The way that an opponent could prevent this is by this:
Active Player A puts welder ability on the stack Targeting artifact A (in Play) and CoW(in graveyard): active player A passes priority.
Active Player B puts own welder ability on the stack targeting artifact A (in play) and artifact B (in graveyard). Active player b passes priority.
To sum it up, your opponent has switched out the original target for your welder making your exchange action fizzle (counter upon resolution).
Way to state information that was posted yesterday.
An it isn't countered on resolution, it just fails to do anything.
This thread is done.