Note
Microstructural Evolution of Brazed Inconel 625 Using Amorphous VZ-2106 Foil
2013 Volume 53 Issue 5 Pages 923-925
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2013 Volume 53 Issue 5 Pages 923-925
Inconel 625 (IN-625) has the nominal composition of 61Ni–21.5Cr–2.5Fe–9Mo–3.6Nb in wt%, and it is versatilely used in different industries such as aerospace, marine system and chemical plant … etc.1,2) IN-625 is solid-solution strengthened by alloying molybdenum and niobium, and it is easily welded without cracking.3) Furthermore, IN-625 has excellent mechanical properties at elevated temperature and superior corrosion resistance to pitting, especially for the existence of chloride ion environment.2) For example, traditional plate heat exchanger is made by Cu brazing 316 stainless steel, and it is not suitable applied in corrosive environment.4) IN-625 alloy is suitable to be applied in making the plate heat exchanger used in sea water.
Ni-based fillers are often used in brazing Ni-based alloys for the purpose of high-temperature and corrosive environment applications.4,5,6) Boron and/or silicon are often alloyed into the Ni-based filler in order to decreasing the melting point of the braze alloy.5,6) However, too much boron and/or silicon in the braze alloy lead to form continuous brittle intermetallic compound(s) at the interface between filler metal and substrate, and they should be confined in order to improve bonding strength of the brazed joint.7,8)
Because the amorphous foil is featured with uniform chemical composition, it is currently widely used in brazing.5,6) Amorphous VZ-2106 (44Ni–35Fe–11Cr–1.5Mo–1.0Cu–6.4Si–1.5B in wt%) foil is a Ni/Fe-based filler foil, and contains less boron as compared with traditional Ni-based fillers.8) Solidus and liquidus temperatures of VZ-2106 foil are 1044 and 1154°C, respectively. Based on previous study, the presence of brittle boride and/or silicide is strongly related to bonding strength of the brazed joint.8,9) The amount of boride and/or silicide in the brazed zone are affected by brazing process variables such as thickness of filler metal, brazing temperature and time. The purpose of this investigation is concentrated on microstructural evolution of brazed IN-625 joint using amorphous VZ-2106 foil, and optimized brazing conditions are obtained from the experiment.
IN-625 alloy were machined with the dimension of 15 mm × 6 mm × 3 mm. Each brazed surface was ground by SiC papers up to grit 600 and ultrasonically cleaned by acetone before brazing. Amorphous VZ-2106 foils with the thickness of 40 μm, 80 μm and 120 μm were chosen as the brazing filler, respectively. Vacuum brazing IN-625 using VZ-2106 foil was performed under the vacuum of 5 × 10–2 Pa and the heating rate was set at 0.33°C/s throughout the experiment. For the purpose of equilibrating temperature gradient of brazed specimens, all samples were preheated at 1000°C for 1800 s prior to brazing. Table 1 illustrates brazing conditions used in the experiment.
| Brazing Temperature/Time | Thickness of VZ-2106 Foil |
|---|---|
| 1150°C/1800 s | 40, 80, 120 μm |
| 1180°C/1800 s | 40, 80, 120 μm |
| 1200°C/1800 s | 40, 80, 120 μm |
Cross-sections of brazed joints were cut by a low-speed diamond saw, and examined by using JEOL JSM 6510 scanning electron microscope (SEM). Quantitative chemical analyses of selected areas of the brazed joint were carried out by using JEOL 8600SX electron probe microanalyzer (EPMA) equipped with the wavelength dispersive spectroscope (WDS). Its operation voltage was 15 kV, and the minimum spot size was set at 1 μm. Shear tests were performed to evaluate bonding strengths of selected brazed joints.10) Figure 1 shows the shear test specimen used in this experiment. The shaded area is IN-625 base metal, and the outer part of the layout is the graphite fixture used in brazing. Two bold black lines 3.0 mm wide in the middle of the graph indicate the brazing filler metal, and the specimen is sandwiched between two graphite plates. The specimen was compressed by a Shimadzu AG-10 universal testing machine with a constant speed of 0.0167 mm/s. The experimental data were averaged from at least three measurements for each brazing condition, and fractured surfaces were examined by using JEOL JSM 6510 SEM.
