Showing papers on "Powder metallurgy published in 1995"

Ceramic Processing and Sintering

Abstract: Ceramic fabrication processes - an introductory overview synthesis of powders powder characterization science of colloidal processing sol-gel processing powder consolidation and forming of ceramics sintering of ceramics - fundamentals theory of viscous sintering grain growth and microstructural control liquid-phase sintering problems of sintering densification process variables and densification practice.

General aspects and limits of conventional ultrafine WC powder manufacture and hard metal production

Abstract: Ultrafine WC/Co hard metals (average WC grain sizes ≤ 0.5 μm) can be successfully and reliably obtained by conventional hard metal manufacturing techniques. In this paper, some of the crucial aspects of conventional powder manufacture, powder milling and liquid phase sintering are discussed. Conventional ultrafine WC powder manufacture is based on the production of tungsten powder by hydrogen reduction of tungsten oxides and subsequent carburization. Alternatively, direct carburization can be carried out. However, inherent to the powder processing techniques used and the particle growth mechanisms involved (oxide precursors used, reduction and carburization history), there exists a lower limit beyond which-finer WC powders cannot be produced. This limit lies in the particle size range of 50–150 nm (0.05–0.15 μm). Powder milling is carried out to obtain an even dispersion of the Co binder in the ultrafine WC matrix. The more uniform the phase distribution (WC, Co, grain growth inhibitor) within the green powder compact, the more uniform will be the material transport during sintering, and hence the uniformity of the WC grain growth/growth inhibition during sintering. Enhanced WC grain growth occurs early in the sintering cycle, even below the temperature at which the liquid phase is formed. This growth can be largely restricted by the addition of VC. However, effective grain growth inhibition has to take place already during this early period of solid-state sintering. The ‘early’ availability of the grain growth inhibitor at the WC/Co interface can, therefore, determine the degree of growth inhibition. Ultrafine hard metals are in particular prone to discontinuous grain growth of the WC. Different reasons for this local growth mode are propounded relating to both the chemical as well as the geometrical departures from uniformity in the green powder compact. While it is still not possible to predict exactly an ultimate WC grain size limit, below which WC grain growth can no longer be restricted, even with proper inhibitor additions, experimental evidence indicates that this average WC grain size limit lies in the range of 200–300 nm. This limit is inherent to the existing conventional processing techniques (powder manufacture, milling, liquid phase sintering) and the WC growth mechanisms involved and can be overcome only by establishing a completely new route in hard metal manufacture.

Thermal Spray Processing of FGMs

Abstract: Functionally gradient materials (FGMs) display continuously or discontinuously varying compositions and/or microstructures over definable geometrical distances. The gradients can be continuous on a microscopic level, or they can be laminates comprised of gradients of metals, ceramics, polymers, or variations of porosity/density. Several processing techniques have been explored for the fabrication of FGMs for structural applications, e.g., powder metallurgy, thermal spraying, in situ synthesis, self-propagating high-temperature synthesis, reactive infiltration, etc. Physical and chemical vapor deposition (CVD) techniques are also being explored to process FGM films with nanometer level gradients in composition. This article addresses the issues related to thermal-spray processing of FGMs and will only peripherally compare the advantages and limitations of thermal spray versus other processing techniques as reported in the literature.In thermal spraying, feedstock material (in the form of powder, rod, or wire) is introduced into a combustion or plasma flame. The particles melt in transit and impinge on the substrate where they flatten, undergo rapid solidification, and form a deposit through successive impingement. Thermal spraying has been traditionally employed to produce a variety of protective coatings of ceramics, metals, and polymers on a range of substrates. More recently, the process has been used for spray-forming structural components.Arc spray, combustion, and plasma are the major techniques comprising thermal spray. These classifications are based on the type of heat source and the method by which feedstock is injected. Arc-spray processes use electrically conductive wire as feedstock, while combustion methods use powder or wire.

Soft magnetic composite materials-use for electrical machines

Abstract: Powder metallurgy has long been recognised as a cost effective method of producing high volumes of consistent parts for magnetic applications, particularly for DC applications and for hard magnetic materials. Powder metallurgy offers advantages including high material utilisation, precise material control, and the ability to produce relatively complex shapes. The structure of the paper is to first describe the nature, production and properties of the material, then move on to a discussion of its application in electrical machines. The paper ends with a description of a prototype machine, used to explore the material in a practical setting.

