Research Papers

Measurement of the Melt Pool Length During Single Scan Tracks in a Commercial Laser Powder Bed Fusion Process

[+] Author and Article Information
J. C. Heigel

Intelligent Systems Division,
Engineering Laboratory,
National Institute of Standards and Technology,
Gaithersburg, MD 20899
e-mail: jarred.heigel@nist.gov

B. M. Lane

Intelligent Systems Division,
Engineering Laboratory,
National Institute of Standards and Technology,
Gaithersburg, MD 20899

1Corresponding author.

Manuscript received July 19, 2017; final manuscript received August 4, 2017; published online March 7, 2018. Editor: Y. Lawrence Yao.

J. Manuf. Sci. Eng 140(5), 051012 (Mar 07, 2018) (7 pages) Paper No: MANU-17-1453; doi: 10.1115/1.4037571 History: Received July 19, 2017; Revised August 04, 2017

This work presents high-speed thermographic measurements of the melt pool length during single track laser scans on nickel alloy 625 substrates. Scans are made using a commercial laser powder bed fusion (PBF) machine while measurements of the radiation from the surface are made using a high speed (1800 frames per second) infrared camera. The melt pool length measurement is based on the detection of the liquidus–solidus transition that is evident in the temperature profile. Seven different combinations of programmed laser power (49–195 W) and scan speed (200–800 mm/s) are investigated, and numerous replications using a variety of scan lengths (4–12 mm) are performed. Results show that the melt pool length reaches steady-state within 2 mm of the start of each scan. Melt pool length increases with laser power, but its relationship with scan speed is less obvious because there is no significant difference between cases performed at the highest laser power of 195 W. Although keyholing appears to affect the anticipated trends in melt pool length, further research is required.

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Fig. 1

Thermography setup on the commercial laser PBF system with the thermal camera set adjacent to the testing position and the custom door open, exposing the sample holder and video camera inside the chamber

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Fig. 2

Calibration results of the infrared camera

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Fig. 3

The five samples held by the rotary stage as seen from the camera's perspective through the viewport on the custom door

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Fig. 4

A comparison of infrared images acquired from each case. Each image is representative of those acquired during steady-state for each case. Temperatures below the detectable range of the camera (550 °C radiant) are not shown.

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Fig. 5

Radiant temperature profile extracted along the horizontal lines in Fig. 4

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Fig. 6

The example temperature curves from Fig. 5, isolated to the radiant temperature profile behind the 1038 °C isotherm

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Fig. 7

An example of the algorithm used to detect the discontinuity at the liquidus–solidus transition. The radiant temperature profile is from the case 7 profile presented in Fig. 6. The minimum of the second derivative is used to identify the transition (vertical line through each plot).

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Fig. 8

The radiant temperature associated with the liquidus–solidus transition discontinuity in every frame for case 7. Outliers are excluded from the analysis.

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Fig. 9

Melt pool length as a function of laser scan distance for cases 5–7. The black solid line represents the average melt pool length during steady-state while the black dashed lines are±1σ.

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Fig. 10

Comparison of the melt pool lengths of each case. Cases with the same nominal laser power are grouped together to highlight the similarities in melt pool length. Error bars represent measurement variance of ±1σ.

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Fig. 11

The representative images and average melt pool length measurement mapped onto the P–V space. The shaded region in the upper left hand corner indicates where keyholing is expected according to Montgomery et al. [18].



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