Fabrication of Porous Copper with Directional Pores by Continuous Casting Technique Through Thermal Decomposition of Hydride

Lotus-type porous copper with aligned long cylindrical pores was fabricated by continuous casting technique through thermal decomposition method (TDM) in an argon atmosphere of 0.1 MPa. A pellet of titanium hydride was supplied into molten copper with adjusting the time interval to maintain the constant concentration of hydrogen to be dissolved in the melt, when the transfer velocity of the unidirectional solidification is changed. Long lotus-type porous copper slabs were fabricated with constant solidification velocity. The effect of the transfer velocity on the porosity and pore size was investigated. The average pore diameter was independent of the transfer velocity, but the porosity is slightly dependent on the velocity. It is apparent that the continuous casting technique can be applicable for production of lotus metals through TDM.     

Fabrication of Aluminum Foams with Small Pore Size by Melt Foaming Method

This article introduces an improvement to the fabrication of aluminum foams with small pore size by melt foaming method. Before added to the melt, the foaming agent (titanium hydride) was pretreated in two steps. It firstly went through the traditional pre-oxidation treatment, which delayed the decomposition of titanium hydride and made sure the dispersion stage was controllable. Then such pre-oxidized titanium hydride powder was mixed with copper powder in a planetary ball mill. This treatment can not only increase the number of foaming agent particles and make them easier to disperse in the melt, which helps to increase the number of pores, but also reduce the amount of hydrogen released in the foaming stage. Therefore, the pore size could be decreased. Using such a ball-milled foaming agent in melt foaming method, aluminum foams with small pore size (average size of 1.6 mm) were successfully fabricated.     

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.     

Pore nucleation in solidifying high-purity copper

High-purity copper (6 or 7 N) was melted and solidified unidirectionally in the atmosphere of a H₂-Ar gas mixture for the purpose of studying the mechanism of pore nucleation in solidifying metal. Hydrogen content in the melt was controlled by changing the partial pressure in the atmosphere. Pores were formed when the hydrogen partial pressure in the atmosphere was 0.3 atm or more. Oxides of aluminum and silicon were observed at the bottom of the pores and the pores were nucleated heterogeneously. Water vapor with a very low partial pressure existed in the furnace atmosphere, and the melt must have contained a small amount of oxygen in equilibrium with this water vapor. The solid/liquid (S/L) interface was planar and convection was eliminated. The redistribution of the solute during solidification can, therefore, be estimated. The concentration of oxygen in the liquid at the S/L interface is estimated to be much larger than its initial concentration, due to the very small equilibrium distribution coefficient of oxygen in copper, and aluminum and silicon were oxidized even though their concentrations were very low. The probability of homogeneous nucleation by α particles was very small in these experiments.     

Metal-gas eutectic growth during unidirectional solidification

Metal-gas eutectic growth is a novel method for fabricating porous metals in which the gas pores are rodlike and aligned with the solidification direction. A new model was developed to predict the effects of the gas pressure and the solidification velocity on the porosity, pore diameter, and interpore spacing in metal-gas eutectic unidirectional solidification. The pore size and the interpore spacing are primarily dependent on the total gas pressure, but the porosity is dependent not only on the total gas pressure but also on the ratio of the partial pressures of hydrogen to argon.     

Processing and Pore Growth Mechanisms in Aluminum Gasarites Produced by Thermal Decomposition

Gasarites are a subclass of metallic foams that have a cylindrical pore morphology created by directional solidification of metals saturated with a gas. Thermal decomposition is an alternative process in which the soluble gas is delivered by decomposition of a particulate gas source. Aluminum gasarites formed through decomposition of titanium and zirconium hydrides were studied to both replicate the results of a previous study and discern pore-formation mechanisms. Replication of the previous study was not achieved, and additional processing enhancements were required to produce gasarite pore morphologies. For the first time, zirconium hydride was utilized to produce gasarites, with porosity levels and pore sizes lower than that from titanium hydride. Maximum average porosity levels of 10 and 6 pct were observed for titanium hydride and zirconium hydride, respectively. Pore-formation mechanisms in aluminum gasarite foams created via thermal decomposition of titanium and zirconium hydrides were evaluated through metallographic analysis and scanning electron microscopy. Definitive evidence of gas–metal eutectic pore growth was not found, but pore morphological characteristics and chemical analysis of particulate at pore surfaces support direct gas evolution from the hydride particles as a contributor to pore formation and growth.     

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 Cr_1.18N. 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 Cr_1.18N 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.     

