SINTERING OF METAL POWDER COMPACTS CONTAINING CERAMIC OXIDES

Abstract

The dimensional change during sintering of cobalt and nickel powders containing an oxide dispersion has been studied. Generally, the presence of MgO or CaO in cobalt and nickel retards the densification of the metals, and this retarding influence has a maximum for a certain proportion of oxide in the metal. The wettability between the dispersed phase and the metal may explain differences in sintering behaviour.     

THE HOT EXTRUSION OF METAL POWDERS

Abstract

The procedure of consolidating metal powders by isostatic compaction, canning, and hot extrusion is described. The properties of a number of nickel. base alloys and stainless steels prepared by this method are given.

It is concluded that, even when no precautions are taken to eliminate air (oxygen) from the canned billet, the resulting oxide content of the extruded bar is insufficient to exert a serious influence on the mechanical properties. The corrosion-resistances of certain stainless-steel samples, extruded from powders, were found to be somewhat lower than the corrosion-resistance of equivalent conventionally wrought products.     

THE PROPERTIES AND SOME APPLICATIONS OF CARBONYL-NICKEL POWDERS

Abstract

Nickel carbonyl, formed by the reaction of carbon monoxide with a nickel concentrate, can be decomposed in a heated space to produce pure nickel in powder form. By varying the decomposition conditions it is possible to achieve a wide range of powders, differing in particle shape and size distribution.

The properties of carbonyl-nickel powders now commercially available or in an advanced stage of development are discussed, and it is shown how such powders can be used for some applications in which specific properties are required.     

Swelling of Precipitated Nickel Powder

Abstract

When nickel powders produced by precipitation are heated at elevated temperatures, cracks and voids form within the particles causing them to swell. This swelling has been correlated with the liberation of gases generated by trace impurities in the metal. Using gas volumetric, gas density balance, infrared, and microchemical analyses, carbon dioxide and nitrogen have been identified as the gases mainly responsible for the swelling when the powders are heated in nitrogen. On the other hand, hydrogen sulphide has the major influence when they are heated in hydrogen.     

THE PROPERTIES OF POROUS NICKEL PRODUCED BY PRESSING AND SINTERING

Abstract

The effects of compacting pressure and of sintering temperature and time on the properties of porous sintered nickel compacts have been studied, using three carbonyl and two reduced nickel powders. For all five powders, the density of the green compacts and the porosity of the sintered compacts were linearly related to the log compacting pressure. Similar relationships with pressure were observed for strength and electrical conductivity.

Photomicrographs of sections through the sintered compacts made from the reduced nickel powders show that there are pores in two different size ranges, originating from the porosity between the original powder particles and the pores within the particles. It is concluded that sintered compacts from all five powders containing 40–50% porosity have adequate strength and conductivity for use in fuel-cell electrodes.     

PRODUCTION OF SOME METAL POWDERS BY HYDROMETALLURGICAL PROCESSES

Abstract

Nickel, cobalt, copper, and iron powders can be manufactured by hydrometallurgical processes. It is possible to use a wide variety of materials, including waste solutions, as the metal-containing feed. Estimates of the capital cost of reduction autoclaves and ancillary equipment show the advantage of increasing the scale of the operation from 2 to 50 tons of metal produced per day.

A major factor in the economics of producing the powders is the cost of purifying the solutions to give a liquor from which metal of the desired purity can be precipitated. Liquid–liquid extraction does not appear to have been used so far in a refinery in which metal is precipitated by hydrogen. The possible application of the technique is examined briefly.

Metal powders can be precipitated directly by reducing with hydrogen some organic phases produced by liquid–liquid extraction of aqueous solutions containing several metals. A suitable solvent mixture is acarboxylic acid with hydrocarbon diluent. The total pressure necessary for reduction is lower than that required when water is the solvent.

An important factor in considering the use of hydrometallurgical methods for producing metal powders is that in some cases the cost of obtaining metal is competitive with any other kind of process, and the fact that it is formed as powder is an added advantage. Also it is possible to make powders with physical characteristics that can be controlled withinnarrow limits over a very wide range.     

