assembly vs weldment

assembly and weldment both are nouns.

Word	Noun	Adjective	Verb	Adverb
assembly	Yes	No	No	No
weldment	Yes	No	No	No

As nouns, weldment is a hyponym of assembly; that is, weldment is a word with a more specific, narrower meaning than assembly:

assembly: a group of machine parts that fit together to form a self-contained unit
weldment: an assembly of parts welded together

Other hyponyms of assembly include sub-assembly.

assembly (noun)	weldment (noun)
the social act of assembling	an assembly of parts welded together
the act of constructing something (as a piece of machinery)
a group of machine parts that fit together to form a self-contained unit
a public facility to meet for open discussion
a unit consisting of components that have been fitted together
a group of persons who are gathered together for a common purpose