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Thermal Stability and Microstructure of Nanometric 2024 Aluminum Alloy Powder Obtained by Cryogenic Milling and Spark Plasma Sintering

Thermal Stability and Microstructure of Nanometric 2024 Aluminum Alloy Powder Obtained by Cryogenic Milling and Spark Plasma Sintering

Metals and nanocrystalline alloy powders can be produced by high-energy milling and be consolidated by sintering techniques; the use of relatively low sintering temperatures maintains nanometric grains by avoiding grain growth. In the present study, 2024 aluminum alloy powders were produced by milling at cryogenic temperatures (cryomilling) and consolidation using spark plasma sintering (SPS). The thermal stability and microstructure of the nanometric 2024 Al alloy powder produced by cryomilling were analyzed. The milled powder was degassed to improve its sintering behavior and avoid the presence of pores in the final sintered piece. When the material was consolidated by SPS at a temperature of 525 °C and under a pressure of 60 MPa, the resulting alloy density was 98% and an ultra-fine grained (100-300 nm)/micrometric microstructure. The hardness and microhardness of the milled degassed sintered samples showed values superior to atomized samples. The material also presented nanopores in the grain boundaries, very fine precipitates of AIN, rounded θ phases (Al 2 Cu),
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"Spark plasma sintering" (SPS) de nanocompósitos de Al2O3-ZrO2.

"Spark plasma sintering" (SPS) de nanocompósitos de Al2O3-ZrO2.

Os procedimentos propostos, combinando a desaglomeração dos pós de alumina e de zircônia separadamente, por moagem em moinho de bolas, preparação de suspensões alcoólicas defloculadas desses pós e mistura das suspensões, permitiu a obtenção de dispersão homogênea e completa das partículas nanométricas de zircônia na matriz de alumina. O processo de Spark Plasma Sintering se mostrou uma excelente técnica na sinterização de nanocompósitos cerâmicos. Foi possível a obtenção de amostras com densidades próximas a densidade teórica, com microestruturas bem homogêneas e com tamanho de grãos semelhantes ao tamanho das partículas iniciais do pó com temperatura de sinterização de 1300 o C, temperatura
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Nanocompósitos de Al2O3-SiC sinterizados por "spark plasma sintering" (SPS).

Nanocompósitos de Al2O3-SiC sinterizados por "spark plasma sintering" (SPS).

Uma técnica mais recente para a sinterização de cerâmicas nanocristalinas é a sinterização por plasma (Spark Plasma Sintering - SPS) [14]. Esse processo permite a utilização de temperaturas mais baixas e tempos menores de sinterização, além de produzir densificação com pequeno crescimento de grãos [15, 16]. O método de sinterização por plasma é similar ao método de prensagem a quente; nesse método o pó é prensado uniaxialmente em um molde de grafite e pulsos de voltagem com altas correntes são aplicados diretamente no molde e no pó. O pó é aquecido pela descarga entre as partículas e pela corrente através do molde de grafite. Devido às descargas, as superfícies das partículas são ativadas e purificadas e um fenômeno de auto-aquecimento é gerado entre as partículas e assim, a transferência de calor e de massa ocorre rapidamente [14, 15]. O processo de SPS é caracterizado pela alta eficiência térmica devido ao aquecimento direto do molde de grafite e do pó a ser sinterizado pela passagem de altas correntes alternadas. O processo de SPS oferece várias vantagens em relação aos processos convencionais, tais como prensagem a quente (HP), prensagem isostática a quente (HIP) e sinterização sem pressão seguida de prensagem isostática a quente, incluindo a facilidade de operação, alta velocidade de sinterização, alta reprodutibilidade, segurança e confiabilidade, além de apresentar menor consumo energético, que é em torno de um terço a um quinto menor do que nos processos apresentados acima [14]. Devido às vantagens apresentadas por essa técnica é esperado que ocorra um aumento no uso do processo de SPS na fabricação de vários materiais, tais como: compostos intermetálicos, cerâmicas reforçadas com fibras, compósitos de matriz metálica e materiais nanocristalinos, os quais são difíceis de sinterizar utilizando métodos convencionais [14].
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Master Sintering Surface: A practical approach to its construction and utilization for Spark Plasma Sintering prediction

