041. Introductionmaterials manufacturing technologies. This method got itsPorous and foamed metallic materials have become an fame in 1993, after that it was successfully tested andattractive research field from either the scientific studied in Ukraine, USA, Japan, and China 3-11. Thviewpoint or the industrial application prospect, because paper is to introduce our research work in this fieldthey exhibit many special combinations of physical andmechanical properties, such as impact energy absorption 2. Gasar principlecapacity, air and water permeability, unusual acousticThis processing technique utilizes an invariant reactionenergy absorption ability, lower thermal conductivity etc of the so-called 'metal-gas eutectic transformation'inVarious fabrication methods for porous materials have which the melt is solidified into a solid solution and a gasbeen developed. They include the foaming method with phase 8, as shown in Fig. 1, which is similar to thegas bubbling, the vapor deposition or electro-deposition of traditional eutectic transformation (two solid phasesmetal onto a polyurethane foam precursor, the powder formed from one liquid phase). Numerous Metal-Hydrogensintering method and so on. The spatial distribution of binary systems, including Al-H, Cu-H, Fe-H, Mg-H,pores in the porous materials fabricated by these methods Mn-H, Ti-H, Co-H, Be-H, Cr-H, and Ni-H, etc, exhibitis usually random. Just in 1993, a Ukraine scientist this kind of gas-eutectic transformation Some Metal-OxygShapovalov presented a new method in his patent applied en(Ag-O; Fe-C-O; Cu-O), Metal-Nitrogen systems(Fe-Nin America for producing a new type of porous metalsNi-N: Mn-N) and some kind of ceramic saturated withwhose long cylindrical pores are in an ordered structure 2 ). hydrogen, nitrogen or oxygen are also perspective for theThis method was called 'Gasar process in which an application of the Gasar processinvariant reaction of the so-called 'metal-gas eutecticPressure, Peaction was utilized to fabricate regular porous structureGasar metals with different porosity and structure may besynthesized for filters, metal-matrix composites, bearings,brakes, damping elements etc 3. Compared withtraditional fabrication techniques, this process allowsL+G(H,)effective control of porosity, pore morphology and poreorientation, so it was thought as the revolutionary method中国煤化工 Eutecticand should be distinguished as a new direction in porousCNMHGYuan LIU: Ph. D, being mainly engaged in fabrication ofmaterials, alloy materials and solidification technoE-mail:yuanliu(@tsinghua.edu.cn[Received*ee05-02-23: [Acepted date)2005-05-20Fig 1 Schematic phase diagram for a metalhydrogen systemVol 2 No. 3Directional solidification of metal-gas eutectic and fabregular porous metalsLike the traditional eutectic transformation, during theDuring the following directional solidification process,directional solidification process, the gas-eutectic transfo- as the metal solidifies, the solubility of the hydrogenrmation may result in the formation of ordered structure dissolved in solid goes through a sharp decreasewith two phases one of which is gaseous. This method compared with that in liquid, and then the gas bubblesdemands a specific apparatusform as a result of the supersaturated hydrogen isolatedFigure 2 shows a typical apparatus developed by the from the solid metal. If the process parameters getauthors in the study for making Gasar porous metals. The controlled properlygrowth of the bubbles advancesmain part of the apparatus contains a crucible,concurrently withlid and does not leave from thecoil and a cylindrical mould with a water cooled copper solidification front, thus the porous structure gets formedplate at the base which are all housed in a high-pressure3. Gasar structureDepending on variable heat releasing direction, it ispossible to form Gasar structure with axial or radial poreorientation. As shown in Fig. 2, if the bottom plate of themould is cooled, the melt poured into the mouldsolidified upward from the bottom and simultaneously thegas pores