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.     

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.     

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.     

Improving Conductivity of Copper PM Parts by Pretreatment of Green Compact

Abstract

There is a maximum green density to which pure copper can be compacted and still exhibit no growth upon sintering. This limits the maximum density and thereby the maximum conductivity and strength that can be achieved with sintered copper bodies. The swelling of a part pressed above this critical compacting pressure is caused by premature closure of the outer pores of the compact, thus sealing off the egress of internally generated gases. In this study a number of carefully selected chemical compounds were added to the as-pressed compacts using two methods in an effort to find a way of keeping these outer pores temporarily open until all internal gases are eliminated. These pores are then sealed off in the later stages of sintering, resulting in a higher density body with improved electrical conductivity throughout. Two compounds proved to be most effective in accomplishing this without leaving a residue deleterious to conductivity. Two commercial copper powders were die pressed into thick discs and then impregnated with an aqueous salt solution, the preferred treatment. After a special sintering procedure, the best conductivity obtained in an 18 mm diameter × 10 mm thick disc using the most responsive powder and treatment was 93·9% IACS at the centre and 94·2% IACS at the surface. Untreated, the same powder showed a maximum centre conductivity of 89% IACS.     

CeO2-Gd2O3纳米复合粉的模压成形行为研究

测定了CeO2-Gd2O3纳米复合粉压制过程中压坯密度与成形压力的关系曲线。成形压力低于100MPa时。压坯密度随压力的增大而急剧增加；成形压力继续增大，压坯密度随压力的变化减慢。由压坯密度一压力对数关系曲线可知，粉末中团粒的屈服强度为44．7MPa。另外，粉末的压制规律遵循黄培云双对数方程。     

THE PREPARATION AND SOME PROPERTIES OF CHROMIC OXIDE-CHROMIUM CERMETS MADE BY PARTIAL REDUCTION OF THE OXIDE

Abstract

Compacts of chromic oxide/carbon mixtures have been sintered in vacuum to produce oxide-metal cermets. The effect of carbon addition, type of carbon, purity of oxide, compacting pressure, and sintering temperature on the green and sintered density has been studied, and this has been supplemented by tests of hardness and compressive strength.

Considerable densification can be attained by a small addition of carbon to the powder mixture, and this is accompanied by an increase in compressive strength to 20 tons/in², compared with 3 tons/in² for the pure oxide sintered to the same temperature.     

THE EFFECT OF INTERPARTICLE CONTACT AREA ON THE STRENGTH OF COLD-PRESSED ALUMINIUM POWDER COMPACTS

Abstract

Measurements of the tensile strength of spherical cold-pressed aluminium powder, pressed to various densities up to the theoretical maximum, have shown that compaction is a two-stage process. At some high, intermediate pressure, interparticle sliding occurs in a way that does not itself increase densification but makes it easier for further deformation to occur. It is likely that the pressure at which this sliding takes place is dependent on the work-hardening rate of the powder as well as the powder size and morphology. In any case, it is shown to be important to the densification and strength reached by the compact.

It is concluded that the strength of a green compact is dependent upon the interparticle metallic contacts made during compaction. However,the green strength is well below that of wrought aluminium, probably due to the presence of broken-up oxides,which act as stress-concentrators at the interparticle boundaries.     

THE ISOSTATIC PRESSING,VACUUM SINTERING,AND SWAGING OF THORIUM POWDER

Abstract

The isostatic pressing of calcium-reduced thorium powder into round bars and tubes, prior to vacuum sintering and swaging, is described. The simple equipment allows powder filling to be carried out in an enclosed system, thus avoiding fire and health hazards at this stage. A number of bars or tubes, of varying length and diameter, can be pressed simultaneously. The technique has certain advantages over conventional die compacting.

The effect of pressure on density and hardness through the bar section before and after sintering at various temperatures has been studied, and the optimum conditions determined for the manufacture of dense bars with high metal efficiencies (98%). Details are given of the microstructure and mechanical properties of thorium bars in the sintered, swaged, and annealed conditions.     

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.     

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.     

EVALUATION OF IRON POWDERS FOR SINTERED STRUCTURAL COMPONENTS

Abstract

A new method for evaluation of iron powders is suggested. Ultimate tensile strength is chosen as a base parameter, and the relations between this property and compacting pressure and raw material cost, respectively, are shown. For this purpose it has been necessary to deduce two supplementary parameters, Relative Pressure Response (P_r) and Relative Raw Material Requirement (M_r), which are functions of compacting pressure and ultimate tensile strength, and of compacting pressure and density, respectively.

