Fig. 5 Comparison of experimental SEM image and CtFD simulated melt pool with beam diameters of(a)700 μm,(b)1000 μm, and(c)1300 μm and an absorption rate of 0.3. Electron beam power and scan speed are 900 W and 100 mm s-1, respectively

Melting and Solidification Behavior of 316L Steel Induced by Electron-Beam Irradiation for Additive Manufacturing

付加製造用電子ビーム照射による 316L ステンレス鋼の溶融・凝固挙動

奥 川 将 行*・宮 田 雄一朗*・王     雷*・能 勢 和 史*
小 泉 雄一郎*・中 野 貴 由*
Masayuki OKUGAWA, Yuichiro MIYATA, Lei WANG, Kazufumi NOSE,
Yuichiro KOIZUMI and Takayoshi NAKANO

Abstract

적층 제조(AM) 기술은 복잡한 형상의 3D 부품을 쉽게 만들고 미세 구조 제어를 통해 재료 특성을 크게 제어할 수 있기 때문에 많은 관심을 받았습니다. PBF(Powderbed fusion) 방식의 AM 공정에서는 금속 분말을 레이저나 전자빔으로 녹이고 응고시키는 과정을 반복하여 3D 부품을 제작합니다.

일반적으로 응고 미세구조는 Hunt[Mater. 과학. 영어 65, 75(1984)]. 그러나 CET 이론이 일반 316L 스테인리스강에서도 높은 G와 R로 인해 PBF형 AM 공정에 적용될 수 있을지는 불확실하다.

본 연구에서는 미세구조와 응고 조건 간의 관계를 밝히기 위해 전자빔 조사에 의해 유도된 316L 강의 응고 미세구조를 분석하고 CtFD(Computational Thermal-Fluid Dynamics) 방법을 사용하여 고체/액체 계면에서의 응고 조건을 평가했습니다.

CET 이론과 반대로 높은 G 조건에서 등축 결정립이 종종 형성되는 것으로 밝혀졌다. CtFD 시뮬레이션은 약 400 mm s-1의 속도까지 유체 흐름이 있음을 보여 주며 수상 돌기의 파편 및 이동의 영향으로 등축 결정립이 형성됨을 시사했습니다.

Additive manufacturing(AM)technologies have attracted much attention because it enables us to build 3D parts with complicated geometry easily and control material properties significantly via the control of microstructures. In the powderbed fusion(PBF)type AM process, 3D parts are fabricated by repeating a process of melting and solidifying metal powders by laser or electron beams. In general, the solidification microstructures can be predicted from solidification conditions defined by the combination of temperature gradient G and solidification rate R on the basis of columnar-equiaxed transition(CET)theory proposed by Hunt [Mater. Sci. Eng. 65, 75(1984)]. However, it is unclear whether the CET theory can be applied to the PBF type AM process because of the high G and R, even for general 316L stainless steel. In this study, to reveal relationships between microstructures and solidification conditions, we have analyzed solidification microstructures of 316L steel induced by electronbeam irradiation and evaluated solidification conditions at the solid/liquid interface using a computational thermal-fluid dynamics (CtFD)method. It was found that equiaxed grains were often formed under high G conditions contrary to the CET theory. CtFD simulation revealed that there is a fluid flow up to a velocity of about 400 mm s-1, and suggested that equiaxed grains are formed owing to the effect of fragmentations and migrations of dendrites.

Keywords

Additive Manufacturing, 316L Stainless Steel, Powder Bed Fusion, Electron Beam Melting, Computational Thermal
Fluid Dynamics Simulation