脱氮热导法测定钛中氩

钛及其合金对气体元素溶解度高,氩作为保护性气体在钛基材料制备过程中可能进入钛材料从而对其性能产生影响。实验将自制脱氮快速转换装置应用于现有的脉冲热导气体分析仪,转换装置脱氮效率高达97%以上,氮的干扰可量化、可排除,从而实现了钛中氩的准确测定。脱氮热导-程序升温法确定钛中氩可以在1 400 ℃完全释放,氩为非化合态。将脱氮热导法与色谱热导法及飞行时间质谱法对比,氩测定结果一致,因此,脱氮热导法可作为主要参比方法,辅助飞行时间质谱法同时测定金属中氢、氧、氮和氩的新方法建立和完善。     

Oxidation rates of liquid copper and liquid copper sulfide

The kinetics of oxidation of liquid copper and copper sulfide were determined when the rate was controlled by the diffusion of the oxidizing gas to the melt interface. The method incorporated a capillary tube sample holder and the flux rate was measured by a quartz spring and cathetometer. The sample of copper sulfide was suspended from the balance in a tube furnace through which was passed a mixture of oxygen and argon gas. The oxidation of molten copper sulfide was found to proceed by the formation of copper followed by the formation of copper oxide. From the results, the diffusion coefficients of SO₂ and O₂ in argon were calculated over a temperature range of 1150° to 1350°C. The data compare well with the empirical equations for the diffusion of these gases. At lower temperatures, a different oxidation behavior is observed due to solidification of the sample.     

Effects of dispersal and mechanical boron alloying on the thermal stability of hydride phases in the Ti-B-H system

Methods have been applied from scanning electron microscopy, hydrogen thermal desorption, XRD, and differential thermal analysis on the effects of grain size and alloying with boron as regards the thermal stability and decomposition temperatures of hydride phases in mechanical alloys in the Ti-B-H system. The alloys were prepared by high-energy processing for 50 h in a planetary ball mill with mixtures of TiH_1.9 + 9 mass% B + 13 mass% Ti and also with TiH_1.9 + 50 mass% TiB₂ at speeds of 1000 rpm, in addition to mixtures of TiH_1.9 + 40 mass% B and TiH_1.9 + 50 mass% TiB₂, which were treated for 20 min at speeds of 1680 rpm. The dispersal on mechanical treatment and the addition of boron to the titanium hydride powder have substantial effects on the thermal stability. The processing of the mixture TiH_1.9 + 9 mass% B + 13 mass% Ti lasting 50 h in argon gave temperatures for the dissociation of the Ti(B, H)_x hydride phase in the mechanical alloy lower by 300 deg than the decomposition temperature for the initial titanium hydride TiH_1.9. The mechanisms have been identified for the effects of the dispersal and boron alloying on the thermal stability of the titanium hydride.     

Effect of Coating on Instantaneous Interracial Heat Transfer During Near-Rapid Solidification

 For many rapid or near-rapid solidification processes, the interfacial heat transfer between the melt and the substrate is a key issue on the cooling and solidification rate of castings. For the purpose of controlling and adjusting of the interfacial thermal resistance, the effects of C/BN, Zn and organic coatings on the instantaneous interfacial heat flux and the solidified structure of AISI304 stainless steel solidification on copper substrate have been investigated by using an experimental simulator. The results show that C/BN coatings can improve the uniformity of heat flux and solidified structure; Zn coating can increase the heat flux and solidification rate in the growth stage of the solidified shell; organic coating will decrease the heat flux and the solidification rate and make re-melted structure on the surface of the solidified shell.     

电解铜中的氢对铜线坯拉丝影响的实践（上篇：电解铜与熔炼）

阴极铜电解过程中使用的添加剂是阴极铜增氢的一个主要因素，阴极铜表面有大量凸瘤时，氢含量可增加数倍以上，感应电炉采用微还原性气氛熔炼，对氢无能为力，氢在铸造凝同时析出，过量气体的逸出会造成品界脆弱，导至轧制中开裂，影响线坯拉制微细线的性能。不能逸出的氢气体在铸坯中形成气孔，是拉线断线的一个主要原因。根据阴极铜质量和熔炼方式，在阴极铜熔化过程中，最大限度的降低氢的含量。     

减压试样密度法预测铝合金铸件的氢针孔度

利用砂型样杯取代传统的金属型样杯制作了铝合金减压凝固试样.结果表明,该试样可以预测铝合金铸件的氢针孔级别,控制铝合金的熔炼质量.测试仪器中还设置了预真空室,可使凝固室在最短的时间内达到预定的真空度,提高测试精度.试验还发现合适的砂型减压凝固压力约为1.13kPa.     

