THE BRIQUETTING OF GRAPHITE

Abstract

Natural-graphite and finely ground artificial-graphite powders have been subjected to improved compacting ( “Shape”) techniques, developed by the National Coal Board, in an attempt to produce strong, dense materials suitable for use in atomic reactors. The densities of the natural-graphite and artificial-graphite compacts were 2.18 and 2.06 g/ml, respectively; the strengths were ～2000 Ib/in² in each case. Whereas “Shape” techniques improved the compact quality of briquettable materials (e.g. artificial graphite ground to 12 μ dia.), non-briquettable materials (e.g. +350 B.S. mesh artificial graphite) could not be satisfactorily compacted by any means.

Compact quality was very sensitive to particle size and size distribution in the sub-sieve range. On the whole, compact density decreased with decreasing particle size, whereas the strength increased. An empirical relationship S = KA ^5/2, between the strength (S) and the specific surface area (A) of the powder, was obtained for electro-graphite powder compacted at 10 tons/in²

Annealing these compacts at 800°C reduced their density by 1-2% but increased their strength by ～30%.

The compacts were extremely anisotropic and experiments which were aimed at reducing this anisotropy are described.     

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.     

Synthesis of nanosized WC/MgO powders by high energy ball milling and analysis of reaction thermodynamics

Abstract

In the present work, a powder mixture of pure WO₃, graphite and Mg with a definite atomic ratio was milled at room temperature using a high energy ball mill method, and ball milled powders were analysed by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The results indicated that after ball milling for a period of time, an oxidation–reduction reaction was successfully achieved among the Mg, graphite and WO₃ powders to obtain MgO and WC. The extension of the ball milling led to the refinement of the powders. After ball milling 50 h, nanocrystalline WC grains (25 nm) were embedded into the fine matrix of MgO and formed fine nanocomposite MgO/WC powders (～100 nm in diameter). The experimental results and thermodynamic analysis showed that the formation of nanocomposite MgO/WC was a mechanically induced self-propagating reaction, and very short milling time was needed to complete the reaction.     

Rapid sintering of iron powders under action of electric field

Abstract

A new rapid sintering technique for iron powders compacted under the action of an electric field with high current density has been advanced. The results show that the sintering densification of iron powder could be finished in less than 6 min at a temperature of 800^u C reached at a heating rate of 600 K s^?1, and the relative density of the sintered compact was over 95%. Moreover, the sintering densification was almost finished in the heating stage of the compact.     

Comparison of sintering of titanium and titanium hydride powders

Abstract

The cold compaction and vacuum sintering behaviour of a Ti powder and a Ti hydride powder were compared. Master sintering curve models were developed for both powders. Die ejection force, green strength and green porosity were lower for hydride powder than for Ti powder, all probably resulting from reduced cold welding and friction during compaction. For sintering temperatures above ～1000°C, most of the difference in the sintered density of Ti and hydride is explained by assuming equal densification, while taking into account the lower green porosity of compacts made from hydride powder. However, there is evidence that particle fracture during compaction also contributes to increased sintered density for hydride powder. The Ti powder conformed to a master sintering curve model with apparent activation energy of 160 kJ mol^?. The activation energy for Ti hydride also appeared to be about 160 kJ mol^?, but the model did not fit the experimental data well.     

Characterisation of functionally graded and VC/steel surface alloyed composites produced using powder metallurgy

Abstract

In this study, low carbon steel specimens with surface alloyed composites were produced by means of powder metallurgy. Vanadium carbide, graphite (1·2 wt-%) and Fe were used for the surface alloyed composite, while Fe and graphite (0·2 wt-%) were used for the low carbon steel side. The powder mixtures were compacted together in the same mould. On the surface alloyed side the vanadium carbide content was changed from 5 to 25 wt-%. Microstructural investigations including EDX and X-ray, hardness measurement and abrasive wear tests were performed. The results showed that V₈C₇ formed in the alloyed surface and carbon diffusion from the alloyed surface to the parent metal created a functionally graded material. The hardness values decreased towards the parent metal. Wear resistance increased as the vanadium carbide increased in the surface alloyed composite. Thus, a functionally graded steel having a surface composite that is resistant to abrasive wear can be obtained via the powder metallurgy route.     

Nanostructure Cu–Cr alloy with high dissolved Cr contents obtained by mechanical alloying process

Abstract

A nanostructural solid solution of Cu–Cr was prepared by the mechanical alloying process. Three mixtures of Cu powders with 1, 3 and 6 wt-%Cr powders were milled under 250 rev min^?1 for different milling times of 4, 12, 48 and 96 h. The mixtures were subsequently compacted and sintered at 450, 600 and 750°C for half an hour. Milled powder mixtures were examined by X-ray diffraction technique, which showed the presence of nanoscale crystallites in the samples and the decrease of lattice parameter of Cu crystals. Sintered powders were investigated by optical microscope and their hardnesses were measured by microhardness. Results showed increasing trends in hardness of the compacted powder mixtures with increasing milling time. Sintering temperature had also evident effects on the behaviour of powder mixtures. As sintering temperature increased, microhardness increased and a peak appeared then a decreasing trend was observed.     