Master Sintering Surface: A practical approach to its construction and utilization for Spark Plasma Sintering prediction

The Spark Plasma Sintering (SPS) is very promising technique for sintering of ceramics, metals, composites etc. In SPS the pulsed DC powder is combined with an applied pressure for the purpose to enhance sintering process. Pulsed Joule heat provides the advantage of rapid temperature increase (up to several hundred °C/min) and possibility to electro- migration or electro-plasticity [15]. If the sample is not conductive (as alumina in this paper) the passing current heats the die and material is heated due to convection of heat. The SPS can usually record the motion of pressing ram during the sintering process. The SPS thus can work as a simple high temperature and pressure dilatometer, which allows construction of MSS from SPS experiments.
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Fabricação de ferramentas de corte em gradação funcional por Spark Plasma Sintering...

Fabricação de ferramentas de corte em gradação funcional por Spark Plasma Sintering...

TiC) graded with cemented carbide. The experimental methodology was developed from thermo-mechanical model application to estimate thermal residual stress along with tool thickness, study into the influence of SPS sintering parameters (Temperature and Pressure) over sintered quality (physical properties characterization, density), on the basis of these data, the best operating condition was chosen to fabricate workpieces for mechanical tests of flexural strength, hardness and fracture toughness, besides FGM inserts to machining tests in grey cast iron using turning operation. The results showed the machine parameter that mostly influenced density was Temperature; the AlTiC and AlZr FGMs got an increase of 126 and 73% in flexural strength in relation to their homogeneous ceramics. Following the materials sequence, the hardness was evaluated at 13.8 and 15.8 GPa, whereas the fracture toughness was 4.91 and 5.04 MPa.m 1/2 . For the machining tests, FGM AlZr cutting tools
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Mat. Res.

Mat. Res.

Spark plasma sintering is used to consolidate a WC-Co powder produced from milled discharged hard metal cutting inserts. The powder was mixed to 10%wt Co powder and spark plasma sintered (SPS) under 80Mpa pressure, at 1200ºC with heating rate of 160ºC/min. A reference alloy was prepared with a commercial WC powder and 10%wt. Co under the same conditions. The structures of both alloys were investigated by means of scanning electron microscopy and Vickers hardness. The results show structures with densities and hardness at the same level. SPS can be used to produce hard metal pieces directly from scrap powder without any prior chemical and thermal treatment.
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Structural, Dimensional and Thermoelectric Properties of Melt Spun p-Bi0,5Sb1,5Te3

Structural, Dimensional and Thermoelectric Properties of Melt Spun p-Bi0,5Sb1,5Te3

studied. It was established that the crystallites constituting the powder particles are nanofragmented. Powders were compacted by vacuum hot pressing and spark plasma sintering. Thermoelectric characteris- tics of obtained samples were investigated in 100 K-700 K temperature range. The samples prepared by above methods posses low thermal conductivity while retaining values of the Seebeck coefficient and elec- trical conductivity comparable to conventional crystallized materials, thereby thermoelectric efficiency ZT reaches 1.05-1.15 at 330-350 K.

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Three-dimensional computer simulation of grain coarsening during sintering

Three-dimensional computer simulation of grain coarsening during sintering

Coarsening and grain growth accompany skeletal structure evolution during sintering of powder compacts [1,2]. In general, the sintering process is driven by a decrease in the overall potential energy via reduction of the solid-vapor interfacial energy balanced with the growth of solid-solid interfaces within the sintered part. The growth of a neck between touching solid grains can occur via surface, grain boundary and volume diffusion or vapor transport via evaporation and condensation. Based on these diffusion mechanisms, a number of sintering models describing the evolution of neck geometry between two grains or an array of connecting grains (skeletal structure) have been developed [3-6]. Such sintering models introduce an assumption that the driving force can be principally modeled on the basis of the local grain and neck curvature. For traditional two- and three-grain models good agreement between predicted evolution of neck geometry and neck growth experiments was obtained, but extension from simple models to multi-grain models for densification of a powder compact was usually failed due to unaccounted geometric changes and topological constrains [7], such as local grain rearrangements connected with formation of new contacts between grains involved into the same or different solid skeletons [8].
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Phenomenological theory of sintering and its application to swelling

