Can I solder high power 1W LEDS on aluminum with 60/40 solder?
In short, no. Aluminum needs a special kind of solder because it doesn't tin like many other metals with normal solder. Aluminum oxide forms very fast when aluminum is exposed to oxygen, and aluminum oxide doesn't stick to normal solders. 

Aluminum solder melts at such a high temperature, that you'll probably burn your LEDs out before you ever attach the LEDs to the heat-sink.

As Phil states, it's better to use a thermal adhesive or use a thermal paste and then mechanically clamp the items together.