Typically used with metals but applicable to ceramic materials too, vacuum brazing is used to join two or more similar or dissimilar material parts in a vacuum environment by melting and flowing a filler material into the gaps between the parts to be joined by capillary action.
In order to melt and flow as required, the braze filler material must have a lower melting point than the adjoining parts so that the joint can be heated to a temperature above the filler material’s liquidus temperature. But below the solidus temperature of the parent materials.
Projects related to vacuum brazing undertaken at TWI over the decades include core research for the benefit of our Industrial Members, public-funded projects where TWI worked as part of a consortium of organisations and dedicated support projects for individual Industrial Member companies.
Core Research Programme (CRP)
Through the core research programme (CRP), TWI conducts projects to address challenges faced by our Industrial Members, advancing technologies and processes, and finding new solutions for industry problems. These projects are jointly funded by our Industrial Members, with the outcomes made available to all our Members.
- Brazing Oxide-Dispersion-Strengthened Nickel Alloys via Sputter Coatings
This 1987 CRP project investigated brazing for oxide dispersion strengthened (ODS) alloys that cannot be fusion welded without local destruction of the dispersion of fine oxide particles and interruption of the grain structure that develops their creep resistance. Brazing offered an alternative joining method whereby joints can be made in which the degradation of high temperature properties is within acceptable limits. This work saw our experts join two nickel ODS alloys using brazing alloys containing boron and silicon in the form of foils and sputtered coatings as well as with brazes that contained no boron but instead had enhanced silicon content to act as the melting point depressant. The joints were examined metallographically for the presence of porosity, agglomerations of dispersoid, second-phase particles, and effects upon the parent metal microstructure. In addition, techniques for making sputtered braze coatings of different compositions were explored.
- A Study of Ceramic/Metal Bonding
This 1990 CRP project was created to address increased interest across industry in the use of ceramics as engineering materials and specifically ceramic-to-metal joints whose formulation posed problems not encountered in metal-to-metal joints. Investigating two combinations of interest to the automotive industry - partially stabilised tetragonal zirconia (PSTZ) to a spheroidal-graphite (SG) cast iron and reaction-bonded silicon carbide (RB-SiC) to a 0.4%C steel – these combinations also embodied many of the problems that could arise in other ceramic / metal joints. For this work, our experts created simple vacuum brazed and diffusion bonded butt joints between ceramic discs and metal stubs. These joints were then tested with a specially-built shear tester as well as being assessed by sectioning and metallography, and by fractography of the sheared faces.
- Development of a Brazed Ceramic-Faced Steel Tappet
This 1992 work, which was part-funded under the CEC EURAM initiative, saw TWI partner with Peugeot (France) and Johnson Matthey with the aim of producing 80 ceramic-faced steel tappets for trials in a Peugeot car test engine. The ceramic, Syalon101, was brazed to the steel, which required our experts to overcome the large difference in thermal contraction between the materials during cooling. TWI explored strain accommodation by including solid interlayers of thin convoluted discs of nickel or iron, and also by titanium honeycomb. Finite element analysis was employed to assist in the selection of interlayer materials and in the design of the convoluted discs. A brazing programme to produce a satisfactory brazed joint at the same time as achieving a specified steel hardness was formulated, and shear testing of the completed joints was carried out.
- Brazing of Zirconia for Structural and Sensor Applications
The TWI CRP programme returned to vacuum brazing of ceramics in 2000 with an investigation of zirconia (ZrO 2), whose low thermal conductivity, high strength and fracture toughness, and ionic conduction at elevated temperatures had garnered interest from industry. Although adhesives and mechanical bonding was suitable for low temperature applications, brazing was deemed the best solution for higher temperature applications. However, during the brazing process, the active element, titanium, reacts with the zirconia, depleting the surface region of oxygen. This causes discolouring (darkening) in the zirconia, which this project sought to investigate by quantifying the extent and effect of the known discolouration phenomenon. TWI’s expert team also determined the effect of braze alloy composition and brazing parameters on the strength of zirconia-zirconia and zirconia-metal joints in order to provide guidance for our Industrial Members.