grow along the axial direction, too. This kind ofregular porous structure with an axial pore distribution isalso called as lotus-type porous metal or lotus metalbecause it looks like lotus roots. Figure 3 shows a typicallotus-type of porous magnesium sample produced by theauthors. If the lateral surroundings of the mould areapplied by cooling, the melt is solidified inwards and thepores grow along the radial direction. Figure 4 shows atypical regular porous magnesium sample with a radialpore distribution produced by the authors, too1.Graphite stopper; 2. High pressure chamber,A few general observations from the Gasar structure3. Heating coil; 4. Molten metal; 5. Graphite crucibleunder the study are6. Ceramic mould; 7. Copper chiller; 8. Cooling waterThe pore size distribution is non-uniform because ofFig.2 A schematic of the fabrication principle andconcurrent growth of small and large pores andapparatus for Gasar metalscoalescenceNo branching of pores is ever observed to occurThe apparatus makes it possible to melt metals in a No pores are nucleated on the mould surface and a non-rucible and to solidify them in a casting mould under porous metal skin in the range of 0.05-5mm thicknesscontrollable gas pressure(typically 1-50 atmospheres). By forms firstchanging the partial pressure of H, variable concentrations Porosity 10%-55%and pore diameters 10-1 500 umof hydrogen in the melt are obtainedPore shape: cylindrical, spherical, and ellipsoidalLLR中国煤化工CNMHGFig 3 Gasar magnesium(Lotus-type structure)with axial pores distribution( PH2=0.2 MPa, PAr =0.1 MPa, T=1 023 K)186· CHINA FOUNDRYAug.2005Fig 4 Gasar magnesium with radial pores distribution( PH2=0. 2 MPa, PAr=0 MPa, 71 023 K)4. Control of porosity and pore sizeand pore sizes, as shown in Figs. 5-6. In contrast,The average porosity(e)of the entire ingot is measured solidification velocity and pouring temperature are lessthrough Archimedes'principle. The average pore size is effective parameters in Gasar process. Figure 5(a)and(b)evaluated by an image analysis system. Generally, for the show the experimental porosities together with theGasar structures with an axial or radial pores distribution, predicted ones on the Mg/Hy system. It can also be foundthe porosities at different sites of the ingots producedthe studpproximately homogeneous and have The porosity decreases with increasing partial pressureinsignificant deviation from the measured average of hydrogen when only single hydrogen is usedporosity of the entire ingot. In addition, the distribution of The porosity increases with increasing partial pressureaverage pore sizes also has less significant fluctuation at of hydrogen when the total gas pressure keeps constant.different positions of the produced ingotThe porosity decreases with increasing partial pressureIn Gasar solidification, besides the hydrogen, inert gas of argon when the partial pressure of hydrogen keepsuch as argon is often added to the Gasar apparatus. The constant. As shown in Fig. 6, the mean diameter Dm. ofaddition of inert gas can realize the adjustment of the final pores is mainly dependent on the total solidificationporosity, the average pore size as well as the pore size pressure Ps under the same pouring temperature anddistribution scope. The gas pressure is a very powerful cooling conditions, namely Dm decreases with the increasetechnological parameter for the proIt Isof the solidification pressuredjustable to achieve many different kindporositiesExperimental valueExperimental values- The model in [15]T=1023K, P =0, 4MPaT=1023K, PRPP.T=1023K.户=0.2MPa中国煤化工CNMHGLFig. 