Kinetics of interparticle contact formation during the laser sintering of two-component powders

The kinetics of interparticle contacts formation during the liquid-phase sintering of unlike materials under the action of laser irradiation was investigated. Trios of particles arranged in a row according to the schemes glass—polymer glass, metal—polymer—metal, and metal—glass—metal were sintered. The processes of particle approach and liquid neck formation between particles were studied. Possibilities for the manufacture of powder components with polymer and glass binders by the use of selective laser sintering followed by heat treatment are considered]. Institute of Technical Acoustics, Academy of Sciences of Belarus', Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(407), pp. 37–41, May–June, 1999.     

Experimental modeling of the joining of two superficially melted particles

The kinetics of surface melting and solidification of particles, and the joining of two superficially melted particles, were studied using model experiments. The experimental results confirmed the welding mechanism of particle joining in laser sintering. Institute of Technical Acoustics, Byelorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 42–46, May–June, 1998     

Selective laser sintering of compacted powders

The selective laser sintering of preliminarily compacted powders was investigated. Depths of sintering lower than those with freely poured powders were attained. This has a negative effect on the production of multilayered articles. The main disadvantage of the process, however, was the difficulty in separating the article from the unsintered compacted powder. Institute of Technical Acoustics, Byelorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 27–31, July–August, 1998.     

The role of particle size on the laser sintering of iron powder

The effects of powder particle size on the densification and microstructure of iron powder in the direct laser sintering process were investigated. Iron powders with particle sizes ranging from 10 to 200 μm were used. It was found that the sintered density increases as the laser energy input is increased. There is, however, a saturation level at which higher density cannot be obtained even at very intensive energy input. This saturation density increases as the size of the iron particles decreases. Meanwhile fine powders with narrow particle size distributions have a tendency toward agglomeration, and coarse powders with broad particle size distributions have a tendency toward segregation, both of them resulting in lower attainable density. In order to investigate the role of particle size, a “densification coefficient (K)” was defined and used. This coefficient depends on the particle size and the oxygen content of iron powder. The results of this investigation demonstrate that the presence of oxygen significantly influences the densification and pore morphology of laser-sintered iron. At higher oxygen concentrations, the iron melt pool is solidified to agglomerates, and formation of pores with orientation toward the building direction is more likely to occur. When the oxygen concentration is kept constant, the densification coefficient decreases with decreasing the particle size, meaning the densification kinetics enhances. This article presents the role of powder characteristics and the processing parameters in the laser sintering of iron powder as a model material. The mechanism of particle bonding and microstructural features of laser-sintered parts are addressed.     

Sintered ceramics based on boron nitride and carbide

The formation of BN-B₄C composite materials by sintering in nitrogen is investigated. Structural, mechanical, and chemical characteristics of these materials are examined. Excellent dielectric properties, thermal and chemical stability, and erosion resistance in high-intensity laser beams enable high-temperature application of BN-B₄C composite materials. __________ Translated from Poroshkovaya Metallurgiya, Vol. 46, No. 1–2(453), pp. 58–63, 2007.     

Investigation into the Influence of the Structure Dispersion and Homogeneity on the Properties of Powder Metastable Austenitic Carbide Steels and Diamond Tools

Diffusion and homogenization in “iron (5 μm–nickel (5 μm or 50 nm)” powder systems of various degrees of dispersion during sintering (900 and 1000°C), as well as spark plasma sintering, are investigated using the Matano–Boltzmann method. Calculated diffusivities in pairs of micron powders sintering without applying pressure (900°C, 6 h) and by the spark plasma method (900°C, 5 min) in these systems are equal to 7 × 10–10 cm2/s. It is shown that the use of nanodispersed nickel powder in diffusion pairs based on finely dispersed iron powder promotes a twofold increase in diffusivity at 900°C in contrast to the pair with the microdispersed nickel powder. Constants in the Ivensen sintering kinetics equation are calculated for the “iron–nickel” powder systems, by which the factors activating sintering of these systems are established. The dependences of the structural phase composition and physicomechanical properties of carbide steels of the Fe(base)–14 wt % Ni–8 wt % TiC system on the sintering temperature in range t = 900–1200°C and structure dispersity and homogeneity are determined. The dependences of the grain size, porosity, hardness, microhardness, fracture toughness, and bending ultimate strength on the sintering temperature are shown. Dependences of tribotechnical properties on the degree of homogeneity of the solid solution and volume of the phase transformation of metastable austenite into deformation martensite during abrasive friction turn out similar for carbide steels and diamond tools based on carbide steel. Optimal values of the variation coefficient of the nickel concentration in austenite and carbide steels of the same chemical composition but with different degrees of dispersity, which provide the maximal volume of the austenite decomposition and high values of the diamond-tool grinding coefficient, turn out equal to 5 in both systems, but the sintering parameters are different. It is shown that the physicomechanical properties of the studied systems depend on the structure porosity and dispersity, while tribotechnical properties depend on the structural homogeneity of steels.     

