Friction stir rivet method of joining

A method of joining upper and lower workpieces fluidly bondable at a point of engagement, the method comprising:

• positioning a fiction stir rivet at the point of engagement of the workpieces, the rivet comprising:
• a body having an elongated shank, a cap at a first end, and an axial hole therethrough, a mandrel having an elongated shaft defining an axis and a head at one end thereof, the head having an effective outside diameter greater than the effective outside diameter of the shaft, the shaft being disposed within the axial hole of the body, the mandrel head being disposed at an opposite end of the body to that of the cap, and the end of the mandrel head having a substantially pointed surface aligned with the axis of the shaft, and a mechanical interface between the body and the mandrel, wherein at least one of the body and the mandrel is configured to engage with the other to provide a rotational inhibitor such that axial rotation of the mandrel is capable of causing axial rotation of the body;
• rotating the mandrel about its rotational axis and causing rotation of the body, driving the rivet such that the substantially pointed surface of the mandrel is driven toward and into the workpieces such that resultant frictional heating between the rivet and the workpieces causes the materials of the workpieces to soften at a process temperature that is substantially lower than the melting temperature of the rivet thereby providing a fiction stirred displaceable path for the rivet to traverse, and driving the rivet along the displaceable path until the substantially pointed surface of the mandrel pierces through the workpieces and the cap is seated against the workpieces;
• stopping further rotation of the mandrel and allowing the workpieces and mandrel to cool below the process temperature, thereby permitting the softened workpieces to harden;
• and
• axially loading the mandrel with sufficient force to drive the mandrel head into the end of the body thereby upsetting and expanding the body end to create an interference between the body and the underside of the lowermost workpiece and to cause the mandrel shaft to sever at a point internal to the body and proximate the cap, thereby resulting in the workpieces being held together by the fluidly bonded materials of the workpieces, the differential thermal contraction of the workpieces and the rivet, and the mechanical loading between the mandrel and the body, at the point of engagement;
• wherein 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.