Effect of substrate cooling on the epitaxial growth of Ni-based single-crystal superalloy fabricated by direct energy deposition

doi:10.1016/j.jmst.2020.05.041

Abstract

Abstract:

The columnar-to-equiaxed transition (CET) or the formation of stray grains in the laser melting deposition is the least desirable for the repair of single-crystal blades. In this work, the forced water-cooling was conducted on a single-crystal René N5 substrate during the direct energy deposition (DED). The single track remelting, one-layer, two-layer, and eight-layer depositions were investigated to explore the grain growth mechanism. The solidification conditions of the DED process, including temperature field, temperature gradient, and solidification speed, were numerically analyzed by a finite element model. The single-track remelting results showed that the fraction of columnar crystal regions increases from 55.81 % in the air-cooled sample to 77.14 % in the water-cooled one. The single-track deposits of one- and two-layer have the same trend, where the proportion of columnar crystal height was higher under the forced water-cooled condition. The electron backscattered diffraction (EBSD) grain-structure maps of an eight-layer deposit show that the epitaxial growth height increases from 1 mm in the air-cooling sample to 1.5 mm in the water-cooling one. The numerical results showed that the temperature gradient in [001] direction was significantly increased by using forced water-cooling. In conclusion, the in-situ substrate cooling can become a potential method to promote epitaxial growth during DED via the influence on CET occurrence.

Key words: Direct energy deposition, Epitaxial growth, Columnar-to-equiaxed transition (CET), Temperature gradient

Jianwen Nie, Chaoyue Chen, Longtao Liu, Xiaodong Wang, Ruixin Zhao, Sansan Shuai, Jiang Wang, Zhongming Ren. Effect of substrate cooling on the epitaxial growth of Ni-based single-crystal superalloy fabricated by direct energy deposition[J]. J. Mater. Sci. Technol., 2021, 62: 148-161.

Figures/Tables 19

Fig. 1. SEM images of feedstock powder used in this study: (a) overview and (b) magnified view.

Table 1 Chemical composition (wt%) of feedstock powder and substrate.

Sample	Cr	Co	W	Mo	Al	Ta	Hf	Re	Ni
Substrate	7.00	8.00	5.00	2.00	6.20	7.00	0.20	3.00	Bal.
Powder	7.09	7.60	4.76	2.05	7.10	6.78	0.18	0	Bal.

Fig. 2. (a) Schematic of the laser powder deposition process and (b) deposition scanning strategy.

Fig. 3. Structure diagram of the forced water-cooling installation in DED.

Fig. 4. Finite element mesh used in the numerical simulation. (a, a1) The mesh grid under un-deformed condition, (b, b1) the deformed mesh structure with one-layer deposition, and (c, c1) the specific boundary conditions.

Table 2 Thermo-physical property parameters of substrate and powder material.

Property	Values
Density of liquid (kg/m³)	7634
Density of solid (kg/m³)	8630
Specific heat (J/(kg K))	400 + 0.263×T
Conductivity (W/(m K))	9.8 + 0.0138×T
Liquid temperature (K)	1672.4
Solidus temperature (K)	1446.8
Latent heat of fusion (J/kg)	3.0 × 10⁵

Fig. 5. OM images of the microstructures of the laser-remelting tracks: (a) 3D overview, magnified views on the (b) horizontal, and (c) vertical microstructure.

Fig. 6. Cross-sectional microstructures of laser remelting track with different substrate conditions: (a) air-cooling and (b) forced water-cooling.

Fig. 7. Temperature field of the substrate (a) and the molten pool (b) during laser remelting, and the temperature gradient simulation results under air-cooling (c) and forced water-cooling.

Table 3 Geometry of the laser remelting molten pool and the ratio of epitaxial columnar dendrites.

Parameter	Experimental result		Numerical result
	Air-cooling	Water-cooling	Air-cooling	Water-cooling
W (mm)	1.64	1.23	0.98	0.75
D_r (mm)	0.33	0.30	0.32	0.22
R_e (%)	55.81	77.14	62.50	75.00

Fig. 8. Calculation result of solidification in the remelting pool.

Fig. 9. OM images of single-track and one-layer deposit under different substrate conditions: (a) air-cooling and (b) forced water-cooling, and their magnified views on the CET are shown in (c) and (d), respectively.

Fig. 10. OM images of single-track and two-layer deposit under different substrate conditions: (a) air-cooling and (b) forced water-cooling, and their magnified views on the CET are shown in (c) and (d), respectively.

Table 4 Geometric size of the deposition layer and the ratio of epitaxial columnar.

Deposit layer	One-layer		Two-layer
Substrate condition	Air-cooling	Water-cooling	Air-cooling	Water-cooling
W (mm)	1.03	0.96	1.09	1.01
D_r (mm)	0.88	0.81	0.13	0.13
H_d (mm)	0.90	0.53	0.14	0.13
R_e (%)	52.62	84.86	48.23	76.19

Fig. 11. Calculated molten pool (a) and the temperature gradient (G) of the first layer deposition sample toward the X-axis, Y-axis, and Z-axis with air-cooling (b-d, b1-d1) and water-cooling (e-g, e1-g1).

Fig. 12. Calculated molten pool (a) and the temperature gradient (G) of the second layer deposition sample toward the X-axis, Y-axis, and Z-axis with air-cooling (b-d, b1-d1) and the forced water-cooling (e-g, e1-g1).

Fig. 13. Optical micrographs and corresponding electron backscattered diffraction (EBSD) grain-structure maps of a transverse section of DED sample with (a) air-cooling and (b) forced water-cooling.

Fig. 14. Observation of the dendritic microstructure at the fifth (a, b) and second (c, d) layer of multi-layer deposition on the horizontal section under air-cooling (a, c) and the forced water-cooling (b, d) conditions.

Fig. 15. SEM images of air-cooling DED samples at different positions: (a, b) interface, the morphology and dimension of the γ' phase at the same height as the air-cooling DED sample (c) and water-cooling DED sample (d).

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