VIBRATORY COMPACTING OF METAL POWDERS

Abstract

The vibratory compacting of copper powder has been studied using a mechanical vibrator. The major factors influencing the green density of the compacts were the amplitude and frequency of vibration, and the applied pressure. A minimum time of 10 sec on the vibrator was necessary to achieve the maximum density value. Other factors examined were the effects of vibration on blended powders with constituents of widely different densities, and the suitability of this method to compact various materials. Vibratory compacting produced compacts of improved uniformity and green density.     

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.     

VARIOUS FACTORS AFFECTING THE WEAR OF STEEL DIES USED FOR THE COMPACTING OF IRON POWDER

Abstract

Small cylindrical iron powder compacts have been produced in series of ～ 75 000 specimens, using an automatic compacting press running at a speed of 33 strokes/min. Die-wear rates were determined, as influenced by powder type, compacting pressure, compact density, type and hardness of die steel, and punch/die clearance. Simultaneously, the ejection forces were continuously recorded.

No substantial difference was found between the die-wear rates of common iron powder grades of an atomized or a sponge type, but an electrolytic grade of especially compact particle structure gave a lower rate. Die wear increases in roughly linear proportion both with the number of compacts and with the compacting pressure.

Steel type and hardness of the die had a pronounced influence upon die-wear behaviour, dies of higher hardness yielding lower wear rates. Unsuitable heat-treatment can cause high wear rates even though it might produce high die hardnesses.

A punch/die clearance of 10 μm resulted in the most favourable die-wear behaviour. At a clearance of only 5 μm the punches became stuck in the die very quickly; at clearances of 25 and 45 μm severe cladding of the die-bore surface with iron from the compacts occurred after 40 000-50 000 strokes.

The following, partly interacting, phenomena were found to be contributory factors in the die-wear mechanism: abrasion, cladding, surface cracking, and chipping. It proved impossible to establish a reliable correlation between ejection forces and die-wear rates.     

Bearing Materials by Powder Metallurgy

Abstract

The dependence of green strength on green density and on compacting pressure was investigated for the bidirectional die pressed and isostatically pressed Cu powder compacts. The breaking strength of the pressed Cu compact was found to increase with green density and also with compacting pressure. The green strength seemed to be directly proportional to the contact area between powder particles. A theoretical equation for the relationship between green density and contact area was derived from a geometrical consideration, and agreed well with experimental findings. PM/0272     

The Effect of Density Distribution on the Radial/Axial Shrinkage Ratio of Iron Powder Compacts

Abstract

The effect of compacting pressure on the radial/axial (R/A) shrinkage ratio for iron powder compacts was studied. It was observed that R/A and compacting pressure have a linear relationship. The effect of density distribution on R/A ratio inside the green compact was determined. The observed changes in R/A are attributed to the change in density distribution in the green compact.     

Electrical resistivity of green powder compacts

Abstract

Different parameters affect the electrical resistivity of green specimens. This paper presents the effect of the particle size distribution, the compacting pressure, and the oxidation of the powder on the electrical resistivity of green specimens fabricated with different powders (Fe, Zn, Ni, and Cu). The results show that the electrical resistivity increases when the compacting pressure decreases, the particle size is reduced and the oxidation increases. It indicates that the electrical resistivity is sensitive to powder surface characteristics and particle interfaces in green compacts. Electrical resistivity may therefore be used to study particle interfaces, evaluate green powder compact characteristics, and monitor powder oxidation.     

LUBRICATION EFFECTS IN THE COMPACTION OF SPONGE-IRON POWDER AT LOW AND HIGH SPEEDS

Abstract

The effects of the amount and method of lubrication have been investigated when compacting Höganäs sponge-iron powder, NC100-24, at both low and high speeds. Pressing characteristics, ejection loads, and the final properties of the sintered compacts were markedly affected by both the amount and the method of lubrication. From the results obtained, an optimum amount of admixed zinc stearate is recommended for both low and high-speed conditions.     

