do I need duct tape or self drilling screwsYou have a choiceYou can slip-fit them together and space (3) self threading sheet metal screws 120 degrees apartWrap the joint with self-adhering backed,
aluminum foiled tapeOr, you can fit together by using a duct connectorThis is usually used for making bends or slight curvesA sturdy joint to secure a duct hanger fromCan secure with screws also but not necessary but wrap with above tapeDuct tape is cheaper and ok but will peel off within a few years.Other related question