Effects of wet and dry milling conditions on properties of mechanically alloyed and sintered W–C and W–B4C–C composites

Abstract

Tungsten based W–1C and W–2B₄C–1C (wt-%) powders synthesised by mechanical alloying (MA) for milling durations of 10, 20 and 30 h, in wet (ethanol) and dry conditions, were characterised. X-ray fluorescence spectroscopy investigations revealed Co contamination which increased with increasing milling time during wet milling. X-ray diffraction investigations revealed the presence of W and WC phases in all powders, Co₃C intermetallic in the wet milled W–1C powders and W₂B intermetallic phase in both wet and dry milled W–2B₄C–1C powders. As blended and MA processed powders were consolidated into green compacts by uniaxial cold pressing at 500 MPa and solid phase sintered at 1680°C under hydrogen and argon atmospheres for 1 h. X-ray diffraction investigations revealed the presence of W₂C intermetallic phase in sintered composites produced from both wet and dry milled W–1C powders and the W₂B intermetallic phase in sintered material from the wet milled W–2B₄C–1C powder. Sintered composites from wet milled powders showed relative densities >91%, with the maximum density of 99·5% measured for the sintered 30 h wet milled W–2B₄C–1C composites. Microhardness values for the wet milled W–1C and W–2B₄C–1C composites were 2–2·5 times higher than those for dry milled composite powders. A maximum hardness value of 23·7±2·1 GPa was measured for the sintered W–2B₄C–1C composite wet milled for 20 h.     

Reactive hot pressing of nonstoichiometric uranium dioxide

High density UO_2+x pellets have been produced by reactive hot pressing uranyl oxalate at temperatures up to 700°C. Rapid densification occurred during the decomposition reactions resulting in densities in the range 90 to 92 pct of the theoretical. A density of 98 pct of the theoretical value was achieved by further hot-pressing at 650° to 700°C for 30 min. This densification behavior can be related to the nonstoichiometry and submicron sized particles of UO_2+x produced in the decomposition reactions. The kinetics of hot-pressing of powder compacts of this UO_2+x were studied in the temperature range 500° to 700°C. The results were analyzed utilizing models proposed by Fryer. The activation energy of 53 kcal per mole, obtained from this analysis is the same as that for creep of nonstoichiometric urania in the temperature range 975° to 1400°C, suggesting that the mechanism controlling the rate of the final stage of densification may be a creep process.     

Combustion synthesis of SiC nanosized powders at low nitrogen pressure

Abstract

Combustion synthesis of β-silicon carbide (SiC) powders was accomplished at a nitrogen pressure lower than 2 MPa. The combination of mechanical activation and chemical stimulation was effective in enhancing the reactivity of Si powder reactants, which was responsible for the reduction in the minimum nitrogen pressure normally required for the combustion synthesis of SiC. Nanosized β-SiC powders with spherical particles were synthesised at nitrogen pressure as low as 1 MPa. The combustion synthesised SiC powders have a narrow particle size distribution in the range of 50–100 nm and could be hot pressed to 99·1% theoretical density with 10 wt-%Y₂O₃ and AlN as additives.     

The effect of Mo addition on the liquid-phase sintering of W heavy alloy

The morphological and compositional changes of grains have been investigated in the initial stage of liquid-phase sintering of W-Mo-Ni-Fe powder compacts. Both large (5.4-μm) and small (1.3-μm) W powders have been used to vary their time of dissolution in the liquid matrix. When 8OW-10M0-7Ni-3Fe (wt pct) compacts of fine (about 1- to 2-μm) Mo, Ni, and Fe and coarse (5.4-μm) W powders are liquid-phase sintered at 1500 °C, the Mo powder and a fraction of the W powder rapidly dissolve in the Ni-Fe liquid matrix. The W-Mo grains (containing small amounts of Ni and Fe) nucleate in the matrix and grow while the W particles slowly dissolve. In this transient initial stage of the liquid-phase sintering, duplex structures of coarse W-Mo grains and fine W particles are obtained. As the W particles dissolve in the liquid matrix during the sintering, the W content in the precipitated solid phase also increases. The dissolution of the small W particles is assessed to be driven partially by the coherency strain produced by Mo diffusion at the surface. During sintering, the W particles continuously dissolve while the W-Mo grains grow. When the compacts are prepared from a fine (1.3-μm) W powder, the W grains dissolve more rapidly, in about 1 hour, and only W-Mo grains remain. These observations show that the morphological evolution of grains during liquid-phase sintering can be strongly influenced by the chemical equilibrium process. formerly Graduate Student, Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology     

Sintered copper–graphite powder compacts forindustrial applications

Abstract

Studies were made on copper/graphite based powders and sintered compacts for industrial applications. The dependence of particle shape on friction in the powder mass, compression ratio, and electrical receptivity of powder metallurgy components was studied using near spherical precipitated copper powders and angular or flakelike powders generated by mechanical comminution. Results reveal that powders with particles that are nearly spherical in shape have lower friction, lower compression ratios, and higher electrical resistivities in sintered compacts than powders with acicular or flakelike particles. Also, the effects produced by the small additions of lead and zinc (up to 2·5 wt-%) on the electrical resistivity and hardness of sintered copper–graphite compacts are also presented, and the influence of variation of briguetting pressure is discussed.     

