Aluminium wire became popular in the 1970s because it conducts electricity well and is less expensive than copper. It quickly lost its popularity went it was deemed responsible for a large volume of home electrical fires. The problem was that the aluminium connections expanded and contracted much greater than copper. This slowly separated the connections creating an electrical short. Solutions to the problem were developed, but aluminium wire never became popular again. Properly joining aluminium and copper wire reduces the chances of a separation of the connection.
Turn the power off to the circuit you are working on at the distribution panel. Verify the power is off by touching a non-contact voltage detector to the wire's insulation.
Strip 3/8-inch of insulation from the aluminium and copper wire with wire strippers. Make sure the copper wire has no oxidation on the conductor. Oxidation changes the colour of copper to white.
Apply a small amount of antioxidant cream to the aluminium wire and wipe off any excess. Twist the two wires together tightly with pliers
Squeeze aluminium wire antioxidant cream into the twist-on wire connector. Install the connector on top of the two wires.
Only use a twist-on wire connector that is rated for an aluminium to copper connection and the gauge of wire you are using.