Showing papers in "Powder Metallurgy in 1976"

Densification Mechanisms in the Tungsten Carbide—Cobalt System

Abstract: An investigation has been made into the sintering of tungsten carbide in the presence of small amounts of various metal additives. It has been found that a very marked degree of densification occurs when a small amount (1 vol.-% or less) of iron-group metal is added to WC. This arises from a process of activated sintering due to enhanced surface and interfacial diffusion, which has been observed by other workers to take place when nickel or other Fe-group metals are added in small quantities to W. Tungsten carbide forms low-energy prismatic interfaces in the presence of cobalt and other Fe-group metals. Densification takes place by the close-packing of particles along irregular boundaries which comprise segments of these low-energy interfaces. This configuration is far removed from that required by the usual sintering criterion of minimum surface and interfacial area. The segments of low-energy interface set up activity/diffusion gradients which bring about rapid material transport to achieve this...

A Comparative Assessment of Explosive and Other Methods of Compaction in the Production of Tungsten—Copper Composites

Abstract: In the production of tungsten-copper composites by pressing and sintering mixed powders followed by hot working, or by liquid-phase infiltration techniques, difficulties arise in obtaining a fully dense material, particularly at tungsten contents > 70 wt-% W–Cu composites containing between 70 and 90 wt-% W have been made by various routes including liquid-phase sintering, hot forging, triaxial compaction, isostatic re-pressing, and explosive compaction, and a comparative assessment of their mechanical and electrical properties has been made In the case of composites containing 70 wt-% W, it has been shown that the maximum density achieved by liquid-phase sintering at up to 1753 K increases as the Cu particle size decreases A density of 95·5% theoretical was obtained using −350 mesh Cu powder Isostatic repressing at 1·4 GPa of liquid-phase sintered composites yielded a maximum density of 95·5% theoretical, irrespective of initial Cu particle size Triaxial compaction of the mixed powders wa

The Liquid–Phase Sintering of Titanium Carbide

Abstract: (1976). The Liquid–Phase Sintering of Titanium Carbide. Powder Metallurgy: Vol. 19, No. 3, pp. 162-170.

Wear of Sintered Metals

Abstract: A pin-on-disc technique is described which produces a graphical record of the whole duration of a wear experiment. Small changes to the experimental conditions produce large differences in wear rates which are related to oxidative and metallic wear mechanisms. This technique has been used to evaluate materials pressed and sintered under various conditions and to investigate the effect of alloying and oil impregnation. Increases in pressing pressure and sintering temperature improved the wear resistance. The addition of phosphorus in the form of a hard phosphide eutectic brought about an appreciable reduction in wear rate. The presence of lubricant impregnated into the pores of the sintered compact reduced the wear rate to a very low value until all the lubricant had been used up. A correlation is presented between wear of piston rings and cylinder liners in a diesel engine and wear of the same materials investigated on the pin-on-disc machine, on which it was possible to reproduce the wear mechani...

Pyrophoricity of Fine Metal Powders

Abstract: Facilities exist at AERE Harwell for the preparation and handling under inert atmospheres of fine metal powders in sizes down to 0·1 μm. Such powders may however require handling in air at some sta...

Compatibility Equations for the Powder-Forging Process

Abstract: The three principal modes of deformation encountered during powder forging are (i) axisymmetric compressiop, (ii) plane-strain compression, and (iii)re-pressing. To establish compatibility or continuity equations to describe the geometry of deformation of porous preforms during each of these processes, the authors resort to the artifice of assuming an apparent plastic Poisson's ratio (va), which experimental observation suggests is a function of the relative density. (p *) of the preform. However, since the rate of densification is dependent upon the degree of lateral restraint imposed by the mode of deformation, the empirical relationships connecting va and p* will vary accordingly. Compatibility equations are therefore derived for each mode of deformation, and their predictions compared against a selection of previously published experimental data. no lateral constraint; (ii) plane-strain compression, i.e. restraint in one lateral direction; and (iii) re-pressing, i.e. total constraint. For the initial part of this study it has been assumed that. frictionless, homogeneous compression occurs throughout, so that there is no barrelling, and where applicable, all points on the periphery of the preform make contact with the die walls simultaneously. Although this may not be strictly true in practice, the amount· of' unsupported lateral flow is generally so small that this assumption will not introduce appreciable errors. As a further consequence of homogeneous compression it seems reasonable to assume an average density for the preforms as given by p*, which is the ratio of actual density to solid density, so that the porosity is given by (I - p*). The approximate density changes which take place with a typical commercial iron powder during the various stages in the production of a powder forging are summarized in Table I.

