I'm not sure if this is right.. just of the top of my head.. I think what you're confusing is the term "fusion" for phase change with the term "fusion" as in collision of two nuclei that join together.. I guess another word for it is "nuclear fusion".
The second one is where there is no elasticity.. if there were, the two nuclei would "bounce" of each other and wouldn't actually join together. Easy way to look at it is throwing a ball at the wall - it bounces of, so the collision is elastic. Throw play doh at the wall and it will stick, that collision wouldn't be elastic just as in the "nuclear fusion" case.
