Influence of atomising gas on particle characteristics of Al,Al–1 wt-%Li,Mg, and Sn powders

Abstract

The influence of physical and flow properties of atomising gas on the particle characteristics of gas atomised Al, Al–1 wt-%Li, Mg, and Sn powders was investigated in a pilot plant gas atomiser with IN4/ON18/3·5–4·0 type ‘confined design’ nozzle. In the tests, Al powders were produced under high and low pressures of argon, under air, (N₂ + O₂ ) mixture, nitrogen, and helium; Al–1 wt-%Li binary alloy powders were produced under argon and helium; Mg powders were produced under high and low pressures of argon and helium; and finally Sn powders were produced under argon, nitrogen, and helium. The morphology, size, size distribution, and surface features of the powders used in the present study were examined under SEM together with dry and wet sieving, used for sizing the powders. It was observed that high gas velocities and/or low atomising gas densities not only affect powder particle size, but also shape and surface texture. The oxygen content of the atomising gases also has an influence on the powder particle shape. In this context, powders produced under helium are finer in size owing to efficient secondary breakup; more spherically shaped in their fine size fraction in non-oxidising or difficult to oxidise atomising liquids (such as Sn and Al), because the time to breakup is shorter than that for solidification; and more irregularly shaped in their coarse size fraction in oxidising atomising liquids (such as Mg and Al–Li) owing to oxygen (the time to breakup is longer than that for solidification) compared with other atomising gases such as argon, air, (N₂ +O₂ ) mixtures, and pure nitrogen.     

THE ROLE OF ARC-PLASMA IN METALLURGY

Abstract

Arc-plasma is introduced by a comparison with conventional heat sources. The mechanism and requirements of arc-plasma devices and equipment are then considered. An arc-plasma materials-deposition unit for automatic operation in an atmosphere of controlled composition and pressure is described. Photographs of objects plasma-spray-formed in tungsten are shown, together with microstructures of plasma-sprayed refractory deposits.

Metallurgical applications of arc-plasma technology are considered under four headings:

Plasma as a user: the requirements of powders for arc-plasma purposes are discussed.

Plasma as a producer: extractive metallurgy, production of ultrafine–and spherical–particles, powders of intermetallic compounds and alloys.

Plasma as a fabricator: consideration of spraying, spray-welding, spray-forming, cutting, and arc-plasma furnaces, including King’s rotating-plasma furnace.

Plasma as a tester: pla,sma for the analyst, application to thermal endurance, ablation, and thermal-shock testing.     

Synthesis and characterisation of Si3N4p/Cu nanocomposite powders by high energy ball milling

Abstract

The present work reported the preparation of Cu–25 wt-%Si₃N₄ nanocomposite powders via high energy ball milling (HEBM). The phases and morphologies of as-milled powders with various milling times were investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The experimental results showed that with increasing the milling time, the irregularly shaped Cu powder became flattened, and, subsequently, refined and near spherical. After 12 h milling, the particle size of Cu–Si₃N₄ composite powders was in the range of 200–300 nm, while the grain size of Si₃N₄ particulates, 10–25 nm, was well within a nanometre scale. A uniform distribution of the nanosized Si₃N₄ reinforcing phase throughout the Cu matrix was successfully obtained. A reasonable mechanism for the formation of Cu–Si₃N₄ nanocomposite powders during HEBM was also proposed.     

Experimental investigation of yield behaviour of metal powder compacts

Abstract

In this paper the compaction and yield response of two steel and two copper powders are examined. These were chosen to determine how the material response depends on the type of material and the morphology of the powder particles. Experiments were conducted in a computer controlled triaxial cell. Here, concentration is on the response during simulated, frictionless closed die compaction, whereby the radial stress is controlled so as to keep the radius of the sample constant. The compaction process was stopped at regular intervals and a series of probing paths were followed in stress space to construct the yield surface for the compact.

The experimentally determined yield surfaces are compared with yield surfaces predicted by empirical models and micromechanical models of the Fleck type, which assume that the compact consists of monosized spherical particles. During the early stages of compaction the form of the yield surfaces for spherical powders are consistent with Fleck's micromechanical model, but the surfaces become less elongated in the direction of loading at high densities. The yield surfaces for irregular shaped powders are significantly different from the predictions of the Fleck micromechanical model. A modified anisotropic Cam-Clay model is proposed, which is able to predict yield surfaces for the four powders at all densification levels.     