Production of an Aluminium-Rich Magnesium Silicide Alloy from Atomized Powder

Abstract: 0_ Mg EXPERIMENTAL Material The air-atomized AI-Mg-Si (HE9) alloy powder, supplied by Borax Consolidated Limited, nominally contained 0·5 wt.-%Mg, 0·5 wt.-%Si, balance AI. Initially the powder particle size distribution was obtained using standard 200 mm BS sieves stacked on an Endrock mechanical shaker to sieve a standard weight of powder (175 g) for a specified time (1 h). The results are shown in Table I. Samples of the powder were analysed by the Imperial College Analytical Services Laboratory. The chemical composition is given in Table II. precipitates may together be expected to alter the heattreatment reaction. There appears to be no reported literature on this subject. The AIMg-Si system, when aluminium-rich, is among the most widely used aluminium alloys, having architectural and structural applications. The object of the present work was to examine the rate-controlling deformation mechanism, the nature of product densification, the alloy structure, and the effect of oxide dispersions on the ageing process.

The Effect of Cr, Co, and Sr Additions on the Strength of an Al–35%Si, Powder Alloy

Abstract: Hypereutectic AI-Si alloys with a refined primary silicon' phase' and containing up to 45%Si have been prepared in these laboratories by PM techniques.1-2 The rapidity of cooling during production of the powder by atomization, and the size of the powder particles themselves, effectively prevent growth of the primary Si. The eutectic is also considerably refined in the PM product. Hypereutectic AI-Si alloys have a low coefficient of thermal expansion (which decreases with increasing Si content), excellent . wear resistance, good machinability (when the primary Si and eutectic are refined), low density, and high strength/weight ratio. They have been used as ,piston alloys for some time3 and are attractive for other automotive applications such as piston rings and cylinder liners. They also have potential for use in applications where lightness and wear resistance are important, such as moving parts of business machines. . Although the tensile properties of the PM product compare favourably with those of cast material it was thought that the strength of the former could be improved even more by addition of the alloYingelements Cr, Co, and Sr.

Oxide-Dispersed Iron Alloys

Abstract: As part of a programme to develop novel iron-base superalloys, several oxide-dispersed pure iron alloys were prepared by the technique of mechanical alloying and hot extrusion of powders. The refractory oxides (AI 203, Ce02, Cr203, Si02, and Y203) were added at the 2 wt.-% level. Control alloys were also prepared without refractory oxide additions. The distribution of the oxide phase was observed metallographically in . as-extruded material and in material. exposed at 1250° C for 72 h. There was good correlation between .the observed microstructure and the roomtemperature mechanical properties. The dispersed oxides were extracted from the metal matrix' and identified by X -ray diffraction as mixed oxides with iron (spinels and perovskites) formed by reaction between the added refractory oxides and contaminant oxygen present during fabrication. The degree of oxide coarsening observed after heat-treatment varied with the oxide addition and was least for the chromiacontaining alloy. Attempts to correlate this effect with the parameters in the- Wagner equation, which is usually used: to describe such behaviour, proved unsuccessful. The coarsening could not be related to the known~thermodynamic stabilities of either the added. single oxides or the mixed oxides. However, a correlation with the melting temperatures of the mixed oxides was observed. Some theoretical implications, and practical consequences for the design of oxidedispersed-super alloys, are discussed.

