Effect of Nanopowder Addition on the Sintering of Water-Atomized Iron Powder
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E press and sinter route offers cost-effective solutions in the manufacturing of powder metallurgical (PM) steel components for structural applications. There is a constant drive to improve the density of PM steel, which expands its usage in applications demanding higher performance than what it currently delivers. Sintering is an important step in the PM manufacturing route and involves the transfer of powder compact into a high-density body that minimizes residual porosity while controlling tolerances. The process of so-called solid-state sinter occurs at temperatures below the melting point of the particles, and the properties of the product are a direct function of the sinter neck development and the degree of SWATHI K. MANCHILI, JOHAN WENDEL, EDUARD HRYHA, and LARS NYBORG are with the Department of Industrial and Materials Science, Chalmers University of Technology, 41258 Gothenburg, Sweden. Contact e-mail: [email protected] ABDELHAFID ZEHRI and JOHAN LIU are with the Department of Microtechnology and Nanoscience, Chalmers University of Technology, 41258 Gothenburg, Sweden. Manuscript submitted January 16, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
densification.[1] Understanding sintering is therefore important, because the performance of the component largely depends on this process. For densification to occur, material flow is required. This largely depends on the sintering temperature and particle size. Surface diffusion mechanisms for material flow facilitate neck formation between the metal particles, whereas bulk diffusion mechanisms are required for the densification of the component.[2] Densification is accomplished by increasing the temperature of the powder compact to enable material transport. The metal particles change their morphology to reduce the total energy of the system. This has been explained at both a coarse and fine scale. Necks are formed between the particles, comprising metallurgical bonds in nature, and they gradually approach one another, which in turn leads to shrinkage and densification. Mass transport mechanisms describe sintering on a fine scale. Sintering not only results in the densification of the compact but also leads to grain growth because the process is followed within the grain growth temperature range. Grain growth or coarsening occurs during the final stage of sintering as the temperature is sufficient to drive the coarsening process.[3] A promising method of improving densification is the use of bimodal mixes. A high
surface-to-volume ratio enhances the reactivity of the nanoparticles. Because the large surface area enhances the surface energy, nanopowder is considered to have lower activation energy for sintering.[4,5] However, with this advantage, there is also the problem of strong agglomeration accompanying the high surface energy and the problem of increase in inter-particle friction.[6,7] Tapping the full potential of these nanostructured materials across the spectrum of applications requires a simple and cost-effective method of producing and processing them in bulk f
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