Most modern airliners use turbofan engines because of their high thrust and good fuel efficiency. A turbofan gets some of its thrust from the core and some from the fan. Incoming air is captured by the engine inlet. Some of the incoming air passes through the fan and continues on into the core compressor and then the burner, where it is mixed with fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then out the nozzle. The rest of the incoming air passes through the fan and bypasses the engine, similar to air through a propeller. The air that goes through the fan has a slightly increased velocity.
Figure 1 is a diagram of a typical turbofan engine, showing the most common locations for use of alloys, including those used for the engine’s “cold section” (air inlet and compressor) and its “hot section” (turbine and combustion chamber).
Morgan Technical Ceramics’s Wesgo Metals site in Hayward, CA, produces more than 15 braze alloy compositions for use in the compressor section. Nioro is used on Inconel X750 or 718 to meet the solution anneal temperature without excess grain growth that occurs from nickel-based alloys. Nioro is a high-purity gold/nickel alloy for vacuum brazing. Nickel braze alloys are used in compressor and turbine section brazing. In its foil form, it can be used for brazing honeycomb and metal seal strips.
In the stator section of a turbofan engine, the stator pulls the cold air in and bypasses the engine, creating the extra thrust. The stator also has a role in reducing turbulence, so air pitching and rolling is minimized.
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In turbofan fuel systems, gold nickel and platinum gold nickel are used to braze the fuel system tubes and nozzles. The fuel nozzle, located where the first and second combustion stages take place, sees a considerable amount of heat. Ductility in the braze joints is needed to help with the expansion and vibration in the combustion section. Gold and platinum braze alloys also exhibit superior contrast in the braze joint, allowing the use of X-ray technology to check braze joint integrity. In addition, these alloys demonstrate extremely good corrosion resistance. This area is one where engine manufacturers have expressed a tremendous amount of interest in materials that can withstand highly extreme temperatures, where conventional super alloys fail.