I've ordered a reel of silicone bronze wire from www.welduk.com . MIG brazing seems to be a new thing - welduk were the only suppliers I could find in the UK, but it seems to be catching on in America so I thought I'd give it a try.
Like gas brazing the idea isn't to melt the parent metal, just kind of glue it together using molten brass. It isn't as strong as ordinary welding, but the advantage is it can be used at a lower temperature so less distortion.
I'm planning to experiment at the weekend. I'll let you know how I get on.
Edit - the other big advantage is you can braze galvanised steel without too much of the coating burning off. Makes for less rust and less zinc poisoning.
My bottle is filled with Argon at the moment - left over from some Aluminium welding. People do recommend Argon for MIG brazing. But then they recommend Argon for Stainless too which as far as I can see is for cosmetic reasons. I've no idea what would happen if you tried using CO2 with brazing.
The argon keeps the temperature of the molten wire down i think. If it gets too hot it will look like a load of honeycomb when it cools.
Stainless is supposed to be done with an argon / helium mix. I always use pure argon though.
If you are using a flux cored ss wire you can use a mix of Argon, carbon dioxide & oxy, or just a argon & carbon dioxide.
If using normal ss wire, yes, you should be using an argon/ helium mix.
I cant remember why at the moment though.
As for the mig brazing, some car manufactures now specify that it must be used instead of normal mig/mag welding for repairs.
Its main advantage is it requires much less heat, so for thin materials it will minimise or even eliminate any distortion in the haz.
Im trying to pursuade work to try and get a synergic mig for brazing on demo, as they we do a lot of work with thin material.
The brazing wire arrived this morning. Seems to handle very much like steel (in fact it looks like steel wire too and I was at the point of returning it when I spotted the weld was brass coloured).
I've only managed a small length but have noticed some transverse hot cracking in the braze. The technique for gas brazing seems to involve pre-heating, but I can't believe MIG brazing can be the miracle answer to car repairs if you have to put the car in a big oven first
Pure argon. Funnily enough my first braze had hot cracking, but the rest have been OK.
I'm not completely sure that I'm brazing rather than welding despite welding quickly at the lowest power setting - trying a few different things to make the braze flow a little better, but I tried a butt joint and did a fatigue test in a vice - the sheet broke some distance from the braze which surprised me.
Here's the weld test - it's a short and untidy butt weld in 1.2mm steel. Took some breaking in a vice. I think there is some melting of the original steel, but only right at the edge.
The braze - seems to work best when there is a gap between the two sheets.
Clamped one sheet in the vice, then bent 30 degrees one way and the other until it finally8 broke - took some surprising effort.
For best results its reccomended to use a pulse mig, which will give much more control of material tranfer..... making for an almost perfectly smooth weld with very little or no spatter.
The test piece looks good Malc, have you tried any thicker materials and fillet welds yet?
I thought I'd try a bit of 10mm cast iron that doesn't matter very much (one of the doors on that lathe). Apparently it's possible to gas braze cast iron when cold. Made a 90 degree V and did a root weld. Picked it up and it seemed to have stuck together. Then I tapped it against the floor and it fell off too easily. Oops.
The braze had stuck to the cast OK, but the cast had cracked. There were a few tinkling noises after I welded so I guess it had cracked behind the weld while cooling down.
To be honest I hadn't been too careful about distortion and it had distorted. Also the braze seemed to be cooling too quickly - I couldn't get it to flow very well. Might be worth trying it hot.
I'm getting there - I needed a wider root gap (about 1.5 to 2mm between the bits of casting. Still the braze is only sticking in places, so I'm guessing I'm going to have to either turn up the power or heat up the cast iron. Fun for tomorrow.
For fun I tried some mild steel wire - it went very well, although there were a lot of tinkling sounds as the metal cooled :lol: Still a lot stronger than my brazing attempts.
My welding book (Practical Welding) suggests a few filler options. For gas: brazing or cast rod. For stick: cast, monel metal (whatever that is); nickel/iron mix; mild steel; bronze (again). I gather that nickel is best.
Brazing they reckon can be done cold (I guess because it's soft and won't stress the cast iron too much when it cools. Certainly there isn't too much tinkling on cooling so I can believe that. Just need to get it to stick.)
For the mild steel option they prefer preheating to 600 degrees plus or peening the weld like mad while it cools to reduce contraction stresses. With the amount of tinkling on cooling it does seem that welding cold will cause a whole load of cracks.
I've only got a MIG and the work I'm doing is not stress bearing. Thought I'd mess around and see what works. I can only learn. At least I'll keep warm while pre-heating
I,ve got a carbon arc torch but never tried it yet, has anyone used one? you can pre- heat with these so might be useful for some jobs, e.g. cast iron. what filler rods would I use?
It's apparently possible to weld cast iron nearly cold (60 degrees C) with a stick welder using Nickel rods. They are sold in the USA as "cold cast iron" rods. My book suggests the weld might need to be peened (tapped with the round end of a hammer) as it cools to reduce the contraction stress.