2019 September 2     Metal Shop     Brazing & Soldering


Introduction


A reader asked about silver brazing copper cables, instead of soldering.  Can you use MAPP gas?  What type of filler metal to use?

He said he only wants to have to do this once.  So let's see how to go about that.


Warning    Metalworking can be dangerous.  Before trying to weld or braze, make sure you're familiar with the potential hazards.  The quality of result depends in large part on your level of skill and experience.  Please read the Disclaimer.



In This Article

The Silver Braze

The Flux

The Heating

Conclusion



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The Silver Braze


Medium silver braze wire, often called "medium silver solder", is 70% Ag.  This tends to be offered in very thin wire, too thin for "heavy duty" work. 

If you're just going to braze copper to copper, as with battery cables, it's possible to use this or this.  These sil-phos alloys are made to be used without flux.  They're only for copper-to-copper brazing. 

Harris Safety Silv 45 (available here) is normally for copper-to-steel or copper-to-brass brazing, but it will work with copper-to-copper.  The main reason why it's not recommended is because it costs more.  Also, in some HVAC copper-to-copper joints, they don't like to use flux;  that's why they use the sil-phos compositions.  But if you use proper technique, the 45% will work with most types of metals that can be brazed.  I'd actually just use this type of braze, not sil-phos, because I don't want to buy something that could only be used on copper-to-copper.  It just seems like there are many more times when two different metals have to be stuck together.

Harris Safety-Silv 56 is another option, but I haven't tried that.  It's supposed to flow very well.  56% seems to be the highest percentage made by the big welding-brazing supply companies.  Oddly enough, the 56 has a lower flow temperature than the 45.


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The Flux


Before you even apply flux, the metal should be clean.  Remove dirt, grease, oils, and tarnish.

Don't use regular borax for silver brazing. 

Harris makes a white flux and a black flux for brazing.  Either of these should work for this.  The black flux is good when there's a lot of oxidation. 

Don't use huge amounts of flux.  Just brush on enough to make a thin coating.  The flux should be brushed on to the clean metal before heating. 


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The Heating


The 45% braze melts and flows above 1300 F.  An air-acetylene torch would be OK for this.  You can use MAPP (MAP Pro) gas for this, with a torch such as this one.  Use some fire brick to keep the heat concentrated.  And just remember, those bricks get HOT and stay that way for quite a while.

Clamp the cable a few inches back from the bare copper end.  For brazing I'd heat the copper lug a slight distance from the joint.  (Otherwise you may start melting the cable insulation...)  There has to be enough heat soaked in that the braze will flow in by capillary action. 

If the joint is very hot, but the metal a few millimeters away is cooler, it could solidify before it gets in there.  Brazing is a bit more tricky than soldering, because of the higher melting point.

Once the flux turns clear and quiet, it's at or near the melting temperature of the filler metal;  go ahead and apply the braze rod.  The metal should flow in by capillary action.  This will happen only if the copper lug is hot enough.  Too cold, and it weakens the joint;  too hot, and it can also weaken the joint.  This is why you should braze a few sample projects before you try something important. 


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Conclusion


If you do a silver braze correctly, the metal forms a shiny, thin joint that's very smooth.  If it didn't go quite right, it will look frosted or pitted and won't be as strong. 

So if you want to do this only once, your best approach is to braze a few things of similar type and size... then do the "good" project when you have built up some experience.  A lot of it has to do with the heating method you use.  There is a learning curve for any of them. 


This was the quick version of "how to braze heavy wires and cables". 

See also the soldering article.


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