Growth of Mo grains around Al2O3 particles during liquid phase sintering

The growth behavior of Mo grains in contact with Al₂O₃ particles during liquid phase sintering was studied in 95.4 Mo-4 Ni-0.6 Al₂O₃ (wt%). Sintering of the powder compacts at 1460°C was repeatedly interrupted by cooling to 1300°C and reheating to 1460°C. Layers grown on Mo grains during each sintering interval between the interruptions were revealed by etch boundaries. The Mo grains grew around Al₂O₃ particles by solution-reprecipitation. In contact areas the Mo grains and the Al₂O₃-particles were separated by thin liquid films. Deposition or loss of material in contact areas resulting from material transport in the thin liquid film was found to be slow. During prolonged sintering some Al₂O₃ particles were trapped within growing Mo grains.     

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.     

SOME ASPECTS OF THE MILLING OF CEMENTED CARBIDE POWDER

Abstract

Cemented carbide powders milled for various times have been studied with respect to the shape and internal structure of the grains. The shrinkage of powder compacts during heat-treatment was recorded dilatometrically. The major effects of milling seem to be to alter the grain size and morphology of the powder and to form a fine dispersion of small particles of Co₃O₄.     

Mechanical properties of green compacts. II. Effect of powder relative bulk density on the strength of compacts with different forming temperature conditions

Mechanisms of strength for green compacts made from powders of iron, nickel and its alloys, copper, tin, and zinc are analyzed. The strength of green compacts prepared from metal powders of medium fineness with a relative bulk density (RBD) from 0.119 to 0.568 by two-way compaction in rigid dies with homologous temperatures from 0.15 to 0.59 (pressure from 200 to 800 MPa, powder deformation rate 10^?2–10^?3 m/sec) is studied. Compact strength is determined by diametric compression of cylindrical compacts. The dependence of strength on compact porosity is studied by the Bal’shin equation. The possibility is demonstrated of using this relationship in order to describe hot compaction and formally describe cold compaction of powders with RBD up to 0.40. The effect of homologous temperature and powder RBD on compact strength is determined. The homologous temperature for transition from warm to hot compaction and the effect of compact density (degree of deformation) on this temperature is studied. It is shown that linear approximation is possible for the dependence of compact strength on powder RBD according to the equation σ _f.c = 87–217?RBD.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5/FeAl2 powders Part 1: Experimentation and results

Abstract

High density Fe₃Al was produced through transient liquid phase sintering, using rapid heating rates of greater than 150 K min^-1 and a mixture of prealloyed and elemental powders. Prealloyed Fe₂Al₅/FeAl₂ (50Fe/50Al, wt-%) powder was added to elemental iron powder in a ratio appropriate for producing an overall Fe₃Al (13·87 wt-%) ratio. The heating rate, sintering time, sintering temperature, green density and powder particle size were controlled during the study. Heating rate, sintering time and powder particle size had the most significant influence upon the sintered density of the compacts. The highest sintered density of 6·12 Mg m^-3 (92% of the theoretical density for Fe3Al) was achieved after 15 minutes of sintering at 1350°C, using a 250 K min^{- 1} heating rate, 1-6 μm Fe powders and 5·66 μm alloy powders.

SEM microscopy suggests that agglomerated Fe₂Al₅/ FeAl₂ particles, which form a liquid during sintering, are responsible for a significant portion of the remaining porosity in high sintered density compacts, creating stable pores, larger than 100 μm diameter, after melting. High density was achieved by minimising the Kirkendall porosity formed during heating by unbalanced diffusion and solubility between the iron and Fe₂Al₅/FeAl₂ components. The lower diffusion rate of aluminium in the prealloyed powder into the iron compared with elemental aluminium in iron, coupled with a fast heating rate, is expected to permit minimal iron-aluminium interdiffusion during heating so that when a liquid forms the aluminium dissolves in the iron to promote solidification at a lower aluminium content. This leads to a further reduction in porosity.     

Characteristic of interconnected porosity in ferrous PM compacts

Abstract

The purpose of this work was to quantitatively describe the interconnected porosity in iron compacts, both in macro- and microscale. Size and volume fraction of micro-, meso- and macropores were examined in the compacts with density within 5˙6–6˙4 g cm^?3, made in laboratory conditions of two iron powders: NC100˙24 and ASC100˙29 manufactured by Höganäs Company. The interconnected porosity was determined using the method based on measuring the sorption isotherms of CO₂ and benzene at T=25°C in static conditions in a high vacuum gravimetric appliance equipped with McBain–Bakr weighers.

