Formation Mechanisms of Hot Cracks in Laser Additive Repairing Single Crystal Superalloys
LU Nannan1, GUO Yimo1,2, YANG Shulin3, LIANG Jingjing1, ZHOU Yizhou1, SUN Xiaofeng1, LI Jinguo1()
1Shi -changxu Innovation Center for Advanced Materials, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China 2School of Materials Science and Engineering, Shenyang University of Technology, Shenyang 110870, China 3AECC Shenyang Liming Aero-Engine Co., Ltd., Shenyang 110043, China
Cite this article:
LU Nannan, GUO Yimo, YANG Shulin, LIANG Jingjing, ZHOU Yizhou, SUN Xiaofeng, LI Jinguo. Formation Mechanisms of Hot Cracks in Laser Additive Repairing Single Crystal Superalloys. Acta Metall Sin, 2023, 59(9): 1243-1252.
Hot cracking is a prevalent defect in metallurgy that often occurs during the laser additive repair of single crystal superalloys. The understanding of the cracking mechanism is vital for defect prevention. Consequently, this study entails combining experimental analysis and theoretical calculations to investigate the hot cracking mechanism in a second-generation single crystal superalloy, DD432, during laser additive repairing. The incident of hot cracking was observed predominantly at high-angle grain boundaries (HAGBs). High-magnitude stress concentrations were identified on both sides of the crack, accompanied by an extensive distribution of MC-type carbides in the crack initiation region. Hot cracking depended on factors such as liquid film stability, stress concentration, and MC-type carbide precipitates. The stability of the liquid film depended on dendrite coalescence undercooling, which in turn was related to the angle of grain boundaries. According to Rappaz's theory of dendrite coalescence undercooling, the calculated dendrite coalescence undercooling at HAGBs was 395 K. This figure was substantially higher than the 38 K liquid film undercooling found within a single dendrite, and far exceeded the undercooling at a low-angle grain boundary (3.6°) with a value of 56 K. The elevated level of stress concentration served as a driving force for crack initiation and propagation. MC-type carbide precipitates promoted crack initiation through a pinning effect on the liquid feed, thereby weakening the interface bonding strength with the substrate.
Fig.1 Macro-morphology of cracks in the additive manufactured DD432 single crystal (SX) superalloy thinwall specimen
Fig.2 OM image of micromorphology for longitudinal section cracks in the deposition
Fig.3 SEM images of longitudinal crack fracture morphology (a) and liquid film distribution at the crack tip (b)
Fig.4 Morphology and orientation characteristics of the crack initiation region (a) back scattered electron (BSE) image of the crack initiation region (b-d) inverse pole figures (IPFs) along the X (b), Y (c), and Z (d) directions, respectively
Fig.5 Kernel Average misorientation (KAM) map of the crack initiation region
Fig.6 Localized amplified microstructure and orientation characteristics in the crack source region (HAGB—high-angle grain boundary, LAGB—low-angle grain boundary) (a) BSE image of the crack initiation region (b) enlargement of the selected area in Fig.6a (c-e) IPFs along the X (c), Y (d), and Z (e) directions, respectively (f) KAM map of the selected area in Fig.6a
Fig.7 Elemental distribution in the crack source region
Fig.8 BSE image of microstructure (a) and EDS analysis of point 1 (b) near crack initiation
Physical parameter
Value
Unit
Ref.
Shear modulus G
86.3
GPa
[29]
Burgers vector b
3.578 × 10-10
m
[30]
Solid/liquid interfacial energy γsl
310
mJ·m-2
[31]
Poisson ratio ν
0.467
-
[29]
Table 1 Physical parameters used to calculate the undercooling ΔTb and grain boundary energy γgb[29-31]
Fig.9 Relationships between GB angle (θ) and the dendrite coalescence temperature and GB energy (γgb) (T—temperature, θc—critical transition angle, θm—the angle corresponding to the maximum GB energy, —the minimum dendrite coalescence temperature for a repulsive boundary, —the dendrite coalescence temperature for an attractive boundary, ΔTb—dendrite coalescence undercooling ΔTb,marx—the max dendrite coalescence undercooling)
Fig.10 Relationships between the solidification temperature and the solid fraction (fs) for DD432 SX superalloys (ΔTDD432—interdendritic liquid film undercooling in DD432 alloy)
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