Microstructure and Mechanical Properties of Fiber Laser Welded GH3535 Superalloy

doi:10.1016/j.jmst.2016.11.026

Abstract

Abstract:

As a primary material of the thorium molten salt reactor (TMSR) that is a suitable candidate reactor of the Generation IV nuclear reactors, GH3535 superalloy was successfully welded. The effect of laser beam welding (LBW) on microstructure evolution of fusion zone (FZ) and heat affected zone (HAZ), such as element segregation, precipitate behavior and grain evolution, was investigated. The microhardness and tensile properties were tested and discussed. The results of microstructure evolution showed that a number of fine M₆C-γ eutectic phases precipitated at solidification grain boundaries and interdendritic region in FZ. Compared to base metal zone (BMZ), the grain size of HAZ has no obvious change. While a few of M₆C-γ eutectic phases were observed in partially melted zone (PMZ) of HAZ. The results of microhardness indicated that the hardness of FZ was higher than that of HAZ and BMZ. The results of tensile test showed that the ultimate tensile strength of joints at room temperature, 650 and 700 °C were 98%, 97% and 99% of that of BM, respectively. All the tensile specimens of joints failed in BMZ rather than in PMZ where M₆C carbides had been transformed into M₆C-γ eutectic phases.

Key words: Laser beam welding, GH3535 superalloy, Microstructure, M₆C-γ eutectic phases, Microhardness, Tensile properties

Yu Kun, Jiang Zhenguo, Li Chaowen, Chen Shuangjian, Tao Wang, Zhou1 Xingtai, Li Zhijun. Microstructure and Mechanical Properties of Fiber Laser Welded GH3535 Superalloy[J]. J. Mater. Sci. Technol., 2017, 33(11): 1289-1299.

Figures/Tables 23

Table 1 Chemical composition of GH3535 superalloy (wt%)

Ni	Mo	Cr	Fe	Mn	Si	Al	C	P	S	B
Bal.	17.20	6.95	4.06	0.628	0.43	0.07	0.054	0.005	0.001	0.0008

Fig. 1. Microstructure of GH3535 superalloy: (a) optical micrograph of microstructure, (b) scanning electron micrograph of microstructure.

Fig. 2. Schematic of laser beam welding setup.

Table 2 Laser beam welding parameters

Peak power per pulse (kW)	Pulse frequency (Hz)	Duty cycle of laser (%)	Welding speed (m/min)	Defocus amount (mm)	Shielding gas	Top shielding gas flow (L/min)	Back shielding gas flow (L/min)
2.8	35	60	0.8	-2	Ar	20	15

Fig. 3. Weld appearance and radiography image: (a) weld appearance of the top side; (b) weld appearance of the back side; (c) radiography image of the joint.

Fig. 4. Size of tensile specimens.

Fig. 5. Optical photographs in the weld: (a, b) cross-section of weld bead; (c, d) fusion zone.

Fig. 6. Grain morphology of the fusion zone by EBSD analysis (step size 2 μm).

Fig. 7. Precipitates in the fusion zone: (a) interior grain, (b) SGB.

Fig. 8. Elements distribution of precipitate by EDS analysis.

Table 3 Chemical composition (wt%) of dendrite core by EPMA analysis and calculated equilibrium partition coefficient (k)

Elements	Si	Cr	Mn	Fe	Ni	Mo
Dendritic core, C_s	0.321	7.189	0.555	4.305	72.333	15.296
k	0.747	1.034	0.884	1.060	1.025	0.889

Fig. 9. Back scattering electron image (a) and EPMA mapping showing relative concentration for the dendritic core and interdendritic region, (b) C, (c) Si, (d) Mo, (e) Ni, (f) Fe, (g) Cr.

Fig. 10. TEM analyses of carbide of interior grain: (a) image of carbide; (b) SAED pattern of the carbide; (c) spectrum of EDX point analysis at the carbide marked as 1 in (a).

Fig. 11. TEM analyses of carbide of grain boundary: (a) image of carbide; (b) SAED pattern of the carbide; (c) spectrum of EDX point analysis at the carbide marked as 2 in (a).

Fig. 12. Optical photographs of microstructure: (a) HAZ of the joint; (b) BMZ of the joint.

Fig. 13. Grain morphology by EBSD analysis (step size: 2 μm): (a) HAZ of the joint, (b) BMZ of the joint.

Fig. 14. Microstructure of HAZ by SEM analysis: (a) morphology of carbide evolution; (b) morphology of eutectic phase; (c) morphology of grain boundary.

Fig. 15. Microhardness distributions of the cross-section of the joint.

Fig. 16. Stress versus strain curves of BM and welded joint under different conditions.

Fig. 17. Tensile properties of BM and welded joint under different conditions: (a) ultimate tensile strength of specimens; (b) elongation of specimens.

Fig. 18. Macrographs of tensile fracture under different conditions: (a) specimens of BM; (b) specimens of welded joints.

Fig. 19. Micrographs of cross sectional welded specimens: (a) fracture specimen at room temperature; (b) fracture specimen at 650 °C; (c) fracture specimen at 700 °C.

Fig. 20. Fracture morphology of specimens: (a) fracture surface of BM at room temperature; (b) fracture surface of BM at 650 °C, (c) fracture surface of BM at 700 °C; (d) fracture surface of welded joint at room temperature; (e) fracture surface of welded joint at 650 °C; (f) fracture surface of welded joint at 700 °C.

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