roysallis, are you talking about the warranty on the tractor, the loader, or the bucket itself? I don't know of any effect welding or drilling on the bucket would have on any warranty, but I would assume there are ways you could damage, bend, or break the boom arms, hoses, cylinders, bucket, etc. that would be considered abuse and not be covered by the warranty.
What I actually did was drill two holes in the top of the bucket at either end and mounted my chain hooks with u-bolts (washers and nuts on both top and bottom of the u-bolts) so the hooks lay back on the top of the bucket when not in use. Then I welded a rectangular half inch steel plate on either end of the bucket so they stick up above the bucket, with a big hole through each to slide a pipe through to which I mount my fork lift forks when I use them. I also drilled one hole in the top of the bucket in the center, and bolt on a tow hook that holds and supports the center of the pipe when using the forks.
Now improper use with the forks could certainly damage something that I wouldn't expect to be covered by any warranty.
Bird