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Research Papers

Microhole Drilling by Double Laser Pulses With Different Pulse Energies

[+] Author and Article Information
Ze Liu

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: liu1583@purdue.edu

Benxin Wu

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: wu65@purdue.edu

Rong Xu

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: xu666@purdue.edu

Kejie Zhao

School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: kjzhao@purdue.edu

Yung C. Shin

Fellow ASME
School of Mechanical Engineering,
Purdue University,
585 Purdue Mall,
West Lafayette, IN 47907
e-mail: shin@purdue.edu

1Corresponding author.

Manuscript received December 9, 2017; final manuscript received May 24, 2018; published online July 5, 2018. Assoc. Editor: Hongqiang Chen.

J. Manuf. Sci. Eng 140(9), 091015 (Jul 05, 2018) (8 pages) Paper No: MANU-17-1771; doi: 10.1115/1.4040483 History: Received December 09, 2017; Revised May 24, 2018

Previous investigations on “double-pulse” nanosecond (ns) laser drilling reported in the literature typically utilize double pulses of equal or similar pulse energies. In this paper, “double-pulse” ns laser drilling using double pulses with energies differing by more than ten times has been studied, where both postprocess workpiece characterizations and in situ time-resolved shadowgraph imaging observations have been performed. A very interesting physical phenomenon has been discovered under the studied conditions: the “double-pulse” ns laser ablation process, where the low-energy pulse precedes the high-energy pulse (called “low-high double-pulse” laser ablation) by a suitable amount of time, can produce significantly higher ablation rates than “high-low double-pulse” or “single-pulse” laser ablation under a similar laser energy input. In particular, “low-high double-pulse” laser ablation at a suitable interpulse separation time can drill through a ∼0.93 mm thick aluminum 7075 workpiece in less than 200 pulse pairs, while “high-low double-pulse” or “single-pulse” laser ablation cannot drill through the workpiece even using 1000 pulse pairs or pulses, respectively. This indicates that “low-high double-pulse” laser ablation has led to a significantly enhanced average ablation rate that is more than five times those for “single-pulse” or “high-low double-pulse” laser ablation. The fundamental physical mechanism for the ablation rate enhancement has been discussed, and a hypothesized explanation has been given.

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References

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Figures

Grahic Jump Location
Fig. 4

The hole exit diameter produced by “double-pulse” laser drilling versus the interpulse separation time (1000 pulse pairs are used in the drilling)

Grahic Jump Location
Fig. 1

(a) Schematic diagram of the setup for the “double-pulse” laser drilling and the related shadowgraph imaging experiments in this work (not drawn to scale and not drawn to denote all the exact actual shapes, sizes, and/or details; only some major components are shown), (b) schematic of laser pulse format (i.e., laser beam power versus time) for the “double-pulse” laser drilling experiment, and (c) schematic of laser pulse format for the “single-pulse” laser drilling experiment. The plots in (b) and (c) do not represent the exact actual temporal shapes of the laser pulses.

Grahic Jump Location
Fig. 5

Scanning electron microscope images of the entrance or exit side for holes drilled by “double-pulse” laser ablation using 1000 pulse pairs ((a)–(e)), or by “single-pulse” laser ablation using 1000 pulses (f) (workpiece: ∼0.93 mm thick aluminum 7075): (a) entrance, and (b) exit for a hole drilled through “double-pulse” laser ablation with an interpulse separation time of ts = −15 μs; (c) entrance, and (d) exit for a hole drilled through “double-pulse” laser ablation with ts = −1 μs; (e) entrance for a hole drilled through “double-pulse” laser ablation with ts = +10 μs; and (f) entrance for a hole drilled through “single-pulse” laser ablation using the pulse format in Fig. 1(c)

Grahic Jump Location
Fig. 3

The number of pulse pairs that is needed to perforate the workpiece plate versus the interpulse separation time for “double-pulse” laser drilling (workpiece: aluminum 7075 plate that is ∼0.93 mm thick). When the interpulse separation time is larger than or equal to zero, the plotted dashed line means that the workpiece plate was not perforated even using 1000 laser pulse pairs. The inset shows the number of pulse pairs needed for perforation versus the interpulse separation time in the range of −0.05 μs to −3 μs (in the figure, “pulses” means “pulse pairs” for “double-pulse” laser ablation).

Grahic Jump Location
Fig. 2

(a) The depths of holes drilled through “double-pulse” laser ablation using 5, 10, and 15 pulse pairs versus the interpulse separation time in the range of −15 μs to +15 μs (as a comparison, the depths of holes drilled by “single-pulse” laser ablation are also shown; workpiece: aluminum 7075), and (b) an enlarged view of (a) when the interpulse separation time is in the range of −0.5 μs to +0.5 μs (in (a) and (b), “pulses” means “pulse pairs” for “double-pulse” laser ablation)

Grahic Jump Location
Fig. 6

Shadowgraph images for laser drilling through “double-pulse” laser ablation with an interpulse separation time of ts = −1 μs (the left column) and ts = +1 μs (the center column), and through “single-pulse” laser ablation (the right column), at delay time of 3, 5, 10, 20, and 30 μs relative to the incidence of the corresponding laser pulse, before which a hole is already formed due to ablation by roughly around 40 preceding laser pulse pairs or pulses. In the first image, the workpiece surface is approximately indicated by the dashed line and the laser incidence direction is schematically shown by the arrow. Each image corresponds to an actual physical domain of ∼2.25 × 1.41 mm. For “double-pulse” laser ablation, the given delay time is relative to the corresponding high-energy pulse.

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