Showing papers in "Powder Metallurgy in 2004"
TL;DR: In this paper, the production of highly porous titanium parts of complex shape by powder metallurgical technology is described, where well defined porosity and pore sizes are achieved using a pore forming additive, which enhances the strength of unsintered compacts, allowing machining in the green state.
Abstract: The production of highly porous titanium parts of complex shape by powder metallurgical technology is described. Well defined porosity and pore sizes were achieved using a pore forming additive. This additive also enhances the strength of unsintered compacts, allowing machining in the green state. The requirements of the initial powders and pecularities of each production step are discussed in detail. The microstructure of porous materials and examples of net shape parts are given, aiming preferentially at biomedical applications. The impurities of the obtained titanium parts are compared with the requirements of the ISO standard for titanium implants.
135 citations
TL;DR: In this article, the powder behavior during die filling is discussed in two successive stages: the flow of powder from a shoe and the packing of powder inside a die, and the typical behaviour of powder during die-filling is summarised.
Abstract: In this paper, the powder behaviour during die filling is discussed. Initially, methods used to test powder flow properties are reviewed and a novel method based on the measurement of critical shoe velocity is emphasised. It is shown that this method not only can be used to characterise the flow of powders, but it can also be used to assist process design in powder metallurgy and similar operations. The powder behaviour during die filling is then discussed in two successive stages: the flow of powder from a shoe and the packing of powder inside a die. Previous studies are reviewed and typical behaviour of powder during die filling is summarised. Finally, segregation during die filling is also discussed and further studies on die filling are suggested.
79 citations
TL;DR: In this article, the role of porosity on the wear behavior of sintered iron was investigated and it was found that the wear mechanism is delamination or mechanical wear, which is basically similar to that of wrought materials according to subsurface crack generation and crack propagation processes.
Abstract: The role of porosity on the wear behaviour of sintered iron was investigated. Sintered iron was used in preference to steel because the various alloy additions and matrix heterogeneity in the latter might affect the microwear behaviour and the role of porosity. The wear tests were performed in a pin on disc test bed under 10-40 N loads at 0·56 m s-1 sliding speed in air. For the given tribological conditions, it was found that the wear mechanism is delamination or mechanical wear, which is basically similar to that of wrought materials according to subsurface crack generation and crack propagation processes. However, the open pores on the surface act as a site for generation and collection of wear debris. Furthermore, the pores affect the stress distribution and the metal deformation of subsurface layers during dry sliding. Accumulation of the plastic shearing on the subsurface results in nucleation of cracks that further grow by shearing: the fracture process. The propagation of subsurface cracks...
64 citations
TL;DR: In this article, the effects of atomiser design and processing parameters on the morphology and size distribution of centrifugally atomised tin powder were investigated and the main causes of unsuccessful atomisation were found.
Abstract: This paper investigates the effects of atomiser design and processing parameters on the morphology and size distribution of centrifugally atomised tin powder. Premature solidification of the melt on the atomiser and poor wetting of the atomiser by the melt were found to be the main causes of unsuccessful atomisation. The particle size distributions of the powders follow a lognormal distribution. The median particle size increased with decreasing atomiser rotation speed and with increasing melt flowrate. Cups with a high included angle made significantly finer powders than flat discs under the same operating conditions.
43 citations
TL;DR: In this article, the role of carbon in the reduction of oxides is discussed with reference to Boudouard's reaction, proposing the indirect carbothermal reduction as the controlling mechanism, and microstructural characterisation of the material included inspection of the powder particles as well as sintered specimens.