5 The experimental porosities(the scattered circle dots) of lotus-type porous magnesium under different partial pressures ofThe solid line shows the predicted porosities by the model developed by the present authors in男毁Vol 2 No. 3Directional solidification of metal-gas eutectic and fabrication of regular pe[2]V. I. Shapovalov. Method for manufacturing porous articles [Pl· Experimental valuesU. S. Patent, No 5, 181, 549(January 26, 1993)[3]V. I. Shapovalov and L.V. Boyko. Gasar- A new class of porousmaterials [J]. Advanced Engineering Materials, 2004, 6(6):407-410[4]V. I. Shapovalov. Structure formation behavior of alloys duringNo.3gas-eutectic transformation and prospects of the use ofhydrogen in alloying. Microstructural Design by SolidificationProcessing (Edited by Enrique J. Lavernia and Mehmet Nungor)[M]. The Minerals, Metals& Materials Society, 1992.No8No.10No 12207-216[5 D. Ludmil, S. Jerzy and A. Rajiv. Mathematical modelling arP(P,+P)(MPa]numerical simulation of ordered porosity metal materialsrmation [J]. Journal of Computer-Aided Materials DesignFig. 6 The evolution trend of the mean diameter of pores2003,10:3554in lotus-type magnesium with the solidification[6] J. M. Apprill, D. R. Poirier and M. C. Maguire. Gasar porouspressure P (P=PH2+PAr ),T-1 023 Kmetals process control [A]. Mat. Res. Soc. Symp. Proc. [CI998,521:2912965. SummaryC. J. Paradies and A. Tobin. The effect of Gasar processingparameters on porosity and properties in aluminum alloys [AGasar technology is based on a new scientificMat. Res. Soc. Symp. Proc. [C]. 1998, 521: 297-302about gas-eutectic reaction in metal-gas [8] P. Chanman and R. N. Steven. Metallographic study of Gasarsystem. Although the researches in this field have beenporous magnesium [A]. Mat. Res. Soc. Symp. Proc. [C]. 1998521:315320carried out for 12 years in several countries, unfortunately, (9) A. Patanaik, s. C. Sanday and C. L. Vold. Microstructure ofa well understanding on Gasar's manufacturing parametersGasar porous copper ingots [A]. Mat. Res. Soc. Symp Proc. [C]as well as its processing control system has not been995371:371-376established, yet. The current research on the Gasar process [10/H. Nakajima, S. K. Hyun and K. Ohashi. Fabrication of porouscopper by unidirectional solidification under hydrogen and itshas just been carried out for 3 years by our group. TheColloids and Surfaces A: Physicochemical anGasar structures with axial and radial pore distributionEngineering Aspects, 2001, 179: 209-214have been fabricated successfully with an apparatus[11]S. K. Hyun and H. Nakajima, Fabrication of porous iron byunidirectional solidification in nitrogen atmosphere [J. Materdeveloped by the authors in the study. The experimentalans.2002,43:52653results indicate that the gas pressure is a very powerful [12]S. Yamamura, H. Shiota, K. Murakami and H. Nakajimatechnical parameter that can be applied to achieve variousEvaluation of porosity in porous copper fabricated byunidirectional solidification under pressured hydrogen [J]. Materkinds of porosities and pore sizes. In contrast, thec.Eng,2003,A318:137-143parameters such as solidification velocity and pouring [13] Yuan LIU and Yanxiang LI. A theoretical study of Gasaritetemperature are less important and relatively ineffectiveeutectic growth [J]. Scripta. Metall., 2003, 49(5): 379-386[14 Yuan LIU and Yanxiang Ll. Theoretical analysis of bubblefor the Gasar processucleation in GASAR materials [J]. Trans. Nonferrous Met. Soc.China,200313(4}830834Acknowledgments[15] Yuan LIU, Yangxiang LI and Huawei ZHANG. Effect of Gasarprocessing parameters on structure of botus-type porous metalThe present research was supported by the NaturalRare Metal Materials Engineering, 2005, 34(7): 1128-1130Science Foundation of China (No. 50404002) and[16] Yuan LIU, Yanxiang LI and Huawei ZHANG. Fabrication ofNational Program on Key Basic Research Projects(Nolotus-structured porous magnesium with Gasar process [J]2004CCA05100)Acta Metallurgica. Sinica, 2004, 40(11): 1121-1126[17 Yuan LIU, Yanxiang LI and Huawel ZHANG Experimental studyn fabrication of lotus-type porous metals with metal-gasReferencesutectic directional solidification process [J]. Special CastingBanhart. Manufacture characterization and application ofNonferrous, 2005. (1): 1-4( in ChineseScience, aoos and metal foams JJ. Progress in Materials [18] H. Nakajima, Invariant reaction of liquid-solid+gas-gas-evolutioncrystallization reaction [J]. Mater. Trans., 2001, 42: 1827-1829中国煤化工CNMHG

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