I recently de-potted a 2x F40T12 residential ballast. I then soldered wires onto the windings so I could figure out what everything was. In the end I did get it working outside of the potting, but to do this I had to solder to the aluminum windings to do that. This is how I did it:

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• Scrape enamel off of the wire, exposing bare aluminum all around the wire
• Flux the wire and put a big drop of solder on the soldering iron
• Apply the solder while scratching the wire with the soldering iron tip until all areas of the bare wire have been scratched and now have solder adhering to them
• Continue to solder as normal, acting like this wire is tinned copper when in reality it is tinned aluminum

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Now, those steps do indeed work, but still present the following annoying challenges:

• After doing this multiple times, the solder on your iron will contain dissolved aluminum that will raise the melting temperature of your solder to above the temperature of the iron, causing it to solidify on the hot iron
• If you don't scratch the wire enough with the iron, the solder could just pop right off under any strain

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So, I know my technique is probably not acceptable for long-term reliability. The question is:

How do you do this? Do you just crimp the wire and call it a day, or is there some sort of easily-accessible electronics solder + flux for aluminum?

Good question. I actually have non-electronics suitable aluminium solder, but that stuff has flux that is super aggressive. You can fairly easily solder copper wire to aluminium substrates with it, but i am unsure of the long term effects of the flux (which might get wicked into the copper stranded wire).

One commonly quoted way is to sand the aluminium while being continuously soaked in paraffin oil/kerosine/similar. As long as the wire is coated with kerosine it can't form the oxyde layer that prevents soldering. This theoretically should work but my succes is limited. The kerosine will vaporise while soldering and stink, so ensure good fume extraction.

It would be better to copper plate the aluminium galvanically, and then just solder to the copper plating. But in the end it's a big hassle.

@Laurens
I saw a video of this being done, I tried it with mineral oil and had zero success. Maybe it only works with certain aluminum alloys and not others, IDK.

I know that copper-clad aluminum wire does exist, which means that this is possible, but I don't know how they get the copper to adhere. If i were to take a wild guess, doing this at home is almost certainly not possible because I don't think conventional electroplating techniques would really work on aluminum. Though maybe someone else could figure it out.

This is a big bloody problem. For thick (1mm+) wires I use aluminium-rated gel-filled clamp or push-in connectors. Special technique is also required. Thin Al wires are just PITA. I'd try remove the enamel carefully and make a long (~2cm) and tight twist connection with a clean copper wire. That works most of a time though long-term reliability is questionable.

@RRK
Yes, for larger wires I have seen connectors rated for that with special instructions.

Maybe it isn't worth it trying to bond solder to the wire. Maybe you could twist the aluminum and copper together and then wrap fine copper wire around it. This way when it is soldered there should be a strong mechanical (and hopefully electrical) encapsulation of solder around the aluminum even though it almost certainly isn't bonding. I feel like that would work.