Abstract: Several atmospheres based on N2–H2 gaseous mixtures, with occasional additions of CH4 were used to study the sintering behaviour of Astaloy CrM at temperatures of 1100 and 1240°C Theoretical thermodynamic calculations and sintering experiments, with and without admixed graphite additions, were carried out devoting particular attention to the oxide reduction reactions The role of carbon in the reduction of oxides is discussed with reference to Boudouard's reaction, proposing the indirect carbothermal reduction as the controlling mechanism The microstructural characterisation of the material included inspection of the powder particles as well as sintered specimens It was established that the Astaloy CrM particles contain two distinct types of oxides One associated with the particle surface and another, mainly constituted by Cr, forming a dispersion of internal oxides These internal oxides were microstructurally characterised, both directly and by carbon extraction replicas A selection of powd
38 citations
TL;DR: In this article, the authors compared prealloyed and master alloyed gas atomised powders of − 16 μm and − 22 μm sizes, respectively, for green strength, green defects, sintered strength and microstructure.
Abstract: Stainless steel 316L MIM components can be made from either prealloyed powders or from master alloys blended with carbonyl iron powder. In this study these two techniques were compared using prealloyed and master alloyed gas atomised powders of − 16 μm and − 22 μm sizes. Four different compounds were prepared, characterised and injection moulded into tensile bars. The bars were compared for green strength, green defects, sintered strength and microstructure. The green components are stronger when carbonyl iron powder is used with the gas atomised master alloy. This material also seems to be less susceptible to moulding defects. The sintering strength of the material produced using the pre-alloyed powder was higher than the master alloyed prepared material. Little difference in mechanical properties existed between the materials fabricated from gas atomised prealloyed − 16 μm and the − 22 μm powders. Also, the viscosity of the mixtures was higher for the − 16 μm material and the master alloy mixtur...
33 citations
TL;DR: In this paper, a correlation between green part dimensional variation and feedstock viscosity variation is presented for the powder injection molding (PIM) manufacturing process, and the correlation was independent of powder type (316L gas atomised and water atomised) and mixing technique (batch and continuous).
Abstract: In this study, a correlation between green part dimensional variation and feedstock viscosity variation is presented for the powder injection moulding (PIM) manufacturing process. A correlation of an increase in green part dimensional variation as feedstock viscosity variation increases has been found and the correlation was independent of powder type (316L gas atomised and water atomised) and mixing technique (batch and continuous). The variation of feedstock viscosity was lowest over the greatest temperature range for high shear continuous compounding with a broad distribution of irregularly shaped powder. Thus, this feedstock material would have the greatest process window for injection moulding with the least variation.
33 citations
TL;DR: In this paper, the potential of PM Mn steels has been established in laboratory experiments, and the sintering of Fe(2)-4)Mn-(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of −55 °C.
Abstract: The potential of PM Mn steels has been established in laboratory experiments. This paper deals with sintering of Fe–(2–4)Mn–(0.3/0.7)C, also with 0˙85%Mo addition, in an industrial pusher furnace at 1180°C in an atmosphere of 25% hydrogen plus 75% nitrogen, obtained from a cryogenic liquid, giving an inlet dew-point of −55 °C. Tensile, bend (including fatigue) and miniature Charpy specimens were sintered in flowing gases and in semiclosed containers with a getter of ferromanganese, carbon and alumina. The quenched and tem- pered state was investigated, as was sinter hardening (cooling rate of 55 K min −1), simulated for comparison with slow cooling at 10 K min −1. As there was no forma tion of oxide networks at the combination of sintering temperature and dewpoint, in accordance with the Ellingham–Richardson diagram for Mn oxidation/reduction, the use of semiclosed containers was superfluous. The quenched and tempered specimens were brittle. Sinter hardening lead to an improvement in mechanical pr...
30 citations
TL;DR: In this paper, elemental powders of Al and 2 wt-% graphite were mechanically alloyed in a high energy horizontal attritor under purified argon atmosphere for 0·5-2 h.
Abstract: In this work elemental powders of Al and 2 wt-% graphite were mechanically alloyed in a high energy horizontal attritor under purified argon atmosphere for 0·5-2 h. Powder mixes were then cold pressed at 1200 MPa and sintered at 550°C for between 2-32 h under the same protective atmosphere. Structural evolution was characterised by X-ray diffraction, scanning electron microscopy and transmission electron microscopy techniques. Results revealed that mechanical alloying was very effective in pulverising the powder mix, where after 2 h, the mix was fine enough to oxidise rigorously when exposed to open air. In general however, mechanical alloying was found to be inefficient to synthesise Al with C. But after sintering, Al4C3 phase nanosized particles were formed in the microstructure. When the duration of sintering was prolonged, the particle population multiplied in number. Hence because of improvement in dispersion strengthening, the room temperature hardness of the material increased gradually.
