Rheological and thermal activation analyses of the sintering kinetics of tungsten powders

Conclusions At any given sintering temperature the relative rate of densificaron of compacts of porosities 29–44% from reduced and atomized tungsten powders of particle sizes 1.5–5m is determined by their capillary pressure, the character of this dependence being close to linear. Densification during sintering is a threshold process, i.e., porous skeletons behave during sintering like Bingham solids. For each sintering temperature there exists a critical pore size above which the rate of densification sharply changes, approaching zero. Assessments of coefficients of Bingham viscosity have shown that at 1600 and 1900°C they are linear functions of sintering time. The energy of activation for the densification of the powders investigated during sintering in the range 1600–2000°C is equal to the energy of activation for grain-boundary self-diffusion in tungsten and amounts to 90 ± 5 kcal/mole.Translated from Poroshkovaya Metallurgiya, No. 10(262), pp. 27–32, October, 1984.     

Effect of heat treatment of the densification behavior of porous bodies from fine tungsten powders during sintering

Conclusions It has been established that the fall in the activity of a tungsten powder produced by reduction at 950°C and annealed, before pressing, at a temperature of 1700°C or higher is due not to growth of its particles during the annealing but to a decrease in the internal porosity of the particles and their agglomeration and spheroidization. With single-fraction as-reduced and annealed powders their is a correlation between the maximum size of pore channel constructions in a compact and the letter's sintering behavior. Prior heat treatment of tungsten powders produced by reduction at an end temperature of 700°C exerts a nonmonotonic influence on their sintering activity. Shorttime (=30 sec) sintering of compacts from such tungsten powders has been found to result in the formation of a new porous structure, with enlarged maximum pore channel constrictions, responsible for their sintering behavior. A correlation has been discovered between the volume shrinkage of compacts and the maximum size of constrictions in the enlarged capillaries.Translated from Poroshkovaya Metallurgiya, No. 11(227), pp. 25–31, November, 1981.     

Correlation between the particle size,pore size,and porous structure of sintered tungsten

Conclusions It has been established that the maximum size of pore channel constrictions D₁ is close to the mean size of pore sections in microsections of porous skeletons (-=22–44%) from tungsten powders of 1- to 5-m mean particle size. A rapid method of assessment of an integral fineness characteristic of a tungsten powder is proposed consisting in the determination of the pore size D₁ in a compact (-_c=25–45%), followed by the calculation of the mean size of agglomerated particles with Kozeny's formula. The densification of compacts from tungsten powders of 4-m particle size at sintering temperatures of about 0.6 T_melt is a result of decrease in the number of pores and increase in the equivalent size of agglomerated particles. In this process the mean pore section size determined by the metallographic method remains unchanged during sintering, which corresponds to a statistical model of a porous solid in the rheological theory of sintering.Translated from Poroshkovaya Metallurgiya, No. 12(312), pp. 24–31, December, 1988.     

Physicotechnological properties of VA grade tungsten powders

Particle size properties for the VA tungsten powder grade have been determined, using the Fischer method, scanning electron microscopy, and liquid displacement from pore channels, and analyzed. It can be shown that the particle size properties and material compaction during sintering, as well as the porous material structural parameters, can be correlated. A specification (TU 48-4206-413-89) has been established for the VA tungsten powder grade with a specific particle size.Translated from Poroshkovaya Metallurgiya, No. 11(359), pp. 11–16, November, 1992.     

Volume changes accompanying the sintering of compacts from mixtures of titanium and iron powders

Conclusions The sintering of compacts from mixtures of titanium and iron powders at temperatures below the eutectic point is accompanied by their shrinkage, the sintering process being mainly determined by the particle size of the starting powders. During the sintering of compacts from mixtures of titanium and iron powders at temperatures exceeding the eutectic point the compacts grow in size, the extent of the growth depending on the particle size of the powders. This phenomenon may be attributed to the effect of heterodiffusion on sintering processes and to the crystallization pressure generated during the formation of intermetallic compounds.Translated from Poroshkovaya Metallurgiya, No. 5(233), pp. 17–21, May, 1982.     

