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Abstract

The development of multilayered polymeric/metallic materials by a PM route employing spark plasma sintering (SPS) has been explored. The aim is to produce composites that join a polymer with a metal for structural applications. In a first approach, the consolidation of polyimide-based composites by SPS was studied. For temperatures as low as 350°C, homogeneous mechanical properties were achieved in compression tests. In a second set of experiments, multilayered polyimide/aluminium composites were consolidated by SPS. These polymer-based composites could be used as an interlayer material to join effectively polyimide and aluminium.     

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.     

Densification of Al-2024 and Al-2024/Al2O3 Powders by Conventional P/M Route and ECAP: A Comparative Study

In this study, mechanically alloyed Al-2024 and Al-2024/Al₂O₃ powders are densified by conventional sintering and by equal channel angular pressing (ECAP) with and without back pressure. The powder was encapsulated in an aluminium can for consolidation through ECAP. The properties obtained in the compacts by conventional sintering route and by ECAP are compared. The effect of conventional sintering and ECAP on consolidation behaviour of powder, microstructure, density and hardness is discussed. Room temperature back pressure aided ECAP results in nearly full denser (97?% of its theoretical density) compact at room temperature. Nano Indentation technique was used to determine the modulus of the consolidated compacts.     

Effect of pulsed current on reactively synthesised TiB2 consolidated by plasma pressure compaction

Abstract

The effect of pulsed current on TiB₂ formed by reactive consolidation between titanium and boron is reported in this paper. This consolidation was performed using the plasma pressure compaction (P²C) technique. A comparison between the pulsed and control samples reveals that pulsed current reduces grain growth (pulsed samples had an average grain size of 2·79 μm compared to 5·99 μm) while increasing sintering rates (pulsed samples were on average 15·5% more dense). The reduced grain growth and increased densification is due to the removal of adsorbed oxygen from the surface of the powder.     

Microstructure and properties of molybdenumprinciple-copper composite metal samples consolidated by plasma pressure compaction

Abstract

In this study, a composite metal powder mixture using molybdenum and copper powders was consolidated tonear theoretical density using the rapid consolidation technique of plasma pressure compaction. Rapid consolidation of the mixture of metal powders is an essential requirement for better microstructural control and mechanical properties in the consolidated product. The microstructure and hardness of the composite samples are compared with monolithic samples made by consolidating pure molybdenum powders under identical conditions. Microhardness measurements revealed an increase in hardness of molybdenum when mixed with small amounts of copper. The role of consolidation parameters on hardness and microstructural development is presented and discussed.     

Fabrication of Ni-Ti Alloy by Self-Propagating High-Temperature Synthesis and Spark Plasma Sintering Technique

This work is focused on the possibilities of preparing Ni-Ti46 wt pct alloy by powder metallurgy methods. The self-propagating high-temperature synthesis (SHS) and combination of SHS reaction, milling, and spark plasma sintering consolidation (SPS) are explored. The aim of this work is the development of preparation method with the lowest amount of undesirable phases (mainly Ti₂Ni phase). The SHS with high heating rate (approx. 200 and 300 K min^?1) was applied. Because the SHS product is very porous, it was milled in vibratory disk milling and consolidated by SPS technique at temperatures of 1173 K, 1273 K, and 1373 K (900 °C, 1000 °C, and 1100 °C). The microstructures of samples prepared by SHS reaction and combination of SHS reaction, milling, and SPS consolidation are compared. The changes in microstructure with increasing temperature of SPS consolidation are observed. Mechanical properties are tested by hardness measurement. The way to reduce the amount of Ti₂Ni phase in structure is leaching of powder in 35 pct hydrochloric acid before SPS consolidation.     

Characterisation of nickel alloy powders processed by spark plasma sintering

Abstract

In this study, nickel alloy powders were consolidated by spark plasma sintering. Experiments were performed between 700 and 750°C temperature range under 50 MPa pressure with holding times from 5 to 10 min. In addition to these main spark plasma sintering parameters three different heating rates ranging from 100 to 235°C min^?1 and two different particle size ranges (75–106 μm narrow size distribution and ?45 μm wide size distribution) were used for the experiments. After sintering, the sliding wear behaviour of the samples was investigated. The results revealed that the density of the material increased with raising the sintering temperature and holding time. However, heating rate and particle size also played an important role in the densification and these parameters were investigated in detail.     

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.     

Formation of aluminium nitride during sintering of powder injection moulded aluminium

Abstract

A detailed transmission electron microscopy study of the structure of aluminium nitride formed during sintering of powder injection moulded aluminium is presented. A polycrystalline layer formed on Al particle surfaces exposed to a nitrogen atmosphere. This layer consisted of fine, rod-like crystallites of hexagonal AlN typically aligned normal to the Al surface. A double layer of AlN separated by a thin layer of Al was observed at the interfaces between Al grains. In this report, the structure of the nitride is characterised and its influence on sintering is discussed.     