INFLUENCE OF THIN OXIDE FILMS ON THE MECHANICAL PROPERTIES OF SINTERED METAL-POWDER COMPACTS

Abstract

The influence of thin oxide films, in the range 200–1200Å thick, on the mechanical properties of sintered iron, copper, and nickel powder compacts has been investigated. As the thickness of the oxide film on the metal powders increased, the properties studied, namely, densification parameter, hardness, and tensile strength improved and attained a maximum at a critical oxide-film thickness, the value of which was ～ 625 Å for iron and nickel and ～ 500 Å for copper. Further increase in thickness to ～ 1200 Å led to a gradual decline in the properties. The improvement in the properties obtained with powders having the optimum oxide thickness was independent of the sintering atmosphere. A probable explanation in terms of activated sintering is given.     

FEASIBILITY OF PRODUCING DISPERSION-STRENGTHENED CHROMIUM BY BALL-MILLING IN HYDROGEN HALIDES

Abstract

Chromium, with and without 4 vol.-% thoria, and nickel powders were ground to fine powder sizes by ball-milling in gaseous hydrogen halides. After reducing the milled chromium in flowing hydrogen under pulsating pressure at ～680°C, submicron-size powders with 4–500 ppm residual halogens were obtained. The compacted chromium–thoria alloys had interparticle spacings ranging from 2·1 to 6·5 μm. After 100 h at 1318°C the interparticle spacing of the 2·1-μm alloy increased to 5·2 μm. Submicron-size chromium and nickel powders were also obtained by pulsating hydrogen reduction of their chlorides.     

CONTINUOUS COMPACTION BY CYCLIC PRESSING

Abstract

A process recently developed produces wide, thick bars of unlimited length from powder, granular, or sponge raw materials by a cyclic pressing operation. The process can be adapted readily for laboratory study or for large-scale production.

Many materials have been pressed by this technique, and in all cases compaction was similar to that experienced in conventional pressing. Work on the pressing, sintering, and subsequent working of various nickel powders and of a nickel-iron-molybdenum magnetic alloy has been carried out. Carbonyl nickel powder produces nickel strip of high quality. A high-permeability alloy containing nickel 79, iron 17, and molybdenum 4% can be made into strip with good magnetic properties by compacting the powders with the cyclic-pressing technique and then sintering and rolling the pressed bar into strip.     

THE PREPARATION AND PROPERTIES OF COPPER,NICKEL, AND IRON CONTAINING A DISPERSED OXIDE PHASE

Abstract

Three methods are described for the preparation of a stable oxide in copper, nickel, or iron. The first, which has been applied to all three metals, requires a reaction between the matrix metal oxide and the refractory oxide at high temperatures such that subsequent reduction will yield a dispersion of oxide in the matrix metal powder. In the second method, copper hydroxide or carbonate is precipitated in the presence of an aluminium salt. Ignition of the precipitate to the oxide followed by hydrogen reduction gives a copper powder containing a dispersion of alumina. In the third process, a salt that can be decomposed to a stable oxide is dissolved in molten nickel nitrate, which on rapid cooling eitherretains the additive in solid solution or as an intimate dispersion. Nickel powder containing the dispersed oxide is obtained on subsequent decomposition.

The results on sintered and worked materials show a substantial improvement in mechanical properties compared with the pure metals. Improvements are also achieved in the sintered nickel alloys, which can be sintered to high densities. The effect of the dispersion on the structure and recrystallization behaviour of these materials is discussed.     

PRESSURE-DENSIFICATION OF POWDER COMPOSITES

Abstract

A preliminary investigation of the pressure-densification behaviour of iron and nickel powders both with and without added alumina or zirconia particles has been made. The importance of some relevant parameters is assessed in the light of experimental results obtained on different powder composites, prepared by mechanical mixing of powders followed by compaction in a die using a hydraulic press.

The degree of pressure-densification decreases with increasing volume fraction of the added oxide particles, the decrease being also a function of the matrix and oxide particle-size ratio. These data are interpreted in terms of reductions in the number and area of the contact surfaces deforming as a result of the presence of non-deforming particles.     

Euro PM 2014: special interest seminars

Abstract

It is shown that metal powder and particle technology extends beyond conventional Powder Metallurgy to include the hot working of metal powders and several integrated processing routes to semi-finished products involving metal powder or particles. The salient features of the processes are examined and an assessment is made of likely future developments in each case.     