Formation of Nanostructural Materials Induced by Mechanical Processings ( Overview )

Abstract: Mechanical alloying (MA) was firstly developed to synthesize metallic matrix composite by mechanically incorporating preformed oxide and or carbide particles into a metallic matrix. A consecutive compaction process is applied to obtain bulk materials. During MA, powders are repeatedly welded, fractured and rewelded in a high energy mill leading to an intimate mixing on a nano/micro-scale with the possible formation of far from equilibrium phases. The versatility of MA is well known ; high volume, low energy mills can be used to commercially produced dispersion strengthened Al, Ni and other transition metal alloys. Various intermetallics and inorganic compounds (amorphous and/or nanocrystalline) have been synthesized by using higher energy mills which have been specially developed in some cases. Mechanical alloying, it appears, as suggested by T. H. Courtney et al., is the Alladin's lamp of powder processing. All the published works have shown that the reaction and end products of the MA process strongly depend on the milling conditions. As a consequence, it is obvious that an improved understanding of the dynamics of MA process is required to gain a full appreciation of the industrial potential of the technique for synthesizing materials. Recently, M. Abdellaoui and E. Gaffet have shown that the crystal to amorphous phase transition (at least in the case of the model Ni 10 Zr 7 ) only depends on the injected mechanically power, allowing a direct comparison among experiments performed using distinct type of milling apparatus (planetary milling machine, horizontal apparatus). An alternative method has been recently proposed by N. Malhouroux-Gaffet and E. Gaffet, for the solid state synthesis of disilicide powders exhibiting a wide contamination during the direct MA preparation : the mechanically activated annealing process (M2AP). Such a M2AP method has been applied to the synthesis of FeSi 2 , MoSi 2 , WSi 2 compounds. Such a method appears as being a well suitable one for the low temperature synthesis of refractory nanomaterials. Recent applications have been successfully performed to mechanically activated sintering (MAS).

Application of Mechanical Alloying to Chemical Refining (Overview)

Abstract: Since its discovery the process of mechanical alloying has demonstrated significant potential for the synthesis of materials with novel structures and properties. In this paper recent studies of the application of mechanical alloying to solid state refining processes are reviewed. These studies show that mechanical alloying enables displacement reactions, which conventionally require high temperatures to be thermally activated, to be mechanically activated during milling in a ball mill at ambient temperatures. It is shown that mechanical alloying can provide the basis of a single stage process which combines the separate processes of refining, alloying and powder manufacture. Of particular interest is the application of the process to the synthesis of rare earth permanent magnet materials.

Selective laser sintering with composite plastic material

Abstract: A composite powder specially adapted for use in selective laser sintering is disclosed. The composite powder includes a polymer powder dry mixed with a reinforcement powder, where the polymer powder has a melting temperature substantially lower than that of the reinforcement powder. In the case where nearfully dense parts are to be formed, the first constituent powder is preferably a semi-crystalline powder, for example nylon 11, of a composition suitable for forming near-fully dense parts when used unblended in selective laser sintering; if porous parts are desired, the polymer powder is an amorphous powder, such as polycarbonate, polystyrene, acrylates, and styrene/acrylate copolymers. The reinforcement powder is preferably microspheres of glass, preferably coated to enhance wetting and adhesion with the polymer powder when selective laser sintering is performed. Besides improving the stiffness and heat resistance of the part produced, the composite powder widens the process window over that provided by unblended powder, provides improved dimensional accuracy in the part produced, and facilitates roughbreakout and smooth finishing of the part produced.

Powder deposition in selective metal powder sintering

Abstract: Selective metal powder sintering is a layer‐by‐layer manufacturing system producing metallic parts with good mechanical properties. Describes why an Fe‐Cu powder mixture has been selected as the basic material for the process. Deals with the powder deposition issue and proposes a mechanism which can deposit thin powder layers on top of a recipient. Shows that the powder deposition mainly depends on the powder properties. States that the required powder properties are partially compatible with the specifications set by the technology of selective sintering but that some properties are in conflict with one another. Discusses the resulting compromises needed in the powder mixtures and the required modifications to the deposition mechanism.