An integrated model for dendritic and planar interface growth and morphological transition in rapid solidification

Rapid solidification can be achieved by quenching a thin layer of molten metal on a cold substrate, such as in melt spinning and thermal spray deposition. An integrated model is developed to predict microstructure formation in rapidly solidified materials through melt substrate quenching. The model solves heat and mass diffusion equations together with a moving interface that may either be a real solid/liquid interface or an artificial dendrite tip/melt interface. For the latter case, a dendrite growth theory is introduced at the interface. The model can also predict the transition of solidification morphology, e.g., from dendritic to planar growth. Microstructure development of Al-Cu alloy splats quenched on a copper substrate is investigated using the model. Oscillatory planar solidification is predicted under a critical range of interfacial heat-transfer coefficient between the splat and the substrate. Such oscillatory planar solidification leads to a banded solute structure, which agrees with the linear stability analysis. Finally, a microstructure selection map is proposed for the melt quenching process based on the melt undercooling and thermal contact conditions between the splat and the substrate.     

Thermal stability of hydride phase obtained by mechanical treatment of Mg-10 mass% Fe in hydrogen under pressure

The formation of hydride phases under high-energy ball-milling of Mg and Mg + 10 mass% Fe powders in hydrogen under a pressure of 1.2 MPa has been studied. The influence of iron content and dispersion degree of the Mg-alloys upon thermal stability and decomposition temperature of the hydride phase of the alloys was investigated by thermal desorption, x-ray diffraction, and differential thermal analyses. It was established that addition of 10 mass% of iron to magnesium contributes to higher dispersion degree of the magnesium hydride obtained via milling in hydrogen under pressure. The hydride contains maximum amount of weak-and medium-bound hydrogen. The latter is mainly concentrated in the region of grain boundaries, the quantity of which increases with increase in dispersion degree. The formation of new boundaries and accumulation of defects in these regions are accompanied by an increase in thermodynamic potential, which causes a decrease in both the temperature of decomposition of the hydride phase by 100 °C and its thermal stability. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 99–106, July–August 2006.     

Fabrication of High-Porosity Lotus-Type Porous Aluminum in Vacuum

Lotus-type porous aluminums with porosities from 10 to 26 pct were fabricated with the Bridgman-type directional solidification method (Gasar). A vacuum atmosphere is critical to obtain high-porosity lotus-type porous aluminum by the Gasar process. The lotus-type porous aluminum was directionally solidified under a pure hydrogen pressure of 0.2 to 16 kPa. The influence of hydrogen pressure on the porosity and pore size in vacuum was investigated. The porosity and pore size increase with decreasing hydrogen pressure, but there exists a maximum porosity at some critical hydrogen pressure. Since a low hydrogen pressure is adopted, the effect of capillary pressure and hydrostatic pressure on the porosity becomes important. With the decreasing of hydrogen pressure, the influence of hydrostatic pressure and capillary pressure on porosity becomes stronger and stronger. The influence of melt height, which is proportional hydrostatic pressure, on porosity and pore size was investigated. The calculated porosities considering capillary pressure and hydrostatic pressure are in good agreement with experimental results.

     

急冷对Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金储氢性能的影响

为了研究急冷对储氢合金残余氢量的影响,利用真空电弧熔炼炉和铜模喷铸制备了Ti_(0.32)Cr_(0.345)V_(0.25)Fe_(0.03)Mn_(0.055)合金,采用XRD、PCT(压力-容量-温度)、TG/DTA等手段分析了急冷对储氢合金吸放氢性能的影响。结果表明,铸态合金和急冷合金均由BCC固溶体主相和Laves第二相组成;急冷对首次吸氢动力学行为影响较大,由铸态时的化学反应控制变为急冷时的新相晶核形成长大控制;急冷后,合金吸放氢平台压得到提高,且吸氢起始点左移,但吸放氢滞后性增大。TG/DTA曲线表明,急冷并没有改变合金的残余氢量,但氢化物放氢温度升高。     

Use of titanium hydride for the synthesis of titanium aluminides from powder materials

Titanium aluminides Ti₃Al and TiAl were synthesized by the use of TiH₂+Al powder mixtures. Phase transformations occurring upon the decomposition of titanium hydride in vacuum and inert gas and synthesis of titanium aluminides were studied. It was shown that the use of titanium hydride instead of titanium in powder mixtures with aluminum results in a significant activation of diffusion processes, and leads to an accelerated production of single-phase titanium aluminides upon isothermal heating. This is attributable to the small particle size of the charge, high density of defects (including those due to hydrogen-phase hardening) in the titanium and aluminum lattices, and possible reduction of Al₂O₃ films by atomic hydrogen. The type of atmosphere did not noticeably effect the rate of phase formation, however synthesis in vacuum appears to be more practical since hydrogen is completely removed with simultaneous formation of the aluminides. Due to the fact that aluminum reacted completely with titanium in the solid state, final products close to theoretical densities were obtained without the application of high pressure.