Powder,sintering,theory,The,po business, insurance Powder sintering theory
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The powder sintering theorybegan in the early 1820s. In 1942, scientists using physical and chemicalresearch methods determined the effects of sintering temperature to thesintered body electromotive force, density, microstructure, mechanicalproperties, found that sintering is a very complex process. Chinese PowderMetallurgy scientists in 1961 come up with the combined effect sintering theory,think that the sintered powder is the result of spreading, flow and physicaland chemical reactions combined effects.The decrease of sintering thermodynamics sinteringsystem free energy is the driving force of the sintering process, including:the increases binding surface (sintering neck) and the straightness of particlesurface, the powder bodys total surface area and total surface free energy canbe reduced; the total volume of sintered bodys internal pores and the totalsurface area decreases; within powder particles, the lattice distortioneliminated.According to metal powder supplier, sintered motivitys quantitative analysis can proceed from threeaspects: considering the effects of surface tension from two goals model,deduced in the surface of the sintering neck exist tensile stress, make thesintering neck growing. The tensile stress is inversely proportional with the curvatureof the sintering neck radius. This shows that with the sintering neck grew up,the driving force decreased. The sintering neck surface exist tensile stress, sothat this areas vacancy concentration C will be higher than the no-stress areasvacancy concentration a. So that a small area under the surface of thesintering neck appears vacancy concentration gradient, cause the vacancieswithin the small region of the sintering necks surface diffuse to the sinteredbody, while the atoms remove to the sintering neck, make the sintering neck growing.Consider the equilibrium vapor pressure of the substances surface is relatedto its surface curvature radius, deduced that the sintered powder particles sphericalsurface (convex surface) and the surface of the sintering neck (concave) exist saturatedvapor pressure, make the atoms evaporated from the sphere while the cohesion inthe sintering neck concave down caused the sintering neck grew up.Sintering kinetics of sintering theory is mainlyconducted around this problem, through the model to determine the materials migrationpatterns and the rate of sintering process. Be summarized in the following:flow mechanism, think that the bonded and sintered neck grew up occurs in the powdersintering is due to the results of under surface tension caused viscous flow orplastic flow. Evaporating-cohesion mechanism, think that the substancemigration in the powder sintering process is mainly due to the surface of thepowder particles (convex surface) and the surface of the sintering neck(concave) exists the saturated vapor pressure differential, make the atomsevaporated from the particle surface and cohered in the sintering neck concave,make the sintering neck grew up. The relationship is the sintering neck radiusz 3 power proportional to time. Diffusion mechanism, think that the substancesmigration in the powder sintering process is mainly the diffusion take effect.The combined effects sintering theory. Therelationship between sintering neck grow and time derived from ideal sinteringmodel, although be verified under certain experimental conditions, but undersome other conditions there are exist big differences with the actual sinteringprocess. Reflect on the mathematical relationship is in the same kind ofsintering mechanism, in the relationship of xm- t, the m value is inconsistent.This reflects the complexity of the actual sintering process. The combined effectssintering theory deduced that the dm-d0/dm-d values double logarithmic lnln(dm-d0/dm-d) and the sintering temperatures reciprocal 1/T show linearrelationship. Dm is the theoretical density of the powder, d0 is the densitybefore sintering compacts, d is the density of the sintered body. Use metallic nickel powder, cobalt powder, copper powder, molybdenum powder, tantalum powder toconduct the powder sintering experiments, use tungsten powder to conduct the activatedsintering experiments, use copper powder, beryllium oxide powder to conductheat pressing test and other data experiments, are consistent very well.Source:http://www.mhcmp.com
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