Wales coach Phil Davies has drafted in Llanelli outside half Steve Shingler to play at No 10 and has also handed a first start to lock Joel Galley.
"We were unable to consider Lloyd Peers, Ben John and Rhodri Jones for this match due to injuries, while James Thomas is suffering from a slight dead leg and has been placed on the bench," said Davies.
"We hope to have everyone fit for our final game on Monday, when we play either France or Fiji. We know that Argentina will pose a big physical threat up front and kick the ball a lot.
"But it is a game that all the players are really looking forward to and is a massive opportunity for guys like Shingler and Galley to express themselves on the big stage. I've been extremely pleased with the attitude of the squad throughout the tournament - they have shown great professionalism and an ability to compete well on the world stage."
France and Fiji clash in the other semi-final immediately after the Wales game with the winners meeting to settle 5th place next week.
Wales Under 20: Dan Fish; Kristian Phillips, Owen Williams, Scott Williams, Adam Hughes; Steven Shingler, Rhys Downes; Dan Watchurst (captain), Ieuan Davies, Will Taylor, Joel Galley, James King, Edward Siggery, Josh Navidi, Morgan Allen
Replacements: Rhys Williams, Joe Rees, James Thomas, Rhys Jenkins, Gareth Davies, Ashley Beck, Matthew Jarvis