Diffusion Bonding is a solid-state joining process which is applicable to similar and dissimilar materials, primarily metals, although ceramic materials can also be joined using this process. The process works through the diffusion of atoms across the joint interface at elevated temperature. Diffusion bonding does not require any filler materials, and for a similar materials joint, it is completely autogenous. When diffusion bonding dissimilar materials, interlayers can be used.
A typical commercial diffusion bonding furnace is a vacuum furnace, with hydraulic rams present to apply pressure through graphite tooling and into the parts to be joined. This process is known as uniaxial diffusion bonding. As the process relies on diffusion, applying pressure is required to bring the two surfaces into intimate contact, and thus promote increased diffusion across the interface(s). For this reason, the surface roughness and flatness of the parts being joined are important process parameters. For example, smoother surfaces have fewer asperities and yield joints that have a reduced number of voids and are therefore higher in strength. Diffusion bonding is commonly performed under vacuum at a level <1x10-2 mbar and at temperatures up to 1300°C. Higher temperatures are required for materials with a higher melting point, as the diffusion bonding temperature is typically 50-80% of the material’s melting point (Tm), although at TWI we tend to operate around 70-90% of Tm due to the maximum load limit of our furnace. Certain materials are not compatible with high vacuum conditions, in which case, diffusion bonding under the partial pressure of an inert gas, such as Argon or N2, is common.
Diffusion bonding is a batch process and is generally used where it is difficult or impossible to form a joint, for example, in components that have a complex internal structure. A common application for diffusion bonding is compact heat exchangers. As shown in the video, a series of patterned shims are stacked to form the final heat exchanger geometry, which is effectively an additive manufacturing process. Diffusion bonded joints have been shown to have good bond strength, which allows the heat exchangers to run at high pressure. In addition, the process parameters can be developed to minimise the presence of pores at the bondline, thus allowing the diffusion bonded heat exchanger to be helium leak-tight. The channels can be made to be very small in diameter and depth, which, in combination with the higher service pressures, allows heat exchangers with a favourable heat transfer to weight ratio. Finally, for a heat exchanger made using similar materials, due to the lack of a filler, galvanic corrosion mechanisms do not occur, so when a corrosion resistant alloy is selected, a heat exchanger for highly corrosive fluids can be designed.
TWI has over 60 years of experience in diffusion bonding. Materials that TWI have diffusion bonded include aluminium alloys, titanium alloys, steels (carbon, stainless and ODS), nickel superalloys, Haynes alloys, Fe-Co alloys, zirconium alloys, copper alloys, silicon carbide (SiC), Silicon Nitride (Si3N4) and metal matrix composites (MMC’s).
Find out more about brazing and diffusion bonding at TWI.