Showing papers in "Powder Metallurgy in 1979"
TL;DR: The amorphous citrate process has been used to produce Co3O4, CoO, Cr2O3, LaCoO3 and LaCrO3 as discussed by the authors.
Abstract: The amorphous citrate process has been used to produce Co3O4, CoO, Cr2O3, LaCoO3, LaCrO3, SrCoO3, and strontium-doped LaCoO3, and LaCrO3. The citrate-nitrate gels were produced at room temperature and dehydrated at 70°C to yield solid precursor materials. The decomposition of the precursors was studied at temperatures of 220, 310, 500,710, 880, and 1060°C. The yield of the process for strontium-doped lanthanum cobaltite is of the order of 90% which compares extremely well with the yield of 60% obtained by the conventional powder technique. Thermogravimetry, X-ray diffraction analysis, scanning electron microscopy, and analytical electron microscopy have been used to characterize the precursors and oxide materials produced by the citrate process.
89 citations
TL;DR: In this article, the authors demonstrate that small amounts of porosity, even 1 or 2%, can cause drastic reduction of the mechanical properties in tungsten heavy alloys, and large irregular pores are formed with a sharp decrease of strength and ductility.
Abstract: W–Ni–Fe heavy alloy tensile specimens were sintered at 1450°C for various times up to 44h. The W content varied between 90 and 96 wt-%, and the Ni to Fe weight ratio was 1:1. The specimens are fully densified after 15 min to 1 h of sintering and show high strength and ductility. During the tension test, cracks are formed at the interface between tungsten grains when the grain deformation reaches critical levels. The number of these intergranular cracks increases with deformation until the specimens fracture. When the specimens are over-sintered for 4 and 8 h, large irregular pores are formed with a sharp decrease of strength and ductility. Upon further sintering, the porosity decreases again with a recovery of the mechanical properties. The results demonstrate that small amounts of porosity, even 1 or 2%, can cause drastic reduction of the mechanical properties in tungsten heavy alloys.
42 citations
TL;DR: In this article, the effects of pore geometry on tensile strength of porous materials were investigated and the results showed that pore geometry has a significant effect on the strength of porous materials.
Abstract: (1979). Analytical Study of Effects of Pore Geometry on Tensile Strength of Porous Materials. Powder Metallurgy: Vol. 22, No. 3, pp. 119-123.
37 citations
TL;DR: In this article, the substitution of Co for Ni would improve their sintering characteristics and their properties as contact materials, but no significant differences were detected in the contact angles of the liquid phases in low-Cu alloys containing Co appears to be related to the formation of W6Co7 intermetallic compound on the surface of the W particles.
Abstract: Nickel or cobalt additions were made to W–Cu compacts to determine whether the substitution of Co for Ni would improve their sintering characteristics and their properties as contact materials. For alloys with small proportions of Cu, Co was more effective in promoting sintering than Ni, whereas in those alloys with a high Cu content neither addition affected the sintering process. Despite their different sintering characteristics, no significant differences were detected in the contact angles of the liquid phases in W–Cu, W–Cu–Co or W–Cu–Ni alloys. The improved sintering of low-Cu alloys containing Co appears to be related to the formation of W6Co7 intermetallic compound on the surface of the W particles.
26 citations
TL;DR: In this paper, the freeze-drying of mixed nitrate solutions has been used to produce lanthanum cobaltite and strontium-doped cobaltites.
Abstract: The freeze-drying of mixed nitrate solutions has been used to produce lanthanum cobaltite and strontium-doped lanthanum cobaltite. The freeze-dried nitrates were decomposed in vacuum at 500°C and the residue heat-treated for one hour at 1060°C. The yield of the freeze-drying process is shown to be the same as that of a commercial process involving the repeated sintering and milling of a carbonate/oxides mixture. X-ray diffraction analysis, scanning electron microscopy, and analytical electron microscopy have been used to characterize both the freeze-dried material and the commercial material.
17 citations
TL;DR: The behavior of zinc stearate (0.4 wt-%) during mixing with iron powder has been studied by monitoring changes of apparent density and comparison with powder prepared by depositing zinc Stearate from solution onto the iron particles.
Abstract: The behaviour of zinc stearate (0–4 wt-%) during mixing with iron powder has been studied by monitoring changes of apparent density and comparison with powder prepared by depositing zinc stearate from solution onto the iron particles. Apparent density was increased by the development of thin surface films at low lubricant contents but was decreased by the formation of thick layers and free lubricant at high contents. Pressure/density data were obtained from floating-die compaction of cylindrical iron powder compacts containing deposited zinc stearate with and without die-wall lubrication to determine the relative significance of interparticle friction, particle/die-wall friction, and compaction inhibition. An optimum lubricant addition minimized interparticle friction during compaction. This was greater than the optimum observed for loose packing owing to the more arduous conditions prevalent in compaction. Compaction pressure losses associated with interparticle friction were lower than those ca...
17 citations
TL;DR: In this paper, the ageing in SAP-type (Al-Mg-Si)-Al2O3 composites was investigated using resistivity measurements, hardness measurements, mechanical tests, and electron microscopy observations.