The schematic diagram of the specimen used in the shear test.10)
Figure 2 illustrates EPMA backscattered electron images (BEIs) and WDS chemical analysis results in atomic percent of IN-625/VZ-2106/IN-625 joint brazed at 1180°C for 1800 s using 80 μm brazing foil. The solidification of braze melt results in forming coarse Nb6Ni16Si7 intermetallic compound as marked by E and Ni/Cr/Fe-rich matrix as marked by D.9,11) Boron atoms are depleted from VZ-2106 braze melt into IN-625 substrate primarily along its grain boundaries, and react with IN-625 to form grain boundary borides. In Fig. 2, the position of F is located in grain boundary of IN-625 substrate. Based on the EPMA chemical analysis result, the grain boundary boride is mainly comprised of B, Cr, Ni and Mo. However, chemical formula of the quaternary boride cannot be accurately identified due to lack of related phase diagram. Both borides as marked by B and fine Nb6Ni16Si7 precipitates as marked by C are observed from the interface between VZ-2106 braze and IN-625 substrate, and it is originated from interfacial reaction between the braze melt and IN-625 substrate during brazing.
EPMA BEIs and WDS chemical analysis results in atomic percent of IN-625/VZ-2106/ IN-625 joint using 80 μm VZ-2106 foil brazed at 1180°C for 1800 s.
Figure 3 shows microstructural evolution of IN-625/VZ-2106/IN-625 joints under different brazing temperatures and braze thicknesses. Increasing the brazing temperature to 1200°C results in coarsening grain boundary boride as displayed in Figs. 3(g)–3(i). Meanwhile, coarse Nb6Ni16Si7 compound in the brazed zone and interfacial Nb6Ni16Si7 precipitates are disappeared from the joint when the brazing temperature is increased to 1200°C. IN-625 can dissolve more silicon than boron. Based on related binary alloy phase diagrams, silicon is dissolved in nickel up to 10 at%, but nickel has almost no solubility of boron.12) It has been reported that the solubility of boron in austenitic matrix is very low, and boron can easily segregate along the grain boundary to form boride precipitates upon cooling cycle of heat treatment.13,14) It is consistent with the experimental observation that Ni/Cr/Fe matrix is alloyed with 5.7 at% silicon but no boron as shown in Fig. 2. Therefore, Nb6Ni16Si7 phase is dissolved into the IN-625 substrate for specimens brazed at 1200°C. In contrast, grain boundary borides become discontinuous due to significant coarsening of these borides as demonstrated in Figs. 3(g)–3(i). The amount of grain boundary boride and Nb6Ni16Si7 intermetallic compound in the joint is not affected by increasing the thickness of VZ-2106 foil as illustrated in Fig. 3.
Microstructural evolution of IN-625/VZ-2106/IN-625 joints brazed at (a) 1150°C, 1800 s, 40 μm, (b) 1150°C, 1800 s, 80 μm, (c) 1150°C, 1800 s, 120 μm, (d) 1180°C, 1800 s, 40 μm, (e) 1180°C, 1800 s, 80 μm, (f) 1180°C, 1800 s, 120 μm, (g) 1200°C, 1800 s, 40 μm, (h) 1200°C, 1800 s, 80 μm, (i) 1200°C, 1800 s, 120 μm.
