Sintering Simplified: Surface Area, Density, and Grain Size Relations
TL;DR: In this paper, an approach that links to global energy reduction during sintering to simple monitors is presented, showing that grain boundary area peaks at intermediate sintered densities, while surface area continuously declines.
Abstract: Sintering involves several interactions as particles bond and enable microstructure evolution toward a minimized energy condition, resulting in a complex interplay of measurement parameters. Overriding the evolution is energy minimization, and from that perspective some simple relations emerge. The natural progression is determined by energy reduction, measured by surface area, density, and grain boundary area (grain size). Contrary to the usual sintering analysis that starts with atomic level mass transport mechanisms, presented here is an approach that links to global energy reduction during sintering to simple monitors. Initially sintering converts surface area into lower energy grain boundary area. Subsequently grain growth annihilates grain boundary area. Thus, grain boundary area peaks at intermediate sintered densities, while surface area continuously declines. The trajectory follows a straightforward dependence on density as illustrated using data for a wide variety of materials and consolidation conditions.
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TL;DR: In this paper, the effect of powder size distributions on green and sintered part qualities (bulk density, relative density, particle size, pore size, sinter neck size) was investigated.
52 citations
TL;DR: In this paper, the sintering process is treated using a matrix of mathematical relationships that include at least seven atomic transport mechanisms, several options on powder characteristics, and three pore-grain morphology options.
Abstract: Sintering is a mainstay production step in forming metal, ceramic, polymer, and composite components from particles. Since the 1940s, the sintering process is treated using a matrix of mathematical relationships that include at least seven atomic transport mechanisms, several options on powder characteristics, and three pore–grain morphology options. The interplay of these relationships is handled by numerical solutions to predict property development. An alternative approach is to track the sintering trajectory using relatively simple relationships based on bulk measures. Energy minimization dictates that initial stage sintering acts to reduce surface area. In late stage sintering, the energy minimization turns to grain boundary area reduction via grain growth. Accordingly, relationships result between density, surface area, and grain size, which largely ignore mechanistic details. These relationships are applicable to a wide variety of materials and consolidation conditions, including hot pressing, and spark sintering.
40 citations
TL;DR: In this paper, a characterization of graphene-reinforced aluminium alloy 7075 (AA7075) microcomposites and nanocomposites is reported, which shows that the hardness of the composites proportionally increases with the graphene addition.
Abstract: Processing and characterization of graphene (Gr)-reinforced aluminium alloy 7075 (AA7075) microcomposites and nanocomposites are reported in this work. Composites are fabricated by mechanical alloying process at wet conditions. The bulk composites are prepared by uniaxial die pressing to get higher densification and sintered in an inert atmosphere. Density of the nanocomposites is higher than the microcomposites due to the reduction of grain size by increased milling time. X-ray diffraction (XRD) analysis confirms graphene interaction with the AA7075 matrix lattice spaces. The effective distribution of graphene with aluminium alloy is further confirmed by the Transmission Electron Microscopy (TEM) analysis. The hardness of the composites proportionally increases with the graphene addition owing to grain refinement. Wear morphology is characterized using Scanning Electron Microscopy (SEM) and Atomic Force Microscopy (AFM). Microcomposites reveal abrasive and ploughing wear mechanism of material removal from the surface. Nanocomposites show adhesive wear with delamination and particle pull-out from the material surface.
34 citations
Cites background from "Sintering Simplified: Surface Area,..."
...During sintering, the grain growth diffuses the grain boundaries and the shrinkage of compact yields better densification [26]....
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TL;DR: The sintering densification trajectory for titanium powder is identified in terms of the interaction between mass transport processes and microstructure evolution as discussed by the authors, which is a means to link densification to process parameters without concern over detailing this cascade of transport mechanisms and micro structure changes.
Abstract: The sintering densification trajectory for titanium powder is identified in terms of the interaction between mass transport processes and microstructure evolution. During initial heating, as surface oxides dissolve, surface diffusion forms bonds between contacting particles without densification. Grain boundaries form in the bonds due to random crystal orientations at the contacts. Except for mixed powder Kirkendall swelling, subsequent diffusion in these interparticle grain boundaries leads to densification. Most importantly, the alpha-beta transformation provides strain, defects, and interfaces that accelerate densification in the 800–1100 °C temperature range. This is below a typical peak sintering temperature. Final densification involves beta phase volume diffusion and grain boundary diffusion. Densification slows due to grain growth and the loss of grain boundary area. Pores close near 92% density to trap impurities and reaction products inside the closed pores, often limiting sintered density to about 95% of theoretical. High final density requires slow heating or long holds at intermediate temperatures to evaporate impurities prior to pore closure. The master sintering curve is a means to link densification to process parameters without concern over detailing this cascade of transport mechanisms and microstructure changes.