Phenomenological theory of sintering and its application to swelling

A general phenomenological theory of initial stages of ohmic sintering and final stages of a sintering process with and without bulk heating mechanisms is presented in this paper. The role of various sinks and sources of vacancies is analyzed. Mobile and immobile dislocations and their role in the sintering process are considered. The diffusional relaxation parameters for ensembles of grain boundaries, pores and dislocations are calculated. The role of the quality of the surface is discussed. Cycling and pulsing regimes of sintering are described. Specific driving forces of the bulk heating origin are analyzed in detail. The same approach is applied to the problem of swelling under penetrating radiation.
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Microstructure evolution during pressureless sintering of  bulk oxide ceramics

Microstructure evolution during pressureless sintering of bulk oxide ceramics

The Two-Step Sintering of isostatically pressed TAI, REY, Z3Y and Z8YS ceramics was performed in a su- perkanthal resistance furnace (K 1700/1, Heraeus, Ger- many) in air atmosphere. According to the data from the literature [11] temperatures higher than those guar- anteeing a relative density of 75% t.d. were chosen for the fi rst sintering step. Several combina tions of temper- atures of the fi rst sintering step (T 1 ) and the second sin- tering step (T 2 ) were tested for each material. The dwell times at the T 2 were between 0 and 20 hours. Sintering proceeded at a heating rate of 10°C/min up to a temper- ature of 800°C, and at 5°C/min up to a temperature of the fi rst sintering step. The temperature decrease from the fi rst to the second sintering step proceeded at a rate of 60°C/min.
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Matéria (Rio J.)  vol.12 número2

Matéria (Rio J.) vol.12 número2

Variações na tecnologia SLS tem levado à confecção de scaffolds em biometais (aço inox, ligas Co- Cr e Ti-6Al-4V ). Para diferenciar das aplicações de SLS para polímeros foram cunhados dois outros acrônimos: DMLS (Direct Metal Laser Sintering) da empresa EOS e SLM (Selective Laser Melting ) da empresa MPC-HEK. Outra tecnologia recente disponível no mercado é conhecida como LENS (Laser Engineering Net Shaping) que, diferentemente da tecnologia SLS e suas derivações para uso de metal, não utilizada camada espalhada e o pó metálico é injetado sob o foco do laser onde é fundido. Uma outra técnica recente para prototipagem em metal, embora não utilize laser, é o EBM (Electron Beam Manufacturing ) onde o pó é dispensado sob um feixe de elétrons para ser fundido de acordo com o desenho da peça. Com estas tecnologias de prototipagem rápida de metais é possível grande controle de geometrias e dimensões dos poros (Functionally Graded Pore Distribution - FGPD) [12]. Estas evoluções utilizando feixes de alta energia tem propiciado também a construção de peças em cerâmica o que pode levar, em breve, à construção de scaffolds de pura hidroxiapatita com alto controle dimensional e geométrico dos poros e com controle espacial no volume do implante.
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High pressure sintering of TiB2 ceramics at different temperatures

High pressure sintering of TiB2 ceramics at different temperatures

High pressure sintering kinetics of TiB 2 powder with a mean particle diameter of 60 μm was studied earlier [6]. It was shown that the densifi cation process consists of two stages which proceed with different rates. On the fi rst stage, densifi cation rate is fast and

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Master sintering curve: A practical approach to its construction

Master sintering curve: A practical approach to its construction

Disks of 30 mm in diameter and ca. 5 mm in height were prepared from the above materials, via cold isostatic pressing (CIP). Pressing was carried out in an isostatic press (Autoclave Engineering, Inc., USA) at a pressure of 300 MPa with a dwell time of 5 minutes. The CIPed samples were presintered at 800°C/1h, then cut and ground in the shape of prisms of ca. 4x4x15mm for sintering in the dilatometer.

