Grain Structure and Metallurgical Defects Regulation of Selective Laser Melted René 88DT Superalloy
LIU Jian1,2, PENG Qin1,2, XIE Jianxin1,2()
1.Institute for Advanced Materials and Technology, University of Science and Technology Beijing, Beijing 100083, China 2.Key Laboratory for Advanced Materials Processing, Ministry of Education, University of Science and Technology Beijing, Beijing 100083, China
Cite this article:
LIU Jian, PENG Qin, XIE Jianxin. Grain Structure and Metallurgical Defects Regulation of Selective Laser Melted René 88DT Superalloy. Acta Metall Sin, 2021, 57(2): 191-204.
Columnar grain and hot crack restrict the performance of laser melting deposited René 88DT superalloy when applied to turbine disks, requiring equiaxed grain to improve the fatigue property. Selective laser melting can fabricate material with low composition segregation, reduced defects, and equiaxed or near-equiaxed grains due to the fast cooling rate and vastly varied local heat flow direction during processing. In this study, a selective laser melted René 88DT was fabricated under variable processing parameters. The formation mechanism and control methods of grain structure and metallurgical defects were investigated. Results showed that changing the processing parameters can affect the preferred growth direction of crystal; thus, affecting the grain morphology and size. Processed with low heat input and 67° scan vectors rotation between deposited layers, cellular dendrites with different orientations grow competitively with each other, leading to the formation of near-equiaxed grains. The cellular dendrites can grow epitaxially across multiple deposited layers due to high heat input and 0° scan vector rotation, forming columnar grains. The columnar grains with a relatively low aspect ratio can be fabricated with 90° scan vectors rotation. However, anisotropy exists between the two orthogonal scanning directions due to the shielding effect of metal vapor dust. The primary defects in specimens are solidification cracks along grain boundaries caused by remelting of low-melting-point eutectic. Specimens with equiaxed grains showed much better ability in preventing solidification cracks from propagation than that with columnar grains. The defect density in the columnar grains is about 21 times higher than the equiaxed grains. Suitable processing conditions for selective laser melted René 88DT superalloys are low heat input and 67° scan vectors rotation based on the forming quality and service conditions.
Fig.1 Morphology of René 88DT superalloy powder particles
Specimen No.
P
V
A
Defect density / %
W
mm·s-1
(°)
Incomplete fusion
Pore
Crack
1
200
960
90
0.513
0.025
0.054
2
250
960
90
0.018
0.011
0.107
3
300
960
90
0
0.015
0.403
4
350
960
90
0
0.017
0.485
5
400
960
90
0
0.023
0.771
6
300
720
90
0
0.050
0.646
7
300
840
90
0
0.018
0.397
8
300
1080
90
0
0.027
0.187
9
300
1200
90
0
0.012
0.069
10
300
960
0
0
0.030
2.213
11
300
960
67
0
0.012
0.101
Table 1 Experimental parameters, defect types and defect densities of selective laser melted specimens
Fig.2 Schematics of scan strategies with different scan vector rotations between layers
Fig.3 Defect morphologies of selective laser melted René 88DT specimens
Fig.4 Crack morphologies of selective laser melted René 88DT No.5 specimen
Region
Cr
Co
Mo
W
Nb
Al
Ti
O
Ni
Matrix
15.83
12.31
5.15
3.72
1.77
2.91
4.17
-
54.14
Crack
11.56
10.54
3.36
2.31
0.40
2.39
2.87
20.75
43.73
Table 2 EDS results of selective laser melted René 88DT superalloy specimen
Fig.5 Deposition track morphologies in XOZ section fabricated with different scan vector rotations (The dash lines indicate the boundaries of deposition tracks, and arrows indicate the growth directions of cellular dendrites, the same bellow)
Fig.6 Grain structures of selective laser melted René 88DT specimens fabricated with different scan vectors rotation in YOZ sections (a, d, g), XOZ sections (b, e, h), and XOY sections (c, f, i)
Fig.7 Deposition track morphologies in XOZ section fabricated with laser powers of 200 W (a), 300 W (b), and 400 W (c), and changes of the ratio of depth to width and curvature radius with laser power (d)
Fig.8 Morphology of unmelted powders in selective laser melted René 88DT specimen
Fig.9 Grain structures of selective laser melted René 88DT specimens fabricated with different laser powers in YOZ sections (a, d, g), XOZ sections (b, e, h), and XOY sections (c, f, i)
Fig.10 Deposition track morphologies in XOZ section fabricated with scanning speeds of 720 mm/s (a), 960 mm/s (b), and 1200 mm/s (c), and changes of the ratio of depth to width and curvature radius with scanning speed (d)
Fig.11 Grain structures of selective laser melted René 88DT specimens fabricated with different scanning speeds in YOZ sections (a, d, g), XOZ sections (b, e, h), and XOY sections (c, f, i)
Fig.12 Low (a) and high (b) magnified SEM images, and TEM image (c) of selective laser melted René 88DT No.3 specimen
Fig.13 Changes of the ratio of depth to width and curvature radius with volume energy density (E)
Fig.14 Schematics showing the dendritic growth and grain evolution of selective laser melted René 88DT superalloy under A=90° and heat inputs of 47 J/mm3 (a), 57 J/mm3 (b), and 95 J/mm3 (c) (The red arrows indicate the directions of maximum temperature gradient)
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