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Research Papers

Shallow Angle Drilling of Inconel 718 Using a Helical Laser Drilling Technique

[+] Author and Article Information
Joonghan Shin

Center for Lasers and Plasmas for
Advanced Manufacturing,
University of Michigan,
Ann Arbor, MI 48109-2125

Jyotirmoy Mazumder

Center for Lasers and Plasmas for
Advanced Manufacturing,
University of Michigan,
Ann Arbor, MI 48109-2125
e-mail: mazumder@umich.edu

1Corresponding author.

Manuscript received May 20, 2016; final manuscript received August 24, 2016; published online October 3, 2016. Assoc. Editor: Y. B. Guo.

J. Manuf. Sci. Eng 139(3), 031004 (Oct 03, 2016) (10 pages) Paper No: MANU-16-1289; doi: 10.1115/1.4034718 History: Received May 20, 2016; Revised August 24, 2016

This study reports an experimental investigation for the shallow angle laser drilling of Inconel 718. In this study, a helical laser drilling technique was used to effectively produce holes with a diameter of several hundred microns. The design of experiment (DOE) using the Taguchi method was employed to examine the influence of various process parameters on the geometrical and metallurgical features of drilled holes. A higher laser power, lower speed, and closer focal position to the workpiece surface contributed to the further removal of material by the absorption of more laser energy and larger beam intensity. This resulted in a larger exit hole diameter and less hole taper. The increase in laser power reduced a thickness of the recast layer due to material removal by vaporization. From the DOE result, a regression model to estimate a correlation between experimental factors and hole quality was also suggested. In the second stage of this study, trials to improve drilling performance were made. Using the O2 assist gas of 50 kPa significantly enhanced the drilling performance owing to the delivery of more energy to the workpiece by an exothermic reaction. However, the further increase of O2 gas caused rapid cooling of the workpiece, which lowered the drilling performance. The drilling performance was greatly improved as well using the high laser duty cycle to provide more laser energy. The moving focal position was only beneficial to the drilling performance when a focusing of the beam was moderately maintained on the interaction region of the laser–workpiece.

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References

Figures

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Fig. 1

Pulse formats used in this study: (a) for 7.5% duty cycle and (b) for 10% duty cycle

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Fig. 2

The real images of laser, beam delivery optics, and CNC stage: (a) laser and beam delivery optics (the end of the delivery optics connected to the inlet of the beam delivery pipe), (b) beam delivery pipe and CNC stage, and (c) laser beam nozzle and 60 deg wedge for sample holding

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Fig. 3

Schematic diagram for laser process area

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Fig. 4

Schematic representation of helical drilling

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Fig. 5

Schematic representation of the hole cross section (dentry, entry hole diameter; dexit, exit hole diameter; and L, hole length)

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Fig. 6

Effect of parameters on (a) drilling time, (b) exit hole diameter, (c) hole taper, and (d) recast layer thickness

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Fig. 7

(a) Entire hole cross section (generated from experiment no. 13 in Table 2) and recast at the (b) entry side, (c) middle, and (d) exit side of the hole

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Fig. 8

(a) High-magnification SEM image of the recast (generated from experiment no. 13 in Table 2) and recast at the (b) entry side, (c) middle, and (d) exit side of the hole

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Fig. 9

Effect of the assist gas on the exit hole diameter (laser power: 70 W, focal position: 0 mm, speed: 8 mm/s, and number of passes: 30)

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Fig. 10

Weight percentage of the O2 in recasts (laser power: 70 W, focal position: 0 mm, speed: 8 mm/s, and number of passes: 30), inset: composition of three main elements of Inconel 718 in the recast (for O2 assist gas)

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Fig. 11

Investigation of the drilling performance: (a) 7.5% duty cycle (70 W, 8 mm/s, and 30 passes) and (b) 10% duty cycle (90 W, 8 mm/s, and 30 passes) (experiment 1: fixed focal position, 0 mm, and no assist gas; experiment 2: moving focal position and no assist gas; experiment 3: fixed focal position, 0 mm, and O2 50 kPa; and experiment 4: moving focal position and 50 kPa)

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Fig. 12

(a) Entire cross section of the hole produced at a 10% duty cycle laser setting (90 W laser power, 0 mm focal position, 50 kPa O2 assist gas pressure, 8 mm/s speed, 30 passes, and ∼4.97 s drilling time), (b) recast at entry side, (c) interface between parent material and nonoxide layer, and (d) interface between nonoxide layer and oxide layer

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