Diffusion bonding (welding or brazing)

Diffusion joining is a generic term covering a family of processes that rely on diffusion [M 55] to establish strong bonds between two metal parts. They can therefore be compared to either fusion or mechanical joining. These processes are generally slower than those involving the liquid state of part of the assembly at the joint, such as fusion welding or conventional brazing. While they do not generally offer the advantage of high productivity, "diffusion joining" processes do have other, related advantages:

• absence of microporosity and segregation - total for diffusion welding, partial for diffusion brazing;

• absence of discontinuities, molten zone (MZ) or heat-affected zone (HAZ);

• the absence of massive deformation and the virtual absence of residual stresses, making precision assembly possible;

• the high mechanical strength of the interface (stress at break generally greater than 80% of the stress at break of the least resistant material in the assembly).

All parts in a diffusion assembly are subjected to the same thermomechanical cycle. It may therefore be necessary to complete or repeat the heat treatment. The formation of stable compounds at the interface can pose a serious quality problem and prohibit diffusion welding, or at least guide the choice of technologies.

Application areas for diffusion assembly processes range from electronics and sensor manufacturing, to nuclear engineering and aerospace construction. These are often "niche" markets. Assemblies of advanced materials such as metal matrix composites or dispersoid hardened alloys (ODS), and metal/ceramic assemblies that cannot be joined by conventional welding or brazing (with a massive liquid phase) can often be joined using diffusion joining processes.

We'll see that there's quite a wide variety of diffusion joining technologies, even if we limit our discussion to diffusion welding and diffusion brazing. At a fundamental level, we distinguish between solid-phase assemblies and diffusion assemblies with a transient liquid phase, known as diffusion brazing. We will mainly consider processes with a low level of generalized plastic deformation (< 1%), but we will quickly compare them with related processes covered elsewhere in the Techniques de l'Ingénieur collection, such as colamination, explosion welding and friction welding...