Infiltration of graphite by aluminum during electric-discharge sintering

Conclusions During the electric-discharge sintering of an aluminum powder in contact with graphite, infiltration of the latter by molten aluminum takes place. A similar phenomenon is observed during sintering in a graphite-bronze system. The intensity of infiltration growswith increasing effective current density and with rising external mechanical pressure. Infiltration by aluminum increases the strength and wear resistance of graphite and decreases its coefficient of friction on steel.Translated from Poroshkovaya Metallurgiya, No. 3(231), pp. 26–28, March, 1982.     

Wear-resistant bronze-base composite materials produced by electric-discharge sintering

Conclusions Sintered bronze-stellite and bronze-sormite materials produced by EDS are superior in frictional properties and wear resistance to BrO10 bronze. Under conditions of friction without lubrication the advantages of such bronze-stellite and bronze-sormite materials manifest themselves most clearly at high sliding speeds (in excess of 4 m/sec). In operation with lubrication the load-carrying capacity of a sintered bronze-sormite material is two and a half times that of BrO10 bronze.Translated from Poroshkovaya Metallurgiya, No. 9(213), pp. 6–8, September, 1980.     

Densification kinetics of a copper-tin powder composite during electric-discharge sintering

Conclusions In the electric-discharge sintering of a copper-tin composite raising the prepressing pressure from 4.17 to 12.5 MPa decreased the extent and rate of shrinkage by more than half. To attain the same hardness in the sintered copper-tin powder composite on passing from a low (4.17 MPa) to a higher prepressing pressure (12.5 MPa), it was necessary to resort to increasing the current density by 100 A/cm² (from 595 to 695 A/cm²).Translated from Poroshkovaya Metallurgiya, No. 7(295), pp. 19–21, July, 1987.     

Reactions in the TiN-TiB2 system

Conclusions Using data on melting points, phase and chemical compositions, lattice constants, and microhardnesses of individual phases and alloys, a constitution diagram has been constructed for the quasibinary eutectic system TiN_0.96-TiB₂. The system has a eutectic temperature of 2600±40°C and a eutectic composition 40 wt. % TiN_0.96 + 60 wt. % TiB₂. It is characterized by a negligible intersolubility of the components (up to 3–4 wt. % TiB₂ in TiN_0.96 and a virtual insolubility of TiN₂ in TiB₂).Translated from Poroshkovaya Metallurgiya, No. 1(217), pp. 62–66, January, 1981.     

Reactive sintering in Fe-Co system

Abstract

Low porosity powder metallurgy compacts have been manufactured from treated elemental iron and cobalt powders sintered at 1150°C under an H_2(g) atmosphere. Their microstructures consist of an interconnected mixed oxide network which encapsulates both the iron and cobalt phases. The production technique employed is an innovative process termed reacto-thermitic sintering (RTS), which leads to near full density and near net shape parts utilising conventional uniaxial compaction and mesh belt furnace practices. The RTS technique relies on microscale exothermic reaction between small quantities of added elemental Al and oxides present on the surface of the bulk powder, together with the bulk powder itself. This results in the production of a transient liquid phase which freezes rapidly and consolidates the compact without slumping. In order to generate an interconnected mixed oxide network, experiments were designed such that the Al powder reacts with the cobalt and the surface of the iron powder which is artificially doped with Fe and Cr oxides.

Differential thermal analysis (DTA) and energy balance calculations revealed that the Al and the oxide coating reaction does not proceed directly. Instead the main contribution to the exothermic process is the reaction between Al and Co/Fe. The system does not exhibit true RTS behaviour and the interconnected network of mixed Al, Cr, and Fe oxides is created by subsequent reaction of Co-Al and Fe-Al intermetallics with the artificial Fe-Cr oxide coating on the Fe. The microstructure obtained exhibits negligible porosity with the metallic particles on the whole fully encapsulated by the oxide.     