Refractories for Copper Production

Abstract

The furnaces used for producing molten copper from concentrates and scrap - flash: smelters, converters, and anode and fire-refining furnaces - present a unique combination of challenges to refractory life. Highly aggressive slags, mechanical stresses, batch operation, and increasingly higher operating temperatures all combine to destroy most refractory materials. Over the past generation, copper producers have adopted refractory materials biased on the MgO-Cr₂O₃ system to meet these challenges, to the point where ‘mag-chrome’ brick is used almost exclusively in the industry. However, recent environmental imperatives have created a need to develop chrome-free alternative refractories for copper smelters, converters and refining furnaces. Some possible alternative materials are introduced, and speculation is made about the locations where they might be used most advantageously.     

Control of open slag bath furnaces at Highveld Steel and Vanadium Ltd: development of operator guidance tables

Abstract

Several electric furnaces, which are used to melt down partially prereduced vanadiferous magnetite at Highveld Steel and Vanadium, have recently been converted from submerged arc furnaces (SAFs) to open slag bath furnaces (OSBFs). The self-regulating nature of SAFs is largely absent from the OSBFs, and hence the OSBFs rely more strongly on process control. As a first step towards implementing a process control system, a mass and energy balance was drawn up for the OSBFs, and this was used to develop operator guidance tables. The tables show the required changes in feed rate and power, to accommodate changes in prereduction, and to correct overcharged and undercharged furnace conditions. Implementation of the tables has contributed to improved process stability.     

Netshaping concepts for tungsten alloys and composites

Abstract

The conventional powder metallurgy (PM) approach of compaction and sintering has been used extensively in the fabrication of tungsten alloys and composite hardmetals based on WC-Co. In fact, these are some of the earliest known materials to have been fabricated by the PM route. The last 15-20 years have seen the emergence of a new shaping technique of powder injection moulding (PIM) which can shape such tungsten metal alloys and composites into complex near net shaped components. The PIM process starts with the mixing of an organic binder with the desired powders in the form of a homogeneous mixture, known as a feedstock. The feedstock, like plastics, can be moulded into near net shapes from which the organic part is removed and then the material can be sintered to almost theoretical density. This produces complex, near net shaped parts that have properties that are comparable to that of the press and sintered materials. This paper will provide a brief overview of the use of PIM in tungsten based alloys and composites and discuss some of the applications of these materials.     

Driving force for low temperature sintering of interstitial and intermetallic powders and induced applications

Abstract

Intermetallic as well as (carbides and nitrides) interstitial compounds present functional and structural properties which make these materials necessary for advanced technologies. Meanwhile fabrication routes based on melting and casting or plastic deformation, are usually far to compete with powder metallurgy processes likely to provide near net shaped parts and components. The present study is devoted to a model system exemplified by nitrided iron and steel powders for which the thermal stability of nitrides is severely decreasing when temperature exceeds critical values during densification treatments. Thanks to the analysis of the thermal treatment induced transformations mainly characterised by Mössbauer spectroscopy, the consequences and impact on driving forces for densification under critical thermal conditions are discussed in order to achieve an optimised sintering process and an actual development.     

ISOSTATIC HOT-PRESSING: PRACTICAL EXPERIENCE AT AWRE,ALDERMASTON

Abstract

The technique of isostatic hot-pressing and its uses are briefly considered and an account is given of practical experience in the commissioning and operation of an installation of moderate size. The main problems encountered were those of temperature control and uniformity in the furnace in the high-pressure gas; these were overcome by the use of furnaces of special but simple design, yielding excellent control. The principles of this design are described and some performance figures cited.     

Processing and performance of Cr2O3–Fe2O3 reference electrode powders prepared by oxide coprecipitation

Abstract

Cr₂O₃–Fe₂O₃ based oxide mixtures for reference electrode powders of oxygen sensors were processed using oxide coprecipitation route. A special method for preparing reference electrode powders has been developed by mixing coarse Cr particles with the oxide mixture in the form of Cr–Fe hydroxide. Morphology and size of the mixed oxide powders were characterised by means of scanning electron microscopy and laser diffraction method. With the coprecipitation process, chemically homogeneous and very fine powders with a mean particle size of 1.53 μm were prepared. This powder mixture adhered and loosely coated to Cr particles. The processed reference electrode powders were tested in low level oxygen concentration measurements of steelmaking process under industrial scale. The reference electrode powders showed excellent results in terms of electromotive force reproducibility, response time and accuracy in soluble aluminium predictions at the oxygen concentration measurements. Most of the particles of the reference electrode powder remained separated after dipping to molten steel.     