Volume distributions of individual size classes of micro- and mesopores in the compacts made of both iron powders with fixed density were compared.

Relationships between density and the interconnected micro- and macroporosity of the examined compacts were determined.     

THE IMPORTANCE OF SURFACE PROPERTIES AND PARTICLE SHAPE IN THE COMPACTION OF GRAPHITE AND MoS2

Abstract

Adsorptive studies of the surfaces of graphite and MOS₂have shown that these consist of two distinct types of site. The sites on the basal-plane surface differ from those on the edge surface with respect to their relative affinities for different organic compounds. These findings led to the development of grinding techniques to produce graphite and MoS₂ powders possessing different ratios of basal-plane:edge-surface area.

Grinding graphite and MoS₂ in the presence of low-viscosity, volatile hydrocarbons produced very thin flake-like powders, consisting predominantly of basal-plane surface. These fine flakes showed a high affinity for long-chain n-paraffins and were therefore termed oleophilic solids. Grinding under reduced pressure also produced very fine powders, having, however, a more granular structure exhibiting a far lower ratio of basal-plane: edge-surface area. These were termed polar solids to distinguish them from the solids ground in liquid hydrocarbons.

The cold-forming properties of the various powders have been compared under uniaxial compaction. The conversion of synthetic and natural graphite powders to the oleophilic form resulted in marked improvements in both compact strength and modulus. Synthetic graphite converted to the polar form would not form a compact at cold-forming pressures up to 800 MN/m².

The cohesive properties of the oleophilic graphite powders were improved by heating to 900°C in hydrogen. Electrical-resistivity measurements showed that cold-formed oleophilic graphite compacts exhibited a marked anisotropy. The improved cold-forming properties of the powders are ascribed directly to improved cohesion via basal-plane site interactions, coupled with the facility of the flake powders to take up a preferential orientation during compaction in order fully to utilize the extensive basal-plane sites available for cohesion.

The differences between the oleophilic and polar forms of MoS₂ were less marked. It is believed that interparticle cohesive junctions are more readily formed via edge/edge interactions, and basal-plane junctions do not play as important a role in the cohesion of MoS₂ as in that of graphite.

The corrosion and abrasion of metal surfaces by graphite and MoS₂ have been examined. In all cases the powders converted to the oleophilic form showed reduced abrasive and corrosive characteristics when compared to similar powders converted to the polar form. These improvements are believed to result from the reduction of the possibilities of edge interactions with the metal surfaces.     

FACTORS AFFECTING THE COMPACTION OF TUNGSTEN POWDERS

Abstract

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

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

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

Microstructural characterisation and mechanical properties of spark plasma-sintered TiB2-reinforced titanium matrix composite

Titanium and titanium matrix composites, reinforced with TiB₂ particles, have been synthesised by the spark plasma sintering method at 1050°C under 50?MPa pressure, using mixtures of 2.4?wt.-% TiB₂ and 97.6?wt.-% Ti powders. The changes in microstructural features and mechanical properties were investigated. XRD results and SEM observations confirm the formation of TiB whiskers as a result of the reaction between Ti and TiB₂. However, some unreacted TiB₂ particles have remained in the composite owing to the incomplete chemical reaction between matrix and additives. The measured mechanical properties demonstrate that the increase in hardness and tensile strength with TiB₂ addition is mainly attributed to the generation of TiB whiskers, increase in relative density and decrease in grain size, while the reduction in bending strength is possibly due to the plastic restraint imposed on the matrix by the TiB whiskers and unreacted TiB₂ particles.     

Magnetic and structural properties of hot pressed Cu–SmCo5 composites obtained by mechanical alloying

Abstract

Cu–8 wt-%SmCo₅ alloys were obtained through mechanical milling for novel industrial applications. Copper and SmCo₅ powder mixtures were mechanically alloyed in a planetary ball mill to disperse SmCo₅ fine particles in the copper matrix with the aim to modify the structural, mechanical, electrical and magnetic properties. The resulting alloyed powders were characterised as a function of milling time. Under the magnetic field, SmCo₅ particles achieved M_s to improve the soft magnetic properties of copper–8 wt-%SmCo₅ to be used in dielectromagnetic components. The magnetic properties of Cu–8 wt-%SmCo₅ powders reached their optimum values after milling time ranging from 10 to 15 h. The consolidation of milled alloy powders was performed by uniaxial hot pressing at 923 K for 2 h under argon atmosphere to obtain dense compacts. The consolidation process resulted in good dense metal matrix composite materials with adequate properties of compression strength >900 MPa, 95 HRB in hardness, electrical conductivity up to 43% of that of the International Annealed Copper Standard (IACS) and magnetic properties such as coercive field, saturation and remanent magnetisation obtained at 218 Oe, 70·23 emu g^?1 and 6·09 emu g^?1 respectively at 300 K. The existence of a coercive field and a little magnetic memory of the consolidated system is a typical behaviour of magnetically soft materials. The variation of electric and magnetic properties and its dependence on structure strength change with milling time were discussed.     

