3. Results and Discussion
For the induction brazing process at temperature 950 °C, the microstructure obtained after brazing duration of 10 min was found to contain numerous defects, as shown in
Figure 2. The temperature of 950 °C was found to be too low for the induction brazing, so further investigations focused on higher temperatures.
The microstructure images of specimens S2 to S7 are shown in
Figure 3. The figure shows that the holding temperature of 1050 °C and 1150 °C produce good brazing layers that do not contain any observable defects. A significant number of Ni-enriched solid solution matrix (Ni-matrix) was observed to have grown upward from the brazing interface of the specimens. This could be attributed to the directional cooling of the specimen down to the unheated platform where the specimen was placed. These Ni-matrixes form in the brazed layer when the melting point depressants such as silicon, boron and phosphorus in that region of the layer have fully dissolved or have diffused to the base material.
There is a distinct difference in Ni-matrix growth from the six investigated parameter combinations. The percentage of Ni-matrix in the brazed layer was observed to increase largely with increasing brazing temperature and increasing brazing duration. The effect of brazing temperature was found to be relatively more significant as compared to the effect of brazing time.
The microhardness results of the specimens are also shown in
Figure 3. Microhardness was obtained as it is one of the key properties in evaluating brazing repair. The results show that the grown Ni-matrix in the brazed layer reduces the overall hardness of the brazed layer. As shown in the figure, for specimen S3, the microhardness of the grown Ni-matrix was tested to be at around 253 HV, whereas the non-Ni-matrix region was at about 664 HV. The Ni-matrix in the brazed layer reduced its overall microhardness. Therefore, for specimen S2, as there was not much of Ni-matrix in the brazed layer, the overall microhardness was found to be the highest. Similarly, for the substrate’s microhardness, specimen S2 was also found to have the highest microhardness as compared to specimens S3 to S7. More detailed microhardness measurements at different depth locations of the specimens’ substrates are shown in
Figure 4.
Microhardness was also measured in the Inconel 718 substrate at 200 μm, 400 μm and 600 μm below the brazing interface. The original as-received substrate’s microhardness was 326 HV. As shown from the figure, after induction brazing, the substrate’s microhardness of all specimens has reduced. This is related to the grain size growth of the substrate after the induction brazing process. Comparing the different depths, no significant difference in microhardness for the depth of 200 μm, 400 μm and 600 μm was observed. Comparing the different process parameters, the brazing duration of 2 min specimens (S2 and S5) showed a higher microhardness as compared to the longer brazing durations of 10 min and 20 min (S3, S4, S6 and S7). A further evaluation on the substrate’s microstructure was then carried out.
Figure 5 shows the microstructure images of the specimens’ substrate after etching. The base material’s grain boundaries can be observed. From the observation, a significant difference in the microstructure’s grain size was observed. The average grain size of the as-received material was measured using linear intercept method to be about 9.9 μm. However, after induction brazing at 1050 °C for 2 min (specimen S2), the average grain size increased to 45.5 μm (see
Figure 5a). As the brazing duration increases (10 min for specimen S3 and 20 min for specimen S4), the average grain sizes were found to have increased even further, up to about 84.3 μm. Similarly, the substrate’s average grain size was found to increase as the brazing temperature increases (see
Figure 5d–f). The specimens brazed at 1150 °C were found to have grain sizes larger than that of the specimens brazed at 1050 °C. This increase of substrate’s grain size reduces its microhardness.
From the optical microstructure and microhardness analysis of all the specimens, it is concluded that the brazing temperature of 950 °C is not sufficient for the induction brazing of Inconel 718 with the brazing paste AMS 4777. To obtain good brazing result with no obvious defects in the brazed layer, the brazing temperature of 1050 °C and 1150 °C are recommended. Moreover, it is to be noted that as the brazing temperature or time increases, the microhardness of the brazed layer and the substrate would decrease. This is mainly due to the more percentage of Ni-matrix grown in the brazed layer. To obtain the highest microhardness property of a brazing result, temperature and time that is high enough to ensure good brazing layer and bonding yet low enough to prevent the decrease in hardness are recommended. In this experiment, specimen S2 (1050 °C for 2 min) was found to be the optimum parameter.