Effects of vacuum hot pressing parameters on the tensile properties and microstructures of SiC-2124 Al composites

Abstract: The effects of vacuum hot pressing temperature and pressure on tensile properties and microstructures of 20 vol.% SiC whisker (SiC w )-2124 Al composites were investigated. The tensile strength of SiC w -2124 Al composite increased rapidly with increasing vacuum hot pressing temperature up to 570 °C and became saturated above 570 °C. The increase in tensile strength with increasing vacuum hot pressing temperature is due to the enhanced densification of composite and reduced damage of whiskers from the softening of 2124 Al matrix with increasing volume fraction of liquid phase. The vacuum hot pressing pressure needed to be higher than 70 MPa to achieve densification of SiC w -2124 Al composite above 99%. A vacuum hot pressing pressure above 70 MPa was not helpful to enhance the tensile strength, since the aspect ratio of whiskers decreased owing to the damage during vacuum hot pressing. Combining the effects of the aspect ratio of whiskers and the density of composite, which are dependent on the vacuum hot pressing parameters, an equation is proposed to estimate the tensile strength of SiC w -2124 Al composite. The effect of porosity on tensile strength of SiC w -2124 Al composite was found to be more sensitive compared with powder metallurgy alloys or ceramics, since the most pores were located at whisker-matrix interfaces and reduced the load transfer efficiency between SiC whiskers and 2124 Al matrix.

A cost-effective P/M titanium matrix composite for automobile use

Abstract: The aim of the present study is to obtain a new high-performance titanium matrix composite appropriate for automobile parts using a new low-cost powder metallurgy process. The results can be summarized as follows: (1) A production process was developed for a sintered titanium alloy from cheap, low-purity titanium powder (sponge fines) which in its as-sintered form (without expensive hot isostatic pressing or heat treatment) achieves superior fatigue properties to hot-isostatic-pressed titanium alloy made from expensive high purity hydride-dehydride titanium powder. (2) TiB was found to be a superior reinforcing compound for blended elemental titanium matrix composites than SiC, B4C, TiAl, TiB2, TiN and TiC tested previously and it was used in the above low-cost production process to make the new disperse-particle titanium matrix composites. (3) The developed titanium matrix composite allows considerably cheaper production of parts from titanium alloy than by conventional ingot forging methods and was confirmed to be far superior to conventional titatium alloys in tensile strength, fatigue properties, rigidity, heat resistance, and wear resistance.

Preparation of Thermoelectric β-FeSi2 Doped with Al and Mn by Mechanical Alloying (Overview)

Abstract: MA has been applied to the Fe-Si system near the composition FeSi 2 starting from elemental powders using a horizontal ball mill. Powders were examined by means of XRD and DSC. After substantially long time milling, the premixed powder of nominal composition Fe 3o Si 70 was found to transform to a homogeneous β-FeSi 2 phase in the as milled condition. The appropriate milling time of a powder for sintering was considered to be 720 ks. At this stage the powder consists of very fine Fe and Si crystals in the as milled condition. This transforms into a homogeneous β phase with an exothermic reaction at around 720 K. The transformation rate from (α + e) to β in the MA powder was found to be faster than that in the specimen prepared from an eutectic alloy ingot. Specimens prepared by hot pressing from MA powder showed smaller grain size and lower thermal conductivity than those prepared by ingot metallurgy. The p-type thermoelectric properties were measured for β-FeSi 5 hot-press sintered specimens doped with both Al and Mn. Due to a small grain size, the specimen prepared from the MA powder showed a higher figure of merit and hence is, higher in conversion efficiency than that prepared by ingot metallurgy. The sequence of phase formation by mechanical alloying and post-milling annealing is considered. The effects of grain size on the thermal conductivity and Seebeck potential are discussed.