Hot-Forged Copper Powder Compacts

Abstract: The effects of powder size, preform density, sintering treatment, preheat temperature, ..forging rate, and forging load on the properties and structure of hotforged copper compacts have been investigated. The most important effect is produced by the die temperature, irrespective of the preheat temperature, because of the chilling of the Cu preform before hot forging. Sintering time and powder size have smaller effects. Major variations which arise in the occurrence and extent of recrystallization during hot· forging are accompanied by changes in the rate of densification. The variation in mechanical properties is discussed in terms of the structural observations.

Glass-Fibre-Reinforced Aluminium Alloys Formed by Hot Extrusion: Part I: Preparation and Properties

Abstract: .The preparation, and properties of glass-fibrereinforced alloys' have been studied. The composites were prepared by the hot (720-820 K) extrusion of pressed billets, consisting ofa mixture of metal powder and glass particles. The softening-point of the glass used was· below..the temperature of extrusion, hence the glass particles were elongated .on extrusion to form discontinuous fibres. The tensile strength of composite extrudes produced in this way was significantly greater than the tensile strength of extrudes formed from metal powder only. The effect of glass content on the UTS, 0·2% proof stress, Young's modulus, elongation, impact strength, and microstructure. of the composites was determined. The relationship with glass composition was also studied: effective reinforcement was only obtained using glasses of low softening-point and high boric oxide content.

Handling, Characterization, and Monitoring of Condensates Produced by Atomization from the Molten State

Abstract: In any process of atomization from the molten state it is inevitable that vapour species are produced. It has been found that these vapours condense to extremely small particles of the order of 0·04 μm dia. and larger. When such particles are produced under high-purity inert atmospheres, they are extremely pure, remain in suspension, and are therefore intensely pyrophoric when introduced to oxidizing atmospheres. In addition such particles are often toxic. The dusts resulting from the processing of ferrous, nickel, and titanium alloys have been characterized by electron microscopy in order to assess the problem of safe removal. The procedures developed to minimize the handling risks of such condensates are described in the context of operation of an arc-melting centrifugal atomizer. Sensitive monitoring techniques are available for specialized materials such as beryllium and uranium as well as the more common metals, and these can be utilized to assess the levels of both airborne and surface cont...

Hot Re-pressing of Powder Made from Mild Steel Machining Swarf

Abstract: Since economic studies indicate that powder produced from mild steel machining swarf in a cryogenic hammer mill is significantly cheaper than that made by any other method, it was decided to investigate whether powder-metallurgy products of acceptable properties could be obtained from such powder by a number of alternative methods. The powder was first isostatically compacted, either with or without prior heat-treatment, at room temperature at pressures of up to 1·38 GN/m2 (200000 Ibf/in2). The cylindrical preforms were then used to produce specimens by the alternative routes of sintering, or hot forging, or sintering followed by hot forging at small strains. The density and tensile properties of the .swarf powder metal were determined as a function of the process variables. By using certain processing routes the density closely approached that of the parent metal while elongation and ultimate tensile strength remained relatively lower, partly because the limited capacity of the forging machine restricted the plastic strain and hence bonding of the particles, partly owing to contamination of the swarfpowder. The presence of non-metallic inclusions caused considerable scatter in the results.

The Production of Mild Steel Rings by a Combined Powder Metallurgy and Cold Forging Process

Abstract: (1976). The Production of Mild Steel Rings by a Combined Powder Metallurgy and Cold Forging Process. Powder Metallurgy: Vol. 19, No. 3, pp. 141-152.

Fabrication of Low—Permeability Gas-Flow Control Devices

Abstract: A commercially practicable P/M process for producing gas-flow control devices is outlined. Considerations favouring a P/M approach are presented and the importance of powder characteristics is stressed. The technology for producing units with a gas flow of the order of 10−4cm3/s is outlined and the influences of sintering, compaction, and coining are discussed in detail. The generalizations resulting from this study are applicable to units ranging in flow rates from 10−2 to 10−8 cm3/s. Density reproducibility through compaction-pressure control is shown to be the most important factor in producing repeatable flow rates. Coining-tool geometry, compaction pressure, sintering kinetics, permeability, density, and pore size are variables considered in the analysis.