Study of contact interaction between metalline borides and graphite at high temperatures in a vacuum

Summary We studied the contact interaction between compacted boride specimens and boride powders: TiB₂, ZrB₂, HfB₂, NbB₂, TaB₂, Mo₂B₅, and W₂B₅ and graphite powder and graphite at temperatures up to 2200°C in a vacuum. It was established by metallographic and chemical analyses together with microhardness test that the borides of hafnium, niobium, tantalum, and tungsten were stable in contact with graphite right up to 2200°C, and that molybdenum boride was stable up to its melting point, after a contact period of up to 5 h. In the case of titanium boride, we observed a slight interaction with graphite at 2200°, as a result of which the boride loses up to 1% boron, which transfers to the graphite charge. At 2200° after 5 h soaking, clear signs of interaction with graphite are detected in zirconium boride; during contact between compacted boride and graphite powder, the weight of the boride specimen and its microhardness decline, and traces of zirconium and boron are detected in the graphite charge. When compacted graphite is in contact with ZrB₂ powder, we observe a change in the chemical composition of the boride, an increase in the microhardness of the graphite, and a transition layer is detected on the contact boundary.     

THE PREPARATION OF CARBIDE-ENRICHED TOOL STEELS BY POWDER METALLURGY

Abstract

Two PM methods of increasing the carbide content of M2 high-speed steel have been investigated: (1) By the mechanical mixing of tool-steel powder with up to 15 wt.-% of either VC or TiC powders of two different size ranges. (2) By the production of fully prealloyed carbide-enriched powders by gas atomization.

The mixed powders were consolidated by either cold isostatic pressing or explosive compaction, and then vacuum-sintered. Fully dense specimens, however, could be obtained only by subsequent hot working. The pre alloyed powders could not be cold compacted and were densified by hot working the canned loose powders.

With the mixed powders, the carbide dispersion depended closely both on the relative sizes and amounts of the tool-steel and carbide particles and onthe total reduction during hot working. With the fully prealloyed powders a very fine uniform carbide dispersion was obtained in all samples. It was found that with proper composition control the new materials could be heat-treated in a manner similar to that applied to M2 tool steel; significant increases were thus obtained in hardness and wear-resistance. The preferred production methods would be to employ mixed powders for TiC-enriched materials and fully pre alloyed powders for VC-enriched materials.     

Optimisation of mechanical milling process for production of AA 7075/(SiC or TiB2) composite powders

Abstract

The present work concerns the processing of composite powders based on 7075 aluminium alloy by mechanical milling. A premixed powder (Alumix 431D, Ecka Granules, Germany) was used as the matrix material, and two different ceramic reinforcements (SiC and TiB₂) were chosen as reinforcements. The main objective was to evaluate the effect of the content and addition method of the process control agent as well as the content and type of reinforcement on the microstructural and morphological evolutions of the powder particles during milling process and the as milled properties of the processed materials. Results showed that regardless of the starting composition, alloying took place through three stages, in which deformation, cold welding and fracturing of powder particles were the main mechanisms involved respectively. The mechanically milled composite powders showed a fine and homogenous distribution of reinforcement particles. A higher content of reinforcement resulted in a lower crystalline size for the milled powders (～18 nm for composite powders containing 15 vol.-% ceramic particles).     

Powder reaction mechanism in fabrication of high silicon iron alloy

Abstract

In the present paper, the reaction mechanism of silicon and iron powders under different sintering conditions during the fabrication of high silicon iron sheet (～6·5 mass-%Si) is clarified. It is indicated that the phases, Fe₃Si (Si) and FeSi, play an important role in the reaction between iron and silicon powders. Two temperature regions of the powder reaction are very important for producing commercial high silicon iron sheets: the temperature region of ～1000°C in which the ductile composite structure can be produced, and the temperature region of ～1200°C in which the density and homogeneity can be improved.     

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.     

THE COMPACTING OF METAL POWDERS BY EXPLOSIVES

Abstract

The paper describes a technique developed for compacting metal powders by the use of detonating explosives under controlled conditions. Aluminium, iron, copper, and steel powders were originally compacted by this means into solid cylinders. Graphite and other non-metal powders have recently been compacted, and modifications have been introduced whereby annular cylinders may be formed. This novel technique may find application in cases where existing mechanical methods of powder compacting are not adequate.     

SINTERED IRON-BASE ALLOYS WITH HIGH STRENGTH USING SELECTED NITRIDES AND SILICIDES

Abstract

The techniques used in alloying in iron powder metallurgy have been extended by employing special compounds. The introduction of the alloying elements in this form and the decomposition of selected nitrides and silicides are described. Elements that oxidize readily at high temperatures (e.g. Cr, Si) can be added in a relatively pure and homogeneous state. These elements stabilize the α phase and thus improve the sintering behaviour.

The paper deals mainly with the preparation of binary Fe-Cr, Fe-Si, and also ternary Fe-Cr-Si alloys obtained by ‘in situ’ decomposition of Si₃N₄, Cr₂N, and CrSi₂ in an iron matrix (WP-150).

