Novel Brazing Filler Metals using High Entropy Alloys

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Brazing is an important process for joining materials. It is quick and permits high strength, and is unique among high-temperature permanent joining methods in leaving the materials being joined largely unchanged; hence it can make complex joints and join dissimilar and difficult to weld materials (e.g. metals to ceramics and high Al/Ti content nickel superalloys respectively). It works by having a specific alloy, called a Brazing Filler Metal (BFM), introduced between the parts to be joined. Thermal treatment of the assembly is used to melt and solidify the BFM, forming a bond. These BFMs are designed specifically for different types of bonding situation, and can have many different compositions.Brazing is a key technology for many advanced applications, including the aerospace and nuclear sectors, but it has limitations. As the service requirements become more demanding, and base metals are refined, new BFMs must be developed. Some specific problems facing brazing technology today include:1) Widening the spectrum of materials that can be joined (including higher temperature materials, bonding metals to ceramics, and also lower process temperatures for materials that cannot survive those of existing brazing alloys; functional ceramics and high strength 7000 series aluminium alloys, for example), would open up a whole host of novel technologies, using both existing and advanced materials in new ways2) High temperature brazing uses additions such as boron or silicon to suppress the BFM melting point. They do this well, but also introduce brittle intermetallic phases in the joint region, limiting mechanical performance.3) In practice, the parameters for brazing are determined on an application-specific basis, by experimental trial and error. Greater fundamental understanding of the brazing process will render this more efficient, permitting the brazing conditions to be designed.This project builds the understanding to address such challenges.A new type of alloy, High Entropy Alloys (HEAs) has recently come to the fore for alloy design. In these alloys, similar amounts of many elements are combined, rather than the typical approach of main solvent element with small additions of other elements to adjust the properties. Some HEAs have reported properties desirable for BFMs; e.g. the ability to add large amounts of elements to control melting point or wetting and flow behaviour without inducing brittle phases, and the multicomponent nature could mediate the transition in a joint between dissimilar materials. However, the physical metallurgy of HEAs is still relatively poorly understood, and their use in brazing has only been explored to a very limited extent.In this work we are investigating systematically the design, understanding and use of HEAs as BFMs. This both adds to our fundamental understanding of this intriguing new class of alloys, and provides the knowledge and skills to permit the design of new products for industry. The data and computer models of the brazing process we will generate give the design methods and data for the development of brazing parameters, which is currently done on a case-by-case basis.The project brings together the UK academic and industrial community on brazing for the first time, and will act as a focus for brazing interest. Aided by our industrial partners we will demonstrate the outcome of this work by two example case studies of alloy development:I) Reduced cost BFM for aero engines; current alloys contain significant amounts of Au and so a noble metal-free BFM, with appropriate performance, would reduce costs.II) Fusion BFM; to build advanced fusion reactor designs, it is necessary to join tungsten blocks on the reactor interior to copper pipes for coolant. This is currently done with BFMs with melting points <325degC; this limits operating temperatures. A new BFM would improve the performance and give more design flexibility for fusion reactor components.