Efficient, High Aspect Ratio Microhole Drilling with Rapidly Repeated Pulses

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Title: Efficient, High Aspect Ratio Microhole Drilling with Rapidly Repeated Pulses
Author: Lehman, Ted Eugene
Advisors: Dr. Juei-Feng Tu, Committee Chair
Dr. Fuh-Gwo Yuan, Committee Member
Dr. Tarek Echekki, Committee Member
Abstract: The necessity for micro holes spans across a wide range of industrial applications with nearly endless boundaries. From electronics and X-Ray apertures to micro fluidics and heat transfer phenomenon, the advancement of precision ablation will undoubtedly be of benefit. Currently, three major techniques are employed in the fabrication of micro-holes: Conventional drilling methods, electrical discharge machining (EDM), and laser drilling. The flexibility to drill a wide range of materials with a single device in combination with high consistency and accuracy lead many applications to the necessity of laser drilling. Major drawbacks of laser ablation are low material removal rates and heat affected zones around the holes. Much research has been performed in the pico- and femtosecond paradigm with less insight into longer pulse durations. Investigation was conducted into four variables affecting microsecond pulses on the material removal of steel: pulse length, high power density spikes, pulsing frequency, as well as group multiple pulsing. The optimal pulse length of 3 µs created unblocked holes with aspect ratios of 6 and depths of 196 µm. Multiple pulsing increased the aspect ratio to 10, deepening the cavity to 236 µm. Optimum depth occurred with the maximum number of pulses tested, 10; producing an aspect ratio of 12 and a depth of 295 µm. A major drawback produced by 10 pulses was the signs of hole blockage. A process anatomy of hole creation explains causes of the blockage. The second variable, high power density spike exhibited by the laser increased hole depth 5% over instances lacking this spike. Major improvements in depth were demonstrated as the pulsing frequency was increased to a laser capable maximum of 48.7 kHz. Holes created under this condition experienced major blockage near the exit. Group multiple pulsing deepened the hole to 438 µm with three sets of ten pulses at 48.7 kHz. Holes exhibited extreme blockage throughout their depths.
Date: 2009-12-02
Degree: MS
Discipline: Mechanical Engineering
URI: http://www.lib.ncsu.edu/resolver/1840.16/2413

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