Effects of nickel addition on properties of Ag–W electrical contact materials

Abstract

Tungsten and silver powders doped with small amounts of nickel were milled in a heavy duty attritor for 150 min to constitute the W–35 wt-%Ag composition. Milled powders were compacted, sintered, furnace cooled, and re-sintered using industrial conditions. Scanning electron microscopy (SEM) investigations revealed homogeneous dispersions of silver and tungsten regions both in the as milled powders and the as milled compacts. The X-ray results taken from the milled powders and the as milled compacts showed the presence of the characteristic W and Ag peaks. Bulk hardness values of the sintered compacts increased with increases in nickel additions. The arc erosion characteristics of the W–35 wt-%Ag com pacts indicated that the erosion rate of the contact material declined with nickel additions. Contact life expectancy increases about 10 fold for the W–35 wt-%Ag compact containing 1 wt-%Ni from the one with no Ni content.     

Production of Metallic Oxide and Metal Powders by Crystallization Stripping Applying Solvent Extraction

Abstract

The fundamental studies on production of metallic oxide and metal powders with crystallization-stripping using NH₄HF₄, (NH₄) ₂S04 and pressurized C0₂ gas as crystallization-stripping agent in solvent extraction were carried out in order to develop a new production method of fine powders.

The reaction significantly relates to the stripability of the extracted metal tons and the solubility of the metal compounds formed in crystallization process. Furthermore, the reaction depends on the various factors, such as extractant concentration, metal concentration in the organic phase, the sort of stripping solution and still more CO₂ pressure in the pressurized CO₂ process.

The resultant crystallized products can be converted to fine powders of high-purity metallic oxide and metal in micron order by thermal decomposition in air or hydrogen atmosphere, respectively.     

Production of rapidly solidified metal powders by water cooled rotating disc atomisation

Abstract

In this study production of rapidly solidified metal powders by water cooled rotating disc has been investigated. A rotating disc was cooled by water which was not in contact with the molten metal and the produced powders during atomisation. This method is an effective process to produce rapidly solidified powders and ribbons.

Tin, lead, aluminium, zinc, and AA 2014 aluminium alloy powders and ribbons were produced using different types of discs. Parameters such as disc rotation velocity, disc shape, diameter of liquid metal stream, and superheat were investigated. The results showed that increasing disc rotation velocity, decreasing liquid metal stream diameter, and increasing superheat resulted in finer particles as expected.

Different types of discs such as flat, flat with fins, and cone with fins were used to investigate the effects of the disc geometry. Performance of the flat type disc with rectangular fins was better than that of the others.

Metallographic examinations showed that estimated cooling rates of atomised AA 2014 powders were between 10⁴–10⁵ K s^-1 depending on particle size. With the ribbons cooling rates were relatively higher, of the order of 10⁵–10⁶ K s^-1.     

THE IMPORTANCE OF SURFACE PROPERTIES AND PARTICLE SHAPE IN THE COMPACTION OF GRAPHITE AND MoS2

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 various powders have been compared under uniaxial compaction. The conversion of synthetic and natural graphite powders to the oleophilic form resulted in marked improvements in both compact strength and modulus. Synthetic graphite converted to the polar form would not form a compact at cold-forming pressures up to 800 MN/m².

The cohesive properties of the oleophilic graphite powders were improved by heating to 900°C in hydrogen. Electrical-resistivity measurements showed that cold-formed oleophilic graphite compacts exhibited a marked anisotropy. The improved cold-forming properties of the powders are ascribed directly to improved cohesion via basal-plane site interactions, coupled with the facility of the flake powders to take up a preferential orientation during compaction in order fully to utilize the extensive basal-plane sites available for cohesion.

The differences between the oleophilic and polar forms of MoS₂ were less marked. It is believed that interparticle cohesive junctions are more readily formed via edge/edge interactions, and basal-plane junctions do not play as important a role in the cohesion of MoS₂ as in that of graphite.