INVESTIGATION OF THE DIMENSIONAL CHANGES IN SOFT METAL POWDER COMPACTS

Abstract

The variables affecting the radial/axial (R/A) shrinkage ratio in compacts made from spherical copper powder have been investigated, also the linear dependence of R/A on compacting pressure and sintering temperature. The values of R/A for spherical powder are higher than those for irregularly shaped powder. The effect of particle shape and height/dia. ratio of the compact on R/A have been studied. The differences in green-density distributions have been determined, together with the effect on these of pressure and height/dia. ratio of the compact. The observed variation of R/A is attributed to differences in density distribution in the green compacts, resulting in anisotropic stresses during sintering.     

FACTORS AFFECTING THE RADIAL AND AXIAL SHRINKAGE IN SOFT-METAL POWDER COMPACTS

Abstract

The variables affecting the radial: axial (R/A) shrinkage ratio in copper-powder compacts have been investigated. The value of R/A is linearly dependent on compacting pressure, green density, and sintering temperature, and also increases with decrease in the particle size of the powder. The observed variation of R/A is attributed to the differences in density in the green compacts, which result in anisotropic stresses in sintering. Surface-tension forces or residual stresses introduced during compaction cannot alone be regarded as the main driving forces responsible for shrinkage; anisotropic stresses also play an important role in the densification of metal-powder compacts. By proper control of these variables, parts can be produced from the compacts to close dimensional tolerances.     

THE POWDER METALLURGY OF RUTHENIUM

Abstract

An investigation of the powder metallurgy of ruthenium is described, from the reduction of ammonium ruthenium chloride to the working of sintered compacts. The powder properties measured were specific surface area, by a simplified BET method, and tap density. The dependence of these properties on the conditions of reduction has been determined. The surface area of powders varies from 1 to 10 m²/g in the temperature-of-reduction range 700-350°C. The tap density is also variable (1–3 g/c.c.) and is generally related to the surface area. The effects of compacting pressure and temperature on sintering are described, the progress of sintering being observed by measurements of the “open” and “closed” porosity present in samples. Compact densities up to 95% of theoretical can be obtained by sintering at 1500°C. The selection of powder properties and compacting pressures to be used in the production, by vacuum sintering at 1500°C, of high-density compacts for working, is governed by the necessity to maintain open porosity during the heating cycle up to at least 1200°C, as considerable gas evolution occurs at this temperature; at the same time it is essential that good densification shall have occurred even at this stage. These conditions can be met by using powder with a surface area of 2–5 m²/g and compacting pressures in the range 0·5–25 tons/in ².

Observations on the hot working of sintered compacts indicate that ease of working is related to the surface area of the powder.     

FACTORS AFFECTING THE COMPACTION OF TUNGSTEN POWDERS

Abstract

It is difficult to form tungsten powders into compacts by pressure-forming methods. The brittleness of the powder particles causes them to fracture under pressure instead of producing the typical “point welds” exhibited by more ductile particles. Because of this, the powder characteristics such as particle size, size distribution, and particle shape play a most important role in the compacting of tungsten powders.

Both regular- and irregular-shaped particles of tungsten powder are discussed as regards the formation of strong and dense compacts from these powders. Powders composed of irregular-shaped particles gave stronger, but less dense compacts. The effects of particle size and particle-size distribution are also considered. Each of these factors has individual as well as combined effects. It was found that certain critical particle-size distributions produced the densest compacts.

It is concluded that interlocking of particles, which is brought about by surface irregularities, and interfit, which is determined by correct particle-size distribution, are the determining factors in the compaction of tungsten powders.     

THE CORRELATION BETWEEN RAW MATERIALS,PREPARATION CONDITIONS,AND PROPERTIES OF SINTERED IRON

Abstract

Three plain iron powders of different types (sponge-iron, atomized and electrolytic iron powder) were studied with respect to their sintering behaviour and to the influence of manufacturing parameters—i.e., compacting pressure, sintering temperature, and sintering atmosphere—on the microstructure and the properties of sintered compacts. The changes of length, electric conductivity, and strength during sintering are explained in physical and chemical terms. Technical sintering diagrams are presented. The influence of sintering atmospheres on the mechanical properties of sintered compacts is shown for the three types of powder. The correlation between pore structure and strength is discussed; analytical relationships are developed which are in agreement with the experimental results.     

