How to Braze copper to brass using Uniweld UNI-4300.
Uniweld UNI-4300 is actually an aluminum solder that melts at around 500f.
Is Uniweld uni-4300 a brazing or soldering alloy?...and who really cares as long as it works?
OK here is some brief technical info about brazing vs soldering...
The terms brazing and soldering are often used incorrectly but technically speaking , brazing begins at about 840 F and anything under that is called soldering.
The principles of soldering and brazing are the same in that the base metal never melts and a bond is made by heating a base metal that has been fluxed to a point where another metal with a lower melting point can flow into nooks and crannies by capillary action and form a bond. Sometimes that bond can be even stronger than a weld depending on the joint design.
A common problem for HVAC technicians as well as automotive mechanics is a small pinhole in an Air Conditioning System's evaporator coil or other heat exchanger of some kind. UNI-4300 can be used for pinhole repairs on aluminum and aluminum to copper or brass due to it’s low melting point of 500 °F (260 °C) and 20,000 PSI tensile strength.
When fixing a pinhole leak, the trick is not to cause more damage or more leaks by heating things up too much. You have to consider what caused the pinhole leak in the first place. If corrosion is the cause, you have to be very careful not to disturb things too much or you might be fixing more leaks all around the first one... And if there are nearby soldered joints, you can melt them if you are not careful.
A low melt braze like uni-4300 can sometimes pull your butt out of a bind for tedious pinhole repairs.
As with any brazing or soldering task, the trick lies in cleaning and fluxing and avoiding oxidation anywhere on the base metal or filler metal.
In this video, its easy to see that the filler metal and honey flux react together to let the braze metal flow and wet into the joint easily.
Using real world methods of hand feeding the brazing wire would probably require fluxing the braze wire to obtain results close to what is seen here in this brazing video.
Pre-placed filler metal makes things easier and the guy demonstrating this has done this a few times before...
demonstrations like this are always going to look easier than real world applications.
But with a little practice on scrap material, you can make it work too.