I believe that Ron,... who started welding the year I was born:) is essencially correct.
I've never seen solid copper fittings come loose due to expansion and contraction.
Having said that, I have seen plenty of work clamp connections become poor, where copper fibers are gripped by pressure begin to corrode and become brittle, when the connection becomes poor the electrical resistance (and heat) increase, the copper fibers become even more brittle and more of them break. Soon you have a much less efficient conductor and weld performance is affected. To an inexperienced worker this may appear like a loose connection.
How many times have you seen field service folks come in at a very high pay rate and change the work lead on a Robot or even a plain old MIG and fix a problem that was causing major rejects?