Effect of compacting pressure on microstructure and mechanical properties of carbide cutting tools

Abstract

The effects of compaction pressure on properties of carbide cutting tools containing 80·5 wt-%WC, 5 wt-%TiC, 5 wt-%TaC–NbC and 9·5 wt-%Co were studied. These tools were formed by powder metallurgy with different compacting pressures ranging from 77 to 231 MPa (5–15 tons in^?2) and sintered in a vacuum furnace at a constant sintering temperature (1450°C) and at a constant heating and cooling rate of 5°C min^?1. Green and bulk densities, shrinkage and hardness of the produced compacts were measured. Tool cutting performance has been assessed based on machining a medium alloyed steel workpiece under different cutting conditions and measuring the tool flank wear and the workpiece surface roughness. The microstructure of the compacts was metallographically examined using scanning electron microscopy. The results have revealed that both densities and hardness figures increase with increasing compaction pressures, while shrinkage decreases. Cutting performance has not demonstrated a substantial improvement of the tool's performance and life due to the increasing compacting pressure of these tools.     

DISPERSION-STRENGTHENED IRON BY GEL PRECIPITATION

Abstract

Metastable iron oxide particles containing calcium, magnesium, or zirconium in solid solution were produced by the gel precipitation method. These were reduced to metal with hydrogen/nitrogen mixtures in a fluidized-bed furnace at 800°C to yield iron powders containing dispersed oxide phases within each iron particle. The oxide phases were either 2CaO.Fe₂O₃, a solid solution of MgO and FeO, or ZrO₂, which appeared to be free from iron. Consolidation by compacting the powders into cans, sealing under vacuum, and hot extrusion yielded bars in which the oxide particles were dispersed. Hardness and tensile-test data for material heated to 1000°C for up to 100 h suggest that the oxides containing iron coarsen rapidly and contribute to strengthening only by maintaining a small matrix grain size. The iron-free ZrO₂ appears to be a true dispersion hardener and to restrain grain growth more than do the other oxides investigated.     

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.     

Current PM Literature for Engineers and Users

Abstract

Fine tin powders were produced in a pilot plant gas atomiser. Nitrogen gas at 1·56 MPa pressure was used as the atomising agent in a ‘confined design’ nozzle which operated vertically upwards. A range of metal flowrates from 0·864 to 1·425 kg min^?1 was studied at a melt temperature of 450°C. Powders were sized using dry sieving down to 45 μm and wet sieving for smaller sizes. The Sauter mean diameter of the powders varied from 9·01 to 10·28 μm, depending on the rate of production. The size distribution was bimodal (albeit not very well defined) with the peak separation at ～44 μm. In the fine size range, particles were spherical, while those in the coarse range were more elongated or irregular in shape and free of satellites. Comparison of the tin powders with copper powders from another study, AA 2014 aluminium alloy powders, and magnesium and zinc powders from previous work showed that the differences in mean diameter and standard deviation are small among these common metals at a given volumetric production rate. This confirms the overriding importance of liquid metal volume flowrate under fixed gas flow conditions in gas atomisation, while the actual physical properties of the liquid playa secondary role. Although surface tension is secondary to volume flowrate in importance for controlling particle size, the study has shown that a liquid metal with lower surface tension and viscosity than AA 2014 alloy, together with a higher density, yields finer particles. PM/0667A     

Influence of ammonium sulfate on YAG nanopowders and Yb:YAG ceramics synthesized by a novel homogeneous co-precipitation method

Homogeneous and dispersed Y_3 Al_5 O_(12)(yttrium aluminum garnet,YAG) nanopowders were synthesized via a homogeneous co-precipitation method from the mixed solutions of yttrium nitrate,aluminum nitrate and a small amount of ammonium sulfate using hot urea as the precipitant.The method has the superiorities that co-precipitation of cations is ensured and continuous decomposition of the hot urea is achieved to obtain the narrow size distribution particles.The addition of small amount of ammonium sulfate surfactant,although has no influence on YAG garnet phase formation,has significant effect on dispersion,particles distribution and sinterability of the resultant YAG and Yb:YAG powders.Compared with the undoped sample,the green body of Yb:YAG doped with ammonium sulfate has higher total shrinkage,linear shrinkage rate and relative density through sintering at 1600 ℃.The resultant Yb:YAG powders can be sintered into transparent ceramics at 1700 ℃ through vacuum sintering.The influence of the sulfate ions on characteristics of the resultant powders was thoroughly studied.