It is in my opinion, in this case, as tazmannusa stated, the problem of an electrolytic cell since we are talking about, I assume, manufacturing contamination.
If it were weldment related, since the contaminating Cu can find its way into weldments, the Cu forms Theta phase with Al to provide the strengthening intermetallic for generally 2000 Series Al alloys. Too much Theta phase and the alloy becomes more and more difficult to weld, even to the point, as Al said, Cu insolubility.
Also, I would think that the existence of other alloying elements, Mg, for example, since I do not believe we are talking about 2000 Series alloys, will create more complicated and problematic metallurgies. Though I'd have to look this one up.
Hello js55;
I thought about the galvanic couple, but then I didn't see that this was going to be used in a wetted environment necessary to provide act as an electrolyte. It was mentioned this was for an application involving tooling, which I assume is in a dry environment. Silly me.
I also assumed that the nature of the contamination was surface contamination due to fretting, dragging the copper across the aluminum surface, etc. It was my thought that some of the copper on the surface could be consumed during welding and manifest itself as a contaminant due its excess in the weld puddle. Am I on the right track in assuming the combination would most likely be an intermetallic which would be brittle and prone to cracking upon cooling to ambient temperatures?
Best regards - Al
Al,
"Silly me." :)
"Am I on the right track in assuming the combination would most likely be an intermetallic which would be brittle and prone to cracking upon cooling to ambient temperatures?"
I think so. The intermetallic would be in all likelyhood Theta. And, although not being an Al (all these Al's are getting confusing) metallurgist, far from it, it seems to me the Theta design for 2000 Al alloys is very carefully considered. The mix from contamination would indeed be embrittling since it cold create volume percents in areas far in excess of Cu's solubility. IMO.
Your opinion and knowledge is always welcomed as a valued resource from a good friend.
I'm in Houston this week. The summers are not any cooler than the last time I visited. It was 52 degrees when I left New England this morning. I'm afraid to look at a thermometer down here!.
Best regards - Al
Al,
If I was still in Houston I would have bought you dinner. There were still many places there I never got the chance to try.
And no, it ain't cool in the summer. Houston essentially has two seasons. Warm and hot. The warm lasting maybe 5 days.
Of course, I lived on the north side so we were a 1/2 degree or so cooler than the south side. :)
By 803056 
Date 07-13-2009 21:41
Edited 07-13-2009 21:44
Hello js;
It figures you're on the road while I'm visiting the "Great State of Texas". I was looking forward to sharing a table with you at one of the many fine establishments near the Johnson Space Center.
Hey, I may not be a rocket scientist, but who says I can't preach the "Gospel of Welding" to them?
It reminds me of the last time I was in Minnesota, I missed their summer by one day. I arrived in Minneapolis/St Paul on July 5Th.
Best regards - Al
The copper in the 2xxx alloys does lead to "intergranular corosion", a galvanic cell set up within the material. This is an issue in aircraft parts, and the reason You don't find 2xxx alloys in marine use.
I think too much copper concentrated in the grain boundries causing cracks while/after cooling is the more likely problem with the 6061 part in question.
I Know I,m joining late, this interests me. 