28 citations
TL;DR: In this paper, four grades of sinter hardening materials have been compared, using industrial equipment, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties.
Abstract: Four grades of sinter hardening materials have been compared, using industrial equipment. Three powder types were completely prealloyed; the last one was a hybrid, combining prealloying and diffusion bonding. Different amounts of Cu have been added by mixing. The lubricated mixes, containing 0.6% graphite, have been compacted at different pressures, to form gears at green densities ≥7.0 g cm−3 ; the compacts have been sintered at 1120°C, under endogas from methane and fast cooled (at least 7 K s−1 within the range 850–400°C). The final step has been stress relieving, at 180°C, for 1 h. Material properties have been investigated, focusing on porosity, pore shape, hardness, microhardness, microstructure, local chemical composition and mechanical properties. For comparison, other gears, compacted in the same tool and at the same density level, but manufactured according to a more conventional cycle, i.e. starting from less alloyed powders and adding carbonitriding, quenching and stress relieving, ha...
22 citations
TL;DR: In this article, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel, and the results show that the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering.
Abstract: In this study, the effect of powder characteristics and their variability on the dimensional variability of green and sintered PIM components has been examined for 316L stainless steel. Three lots of gas atomised and three lots of water atomised powders were characterised and used to make six batches of PIM compound. These compound lots were injection moulded using a cavity pressure transducer and screw position regulation controls. The moulded geometry was measured in the green state and sintered state for dimensional variability. The general findings are that gas atomised powder produce less dimensional variability than the water atomised powder from lot to lot, however, the water atomised powders produce less in lot dimensional variability and are generally less susceptible to distortion of cantilevered members during sintering. Also, the lot to lot variation in the powder characteristics, such as particle size and pycnometer density, have an effect on dimensional stability whereas variations ...
TL;DR: In this article, a mathematical link between green body mass homogeneity and sintered component dimensional scatter is created, and data from die pressed and injection moulded powders are analyzed for green mass variations and size variations, providing guidelines for assessing research needs for process control, green body inspection and computer simulation with respect to dimensional tolerance goals.
Abstract: A mathematical link is created between green body mass homogeneity and sintered component dimensional scatter. In the light of the model, data from die pressed and injection moulded powders are analysed for green mass variations and sintered size variations. The model provides guidelines for assessing research needs for process control, green body inspection and computer simulation with respect to dimensional tolerance goals.
TL;DR: In this paper, the effects of master alloy composition and sintering atmosphere on the mechanical properties and dimensional change of sintered steels were studied, and the highest tensile strength and smallest dimensional change were obtained in a steel with a composition of Fe −0.85%Mo−1.40%Mn−0.8%Si− 0.7%C.
Abstract: Four Fe–Mn–Si master alloy powders with the compositions Fe–35%Mn–14%Si, Fe–35%Mn–20%Si, Fe–45%Mn–20%Si and Fe–60%Mn–14%Si were made by the cast milling method, and used to produce Fe–Mo–Mn–Si–C sintered steels. The effects of master alloy composition and sintering atmosphere on the mechanical properties and dimensional change of the sintered steels were studied. The steels were sintered either at 1250°C for 30 min in hydrogen–30% nitrogen or at 1120°C for 30 min in hydrogen and nitrogen–10% hydrogen with different dewpoints. After transient liquid phase sintering, the ultimate tensile strength, yield strength and hardness increase and elongation decreases with increasing contents of manganese and carbon. The dewpoint of sintering atmospheres had little effect on the properties, probably as the addition of manganese and silicon was in the form of a master alloy. The highest tensile strength and smallest dimensional change were obtained in a steel with a composition of Fe–0.85%Mo–1.40%Mn–0.8%Si–0.7%C.
