Showing papers in "Powder Metallurgy in 1972"
TL;DR: In this paper, the authors show that the carbide contiguity in a given alloy develops towards an equilibrium value, which is directly related to the dihedral angle between carbide grains and therefore to the relative values of the interfacial energy and the contiguous boundary energy.
Abstract: Measurements have been made of the contiguity, the dihedral angle, and the continuity of the carbide phase in alloys of a number of transition-metal carbides with cobalt, which were prepared by liquid-phase sintering. During sintering, the carbide contiguity in a given alloy develops towards an equilibrium value, which is directly related to the dihedral angle between carbide grains and therefore to the relative values of the interfacial energy and the contiguous boundary energy. A direct relationship between the contiguity and the angle of contact of the liquid on the carbide, however, does not exist. The contiguity at the start of sintering depends to some extent on the history of the specimen before liquid-phase sintering and in most cases is higher than the equilibrium value. Observations were made which indicate that the carbide grains in the majority of cemented carbides exist as a continuous skeleton. Arguments are presented to show that neither the presence of such a skeleton nor the prese...
138 citations
TL;DR: The binder phase of WC-Co cemented carbide is a Co-rich alloy in which tungsten and carbon are in solid solution as mentioned in this paper, and it is supersaturated with respect to Tungsten even after slow cooling from the sintering temperature.
Abstract: The binder phase of WC-Co cemented carbide is a Co-rich alloy in which tungsten and carbon are in solid solution. The binder phase is supersaturated with respect to tungsten even after slow cooling from the sintering temperature. In this study the binder phase contained 6% W in solid solution before heat-treatment at 650°C (923 K). Transmission electron microscopy on thin foils of binder phase showed that a finely dispersed. phase, α′, precipitated in the cubic binder phase. After long ageing times Co3W could be identified by X-ray diffraction methods. As no discontinuous , ‘cells’ of Co3W and ∊-Co could be observed, the following reaction is suggested.α-Co(W, C)→α-Co+α′→α-Co (C)+Co3W (needles)The precipitation of α′ was accompanied by an increase in hardness and a decrease in transverse rupture strength. The effects observed are consistent with those found during annealing of Co-rich cobalt-tungsten-carbon alloys (> 85% Co).
29 citations
TL;DR: In this paper, modern atomizing techniques and parameters are reviewed in terms of their economics and influence on powder properties, chemistry, ultimate structural control, property reliability, and alloy and process flexibility, especially with materials difficult to fabricate.
Abstract: Reduced iron, copper-base, and refractory metal powders have dominated the raw-material field during the rapid growth of the PM industry in the last 30 years. Emphasis has been on the economic mass production of structural parts, rather than on properties. New material concepts have brought about the need for new and improved powders possessing enhanced properties and performance reliability, i.e. atomized powders. Modern atomizing techniques and parameters are reviewed in terms of their economics and influence on powder properties, chemistry, ultimate structural control, property reliability, and alloy and process flexibility, especially with materials difficult to fabricate.
28 citations
TL;DR: More than a hundred experiments have been performed to investigate the extrusion mechanism involved and the effect of process variables on the structure, substructure, and mechanical properties of the extrude when processing atomized aluminium powder of varying mesh size.
Abstract: More than a hundred experiments have been performed to investigate the extrusion mechanism involved and the effect of process variables on the structure, substructure, and mechanical properties of the extrude when processing atomized aluminium powder of varying mesh size. It is demonstrated that extrusion pressure is largely independent of the final properties and that the ductility of the extrude may be controlled by the process variables. A mechanism that agrees with experimental observations is suggested to explain the extrusion mechanics. Transmission and scanning electron microscopy results also support the theses presented.
24 citations
TL;DR: In this paper, a reduced iron powder mixed with a number of zinc stearates having median particle sizes between 4 and 22μm was used for laboratory compaction and ejection studies.
Abstract: Laboratory compaction and ejection studies have been made using a reduced iron powder mixed with a number of zinc stearates having median particle sizes between 4 and 22μm. Comparable experiments were carried out on a fully instrumented production press, which was operated at compacting pressures between 300 and 500 MN/m2 to produce compacts with true densities ranging from 5·90 to 6·70 g/cm3. Determination of ejection forces by the two methods enabled calculations of the coefficients of friction between compact and die wall to be made for mixtures containing 0·5–2·0 wt.% zinc stearate. These showed that the behaviour during compaction and ejection was comparable on both laboratory and production scales and gave very similar results. An interpretation of the results is given and values of coefficients of friction are presented which show that these are dependent on the type of zinc stearate used.