Characteristics of rapidly solidified Cu–10%Sn alloy powders produced by water jet cooled rotating disc atomisation

Abstract

In the present work, an experimental water jet cooled rotating disc centrifugal atomiser was designed and constructed and used to produce rapidly solidified Cu–10%Sn alloy powders. The characteristics of rapidly solidified Cu–10%Sn alloy powders have been investigated with respect to powder size and disc surface condition. Uncoated and ZrO₂ coated copper discs were used to investigate the effect of disc surface conditions on the microstructure and cooling rate of the powders. The produced powders appeared in the shape of sphere, rounded, ligament, irregular and flaky, depending on the particle size. The powders exhibited fine grained microstructure, cell size increased with increasing powder size and higher cooling rates were obtained using uncoated disc. The results indicated that cooling rates of 20 μm powder produced with uncoated and ZrO₂ material coated discs were estimated as 5·82×10⁵ and 1·44×10⁵ K s^?1 respectively.     

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.     

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.     

Recycling of WC–Co from scrap materials

Abstract

WC-Co scrap generated by the cutting tool industries was electrochemically broken down to cobalt, which was deposited at the cathode and a mixture of tungsten oxide and tungstic acid was collected at the anode with an overall recovery efficiency of about 90%. The tungsten oxide/tungstic acid was reduced to produce nanostructural tungsten powders, which were subsequently carburised and chemically coated with cobalt to produce WC-Co powders. The powders synthesised were characterised for purity and size.The WC-Co powders, thus obtained were consolidated to near theoretical densities using a novel plasma pressure compaction (P²C) technique. The microhardness of the consolidated sample was measured to be 2200 HV, which is 20% higher than the reported literature values.     

Structural heterogeneity and its effects on the properties of an in situ TiB2–TiC/Cu composite synthesised by hot pressing

ABSTRACT

In situ TiB₂ and TiC reinforced copper matrix composites with tailored heterogeneous structure were fabricated via high-energy ball milling of Cu, TiH₂ and B₄C powders followed by hot pressing. The microstructures of both ball-milled powders and hot-pressed composites were compared. Although the dislocation density of Cu matrix was changed after hot pressing, the mode of distribution of ceramic phases in the Cu matrix was noted to transmit from the ball-milled powders to hot-pressed composites in case of the TiH₂ particles synthesised by the in situ reactions. The structural inheritance between the ball-milled powders and hot-pressed composites could be used to control microstructural features and thus to tune properties. The hot-pressed TiB₂–TiC/Cu composites with tailored heterogeneous structure exhibited better performance than those of homogeneous counterparts.     

Microstructural studies of W – Cu nanostructured precursor composite powders prepared by mechanical alloying

Abstract

The present work reported the fabrication of the W–Cu nanocomposite precursor powders via high energy ball milling. The W–25 wt-%CuO powders were taken as the raw materials, and the following process condition was used: ball to powder weight ratio of 20 : 1, the rotation speed of 500 rev min^&minus1, the milling time of 15–45 min and 1–40 h, and the mode of milling 10 min, air cooling 30 min. The phase and microstructure of the as milled powders with variation of milling time was investigated, using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The experiment results show that the nanocomposite powders can be successfully synthesised by mechanical alloying using a short time of 1 h. During the ball milling, CuO powders were reduced by W, and a portion of the W powders were oxidised into WO_x (x=2 to 3). The possible mechanism of the reaction was detected.     

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.     

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.     

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.     

Effects of adding overspray powder on microstructures and mechanical properties of spray deposited 8009 aluminium alloy

Abstract

Under normal condition the cooling rate in a spray deposition process is too low to form 8009 aluminium alloy. In order to increase the cooling rate, overspray 8009 aluminium alloy powders were added. The effects of adding overspray powder on microstructures and mechanical properties of the spray deposited 8009, with and without addition of the overspray powders, were studied. It is shown that adding proper content of overspray powders significantly improves the mechanical properties of the spray deposited 8009 alloy. The mechanism involved in modification of the microstructures and the mechanical properties of the spray deposited 8009 alloy produced with the overspray powder addition approach was discussed.