A method of joining workpieces includes rotating a friction stir rivet via a mandrel and driving the rivet into the workpieces causing frictional heating between the rivet and the workpieces and causing the materials of the workpieces to soften, thereby providing a fiction stirred displaceable path for the rivet to traverse, and driving the rivet along the displaceable path until the rivet mandrel pierces through the workpieces and a cap of the rivet is seated against the workpieces. Subsequent to seating the cap, further rotation of the mandrel is stopped and the workpieces are allowed to cool and harden. An axial load is then applied to the mandrel sufficient to provide mechanical loading between the rivet body and the workpieces. A resultant volume of displaced material from the workpieces is fixedly attached to the workpieces, thereby avoiding the creation of a potentially detachable slug of the displaced material.