Neck growth kinetics during microwave sintering of nickel powder

Model experiments on initial stage of microwave sintering of nickel powder showed anomalous neck-growth rate during isothermal soaking, which is not the case for conventional sintering. Neck growth was determined as a function of time. Values for the neck growth exponent in the neck growth equation, (x/a)ⁿ = Bt, of 5.2, 5.4, 5.8, and 5.9 were found for within the temperature range 700-950 °C, respectively.The evidences of formation of liquid phase during microwave sintering have been revealed, that may support enhancement of mass transfer during sintering process. The activation energy of 48 kJ mol⁻¹ was found for microwave sintering of nickel, according to sphere-to-sphere model. Value revealed is significantly lower then values for conventional sintering (136 kJ mol⁻¹), and is on same level with activation energy for diffusion of metals in liquid state. An explanation and analysis of this phenomenon has been attempted.     

Gravitational effects on compact shaping and microstructure during liquid-phase sintering

Liquid-phase sintering is routinely used to consolidate dense components. However, there are several problems during liquidphase sintering that have their origin in gravitational effects. Under terrestrial conditions, this technique is generally limited to high solid contents to ensure structural stability and net-shaping. In addition, there are several unresolved issues pertaining to microstructural evolution and compact reshaping during sintering. Experimental conditions present during microgravity processing have allowed liquid-phase sintering over a wider range of solid-liquid ratios than is possible on Earth. A rare opportunity was provided to conduct extensive liquid-phase-sintering experiments on tungsten heavy alloys aboard the space shuttle Columbia as part of the International Microgravity Lab and Microgravity Space Lab. Early results from these experiments show novel behavior associated with microgravity both in microstructural and macrostructural evolution. For more information, contact A. Upadhyaya, Pennsylvania State University, P/M Lab, 118 Research West, University Park, Pennsylvania 16802-6809, (814) 865-2121; fax (814) 863-8211; e-mail axu3@psu.edu.     

The effect of particle size on the densification kinetics of tungsten powder during spark plasma sintering

The influence of particle size on the densification kinetics of tungsten powder during spark plasma sintering was investigated. The densification rate of tungsten powder in the intermediate sintering stage decrease with increasing particle size, resulting in a delay in the sintering stages of coarse powder. The isothermal densification kinetic behaviors of tungsten powder show that the densification of tungsten powder can be divided into two kinetic stages: a low-stress exponent segment (n = 1.5) and a high-stress exponent segment (n = 3 or 4). With increasing of particle size, n increases from 3 to 4, and the activation energy decreases from 304 to 254 kJ/mol for the high-stress exponent segment. This is because the densification mechanism has a tendency to change from diffusion creep to dislocation creep or dislocation glide as the particle size increases. The evolution of the activation energy exactly matches the transformation of the deformation mechanism, indicating that the densification activation energy does not reflect a barrier to densification, but rather a barrier to deformation with different deformation mechanisms.     

Modelling of metal-binder migration during liquid-phase sintering of graded cemented carbides

Sintering of cemented carbides with graded metal binder content is modelled in this article. Production of graded cemented carbides by traditional liquid-phase sintering is hampered by high rate of liquid-binder migration with inevitable subsequent homogenisation of binder content in specimens. The driving forces of the migration are analysed. It is shown that the migration is governed not only by capillary forces but also by additional stresses that arise due to surface tension at solid–liquid interfaces. As a result, the migration takes place even in pore-free graded specimens. A numerical approach for the prediction of binder flow with coupled deformation of the refractory skeleton during liquid-phase sintering is put forward. Calculations predict that complete densification of specimens with retention of graded-binder distribution can be achieved by sintering with long solid-state sintering stage ended by comparatively short high temperature liquid-phase stage.     

The effect of scanning strategies on electron beam sintering

The electron beam technology offers a wide range of possibilities for the use with industrial production processes. In addition to welding applications the fast beam deflection with electromagnetic coils can be used within additive layer manufacturing to gain a higher scanning speed and to realize new deflection figures compared to laser based systems. Therefore, the electron beam sintering exhibits a high application potential in order to achieve higher build-up rates. After explaining the functional principle and the relating process steps the examination of different scanning strategies is presented. By means of part analysis and visual observation, determining effects could be identified. In the following, the analysis of temperature profiles by means of thermal simulation was carried out in order to optimize the sintering of one layer. Based on these results the adaptation of process parameters could be derived. It is shown that the scanning strategies have a significant influence on the process and the part quality respectively.     

掺碳碳化硼活化烧结及其动力学

研究了在中位径为０．４２μｍ的碳化硼微粉中添加１％～４％碳为活化剂的烧结过程。研究了烧结坯的密度、抗弯强度、晶粒度与掺碳量、烧结温度、保温时间的关系。在一定烧结温度以上,少量碳的掺入明显提高了烧结活性,烧结坯密度明显提高,在本实验条件下最佳掺碳量为１％～２％,最佳烧结温度为２１６０～２２００℃。研究了掺碳碳化硼烧结动力学,得出其物质迁移机制主要为晶界扩攻。最后研究了其相组成,掺入的碳除溶解于碳化硼     

Densification and grain growth kinetics of boron carbide powder during ultrahigh temperature spark plasma sintering

Dense B₄C material was fabricated using spark plasma sintering (SPS), and the densification mechanisms and grain growth kinetics were revealed. The density, hardness, transverse flexure strength and toughness of samples were investigated and the model predictions were confirmed by SEM and TEM experimental observations. Results show that SPSed B₄C exhibits two sintering periods: a densification period (1800–2000 °C) and a grain growth period (2100–2200 °C). Based on steady-state creep model, densification proceeds by grain boundary sliding and then dislocation-climb-controlled mechanism. Grain growth mechanism is controlled by grain boundary diffusion at 2100 °C, and then governed by volume or liquid-phase diffusion at 2200 °C.     