Effects of sintering conditions on the physicomechanical properties of porous tungsten

Conclusions An investigation into the sintering of porous tungsten has shown that good-quality parts can be produced by sintering compacts pressed from mixtures of nickel-coated (1 wt.%) tungsten and filler powders in hydrogen at 1250°C. Compressive strength determination may be recommended as a method of assessing the mechanical properties of porous materials sintered from powders of brittle metals.Translated from Poroshkovaya Metallurgiya, No. 10 (154), pp. 31–33, October, 1975.     

Effect of nickel on sintering processes in the Ti-Fe system. Part I

Conclusions Diffusional interaction in the Ti-Fe-Ni system is more vigorous than that in the Ti-Fe system. The temperature of the first ternary eutectic point in the Ti-Fe-Ni system, determined by the contact melting method, is 1040°C. During the sintering of compacts from a mixture of Ti, Fe, and Ni powders at temperatures above the eutectic point growth is observed, whose extent depends on the sintering temperature and the particle size of the powder employed: The smaller the particle size and the higher the sintering temperature, the smaller is the growth.Translated from Poroshkovaya Metallurgiya, No. 7(247), pp. 34–39, July, 1983.     

Particle growth during the reduction of tungsten and molybdenum and activation of the sintering of these metals

Conclusions Particle growth during the reduction of tungsten and molybdenum powders and the shrinkage kinetics of these powders depend on two processes taking place simultaneously, namely, partial melting and vaporization of surface layers of the dioxides of these metals. During high-temperature reduction (> 1100°C), fine particles of tungsten and molybdenum dioxides are vaporized and redeposited on large particles; on the surface of the particles, a liquid phase may form, resulting in the generation of capillary forces which create a bond between separate particles. Melting of the oxides covering the surface of metallic tungsten and molybdenum particles promotes shrinkage during sintering, while conversely vaporization of the oxides inhibits the shrinkage of these metals.Translated from Poroshkovaya Metallurgiya, No. 9(69), pp. 34–38, September, 1968.     

Sintering of very fine molybdenum and tungsten powders

Conclusions Vacuum annealing at a temperature above 900°K enables the specific surfaces of very fine loose tungsten and molybdenum powders to be varied in a wide range. The vacuum sintering of compacts pressed from very fine (particle size less than 0.05m tungsten and molybdenum powders is accompanied by severe cracking. In the hot pressing of very fine Mo and W powders produced by the pyrolysis of carbonyls in a stream of high-temperature plasma, a specimen density close to theoretical is reached at 1600°K i.e., at a temperature not less than 400°K lower than the sintering temperatures of powders of particle size more than 1 m. Sintering lowers the amounts of carbon and oxygen in Mo and W by more than half compared with the starting condition.Translated from Poroshkovaya Metallurgiya, No. 1(229), pp. 47–51, January, 1982.     

Effect of the method of production of porous powder metallurgy materials on their capillary properties (review)

The unique features of liquid infiltration in capillary porous powder metallurgy materials (CPPM) are discussed. The effect of the production method on the maximum height and rate of capillary rise of a liquid in CPPM is clarified. It is shown that in materials with a uniform pore structure, and the same average pore size, the maximum value of H_max is attained when the form factor of the initial powder particles is a minimum. The highest values of liquid infiltration rate at closely similar heights of capillary rise (within 5–7%) were obtained in CPPM with a variable pore distribution prepared from mixtures of powders with dtiferent particle sizes (pseudo fluidization method), or in CPPM with a biporous structure prepared by the sintering of a previously oxidized powder, by compacting metal powder with a pore forming agent, as well as by the cathodic deposition of a porous copper structure from electrolytic solution.Scientific Research Institute for Powder Metallurgy, Minsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 30–39, May-June, 1996. Original article submitted July 14, 1994.     

大尺寸高相对密度钨管的制备

大尺寸高相对密度钨管制品对于石英熔炼行业具有重要的应用价值,为了克服大尺寸钨管在制备过程中易出现压坯开裂和相对密度不高的问题,对不同粒度钨粉进行了掺混实验,研究了混合时间对压坯强度的影响.研究表明,对掺混钨粉进行气流破碎处理,可以有效地缩小粒度分布范围,提高颗粒均匀性.通过对压制压强和保压时间的研究,发现压制压强>23...     

