Effects of Overlapping Process on Grain Orientation and Microstructure of Nickel-Based Single-Crystal Superalloy DD491 Fabricated by Selective Laser Melting

doi:10.11900/0412.1961.2023.00230

Acta Metall Sin

2024, Vol. 60

Issue (11): 1471-1486 DOI: 10.11900/0412.1961.2023.00230

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Effects of Overlapping Process on Grain Orientation and Microstructure of Nickel-Based Single-Crystal Superalloy DD491 Fabricated by Selective Laser Melting

ZHANG Zhenwu¹, LI Jikang¹(

), XU Wenhe¹, SHEN Muyu¹, QI Leiyi¹, ZHENG Keying¹, LI Wei², WEI Qingsong¹(

)

1 State Key Laboratory of Material Processing and Die & Mould Technology, School of Material Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China
2 Key Laboratory of Metallurgical Equipment and Control Technology, Ministry of Education, School of Mechanical and Automation, Wuhan University of Science and Technology, Wuhan 430081, China

Cite this article:

ZHANG Zhenwu, LI Jikang, XU Wenhe, SHEN Muyu, QI Leiyi, ZHENG Keying, LI Wei, WEI Qingsong. Effects of Overlapping Process on Grain Orientation and Microstructure of Nickel-Based Single-Crystal Superalloy DD491 Fabricated by Selective Laser Melting. Acta Metall Sin, 2024, 60(11): 1471-1486.

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Abstract

Aero-engine turbine blades are operated under harsh conditions such as high temperature, pressure, and load. Therefore, weak grain boundaries at high temperatures should be eliminated from the turbine blades, whereas convection channels inside the blades should be added to dissipate heat. Achieving integrated manufacturing of specialized microstructure in complex components has been a long-term research priority in turbine blade manufacturing. Nickel-based single-crystal superalloys are key materials for manufacturing single-crystal turbine blades for aero-engines, and selective laser melting (SLM) is feasible and technically advantageous for manufacturing complex components with single-crystal microstructures. Owing to the extremely high temperature gradient and scanning speed during SLM, the melt pool is unstable, thereby interrupting directional crystal growth. The metallurgical environment of SLM is further complicated by the large number of overlapping tracks and stacking layers. The quality of the overlaps is critical for the integrity of the single-crystal structure during SLM. Herein, the effects of scanning hatch (h = 0.06, 0.09, 0.12, and 0.15 mm) on the melt track morphology, metallurgical defects, crystal orientation, and microstructure of DD491 fourth-generation nickel-based single-crystal superalloy were investigated. Directionally solidified and solution-aged DD6 single-crystal superalloy rods were used as the substrate, and DD491 powder was coated to a thickness of 40 μm. Electron backscatter diffraction was used to characterize the crystal orientation of the samples. Results show that low power/low speed (S1) and high power/high speed (S4) combinations of laser power and scanning speed provide geometrically and metallurgically stable conditions for directional crystal growth, and the grains at the bottom of the melt track can orient the substrate to achieve [001] directional growth. Different types of crystal orientation defects were observed in different regions, including equiaxed stray grains in the top middle region, [010] and [100] columnar stray grains in the top side regions, and small orientation deviation in the internal region. The scanning hatch affected the crystal orientation in the overlapping regions mainly through the remelted proportion of the old melt pool and the substrate microstructure of the new melt pool during solidification. The higher overlapping ratio with a smaller scanning hatch was beneficial for reducing stray grain defects on both sides of the melt tracks. The role of residual heat on solidification conditions was related to the heat gradient vector of laser input, and multitrack overlapping samples under the S1 process accommodated higher residual heat without causing orientation deviation in the overlapping regions. The multitrack overlapping samples under S1, h = 0.06 and0.09 mm, had maximum pole densities along the y-z plane as high as 47.66 and 46.85, respectively, exhibiting a typical [001] single-crystal structure.

Key words: selective laser melting nickel-based single-crystal superalloy crystal orientation overlap

Received: 24 May 2023

ZTFLH:

TG146.1

Fund: National Natural Science Foundation of China(52275333);Stabilization Support Project of AVIC Manufacturing Technology Institute(KZ571801);Knowledge Innovation Special Project of Wuhan(2022010801010302)

Corresponding Authors: WEI Qingsong, professor, Tel: (027)87558155, E-mail: wqs_xn@hust.edu.cn;
LI Jikang, Tel: (027)87558155, E-mail: lijikang@hust.edu.cn

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	ZHANG Zhenwu
	LI Jikang
	XU Wenhe
	SHEN Muyu
	QI Leiyi
	ZHENG Keying
	LI Wei
	WEI Qingsong

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https://www.ams.org.cn/EN/10.11900/0412.1961.2023.00230 OR https://www.ams.org.cn/EN/Y2024/V60/I11/1471

Table 1 Nominal chemical compositions of the DD491 and DD6 Ni-based single-crystal superalloys