Abstract: The ageing in SAP-type (Al–Mg–Si)–Al2O3 composites is investigated using resistivity measurements, hardness measurements, mechanical tests, and electron microscopy observations. An oxide content of about 3% completely inhibits GP zone formation during natural ageing because of the absence of quenched-in vacancies. In contrast, GP zone formation occurs during artificial ageing, the process being maintained by thermal vacancies and by diffusion along grain boundaries. To take advantage of both dispersion and precipitation hardening, it appears advisable to keep the oxide content below 1·5%. A considerable improvement in mechanical properties of the present composites over the conventional alloy is observed.
17 citations
16 citations
12 citations
TL;DR: Two types of Hoganas iron powder, sponge and atomized with very high compressibility (ASC), after compaction to densities of 6·0, 6·4, and 6·8 Mg m−3 and sintering under standard conditions were subjected to steam oxidation at 450, 525, and 600°C as discussed by the authors.
Abstract: Two types of Hoganas iron powder – sponge, and atomized with very high compressibility (ASC), after compaction to densities of 6·0, 6·4, and 6·8 Mg m−3 and sintering under standard conditions were subjected to steam oxidation at 450, 525, and 600°C. The progress of oxidation was studied by measurement of weight gain and hardness. X-ray methods were used to determine the type of oxide present after treatment. During steam oxidation the type of powder has an important influence on the extent of pore closure and on the morphology of the oxide produced. The kinetics of oxidation were always faster for sponge iron than for atomized iron and there was a corresponding increase in the rate of pore closure and in surface hardness. For effective sealing of surface pores components should be of high density and be steam treated at 600°C but for attainment of maximum hardness components should be of low density and be steam treated at 525°C.
11 citations
TL;DR: In this article, the influence of zinc stearate on the shrinkage of uniaxially pressed iron powder compacts has been studied and it was shown that green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing.
Abstract: The influence of admixed zinc stearate on the shrinkage of uniaxially pressed iron powder compacts has been studied. For pressing conditions which caused inhibition of compaction the removal of the stearate during sintering produced an increase in shrinkage parallel to the pressing axis and in direct proportion to lubricant content. Additions of stearic acid (varying particle size), zinc stearate, lithium stearate, stearamide, and Cosmic 64 wax were used to investigate the influence of lubricant on mechanical properties of green and sintered iron powder compacts. Green strength was reduced relative to unlubricated material only by lubricants whose physical and chemical properties enabled them to produce and maintain extensive interparticle films during pressing. Vapour from the rapid initial decomposition of lubricants which reduced green strength could have a deleterious physical influence on the tensile strength of dewaxed or sintered Fe compacts. Decomposing lubricants also produced undesirable...
TL;DR: In this article, an improvement in the dimensional tolerances of sintered components by at least one ISO grade was obtained in field tests comprising several thousand parts by using a partially pre-alloyed copper-iron powder instead of an elemental mix.
Abstract: An improvement in the dimensional tolerances of sintered components by at least one ISO grade was obtained in field tests comprising several thousand parts by using a partially pre-alloyed copper-iron powder instead of an elemental mix. This effect is explained by the fact that segregation of the constituents is minimized. Further tests have shown that sintered components made from partially prealloyed powders have a more even microstructure and better distribution of the alloying elements, resulting in components with fewer defects in the microstructure, than do components made from the corresponding elemental mixes.
TL;DR: In this paper, sintered iron specimens are tested in tension and their strength and elongation are compared with a new model which indicates the relationship between the strength or elongation and the porosity content or density of porous metals.
Abstract: Examples of load vs. cross-head displacement curves at room temperature are shown in Fig.2. Both the flow stress and elongation increased with the density ratio of sintered specimens. All specimens were found to have a yield drop at In this paper, sintered iron specimens are tested in tension and their strength and elongation are compared with a new model which indicates the relationship between the strength or elongation and the porosity content or density of porous metals. The elastic-plastic stress distribution around a spherical void in porous metals is calculated to obtain the new model.
TL;DR: In this paper, the extrusion of a range of metal-non-metal particulate mixtures of possible application as materials for contacts and electrodes has been initiated, and very tentative trials extruding aluminium with either alumina or graphite powders have been initiated.
Abstract: ‘Conform’ continuous extrusion is a process invented at the UKAEA Springfields Laboratories. Material fed into the peripheral groove of a rotating wheel is first advanced by friction into the chamber formed between the groove and tooling supported in a fixed shoe, and then extruded through a die. Particulate metal, including aluminium and copper as powders and as comminuted scrap cable which is of interest to the electrical industry, is one type of feed from which wire sections have been extruded. The higher strengths obtained from many particulate feeds, often without a decrease in conductivity, may be attributable to an oxide dispersion hardening effect. As a preliminary to investigating the extrusion of a range of metal-non-metal particulate mixtures of possible application as materials for contacts and electrodes, very tentative trials extruding aluminium with either alumina or graphite powders have been initiated.
TL;DR: The history of the development of Sintered Permanent Magnets can be found in this article, where the authors present a detailed account of the process of finding permanent magnets. Butler et al.