11 citations
TL;DR: In this paper, it was found that growth was most sensitive to: phosphorus content of copper, fineness of tin, synthetic graphite, addition of prealloyed bronze, green density, heating rate, furnace atmosphere, and sintering temperature.
Abstract: 90/10 bronze components are generally manufactured from mixes based on elemental copper and tin powders, which invariably exhibit growth under production sintering conditions. Previous work has shown that many factors affect growth through their influence on the alloying of tin and copper. Experimental work on atomized copper and tin powders has provided quantitative information on the various factors of significance to growth. It was found that growth was most sensitive to: phosphorus content of copper, fineness of tin, synthetic graphite, addition of prealloyed bronze, green density, heating rate, furnace atmosphere, and sintering temperature. Mechanisms for growth are proposed and preferred methods of growth control suggested.
9 citations
TL;DR: In this article, the sintering of porous bodies for catalytic purposes requires a compromise between strength and specific surface properties, since temperature and time have opposing effects on these properties.
Abstract: The sintering of porous bodies for catalytic purposes requires a compromise between strength and specific surface, since temperature and time have opposing effects on these properties. The present investigation on fine powder compacts of Cr2O3 and W was carried out to establish the conditions under which specific surface and strength could be optimized. Hence, sintering temperature and atmosphere were varied. Cylindrical specimens were used to measure specific surface (BET), permeability, and indirect tensile strength. Pore and surface structure were studied by electron-microscopic methods. It has been shown that sintering in nitrogen yields the best results as far as the combination of strength and accessible surface area is concerned. However, if high permeability and high strength are required, hydrogen is the best sintering atmosphere.
8 citations
TL;DR: In this paper, it was shown that the sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds.
Abstract: Adsorptive studies of the surfaces of graphite and MOS2have shown that these consist of two distinct types of site. The sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds. These findings led to the development of grinding techniques to produce graphite and MoS2 powders possessing different ratios of basal-plane:edge-surface area.Grinding graphite and MoS2 in the presence of low-viscosity, volatile hydrocarbons produced very thin flake-like powders, consisting predominantly of basal-plane surface. These fine flakes showed a high affinity for long-chain n-paraffins and were therefore termed oleophilic solids. Grinding under reduced pressure also produced very fine powders, having, however, a more granular structure exhibiting a far lower ratio of basal-plane: edge-surface area. These were termed polar solids to distinguish them from the solids ground in liquid hydrocarbons.The cold-forming properties of the...
6 citations
TL;DR: In this paper, the effects of oxygen, hydrogen, and carbon on tungsten, rhenium, iridium, and iron surfaces were investigated at the atomic level using a model of a small particle of a metal powder.
Abstract: The specimen in a field-ion microscope is in the form of a fine wire,elcctropolished to a sharp point. The observed end region of the specimen is roughly hemispherical, with a radius of curvature in the range 200–1500 A. The end region may be considered as a model of a single (small) particle of a metal powder. This can be cleaned, contaminated with various gases, oxidized, heat-treated, or coated with another metal, as desired. The resulting changes in surface morphology, the occurrence of surface reactions, and the nature of surface-transport processes can be studied at the atomic level. Examples of this approach are drawn from the authors’ work on tungsten, rhenium, iridium, and iron. The effects of oxygen, hydrogen, and carbon on these materials are described. A pronounced surface reaction when a tungsten specimen is coated with approximately one monolayer of nickel is reported. This is believed to be related to the phenomenon of the nickel-activated sintering of tungsten.
4 citations
TL;DR: In this article, the structure of porous nickel plaques produced by the following routes has been examined using scanning electron microscopy: (a) pressureless sintering of nickel powder; (b) slurry sintered of powder, (c) roll-compaction and sinting of powder; and (d) pyrolysis of cloth impregnated with nickel chloride.
Abstract: The structure of porous nickel plaques produced by the following routes has been examined using scanning electron microscopy: (a) pressure-less sintering of nickel powder; (b) slurry sintering of powder; (c) roll-compaction and sintering of powder; (d) pyrolysis of cloth impregnated with nickel chloride. The observations show that measurements of pore-size distribution with a mercury porosimeter can be misleading, and also reveal the existence of fine closed porosity in plaques made from impregnated cloth. The relation of the observed porosity characteristics and inter-particle bond diameters to the mechanical and electrical properties of the plaques is discussed.
