Kevin McKernan and Paul McComiskey scored in each half to seal victory for Down at Pairc Esler, before Declan Carville scored a late consolation goal for Armagh.
Down will go on to play Monaghan in the final four after they were 1-12 to 0-07 winners over Fermanagh in Inniskeen.
Meanwhile Tyrone also progressed as they made it three wins out of three with a comfortable 1-18 to 0-9 victory over UUJ.
Derry and Antrim played out a thrilling 1-13 apiece draw at Celtic Park.
Donegal beat college side St Mary's by one point, 1-14 to 2-10.
Results in full:
Donegal 1-14 St. Mary's 2-10.
Monaghan 1-12 Fermanagh 0-7.
Down 2-11 Armagh 1-10.
Derry 1-13 Antrim 1-13.
Tyrone 1-18 UUJ 0-9.