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Research Papers

A Study of Keyhole Porosity in Selective Laser Melting: Single-Track Scanning With Micro-CT Analysis

[+] Author and Article Information
Subin Shrestha

J.B. Speed School of Engineering,
University of Louisville,
Louisville, KY 40292
e-mail: subin.shrestha@louisville.edu

Thomas Starr

J.B. Speed School of Engineering,
University of Louisville,
Louisville, KY 40292
e-mail: tom.starr@louisville.edu

Kevin Chou

J.B. Speed School of Engineering,
University of Louisville,
Louisville, KY 40292
e-mail: kevin.chou@louisville.edu

1Corresponding author.

Manuscript received January 15, 2019; final manuscript received April 22, 2019; published online May 14, 2019. Assoc. Editor: Tugrul Ozel.

J. Manuf. Sci. Eng 141(7), 071004 (May 14, 2019) (11 pages) Paper No: MANU-19-1032; doi: 10.1115/1.4043622 History: Received January 15, 2019; Accepted April 22, 2019

Porosity is an inherent attribute in selective laser melting (SLM) and profoundly degrades the build part quality and its performance. This study attempts to understand and characterize the keyhole pores formed during single-track scanning in SLM. First, 24 single tracks were generated using different line energy density (LED) levels, ranging from 0.1 J/mm to 0.98 J/mm, by varying the laser power and the scanning speed. The samples were then scanned by micro-computed tomography to measure keyhole pores and analyze the pore characteristics. The results show a general trend that the severity of the keyhole porosity increases with the increase of the LED with exceptions of certain patterns, implying important individual contributions from the parameters. Next, by keeping the LED constant in another set of experiments, different combinations of the power and the speed were tested to investigate the individual effect. Based on the results obtained, the laser power appears to have a greater effect than the scanning speed on both the pore number and the pore volume as well as the pore depth. For the same LED, the pore number and volume increase with increasing laser power until a certain critical level, beyond which, both the pore number and volume will decrease, if the power is further increased. For the LED of 0.32 J/mm, 0.4 J/mm, and 0.48 J/mm, the critical laser power that reverses the trend is about 132 W, 140 W, and 144 W, respectively.

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Figures

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Fig. 1

(a) CAD model of the specimen (unit in mm) and (b) representation of single track designed on top of previously deposited semicylinder

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Fig. 2

(a) Samples after removal from the machine and (b) after support removal

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Fig. 3

(a) Schematic of µCT components and (b) specimen setup in the CT system

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Fig. 4

Selection of region of interest for porosity measurement

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Fig. 5

(a) Coronal (XZ), (b) transaxial (XY), (c) sagittal (ZY) cross-sectional views, and (d) 3D partial cutoff view

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Fig. 6

A 3D pore on right is rendered from the 2D images from the left

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Fig. 7

(a) Pore location definition and (b) detailed view of pores formed at the end of scan track from 195 W 400 mm/s parameters (0.49 J/mm)

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Fig. 8

Single-track morphology from different energy densities

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Fig. 9

3D pore view observed at different process parameters

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Fig. 10

(a) Pore diameter and (b) pore depth summary from different LEDs

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Fig. 11

Total pore volume corresponding to different energy densities used

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Fig. 12

Average pore volume at different power and speed

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Fig. 13

(a) Single-track morphology and (b) track widths at different process parameters for LED = 0.48 J/mm

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Fig. 14

Longitudinal sections from the single tracks with ED = 0.48 J/mm

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Fig. 15

Pore diameter and pore depth summary from process parameters of LED = 0.48 J/mm

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Fig. 16

Pore count and pore volume obtained at different process parameters from different energy densities (a) 0.32 J/mm, (b) 0.4 J/mm, and (c) 0.48 J/mm

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Fig. 17

Stability of keyhole formation affected the evaporation pressure and the pressure due to surface tension at different energy densities [42]

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