Vacuum brazing of GH99 superalloy using graphene reinforced BNi-2 composite filler

doi:10.1016/j.jmst.2018.02.008

Abstract

Abstract:

A novel graphene reinforced BNi-2 composite filler was developed for brazing GH99 superalloy. The interfacial microstructure of brazed joints was analyzed by field emission scanning electron microscope and a transmission electron microscope. The effects of graphene addition on the microstructure evolution and mechanical properties of brazed joints were investigated, and the strengthening mechanism of graphene was analyzed. The results revealed that due to the addition of graphene, M₂₃(C,B)₆ compounds were synthesized in the γ solid solution and brittle boride precipitates near the brazing seam decreased. Graphene was effective in retarding solute atoms diffusion thus impeding the precipitation of borides. Furthermore, the low coefficient of thermal expansion (CTE) of graphene was conducive to relieve stress concentration of the brazed joints during the cooling process. The shear strengths of brazed joints were significantly improved by exerting the strengthening effect of graphene. The maximum shear strengths of the brazed joints were 410.4 MPa and 329.7 MPa at room temperature and 800 °C, respectively.

Key words: Graphene, GH99 superalloy, Brazing, Microstructure, Mechanical properties

Duo Liu, Yanyu Song, Bin Shi, Qi Zhang, Xiaoguo Song, Hongwei Niu, Jicai Feng. Vacuum brazing of GH99 superalloy using graphene reinforced BNi-2 composite filler[J]. J. Mater. Sci. Technol., 2018, 34(10): 1843-1850.

Figures/Tables 16

Table 1 Chemical compositions of GH99 superalloy and BNi-2 (wt%).

	Cr	Co	W	Mo	Si	B	Fe	Al	Ti	Ni
GH99 superalloy	18.21	6.67	8.03	2.86	-	-	0.32	1.15	1.43	Bal.
BNi-2	7.06	-	-	-	4.54	2.85	3.04	-	-	Bal.

Fig. 1. Microstructure of GH99 superalloy.

Fig. 2. Morphologies of BNi-2 filler (a) and BNi-2G filler (b).

Fig. 3. Typical microstructures of GH99 superalloy joints brazed at 1170 °C for 30 min (a) and magnified morphology of DZ.

Table 2 Chemical compositions at each spot shown in Fig. 3 (at.%).

Spot	Ni	Cr	Si	Mo	W	Fe	Other element
A	71.27	14.32	4.42	0.54	1.15	2.56	5.74
B	11.62	46.34	0.01	25.21	14.28	1.53	1.01
C	23.85	40.90	-	19.15	12.94	2.04	1.12

Fig. 4. Microstructures of GH99 superalloy joints brazed at 1090 °C (a), 1130 °C (b), 1170 °C (c) and 1200 °C (d).

Fig. 5. Magnified images of DZ in Fig. 4 for GH99 superalloy joints brazed at 1090 °C (a), 1130 °C (b), 1170 °C (c) and 1200 °C (d).

Fig. 6. Effect of brazing temperature on shear strength of GH99 superalloy joints brazed for 30 min.

Fig. 7. EBSD analysis of grain morphologies (a, b) and distribution of grain size (c, d) of base metal before (a, c) and after (b, d) 1200 °C/30 min brazing process.

Fig. 8. Microstructures of joints brazed with BNi-2G filler (a) and magnified images of DZ (b).

Fig. 9. TEM image of BS (a) and SEAD pattern of γ solid solution and M23(C,B)6 (b).

Fig. 10. Schematic diagram of interfacial microstructure: (a) formation of liquid phase; (b) spreading of liquid phase; (c) isothermal solidification; (d) formation of interface.

Fig. 11. Room-temperature shear strength and high-temperature shear strength of brazed joints.

Fig. 12. Fracture morphologies of joints brazed with BNi-2 filler (a, b) and BNi-2G composite filler (c, d) at low (a, c) and high (b, d) magnification.

Table 3 Chemical compositions at each spot shown in Fig. 12 (at.%).

Spot	Ni	Cr	Si	Mo	W	Co	Fe	Other element
A	46.20	28.59	1.30	6.58	5.27	5.69	1.16	5.21
B	76.07	9.52	6.79	0.31	0.92	2.51	1.91	1.97

Fig. 13. High-temperature (800 °C) shear fracture morphologies of joints brazed using BNi-2 filler (a) and BNi-2G composite filler (b).

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