Aerospace brazing today, and tomorrow

Material scientists and ceramics component manufacturers have been developing new materials and processes that let engines run increasingly hotter, in response to the aerospace industry’s focus on developing products that achieve higher performance and cost less.

In gas turbine engines a large amount of air from the compressor is used to cool the turbine vane and blades. Turbine temperature and the materials that need to be cooled determine the amount of air needed. If the turbine materials need less cooling, or can be made from materials that can withstand higher temperatures, this would make more air available for propulsion. Thus, increasing the turbine’s temperature capability is critical to improving engine efficiency. However, engines run hotter as processing temperature is increased, and this increased heat tends to degrade metals.

Inside turbines, pre-sintered preforms (PSPs) are being used to repair vanes that are breaking down due to excessive heat and wear. PSPs, with a small amount of braze alloy mixed with the parent metal, are used primarily in the turbine section for repairing vane cracks and wear areas.

As temperatures continue to climb in these zones, new materials and technologies are being developed to create a better thermal barrier. This is expected to lower maintenance, repair and overhaul (MRO) costs, significantly.  Examples include the development of advanced braze alloys, the use of ceramics on high temperature metal to ceramic components, and the introduction of active brazing, which allows metal to be bonded directly to ceramic without metallisation.

Braze alloys for high-temp applications

Braze alloys are used in a variety of advanced military aircraft and commercial aerospace engine components and grades 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 (750-850°C).

Alloys are selected to meet the specific service temperature conditions as well as the requirements of all the components to be joined. Examples include alloys used in new turbine hot sections, brazing silicon nitride ceramic to new super alloy engine parts.

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. The engine inlet captures incoming air. 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’ Wesgo Metals in Hayward, Calif., 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.

Creating extra thrust
In the stator section of a turbofan engine, the stator pulls the cold air in and bypasses the engine, creating the extra thrust.KD VIII Elite High