Research Papers

On Process Temperature in Powder-Bed Electron Beam Additive Manufacturing: Process Parameter Effects

[+] Author and Article Information
Steven Price, Bo Cheng

Department of Mechanical Engineering,
The University of Alabama,
Tuscaloosa, AL 35487

James Lydon, Kenneth Cooper

Additive Manufacturing Laboratory,
Marshall Space Flight Center,
Huntsville, AL 35812

Kevin Chou

Department of Mechanical Engineering,
The University of Alabama,
Tuscaloosa, AL 35487
e-mail: kchou@eng.ua.edu

1Corresponding author.

Contributed by the Manufacturing Engineering Division of ASME for publication in the JOURNAL OF MANUFACTURING SCIENCE AND ENGINEERING. Manuscript received April 15, 2014; final manuscript received August 31, 2014; published online October 24, 2014. Assoc. Editor: David L. Bourell.

This material is declared a work of the US Government and is not subject to copyright protection in the United States. Approved for public release; distribution is unlimited.

J. Manuf. Sci. Eng 136(6), 061019 (Oct 24, 2014) (10 pages) Paper No: MANU-14-1218; doi: 10.1115/1.4028485 History: Received April 15, 2014; Revised August 31, 2014

Build part certification has been one of the primary roadblocks for effective usage and broader applications of metal additive manufacturing (AM) technologies including powder-bed electron beam additive manufacturing (EBAM). Process sensitivity to operating parameters, among others such as powder stock variations, is one major source of property scattering in EBAM parts. Thus, it is important to establish quantitative relations between the process parameters and process thermal characteristics that are closely correlated with the AM part properties. In this study, the experimental techniques, fabrications, and temperature measurements, developed in recent work (Cheng et al., 2014, "On Process Temperature in Powder-Bed Electron Beam Additive Manufacturing: Model Development and Experimental Validation," ASME J. Manuf. Sci. Eng., (in press)) were applied to investigate the process parameter effects on the thermal characteristics in EBAM with Ti-6Al-4 V powder, using the system-specific setting called “speed function (SF)” index that controls the beam speed and the beam current during a build. EBAM parts were fabricated using different levels of SF index (20–65) and examined in the part surface morphology and microstructures. In addition, process temperatures were measured by near infrared (NIR) thermography with further analysis of the temperature profiles and the melt pool size. The thermal model, also developed in recent work, was further employed for EBAM temperature predictions, and then compared with the experimental results. The major results are summarized as follows. SF index noticeably affects the thermal characteristics in EBAM, e.g., a melt pool length of 1.72 mm and 1.26 mm for SF20 and SF65, respectively, at 24.43 mm build height. SF setting also strongly affects the EBAM part quality including the surface morphology, surface roughness and part microstructures. In general, a higher SF index tends to produce parts of rougher surfaces with more pore features and large β grain columnar widths. Increasing the beam speed will reduce the peak temperatures, also reduce the melt pool sizes. Simulations conducted to evaluate the beam speed effects are in reasonable agreement compared to the experimental measurements in temperatures and melt pools sizes. However, the results of a lower SF case, SF20, show larger differences between the simulations and the experiments, about 58% for the melt pool size. Moreover, the higher the beam current, the higher the peak process temperatures, also the larger the melt pool. On the other hand, increasing the beam diameter monotonically decreases the peak temperature and the melt pool length.

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Fig. 1

A CAD Model used in EBAM experiments for temperature measurements

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Fig. 2

(a) Actual beam speed and (b) beam current versus build height (SF36)

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Fig. 3

Actual beam speed versus SF at different build heights

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Fig. 4

Typical NIR temperature images at different SF values

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Fig. 5

Measured temperature profiles (averaged) at different SF values (6.65 mm build height)

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Fig. 6

Compensated temperature profiles at different SF values (6.65 mm build height)

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Fig. 7

Measured melt pool length and width versus SF (6.65 mm build height)

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Fig. 8

Melt pool size results at different build heights from different tests

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Fig. 9

Compiled melt pool (a) length and (b) width (from experiment), at different build heights and different SF values

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Fig. 10

Fabricated parts from EBAM Experiments with different SF values

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Fig. 11

EBAM build part surface morphology versus SF: (a) stereoscopic images, (b) white-light interferometric images, and (c) surface roughness (from scanning surface)

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Fig. 12

Microstructure versus SF: optical microscopic images, from X-plane: (a) SF 20, (b) SF 36, (c) SF 50, and (d) SF 65

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Fig. 13

Cooling rates associated with different beam speeds

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Fig. 14

Temperature profile comparisons, simulation versus experiment, for 4 beam speeds

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Fig. 15

Melt pool size comparisons, simulation versus experiment, for four SF values: (a) length and (b) width

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Fig. 16

Melt pool size and shape illustration (simulations) at four different beam speeds

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Fig. 17

Beam current effects on EBAM process temperatures, simulation versus experiment: (a) profile and (b) melt pool size (v = 506 mm/s, d = 0.65 mm)

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Fig. 18

Beam diameter effects on EBAM process temperatures, simulation versus experiment: (a) profile and (b) melt pool size (v = 671 mm/s, i = 6.7 mA)




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