27 citations
TL;DR: In this article, a combination of extrusion of liquid inks containing a binder, solvents, and elemental Ni, Mn, and Ga powders and heat treatments to remove the polymer binder and to interdiffuse and sinter the powders was used to study the microstructural evolution, sintering mechanisms, and grain growth in these wires.
21 citations
References
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Book•
01 Jan 1996
TL;DR: Sintering Measurement Techniques Solid-State Sintering Fundamentals as discussed by the authors Microstructure and Processing Relations in Solid-state Sinterings, Mixed Powders, Pressure-Assisted SinterING.
Abstract: Sintering Measurement Techniques. Solid-State Sintering Fundamentals. Microstructure and Processing Relations in Solid-State Sintering. Solid-State Sintering of Mixed Powders. Liquid-Phase Sintering. Pressure-Assisted Sintering. Novel Sintering Techniques. Sintering Atmospheres. Sintering Practice. Future Directions. Appendix. Index.
2,261 citations
"Sintering Simplified: Surface Area,..." refers background or methods or result in this paper
...7 compares this equation to experimental data, giving a particle coordination of 13 as full density, in agreement with observations [4,30-32]....
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...For comparision, lines show the expected behavior from curvature calculations by German [36], densification data by Kumar [35], computer simulations by Hare [34], and initial stage geometric models [4]....
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...Specific surface area is a linear function of the sintered density [4-6,18-25]....
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...On the other hand, density, surface area, shrinkage, and bulk properties, such as strength, hardness, elastic modulus, magnetic permeability, or conductivity, are measures that average over many neck growth events [4]....
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...The magnitude of the sintering stress depends on the solid-vapor surface energy, particle size, and neck size [4,13,14]....
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TL;DR: In this paper, the pore and grain boundary structures in sintered powder compacts are presented to provide the basis for qualitative description of the important phases of the course of densification.
Abstract: Photomicrographs of pore and grain boundary structures in sintered powder compacts are presented to provide the basis for qualitative description of the important phases of the course of densification. From this guide, appropriate grain shapes and pore shapes and locations are selected for the formulation of diffusion sintering models. The principle models presented are for bulk diffusion transport with the grain boundaries as vacancy sinks when the pore phase is continuous and coincident with three grain edges, and also when the pore phase is discontinuous and located at four-grain corners. These models predict that the rate of density change is constant when the diffusion coefficient and grain size are constant. The need for simultaneous isothermal densification and grain growth data is indicated. The explicit change in densification rate with discontinuous grain growth is predicted in terms of pore spacing and grain size.
961 citations
TL;DR: In this article, a model is presented for the behavior of copper during the initial stages of sintering, which is in agreement with available experimental data, and which requires vacancy elimination at dislocations or grain boundaries.
Abstract: The mechanism of material transport in sintering can be elucidated in some cases by direct observation of the rate of interface growth and approach of centers between spherical particles. Measurements with glass, sodium chloride, and copper indicate that with these materials viscous flow, evaporation-condensation, and self-diffusion are the rate-determining mechanisms. Values of viscosity, vapor pressure, and diffusion constants have been determined, but calculations of diffusion constants from these data are subject to uncertainties of interpretation. A model is presented for the behavior of copper during the initial stages of sintering, which is in agreement with available experimental data, and which requires vacancy elimination at dislocations or grain boundaries. Data for refractory oxides indicate the importance of purity and fabrication pressure, but the sintering mechanism for these materials is not determined by the present data.
797 citations
01 Jan 1945
579 citations
"Sintering Simplified: Surface Area,..." refers background or methods in this paper
...Their treatment assumed the energy required to produce densification came from the surface energy release during sintering, akin to Frenkel’s idea for viscous flow sintering [2] and Sutton and Baluffi’s idea for grain boundary diffusion [12]....
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...Neck size is one monitor for sintering [2, 3]....
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TL;DR: In this paper, the authors studied the mechanisms involved in sintering, a phenomenon of two or more particles with the application of heat only and at temperatures below melting point of any component of the system, although the powder metallurgists use this term in a broader sense, including the presence of molten phase and pressure.
Abstract: Two particles in mutual contact form a system which is not in thermodynamical equilibrium, because its total surface free energy is not a minimum. If such a system is left for a certain period of time, the bonding of the two particles will take place in order to decrease the total surface area, even though the temperature is lower than the melting point. This phenomenon of bonding of two or more particles with the application of heat only and at temperatures below melting point of any component of the system will be called sintering, although the powder metallurgists use this term in a broader sense, including the presence of molten phase and pressure. It is the objective of this paper to study this process and the mechanisms involved in it.
336 citations
"Sintering Simplified: Surface Area,..." refers background in this paper
...Neck size is one monitor for sintering [2, 3]....
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