4 citations
TL;DR: In this paper, different surface contact distributions and their influence on the sintering characteristics of some metal-oxide compacts are considered, and it is shown that the presence of chemically inert second-phase particles always hinders densification of the composite.
Abstract: In compacts containing metal and oxide particles, metal/oxide surface contacts modify the diffusional processes. Different distributions of such dissimilar-surface contacts and their influence on the sintering characteristics of some metal-oxide compacts are considered.By selecting appropriate volume fractions and diameters of matrix and second-phase particles, structures containing different types of dissimilar surface contact distribution have been produced. Isochronal sintering of these structures clearly demonstrates that (a) the presence of chemically inert second-phase particles always hinders densification of the composite, (b) the sintering densification (for a given volume fraction of second-phase particles) is a strong function of (dm/ds), where dm and ds are the diameters of the matrix and second-phase particles, respectively, and (c) the nature of dissimilar-surface contact distribution has a significant effect on the densification of the composite.
TL;DR: A study of the sintering behavior of iron compacts containing additions of tin up to 1 wt.-% has been made in this paper, where a tensile strength of 234 MN/m2 (34 x 103 lbf/in2) has been achieved with an optimum tin addition of 0·5 wt.%, with 10 min at 1100°C (1373 K) in a reactive halide atmosphere.
Abstract: A study of the sintering behaviour of iron compacts containing additions of tin up to 1 wt.-% has been made. A tensile strength of 234 MN/m2 (34 x 103 lbf/in2) has been achieved with an optimum tin addition of 0·5 wt.-%, sintering being carried out for 10 min at 1100°C (1373 K) in a reactive halide atmosphere. Combination of the two ‘activating’ techniques (addition of tin and sintering in a reactive atmosphere) permits current properties to be attained at considerably lower sintering temperatures or sintered densities, and is much more effective than when they are applied individually. A tensile strength of 165·3 MN/m2 (24 x 103lbf/in2), achieved by sintering at 1200°C (1473 K) for 10 min with an addition of 0·5 wt.-% tin can be obtained by reactive-sintering the same composition at 900°C (1173 K) for 10 min. Alternatively, the density of the part can be reduced from 6·7 to 6·2 g/cm3 with no loss of strength or elongation. Tin in excess of 0·5 wt.-% causes deterioration in properties under the si...
TL;DR: In this paper, the effect of particleize, compacting pressure, sintering temperatures, and furnace type on the dimensional changes of the Fe-Ni, Fe-Cu, and FeNi-Cu systems was investigated by using single pressing, double pressing, and hot-forging.
Abstract: Sintered alloys of the Fe-Ni, Fe-Cu, and Fe-Ni-Cu systems have been investigated by using single-pressing, double-pressing and hot-forging techniques. Different iron powders containing 0-6 wt.-% nickel or 0-5 wt.-% copper, also (for the ternary system) 0-5 wt.-% of both nickel and copper, were compacted and sintered and the effect of the additions on dimensional changes was studied. The influence of particlesize, compacting pressure, sintering temperatures, and furnace type on dimensions has been determined.
TL;DR: The strength of 18Ni 350 maraging steel produced from prealloyed powder has been found to be comparable to that of conventional, wrought 18Ni-350 MARaging steel as mentioned in this paper.
Abstract: The strength of extruded and heat-treated bar of 18Ni 350 maraging steel produced from prealloyed powder has been found to be comparable to that of conventional, wrought 18Ni 350 maraging steel. Impact toughness was considerably higher. In contrast, properties of bar Obtained from an elemental powder mix were markedly inferior to wrought 18Ni 350 maraging steel. The high toughness of Charpy V-notch specimens produced from prealloyed powder was partly attributed to the presence of continuous, aluminium-rich stringers.This resulted in delamination of the specimens during testing and led to a fibrous fracture. Charpy V-notch specimens produced from the elemental powders also contained stringers. However, these were not continuous and therefore did not cause delamination. The resulting fracture surface was flat and relatively featureless. Electron-microprobe analysis has shown that the morphology of the stringers is largely determined by their composition and the composition at the stringer/matrix int...
