MIG brazing is a new technology. It's major application is in the repair of modern boron steel automotive panels. Boron steels are high strength steels, and much of that strength is gained through heat treatment. The heat treatment is lost when the metal is heated through welding.
But running MIG brazing wire through an ordinary MIG would also generate too much heat and lose the heat treatment. Body repair shops need to invest in expensive synergic MIG welders with special brazing programs. This page does not cover these welders or modern bodyshop techniques.
Note you need to use a teflon liner for brazing wire. It sticks in a steel liner.
Hobby MIG Brazing
I have an application in mind for MIG brazing - I reckon it could be a good way to repair an iron casting. This page covers some initial experimentation with MIG braze on mild steel sheet just to find out how to use it. Pure argon gas is required for shielding.
A MIG Brazed joint
Silicon-bronze brazing wire feels a lot like mild steel wire to use, though because braze melts at a lower temperature than mild steel it can be applied at much lower power settings.
For brazing a butt joint (as in the photograph) the two sheets of steel should be spaced apart by about the same thickness as the steel. For an overlap joint the two sheets should be spaced apart by at least 1mm. The steel sheets aren't melted during brazing, so there has to be a gap for the filler material.
The MIG Brazing Technique
The technique for mig brazing is to weld in a straight line, moving the torch forward. The forward angle of the torch pre-heats the metal about to be brazed.
During brazing there is a bright blue shimmering light traveling on the surface of the metal ahead of the weld. This is an effect of the arc which apparently does the same job as the flux in gas brazing.
I'm brazing 1.2mm steel, so my welder is set at the minimum power setting (less than for normal mild steel welding), but the wire speed is roughly double that which I'd use when welding.
I found I needed to space two sheets of 1.2mm steel apart by somewhere between 0.5mm and 1mm for the braze to flow all the way between the two sheets of metal.
The gap is important. While ordinary welding will penetrate to the back of the weld through melting through the two sheets of steel, brazing doesn't melt the parent metal, so it needs a gap to get to the rear of the joint.
So How Strong is MIG Brazing?
It certainly took a lot of effort to bend it backwards and forwards in the vice. After bending it backwards and forwards a few times I thought I'd film the fatigue test. It took a further 22 cycles to break the brazed joint, and even then it was the steel that broke not the joint.
I was very impressed. Here's the video of the fatigue test for those with a high boredom threshold.
Results of the Fatigue Test
When the joint finally broke it was the steel rather than the braze that had failed. Suggests the brazed joint is at least as strong as the parent metal. That really impressed me, as I've read that brazed joints aren't as strong as welded joints.
Kind of gets irrelevant when the brazed joint is stronger then the metal you are welding.
Distortion with Brazing
It is still possible to distort panels when brazing as the photograph shows. Distortion can be minimised by brazing in short runs, and allowing the metal to cool between runs. But using an ordinary MIG there doesn't seem to be any advantage over normal welding.
Advantages of brazing might include welding of dissimilar metals. Because the two sheets of metal aren't melted together it doesn't really matter if they are the same type of metal so long as the melting point is above about 950C. Most metals will melt at a higher temperature than bronze. It's even possible to braze galvanised steel without removing the galvanising (although this still needs to be done outdoors or with an extractor).
Brazing is not a technique I would use in place of an ordinary weld (without the appropriate equipment and training), though it has hobby uses for joining other materials.