Progress on grain growth dynamics in sintering of nano-powders

Nanostructured materials, characterized by an ultrafine grain size, have stimulated much research interest by virtue of their unusual mechanical, electrical, optical, and magnetic properties. In this paper, the sintering process of nano-powders were reviewed, to which sintering of the traditional materials compared. The microstructural development, i.e., grain growth and densification during sintering as well as the mechanism of crystal surface diffusion and boundary migration were analyzed, and the dynamic models on sintering process were summarized by the relationship of grain growth and pores size, interface diffusion, densification rate, and sintering temperature. Finally, the research tendency of this major on the basis of above models was discussed.     

The effect of a substrate on the sintering of constrained films

The discrete element method (DEM) is used to simulate the sintering of a powder compact constrained by a substrate. The constrained sintering of micropillars made of 4000–40,000 spherical particles is simulated using appropriate normal contact laws both for particle–particle and particle–substrate contacts. Tangential viscous forces account for the drag of the substrate on the particles. We show that, in accordance with experimental observations, the presence of a substrate has a significant effect on the microstructure of the constrained sintered films. Simulations confirm that anisotropy develops along the thickness of the film. The microstructure is more porous close to the substrate and the pores have their long axis preferentially oriented perpendicular to the substrate. The effects of film thickness and viscous drag at the film–substrate interface have also been investigated.     

The effect of sintering temperature on some properties of Cu-SiC composite

Copper matrix composites reinforced with 1 wt.%, 2 wt.%, 3 wt.% and 5 wt.% SiC particles were fabricated by powder metallurgy method. Cu and Cu-SiC powder mixtures were compacted with a compressive force of 280 MPa and sintered in an open atmospheric furnace at 900-950 °C for 2 h. Within the furnace compacted samples were embedding into the graphite powder. The presence of Cu and SiC components in composites was verified by XRD analysis. Optical and SEM studies showed that Cu-SiC composites have a uniform microstructure in which silicon carbide particles are distributed uniformly in the copper matrix. The results of the study on mechanical and electrical conductivity properties of Cu-SiC composites indicated that with increasing SiC content (wt.%), hardness increased, but relative density and electrical conductivity decreased. The highest electrical conductivity of 98.8% IACS and relative density of 98.2% were obtained for the Cu-1 wt.%SiC composite sintered at 900 °C and this temperature was defined as the optimum sintering temperature.     

The kinetics of growth of copper iodide on copper metal

In the temperature range 370–540°K and with iodine vapor at 1.5×10^–4 bar pressure, compact scales of CuI grow on Cu with parabolic kinetics. The Arrhenius plot of the rate constants shows very pronounced curvature over this small temperature range. This is discussed in terms of a transitionstate reaction-rate model and is ascribed to a temperature-dependent component of the activation entropy which originates in the structural randomness of the copper(I) ion sublattice in the iodide.     

Pressure effects and grain growth kinetics in the consolidation of nanostructured fully stabilized zirconia by pulsed electric current sintering

The effect of applied uniaxial pressure on the densification and grain size of nanocrystalline cubic zirconia (c-YSZ) was investigated during sintering by the pulsed electric current sintering (PECS) method. The role of the pressure depended on temperature, being highly significant at lower temperatures and of little significance at higher temperatures. The kinetics of grain growth were determined under PECS conditions. Analysis of the results indicated a grain growth process that is retarded, probably due to the effect of the current on grain boundary energy or dopant segregation. The activation energy for grain growth of c-YSZ was determined as 252 ± 34 kJ mol^?1, a value that is slightly smaller than reported values for microcrystalline samples.     

Pressureless sintering of La-Y-Sialon ceramics and grain growth in later period of sintering

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Microstructural evolution and sintering kinetics during spark plasma sintering of Fe and Al blended powder

The reaction diffusion between Fe and Al during spark plasma sintering (SPS) was studied. Microstructural evolution was investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) and the sintering kinetics was disclosed. The main interphase of the SPS sample was Fe₂Al₅ at 773–873 K. Ball-milling enabled a large number of lattice defects and grain boundaries thus the reaction kinetics was accelerated, although the direct current can also promote those defects. After milling, the phase transformation kinetics was improved from 0.207 before mill to 4.56×10^?3. Besides, this work provided more details for the generation of Joule heating. The resistance offered to the electric path was considered to be the source of Joule heating, and particularly the resistance offered by the different contact interfaces of die, punch, graphite foil and the sample played a leading role for the generation of Joule heating during spark plasma sintering.