Abutting sections of sheet metal ducts are joined at an interface by flanging the ends of each section adjacent the interface and forming a plurality of spaced tabs in the wall surface of the flanges, which tabs protrude outwardly in a direction away from the interface. A generally V-shaped coupling member, having a portion of the free edges thereof folded back under the corresponding side wall, is then assembled over a pair of opposed flanges with the edges of the folded under portions snapping in behind the tabs to lock the flanges together.