The corrosion and abrasion of metal surfaces by graphite and MoS₂ have been examined. In all cases the powders converted to the oleophilic form showed reduced abrasive and corrosive characteristics when compared to similar powders converted to the polar form. These improvements are believed to result from the reduction of the possibilities of edge interactions with the metal surfaces.     

NOTES ON THE STABILITY OF ALUMINIUM OXIDE IN IRON,NICKEL, AND COBALT

Abstract

Alloys of iron, nickel, and cobalt containing 10 wt.-% alumina have been produced by the technique of mixing fine oxides of iron, nickel, and cobaIt (0·1μ particle dia.) with α-alumina (0·2μ particle dia.). The resultant mixtures were reduced in pure, dry hydrogen at f～ 500°C. After reduction, the powders were quenched in liquid nitrogen to render them non-pyrophoric. The powders were then ball-milled, pressed into slugs, canned, and hot extruded into rod at between 700 and 800°C.

Homogeneous dispersions of essentially unchanged alumina were produced within the metal matrices. The coarsening of the alumina with increasing temperature and time was then followed by hardness measurements and by metallographic means, utilizing both optical and electron microscopy. The kinetics of coarsening was most easily followed in the iron-alumina alloy and so this system was studied in greater detail than the nickel-alumina or cobalt-alumina alloys.

The coarsening of the alumina particles in iron follows a (time) growth law, essentially that predicted by Lifshits and Slezov (Zhur. Eksper. Teoret. Fiziki, 1958, 35,479; Soviet Physics JETP, 1959, 35, 331) for the coarsening of particles in a supersaturated solution.

It is concluded that the growth of alumina particles in iron is accomplished by coalescence, the large grains growing at the expense of the smaller grains. It is implied that nickel-alumina and cobalt-alumina also show the same growth phenomenon. The ability of such apparently stable dispersion-hardened systems to overage could restrict their use in high-temperature applications, a field where it had been hoped that such alloys would excel.     

Microstructure and properties of molybdenumprinciple-copper composite metal samples consolidated by plasma pressure compaction

Abstract

In this study, a composite metal powder mixture using molybdenum and copper powders was consolidated tonear theoretical density using the rapid consolidation technique of plasma pressure compaction. Rapid consolidation of the mixture of metal powders is an essential requirement for better microstructural control and mechanical properties in the consolidated product. The microstructure and hardness of the composite samples are compared with monolithic samples made by consolidating pure molybdenum powders under identical conditions. Microhardness measurements revealed an increase in hardness of molybdenum when mixed with small amounts of copper. The role of consolidation parameters on hardness and microstructural development is presented and discussed.     

A REVIEW OF SINTERED METAL BEARINGS: THEIR PRODUCTION,PROPERTIES, AND PERFORMANCE

Abstract

Five classes of sinter-based bearings are identified: steel-backed materials with compacted non-porous sintered linings for engine bearings and wrapped bushes; steel-backed materials with porous sintered linings impregnated with a plastic, intended for operation dry or with sparse lubrication; unbacked porous sintered-metal parts impregnated with oil to form self-lubricating bearings; unbacked non-porous sintered metal parts incorporating graphite; sintered polytetrafluoroethylene parts incorporating powdered metals. Methods of manufacture are outlined.

Techniques of bearing evaluation are described, the most useful being sophisticated, flexible test-rigs which approach closely the conditions of actual bearing applications.

The relative fatigue strengths of sintered engine-bearing linings are given and sintered copper-lead is shown to be equivalent in fatigue strength to cast linings.

The impregnation of sintered bronze linings with PTFE and lead yields a material with good un lubricated wear-resistance. The effect of load, rubbing speed, and other variables is described. A polyacetal lining bonded to a steel backing via a porous bronze interlayer gives a bearing material with good performance under conditions of sparse lubrication.

The relative merits of oil-impregnated porous metal bearings obtained by the pressing and sintering of copper, iron, or aluminium alloy powders are described. For optimum performance the bearing conditions should favour the formation of a hydrodynamic film of oil over the bearing surface.

The incorporation of graphite into fully compacted powder-metallurgy parts gives improved wear-resistance under dry and sparsely lubricated rubbing conditions.

PTFE parts incorporating metal powders can be moulded to finished size by powder-metallurgy techniques.