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...
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
They also provide this comparison table: (reformatted)
Table 1: Common Reasons for Winding Material Choice for Low Voltage Dry-Type Transformers
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.
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.