Reactive sintering and reactive hot

NbAl₃ has been synthesized from elemental powders by reactive sintering (RS) and reactive hot isostatic pressing (RHIP). Both processes involve a self-propagating exothermic reaction between the constituent powders to form an intermetallic compound. The RHIP approach uses simultaneous external pressurization to make a higher density product. This study focused on developing a method to use reactive synthesis to form high-density NbAl₃ compacts. High RS and RHIP densities were possible with the appropriate raw materials and processing parameters. These include powder purity, particle sizes, degassing, heating rate, furnace temperature, and compaction pressures. Near full density was attained with RHIP, and up to 95 pct density was attained with RS. This article is based on a presentation made in the symposium This article is based on a presentation made in the symposium This article is based on a presentation made in the symposium This article is based on a presentation made in the symposium     

Structure and properties of tungsten carbide obtained by electric-discharge sintering of a disperse powder

Samples of highly dispersed tungsten carbide powders of various origins were obtained by electrical-discharge sintering. The specimens were 8 mm in diameter, 5 mm in height and had a grain size of 1 µm, density 15.5–15.8 g/cm³, hardness 92–93 HRA, and cracking resistance 6–8 MPa·m^1/2. The structure and phase composition of the sintered samples was not observed to change appreciably from those of the initial powder. The salient features of electrical-discharge sintering of tungsten carbide power are described.     

Reactive sintering of porous tungsten for higher homogeneity

Abstract

One of the most important applications of tungsten is its use as a high current density cathode in the form of porous tungsten impregnated with electron emissive salts. Powder metallurgy is the usual processing route to make these parts since melting and casting of tungsten is very tedious due to its high melting point (3410±20°C). The overall porosity and porosity distribution are crucial parameters for the performance and lifetime of porous tungsten cathodes. Although there is an active debate over what constitutes the optimal porosity distribution, lack of homogeneity is observed in conventional parts. This may contribute to a shorter cathode and hence lamp lifetime. These parts are conventionally sintered at temperatures in excess of 2000°C. Furthermore, due to the high temperatures involved, the conventional sintering is very costly and energy consuming. This paper briefly looks into an alternative sintering process being developed for porous tungsten technology and its outcomes in terms of porosity distribution across the parts. As a result, the sintering temperature is dropped down to 1150°C and more homogeneous porous structures have been obtained. A characterisation method previously developed by the authors based on microhardness measurements is proved to be a good measure of the homogeneity of porous tungsten parts.     

Influence of specific energy expenditures in electric-discharge sintering on the structure and properties of a copper-tin-abrasive composite

Conclusions It has been experimentally established that an increase in the specific energy expenditure caused by an increase in the strength of the dc leads to an increase in process temperature in electric discharge sintering of a copper-tin-abrasive composite. By changing the specific energy consumption parts with different structures and properties are obtained from this composite by electric discharge sintering.Translated from Poroshkovaya Metallurgiya, No. 8(284), pp. 67–70, August, 1986.     

Reactive atomization of silicon to formin situ oxide sintering aids

The present investigation demonstrated the feasibility of using reactive atomization to produce Si powder within situ oxide sintering aids. With further process optimization, this powder may be an alternative starting material to the conventional, mechanically blended, Si-plus-oxide powder used to produce commercial sintered reaction bonded silicon nitride (SRBSN). In the reactive atomization approach, yttrium and aluminum additives were introduced into silicon metal during induction melting. Reactive atomization was accomplished using a N₂-5 pct O₂ mixture as the atomization gas. During atomization, oxygen in the atomization gas reacted with Y and Al in the Si melt to produce Y₂O₃ and Al₂O₃, which act asin situ sintering aids. The reactive atomized powder demonstrated a Gaussian distribution with a mean diameter of 36 μm. The powder fines (<38 μm) were used to produce cold isostatically pressed compacts that were subsequently reaction bonded and sintered. The results demonstrate that β-Si₃N₄ formed during reaction bonding and sintering. The density of the SRBSN was 77 pct of theoretical. Transmission electron microscopy (TEM) studies indicated the presence of a glassy phase on the grain boundaries, which is typical in SRBSN and indicative of the presence of thein situ sintering aids. A kinetic model was used to study the influence of processing parameters, such as droplet temperature and oxygen partial pressure, on the kinetics of oxide formation during reactive atomization. The results suggest that the volume fraction of oxides increases with increasing droplet temperature and oxygen partial pressure in the atomization gas mixture. This article is based on a presentation made in the “In Situ Reactions for Synthesis of Composites, Ceramics, and Intermetallics” symposium, held February 12–16, 1995, at the TMS Annual Meeting in Las Vegas, Nevada, under the auspices of SMD and ASM-MSD (the ASM/TMS Composites and TMS Powder Materials Committees).