Effect of reduction parameters on the particle size of tungsten monocarbide obtained from the blue tungsten oxide

Conclusions A study was made, using the methods of particle size analysis and metallographic examination, of the effects of temperature and bed thickness in the reduction of the blue tungsten oxide upon some properties of tungsten and tungsten monocarbide powders and sintered 6 wt.% Co hard alloys produced from the monocarbide powders. Quantitative results were obtained showing the relationship between these reduction parameters on the one hand and the particle size of the resultant powders and the physicomechanical properties of hard alloys on the other. It was established that the particle size grows with rise in reduction temperature and with increase in the thickness of the blue oxide bed.Translated from Poroshkovaya Metallurgiya, No. 11(143), pp. 102–107, November, 1974.     

Properties of R6M5 steel powders of various particle sizes and the structure of the steel in the sintered condition

Conclusions An investigation was carried out into the effect of particle size of powders produced by comminution of R6M5 high-speed steel waste (swarf) on their compressibility and sinterability. It is shown that with decreasing mean particle size the compressibility of such powders deteriorates, but their sinterability improves. The finer the steel powder, the lower is the sintering temperature necessary for the attainment of a given density. Varying the powder particles size brings about marked changes in the microstructure of the steel in the sintered condition. The liquid phase (ledeburitic eutectic) appearing during the sintering of steel powders of different particle sizes varies in form and distribution. With coarse powders (d_m=100 m) the eutectic is similar to that in cast steel, but with powders of small particle size (d_m=22 m) it is finer and evenly distributed throughout the structure in the form of thin lamellae.A 0.85%C-6% W-5% Mo-4% Cr-2% V (nominal composition) high-speed steel — Translator.The authors wish to thank A. N. Nikolaev of the Gorkii Polytechnic Institute for helpful discussion in the experimental part of the work.Translated from Poroshkovaya Metallurgiya, No. 6(234), pp. 9–15, June, 1982.     

The effect of tungsten particle size on the processing and properties of infiltrated W-Cu compacts

Three tungsten powders with average particle sizes of 8.7, 23.2, and 65.2 μm were used to make W-15Cu compacts. The compacting pressure and sintering temperature were adjusted for each powder to attain the desired skeleton density. Sintered skeletons were then infiltrated with oxygen-free copper at 1200 °C in hydrogen and in vacuum. Results showed that as the tungsten particle size decreased, higher compacting pressures and sintering temperatures were required for the same desired skeleton density. The processing parameters and the tungsten particle size caused variations in the amount of closed pores and the W-W contiguity, which in turn resulted in different infiltrated densities and resistivities. Direct infiltration on green compacts was also examined, and higher infiltration densities and lower electrical resistivities were obtained compared to those obtained by infiltrating sintered compacts. These results are discussed based on infiltrated density, differences in microstructure, and the W-W contiguity.     

Liquid-phase sintering of very fine tungsten-copper powder mixtures

Conclusions The sintering of tungsten-copper powder mixtures of low copper content can be intensified by employing tungsten powders of submicron particle size. The process of liquid-phase sintering of very fine tungsten-copper powder mixtures can be quantitatively described within the framework of the theory of local heterogeneous deformation of a polycrystalline solid with allowance for diffusional grain shape accommodation.Translated from Poroshkovaya Metallurgiya, No. 6(234), pp. 27–31, June, 1982.     

Regularities of sintering of zirconium diboride-molybenum alloys

Summary In the process of sintering of mixtures of zirconium diboride with 5, 10, and 15% Mo, specimen growth resulting from heterodiffusion is observed at the instant of formation of solid solution of Mo in ZrB₂ during slow heating to high temperatures or during the initial period of isothermal holding in the case of very rapid heating. At temperatures of up to 1700–1750°C, growth predominates over shrinkage, and specimen dimensions increase with increasing holding time; at temperatures above 1800°C, positive shrinkage takes place, but is very slight in the case of rapid heating to the isothermal holding temperature.During isothermal holding in the temperature range 1800–2200°C, very intensive shrinkage is observed during the initial period (20–30 min). Subsequently, this shrinkage slows down, and may be described as viscous flow caused by diffusional processes. The energy of activation of the densification process, calculated from the shear viscosity values obtained, was found to be 367±48, 352±28, and 379±46 kJ/mole for alloys of ZrB₂ with 5, 10, and 15% Mo, respectively, i.e., less than the energy of activation of densification of zirconium diboride (678±55 kJ/mole).Thus, the presence of molybdenum activates diffusion processes during sintering.     

