Novel braze coating materials & processes for sustainable aeroengine applications

Background
Reaction Engines (https://www.reactionengines.co.uk) are developing expertise in propulsion systems for alternative fuels, such as ammonia, as part of the shift in technologies targeting "sustainable flight". A core aspect of the goal of high speed, energy efficient flight are the heat exchanger designs used by Reaction Engines to cool high speed fluids, and to recover and re-use the heat energy extracted. One of the key design configurations is tubular heat exchangers, which provide light-weight, compact, efficient performance. The primary manufacturing technique enabling the novel designs to be assembled is Vacuum Brazing. Reaction Engines have considerable experience in design, manufacturing, and testing of tubular heat exchangers in both nickel-chromium super alloys, and stainless-steel alloys, brazed in-house using a large state-of-the-art bespoke vacuum furnace. Multiple rows of thin-walled tubing are joined to manifolds in tightly packed complex arrays, in shapes that can be packaged in compact modular designs.

Reaction Engines are developing the vacuum brazed heat exchanger assemblies further, as designs gain traction in the aerospace sectors, and other sectors demanding high levels of performance, in applications requiring certification. As production volumes increase, the level of quality assurance and repeatability will have to increase also. Various aspects of the Vacuum Brazing process must be optimised. The selection of the braze consumable alloy, which flows into the joint gaps, is one of these aspects. Well established, off-the-shelf products are currently used. Improvements are sought via the investigation of novel compositions and formats. As well as the base composition of the braze consumable alloy, the format of it is also important (paste, wire, foil, tape options exist). These different formats offer different advantages in terms of how the braze consumable is locally deposited on to the metal substrates. Both the melting behaviour of the alloy consumable at the brazing temperature, and the ease of application of the alloy consumable at the required locations are key factors to be optimised for the Reaction Engine Tubular Heat Exchanger designs, to increase efficiency of manufacturing in the Assembly and Brazing production areas. Application of uniform layers of braze consumable on to the surfaces via coating technologies are one of the methods whereby process efficiencies will be assessed.

Project Aims
The work will start with the comprehensive undertaking of mapping Reaction Engines' existing assembly and vacuum brazing routes for heat exchanger parts. A baseline material and manufacturing component will be identified for both steel and nickel alloy parts, representative of current tubular heat exchanger designs. An assessment of existing commercial brazing alloys will be done to understand the current limits for repeatability, and performance. Methods for application of the brazing consumable to the component surfaces, at the required locations, will be assessed - laser cladding, thermal spray, cold spray, physical vapour deposition, or hybrid surface additive manufacturing routes are coating methods of interest. The manually intensive placement methods currently in use will be replaced. Materials characterisation will be undertaken to identify optimal process parameters and material properties, including mechanical testing, metallography and thermophysical property measurement. In a final stage novel coating alloys (e.g., high entropy alloys) will be formulated, manufactured, and tested for comparison to the existing commercial systems.