Diffusion bonding

Diffusion bonding is a solid-state welding technique used in metalworking, capable of joining similar and dissimilar metals. It operates on the materials science principle of solid-state diffusion, wherein the atoms of two solid, metallic surfaces intermingle over time under elevated temperature. Diffusion bonding is typically implemented by applying both high pressure and high temperature to the materials to be welded; it is most commonly used to weld "sandwiches" of alternating layers of thin metal foil and metal wires or filaments.

Characteristics

Diffusion bonding involves no liquid fusion or filler metal. No weight is added to the total, and the join tends to exhibit both the strength and temperature resistance of the base metal(s). The materials endure no, or very little, plastic deformation; very little residual stress is introduced; and there is no contamination from the bonding process. It may be performed on a join surface of theoretically any size with no increase in processing time; practically speaking, the surface tends to be limited by the pressure required and physical limitations. It may be performed with similar and dissimilar metals, reactive and refactory metals, or pieces of varying thicknesses.

Diffusion bonding is most often used for jobs either difficult or impossible to weld by other means, due to its relatively high cost. Examples include welding materials normally impossible to join via liquid fusion, such as zirconium and beryllium; materials with very high melting points such as tungsten; alternating layers of different metals which must retain strength at high temperatures; and very thin, honeycombed metal foil structures.

Process

Diffusion bonding is performed by clamping the two pieces to be welded with their surfaces abutting each other. Prior to welding, these surfaces must be machined to as smooth a finish as economically viable, and kept as free from chemical contaminants or other detritus as possible. Any intervening material between the two metallic surfaces may prevent adequate diffusion of material. Once clamped, pressure and heat are applied to the components, usually for many hours. Pressure can be applied using a hydraulic ram at temperature, this method allows for exact measurements of load on the parts. In cases where the parts must have no temperature gradient, differential thermal expansion can be used to apply load. By fixturing parts using a low expansion metal (i.e. Molybdenum) the parts will supply their own load by expanding more than the fixture metal at temperature. Diffusion bonding must be done in a vacuum or inert gas environment when using metals that have strong oxide layers (i.e. copper).

At the microscopic level, diffusion bonding occurs in three simplified stages:

• Before the surfaces completely contact, asperities (very small surface defects) on the two surfaces contact at the microscopic level and plastically deform. As these asperities deform, they interlink forming interfaces between the two surfaces.
• Elevated temperature and pressure causes accelerated creep in the materials; grain boundaries and raw material migrate and gaps between the two surfaces are reduced to isolated pores.
• Material begins to diffuse across the boundary of the abutting surfaces, confusing this boundary and creating a bond.

References

• Schrader, George F.; Elshennawy, Ahmad K. (2000). Manufacturing Processes and Materials (4th, illustrated ed.). SME. p. 319-320. ISBN 0872635171. Retrieved 2013-02-04. 
• Chawla, Krishan K. (1998). Composite Materials: Science and Engineering. Materials research and engineering (2nd, illustrated ed.). Springer. p. 174. ISBN 0387984097. Retrieved 2013-02-04. 
• Jacobson, David M.; Humpston, Giles (2005). Principles Of Brazing (illustrated ed.). ASM International. p. 11-14. ISBN 0871708124. Retrieved 2013-02-04.