Spark Plasma Sintering of Cryomilled Nanocrystalline Al Alloy - Part II: Influence of Processing Conditions on Densification and Properties

In this study, nanostructured Al 5083 powders, which were prepared via cryomilling, were consolidated using spark plasma sintering (SPS). The influence of processing conditions, e.g., the loading mode, starting microstructure (i.e., atomized vs cryomilled powders), sintering pressure, sintering temperature, and powder particle size on the consolidation response and associated mechanical properties were studied. Additionally, the mechanisms that govern densification during SPS were discussed also. The results reported herein suggest that the morphology and microstructure of the cryomilled powder resulted in an enhanced densification rate compared with that of atomized powder. The pressure-loading mode had a significant effect on the mechanical properties of the samples consolidated by SPS. The consolidated compact revealed differences in mechanical response when tested along the SPS loading axis and radial directions. Higher sintering pressures improved both the strength and ductility of the samples. The influence of grain size on diffusion was considered on the basis of available diffusion equations, and the results show that densification was attributed primarily to a plastic flow mechanism during the loading pressure period. Once the final pressure was applied, power law creep became the dominant densification mechanism. Higher sintering temperature improved the ductility of the consolidated compact at the expense of strength, whereas samples sintered at lower temperature exhibited brittle behavior. Finally, densification rate was found to be inversely proportional to the particle size.     

Influence of extrusion ratio and temperature on microstructure and mechanical properties of 2024 aluminium alloy consolidated from nanocrystalline alloy powders via hot hydrostatic extrusion

Abstract

Nanocrystalline 2024 aluminium alloy powders with an average grain size less than 50 nm, prepared by a unique technique which combines rapid solidification and mechanical milling, were consolidated into bulk material under various technical conditions via hot hydrostatic extrusion and the microstructure and mechanical properties of the consolidated alloy were experimentally investigated. The influence of the two main technical parameters, extrusion ratio and temperature, on the microstructure and mechanical properties of the as extruded alloy is made clear and the reasons why these two parameters had such an influence on the microstructure and mechanical properties of the alloy are also discussed. Furthermore, suggestions are given for rationalising the extrusion ratio and temperature for the consolidation of the nanocry stalline 2024 aluminium alloy powders via hot hydro static extrusion.     

Electric-discharge sintering of TiN-AlN nanocomposites

The structurization and properties of TiN-AlN and TiN-AlN-Y₂O₃ nanocomposites consolidated by electric-discharge sintering are examined. TiN-AlN composites with a relative density of about 98 to 99% are produced. Their structure is not homogenous and consists of TiN and AlN grains of about 200 nm in size. There are also large spherical grains of titanium nitride of 2 to 10 µm. This effect is probably caused by microdischarges between particles of the conducting phase and subsequent meltback of the interacting surfaces. The effect of yttrium oxide additives on the material structure and properties is investigated. It is shown that TiN-AlN composites consolidated by electric-discharge sintering have high hardness (HV ～ 25 GPa) and fracture toughness (K_1c ～ 6 MPa · m^1/2).     

DENSE SILICON NITRIDE

Abstract

A method is described for preparing silicon nitride bodies of almost theoretical density. Silicon nitride powder is hot pressed with a small proportion of catalyst, preferably magnesium oxide or nitride, at 1850°C. The bodies have very high flexural strength up to high temperatures, and preliminary measurements of creep rate and thermal shock-resistance indicate that dense silicon nitride should be a suitable material from which to make components required to operate under high stress at temperatures up to ～1200°C.     

Aluminium nitride probes for application in iron melts

Review on high-temperature properties of aluminium nitride AIN. Investigations on aluminium nitride as a possible solid electrolyte at temperatures around 1600°C. Presentation of available data on the electrical conductivity, the thermal conductivity and the thermal expansion coefficient of AIN at elevated temperatures. Experimental study on the oxidation behaviour of AIN in oxidizing gases and oxygen-containing iron melts. EMF measurements on electrochemical cells of the type iron melt ‖ AIN (Al₂O₃) solid electrolyte ‖ reference representing oxygen and aluminium probes in iron melts under long-term conditions.     

Nanostructure and Mechanical Properties of Bulk Al86Ni6Y6Ce2 Alloy Produced by Hot Consolidation of Amorphous Melt-Spun Flakes

Amorphous Al₈₆Ni₆Y₆Ce₂ (at. pct) flakes produced by melt spinning were consolidated using hot pressing at different conditions. The influence of pressing conditions on the crystallization behavior, thermal stability, and mechanical properties of the alloy has been studied through differential scanning calorimetry, scanning electron microscopy, X-ray diffraction, microhardness, and compression test. The results show rapid solidification combined with the hot consolidation produce a highly dense sample (3.41 ± 0.2 g cm^?3) with the ideal interflake bonding, good thermal stability, good microhardness (381 ± 12 HV), and remarkably high strength (910 ± 7 MPa) combined with 20 pct fracture strain were obtained at T = 748 K (475 °C) and P = 1.2 GPa in a lightweight Al-based material. The high mechanical properties mainly result from structural refinement during the controlled consolidation method.     

