TWI has decades of expertise in diffusion bonding, including the creation of new material bonding combinations and process solutions. Diffusion bonding, also known as diffusion welding, is a solid-state joining method performed in a vacuum or inert gas environment. The process works by applying pressure to materials at an elevated temperature, which causes the atoms of the two solid surfaces to diffuse and combine.
When joining similar materials, the bonds are completely autogenous, while dissimilar materials can be joined with the use of cross-sectional interlayers. Diffusion bonding is typically used to join metals but has also been used to join ceramic materials.
Early diffusion bonding work conducted at TWI involved the joining of aluminium to steel in a review of the process alongside friction welding…
- Joining Aluminium to Steel: Friction and Diffusion Bonding
This 1979 core research programme (CRP) project investigated diffusion bonding and friction welding as solid phase welding processes. The review compared and discussed theories and proposed mechanisms for bond formation between aluminium and steel with emphasis on the use of interlayer materials and alloying elements to control joint properties. The research also investigated the formation of intermetallic compounds.
TWI’s core research work also saw us test diffusion bonded joints to provide important feedback to our Industrial Members from a CRP project from 1982...
- Diffusion Bonding of 0.4 Carbon, Chromium/Molybdenum Steel
Our experts conducted tensile and Charpy impact tests on medium carbon, low alloy steel joints created by diffusion bonding. These tests, carried out to BS 910:708M40 (En19), measured tensile strengths against quenched and tempered parent material at different bonding temperatures. Impact properties were also compared during this project, with the addition of heat treatment also being compared to unbonded control specimens. TWI also assessed the use of interlayers and conducted metallographic and fractographic examinations to provide information on joint structure and the extent of bonding for the different conditions and heat treatments investigated.
Other earlier, informative work undertaken at TWI includes…
- Diffusion Bonding of Aluminium and its Alloys to Materials
This 1983 review formed part of an early research programme aimed at finding methods of diffusion bonding aluminium and its alloys. Establishing fundamental information on the effect of process parameters on the properties of the bonds.
- Diffusion Bonding of Aluminium Alloys to Titanium
TWI’s research into diffusion bonding and aluminium (Al) continued into 1986, aiming to solve the challenge of the tenacious oxide layer that hindered bonding. Techniques were broadly based on the principles of removing, disrupting or penetrating the oxide layer in the belief that true metal-to-metal contact would enable higher bond strengths. Titanium (Ti) was chosen for bonding, because the combination of Al and Ti represents a dissimilar metal combination, which would benefit from the lower temperatures involved in diffusion bonding.
- Diffusion Bonding of an Aluminium-Lithium Alloy
Testing the parameters of diffusion bonding on different materials continued, including a 1989 CRP project to investigate three different procedures for producing diffusion bonds in an Al-Li-Cu-Mg-Zr alloy (AA8090). Solid state bonding with commercially pure aluminium foil of 15µm thickness was investigated alongside diffusion bonding with silver interlayers (coatings and foil) in both the solid and liquid phase.
Our research tested bonding parameters (temperature, pressure, time and deformation) and post-bond heat treatments (solution and ageing) using both butt and lap joints. The testing also examined microstructural features and compositional changes in the diffusion zone of joints bonded with the aluminium interlayer and a microhardness measurement was used to follow the increase of strength in the interlayer. Tensile (butt joints) and shear (lap joints) tests were conducted to determine the strength of the bonds.
As industrial interest grew in the use of ceramics as engineering materials in their own right, there grew a need to understand more about the capabilities and creation of ceramic-to-metal joints. This led to several TWI projects to understand ceramic/metal bonding for a range of applications for the benefit of industry…
- A Study of Ceramic/Metal Bonding
This work, which included diffusion bonding alongside vacuum brazing, investigated joining 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 were chosen as being representative of many of the challenges that could arise in other ceramic/metal joints. The specimens were simple butt joints between ceramic discs and metal stubs, which were assessed by sectioning and metallography using a purpose-built shear tester, and by fractography of the sheared faces.
Diffusion bonding has also been assessed as a potential solution for difficult to join materials, including oxide dispersion strengthened (ODS) alloys…
- Joining ODS Alloys
First developed in the 1960s and made using powder metallurgy, ODS alloys promised an attractive combination of properties for industry, including high temperature and operating environment resistance. However, they had proven difficult to join so TWI investigated a number of potential solutions including solid phase bonding, brazing, and TIG and laser fusion welding for a range of matrix alloys including systems based on iron, nickel and intermetallic alloys. As part of this work, TWI technologists developed diffusion bonding methods for iron based alloys. This research determined which joining processes offered the best quality for each alloy type, providing guidance for industry.
However, there was still a need to develop an NDT technique that could reliably detect defects of the required dimensions and size in diffusion bonded joints during manufacture or in-service. TWI’s experts joined a European Commission project to solve the challenge…
- Development of an Ultrasonic Testing Technique to Characterise Diffusion Bonds in Layered Structures
The BondTest project was created to develop a validated NDT technique and system to detect diffusion bond defects in layered structures at the dimensions and size required by industry. The project team worked to create a solution to meet the appropriate detection criteria, before seeking commercialisation to make the solution available to industry.
While much of TWI’s formative research into diffusion bonding began decades ago, our knowledge and in-house expertise continues to be built upon with a number of process breakthroughs and patents as a direct result of our project work…
- TWI Develops New Aluminium Diffusion Bonding Technique
We conducted a research programme to investigate the application of a proprietary new technique for the successful diffusion bonding of aluminium. The process had been shown to create joint strengths comparable to the heat treated parent metal, so the project tested the technique on a range of aluminium alloys and other materials. The research showed that the technique created joints that were significantly stronger than those that did not use the method.
This breakthrough, which seemed to have potential applications including compact heat exchangers, heat sinks, cooling loops, and process intensification units, was tested further leading to the granting of a patent…
- TWI Granted Diffusion Bonding Patent
Originally filed in 2019, TWI was granted a United States patent for the diffusion bonding technique, ‘Diffusion Bonding with a Bonding Surface Coating Material,’ in 2023. As the research had shown, this allowed for diffusion bonding of aluminium with achievable joint strengths comparable to the heat-treated parent material. The technique produces an autogenous joint without the requirement of interlayers or melting point suppressants.