Fatigue and Compression Characteristics of Al 6061–B 4 c Composite Materials

Aluminum matrix composites reinforced with boron carbide are a kind of materials that are widely used because of high strength, low density, and improved tribological properties. In this study, mechanical properties of Al 6061–B4C composites reinforced with B4C of three different particle sizes were investigated. In the Al 6061–B4C composite materials, produced by the powder metallurgy methods (extrusion of billets obtained by sintering at temperature of 550°C under pressure of 450 MPa), the change of mechanical properties such as hardness, compressive strength, and fatigue life, related to B4C particle size and the applied heat treatment mode (aging at 180°C for 5 h), were investigated. The hardness of the materials is increased with B4C grain size and the heat treatment. After the heat treatment, the fatigue life of Al 6061–B4C (3 μm) material increases slightly, while that of the composite materials decreases with larger size of B4C reinforcement. The fatigue life of the composite materials reinforced with a larger grain size B4C is reduced by heat treatment. While the compression test data of untreated composite materials were similar to each other, the heat treatment increased these values in all samples. The highest increase in the compression strength was observed in the composite reinforced with 17 μm sized B4C. The addition of graphite reduces the deformation ability of the composites.     

Selective laser sintering. I. Principles, continual model

he main thermophysical and rheological processes accompanying selective laser sintering are considered. A mathematical model of laser sintering is proposed. The results of computer experiments with the model applied to multilayer composites containing titanium nitride, diboride and carbide, as well as molybdenum, nickel, and steel are discussed. It is shown that densification depends on the heating regime and initial porosity of the composites. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 16–26, July–August, 1998.     

Sintering behavior and strength properties of stomatological porcelain—Titanium nickelide composite materials

The sintering behavior and strength properties of stomatological porcelain—titanium nickelide composites were investigated. The dependence of porosity, linear and volumetric changes, phase composition and structure on sintering time and temperature, component concentrations, and initial porosity have been determined. It was shown that density changes are the result of a competition between shrinkage and volumetric growth. An optimal regime of sintering to ensure open porosity and high strength is recommended. Siberian Physico-Technical Institute, Tomsk. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 31–36, July–August, 1998.     

Effect of carbon on volume changes during the sintering of an iron-chromium material

The effect of carbon added in the form of either cast-iron powder or graphite on the sinterability of chromium steel was investigated. The growth and instability of volume change during sintering produced by cast-iron additions was explained as a result of a thermally activated after-effect in the solid phase, and the formation of FeCr₂O₄ around particles of Kh30. Substitution of graphite for cast iron in the powder charge prevented oxidation of the chromium-rich particles, and resulted in low and stable shrinkage (0.1–0.8%) after sintering in the range 750–1250°C. Scatter in the degree of volume change was reduced from 1.5–2.0 to 0.1–0.3%. Iron-chromium material obtained with the use of graphite had improved strength properties as well as high produceability. Materials Science Institute, Ukrainian Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 5–6, pp. 38–42, May–June, 1998     

Mechanisms of formation of the magnetic properties of powder metallurgy materials

The magnetic susceptibilities of powder titanium and nickel were measured over a wide range of porosities. the effect of consolidation during the sintering of conglomerates on the magnetic properties of the investigated materials was determined. The dependence of magnetic properties on porosity was explained on the basis of a physical analysis of the factors controlling the development of magnetic properties in powder materials. Altai State University, Barnaul. Translated from Poroshkovaya Metallurgiya, Nos. 3–4(400), pp. 36–38, March–April, 1998.     

Kinetics of formation of interparticle contacts in the laser sintering of single component metallic powders

The joining of two spherical particles of nickel alloy powder under the action of lager irradiation, and the lager sintering of thin layers of poured powder were investigated. It was determined that powder particles experience surface melting under the action of lager heating, and join by a welding mechanism Institute of Technical Acoustics, Belorussian Academy of Sciences, Vitebsk. Translated from Poroshkovaya Metallurgiya, Nos. 1–2, pp. 54–62, January–February, 1997.     

Analysis of structure parameters for compacted porous materials. II. Integrated and local compaction during sintering of porous bodies

Integrated and local compaction of porous bodies during sintering are analyzed and an algorithm is developed for calculating the structure parameters of sintered materials. Accurate quantitative estimates are obtained for the efficiency of porous material sintering in different structural models. The most precise procedure is based on a model developed by the author for the structure of porous materials and an approximation according to which Voronoi polyhedra are considered to be unchanged with a decrease in system porosity, and pore volume decreases as a result of an apparent increase in particle diameter. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(408), pp. 106–111, July–August, 1999     

Desorption of gases from the surface of silicon nitride powders

The influence of fabrication and treatment techniques and of carbon coating onto silicon nitride powders on the process of gaseous component desorption has been studied. The N₂, H₂O, CO₂, and CO molecules, as well as OH and NH radicals, were revealed to be absorbed on the particle surface. Different hydrocarbons are the products of reaction in the gas phase and on the surface. Carbon coating on the powder particle surface alloys one to protect the surface from absorbed gases and from chemosorbed compounds, in particular hydroxylic groups. The results obtained permit one to determine the conditions of intended modification of surface and thermal treatment of powders in order to activate their sintering. Institute for Problems of Materials Science, Ukraine National Academy of Sciences, Kiev. Translated from Poroshkovaya Metallurgiya, Nos. 7–8(402), pp. 76–81, July–August, 1998.     

