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On the formation of spherical particles in surface grinding

Dhami, HS and Viswanathan, K (2020) On the formation of spherical particles in surface grinding. In: ASME 2020 15th International Manufacturing Science and Engineering Conference, MSEC 2020, 3 Sept 2020.

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Official URL: https://dx.doi.org/10.1115/MSEC2020-8278


Grinding swarf is conventionally of secondary interest to the process engineer. However, it has long been recognized that it is a useful indicator of process performance�the exact particle morphologies occurring in the swarf contain a wealth of information about the abrasive-workpiece interaction mechanics. In this work, we study the generation of perfectly spherical particles when grinding two plain carbon steels and a grade of stainless steel with an alumina wheel. Similar particles have also been reported in the wear community and several possible formation mechanisms have been discussed including chip curl resulting from electronic charge distributions; melting due to local flash temperatures in the grinding zone; and repeated abrasive wear of the workpiece surface. We postulate that the particles are likely formed as a result of an oxidation-melting-solidification route with small grinding chips. We present spectroscopy and X-ray diffraction data in support of this hypothesis�significant oxygen content, in the form of Fe3O4 was detected on the surface of the spheres. Electron micrographs also show remarkably robust dendrite-like structures on the surface of the particles, indicative of rapid solidification from the melt. Motivated by these results, we present model calculations to support our hypothesis. We first evaluate the initial temperature of chips exiting the grinding zone using a three-way heat partition model for dry grinding. An upper bound for the chip temperature is ? 600?C, well-below the melting point for the metal. Next, we show that the oxidation kinetics at this elevated temperature are such that the formation of a thin oxide layer (? 2 µm) on the surface of an initially curled up chip, with size ? 50 µm comparable to the observed spheres, is enough to melt the entire chip on a timescale of 10?6 seconds. Surface tension then brings the molten chip into a perfectly spherical shape, followed by rapid solidification. We present a preliminary calculation of this solidification process, using a coupled heat conduction model along with a moving interphase interface. By making suitable approximations, we derive an ordinary differential equation describing the temporal evolution of the interface location. Coupling the interface velocity with a Mullins-Sekerka type instability analysis, we argue that solidification of these drops likely starts from a nucleated core in the drop interior, resulting in dendrite-type patterns on the outer surface. Our work is a preliminary attempt to put decades old observations of grinding swarf on a firm quantitative footing. The experimental evidence and related analysis presented here make a strong case for the oxidation-melting-solidification hypothesis for the formation of spherical particles in grinding swarf. Copyright © 2020 ASME.

Item Type: Conference Paper
Publication: ASME 2020 15th International Manufacturing Science and Engineering Conference, MSEC 2020
Publisher: American Society of Mechanical Engineers
Additional Information: The copyright of this article belongs to American Society of Mechanical Engineers
Keywords: Alumina; Aluminum oxide; Drops; Grinding (machining); Heat conduction; Iron oxides; Life cycle; Magnetite; Ordinary differential equations; Oxidation; Rapid solidification; Spheres; Wear of materials, Dendrite-like structure; Electronic charge distribution; Experimental evidence; Interphase interfaces; Particle morphologies; Solidification process; Wealth of information; X-ray diffraction data, Particle size analysis
Department/Centre: Division of Mechanical Sciences > Mechanical Engineering
Date Deposited: 09 Mar 2021 07:25
Last Modified: 09 Mar 2021 07:25
URI: http://eprints.iisc.ac.in/id/eprint/68249

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