Hard Alloy WC – 24% Ni Obtained in the Solid Phase from Ultrafine WC, NiO, and C Powders. Part 1. Density and Structure of Specimens

We have studied the density and structure of specimens of the alloy WC – 24 mass% Ni, obtained by combining into one step the processes of synthesis of the metallic phase and compaction of the ultrafine mixture of WC – Ni powders by high-energy pressing and sintering. We have established that reduction of nickel monoxide by carbon occurs at temperatures of 650-750°C and does not affect the shrinkage process which in the case of sintering begins only at a temperature of 1050°C. High-energy pressing of briquettes sintered at the indicated temperature reduces their porosity from 30-25% down to 8-4%. Specimens of porosity <1% can be obtained by pressing at 1150°C or 1050°C in the case of triple pressing. Raising the temperature at which the briquettes are heated is accompanied by enlargement of the pores together with a decrease in the total porosity, but at temperatures of 1300°C (sintering) and 1250°C (pressing), the pore dimensions are sharply reduced. The high density of the specimens pressed at low temperature does not provide low electrical resistance, which suggests the presence of weakly connected boundaries. When the specimens are sintered and pressed in the solid phase, we observe the growth of tungsten carbide particles. It is most rapid at 1150-1250°C, while at 1050°C the particle growth process slows down. Reduction of the metal oxide when the powders are heated promotes formation of structure in the higher temperature range.     

Effects ofcontaminationonproperties of W-15Cu preparedfrom mechanically alloyed powders

Abstract

To eliminate the contamination of activator elements, such as Fe and Ni, W-15Cu compacts were prepared from mechanically alloyed powders using an attritor with a zirconia tank, balls and agitator arms. Coarse tungsten and copper powders, 9·9 μm and 13·3 μm, respectively, were milled to 1·26 μm composite powders after 145h of milling. The milled powder contained little free copper and was highly combustible in air. After sintering, the 50 vol.-% dense green compacts attained a density of 15·8g cm^-3 or 96·2%. The microstructure consists of uniformly interdispersed tungsten and copper. When stainless steel grinding balls were used, the powder was heavily contaminated with Fe and Ni. The contamination improved the density slightly, but the grain size and the electrical resistivity increased significantly as well. The sintering behaviours of the two composite powders were similar. Most densification occurred during heating before reaching the melting point of copper.     

Microstructural change during liquid phase sintering of W−Ni−Fe alloy

The changes of bulk density and microstructures during heating and liquid phase sintering of 98W-1Ni-1Fe compacts prepared from 1 and 5 μm W powders have been observed in order to characterize the densification behavior. The compact prepared from a fine (1 μm) W powder begins to densify rapidly at about 1200°C in the solid state during heating, attaining about 95 pct density upon reaching the liquid phase sintering temperature of 1460°C. The compact prepared from a coarse (5 μm) W powder begins to densify rapidly at about 1400°C in the solid state, attaining about 87 pct density upon reaching the liquid phase sintering temperature. Thus, the skeleton of grains is already formed prior to liquid formation. During the isothermal liquid phase sintering, substantial grain growth occurs, and the liquid flows into both open and closed pores, filling them sequentially from the regions with small cross-sections. The grains subsequently grow, into, the liquid pockets which have been formed at the pore sites. The sequential pore filling by first liquid thus is shown to be the dominant densification process during the liquid phase sintering of this alloy, as has been demonstrated earlier with spherical model pores and as predicted theoretically.     

Permeability of porous materials from refractory compounds

Summary A study was made of porous materials from spheroidized powders of refractory compounds. A relationship is established linking permeability with porosity and the particle size of the powder from which the materials were prepared. An empirical formula is proposed for determining the dependence of the coefficient of permeability on powder particle size and component porosity under linear filtration conditions. The integral and differential pore radius distribution curves obtained show that such materials are relatively homoporous and exhibit little scatter in pore radius size.Translated from Poroshkovaya Metallurgiya, No. 1(49), pp. 27–30, January, 1967.