Abstract: (1979). History of the Development of Sintered Permanent Magnets. Powder Metallurgy: Vol. 22, No. 3, pp. 132-138.
TL;DR: In this article, the effects of experimental variables such as heating rate, relative sintering temperature, and isothermal hold time on the calculated activation energy are discussed. But, the authors do not consider the effect of these variables on the accuracy of the activation energy determination.
Abstract: The time and temperature dependent morphology changes of a sintering powder compact are quite often used to calculate properties such as mass diffusivities. The fact that non-isothermal conditions exist for a portion of most sintering experiments has prompted the development of data correction routines to aid mechanism identification. 1 However, it is generally recognized that the. simultaneous operation of multiple mechanisms gives ambiguous growth rates.2 As a result, the determination of a sintering mechanism must be substantiated with additional data, such as activation energies. Unfortunately, little attention has been given to the effects of experimental variables such as heating rate, relative sintering temperature, and isothermal hold time on the calculated activation energy. In many sintering experiments, the heating rate to an isothermal condition is reasonably slow. Despite slow heating rates, several studies have produced activation energies in close agreement with tracer determinations.3 The present note discusses some of the errors introduced into experimental activation energy determinations and suggests ways to minimize such errors. previous small step of length At. The final cumulative sintering after an isothermal hold can then be calculated from a combination of eqns. (1)-(3). By varying the isothermal temperatures for such calculations, the cumulative sintering values can be used to extract the observed process, activation energy. Comparison of the observed value with the input value Qo gives a measure of the errors resulting from the different experimental parameters. The res.ults presented here were produced using a digital computer and represent the results of 165 separate simulations, some of which were specifically designed to test for errors. The time increments for eqn. (3)were v~ried from 0·1 to 100 s. Based on such studies it was determined that a At value of 3 s provided a reliable' simulation.
TL;DR: In this paper, 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.
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 a...
TL;DR: In this article, the transmission electron microscopy study of aluminium powder is presented, with a focus on the transmission of the electron microscope to the aluminomethane powder and its properties.
Abstract: (1979). Transmission Electron Microscope Study of Aluminium Powder. Powder Metallurgy: Vol. 22, No. 4, pp. 208-209.
TL;DR: In this paper, the feasibility of alloy forgings from a powder prepared by mixing the alloying additions with iron powder was investigated and it was found that useful mechanical properties could be obtained in forgings made from mixtures of alloying ingredients.
Abstract: An investigation was made of the feasibility of producing alloy forgings (nominal composition Fe–0·5C–0·6% Mn), from a powder prepared by mixing the alloying additions with iron powder. The carbon was added as microcrystalline graphite and the manganese as elemental powder and as ferro-manganese powder. Additions of copper and ammonium chloride powders were also made for the purpose of assisting the manganese to alloy with the iron. The copper addition improved the tensile strength but lowered the ductility. The ammonium chloride had little effect except for an apparent lowering of hardness. It was found that useful mechanical properties could be obtained in forgings made from mixtures of the alloying ingredients.
TL;DR: In this paper, the technology of hard and soft ferrites is discussed and the present technology of soft and hard ferrites are discussed in powder metallurgy: Vol. 22, No. 2, pp. 62-74.
Abstract: (1979). Present Technology of Hard and Soft Ferrites. Powder Metallurgy: Vol. 22, No. 2, pp. 62-74.
TL;DR: In this paper, the boronlzing of commercial unalloyed and alloyed sintered steels has been investigated and an examination of the technological steps involved in the preparation of boride layers and their effect on the production results, such as the thickness of the layer, the level of hardness, the dimensional change and the strength.
Abstract: The increase in wear resistance is in both cases attributable to the materials deposited on the surface. In methods of Type (a) the base material is participating only to the extent that it must form a sufficiently firm bond with the wearresistant layer. An example of this type of method is the flame-spraying of hard materials. For Type (b) methods the base material should have at least partial solubility for the elements to be diffused. Carhurizing, nitriding, and the more recent method of boronizing all belong to this group. The present investigation concerns the boronlzing of commercial unalloyed and alloyed. sintered steels. It includes an examination of the technological steps involved in the preparation of the boride layers and their effect on the production results, such as the thickness of the layer, the level of hardness, the dimensional change, and the strength. Practical tests on boronized sintered steels have not been carried out.
TL;DR: In this article, a composite alloy was produced which consisted of ferrite, martensite, and austenite phases elongated parallel to the rolling direction, and the volume fraction of each phase depended on the amount of Ni powder and the annealing temperature.
Abstract: Fe–Ni heterogeneous alloys were prepared from powders by pressing, sintering, hot rolling, cold rolling, and heat treatment. A composite alloy was produced which consisted of ferrite, martensite, and austenite phases elongated parallel to the rolling direction. The volume fraction of each phase depended on the amount of Ni powder and the annealing temperature. The strength and elongation of the alloy increased with the amount of martensite phase and austenite phase, respectively. Both its yield strength and uniform elongation satisfied a rule of mixtures qualitatively. It was found that inhomogeneous Ni distribution affected the formation of each phase and the mechanical properties of the alloys.