gent going by the name of buter has some copper to steel welding featured in St Brelades wave in the projects section-I know that may not be directly related to your question but it may be of interest.
Warning the piece gets extremely (insert expletive) hot and turns black. you remove the black oxide with brick stain remover i.e. hydrochloric acid or phosphoric acid.
Its really easy to weld. You can also easily weld copper pipe to stainless pipe
thanks to all who posted on my thread you have given me some top tips on this cant wait to give it a go now just need some time to get the tig set up some where