Analysis of nonisothermal sintering of zinc-titanate ceramics doped with MgO

The aim of this work is to analyze nonisothermal sintering of zinc titanate ceramics doped with MgO obtained by mechanical activation. Mixtures of ZnO, TiO₂, and MgO (0, 1.25, and 2.5%) are mechanically activated for 15 min in a planetary ball mill. Nonisothermal sintering is performed in air for 120 min at 800, 900, 1000, and 1100 °C. Microstructure parameters are revealed from an approximation method. Structural characterization of ZnO-TiO₂-MgO system after milling is performed at room temperature using XRPD measurements. The main conclusions are that mechanical activation leads to the particle size reduction, the increase of dislocation density, and lattice strain. Doped zinc titanate samples achieve higher densities after sintering and the diffusion mechanism is dominant during the sintering process. Published in Poroshkovaya Metallurgiya, Vol. 47, No. 1–2 (459), pp. 83–90, 2008.     

Absorption of hydrogen by sintered intermetallics of the Sc — Ni system

The absorption of hydrogen by the intermetallics Sc₂Ni, ScNi, ScNi₂, Sc₂Ni₇, and ScNi₅ produced by sintering was investigated. Hydrogen absorption isobars at the pressures 0.1 and 3 MPa were obtained. The maximum hydrogen absorption capacity of the compounds was determined. The absorption properties were compared with those of specimens produced by arc melting. Translated from Poroshkovaya Metallurgiya, Nos. 5–6(413), pp. 78–82, May–June, 2000.     

Activated sintering of W-HfC composite materials

The features of consolidation of the particles during the activated sintering of tungsten powders with different values of dispersity (d _av = 2–3 and 0.8–1.0 μm) are investigated. Sintering was activated by introducing nickel additives (up to 0.5 wt %), tungsten nanoparticles (up to 30 wt %), and finely dispersed hafnium carbide (5–30 vol %) with subsequent milling in a vibrating mill. The uniaxial compaction of the samples has been performed under pressures from 50 to 1000 MPa, and sintering was performed in vacuum at 1850°C with holding for 1 h. It is shown that the additives of tungsten carbide increase the density of sintered billets and, in combination with dispersed hafnium carbide, tungsten-based composite materials with a grain size up to 2 μm can be obtained.     

Features of skeletal composite compaction and sintering

Skeletal composites are prepared using highly porous cellular nickel as a volumetrically-bonded framework and features of their compaction and sintering are studied. The skeleton improves composite compactability and may slow down volumetric changes during low-temperature sintering, but it does not affect them during high-temperature sintering. In compacts of skeletal composites at skeleton-ceramic boundaries there is formation of pores that by growing during sintering form a volumetric network of microcracks. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(450), pp. 10–18, July–August, 2006.     

Microstructure of W - Ni - Sn Composites in Relation to Amount of Liquid Phase on Sintering

Microstructure transformation is examined for W - Ni - Sn composites made by liquid-phase sintering at 1200 °C for 1 h; the liquid phase was a eutectic containing 67.5 mass% Ni and 32.5 mass% Sn. There is a nonmonotone dependence of the tungsten grain size in the sintered composites on the volume proportion of liquid phase in the range 6.7–36.8%. The mean grain size in relation to volume proportion of liquid phase in liquid-phase sintering is calculated from rheological sintering theory. The form of the empirical function f(v) is quantitatively demonstrated, which determines the rate of tungsten grain growth on liquid-phase sintering and is dependent on the proportion of matrix in the composite and the size of the liquid phase layer. __________ Translated from Poroshkovaya Metallurgiya, Nos. 7–8(444), pp. 53–57, July–August, 2005.     

Finely dispersed Si3N4−SiC powders and materials based on them

The objectives of the present research were to investigate the preparation of homogeneous ultrafine composite Si₃N₄−SiC powders by a plasmochemical process and the properties of ceramics produced from them. The chemical and phase compositions of the powders depended on the particle size of the initial powder, silicon input rate, and ratio of ammonium and hydrocarbon flow rates. The particle size and specific surface area of the compounds depended on the concentration of particles in the gas jet, and the cooling rate of the products. Composite powders containing from a few up to 90 mass % SiC, with specific surface areas of 24–80 m²/g and free silicon and carbon content less than 0.5 mass % were obtained. The main phases present were α-Si₃N₄, β-Si₃N₄, β-SiC, and X-ray amorphous Si₃N₄. Dense materials were prepared both by hot pressing at 1800°C under a load of 30 MPa and gas-pressure sintering at 1600–1900°C under a pressure of 0.5 MPa nitrogen. The plasmochemical composites had smaller pore sizes, were finer grained, and densified more rapidly than materials sintered from commercial powders. Institute of Inorganic Chemistry, Latvian Academy of Sciences, Salaspils. Translated from Poroshkovaya Metallurgiya, Nos. 1–2(405), pp. 7–12, January–February, 1999.