Flux and no flux brazing

[SkyWalker]

hi guys, I have a few questions which I am unable to get consistent answers on, hopefully you guys can help.

when brazing copper to copper using 2% silver content we don't need to use any flux, yet if we braze copper to brass using 40% silver content sticks we do?

why? is it to do with the silver? or the dissimilar metal? if so at what % silver should we start using flux?

last one...

when using flux powder how do you guys apply it? do you mix it into a paste apply to pipe then push into fitting?

or

do you guys heat up brazing sticks and dip the rod into the powder, powder sticks to rod to make a pre fluxed rod and we don't apply anything to the pipe?

I have seen both, which is correct if either?

thanks guys any educational reply much appreciated

[monkey spanners]

The 2% rods which i use on copper to copper, the 5% rods that i use for braze in driers, and the 15% rods that i use for tevs and compressor stubs are ok without flux as all these components are either copper/copper or copper/copper plated. I think these have another metal mixed in which the 40% rods don't which makes these self fluxing.

Its only with the 38% and 40% silver solder that you need flux. I either use the pre coated rods or heat the rod and dip it in the powder. I don't like the idea of mixing the flux with water to make a paste in a refrigeration environment.

Jon

[brunstar]

you should not need flux on copper to copper if the copper is clean and you are using 15%.
The reason flux needs to be used is the impurities in it.
The 40% will melt too easy and this is why it does not stick too well.
Next time you are doing copper to copper use 15% and also clean the copper with emery cloth.
this will work well every time.

[nike123]

I regularly use only two types of rods.
Copper to copper with simmilar thickness (copper pipe to copper fitting ) I braze with 5% phosphorus rod, and all others are brazed with 40% silver rod and borax as flux.
High percent of silver is needed for all connections where different rate of thermal expansion (pipe and big mass part for example) could make crack in brazed connection (because of silver elasticity) and where two dissimilar metals are connecting.
I dissolve borax in distilled water and apply it on connection after male part is placed inside of female part. I also dip hot rod in dry borax powder during brazing. At the end, I clean residual borax with wet rag while it is still hot, but not too hot to crack it with sudden cooling.

Flux is used to prevent oxidation on hot surfaces, and to promote fluidity. 5% phosphorus rod does not need flux because phosphor in it have same effect as flux. For copper surface cleaning, don't use emery cloth or paper. Use dedicated "rags" for copper soldering and don't touch cleaned part after cleaning, or use cotton gloves if you must touch it.

[Magoo]

Well described nike 123
alot of people forget the cleaning off of flux residue, after the welding process.

[RANGER1]

To the best of my knowledge copper to copper in refrigeration is 15% silver no flux .( In Australia anyway ) .
Anything below is probably for plumbers for water pipes .

40% silver content for soldering steel , brass , copper to steel / brass , using flux .
Nearly all heat aimed at brass or steel until solder runs freely on both surfaces .

[nike123]

To the best of my knowledge copper to copper in refrigeration is 15% silver no flux .( In Australia anyway ) .
Anything below is probably for plumbers for water pipes .

40% silver content for soldering steel , brass , copper to steel / brass , using flux .
Nearly all heat aimed at brass or steel until solder runs freely on both surfaces .


Open any of factory manufactured unit from big names (Mitsubishi, Daikin, Fujitsu etc...) and you will not find high Silver alloy nowhere except where I described in my post above that high Silver alloy should be.
These 2%, 5%, and 15% are Silver-Phosphor-Copper alloys, with that percentage of Silver in alloy for better fluidity and lower melting point, and Phosphor 5,9-6,5% as flux.

[nike123]

http://www.harrisproductsgroup.com/pdf/catalogs/brazing_tip_guide_2008_Internat.pdf