merge vs weld together

merge

verb
  • To combine into a whole. 

  • To blend gradually into something else. 

noun
  • The joining together of multiple sources. 

weld together

verb
  • Used other than figuratively or idiomatically: see weld, together. 

  • To forge a social or emotional bond between people. 

How often have the words merge and weld together occurred in a corpus of books? (source: Google Ngram Viewer )