The friction stir welding mechanism has been discussed in numerous publications. Briefly described, two pieces of aluminum sheet are edge-joined using a weld and are placed with edges fitted together. A tool, referred to as a 'pin-tool', actually creates the weld. A retractable pin-tool, developed at Marshall Space Flight Center, consists of a mechanism that can be compared to a plunge router. The shoulder of the tool houses the pin and is placed at the intersection of the edge-joined materials. The shoulder applies pressure and rotates to create frictional heat as it is pressed against the material joint. The frictional heat must reach a temperature that allows the aluminum to flow plastically. The pin is inserted into the material and as it rotates the material is actually stirred, meaning that the material from one plate actually replaces a corresponding amount of material in the second plate. The pin-tool moves with sufficient speed along the joint to continue to heat new material that can be stirred into the weld. Cooling occurs as the tool moves away from the welded material and the aluminum solidifies to become solidly welded. The retractable pin tool enables a weld strip that can end without leaving a hole in the workpiece. The mechanism of the actual placement of the flowable material in front of the pin-tool to a position behind the pin-tool is the primary topic of research.
Classically, theories of mechanics of materials have been proposed that would include all of the phenomena that occur during the process of friction stir welding. Studies have been initiated to determine the applicability of individual theories. The theories in question have been loosely categorized with such titles as thermoelasticity, thermoplasticity, thermo-viscoelasticity and thermo-viscoplasticity. Advances in computational mechanics render it feasible to attempt studies that would have been out of the question in past decades. Tennessee Technological University (TTU) has undertaken a systematic computational study of friction stir welding that will utilize accepted modern theories of continuum mechanics. The three year, three phase approach of this study is intended to result in a model that can be used in government and private section industrial laboratories and that will describe and characterize friction stir welding in computer code.