Relation between pore structure and fatigue behavior in sintered iron-copper-carbon

Abstract: A statistically designed study of the high cycle fatigue of an iron-copper-carbon alloy (Fe-2w/oCu-0.8w/oC or FC-0208) was performed to isolate the controlling pore microstructure factors with respect to endurance strength. The study included four variations in powder characteristics and three variations in density, with a sintering cycle of 30min at 1120 o C in an atmosphere of 11v/o hydrogen and 89v/o nitrogen. Twenty samples were fatigue tested for each of the 12 powder density test combinations to assess the fatigue strength distribution. Quantitative metallography provided insight to the controlling microstructure effects on fatigue life. Regression analysis between the various mechanical properties and microstructure parameters shows that the fatigue endurance strength is linked to the porosity, average pore curvarture, and separation distance between pores. The powder characteristics optimal for fatigue resistance depend in the density level. At the lower density (6.65g/cm 3 ), endurance is enhanced by increasing the proportion of small particles, thereby generating smaller, smoother pores in sintering. But an increased proportion of large particles is desirable for fatigue resistance at the higher densities (7.15g/cm 3 )

Gear wheels rolled from powder metal blanks and method of manufacture

Abstract: A gear wheel is formed from a pressed and sintered powder metal blank in which the metal powder comprises an admixture of iron powder and at least one alloying addition and the gear wheel is surface hardened by densifying at least the tooth root and flank regions to establish densification in the range of 90 to 100 percent of full theoretical density to a depth of at least 380 and up to about 1,000 microns.

Method of producing structural metal matrix composite products from a blend of powders

Abstract: A method of preparing a metal alloy product from a powder blend. The method comprising: (a) cold pressing a blend to form a compact, the metal blend comprising a metal powder phase and at least one reinforcement phase having a hardness greater than the metal phase; (b) heating the compact to form a preheated compact; and (c) hot working the heated compact. In a preferred method, the powder metal blend comprises 50 to 90 vol. % of an aluminum alloy powder and 10 to 50 vol. % of silicon carbide; the heating of the compact perforated in a nitrogen atmosphere to form a preheated compact; and the extruded hot compact is hot worked. Hot working may take the form of forging, rolling, upset forging, exuding, compacting or other processes known in the art.

Gravity induced solid grain packing during liquid phase sintering

Abstract: The effects of gravity on the solid grain packing in liquid phase sintering have been investigated by both theoretical and experimental analyses. This treatment relies on quantitative microstructural measurements to determine the solid volume fraction variation along the direction of gravity. The model assumes that the grain packing coordination is proportional to the gravitational pressure and density difference between solid and liquid phases. It is confirmed by sintering experiments on W-Ni, and is consistent with a computer simulation model previously reported by German.

High density metal components manufactured by powder metallurgy

Abstract: A high density metal component manufactured by powder metallurgy is disclosed The powder metallurgy method provides metal components having a density greater than 95% of theoretical density using a single sequence of uniaxial pressing and heating The metal components are manufactured from substantially linear, acicular metal particles having a substantially triangular cross section

Powders and products of tantalum, niobium and their alloys

Abstract: A powder of tantalum, niobium, or an alloy thereof, having an oxygen content less than about 300 ppm, and the production thereof without exposure to a temperature greater than about 0.7 TH. A powder metallurgy formed product of tantalum, niobium, or an alloy thereof, having an oxygen content less than about 300 ppm, and the production thereof without exposure to a temperature greater than about 0.7 TH.

A mathematical model for gravity-induced distortion during liquid-phase sintering

Abstract: Liquid-phase-sintered materials consist of interconnected crystalline grains in a homogeneous matrix phase that forms a liquid during sintering. These composites exhibit viscous flow during sintering that allows densification. Gravitational forces give rise to compact distortion when there is a large amount of liquid at a high temperature. This article treats kinetic aspects of distortion during sintering of tungsten heavy alloys (W-Ni-Fe). The mathematical model predicts distortion and highlights the important variables influencing this phenomenon. The results provide guidelines for minimizing distortion due to gravity. Experiments conducted at several different sintering times show reasonably good agreement with theoretical predictions using the liquid-phase viscosity as a single adjustable parameter. Theoretical predictions of the model are crucial to designing microgravity experiments aimed at understanding dimensional stability.