Table 2 summarizes average shear strengths of IN-625/VZ-2106/IN-625 joints under various brazing conditions. Increasing the thickness of VZ-2106 foil from 40 μm to 80 μm improves average shear strength of brazed joint due to better fillet formed in the shear test sample as illustrated in Fig. 4.6) The fillet can act as the stress reducer at the edge of joint that benefits overall mechanical properties of the joined assembly.5,6) Shear strengths of 338 and 333 MPa are observed from specimens brazed at 1150°C and 1180°C using 40 μm VZ-2106 foil, respectively. Higher average shear strengths of 495 and 453 MPa are obtained from specimens brazed at 1150°C and 1180°C using 80 μm VZ-2106 foil. Shear strengths of brazed joints are significantly improved for specimens brazed at 1200°C using 40 and 80 μm VZ-2106 foils. The joints are not fractured even yielding of the IN-625 substrate. It is deduced that both disappearance of grain boundary Nb6Ni16Si7 precipitates and coarsening of grain boundary borides are beneficial to bonding strength of the joint.
| Brazing Condition | Thickness of VZ-2106 | Average Shear Strength |
|---|---|---|
| 1150°C/1800 s | 40 μm | 338 ± 25 MPa |
| 1150°C/1800 s | 80 μm | 495 ± 23 MPa |
| 1180°C/1800 s | 40 μm | 333 ± 6 MPa |
| 1180°C/1800 s | 80 μm | 453 ± 9 MPa |
| 1200°C/1800 s | 40 μm | Yielding of IN-625 Substrate |
| 1200°C/1800 s | 80 μm | Yielding of IN-625 Substrate |
SEM BEIs illustrating fillets of IN-625/VZ-2106/IN-625 joints brazed at 1180°C for 1800 s using (a) 40 μm, (b) 80 m VZ-2106 foils.
Figure 5 shows SEM cross-sections and fractographs of IN-625/VZ-2106/IN-625 joints after shear tests under various brazing conditions. For specimens brazed at 1150 and 1180°C, all cracks are initiated from interfacial Nb6Ni16Si7 precipitates and propagate along the interface between braze alloy and IN-625 substrate as displayed in Figs. 5(a) and 5(c). Cleavage dominated fractures are widely observed from their fractographs (Figs. 5(b) and 5(d)). Failure of interfacial Nb6Ni16Si7 precipitates has also been checked by means of EDS chemical analyses of fractured surfaces. In contrast, the joint is not failed even yielding of the IN-625 substrate for the specimen brazed at 1200°C using 80 μm VZ-2106 foil. However, fracture of coarse borides is observed from cross-section of the joint as indicated by arrows shown in Fig. 5(e). The presence of interfacial Nb6Ni16Si7 precipitates demonstrates detrimental effect to bonding strength of the joint brazed at 1150 and 1180°C. Failure mechanism of the joint is changed from fracture of interfacial Nb6Ni16Si7 precipitates into coarsened borides as the brazing temperature increased to 1200°C.
SEM cross-sections and fractographs of IN-625/VZ-2106/IN-625 joints after shear tests: (a,b) 1150°C, 1800 s, 40 μm (338 MPa), (c,d) 1150°C, 1800 s, 80 μm (495 MPa), (e) 1200°C, 1800 s, 80 μm (yielding of IN-625 substrate).
Microstructural evolution of brazed IN-625 using VZ-2106 foil has been evaluated in the experiment. The brazed zone is dominated by coarse Nb6Ni16Si7 in Ni/Cr/Fe-rich matrix, and interfacial Nb6Ni16Si7 precipitates and grain boundary borides are also observed from the joint. Increasing the thickness of VZ-2106 foil from 40 to 80 μm improves bonding strength of the joint for specimens brazed at 1150 and 1180°C. Cracks are initiated from interfacial Nb6Ni16Si7 precipitates and propagate along the interface between VZ-2106 braze and IN-625 substrate. Specimens brazed at 1200°C using 40 and 80 μm VZ-2106 foils demonstrate the best bonding strength. Disappearance of interfacial Nb6Ni16Si7 precipitates is attributed to high shear strengths of these joints, and fracture mechanism is changed from failure of interfacial Nb6Ni16Si7 precipitates into coarsened borides.
The authors gratefully acknowledge the financial support of this research by National Science Council, Republic of China under NSC grant 99-2221-E-002-120-MY3.