A further microstructure evaluation at the interface of the specimen S2 was carried out using electron backscattered diffraction mapping and backscattered electron microscopy as shown in
Figure 6.
Figure 6a presents the EBSD inverse pole figure map of the specimen. Result shows that good microstructure Ni-matrix growth was obtained at the interface of this specimen with no porosity or cracking. The formed Ni-matrix in the brazed layer was observed to be a columnar dendritic grain that grew from the interface with a strong <001> orientation with respect to the growth direction (depicted in red). The pole figures for the subset image over the brazed layer show a strong {001} texture with the maximum multiple of uniform density (MUD) of 17.27, as shown in
Figure 7. This preferred growth direction is common and expected in Ni-based superalloy of face-centered cubic (FCC) crystalline structure and in good agreement with the work conducted on directional solidification of Inconel 718 [
19,
20].
Figure 6b shows the backscattered electron micrograph of the same interface of specimen S2. In the brazed layer, three different phases can be observed (white, black and grey). Chemical compositions of these three phases (points 1, 2 and 3), together with the specimen’s substrate (point 4), were measured using energy-dispersive x-ray spectroscopy (EDS), as shown in
Figure 6b. Points 1, 2 and 3 are likely to be Ni-matrix, Ni-rich boride and Cr-rich boride, respectively, based on the EDS results and result of a previous study [
13]. However, currently, these phases are not confirmed, as EDS cannot detect light elements such as boron. Future works are recommended to identify these phases by X-ray diffraction analysis (XRD).
Author Contributions
Conceptualization, W.Z. and S.C.T.; methodology, W.Z. and S.C.T.; validation, A.A.; formal analysis and investigation, A.A., J.L.T., Y.Y. and W.Z.; data curation, A.A.; writing—original draft preparation, A.A.; writing—review and editing, W.Z., J.L.T., Y.Y. and S.C.T.; visualization, A.A.; supervision, W.Z.; project administration, W.Z. and S.C.T.; funding acquisition, W.Z. and S.C.T. All authors have read and agreed to the published version of the manuscript.
Funding
This research funding was provided by National Research Foundation of Singapore (NRF) through the Industry Alignment Fund (IAF) and by Rolls-Royce for project ARMS 1.2 Repair and Restoration of Airfoils by Induction Brazing.
Institutional Review Board Statement
Not Applicable.
Informed Consent Statement
Not Applicable.
Data Availability Statement
Not Applicable.
Acknowledgments
The authors would like to thank the School of Mechanical and Aerospace Engineering, Nanyang Technological University and Rolls-Royce@NTU Corporate Lab for the research support.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Zhu, D.; Zhang, X.; Ding, H. Tool wear characteristics in machining of nickel-based superalloys. Int. J. Mach. Tools Manuf. 2013, 64, 60–77. [Google Scholar] [CrossRef]
- Çelik, A.; Alağaç, M.S.; Turan, S.; Kara, A.; Kara, F. Wear behavior of solid SiAlON milling tools during high speed milling of Inconel 718. Wear 2017, 378–379, 58–67. [Google Scholar] [CrossRef]
- Yin, Q.; Liu, Z.; Wang, B.; Song, Q.; Cai, Y. Recent progress of machinability and surface integrity for mechanical machining Inconel 718: A review. Int. J. Adv. Manuf. Technol. 2020, 109, 215–245. [Google Scholar] [CrossRef]
- Shakerin, S.; Omidvar, H.; Mirsalehi, S.E. The effect of substrate’s heat treatment on microstructural and mechanical evolution of transient liquid phase bonded IN-738 LC. Mater. Des. 2016, 89, 611–619. [Google Scholar] [CrossRef]