SOME OBSERVATIONS ON THE SHRINKAGE BEHAVIOUR OF COPPER COMPACTS AND OF LOOSE POWDER AGGREGATES

Abstract

The relative shrinkage in the radial and in the axial directions of both conventionally pressed copper powder compacts and of loose copper powder aggregates has been determined. Irregular-shaped electrolytic copper powder and flake copper powder were used. The structures of the green and of the sintered compacts and aggregates were examined metallographically. The results led to the tentative conclusion that the theories previously advanced to explain the differences in the axial and radial shrinkage of compacts and aggregates are not correct. The pores of green and sintered compacts of irregular copper powder are not necessarily disc- or lens-shaped, but rather equiaxed, and the observed difference in shrinkage cannot be attributed to the shape of the pores. On the other hand, the ratio of axial to radial shrinkage of irregular-shaped powder and of flake powder aggregates is nearly the same, in spite of the fact that the pores in the flake powder aggregates are much larger in the radial than in the axial direction. The possibility that forces other than surface-tension forces have an influence upon shrinkage behaviour is discussed.     

THE COMPACTING OF GRAPHITE AND GRAPHITE/URANIUM/THORIUM MIXTURES

Abstract

Fine artificial graphite powders can be cold compacted to give bodies of high density (～ 88% of theoretical), low permeability (B₀ ～10^–14 cm²), and reasonable strength. Such powders, after vacuum annealing, will not compact.

Die-compacted powder has strongly anisotropic properties owing to a high degree of preferred orientation within the compact; this effect is less marked in hydrostatically compacted powder. Minor dimensional changes occur when compacts are annealed in the range 600-1000°C.

The preparation of fuels by incorporation of fissile and fertile materials into graphite powder and cold compacting is described.     

FRICTION COEFFICIENTS BETWEEN IRON POWDER COMPACTS AND DIE WALL DURING EJECTION USING VARIOUS ADMIXED ZINC STEARATE LUBRICANTS

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.     

AN ASSESSMENT OF GLASS-CERAMIC AS AN INSERT MATERIAL FOR DIE COMPACTION OF METAL POWDERS

Abstract

Glass-ceramic inserts have been made and shrink-fitted into steel bolsters to assess the feasibility of glass-ceramic as a die material for the die-compaction of lubricated and unlubricated iron powder. Measurements of compacting pressures and ejection stresses were lower for the glass-ceramic die compared with those for a standard tool-steel die in lubricated conditions, while in unlubricated conditions ejection stresses were appreciably higher, with scoring and brittle fracture of the insert. The results indicate that possibilities may lie in the further development of ceramics as inexpensive die materials for powder compaction in which die-wall friction could be significantly reduced.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

DIMENSIONAL CONTROL 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.     

Improving iron compact green strength using powder surface modification

Abstract

In powder metallurgy, green strength has important consequences for part production rates and product end quality. Mechanical interlocking and interparticle cold welding are the main mechanisms responsible for green strength. These mechanisms are affected by compaction pressure, temperature, amount of lubricant and additives admixed to the powder, and surface characteristics of the powder. The present paper describes the effect of iron powder surface modification on the green strength of compacted specimens. The green properties of compacts fabricated from iron powder treated with diluted sulphuric acid and coated with copper by a non-catalytic displacement plating method are presented. The results indicate that surface modifications strongly influence the green strength of the compacts.     

Investigation of warm compaction and sintering behaviour of aluminium alloys

Abstract

The effects of warm compaction on the green density and sintering behaviour of aluminium alloys were investigated. Particular attention is paid to prealloyed powders, i.e. eutectic and hypereutectic Al-Si alloys, regarding their potential applications in the automotive industry. The effects of chemical composition, alloying method, compacting temperature and the amount of powder lubricant were studied. The compaction behaviour was examined by an instrumented die enabling simultaneous measurement of density, die wall friction coefficient, the triaxial stresses acting on the powder during the course of compaction and ejection pressure. The sintering behaviour was studied via dilatometeric analysis as well as normal batch sintering. The results show that warm compaction could be a promising way to increase the green density of aluminium alloys, especially prealloyed powders, and to decreased imensional instability during sintering. Moreover, it reduces the sliding friction coefficient and the ejection force during the powder shaping process. This paper presents the significant advantages and drawbacks of using the warm compaction process for commercial PM aluminium alloys.