Question:
I have heard the terms "diffusion bonding" and "diffusion brazing" both used in reference to the joining of metallic substrates, and I was wondering if there is really any difference between those two terms.

Answer:
Diffusion brazing (DFB) refers to a process in which the joining together of the metallic components of an assembly is accomplished by heating these metals to the brazing temperature and then melting and flowing a brazing filler metal (BFM) between the faying surfaces (the surfaces inside the joint being brazed). This assembly is then held at brazing temperature for a sufficient amount of time to ensure adequate alloying between the BFM and the base metals being joined, usually until the physical and mechanical properties of the joint become almost identical to those of the base metal. Pressure may (or may not) be applied to the assembly during this BFM melting/diffusion process. It is usually done in a brazing furnace.

The phrase diffusion bonding is a broader term, in that it could refer to either diffusion brazing or perhaps to diffusion welding. The American Welding Society (AWS) now considers this phrase to be a non-standard term and should no longer be used.

Please note that all brazing must involve a certain amount of "diffusion" in order for base-metal "wetting" to occur, i.e. for the BFM to spread between the faying surfaces to form a brazed joint. At the interface between the BFM and the base metal, a certain amount of diffusion (alloying) must take place in order to form a strong "bond" between the base metal and the BFM (i.e. there must be a certain amount of "metallurgical affinity" between the BFM and the base metal). If the BFM and the base metal cannot alloy together to some extent, a brazed joint will not occur. As an example, pure silver does not like to alloy with pure iron – they have no metallurgical affinity for each other. Molten pure silver will merely ball-up on iron surfaces. Copper, on the other hand, has enough affinity (solubility), metallurgically, with iron to be able to alloy slightly with it (up to about 5% max) and can therefore braze iron.

The most advanced diffusion-brazing processes are used in the aerospace industry, particularly for brazements involved with titanium, nickel, cobalt and aluminum alloys.

These specialized diffusion-brazing processes are sometimes given proprietary names by companies seeking to protect patented modifications of the diffusion process. Some descriptive terms found in the aerospace brazing industry today are: activated diffusion bonding® (ADB), activated diffusion healing® (ADH) and transient liquid-phase bonding® (TLP). Each of these is a diffusion-brazing process, even though they may also involve special additions of base-metal powders to enhance their characteristics.

Although diffusion brazing is commonly performed in furnaces specifically set aside for brazing or heat treating, it is also possible to initially braze two components together by induction heating (or even by torch brazing) and then place the brazement into a furnace for an extended diffusion cycle. Diffusion cycles may range from 30 minutes to 80 hours or even longer and may include the additions of weights on the parts to press them together.

The amount of diffusion that occurs will be a function of:

1. The brazing temperature used
2. How long the part is held at temperature
3. The quantity of filler metal available for such diffusion
4. The mutual solubility of BFM and base metals used
5. The use (or non-use) of added weights to apply pressure to the joint

Therefore, where diffusion brazing is required, it should be clearly specified on the drawing or in the specification.