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Research Papers

A Numerical Study on the Keyhole Formation During Laser Powder Bed Fusion Process

[+] Author and Article Information
Subin Shrestha

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

Y. Kevin Chou

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

1Corresponding author.

Manuscript received May 7, 2019; final manuscript received May 15, 2019; published online July 31, 2019. Assoc. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 141(10), (Jul 31, 2019) (9 pages) Paper No: MANU-19-1266; doi: 10.1115/1.4044100 History: Received May 07, 2019; Accepted May 16, 2019

The dynamic phenomenon of a melt pool during the laser powder bed fusion (LPBF) process is complex and sensitive to process parameters. As the energy density input exceeds a certain threshold, a huge vapor depression may form, known as the keyhole. This study focuses on understanding the keyhole behavior and related pore formation during the LPBF process through numerical analysis. For this purpose, a thermo-fluid model with discrete powder particles is developed. The powder distribution, obtained from a discrete element method (DEM), is incorporated into the computational domain to develop a 3D process physics model using flow-3d. The melt pool formation during the conduction mode and the keyhole mode of melting has been discerned and explained. The high energy density leads to the formation of a vapor column and consequently pores under the laser scan track. Further, the keyhole shape resulted from different laser powers and scan speeds is investigated. The numerical results indicated that the keyhole size increases with the increase in the laser power even with the same energy density. The keyhole becomes stable at a higher power, which may reduce the occurrence of pores during laser scanning.

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Figures

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Fig. 1

(a) Powder added to the dispenser platform and (b) powder particles settled over build plate after the recoating process

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Fig. 2

3D computational domain used for single-track simulation

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Fig. 3

Temperature-dependent material properties of Ti-6Al-4V [22]

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Fig. 4

Powder and substrate melting during laser application

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Fig. 5

Melt region formed after complete melting and solidification

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Fig. 6

Melt pool boundary comparison between the experiment [25] and the simulation

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Fig. 7

Equilibrium points during the formation of vapor column [27]

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Fig. 8

Multiple reflection vectors from the keyhole wall

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Fig. 9

(a) Velocity field, keyhole profile, and breakage of the keyhole to form bubble and (b) 2D temperature and velocity field along the longitudinal section

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Fig. 10

Fluid flow in the transverse direction during keyhole melting

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Fig. 11

Melt pool boundary compared with the experiment [21] for 195 W laser power and 400 mm/s scan speed

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Fig. 12

Melt region formed after complete melting and solidification

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Fig. 13

2D images of the pores formed at the beginning of the single track and their 3D-rendered morphology

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Fig. 14

Pore number and volume from a different level of power with LED = 0.4 J/mm [29]

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Fig. 15

Keyhole shape at different time steps from different parameters: (a) P = 100 W, v = 250 mm/s, (b) P = 200 W, v = 500 mm/s, (c) P = 300 W, v = 750 mm/s, and (d) P = 400 W, v = 1000 mm/s

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Fig. 16

Intensity dependence in the relationship between vapor column and evaporation pressure [27]

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Fig. 17

Temperature distribution when laser has moved 0.8 mm with P = 300 W, v = 750 mm/s and P = 400 W, v = 1000 mm/s

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Fig. 18

Melt region with different level of power with LED of 0.4 J/mm

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