What is diffusion bonding?
As said earlier, diffusion bonding is used for joining unlike materials together. If two clean oxide-free surfaces are heated to a high temperature, atoms in one metal may move (diffuse) across the interface. This forms a bond between the two pieces of metal.
Compared to other bonding techniques, the bond area will be very narrow due to the inter-diffusion effect. The alloy composition of the bond region will be a profiled mixture of the two alloys being joined.
Defects in diffusion bonding
The diffusion rate of different atoms in different metals differs. Ultimately, atoms from one metal may move across the boundary quicker than atoms from the other. As no other metal is involved, this can lead to a row of pores across the interface.
Kirkendall porosity actually dates back to some of the earliest studies of metallurgy, specifically proving how atoms actually diffuse!
Some elegant experiments show that increasing the diffusion time increases the interface's porosity level. Read here
Where is diffusion bonding used?
It is often used to join materials of different colors, such as in making billets for mokume gane and for the manufacture of multi-colored rings.
Diffusion soldering is scarcely used but has great potential for jewelry manufacture.
Between the two surfaces, an interlayer is placed.
The join is heated (above the melting point of the interlayer but below the solidus of the alloys on either side of the join and the one that forms at the join), and the interlayer melts and diffuses into the adjacent solid metal.
This results in a coherent, homogeneous bond with no obvious interlayer. Due to the diffusion of the interlayer into both surfaces, the join has a higher melting point than the bonding temperature (we have a different alloy).
On a phase diagram, we can see how the join becomes liquid and then solidifies again at a constant temperature.
Where is diffusion soldering used?
This technique was developed for high-karat golds (see Jacobson). It uses a few micrometers of tin, which is deposited as a thin film on a gold foil (for easy use).
Diffusion soldering is performed at 250˚C/482˚F, which is slightly above Tin's melting point, and a small pressure is applied to ensure contact. The soldering occurs in less than a second. In the second step, the joint is heated to 450˚C to homogenize the joint.
This technique is advantageous in that bonding is accomplished at low temperatures. There is no loss of strength and hardness in the workpiece, and the joint is strong.
It could also be used to join unlike alloys such as white and yellow gold or platinum and karat gold, for example.
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