TL;DR: In this article, a finite element-based simulation is used to evaluate the effect of feedstock rheology and confirm the impact of mixing on the mold filling phase of the PIM process.
Abstract: Numerical modelling of powder injection moulding (PIM) is a powerful tool to optimise mould design based on the rheology of the feedstock. In the present study, a finite element based simulation is utilised in evaluating the effect of feedstock rheology and confirming the effect of mixing on the mould filling phase of the PIM process. The numerical simulation, considering the slip layer model for non-isothermal flow analysis, confirms the experimental results that a feedstock with lower viscosity and higher homogeneity requires lower injection pressures and clamping forces during the mould filling stage. Results amply illustrate the effect of mixing conditions on moulding pressure and temperature related process parameters and flow stability during mould filling stage.
TL;DR: In this article, the authors reported the manufacturing of synchroniser hubs by warm powder compaction, which can substitute for the traditional double pressing and double sintering route for heavy duty gearbox components.
Abstract: Within a project supported by the European Union a consortium consisting of the companies Fraunhofer Institute for Structural Durability LBF, AMES, Miba, GKN, Federal Mogul, Hoganas, RWTH Aachen, Peugeot/Citroen and Fiat worked on the realisation of heavyduty gearbox components by warm powder compaction technology. In this paper, part of this work will be reported, namely the manufacturing of synchroniser hubs by this new technology. Until now these hubs were manufactured by the double pressing and double sintering route.Based on service load measurements in a gearbox of a Peugeot 506, vehicle fatigue tests were carried out with conventionally and warm pressed hubs. The results of fatigue tests with hubs as well as the durability tests carried out in the gearboxes of Peugeot/Fiat revealed the same performance for both technologies. As a consequence, the hubs produced by warm powder compaction can substitute for the ones produced conventionally. Further, the new technology reveals economic advantag...
TL;DR: In this article, the tensile and fatigue properties of the Alumix 431 alloy (Al, Zn, Mg and Cu alloys) produced using the conventional press and sinter processes in different pressures and temperatures are investigated.
Abstract: In this experimental study, tensile and fatigue properties of the Alumix 431 alloy (Al, Zn, Mg and Cu alloys) produced using the conventional press and sinter processes in different pressures and temperatures are investigated. The results clearly showed that the warm compacted specimens can reach the mechanical properties of the cold compacted ones under less pressure. In the fatigue tests it was observed that fracture started from large pores as shown in all scanning electron microscope (SEM) examinations and ductile fracture occurred. 85% of the 180 MPa/80°C and 77% of the 230 MPa/RT specimens fractured at the machined surface. Tensile and fatigue properties of warm compacted (180 MPa/80°C) and cold (230 MPa/RT) compacted specimens are almost equal at these same densities. This result indicates the economic benefit of warm compaction by the much lower applied compaction pressure.
TL;DR: In this paper, the microstructure and mechanical properties of Fe-Cu-Sn alloys containing graphite and talc were investigated and compared with those of wrought bronze by means of optical microscopy and X-ray diffraction.
Abstract: The microstructure and mechanical properties of Fe–Cu–Sn alloys containing graphite and talc were investigated and compared with those of wrought bronze. The influence of sintering temperature on the formation of microstructure and mechanical properties of the sintered alloys was studied. Microstructural investigations were carried out by means of optical microscopy and X-ray diffraction. In the initial stages of sintering (melting temperature of Fe–Cu–Sn peritectic), the liquid is distributed between grain boundaries. During the later stages of sintering, the liquid dissolves the iron and a solid solution of iron in the copper is formed. Microstructure analyses showed that during sintering iron based Fe–Cu–Sn–C solid solutions and copper based Cu–Fe–Sn–C solid solutions are formed.
TL;DR: In this paper, the competitive advantage of ferrous PM will in the future be enhanced by several factors, such as the achievement of much improved precision by effectively eliminating segregation, which will result in more precise composition of the components which in turn give less dimensional scatter as well as closer control of mechanical properties.
