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Looking at wikipedia on resistivity I see that, other than gold, aluminium is a close second to Copper.

Do any transformers use aluminium windings ? I'm guessing they would be less expensive to come by...

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Yes. Aluminium is used for transformer windings and for other wiring use. As you note, Aluminium has higher resistivity to Copper so needs a larger wire size than Copper to achieve the same number of turns OR in a given winding space Aluminium windings will have a larger resistance than if Copper is used.

Aluminium generally takes more care in construction than copper, but when properly implemented no significant problems are caused by using Al rsther than Cu.

Where winding volume is not an issue or where the higher winding resistance is acceptable, Aluminium can be substantially cheaper than Copper for the same application.

Here is an excellent discussion of the pros and cons. This goes into the various differences, their effects on design and how the effects can be made irrelevant or insignificant by good design.

They note that

  • In North America, aluminum is the predominant choice of winding material for low-voltage, dry-type transformers larger than 15 kilovolt-amperes(kVA). In most other areas of the world, copper is the predominant winding material. The primary reason for choosing aluminum windings is its lower initial cost. The cost of copper base metal has historically proven to be much more volatile than the cost of aluminum, so that the purchase price of copper conductor generally is the most expensive choice. Also, because aluminum has greater malleability and is easier to weld, it is the lower-cost manufacturing choice. However, reliable aluminum connections require more discipline and expertise on the part of transformer installers than that needed for copper connections.

They also provide this comparison table: (reformatted)

Table 1: Common Reasons for Winding Material Choice for Low Voltage Dry-Type Transformers

  • FALSE: Aluminum-wound transformer terminations are incompatible with copper line and load cables.

  • TRUE: Properly terminating line and load connections is more difficult for aluminum-wound transformers.

  • FALSE: Line and load connections to copper-wound transformers are more reliable than those to aluminum-wound transformers.

  • TRUE: Aluminum wound transformers are lighter in weight than copper wound equivalents.

  • FALSE: Copper-wound low voltage transformers are better for "high-impact" loads because copper has higher tensile strength than aluminum.

  • FALSE: Aluminum-wound transformers have higher losses because copper is a better conductor.

  • FALSE: Aluminum-wound transformers have higher hot-spot temperatures because copper is a better thermal conductor than aluminum.

Some of those need to be thought about. While Al is a notionally inferior material cost and resistiovity wise, the effects of the differences can in many cases be eliminated by proper design.

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    \$\begingroup\$ What about aluminum oxide forming (over time) on the connection points, causing higher resistance and potentially heating and failure? I've heard of this being a problem in, say, old housing built with Al wire in WWII when copper was in shortage. What about in general usage, does Al cause reliability problems over the long term compared with copper? \$\endgroup\$ – Matt B. Sep 16 '11 at 6:54
  • \$\begingroup\$ Erm. The question should have been more circumspect ; I failed to mention that I had hobbyist/low rating (max 1KW) transformers in mind ... (+: \$\endgroup\$ – Everyone Sep 16 '11 at 10:31
  • \$\begingroup\$ @MattB.: Aluminum oxide forms on bare aluminum immediately when it touches air, so any effects of oxide have already happened when the connection is being made. I don't think the oxide can have any effect afterward. It may slowly grow in thickness, but it's already there. \$\endgroup\$ – endolith Sep 16 '11 at 18:02
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Yes, as Russell says plenty of distribution transformers use aluminium.

However, as far as I know most low power transformers use copper windings still. I had a quick search on Farnell, Mouser, etc, and could not find any aluminium wound transformers or aluminium magnet wire.

As long as it's properties are taken into account though then there should be very little or no difference in performance for any application, apart from an aluminium transformer of the same rating might be a bit bigger. I would think any concerns would be more to do with things like ensuring reliable Cu/Al joints and different mechanical properties (expansion coefficient, embrittlement of Al, etc) rather than performance.

You can even get silver wound transformers too, popular in high end audio gear along with things like oxygen free/silver plated power cables and gold wiring. Yep, really - gold wiring :-)

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    \$\begingroup\$ I don't think any audiophiles use gold wiring, as gold is actually a pretty terrible conductor. They do use a lot of gold-plated stuff, though. Also, lots of snake-oil. \$\endgroup\$ – Connor Wolf Jun 23 '12 at 3:47
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    \$\begingroup\$ Yes, I doubt any technically minded audiophiles do, but with the general audiophile market (probably the other 95%) the technical rarely comes into it ;-) I added a couple of links. It's crazy how much the audio market is snake oil driven (I'm also a musician with a fair amount of gigging/recording experience and I've come across some pretty bizarre stuff over the years) \$\endgroup\$ – Oli Glaser Jun 23 '12 at 4:41
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As you know gold has a low tarnish/oxidation rate unlike copper and especially aluminum which the problem has been spoken of here, that's why gold plating has become commonly used. Low voltage audio connections work better when they are clean and so Audiophiles started using it and it has become an industry standard. Aluminum wiring can work well but needs to be properly crimped at the terminal as proper crimping causes an air tight welding of the two materials.

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    \$\begingroup\$ I have no clue what any of this has to do with the question. \$\endgroup\$ – Ignacio Vazquez-Abrams Mar 12 '14 at 4:19

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