Fig. 1 Width, height, and height differences calculated from laser microscope analysis of melt tracks formed by scanning electron beam. Fig. 2(a)Scanning electron microscope(SEM)image and(b) corresponding electron back-scattering diffraction(EBSD) IPF-map taken from the electron-beam irradiated region in P900-V100 sample. Fig. 3 Average grain size and their aspect ratio calculated from EBSD IPF-map taken from the electron-beam irradiated region.
Fig. 1 Width, height, and height differences calculated from laser microscope analysis of melt tracks formed by scanning electron beam. Fig. 2(a)Scanning electron microscope(SEM)image and(b) corresponding electron back-scattering diffraction(EBSD) IPF-map taken from the electron-beam irradiated region in P900-V100 sample. Fig. 3 Average grain size and their aspect ratio calculated from EBSD IPF-map taken from the electron-beam irradiated region.
Fig. 4 Comparison of experimental SEM image and computational thermal fluid dynamics(CtFD)simulated melt pool with a beam diameter of 700 μm and absorption rates of(a)0.3,(b)0.5, and (c)0.7. Electron beam power and scan speed are 900 W and 100 mm s-1, respectively.
Fig. 4 Comparison of experimental SEM image and computational thermal fluid dynamics(CtFD)simulated melt pool with a beam diameter of 700 μm and absorption rates of(a)0.3,(b)0.5, and (c)0.7. Electron beam power and scan speed are 900 W and 100 mm s-1, respectively.
Fig. 5 Comparison of experimental SEM image and CtFD simulated melt pool with beam diameters of(a)700 μm,(b)1000 μm, and(c)1300 μm and an absorption rate of 0.3. Electron beam power and scan speed are 900 W and 100 mm s-1, respectively
Fig. 5 Comparison of experimental SEM image and CtFD simulated melt pool with beam diameters of(a)700 μm,(b)1000 μm, and(c)1300 μm and an absorption rate of 0.3. Electron beam power and scan speed are 900 W and 100 mm s-1, respectively
Fig. 6 Depth of melt tracks calculated from experimental SEM image and CtFD simulation results.
Fig. 6 Depth of melt tracks calculated from experimental SEM image and CtFD simulation results.
Fig. 7 G-R plots of 316L steel colored by(a)aspect ratio of crystalline grains and(b)fluid velocity.
Fig. 7 G-R plots of 316L steel colored by(a)aspect ratio of crystalline grains and(b)fluid velocity.
Fig. 8 Comparison of solidification microstructure(EBSD IPF-map)of melt region formed by scanning electron beam and corresponding snap shot of CtFD simulation colored by fluid velocity
Fig. 8 Comparison of solidification microstructure(EBSD IPF-map)of melt region formed by scanning electron beam and corresponding snap shot of CtFD simulation colored by fluid velocity

References

1) M.C. Sow, T. De Terris, O. Castelnau, Z. Hamouche, F. Coste, R.
Fabbro and P. Peyre: “Influence of beam diameter on Laser Powder

