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A fracture resistant friction stir welding tool includes a pin for plunging into one or more workpiece(s) to be welded and a shoulder for preventing plasticized material from flowing out of the joint being welded. The friction stir welding tool includes a tool body including a cavity for receiving t

A fracture resistant friction stir welding tool includes a pin for plunging into one or more workpiece(s) to be welded and a shoulder for preventing plasticized material from flowing out of the joint being welded. The friction stir welding tool includes a tool body including a cavity for receiving the shoulder and exerting a compressive stress on the shoulder, and the pin is placed in compression by an axial tension member in the pin.

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We claim: 1. A friction stir welding tool comprising; (1) an integral pin and shoulder, (a) a pin portion designed that plunges into at least one workpiece to perform a friction stir welding operation on the at least one workpiece, (b) a shoulder portion including a working surface designed that en

We claim: 1. A friction stir welding tool comprising; (1) an integral pin and shoulder, (a) a pin portion designed that plunges into at least one workpiece to perform a friction stir welding operation on the at least one workpiece, (b) a shoulder portion including a working surface designed that engages at least one surface of the at least one workpiece, wherein (i) the pin portion includes a proximal end of the pin portion being contiguous with the working surface of the shoulder portion, (ii) a distal end opposite the proximal end, and (iii) a tension member engagement feature in proximity of the distal end; (2) a tool body defining a cavity therein, the cavity for receiving at least a portion of the shoulder portion of the integral pin and shoulder, the tool body further including a shank for engagement with a friction stir welding machine; (3) a tension member having (i) a distal end with an engagement feature complimentary to the tension member engagement feature of the pin portion and (ii) a proximal end with an engagement feature, wherein, the proximal end extends out of the tool body when the tension member is disposed within the integral pin-shoulder and the distal end engagement feature of the tension member is engaged with the tension member engagement feature of the pin portion; and (4) an adjustable tension member engagement device to adjustably engage the proximal end of the tension member to impose a predetermined inward axial force upon the tension member engagement feature of the pin portion and the tool body, thereby placing the pin portion in substantial compression between the distal end and the proximal end of the pin portion. 2. A friction stir welding tool, according to claim 1, wherein the shoulder includes a substantially cylindrical portion and the cavity in the tool body includes a substantially cylindrical portion, the substantially cylindrical portion of the shoulder being disposed within the substantially cylindrical portion of the cavity. 3. A friction stir welding tool, according to claim 1, wherein the shoulder is capable of being assembled into the cavity with positive interference, so that the tool body exerts a radially inward compressive stress on the substantially cylindrical portion of the shoulder. 4. A friction stir welding tool, according to claim 3, wherein the tool body is thermally reactive such that the compressive sleeve expands when it is heated before the shoulder is assembled into the cavity to facilitate an easy assembly with the proximal shoulder and to provide the radially inward compressive stress when the tool body contracts at room temperature. 5. A friction stir welding tool, according to claim 2, wherein the substantially cylindrical portion of the cavity and the substantially cylindrical portion of the shoulder have at least one nonzero release angle to facilitate disassembly of the shoulder from the tool body. 6. A friction stir welding tool, according to claim 5, wherein the release angle is about one degree. 7. A friction stir welding tool, according to claim 1, wherein the adjustable tension member engagement device is at least one nut acting through at least one spring washer. 8. A friction stir welding tool, according to claim 7, wherein the at least one nut and the at least one spring washer are disposed at a proximal end of the tension member. 9. A friction stir welding tool, according to claim 7, wherein the at least one nut and the at least one spring washer are disposed at a proximal end of the shank portion of the tool body. 10. A friction stir welding tool, according to claim 1, wherein the tension member includes a tension member shoulder at a distal end thereof and the pin portion includes a recess at a distal end thereof, the recess for receiving the tension member shoulder. 11. A fiction stir welding tool, according to claim 10, wherein the tension member further includes a passage therein, the passage being for a coolant fluid. 12. A friction stir welding tool, according to claim 1, wherein the tension member includes a pin extension, the pin extension being for extending the pin portion and for exerting a compressive force on the pin portion. 13. A friction stir welding tool, according to claim 12, wherein the pin extension includes threads on a working surface thereof. 14. A friction stir welding tool, according to claim 12, wherein the tension member further includes a passage therein, the passage being for a coolant fluid. 15. A bobbin type friction stir welding tool comprising: a tool body defining a cavity therein, the tool body further including a shank for engagement with a friction stir welding machine; an integral pin shoulder including: (i) a proximal shoulder disposed at least partially within the cavity; (ii) a friction stir welding pin having a proximal end contiguous with a working face of the proximal shoulder; (iii) a distal shoulder having a working face contiguous with a distal end of the friction stir welding pin; a compression sleeve surrounding at least a portion of the distal shoulder, wherein the compression sleeve designed that exerts a radially inward compressive stress on the distal shoulder, a tension member having an engagement feature at both ends thereof, wherein each of the ends of the tension member extends out of the tool body and the compressive sleeve when the tension member is disposed at least partially within the tool body, the integral pin shoulder, and the compressive sleeve, an adjustable tension member engagement device to adjustably engage the tension member engagement feature to impose a predetermined inward axial force directly upon the compressive sleeve and the tool body, thereby the compressive sleeve and the tool body places the friction stir welding pin in substantial compression between the compressive sleeve and the tool body. 16. A bobbin type friction stir welding tool, according to claim 15, wherein the proximal shoulder includes a substantially cylindrical portion and the cavity in the tool body includes a substantially cylindrical portion, the substantially cylindrical portion of the proximal shoulder being disposed within the substantially cylindrical portion of the cavity. 17. A bobbin type fiction stir welding tool, according to claim 16, wherein the substantially cylindrical portion of the proximal shoulder is capable of being assembled into the substantially cylindrical portion of the cavity with positive interference, so that the tool body exerts a radially inward compressive stress on the substantially cylindrical portion of the proximal shoulder. 18. A bobbin type friction stir welding tool, according to claim 17, wherein the tool body is thermally reactive such that the compressive sleeve expands when it is heated before the proximal shoulder is assembled into the substantially cylindrical portion of the cavity to facilitate an easy assembly with the proximal shoulder and to provide the inward compressive stress when the tool body contracts at room temperature. 19. A bobbin type friction stir welding tool, according to claim 15, wherein the friction stir welding pin includes a proximal pin portion and a distal pin portion, a juncture between the proximal pin portion and the distal pin portion being about at a center of the friction stir welding pin, the proximal pin portion including a first thread and the distal pin portion including a second thread, the second thread opposite to the first thread, whereby rotation of the bobbin type friction stir welding tool in an appropriate direction tends to cause plasticized material to move toward the center of the friction stir welding pin. 20. The bobbin type fiction stir welding tool, according to claim 15, wherein the compression sleeve is thermally reactive such that the compressive sleeve expands when it is heated before it is assembled to the distal shoulder to facilitate an easy assembly with the distal shoulder and to provide the radially inward compressive stress on the distal shoulder when the compressive sleeve contracts at room temperature. 21. The bobbin type friction stir welding tool, according to claim 15, wherein the adjustable tension member engagement device is at least one nut acting through at least one spring washer.