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There is provided a method capable of decreasing a friction force in friction stir welding, in which a welding tool is used. In a method of friction stir welding workpieces together by rotating and pressingly plugging a welding tool, formed from a harder material than the workpieces, into a weld zon

There is provided a method capable of decreasing a friction force in friction stir welding, in which a welding tool is used. In a method of friction stir welding workpieces together by rotating and pressingly plugging a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, a power supply device allows a current to flow between the welding tool 1 and the workpieces to cause resistance heating of the welding tool. Combined use of resistance heating makes it possible to decrease a friction force. Preferably, a conductive ceramic is coated on surfaces of the welding tool to cause resistance heating.

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1. A method of friction stir welding workpieces together by rotating and plugging a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a current is allowed to flow be

1. A method of friction stir welding workpieces together by rotating and plugging a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a current is allowed to flow between said welding tool and said workpieces to cause resistance heating of said welding tool, wherein the current allowed to flow between said welding tool and said workpieces is controlled to be made constant in voltage.2. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein said heater comprises a conductive ceramic.3. The friction stir welding method according to claim 2, wherein said conductive ceramic comprises Silicon Carbide.4. The friction stir welding method according to claim 2, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, surfaces of said pin are covered with said heater, and surfaces of said shoulder are covered with an insulating ceramic.5. The friction stir welding method according to claim 2, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, a tip end portion of said pin is covered with said heater, and the remainder of said pin and surfaces of said shoulder are covered with an insulating ceramic.6. The friction stir welding method according to claim 2, wherein said welding tool is formed from an insulating ceramic and surfaces of said tool are covered with said heater.7. The friction stir welding method according to claim 2, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, all surfaces of said pin and surfaces of said shoulder are covered with said heater, and further a coating layer of an insulating ceramic is formed on an outside of said heater except a tip end portion of said pin.8. The friction stir welding method according to claim 2, wherein an outside of a coating layer formed from said conductive ceramic is further covered with a ceramic having a high conductivity.9. The friction stir welding method according to claim 8, wherein said ceramic formed on an outermost surface of said welding tool comprises a material having a more excellent abrasion resistance than that of said conductive ceramic formed therebelow.10. The friction stir welding method according to claim 2, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, materials of a ceramic, which covers a tip end portion of said pin, and of a ceramic, which covers a side portion of said pin and surfaces of said shoulder, are different from each other, and the ceramic covering the tip end portion of said pin is increased in conductivity.11. The friction stir welding method according to claim 2, wherein the current allowed to flow between the welding tool and the workpieces is controlled to be made constant in voltage.12. The friction stir welding method according to claim 2, wherein voltage or current is controlled so that said heater has a constant calorific value.13. A friction stir welding apparatus for performing friction stir welding by rotating and pressing a welding tool, formed from a harder material than workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, wherein there is provided a power supply device, which allows an electric current to flow between the welding tool and the workpieces to cause resistance heating of the welding tool, wherein a coating layer of a heater is formed on at least a portion of said welding tool, which is plugged into the workpieces, and said heater causes resistance heating when an electric current is allowed to flow between said welding tool and said workpieces, and wherein said heater comprises a conductive ceramic.14. The friction stir welding apparatus according to claim 13, wherein said welding tool is held by a spindle, and said power supply device allows an electric current to flow between said spindle and said workpieces.15. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, surfaces of said pin are covered with said heater, and surfaces of said shoulder are covered with an insulating ceramic.16. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, a tip end portion of said pin is covered with said heater, and the remainder of said pin and surfaces of said shoulder are covered with an insulating ceramic.17. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein said welding tool is formed from an insulating ceramic and surfaces of said tool are covered with said heater.18. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein said welding tool comprises a small-diameter pin on a tip end of a large-diameter shoulder, all surfaces of said pin and surfaces of said shoulder are covered with said heater, and further a coating layer of an insulating ceramic is formed on an outside of said heater except a tip end portion of said pin.19. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein the current allowed to flow between the welding tool and the workpieces is controlled to be made constant in voltage.20. A method of friction stir welding workpieces together by rotating and pressing a welding tool, formed from a harder material than the workpieces, into a weld zone of the workpieces and moving the welding tool relatively in a welding direction, characterized in that a coating layer of a heater is formed on at least that portion of said welding tool, which is plugged into the workpieces, and a current is allowed to flow between said welding tool and the workpieces to cause resistance heating of said heater, wherein voltage or current is controlled so that said heater has a constant calorific value.