It is shown that the importance of compressibility of iron powders is overrated in current opinion and, consequently, that it is misleading to judge the overall merits of an iron powder according to its compressibility.

Raw material costs of sintered steels are lower, if sponge-iron powders are used instead of atomized powders, even if the price of all iron powders were equal. This tendency is more strongly emphasized at low densities, where the sponge-iron powder with the lowest apparent density value is preferable. The differences are beginning to lessen and disappear gradually at densities approaching or exceeding 7·0–7·2 g/cm³ (for single-pressed and single-sintered materials).

Alloy composition has a stronger influence on raw-material costs than the choice of iron-powder grades. Close and reliable control of carbon contents and avoidance of oxidation of manganese is essential for lowering of costs in the PM structural-component manufacturing industry.     

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.     

Study of warm compaction of Alumix 123 L

Abstract

Although powder metallurgy (PM) material is dominated by ferrous alloys, there is a growing interest in Al PM. The usage of Al PM in automotive applications depends on the development of higher density and improved dynamic properties. Several approaches have been proposed to increase density of sintered parts. Warm compaction process of Al powder was used to achieve high density. In this study the authors focused on the effect of warm compaction on Alumix 123 L (ECKA Granules) powder blend. It has been found that warm compaction at 110°C led to a reduction in the ejection force by 27·9%, increased green density to 94% of theoretical density and increased sintered strength to 315 MPa as compared to those pressed at room temperature.     

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.     

The effect of FeS film on the surface of iron powder particles on mechanical properties and fracture of Fe–Cu–C alloys

This article provides a new kind of P/M processing by designing thin-layered FeS film coated on the surface of iron powder particles for preparing high density Fe–Cu–C materials. Experimental results showed that FeS lubricating coating on the surface of iron powder particles was significant as a means of reducing friction in the pressing process of Fe–Cu–C alloys. After being pressed, the green density increased from 7.18 to 7.42?g?cm^?3. The sintered density increased from 7.10 to 7.37?g?cm^?3. In the sintering process, the FeS was liquid, and useful in purifying particle surface and strengthening grain boundaries to improve the mechanical properties. The hardness and tensile strength of the Fe–2.0Cu–0.9C–0.5FeS₂ material were 80.5 HRB and 590?MPa. Analysis of fracture showed the main fracture was transgranular fracture. And FeS spherical particles gathered in the pores of the Fe–Cu–C alloys.     

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.     

Effect of Compacting Pressure and 0·5%Mo on Properties of Sintered Iron Powder and Manganese Steels

Abstract

An investigation has been carried out on the effect of compacting pressure, in the range 150–600 MPa, and of the addition of 0·5%Mo on the properties of sintered Hametag iron powder and manganese steels. Higher sintering activity compared with standard iron resulted in higher density and higher mechanical properties in the presence of manganese vapour. The addition of 0·5%Mo in the form of ferromolyb-denum caused an increase in density and strength properties in the Fe–C and Fe–Mn–C steels. PM/0157     

THE INFLUENCE OF PRESSURE,TEMPERATURE, AND TIME ON THE HOT PRESSING OF COMMERCIAL BERYLLIUM POWDER

Abstract

For a particular batch of Brush Super-Pure – 200-mesh beryllium powder the hot-pressability (defined as the length of a standard compact of < 98% theoretical density), increased: (1) continuously with pressure over the range 0–1·25 ton/in² for compacts pressed for 1 h at 1100°C;(2) with temperature from 1000°C to a maximum at 1150°C when pressed for 1 h under 1 ton/in²; and (3) to a lesser extent with time over the range 10–120 min when pressed at 1050°C under 0·25 ton/in² and at 1100°C under 1·0 ton/in². Differences in hot-pressability between various batches of the same powder were small compared with the effects of temperature and pressure.

Compaction during hot pressing occurs in two stages: first, collapse of the powder column causing bulk powder flow; followed, secondly, by sintering of particles forced into close contact. The latter is accompanied by a diminution in both the number of pores and their average size and is associated with grain growth, particularly above 1100°C; after pressing at 1200°C any remaining porosity assumes a thermally stable, spherical configuration.     

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.