In this study, the microstructure and solidus and liquidus of several Ni-Co-Hf-Zr-Co-Ti-Al braze alloys were first examined with the objective to develop a B and Si free low melting braze alloy for narrow gap (NGB) and wide gap brazing (WGB) and repair applications. Among various alloys examined, DSC was used to measure the solidus and liquidus during heating and cooling cycles. Following the measurements of liquidus and solidus, the microstructure was evaluated using SEM. Equations for calculating solidus and liquidus based on alloy’s compositions were established and the functions of each elements on these two temperature were discussed.  One particle alloy with a liquidus of 1201˚C was employed for further NGB and WGB experiments. The results showed that it was able join CMSX-4 at 1240˚C without interfacial voids; and with the use of externally applied pressure and extended homogenization treatment the interfacial intermetallic compounds were substantially removed. Furthermore, the same braze alloy was used to fill a large artificial cavity in a WGB scheme at a reduced temperature of 1200˚C. The braze alloy was able to bond the filler powder alloy in addition to join the two alloys to the substrate IN 738. Finally, oxidation test was conducted at 1050˚C for 100 hours after NGB of CMSX-4 and WGB of IN 738. The results showed that the oxide formed on the standalone braze alloy is very dense and there is no signs of spallation. It contained primarily NiO (+CoO) with no other elements measured. For the NGB joints, large amount of scale spallation was observed on base alloy CMSX-4 while the NGB joint remained spallation free. The oxide formed on the NGB was NiO with partition of Co, Al, Ti, Cr, and W. The WGB joint region in IN 738 showed oxide scale spallation on the substrate IN 738 side, leaving behind steps and depression on the sample surface, as marked by the arrows. In the WGB joint itself, there were three notable phases after oxidation test, however, no scale spallation could be found. For the majority of the surface, a Ni-rich oxide (larger crystals, labelled A) covered the surface. There were areas of smaller oxide particles with higher Cr content. This suggest that the outer layer consists of NiO and an inner layer of perhaps spinel (Ni(Cr,Al)~2~O~4~). There were also small bright particles distributed throughout the surface and EDS results suggested the presence of Ta/Nb/Ti rich oxides (IN 738 filler powder provided Ta/Nb). Overall, the new boron/silicon free braze alloy was found to be able to join several superalloys in both WGB and NGB schemes without occurrence of defects and the oxidation resistance was superior to both substrate alloys.


Microstructure and Oxidation Behaviour of NGB and WGB Joints With Boron/Silicon Free Nickel Base Braze Alloys

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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