TL;DR: In this article, a method for predicting hard-metal contraction and powder preparation and pressing to achieve this end is described. But the authors do not specify the order of dimensional accuracy that can be expected in hard metal manufacture.
Abstract: All hard-metal alloys contract ∼50% by volume during sintering and, in addition, the continuous network of liquid phase makes them extremely weak. Hence, the two major problems in dimensional control are coping with this large contraction and at the same time preventing distortion during sintering.Green compacts of closely controlled and very uniform density are essential to ensure that the contraction is both predictable and uniform. Methods of predicting contraction and of powder preparation and pressing to achieve this end are described.Distortion results not only from mechanical weakness during sintering but also from the carburizing, decarburizing, oxidation, and reduction reactions that occur when the green compact is heated.In conclusion, indications are given of the order of dimensional accuracy that can be expected in hard-metal manufacture.
TL;DR: In this paper, the diabolo effect in sintered nuClear-fuel pellets is investigated as a function of the slope and position of the sinterability curve and the density gradient.
Abstract: As explained mathematically, the magnitude of the ‘diabolo’ effect in sintered nuClear-fuel pellets is a function of the slope and position of the sinterability curve and the density gradient existing in the green pellets. If sinterability is representedby a straight line passing through the origin, the diabolo effect is negligible; this condition can be fulfilled when using a deactivated powder.In any other case, the density gradient must be kept as low as possible to produce sintered pellets of regular shape. This is discussed as a function of powder roughness, lubricant quality, lubrication technique, and compaction characteristics.When the sinterability curve cannot be adapted and powder quality and lubrication conditions are fixed, the possibility still exists of minimizing irregularities in sintered diameter by using a single-action press and compacting the powder in an adapted conical die with a moving lower punch.
TL;DR: In this article, the effect of additions of narrow size-range fractions of atomized tin powder on the dimensional changes and tensile properties obtained on sintering Fe-Sn-Cu compacts made with either sponge iron or atomized copper powders has been determined.
Abstract: In view of increasing industrial interest in the use of tin additions as an aid to the sintering of iron-based powder compacts, an examination has been made of the influence of the characteristics of the tin powder on sintering performance.The effect of additions of narrow size-range fractions of atomized tin powder on the dimensional changes and tensile properties obtained on sintering Fe-Sn-Cu compacts made with –100 mesh (–152 μm) or – 300 mesh (– 53 μm) sponge iron and – 300 mesh (– 53 μm) atomized copper powders has been determined. The compacts contained tin and copper in the ratio 2:3. The narrow size fractions were separated from – 300 mesh tin powder by air elutriation. It was found that the use of coarse tin powder reduced the tensile strength of – 300 mesh iron-based Fe–1% Sn–1 ½% Cu compacts, but had no influence when this mixture was based on –100 mesh iron powder, or when the mixture composition was Fe–2% Sn–3% Cu. The effects have been examined in relation to the sintering mechanism...
TL;DR: In this article, the effect of billet design on the dimensions of the fuel core and cladding is analyzed with a view to its optimization, and the extent of particle deformation as a function of extrusion parameters is measured quantitatively by means of image analysis on particles having different inner porosities.
Abstract: Coextrusion has been used to prepare UO2 (35 vol.-%)-stainless steel cermet fuel-pins (5·6 mm dia. × 1000 mm long), which could be used as fuel elements in fast-flux test reactors. The procedure required to obtain a nearly homogeneous distributionof the fissile phase in the matrix is described. The influence of billet design on the dimensions of the fuel core and cladding is analysed with a view to its optimization. The extent of particle deformation as a function of extrusion parameters has been measured quantitatively by means of image analysis on particles having different (12·7%; 5·8%) inner porosities. Thermal expansion and conductivity as well as tensile strength of the cermets are reported and compared with existing data.
TL;DR: In this paper, a flotation method is described that enables the density and porosity of small specimens to be measured accurately by comparing their densities with those of reference specimens, using the subtle but accurately known, variation of the density of water with temperature to bring about neutral buoyancy of a specimen-float composite.
Abstract: A flotation method is described that enables the density and porosity of small specimens to be measured accurately by comparing their densities with those of reference specimens. The method uses the subtle, but accurately known, variation of the density of water with temperature to bring about neutral buoyancy of a specimen-float composite. Measurements can be made easily and quickly and the expressions derived for density and porosity are readily evaluated numerically.