"Silver solder" questions

[graham-xrf]

Apparently, the term is discouraged. One should be saying "silver brazing" instead.
The aim is to get a very low electrical resistance joint on 22mm copper tubing that might carry high currents (200A).
The recommended stuff is something (probably tin) with 55% silver in it.
I understand that it melts at 650°C , which is 1202°F. That would be sort of reddish hot.
One hopes the butane can torch normally used for copper plumbing joints with tin/lead solder will get it there, but I can augment it with a butane gas ring if need be.

Is Borax OK to use as a flux? I have a tub of Borax.

 

[RJSakowski]

I had occasion to take apart some silver brazed copper plumbing. I used a MAPP gas torch and had to build an improvised oven with firebrick to do the job. The problem with larger pieces of copper is that it sucks the heat away so efficiently. I have used the same torch for brass brazing though.

For the silver soldering that I have done, I used a flux designed for it. I have used borax for forge brazing of iron with brass brazing rod but I don't know if it would work with silver braze.

 

[JPMacG]

Silver solder used to mean silver brazing. Then plumbers stopped using tin-lead solder and started using solder without lead but with a small amount of silver and everyone started calling that silver solder. So now "silver soldering" could mean silver brazing or soft soldering with low-temperature solder containg a small amount of silver.

Solder is generally not a great conductor - it has high resistance compared to copper. In order to minimize the resistance of the solder joint you would want to keep the film of solder as thin as possible and make the surface area of the joint as large as possible.

Interesting thing about metal alloys - the alloy of two good conductors can be a poor conductor. So I would not count on silver brazing being a good conductor just because silver is a good conductor. You would need to look up the conductivity of the particular alloy.

 

[graham-xrf]

Silver solder used to mean silver brazing. Then plumbers stopped using tin-lead solder and started using solder without lead but with a small amount of silver and everyone started calling that silver solder. So now "silver soldering" could mean silver brazing or soft soldering with low-temperature solder containg a small amount of silver.

Solder is generally not a great conductor - it has high resistance compared to copper. In order to minimize the resistance of the solder joint you would want to keep the film of solder as thin as possible and make the surface area of the joint as large as possible.

Interesting thing about metal alloys - the alloy of two good conductors can be a poor conductor. So I would not count on silver brazing being a good conductor just because silver is a good conductor. You would need to look up the conductivity of the particular alloy.

Yes - solder is not a great conductor when it is a low melt temperature tin/lead combination, like used for soldering electrical circuits. Now, we go lead-free, where the alloy is about 3% silver, and 0.5% copper, with the rest being tin. Some include zinc or antimony to broaden the range between solid and melted, but all these melt between 200°C and 240°C

The 55% silver braze, is altogether different. Much higher temperature required, much different strength joint, and with that much silver with tin, also a high conductivity.

 

[graham-xrf]

I had occasion to take apart some silver brazed copper plumbing. I used a MAPP gas torch and had to build an improvised oven with firebrick to do the job. The problem with larger pieces of copper is that it sucks the heat away so efficiently. I have used the same torch for brass brazing though.

For the silver soldering that I have done, I used a flux designed for it. I have used borax for forge brazing of iron with brass brazing rod but I don't know if it would work with silver braze.

I am happy with the conductivity of the 55% silver braze onto the copper. Those little rods are quite expensive, but I won't need much.
I searched on silver solder fluxes, and it seems they are borax based.
One recipe I found is to use one part borax, + 1 part potash + 1/2 part table salt, used dry, or with just enough water to make a paste.
A variation if one does not have potash available is to just leave it out. Apparently, the flux still works.

I have never silver soldered before. I know it is used for brazing steel, but I have no idea how it goes on copper.
Your point about the copper soaking the heat away fast is exactly what I feared.
It's easy enough to get the end of the rod to melt in the blue flame. I am thinking getting the end of the copper pipe up to that temperature may be a different matter! I may be getting in some MAPP gas now.

Can one MIG weld copper with copper wire electrode? (Asked in ignorance - please forgive if it's a stupid notion).

 

[jwmelvin]

One can TIG copper easily enough but I don’t know about MIG.

 

[graham-xrf]

One can TIG copper easily enough but I don’t know about MIG.

That is great to know!
My new welder is still in storage, and temporarily unreachable. I can hardly wait to get to play with it! It does do "Lift TIG", as well as MIG etc.

One thing I do need to invest in is the TIG torch, the gas tube, and some "get started" consumables. At this stage, there is a whole lot I need to know before I end up buying stuff I will regret. It requires a type "17V" torch with a gas valve in the handle.
Also, I would be needing a bottle of pure Argon. The bottle I do have has 5% CO2 in the Argon, intended for MIG.

 

[WaneB]

I have used jewelry silver solder for soldering sterling (.925%), and fine silver (.999%). The solder I used had hard, medium, and easy varieties, hard melting at the highest temperature, then medium, then easy. These were expensive, kind of like sterling expensive, but you don’t need much. I have used propane, acetylene air, and oxy acetylene torches successfully. I used boric acid powder mixed with alcohol as a flux. I also have used white-out or yellow ochre where I didn’t want solder to flow.
I have also joined copper, brass, and occasionally steel with the same materials. I once made a t-handled nut driver by cutting off the plastic handle and silver soldering a steel rod to make the “T” part. Turned out to be an incredibly strong joint.

 

[RJSakowski]

Ersin used to make a soft solder for electronic purposes with 2% silver under the trade name Multicore SavBit. The silver content successfully prevented erosion of the soldering tip. Composition was 62% Sn, 36% Pb, and 2% Ag. Currently, they market a product with the silver replaced with copper although the silver bearing alloy is still available through Newark/Farnell. For many years, my soldering iron was in 24/7/365 at a fairly high temperature and tips lasted for months.

Alpha Metals and Kester also make a 62/36/2 alloy.

 

[jwmelvin]

Oh one thing to keep in mind with copper TIG is that it’s the most current-hungry material (due to the thermal conductivity) so machines quickly run out of oomph as the thickness increases. I only did it briefly as an experiment; my machine is 250A.