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?
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.
Brazing Problems Associated with Tack Welds
By Dan Kay
Dan Kay operates his own brazing consulting practice in Connecticut (since 1996) and has been involved in brazing for almost 45 years. He received his BS in Metallurgical Engineering from Rensselaer Polytechnic Institute in 1966 and his MBA from Michigan State University in 1982.
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