Question: I’ve heard people use the phrase “wiping the joint” when referring to brazing parts together. What does it mean, and when should it be used?
Answer: “Wiping the joint” is a potentially helpful action to use when a person is brazing out in open air (such as in their shop or factory) using either torch-brazing or induction-brazing equipment. It involves slightly twisting/rotating the joint surfaces being brazed while the joint is still being heated and while the brazing filler metal (BFM) is in a fully liquid state.
When someone brazes in air using a torch or induction equipment, it is usually necessary to coat the joint surfaces with a flux prior to brazing so that the surfaces will be protected from oxidation during either brazing process. As metals are heated, there is a strong driving force to react with any available oxygen to form surface oxides. This is true for all metals. Unfortunately, BFM does not like to bond to any oxidized surfaces, and it is therefore essential that all surfaces being brazed must be kept oxide-free.
A flux is a specialized chemical product (usually a paste) that readily absorbs oxygen. Flux pastes, when coated onto the metal surfaces that are to be brazed, can keep surfaces from oxidizing throughout the entire brazing process. However, brazing flux is a very viscous material. When the joint surfaces to be brazed are coated with flux and assembled, it is easy to trap some of the flux (along with air) inside the hills and valleys of the joint-surface roughness during brazing. This can result in increased void content in that joint.
To eliminate the flux more effectively, the process of “wiping the joint” can be used while the BFM is molten and the joint is still being heated, since it is imperative that you never, ever disturb the joint when it is being cooled. So any wiping of the joint must always be done while the joint is being continuously heated and the BFM is fully molten.
While the joint is being heated, gently rotate the two parts relative to each other (only about a quarter-turn or so) and then rotate the assembly back to its original position. At that point, press the two parts firmly together. Only then can you remove the heat and allow the joint to solidify.
The mild rotation (Fig. 1) moves the hills and valleys (characteristic of the roughened faying surfaces) relative to each other. It does so just enough to release any “trapped” air bubbles or flux in those valleys, allowing them to escape from the joint. It is not uncommon that void contents inside the joint can drop from over 50% trapped air/flux bubbles to less than 10% after wiping the joint.