Preheating effects in Electron Beam Additive Manufacturing

1. NUMERICAL THERMAL ANALYSIS IN ELECTRON
BEAM ADDITIVE MANUFACTURING WITH
PREHEATING EFFECTS
Ninggang (George) Shen
Dr. Kevin Chou
8/7/2012
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2. Outline of the contents
1. Background & research objectives
2. Thermal modeling & FE application
3. Study of preheating process
4. Multi-layer cross-raster scan simulation
5. Conclusions
6. Future work
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3. 1. Introduction and research objectives
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4. 1. Introduction and research objectives
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5. 1. Introduction and research objectives
Fig. 1 SEM picture of Ti-6Al-4V powder
Fig 2. SEM picture of sintered Ti-6Al-4V powder
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6. 2. Thermal modeling and FE application
Fig. 3 Actual keyhole example and idealization [2] Fig. 4 Horizontal intensity distribution @ z = 0
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7. 2. Thermal modeling and FE application
Emissivity [3]: Thermal Conductivity [4]:
AH H
1 AH S
2 k kr kc
1
S
2 3 .0 8 2
2
0.908 16
AH 2 H 2 kc l T
3 kr k b u lk x
1.908 2 1 1
1 3 .0 8 2 1 3
S
Emissivity related:
εS – Emissivity of solid material
εH – Emissivity of the hole among adjacent powder particles
AH – The area fraction of the surface that is occupied by the radiation emitting holes
φ – Fractional porosity of the bed
Thermal conductivity related:
l – Mean photon free path between scattering events, the particle diameter in this study
σ – Stefan-Boltzmann constant,
T – Temperature
x = b/R – Ratio of neck radius to particle radius
Λ – Normalized contact conductivity for the three packing structures.
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8. 2. Thermal modeling and FE application
Tab. 1 Truth table of material determination
DTemp > 0 DTemp < 0
Temp < Tmelting 0 0
Temp > Tmelting 0 1
†0 – powder, 1 – solid
• Latent heat of fusion is considered as well
Fig. 5 Flow chart of the user subroutine
coupled UMATH and DFLUX
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9. 2. Thermal modeling and FE application
Tab. 2 Parameters in the melting simulation
Parameters Values
Solidus temperature, TS ( C) 1605
Liquidus temperature, TL ( C) 1665
Latent heat of fusion, Lf (kJ/Kg) 440
Electron beam diameter, Φ (mm) 0.4
Absorption efficiency, η 0.9
Scan speed, v (m/sec) 0.4
Acceleration voltage, U (kV) 60
Beam current, Ib (mA) 2
Powder layer thickness, t-layer (mm) 0.1
Porosity, φ 30%
Beam penetration depth, dP (mm) 0.1
Fig. 6 New FE model configuration
Preheat temperature, Tpreheat ( C) 730
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10. 2. Thermal modeling and FE application
Fig. 7 Schematic of the cross-raster scan pattern
applied in the multi-layer EBAM thermal
analysis.
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11. 2. Thermal modeling and FE application
Fig. 7 Model geometry, ICs and BCs [5]
Fig. 8 Simulation results comparison with Wang et al [6]:
a) Temperature contour; b) Temperature distribution along beam center scan pass
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12. 3. Study of preheating process
Tab. 3 Parameters for Preheating Analysis
Acceleration Voltage Initial Substrate Initial Powder
Beam Current (mA) Scan Speed (m/sec)
(kV) Temperature (°C) Temperature (°C)
60 30 14.6 700 200
Fig. 8 Temperature contour of the preheating simulation in a cut-off cross-sectional view
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13. 3. Study of preheating process
Fig. 9 Simulated temperature profiles for various substrate thicknesses, (i): Temperature profile within the
entire substrate, and (ii): Temperature profile within the 10 mm depth.
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14. 3. Study of preheating process
Fig. 11 The material state transformation denoted with the
material index throughout the simulation
Fig. 10 Applied new thermal initial conditions (ICs): (a) New thermal ICs, (b) Simulated temperature
contour with new thermal ICs, and (c) Simulated temperature contour with old thermal ICs.
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15. 4. Multi-layer cross-raster scan simulation
Fig. 12 Comparisons of simulated temperature
contours and melt pools for (a) the single
straight scan and (b) the multi-layer cross-raster
scan.
Fig. 13 Comparisons of simulated temperature
and cooling rate histories for the single straight
scan and the multi-layer cross-raster scan.
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16. 4. Multi-layer cross-raster scan simulation
Fig. 14 Comparisons of simulated temperature contours and melt pools for the layer of
raster across/along the in solid/powder interface.
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17. 5. Conclusions
• The preheating temperature penetration ≈ 0.5 mm;
the critical substrate thickness ≈ 10 mm.
• New thermal initial conditions really affect the results.
• Differences in the melt pool geometry, temperature distribution, temperature
history, and cooling history due to the multi-layer crossed raster scan pattern.
• Powder substrate has significant effects on the thermal process.
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18. 6. Future work
Fig. 15 IR camera – MCS640 from Mikron Fig. 16 Measurement setup
Fig. 17 Contour melting Fig. 18 Hatch melting
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19. 6. Future work
• Thermal process of manufacturing a part with overhang structure
• Extensive studies on effects of the solid/powder interface in substrate on thermal
process
• Thermo-mechanical analysis
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20. Acknowledgement
Sponsor: NASA, No. NNX11AM11A
Collaborator: Marshall Space Flight Center (Huntsville, AL),
Advanced Manufacturing Team.
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21. Q&A
Thank you for your attention!
Any Question?
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22. Reference
[1] Available from: http://www.arcam.com/.
[2] Lampa, C., Kaplan, A. F. H., Powell, J., and Magnusson, C., 1997, "An analytical thermodynamic
model of laser welding," Journal of Physics D: Applied Physics, 30(9), p. 1293.
[3] Sih, S. S., and Barlow, J. W., 2004, "The prediction of the emissivity and thermal conductivity of
powder beds," Particulate Science and Technology, 22, pp. 291-304.
[4] Kolossov, S., Boillat, E., Glardon, R., Fischer, P., and Locher, M., 2004, "3D FE simulation for
temperature evolution in the selective laser sintering process," International Journal of Machine
Tools and Manufacture, 44(2-3), pp. 117-123.
[5] Wang, L., Felicelli, S., Gooroochurn, Y., Wang, P. T., and Horstemeyer, M. F., 2008, "Optimization
of the LENS process for steady molten pool size," Materials Science & Engineering A (Structural
Materials: Properties, Microstructure and Processing), 474, pp. 148-156.
[6] Hofmeister, W., Wert, M., Smugeresky, J., Philliber, J. A., Griffith, M., and Ensz, M. T., 1999,
"Investigation of solidification in the Laser Engineered Net Shaping (LENS) process," JOM, 51(7).
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23. Appendix I
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24. Appendix II
Other selected conditions comparison
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