[ossiblue]

Copper is very malleable when heated, which is something you may wish to consider, but no matter what method you use, the metal has already been stretched and it will need to be shrunk to get a smooth flat side--if that's how far you want to go. There also appears to be a finish on the copper as well as a crease. Heating will ruin the finish, and you may not ever get rid of the crease completely, so that may be the decider in using heat. Heat and that large pipe might do the trick, however.