Welding/brazing copper problems

I have to join two sheets of copper. Lapped over eachother. Think of a book lying on another book. Bottom book is 6x9. Top on 5x7. They have a 3.5" square hole cut through both. 1/4" thick each.

I brazed the outside together with silver solder and borax type flux, using a rosebud oxy acetylene torch.

Inside absolutely would not go together. Get it hot, melt the rod. It just sits on top. So clean it. reapply flux and do it again. Nothing. Grind to clean metal, endmill, etc. No difference. Bigger torch tip, smaller tip. Nothing.

USed the tig torch. Nothing.

Tried some unkown brazing rod with white flux. No luck

Any thoughts???

I would try to tin each piece separately with silver solder first. I don't know the proper name of the flux, but I use a black flux from member Tom Walz. At temperature the flux isn't active very long so you have to move fast. With copper this will take a lot of heat but I would think a normal rosebud would be large enough.

The phosphorus in regular brazing rod is self fluxing. Ive never used acetylene but with propane, the propane is its own shielding gas and you should see bright copper. Move the flame away and the copper oxidizes

Mebfab said:

1/4" thick each.

Click to expand...

There's your challenge.

Copper that thick normally must be "furnace" brazed. Which works a treat, thankfully.

A "rosebud" is only going to give you contamination and warpage well before it gets to the otherwise rather modest temps SilFlo or such needs.

FWIW - by way of illustration:

1/4" is right ABOUT the thickness of the copper liner a certain British "Banker's Anti-Arc" Diamond safe of my acquaintance, mid-1970's used.

It was enough to transfer away heat fast enough to defeat any torch attack short of a burning-bar (Oxygen lance).

A half-inch of a plastic - specifically formulated with highly lethal combustion byproducts - dealt with the poor sods who DID try an Oxygen lance.

Safes of that sort had "license to kill" - at least as to attacks by "amateurs" - back in the day.

Mebfab said:

I have to join two sheets of copper. Lapped over eachother. Think of a book lying on another book. Bottom book is 6x9. Top on 5x7. They have a 3.5" square hole cut through both. 1/4" thick each.

I brazed the outside together with silver solder and borax type flux, using a rosebud oxy acetylene torch.

Inside absolutely would not go together. Get it hot, melt the rod. It just sits on top. So clean it. reapply flux and do it again. Nothing. Grind to clean metal, endmill, etc. No difference. Bigger torch tip, smaller tip. Nothing.

USed the tig torch. Nothing.

Tried some unkown brazing rod with white flux. No luck

Any thoughts???

Click to expand...

How big is your TIG?
Copper has twice the thermal conductivity of aluminium so I would start somewhere around 60amps/millimeter
For 1/4" copper I wouldn't even try anything smaller than 300amps
TIG brazing with silver braze is possible in theory but you have to be carefull what sort braze alloy you have as most of them contain zinc. Probably better just weld or weld/braze with phosphorous copper if you try TIG.

If you are using cadmium free silver solder and flux, that could be a problem.

I've found using that crap even on normal type jobs that they are vastly inferior to the old cadmium content products and the flux will form an oxide very easily if the part gets too hot.

gbent's suggestion is good; if you have some scraps, try that and ( if it would be acceptable on the finished item ) drill some holes (3/16" ) through the top sheet and see if the solder will wick in through the holes.

Somehow you'll have to clamp the sheets without the clamping becoming a big heat sink.

I have had success with 1/4" copper using pure copper tig rod, and, as mentioned, a LOT of amps, my little machine tops out around 300 amps, barely enough for 1/4" copper, my bigger machine goes more like 400, thats more in the zone for what you want to do.
and the copper gets very very hot while welding- I use the biggest gloves I have, use steel heat shields and standoffs to rest my hand on, and cant do very much welding before the workpiece is so hot you cant keep your gloved hand within six inches of it.

you need lots more heat.
copper wicks it up.

+1 on Tigging it if you have enough machine, 300 amps would be acceptable. I've used a 50/50 Ar/He blend for the gas, gave it a bit more punch like it does for aluminium. With a bottle of 50/50 and some preheat you would have a decent chance.