Fig.1 SEM image (a) and particle size distribution (b) of the DD491 Ni-based single-crystal superalloy powder (Inset in Fig.1a shows the high magnified image. D₁₀, D₅₀, and D₉₀—sizes below which 10%, 50%, and 90% of the particles are con-tained, respectively)

Table 2 Processes for selective laser melting (SLM)-fabricated single-track and multi-track overlapping samples

Fig.2 Samples (a) and schematic (b) of SLM-fabricated single-track and multi-track overlapping samples under different processes

Fig.3 SEM images of single-track samples in the x-y (a-d) and y-z (e-h) planes under S1 (a, e), S2 (b, f), S3 (c, g), and S4 (d, h) processes (White dotted line in Fig.4e indicates boundary of grains in two different orientations, red arrows indicates growth directions)

Fig.4 SEM images of S1 (a1-a4), S2 (b1-b4), S3 (c1-c4), and S4 (d1-d4) multi-track overlapping samples in the x-y plane under 0.06 mm (a1-d1), 0.09 mm (a2-d2), 0.12 mm (a3-d3), and 0.15 mm (a4-d4) scanning hatches (Arrows show cracks)

Fig.5 SEM images of solidification hot cracks (a, b), solid cracks (c, d), and EDS elemental distribution maps of rectangle area in Fig.5d (e) in the multi-track overlapping samples
(a) S1, h = 0.09 mm (b) S3, h = 0.12 mm (c) S1, h = 0.15 mm (d) S1, h = 0.06 mm

Fig.6 SEM images of S1 (a1-a4), S2 (b1-b4), S3 (c1-c4), and S4 (d1-d4) multi-track overlapping samples in the y-z plane under 0.06 mm (a1-d1), 0.09 mm (a2-d2), 0.12 mm (a3-d3), and 0.15 mm (a4-d4) scanning hatches (Dotted lines indicate melt pool boundaries)

Fig.7 Schematic of multi-track overlapping melt pool (a); relationships between process parameters and standard deviation (SD) (b), average melt pool depth ( $D ¯$ ) (c), average overlapping region depth ( $d ¯$ ) (d), and radio of $d ¯ / D ¯$ (e) (D_i —depth of melt pool i, d_j —depth of overlapping region j, l_k —height from the highest point of melt pool/overlapping region k to substrate plane)

Fig.8 Crystal orientation maps of S1 multi-track overlapping samples in the y-z plane under scanning hatches of 0.06 mm (a), 0.09 mm (b), and 0.12 mm (c) (Dotted lines in Fig.8c indicate melt pool boundaries)

Fig.9 Crystal orientation maps of S4 multi-track overlapping samples in the y-z plane under scanning hatches of 0.06 mm (a), 0.09 mm (b), and 0.12 mm (c); distribution of grain misorientation angle from point A to B in Fig.9a (d)

Fig.10 Inverse pole figures (IPFs) of S1 (a-c) and S4 (d-f) multi-track overlapping samples in the y-z plane under scanning hatches of 0.06 mm (a, d), 0.09 mm (b, e), and 0.12 mm (c, f) (a—the lattice parameter along the x₁-axis, b—the lattice parameter along the y₁-axis, c—the lattice parameter along the z₁-axis, δ—the angle difference with respect to the preferred orientation)

Fig.11 SEM images of S1 multi-track overlapping samples under scanning hatches of 0.06 mm (a), 0.09 mm (b), 0.12 mm (c), and 0.15 mm (d) (Insets show the high magnified images)

Fig.12 SEM and corrsponding high magnified images of S4 multi-track overlapping samples under scanning hatches of 0.06 mm (a), 0.09 mm (b), and 0.12 mm (c)

Fig.13 Schematics of the relations between grain evolution (a) and subcooling of the solid-liquid solidification stray interface (b), dendrite interface (c), and parabolic interface (d) front (G—temperature gradient at the solid-liquid solidification interface, T_L—temperature of liquid phase)

Fig.14 Schematics of the relations between the laser scanning direction ( $V$ ), the solidification interface front direction ( $V S - L$ ) (a), and the grain preferred growth direction ( $V u v w$ ) (b) (θ $S - L$ —the angle between $V S - L$ and $V$ , ψ—the angle between $V u v w$ and $V S - L$ )

Fig.15 Schematic of the [001], [010], and [100] grain growth directions in the melt pool (θ—the angle between $V u v w$ and X-axis, ξ—the angle between the projection of $V u v w$ on the Y-Z plane and Y-axis)

Fig.16 Schematics of smaller scanning hatch (a), larger scanning hatch (b), and heat gradient (c) affecting the grain growth in the overlapping region (G₀—residual heat gradient in old melt pool; $G i n p u t$ —heat gradient of laser input; $G u v w$ —heat gradient of crystal solidification; α—the angle between $G u v w$ and [001]-axis; $G 100$ , $G 010$ , and $G 001$ — $G u v w$ component on [100], [010], and [001] direction, respectively; G_∥ and G_⊥—the magnitude of $G i n p u t$ component parallel and perpendicular to [001]-axis)

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