Proposal context Ambitious net-zero targets and society's expectation for a continuous 'on-demand', clean, secure, and sustainable energy commodity necessitates a significant expansion in the UK's electrical infrastructure. The DfT's 2022 report "Taking Charge: the electric vehicle (EV) infrastructure strategy" and the APC's automotive battery end-of-life value chain roadmap, published in June 2023, highlight strategic economic benefits associated with this challenge. Regional and UK-wide prosperity, allied with extending battery life provides the support needed to grow a second hand EV market to allow vehicles to be more affordable, whilst simultaneously improving environmental stewardship through improved recycling and a reduction in demand for critical raw materials, further reducing energy usage. The challenge the project addresses and how it will be applied to this: Extending battery life through tactical replacement or repair of battery cells and / or modules provides a manifold of benefits and offers new market opportunities for the transportation sector. Presently, battery designs and those sub-assembly electrical connections between cells and busbars are created using fusion or solid-state bonded processes producing non-reversible joints; i.e., separation of joints is a destructive activity if they are to be replaced, repaired or recycled. Mechanical methods have been investigated and used for early designs, but these are vulnerable to 'efficiency drop-off' triggered by 'resistance ageing', resulting from thermal and corrosive activities between the connection interfaces and loosening of connections caused by random vibrations. The University of Sheffield, Heriot-Watt University and the University of the West of Scotland will develop a sustainable manufacturing process for battery applications, enabling assembly, non-destructive disassembly and reassembly between electrical connections to achieve full recovery of the cells and busbars. Our EPSRC funding request brings together expertise from across multifarious engineering disciplines: surface engineering and flow dynamics; materials science; joining; AI; and tooling design. We will utilise outputs from previous EPSRC funding projects; e.g., 'NASCENT' to accelerate capability to produce a reversible solution.

The creation of new, 21st Century manufactured products gives us exciting possibilities. However, the number of complex devices and components that consist of one piece of a single material is negligible; almost all manufacturing involves the joining of materials. Joining technology is extensive, but is still challenged by novel designs and new advanced materials. Frequently, these needs could be met by soldering, where a low melting point alloy is introduced in liquid form into the joint, where it solidifies, making a bond. Many people will associate soldering with the electronics industry, where it is widely used, reliably, effectively and at low cost. Yet current soldering is not good at forming bonds with many materials, (for example metals with tenacious oxides and ceramics) and it does not form strong joints which can resist exposure to elevated temperatures where applications demand it. To do this may need an approach used for brazing (very much like soldering, but at higher temperature) of adding an element to the alloy, whose role is to chemically interact with surfaces and improve wetting when liquid and bonding once solidified. Adapting the terminology from brazing, this would be "active soldering". Such a process is not simple however. First we must identify the correct active elements, which may not be the ones used in brazing. These must produce sufficient reaction at low temperatures and be adapted to the materials being bonded. Secondly, a way to introduce a large enough amount of these elements into the solder is required. Solders are based on tin, which may react with the active elements itself if too large quantities are present. Finally, such joints that have been attempted have very poor mechanical properties, and these must be improved. To resolve these challenges, we will deposit the active elements (selected with the aid of thermodynamic modelling) onto a metallic carrier, a Ni or Cu sponge or foam, with fine (~0.5mm) pores, and infiltrate the Sn into this, creating a composite solder. This will keep the active elements and the Sn separate until soldering, when the Sn will begin to dissolve the foam and progressively release the active material to aid in bonding. The residual network of the foam structure across the joint seam will also be effective in increasing the joint strength. We will make and test these composite solders and the joints, and we will also probe the reactions occurring in great detail, to ensure we understand the key step of this new technology. Of immediate use, this approach will improve the strength of bonds achieved in current applications (such as in antennae, heat exchangers and semiconductor devices), give them higher temperature resistance in service and reduce the environmental impact of the process, by removing the need for polluting chemical fluxes or electroplating to prepare the joint and aid bonding. The benefits certainly do not stop there, as the technology would also allow new applications. For example, metals like stainless steel are brazed in vacuum at high temperature; achieving the same goal at lower temperatures and in air would be a much less expensive process. Low process temperatures save energy and cost; for example, some electroceramics (important for, e.g. capacitors) can be processed by cold sintering at temperatures as low as 200degC, but the advantages would be lost without low temperature means to join them in electronic devices. Advanced materials such as graphene also hold much promise in areas like touchscreens and circuitry, and a technique like that developed here would be an essential part of making this a reality. The simple, mass manufacturing nature of solder means that, with our research partners including end users and processors of solder materials, the scalability of the new method created, and the chances of realising these benefits, will be very high.

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 ways 2) 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.