Aerospace brazing today, tomorrow

Aerospace brazing today, tomorrow

The new brazing alloys improve the thermal barrier in the hot section of the jet engine.

Materials scientists and ceramic component manufacturers have been developing new materials and processes that allow engines to run ever hotter in response to the aerospace industry's focus on higher performance and lower costs.

In gas turbine engines, the use of a large amount of air from the compressor cools the turbine blade and blades. The temperature of the turbine and the materials that need to be cooled determine the amount of air required. If the turbine materials need less cooling or can be made of materials that can withstand higher temperatures, this would make more air available for propulsion. Increasing the temperature capacity of the turbine is key to improving the efficiency of the engine. However, engines get hotter as processing temperature increases, and this increase in heat tends to degrade metals.

Inside turbines, the use of pre-sintered preforms (PSP) is to repair blades that break due to excessive heat and wear. PSPs, with a small amount of brazing alloy mixed with the base metal, are used primarily in the turbine section to repair blade cracks and wear areas. As temperatures continue to rise in these areas, the development of new materials and technologies creates a better thermal barrier. This is expected to significantly reduce maintenance, repair and overhaul (MRO) costs. Examples include the development of advanced brazing alloys, the use of ceramics in high temperature metal ceramic components, and the introduction of active brazing, which allows metal to bond directly to ceramic without metallization.

High temperature applications

Brazing alloys are used in a variety of advanced military aircraft and commercial aerospace engine grades and components are being developed that directly bond ceramic to metal or other materials. Alloy compositions vary and include those designed for functional use in very high temperature applications (750 ° C to 850 ° C).

The selection of alloys is to meet the specific service temperature conditions, as well as the requirements of all components that require joints. Examples include alloys used in new turbine hot sections, silicon nitride ceramic brazing in new super alloy engine parts. See Table 1 for an overview of available brazing alloys, showing which part of the motor it is used on and the component / base material.

Most modern airliners use turbofan engines due to their high thrust and good fuel efficiency. A turbofan receives part of the thrust from the core and part of the fan. Capture of the incoming air is done through the engine inlet. Some of the incoming air passes through the fan and continues to the central compressor and then the burner, where it mixes with the fuel and combustion occurs. The hot exhaust passes through the core and fan turbines and then exits the nozzle. The rest of the incoming air passes through the fan and bypasses the engine, similar to air through a propeller. The air going through the fan has a slightly higher speed.

Figure 1 is a diagram of a typical turbofan engine, showing the most common locations for the use of alloys, including those used for the cold section of the engine (air inlet and compressor) and its hot section (turbine and combustion chamber ).

Morgan Technical Ceramics' Wesgo Metals site in Hayward, CA produces over 15 brazing alloy compositions for use in the compressor section. Nioro is used in Inconel X750 or 718 to meet the solution anneal temperature without excessive grain growth that occurs in nickel-based alloys. Nioro is a high purity gold / nickel alloy for vacuum brazing. Nickel brazing alloys are used in brazing sections of turbines and compressors. In its sheet form, it can be used to weld honeycomb and metal sealing strips.

In the stator section of a turbofan engine, the stator draws cold air in and bypasses the engine, creating the additional thrust. The stator also has a role in reducing turbulence, minimizing pitch and roll of the air.

In turbofan fuel systems, the use of gold nickel and platinum gold nickel is to braze the fuel system tubes and nozzles. The fuel nozzle, located where the first and second combustion stages take place, receives a considerable amount of heat. Ductility is needed in brazed joints to help with expansion and vibration in the combustion section. Gold and platinum brazing alloys also exhibit superior contrast in the brazing joint, allowing the use of X-ray technology to verify the integrity of the brazing joint. Furthermore, these alloys demonstrate extremely good corrosion resistance. This area is one in which engine manufacturers have expressed great interest in materials that can withstand very extreme temperatures, where conventional superalloys fail.

Active metal brazing An area of ​​growing interest is active metal brazing, which allows metal to be joined directly to ceramic without metallization, thus eliminating several steps in the joining process and creating an extremely strong hermetic seal that can reach high temperatures. higher operating conditions. Aerospace applications include aerospace and industrial turbine engine nozzles, new turbine blade systems, and engine sensor components.

Active metal brazing can be done with any combination of ceramic, carbon, graphite, metals, and diamond. Active Brazing Alloys (ABA) are used for engine sensors that employ metal-to-ceramic strips to monitor engine functions. Brazing is done with a high temperature ABA, so the sensor can withstand 1000 °C (1830°F) in service.

Active metal brazing makes it easy to join some materials and components, achievements not previously possible, and is especially beneficial in military and aerospace applications.

Fuente: Aerospace Manufacturing and Design

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