Bed Fusion(L-PBF)process”, Addit. Manuf. 36(2020), 101532.
2) J.C. Simmons, X. Chen, A. Azizi, M.A. Daeumer, P.Y. Zavalij, G.
Zhou and S.N. Schiffres: “Influence of processing and microstructure
on the local and bulk thermal conductivity of selective laser melted
316L stainless steel”, Addit. Manuf. 32(2020), 100996.
3) S. Dryepondt, P. Nandwana, P. Fernandez-Zelaia and F. List:
“Microstructure and High Temperature Tensile properties of 316L
Fabricated by Laser Powder-Bed Fusion”, Addit. Manuf. 37(2020),
101723.
4) S.H. Sun, T. Ishimoto, K. Hagihara, Y. Tsutsumi, T. Hanawa and T.
Nakano: “Excellent mechanical and corrosion properties of austenitic
stainless steel with a unique crystallographic lamellar microstructure
via selective laser melting”, Scr. Mater. 159(2019), 89-93.
5) T. Ishimoto, S. Wu, Y. Ito, S.H. Sun, H. Amano and T. Nakano:
“Crystallographic orientation control of 316L austenitic stainless
steel via selective laser melting”, ISIJ Int. 60(2020), 1758-1764.
6) T. Ishimoto, K. Hagihara, K. Hisamoto, S.H. Sun and T. Nakano:
“Crystallographic texture control of beta-type Ti-15Mo-5Zr3Al alloy by selective laser melting for the development of novel
implants with a biocompatible low Young’s modulus”, Scr. Mater.
132(2017), 34-38.
7) X. Ding, Y. Koizumi, D. Wei and A. Chiba: “Effect of process
parameters on melt pool geometry and microstructure development
for electron beam melting of IN718: A systematic single bead
analysis study”, Addit. Manuf. 26(2019), 215-226.
8) K. Karayagiz, L. Johnson, R. Seede, V. Attari, B. Zhang, X. Huang,
S. Ghosh, T. Duong, I. Karaman, A. Elwany and R. Arróyave: “Finite
interface dissipation phase field modeling of Ni-Nb under additive
manufacturing conditions”, Acta Mater. 185(2020), 320-339.
9) M.M. Kirka, Y. Lee, D.A. Greeley, A. Okello, M.J. Goin, M.T.
Pearce and R.R. Dehoff: “Strategy for Texture Management in
Metals Additive Manufacturing”, JOM, 69(2017), 523-531.
10) S.S. Babu, N. Raghavan, J. Raplee, S.J. Foster, C. Frederick, M.
Haines, R. Dinwiddie, M.K. Kirka, A. Plotkowski, Y. Lee and
R.R. Dehoff: “Additive Manufacturing of Nickel Superalloys:
Opportunities for Innovation and Challenges Related to
Qualification”, Metall. Mater. Trans. A. 49(2018), 3764-3780.
11) M.R. Gotterbarm, A.M. Rausch and C. Körner: “Fabrication of
Single Crystals through a μ-Helix Grain Selection Process during
Electron Beam Metal Additive Manufacturing”, Metals, 10(2020),
313.
12) J.D.D. Hunt: “Steady state columnar and equiaxed growth of
dendrites and eutectic”, Mater. Sci. Eng. 65(1984), 75-83.
13) S. Bontha, N.W. Klingbeil, P.A. Kobryn and H.L. Fraser: “Effects of
process variables and size-scale on solidification microstructure in
beam-based fabrication of bulky 3D structures”, Mater. Sci. Eng. A.
513-514(2009), 311-318.
14) J. Gockel and J. Beuth: “Understanding Ti-6Al-4V microstructure
control in additive manufacturing via process maps”, 24th Int. SFF
Symp. – An Addit. Manuf. Conf. SFF 2013.(2013), 666-674.
15) B. Schoinochoritis, D. Chantzis and K. Salonitis: “Simulation of
metallic powder bed additive manufacturing processes with the finite
element method: A critical review”, Proc. of Instit. Mech. Eng., Part
B: J. Eng. Manuf. 231(2017), 96-117.
16)小泉雄一郎: “計算機シミュレーションを用いたAdditive
Manufacturing プロセス最適化予測”, スマートプロセス学会誌,
8-4(2019), 132-138.
17) Y. Zhao, Y. Koizumi, K. Aoyagi, D. Wei, K. Yamanaka and A. Chiba:
“Molten pool behavior and effect of fluid flow on solidification
conditions in selective electron beam melting(SEBM)of a
biomedical Co-Cr-Mo alloy”, Addit. Manuf. 26(2019), 202-214.
18) C. Tang, J.L. Tan and C.H. Wong: “A numerical investigation on
the physical mechanisms of single track defects in selective laser
melting”, Int. J. Heat Mass Transf. 126(2018), 957-968.
19) Technical data for Iron, [Online]. Available: http://periodictable.com/
Elements/026/data.html. [Accessed: 8-Feb-2021].
20) N. Raghavan, R. Dehoff, S. Pannala, S. Simunovic, M. Kirka, J.
Turner, N. Carl-son and S.S. Babu: “Numerical modeling of heattransfer and the influence of process parameters on tailoring the grain
morphology of IN718 in electron beam additive manufacturing”,
Acta Mater. 112(2016), 303-314.
21) S. Morita, Y. Miki and K. Toishi: “Introduction of Dendrite
Fragmentation to Microstructure Calculation by Cellular Automaton
Method”, Tetsu-to-Hagane. 104(2018), 559-566.
22) H. Esaka and M. Tamura: “Model Experiment Using Succinonitrile
on the Formation of Equiaxed Grains caused by Forced Convection”,
Tetsu-to-Hagane. 86(2000), 252-258.