Soldering Aluminum to Steel

This is kind of a product review, kind of a "how to", and kind of a "how not to".
But mostly, it's a request for the RCM brain trust to review what I've
done and figured out, and to straighten me up if I'm going in the wrong direction.
Quite a while ago I got a Harris "Al-Solder 500" kit from the local welding shop. This has some 85-15 tin/zinc solder, some Harris "Stay- Clean" flux, and a rather alarmingly long MSDS (short story: don't breath the white smoke).
I got this for the purposes of soldering aluminum ferrules onto steel music wire, the purpose of the ferrule being to provide a good bonding surface to a carbon-fiber arrow shaft. With that, I can make pushrods for control line airplanes that are more rigid and lighter than ones made all of music wire.
So, partially from inertia, and partially because of the warnings on the MSDS, I've been making the ferrules from brass because I can solder them with plumbers solder relatively free from worry.
Today, I went and made some ferrules out of aluminum, and gave the soldering a whirl.
The first one I tried was completely unsuccessful. The directions implied that the soldering could be done with a torch at the same time they mentioned that the flux became inactive at barely above solder melting temperature. I tried it, with a torch, and things failed miserably. So miserably, in fact, that I've got a couple of pushrod ends sitting on my bench right now with brass ferrules.
After I got the replacement ferrules soldered, and as I was cleaning up the mess, I decided that since the soldering iron was hot anyway, and since the solder is supposed to work at 450F, I'd try it on the stuff. So I slapped some flux on the ferrule, put the iron to it, and melted on some solder. It worked! Rather like magic -- I didn't have to scrape on anything or do anything special other than not snort the smoke (and it is evil-looking: there's something about dense, pure-white smoke that puts me off).
Then once I had a ferrule with solder on it, I took some music wire, sanded it, and stuck it into the hole with the iron on it. This didn't make a good bond -- a light whack with a center punch and a piece of fir 1x2 and the music wire came right out. Crud.
But I saw that the bond to the music wire was pretty crappy, so I slathered flux on the wire, put things together, and tried again. This time I (probably unnecessarily) made sure to pull the wire out of the ferrule a bit, to make sure I had solder on it, then pushed it back in.
This second time was much stronger: I did manage to whack the wire out of the ferrule, but I had to give it a pretty good thump: I'm sure that the impact I gave it was way more than I'd see in normal use, and if it happened in a crash there'd be a lot more damage than one cruddy little solder joint.
So (goodness this is taking me a long time to say), it looks like the proper technique with this stuff is to treat it like regular solder and flux, with the possible exception of being very careful about the fumes, and (also possibly) with the exception that I shouldn't expect the solder to wick into joints with the enthusiasm that tin/lead solder does.
Does that sound about right?
--
My liberal friends think I'm a conservative kook.
My conservative friends think I'm a liberal kook.
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My experience with soldering music wire is that you *must* abrade the oxide layer off the surface before soldering with zinc chloride acid flux (tinners' flux), that flux alone won't work. I use wet-dry sandpaper wet to do the abrading.
I would follow Ed, and pre-tin the music wire before soldering the wire to the aluminum ferrule.
Joe Gwinn
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On Sun, 01 Apr 2012 15:49:26 -0400, Joseph Gwinn

I saw something interesting when I was trying to find out for Tim whether you could use tin-zinc solder with acid flux. It was the claim that the high-zinc (40%) tin-zinc solder is abrasive (zinc crystals) enough that you can scratch-in the solder, mechanically fluxing it, just by rubbing the solder on the work as you tin it.
I sometimes have used stainless brushes to mechanically flux aluminum or steel, but I have little experience with tin-zinc solders. I'll have to give it a try sometime.
Anyway, abrading the oxide (iron oxide, in the case of music wire; it's straight high-carbon steel) makes good sense. Steel can be coaxed to wet well, but sometimes it takes a little extra mechanical fluxing to get it started. Just rubbing the tip of a soldering iron on it often is enough.
--
Ed Huntress

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On Sun, 01 Apr 2012 16:52:46 -0400, Ed Huntress wrote:

Amen to that: I learned to solder wires (copper) to aluminum by putting a drop of machine oil on the aluminum and scratching the heck out of it under the oil layer, to expose clean Al metal. The usual tin/lead solder wets it just dandy.
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On Sun, 01 Apr 2012 15:49:26 -0400, Joseph Gwinn wrote:

Yea, I left out the part about sanding the music wire -- because one always needs to, as you mention...
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Tim Wescott
Control system and signal processing consulting
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