Question:
On some of our parts, prior to induction silver braze, we tack weld the parts to be brazed. The parts are fixtured during the tack weld to hold dimensions and/or position. Unfortunately, this creates some brazing problems. The tack welds create localized oxidation spots that act essentially as stop-off to braze flow. Consequently, we get very poor braze flow around the tacks. This has led to leaks in pressurized braze joints. Have you seen this? What can be done to correct this?

Answer:
Yes, I've also seen this a number of times in the field at different brazing shops where they used heavy tack welds to hold parts together for brazing. I wondered why their furnace-brazing process did not clean up all the oxidation that resulted from the welding process. Many of those parts leaked pressure as a result. The same results would occur to any parts that were induction brazed or torch brazed.  

First of all, please understand that the brazing process will not overcome the contamination problems resulting from poor practice in assembling the parts for braze. Regular brazing processes – with or without flux – are not effective at “deep-cleaning” inside a braze-joint after it has been assembled and thus cannot remove surface oxidation buried deep inside a joint, as is the case with oxidized tack welds.

It must begin with changing the welding process used for fixturing the parts. Standard tack-welding processes must be changed so as to provide only the tiniest possible tack, with no visible oxidation around the welds.

It is very important that only clean parts go into the various brazing processes, and those parts must be free from any trapped welding oxidation. Remember, the tack weld is just a very temporary means of holding the parts gently together until the part is brazed. It is the braze itself that will give the part the robust strength and leak-tightness that it needs to survive end-use service conditions. At that point, the tack welds are no longer needed.

Too many welders make tack welds that are designed to be the sole means of keeping the parts together for the life of the part in service. The welds are big and bold, and they therefore exhibit lots of oxides around them. Someone has to explain to them, and teach them, the difference.  

Folks erroneously think, "Oh, don't worry about that. The furnace atmosphere will clean that up." Wrong! Oxidation inside a braze joint (such as you have) will NOT be cleaned up in a furnace any more than they are being cleaned up now by flux that is trying to reach that spot during your current process. The key, once again, is to get the welders to STOP making large, oxidized tack welds.

An interesting alternative is to use some of the readily available small, hand-held capacitive-discharge spot welders to lightly tack parts together for brazing. These small spot-welders are ideal for brazing prep in many different applications.