- Henderson, M.B.; Arrell, D.; Larsson, M.; Marchant, G. Nickel based superalloy welding practices for industrial gas turbine applications. J. Sci. Technol. Weld. Join. 2004, 9, 13–21. [Google Scholar] [CrossRef]
- Egbewande, A.T.; Buckson, R.A.; Ojo, O.A. Analysis of laser beam weldability of Inconel 738 superalloy. J. Mater. Charact. 2010, 61, 569–574. [Google Scholar] [CrossRef]
- Vitek, J.M. The effect of welding conditions on stray grain formation in single crystal welds—Theoretical analysis. J. Acta Mater. 2005, 53, 53–67. [Google Scholar] [CrossRef]
- Malekan, A.; Farvizi, M.; Mirsalehi, S.E.; Saito, N.; Nakashima, K. Influence of bonding time on the transient liquid phase bonding behavior of Hastelloy X using Ni-Cr-B-Si-Fe filler alloy. Mater. Sci. Eng. A 2019, 755, 37–49. [Google Scholar] [CrossRef]
- Sadeghian, A.; Mirsalehi, S.E.; Arhami, F.; Malekan, A.; Saito, N.; Nakashima, K. Effect of bonding time on dissimilar transient liquid phase (TLP) bonding of IN939 to IN625 superalloys: Microstructural characterization and mechanical properties. Metall. Mater. Trans. A 2021, 52, 1526–1539. [Google Scholar] [CrossRef]
- Arhami, F.; Mirsalehi, S.E. Micostructual evolution and mechanical properties evaluation of IN-939 bonds made by isothermal solidification of a liquated Ni-Cr-B interlayer. Metall. Mater. Trans. A 2018, 49, 6197–6214. [Google Scholar] [CrossRef]
- Kazazi, A.; Ekrami, A. Corrosion behavior of TLP bonded stainless steel 304 with Ni-based interlayer. J. Manuf. Process. 2019, 42, 131–138. [Google Scholar] [CrossRef]
- AlHazaa, A.; Haneklaus, N. Diffusion bonding and transient liquid phase (TLP) bonding of type 304 and 316 austenitic stainless steel—A review of similar and dissimilar material joints. Metals 2020, 10, 613. [Google Scholar] [CrossRef]
- Jalivand, V.; Omidvar, H.; Shakeri, H.R.; Rahimipour, M.R. Microstructure evolution during transient liquid phase bonding of Inconel 738LC using AMS 4777 filler alloy. Mater. Charact. 2013, 75, 20–28. [Google Scholar] [CrossRef]
- Zhang, Y.C.; Yu, X.T.; Jiang, W.; Tu, S.T.; Zhang, X.C. Elastic modulus and hardness characterization for microregion of Inconel 625/BNi-2 vacuum brazed joint by high temperature nanoindentation. Vacuum 2020, 181, 109582. [Google Scholar] [CrossRef]
- Wu, X.; Chandel, R.S.; Pheow, S.H.; Li, H. Brazing of Inconel X-750 to stainless steel 304 using induction process. Mater. Sci. Eng. A 2000, 288, 84–90. [Google Scholar] [CrossRef]
- Blue, C.A.; Lin, A.; Ray, Y. Rapid infrared joining of titanium alloys and titanium matrix composites. In Materials Research Society Symposium Proceedings; Materials Research Society: Pittsburgh, PA, USA, 1993; Volume 314, pp. 143–148. [Google Scholar]
- Wu, X.; Chandel, R.S.; Li, H.; Seow, H.P.; Wu, S. Induction brazing of Inconel 718 to Inconel X-750 using Ni-Cr-Si-B amorphous foil. J. Mater. Process. Technol. 2000, 104, 34–43. [Google Scholar] [CrossRef]
- Jiang, C.; Chen, H.; Wang, Q.; Li, Y. Effect of brazing temperature and holding time on joint properties of induction brazed WC-Co/carbon steel using Ag-based alloy. J. Mater. Process. Technol. 2016, 229, 562–569. [Google Scholar] [CrossRef]
- Bean, G.E.; McLouth, T.D.; Witkin, D.B.; Sitzman, S.D.; Adams, P.M.; Zaldivar, R.J. Build orientation effects on texture and mechanical properties of selective laser melting Inconel 718. J. Mater. Eng. Perform. 2019, 28, 1942–1949. [Google Scholar] [CrossRef]
- Hinojos, A.; Mireles, J.; Reichardt, A.; Frigola, P.; Hosemann, P.; Murr, L.E.; Wicker, R.B. Joining of Inconel 718 and 316 stainless steel using electron beam melting additive manufacturing technology. Mater. Des. 2016, 94, 17–27. [Google Scholar] [CrossRef] [Green Version]
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