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Research Papers

Analytical and Experimental Investigation of Thermocapillary Flow in Pulsed Laser Micropolishing

[+] Author and Article Information
Chao Ma

Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706
e-mail: machao87@gmail.com

Madhu Vadali

Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706
e-mail: vadali.wisc@gmail.com

Xiaochun Li

Professor
Mem. ASME
Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706
e-mail: xcli@engr.wisc.edu

Neil A. Duffie

Professor
Fellow ASME
Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706
e-mail: duffie@engr.wisc.edu

Frank E. Pfefferkorn

Associate Professor
Mem. ASME
Department of Mechanical Engineering,
University of Wisconsin-Madison,
Madison, WI 53706
e-mail: pfefferk@engr.wisc.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MICRO- AND NANO-MANUFACTURING. Manuscript received October 3, 2013; final manuscript received April 10, 2014; published online April 28, 2014. Assoc. Editor: Ulf Engel.

J. Micro Nano-Manuf 2(2), 021010 (Apr 28, 2014) (8 pages) Paper No: JMNM-13-1073; doi: 10.1115/1.4027433 History: Received October 03, 2013; Revised April 10, 2014

The objective of this paper is to define and derive a dimensionless number as a function of material properties and process parameters to quantify the extent (magnitude) of thermocapillary flow in pulsed laser micropolishing (PLμP). Experimental work has shown that thermocapillary flow can tremendously reduce surface roughness (smoothing effect) although it inevitably introduces additional surface features (roughening effect) at the same time. Both the smoothing and roughening effects increase as the extent of thermocapillary flow increases. The extent of thermocapillary flow is the bridge from the available information (i.e., initial surface profile, material properties, and process parameters) to the polished surface profile to be predicted. A dimensionless number, called the normalized average displacement of a liquid particle in a single laser pulse, is proposed and derived via analytical heat transfer and fluid flow equations. The calculated normalized displacement is found to be proportional to the measured slope of the introduced features on Ti6Al4V surface polished with various process parameters, which indicates that the dimensionless number successfully describes the extent of thermocapillary flow. The normalized average displacement will be very useful for prediction of polished surface profile and hence parameter selection and process optimization in the future.

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References

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Figures

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Fig. 1

Schematics of melt pool flow: (a) stationary capillary waves on a rough surface and (b) thermocapillary flow on a flat surface

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Fig. 2

Surface profiles of unpolished and polished Ti6Al4V samples: (a) unpolished, (b) polished in capillary regime, and (c) polished in thermocapillary regime

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Fig. 3

Schematic of fluid flow model of laser-induced surface melting

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Fig. 4

Schematic of one-dimensional heat transfer model of laser-induced surface melting

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Fig. 5

Normalized average melt duration and normalized average melt depth as a function of normalized melting temperature

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Fig. 6

Normalized average displacement as a function of laser power (P), pulse duration (τ), and beam radius (rb) for Ti6Al4V

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Fig. 7

Experimental setup for PLμP

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Fig. 8

A line profile of PLμP in thermocapillary regime (along the center line in Fig. 2(c))

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Fig. 9

Polished surface profiles with various laser beam radii (rb), constant pulse duration (1.56 μs), and power (48.6 W)

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Fig. 10

Normalized average displacement and introduced feature slope as functions of laser beam radius (1.56 μs pulse duration and 48.6 W power)

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Fig. 11

Polished surface profiles with various laser pulse durations (τ), constant beam radius (21.2 μm), and power (25.0 W)

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Fig. 12

Normalized average displacement and introduced feature slope as functions of laser pulse duration (21.2 μm laser beam radius and 25.0 W power)

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Fig. 13

Polished surface profiles with various laser powers (P) and constant beam radius (21.2 μm) and pulse duration (1.56 μs)

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Fig. 14

Normalized average displacement and introduced feature slope as functions of laser power (21.2 μm beam radius and 1.56 μs pulse duration)

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Fig. 15

Introduced feature slope as a function of normalized average displacement

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