The formation and the role of potassium bubbles in NS-doped tungsten

Abstract: The present paper describes the creation of potassium bubbles in non-sag (NS)-doped tungsten ingots during sintering, the behaviour of these bubbles during thermomechanical processing and their role in lamp filaments. Initial discussion will focus on the chemistry changes and densification kinetics of reduced tungsten powder during ingot sintering. A key feature of NS-doped ingot sintering is the generation of potassium bubbles in the tungsten ingot matrix from submicron pores in the reduced powder which contain the NS dopant. The chemistry changes associated with potassium bubble formation are critical to the properties of the ingot, such as potassium concentration, potassium bubble distribution, grain structure and density, and consequently the properties of the final incandescent lamp wire. In addition to the potassium bubble distribution, the density and microstructure of the NS-doped, sintered tungsten ingot are also critical. Consequently, the densification kinetics and microstructural evolution over the temperature range 1200–3000 °C will be discussed together with the predominant sintering mechanisms. Once the ingot is manufactured, one must also understand how the bubbles respond to the thermomechanical processing and how they influence the overall wire micro-structure. The morphological changes of the potassium bubbles have been investigated at the ingot, rod and wire stages of the wire manufacturing process. Potassium bubble growth during filament operation is discussed using analytical models together with microstructural observations.

Powder metallurgy apparatus and process using electrostatic die wall lubrication

Abstract: A method of making a metal composite part by compacting a metal powder composition in a die whose wall surfaces have been electrostatically coated with a lubricant, thereby eliminating or reducing a lubricant in the metal powder composition, resulting in a metal composite having greater density and strength. The method further includes providing an electrostatic charge to the metal powder composition. A powder metallurgy apparatus is also provided.

Characterisation of spatial distribution of reinforcement in powder metallurgy route Al/SiCp metal matrix composites

Abstract: Two techniques which rely on direct microstructural measurements to assess the quality of the spatial distribution of reinforcements in metal matrix composites are outlined in the present paper. The first technique is based on the measurement of the distribution of interparticle spacings of the reinforcement, and the second is based on the Dirichlet tessellation construction. Both techniques are applied to Al–4Cu/20SiCp (wt-%) metal matrix composites produced via a powder metallurgy route. The results show that the widths of the interparticle spacing and Dirichlet cell size distributions are afunction of the level of microstructural homogeneity. The techniques are not sufficiently sensitive to small variations in spatial distribution.MST/2081

Nontraditional Mechanical Alloying by the Controlled Plastic Deformation, Flow and Fracture Processes (Overview)

Abstract: Two aspects of nontraditional mechanical alloying are considered as one of the most promising solid-state materials processing : direct net-shaping into bulk materials from element constituent powders through the combined secondary processing to MA, and, precise description and understanding of mechanically mixing and refinement processes. Through the experimental studies on our developed mechanical alloying in some systems of materials, two key issues are found to promote the above two directions into further advancement. Those are the preclustering or premixing effect induced by MA on the secondary processing and the effect of controlled plastic flow and failure on the mechanical alloying. With respect to the former issue, MA combined with the shock reactive synthesis is developed to demonstrate that the intermetallic compound should be synthesized in the solid-state reaction from the MAed precursor. On the latter, elasto-plastic super-large deformation analysis is applied to the powder compaction and extrusion process in the repeated loading for nontraditional mechanical alloying to show that the refinement process in MA should be attributed to the large configuration change of powders including their plastic failure into fragments.

The relationship between consolidation behavior and particle size in Fe nanometric powders

Abstract: The interest in ultrafine particles with diameters of the order of 10's of nanometers has been growing during recent years. This interest stems specially from the unique physical and chemical properties that matter presents in the ultra dispersed state and the special properties of material having a nanocrystalline structure. Such materials appear to present potentially interesting applications. Recent investigations have been promoted by new methods of ultrafine powder production. A promising aspect of ultrafine powders is the preparation of bulk materials by sintering. As a matter of fact, small particle size decreases the sintering temperature which should allow alloys, ceramics and cermets having attractive mechanical properties to be obtained. The present study is a quantitative analysis of the consolidation process of ultrafine iron powders and compared their behavior with that of micrometric powders. Indeed, few studies have been carried out using metallic powders and there is some work suggesting that products with high green density (the density after pressing prior to sintering) may not be obtainable, whatever the consolidation pressure used.