Abstract: I believe that the competitive advantage of ferrous PM will in the future be enhanced by several factors. Among the technical ones are the achievement of much improved precision by effectively eliminating segregation. This will result in more precise composition of the components which in turn gives less dimensional scatter as well as closer control of mechanical properties. By using computer modelling and digitally controlled multi-axis presses it will further be possible to eliminate defects such as cracks – which have haunted the PM industry for generations. New applications will be developed – among the most interesting are the soft magnetic components for motors and other electric equipment. Finally the growth of the PM industry as such will result in improved economy – lower cost – simply by the economy of scale and by improvements in the manufacturing methods.
TL;DR: In this article, a high energy planetary mill in air, argon and nitrogen atmosphere was used to synthesize ultrafine TiC1-X and Ti(C,N) powders for cutting tool applications.
Abstract: Ultrafine TiC1-X and Ti(C,N) powders 100-200 nm in size were synthesised through mechanical alloying for cutting tool applications. Elemental titanium and carbon were processed by a high energy planetary mill in air, argon and nitrogen atmosphere. With variations in the Ti/C ratios, the characteristics of synthesised powders were investigated. Of three atmospheres, argon was effective in synthesising pure TiC1-X powders. However, the presence of oxygen seemed to interfere with carbon and nitrogen diffusion into TiC lattice. Nitrogen exhibited a stronger affinity with titanium than carbon in forming the Ti(C,N) phase. This study shows the parameters used to determine the final compositions of synthesised TiC1-X and Ti(C1-XNX).
TL;DR: In this article, the wear rate of the sintered Al-Ni PM composite was found to increase with increasing load and decrease with increasing nickel content, which indicated the presence of the Al3 Ni phase in the Sintered alloy.
Abstract: Al–Ni powder mixtures containing 2, 4, 6 and 8 wt-% nickel were compacted at 125, 250, 375 and 500 MPa and sintered at 620, 630 and 640°C in a nitrogen atmosphere. The sintered density, sintered hardness and strength of composites thus produced were determined as a function of compaction pressure and sintering temperature. Wear rates of the composites were evaluated as a function of applied load and sliding velocity. Optical and scanning electron microscopy were used to reveal the morphology of powder and microstructures of green and sintered compacts. X-ray diffraction studies of the sintered compacts were made to confirm the phases formed on sintering. Sintered density, sintered hardness and strength increased with an increase in compaction pressure and nickel content. X-ray diffraction indicated the presence of Al3 Ni phase in the sintered alloy. The wear rate of the sintered Al–Ni PM composite was found to increase with increasing load and decrease with increasing nickel content.
TL;DR: In this paper, the authors analyzed the properties of WC-based cobalt-bonded hardmetals, sintered by two different techniques: vacuum and compression sintering (sinterhipping).
Abstract: This article analyses mechanical properties (strength, toughness) and wear behaviour (in different wear conditions) of WC based cobalt bonded hardmetals (prospectively for metalforming), sintered by two different techniques: vacuum and compression sintering (sinterhipping). It was found that compared with ordinary sintering, the sinterhipping technology enables the improvement of hardmetal resistance to brittle fracture (failure caused by tension stresses) as well as to wear, especially in the conditions of prevailing adhesive surface failure (metalforming, particularly blanking). The results obtained are described by the Griffith's strength theory of brittle materials.
TL;DR: In this paper, Potentiodynamic polarisation and intergranular immersion tests were conducted to study the corrosion behaviour of steel 316L with 0·1 and 0·23%N, prepared by hot isostatic pressing.
Abstract: Potentiodynamic polarisation and intergranular immersion tests were conducted to study the corrosion behaviour of steel 316L with 0·1 and 0·23%N, prepared by hot isostatic pressing. For comparison the corrosion resistance of conventional, nitrogen free rolled 316L steel was also examined. Specimens were subjected to solution treatment at 1050 and 1200°C for 2 h followed by water quenching. The effect of aging temperature and time, in the sensitisation rage 500-900°C, on the microstructure and corrosion behaviour was also investigated. The alloy with 0·23%N showed the highest corrosion resistance in the as hipped, as well as solution treated conditions. Addition of nitrogen causes retardation of intergranular corrosion upon aging at 500 and 900°C. When aging was performed at 700°C for 2 h or more, the alloy with 0·23%N suffered severe intergranular corrosion attack. The changes in corrosion behaviour of nitrogen containing alloys with different heat treatment conditions were attributed to the prese...