The study covers the properties of the powders and their mixtures, the pressing and sintering conditions, the sintering behaviour in the range 1000–1300°C with varying alloying additions, for different sintering times and atmospheres. The tensile strengths observed are ～525 N/mm² at a densityof 6·7 g/cm³, with ～3% elongation at fracture. With respect to the low density and the carbon free state of the alloys, the strength values may be considered as rather high. A study of the homogenization process is being carried out.     

HIGH-VELOCITY EXTRUSION FORGING OF BILLETS COMPACTED FROM IRON AND STEEL POWDERS

Abstract

Billets 25 mm in dia. and weighing ～135 g have been compacted from sponge-iron powder using a single-end-pressing technique. A range of densities from 5·6 to 7·2 g/cm³ was produced. The billets were then hot extrusion forged, at high speed, into tensile specimens of gauge-dia. 10 mm, gauge-length 28 mm, and head dimensions of 13 mm dia. × 12·5 mm long. About half the billets were pre-sintered before heating to forging temperature, while others were hot-forged without a pre-sintering stage. The tensile specimens were then tested and selected ones examined metallographically.

The work was extended to investigate the extrusion forging of alloy steel powder.     

Transient liquid phase sintering of high density Fe3Al using Fe and Fe2Al5–FeAl2 powders Part 2 – Densification mechanism analysis

Abstract

The use of Fe₂Al₅–FeAl₂ prealloyed powders and heating rates >150 K min^?1 overcomes the formation of density restricting Kirkendall porosity in the Fe–Al system. X-ray diffraction, electron probe micro analysis and differential thermal analysis suggest that the absence of a persistent liquid, experienced when liquid phase sintering with elemental powders, is overcome. Homogenisation is greater during heating at a rate of 20 K min^?1 than for 150, 250 or 400 K min^?1 and homogenous Fe₃Al forms across the compact at temperatures below the melting point of the liquid forming constituent, indicating that a liquid will not form under such processing conditions. The maximum density achieved under the processing conditions in the present study is 92% of theoretical density. The presence of large pores shortly after liquid formation suggests that the remaining porosity is largely due to powder agglomeration during mixing.     

FEASIBILITY OF PRODUCING DISPERSION-STRENGTHENED CHROMIUM BY BALL-MILLING IN HYDROGEN HALIDES

Abstract

Chromium, with and without 4 vol.-% thoria, and nickel powders were ground to fine powder sizes by ball-milling in gaseous hydrogen halides. After reducing the milled chromium in flowing hydrogen under pulsating pressure at ～680°C, submicron-size powders with 4–500 ppm residual halogens were obtained. The compacted chromium–thoria alloys had interparticle spacings ranging from 2·1 to 6·5 μm. After 100 h at 1318°C the interparticle spacing of the 2·1-μm alloy increased to 5·2 μm. Submicron-size chromium and nickel powders were also obtained by pulsating hydrogen reduction of their chlorides.     

Relationship between physical structure and machinability of green compacts

Abstract

The dependence of green machinability on compact density and strength was investigated for room temperature and warm compacted steel powder compacts containing two different types of lubricant. Brazilian disc compression tests were employed to determine green strength, while machinability was assessed in terms of response to drilling.

For the room temperature compacted materials, it was found that high compact densities and strength were not, in most cases, associated with improvements in machinability. Furthermore, it was shown that lubrication (both type and quantity) and compaction pressure plays a critical role in determining the level of breakouts observed. In contrast, the use of warm compaction, in conjunction with specially designed lubricants, has been shown to be a suitable method of producing high density, high strength compacts while retaining good green machining characteristics. Mechanisms responsible for the observed behaviours of both the room temperature and warm compacted specimens have been forwarded in the present paper.     

Characterisation and sintering studies of mechanically milled nano tungsten powder

Abstract

Elemental tungsten powder was mechanically milled by planetary mill for 100 h. Particles were thinned down to nanometre scale. The shape of the milled powders was flat cylindrical with average diameter and length 12˙5 and 46˙5 nm respectively. The corresponding crystallite size obtained by X-ray diffraction (XRD) was 26˙96 nm. The results obtained by XRD and small angle X-ray scattering were well supported by transmission electron microscopy and high resolution transmission electron microscopy results. The maximum shrinkage of the compact has been observed at ～1500 K, which has been used as a guideline for sintering experiments. The powders sintered at 1773 K have resulted in 96% relative density.     

THE SINTERING OF URANIUM-CARBON-IRON ALLOYS IN THE PRESENCE OF A LIQUID PHASE

Abstract

The preparation of uranium mono carbide bodies containing ～1% open porosity and with a density of ～12·4 g/c.c. by cold compacting and sintering uranium/graphite mixtures is described. The effect of iron additions in producing a liquid phase during sintering has been studied, and it is shown that the addition of 10 wt.-% UFe₂ to the stoichiometric mixture of uranium and graphite raises the sintered density to ～13·1 g/c.c. and reduces the open porosity to ～0·5%. The mechanism of carbide formation and densification in the presence of a liquid phase is discussed.