- Brazing Ceramics - with a Little Extra
The brazing of ceramics was also the subject of this 2001 project that once-again aimed to solve the challenge of ceramic-ceramic and ceramic-metal joints at higher temperatures or when joining materials with significant mismatch in coefficient of thermal expansion (CTE). In such instances, the interfacial stresses developed become higher, making the production of high strength joints problematic. This project incorporated particulate reinforcement into braze alloys for both moderate and high temperature service, to investigate the potential benefits of increased joint strength and stress management as a result. The strength of ceramic-ceramic and ceramic-metal joints were measured for both moderate (less than 450°C) and high temperature service (800-1000°C).
- A Feasibility Study of Brazing Silicon Carbide to Metals
This 2008 CRP combined experimental work with modelling activities to determine the feasibility of generating a chemical reaction between silicon carbide (SiC) and a range of metal braze alloys utilising titanium as an 'active metal.' In addition, our team sought to establish whether simple butt joints between SiC and a stainless steel or nickel alloy could be fabricated. The investigation highlighted that failure of ceramic-metal joints are most frequently in the ceramic component, with failure stress primarily arising from thermal expansion mismatches between the ceramic, braze and metal components.
Public-Funded Projects
Our expert teams are also called to participate in public funded projects in partnership with other organisations from industry and academia to solve specific challenges, typically for particular industry sectors.
- ‘Adv-Flow’ Mass Flow Meter Project
This Innovate UK-funded project, ‘Development of Advanced, Highly Corrosion Resistant Coriolis Mass Flow Meters (Adv-Flow),’ saw TWI partner with KROHNE Limited and Langley Alloys to investigate advancements in the manufacture of large size Coriolis mass flow meters to meet demanding conditions for KROHNE, with a focus on the development of a suitable manufacturing route to manufacture the flow meters from super duplex stainless steel. TWI’s experts led activities related to heat treatment, vacuum brazing and characterisation (Figure 1). The key project objective was to achieve a sound joint whilst being able to retain the corrosion resistance and mechanical properties of the base materials.
- 5G-ERA: 5G Enhanced Robot Autonomy
TWI was one of 13 project partners involved in the 5G-ERA project, which aimed to take advantage of the improved 5G connectivity to provide an enhanced 5G experimentation facility and the relevant network applications for third party application developers to test and qualify their applications. The applications focused on robotic autonomy, which is essential for many 5G vertical sectors and can deliver multiple benefits in automated mobility, Industry 4.0 and healthcare. This included the collection of sensor data from brazing processes in order to enable digitisation and automation of the processes for the manufacture of safety-critical components, replacing some manual operations with robotic solutions. Field trials were conducted at TWI to test the system including brazing robot operation, edge operation planning services, a central cloud service, a learning service, smart environment service, an intervention service in case of problems, and a security management identity service.
- TWI Assisting in the Development of Novel Braze Fillers
This collaborative project, funded by EPSRC, aligned TWI with experts from the University of Sheffield and the University of Leicester to develop novel brazing filler materials (BFMs) based on high entropy alloy (HEA) systems for brazing nickel-based alloy 718 and CuCrZr alloys. The project aimed to understand the solidification behaviour of the developed HEA fillers and assess their potential benefits when compared to commercially available BFMs. This would widen the spectrum of materials that can be joined through brazing, reduce or eliminate the brittle intermetallic phase formation in the joint region, and achieve a fundamental understanding of the brazing process through experimental trials and numerical modelling. Several commercially available BFMs were brazed and their microstructures were compared with the HEA BFMs. This was also accompanied by advanced in-situ characterisation of the braze joint using the synchrotron X-ray technique at Diamond Light Source (Figure 2).