TL;DR: In this article, the effects of vacuum heat treatment under different cooling conditions on mechanical and structural properties of forged heavy alloys, such as 92.5W−5Ni−2.5Fe and 92.9W−6Ni−25Fe microalloyed with Co.
Abstract: This paper presents the effects of vacuum heat treatment under different cooling conditions on mechanical and structural properties of forged heavy alloys, such as 92.5W–5Ni–2.5Fe and 92.5W–5Ni–25Fe microalloyed with Co. The tungsten composition in the c phase has proved to be higher and more homogenous in the rapidly cooled alloys than in the slowly cooled ones. The effects of chemical composition inhomogeneity on mechanical and structural properties of alloys were also analysed and discussed. The results of tensile and toughness testing have shown an increase in ductility and toughness, while the strength of heat treated alloys decreased in comparison with the strength of forged alloys. The fracture analysis has shown that in the sintered and rotary forged alloys, intergranular fracture of the tungsten phase and transgranular fracture of the γ phase occurred, respectively. The fracture of these phases after heat treatment was characterised by transgranular morphology.
TL;DR: In this article, a new method to prepare titanium dental copings from titanium powder was tested, involving high velocity compaction and various elastic forms, which were used to achieve a semi-isostatic effect during the impact.
Abstract: A new method to prepare titanium dental copings from titanium powder was tested, involving high velocity compaction and various elastic forms, which were used to achieve a semi-isostatic effect during the impact. The tooth preparation die (mandrel) and the powder were placed inside an elastic form. The impact struck the elastic form, and the powder was compacted against the tooth preparation die. Several different elastomers were tested to find the best one. Cross-sections of the powder bodies were studied for density variations. The soft, flexible elastomer worked best to compact the powder. The highest densification could be focused closest to the mandrel where the coping should be milled out. The density in the highest density areas could locally reach 98–99%. If the method with elastic forms could be optimised to give as high a density as without elastic forms, a lower shrinkage and possible deformation of the copings could be reached.
TL;DR: In this article, numerical simulation is used to optimize the hot isostatic pressing (Hipping) manufacturing process using viscoplastic constitutive equations implemented in the finite elements software PRECAD/M.
Abstract: Hot isostatic pressing (hipping) is a manufacturing process capable of producing mechanical parts in only one operation by the hot forming of metal powders. The aim of this process is the production of net shape parts notably for aeronautics or nuclear applications. In the past, the trial and error method was used in order to find the right container and insert geometries giving the net shape parts. Several prototypes must therefore be manufactured before a successful conclusion is reached. This forming procedure results in expensive developing costs due to the quasi unitary production, pressure and temperature environment and the high temperature tooling involved.Numerical simulation raises hopes on the optimisation of this manufacturing process through the use of viscoplastic constitutive equations implemented in the finite elements software PRECAD/M.The rheological functions of this viscoplastic model are usually determined by assuming that the mean pressure in the sample is homogeneous and equ...
TL;DR: In this paper, the effects of milling process and time on mean particle size, distribution and morphology were analyzed by SEM and a laser particle size analyser, and it was found that increasing the milling time resulted in spherical shaped particles in the attritor and flakelike particles, with a reduced particle size in the turbula mixer.