Synthesis and characterisation of quasicrystalline Al based composites

Abstract

Al matrix composites reinforced by Al–Cu–Fe quasicrystalline (QC) phase particles were produced from a mixture of Al and QC powders using electrical current heating and conventional sintering. A combination of X-ray diffractrometry, transmission and scanning electron microscopy was used to characterise the microstructure of consolidated specimens. The metallic bonding of the Al matrix and particles was improved by higher temperature sintering or electrical current heating. However, the dissolution of QC particles into the Al matrix was inevitable during heating and resulted in the formation of ω and/or β phases. The dissolution of QC particles was effectively reduced using prealloyed Al powder containing 2 at.-%Cu. This had led to an increase in microhardness from 96 to 139 HV for specimens using pure Al to prealloyed Al powders. A homogeneous distribution of QC particles within the Al matrix could be achieved by mechanical milling followed by consolidation.     

Oxidation of mechanically alloyed powders during processing: origins and control

Abstract

Oxide dispersion strengthened (ODS) alloys prepared by mechanical alloying (MA) and subsequent consolidation are usually subjected to a series of heat treatments during production, typically comprising a degassing process at ?600°C and a preconsolidation high temperature 'soak' at ?1000°C, both under vacuum. In the current work, the oxidation behaviour of a prototype ODS Fe₃Al alloy and a commercial FeCrAl alloy has been studied during simulation of these temperature and pressure regimes. After the high temperature 'soak' simulation, oxidation had taken place on both alloys with a significantly thicker scale forming on the ODS Fe₃Al. This scale is believed to be the source of much of the high alumina content found in fully consolidated ODS Fe₃Al. Variation in the amount of particulate alumina found in different batches of commercially consolidated powder is discussed. Novel processes involving hydrogen purging and powder precompaction have been employed to decrease oxidation and thereby increase sintering efficiency.     

Wear resistant sintered aluminium parts for automotive applications

Abstract

The automotive industry's focus on weight saving has increased interest in optimised process routes and alloy compositions for light alloy PM components. Conventional pressing, sintering and sizing of aluminium alloys containing about 16%Si has been applied to produce components with high wear resistance and mechanical strength. CISIZE^®, a novel continuous isostatic pressure sintering process, combined with sizing, produces aluminium alloys with finely dispersed Si particles having excellent ductility. Combining parts obtained by these two process routes can give interesting tribological systems. Complete sintered aluminium cam phaser systems, including the sprocket wheel, are being produced in series using this approach, which also shows promise for automotive parts such as oil pumps and rotors.     

Effect of the conditions of sintering W-Ni-Fe-Co heavy alloy nanopowders on the structure and density of compacted samples

A compact material made of a heavy tungsten alloy W-Ni-Fe-Co nanopowder is produced. The nanopowders are synthesized by the treatment of a solid tungstic acid in aqueous solutions of Ni, Fe, and Co salts followed by the reduction of the solid residue by hydrogen at 800°C (the average size of the powder conglomerates is ∼300 nm, and the conglomerates consist of 100-nm particles). Solid-phase sintering is performed in stages. An increase in the temperature at the last stage from 1300 to 1350 and 1450°C increases the density from 16.7 to 17.2–17.4 g/cm³ and the average tungsten grain size to 2.4–4.6 μm. The samples after solid-phase sintering at 1350°C have no porosity. Liquid-phase sintering of nanopowders with high surface and interface energies occurs at 1480°C. Original Russian Text ? K.B. Povarova, M.I. Alymov, O.S. Gavrilin, A.A. Drozdov, E.V. Evstratov, A.I. Kachnov, A.E. Sal’ko, 2007, published in Metally, 2007, No. 6, pp. 65–72.     

Analysis of the spray deposition process

Net or near net shape products can be manufactured by technologies involving solidification processing, metal forming, paniculate processing, and droplet consolidation. One example of droplet consolidation is spray deposition in the Osprey^tm mode. In this process, a stream of liquid metal is atomized by an inert gas to form a spray of molten droplets; these are accelerated towards a substrate where they impinge and consolidate. An integral model for the Osprey^tm spray deposition process has been developed using established theoretical principles. Mathematical models describe the interconnected processes of droplet-gas interactions in flight and subsequent droplet consolidation on the substrate. The models predict droplet velocity and temperature as a function of flight distance, the extent of droplet solidification on arrival at the substrate, and temperature distribution in the consolidated material during deposition. This approach demonstrates the utility of modeling studies in order to establish quantitative guidelines for optimization of the process in terms of the evolution of microstructure in droplet consolidation.