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Pressureless sintering behavior of injection molded alumina ceramics

Pressureless sintering behavior of injection molded alumina ceramics

In this paper, pressureless sinterability of two commonly used commercial submicron alumina (MgO-doped and undoped), produced by injection molding, has been studied. We focused on the influence of solid loading and sintering schedule on the performance of the two different powders. Different solid loading and heating rates were adopted, which allows the shrinkage behavior in the initial, intermediate and final stages of sintering to be observed. The present work was intended to investigate the sintering behavior of pressureless sintered alumina ceramics produced by Injection Molding. The investigation also considered the effect of MgO addition on the densification of the injection-molded-Al 2 O 3.
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Prediction of densification and microstructure evolution for α-Al2O3 during pressureless sintering at low heating rates based on the master sintering curve theory

Prediction of densification and microstructure evolution for α-Al2O3 during pressureless sintering at low heating rates based on the master sintering curve theory

A trial of application of the master sintering curve was carried out to see whether a unique densification curve exists during sintering. Fig.5 shows the relative density continuously determined by a dilatometer during sintering with a constant heating rate as well as those measured by the Archimedes method after sintering with different heating profiles. The relatively small error bars (1.5%) on the MSC for 2.5μm α-alumina shown in Fig.5 are indicative of the small and apparently random scatter of the sintering data about the MSC. The relative densities from dilatometer trace (66.2, 74.2, 85.9, 95.3%) are consistent well with those determined by Archimedes method (67.1, 75.3, 85.1, 94.9.%) after different sintering process (1°C /min to 1250°C, 3°C/min to 1420°C, 4°C /min to 1575°C and 5°C /min to1650°C), even though they undergo a different heating history. It is proved that the density profile versus time–temperature integral values, Θ, can be used to predict the final density processed with a different heating history.
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Analysis of isothermal sintering of zinc-titanate doped with MgO

Analysis of isothermal sintering of zinc-titanate doped with MgO

and 2.5%) were mechanically activated 15 minutes in a planetary ball mill. The powders obtained were pressed under different pressures and the results were fitted with a phenomenological compacting equation. Isothermal sintering was performed in air for 120 minutes at four different temperatures. Structural characterization of ZnO-TiO 2 -MgO system

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High-beta plasma blobs in the morningside plasma sheet

High-beta plasma blobs in the morningside plasma sheet

While the spatial structuring may well be a conse- quence of some sort of mirror instability, this does not explain the origin of the very high beta plasma. Very high beta values, up to 70, have been found in the midnight sector at the end of the growth phase of a substorm and one may wonder what happens to this plasma after substorm onset. Haerendel (1999a) pro- posed on the basis of a ®rst inspection of the here reported measurements that the morningside blobs may be residues of this pre-substorm high-beta plasma near Earth. This may, indeed, be the case for periods following substorm onsets in the midnight sector, but it is hard to believe that this is an explanation for the blobs found during quiet periods with no substorm preceding for many hours.
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Caracterização  e análise de textura cristalográfica do aço maraging 300 produzido por manufatura aditiva

Caracterização e análise de textura cristalográfica do aço maraging 300 produzido por manufatura aditiva

A técnica Direct Metal Laser Sintering (DMLS) é um tipo de manufatura aditiva, que consiste em um processo de sinterização do pó do material por meio de fonte de calor a laser de alta frequência. Nessa técnica, o componente é formado camada por camada, e seu projeto é fornecido para o equipamento por meio de softwares de ilustração tridimensional. O resultado são componentes com dimensões e formatos próximos ou iguais ao do produto final ( near net shape ), por esse motivo essa técnica também é conhecida como prototipagem rápida. Dessa forma, trabalhos de usinagem após fabricação não são necessários, diminuindo tempo de produção, que em escala industrial aumenta produtividade e diminui custos. Por esses motivos, as técnicas de manufatura aditiva vêm desenvolvendo-se muito rápido.
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Físicos mais influentes (teóricos, contando 3 papers)

Físicos mais influentes (teóricos, contando 3 papers)

48 plasma chemistry plasma physics surface chemistry Plasma processing.. 49 Material Vapor Pressure Material Unit Value low high low high glass Mag.[r]

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