butt joint vs butt-weld

butt joint and butt-weld both are nouns.

Word	Noun	Adjective	Verb	Adverb
butt joint	Yes	No	No	No
butt-weld	Yes	No	Yes	No

As nouns, butt-weld is a hyponym of butt joint; that is, butt-weld is a word with a more specific, narrower meaning than butt joint:

Other hyponyms of butt joint include butt weld, fish joint.

butt joint (noun)	butt-weld (noun)
a joint made by fastening ends together without overlapping	a butt joint that is welded

butt joint (verb)	butt-weld (verb)
	join by a butt weld