Oxide-Dispersion-Strengthened Nickel Produced by Non-Reactive Milling

Abstract: It is shown that oxide dispersion strengthened alloys can be produced by a postulated nonreactive milling mechanism whereby the dispersoid is trapped at the interface between welding metal powder particles. This interparticle welding is possible because, without a suitable and sufficiently vigorous chemical reaction between the metal powder particles and the milling fluid, no protective, weld-preventing reaction coating is formed on these particles. Using water as the nonreactive milling fluid, Ni - 1.8-vol % thoria and Ni - 1.8-vol % yttria alloys with 1093 C tensile strengths ranging from 122.3 to 141.5 MN/sq m (17,900 to 20,500 psi) were produced by nonreactive milling.

A Quantitative Assessment of the Mechanical Preparation of Sintered Iron Micrographic Specimens

Abstract: The examination of P/M microstructures by automated image analysis requires a reproducible high level of surface finish from the section of the specimen under investigation. The preparation of mechanically polished sections involves a number of procedures each susceptible to variation. The effect of variations taken together can be large, resulting in erroneous information. A quantitative assessment of each procedure for polishing specimens from an iron P/M system has been conducted using automated polishing machines. The porosity of sintered iron compacts was measured using an automated image analysing computer. The ‘measured’ porosities were compared with the ‘true’ bulk porosity in each case to assess the variation due to each polishing parameter. Although a universal set of polishing conditions cannot be given, the results enable a set of optimized polishing conditions to be derived.

New Areas for Development in Metal Powder and Particle Technology

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.

Some Observations on the Milling Behaviour of Nitrogen-Atomized Cobalt Powder

Abstract: Cobalt powder was dry-milled using a high-energy vibrational ball mill. Examination of the particle appearance using a scanning electron microscope established that the original spherical particles were initially reduced to platelets and that fracture of these platelets resulted in a marked decrease in the average particle size as milling continued. During milling, pressure welding of fine particles onto the surface of the platelets resulted in an increasingly roughened surface appearance with increasing time. After 32 h, the powder consisted essentially of porous agglomerates of fine, almost equiaxed particles.

Glass-Fibre-Reinforced Aluminium Alloys Formed by Hot Extrusion: Part II: Heat-Treatment

Abstract: Procedures for the heat-treatment of aluminium alloys (HE9 and HE30) reinforced with glass fibres were investigated. The fibre-reinforced composites were formed by the hot extrusion of pressed billets consisting of a mixture of alloy and glass powders.(the softening-point of the glass being below the extrusion temperature used, so that discontinuous glass fibres were formed in situ on extrusion). It was found that the composites failed to respond to the standard solution heat-treatment and artificial ageing .procedures for the alloys. This was shown to be due to the removal of magnesium from th~ metal matrix by reaction with the glass phase. A study was made of the kinetics of the glass/metal reactions occurring, and of the effect of time and temperature on the solution heat-treatment. Satisfactory response to heattreatment was obtained by adding excess magnesium to the billet.

Health Hazards and Powder Metallurgy

Abstract: A health hazard may arise in the manufacture and use of certain metal powders. Lead is readily absorbed from the lungs and can become a general poison in the body. Cadmium inhaled from metal spraying may also lead to toxic effects. Certain metals may produce allergic effects. Cobalt can produce an acute asthma in sensitive individuals and a slowly developing fibrosis of the lungs that has occurred in tungsten carbide workers is almost certainly due to cobalt. Aluminium powders as used for fireworks have caused very acute and severe fibrosis of the lungs. Some metals such as tin and iron are deposited in the lungs where they are innocuous but produce marked shadows in radiographs of the chest. The prevention of ill effects requires a knowledge of the possible hazards and the avoidance of levels in the atmosphere above the accepted threshold limit values. In certain circumstances medical supervision is desirable.