Abstract: In this study, Fe-Cr-Al2O3-Ti-Y2O3 powders, used for superalloy production, were milled in a high energy mechanical alloying attritor and processed in a ball containing turbula mixer with a threedimensional action for 1, 2, 10, 24 and 48 h. The effects of milling process and time on mean particle size, distribution and morphology were analysed by SEM and a laser particle size analyser. Mechanically alloyed and turbula mixed composite powders were then mixed with a polyethylene based binder to obtain PIM feedstock. The flow behaviour of the powder feedstock was investigated by capillary rheometer at temperatures between 155 and 200°C. It was found that, increasing the milling time resulted in spherical shaped particles in the attritor and flakelike particles, with a reduced particle size, in the turbula mixer. Attritor milling led to the feedstock flowing more easily and better results for PIM, whereas turbula mixed powders were found not to be suitable for PIM because of the flakelike structure.
TL;DR: In this article, the microstructural evolution upon post-sintering heat treatment on these parts has been explored and the relation between swelling and porosity for different tungsten particle sizes has been demonstrated.
Abstract: It has been shown recently that porous tungsten can be obtained at much lower temperatures T ≤ 1200°C than those in conventional techniques where T ≥ 2000°C. This has been achieved by employing the concept of reactive sintering. The method makes use of the addition of Al, a low melting point phase which decreases the process temperature while acting as a potential sintering aid. As a result, higher homogeneity and uniform porosity distribution have been obtained. In this study, attention has been paid to the swelling phenomenon observed in porous tungsten upon post-sintering heat treatment. It has already been shown that swelling occurred in sintering porous tungsten by this alternative technique, and upon further heat treatment swelling instead of shrinkage became even more pronounced. This article explores the microstructural evolution upon post-sintering heat treatment on these parts. The relation between swelling and porosity for different tungsten particle sizes has been demonstrated. An atte...
TL;DR: In this article, an alternative sintering process was developed for porous tungsten technology and its outcomes in terms of porosity and porosity distribution were discussed. But, lack of homogeneity is observed in conventional parts and may contribute to a shorter cathode and hence lamp lifetime.
Abstract: One of the most important applications of tungsten is its use as a high current density cathode in the form of porous tungsten impregnated with electron emissive salts. Powder metallurgy is the usual processing route to make these parts since melting and casting of tungsten is very tedious due to its high melting point (3410±20°C). The overall porosity and porosity distribution are crucial parameters for the performance and lifetime of porous tungsten cathodes. Although there is an active debate over what constitutes the optimal porosity distribution, lack of homogeneity is observed in conventional parts. This may contribute to a shorter cathode and hence lamp lifetime. These parts are conventionally sintered at temperatures in excess of 2000°C. Furthermore, due to the high temperatures involved, the conventional sintering is very costly and energy consuming. This paper briefly looks into an alternative sintering process being developed for porous tungsten technology and its outcomes in terms of p...
TL;DR: In this article, a neural network model was developed for the prediction of strain hardening and densification constants of sintered aluminium preforms based on a three layer neural network with a back propagation learning algorithm.
Abstract: A neural network model has been developed for the prediction of strain hardening and densification constants of sintered aluminium preforms. The model is based on a three layer neural network with a back propagation learning algorithm. The training data were collected by the experimental setup in the laboratory for sintered aluminium and with various preform densities with different aspect ratios by using MoS2 as a lubricant. The network is trained to predict the values of strain hardening exponent index n i, strength coefficient k i, density power law exponent B i and density constant C i. Regression analysis between experimental and values predicted by the neural network shows the least error. This approach helps in the reduction of the experimentation required to determine these constants.
TL;DR: In this article, the authors aim to anticipate powder behaviour by account of their intrinsic characteristics and thus giving guidelines for modelling industrial cold die compaction, based on complete experimental testing of various industrial powders.
Abstract: Through this study, the authors aim to anticipate powder behaviour by account of their intrinsic characteristics and thus giving guidelines for modelling industrial cold die compaction. Investigations are based on complete experimental testing of various industrial powders. Because of the very distinct material properties (metal or ceramic, presence of binder element or lubricant), as well as the morphology, these powders offer a wide range of compaction behaviours that are analysed in order to establish objective considerations for suitable modelling. A global overview of powder behaviour is then proposed, based on two behaviour subtypes regarding powder hardness. By analysing their main features (a new concept is also detailed) it is then possible to simplify the characterisation and modelling of any powder behaviour.