Hi Mansoo
This obviously changes the picture!
From what you are saying, it sound like you have hot cracking taking place. If you used Monel filler, hot cracking problems become typical when you have 6% or more Cr in your weld bead. Assuming a 20% dilution from the 304, you should end up with around 4% of Cr in your weld pool. This should be OK. If however you have a much higher dilution, say 30%, then you will be at the limit. This may be the problem if you are using Monel filler. This problem will go away if you use a Nickel filler.
With Ni fillers, hot cracking can be a problem if you have relatively high levels of S or P contamination. It is possible that the Cu has P as an alloying element. This may be another reason for cracking.
I would proceed as follows:
1) Weld a "buttering" layer of Ni onto the Cu plate. (Use low dilution welding techniques.) If you experience cracking, then you know that the problem is P contamination.
2) If you have no cracking problem, weld the 304 tube onto the buttering layer, again with Ni filler.
3) If you had cracking problems following step 1, grind the affected area till the cracks are out, and again follow step 1. Following 2 or 3 times of repeating this step, the cracking should cease because you will gradually be removing the P from the weld pool.
Hope this helps.
Regards
Niekie Jooste