Friction stir welding tool member made of silicon nitride sintered body and friction stir welding apparatus using same

1. A welding tool member for friction stir welding comprising a silicon nitride sintered body, wherein the silicon nitride sintered body includes an additive component other than silicon nitride in a content of 3% by mass or more and 12.5% by mass or less, and the additive component includes three or more elements selected from Y, Al, Mg, Si, Ti, Hf, Mo and C,

wherein a surface roughness Ra of a friction surface of the silicon nitride sintered body is 0.05 μm or more and 5 μm or less, and a maximum cross sectional height Rt of the friction surface is 0.04 μm or more and 20 μm or less, and a Vickers hardness of the silicon nitride sintered body is 1400 or more,

wherein the welding tool member comprises a base portion and a protruding portion, in which a surface of the base portion serves as a shoulder portion, a surface roughness Ra of the shoulder portion being 0.06 μm or more and 10 μm or less, and a maximum cross-sectional height Rt of the shoulder portion being 2.9 μm or more and 35 μm or less,

wherein the surface roughness Ra and the maximum cross-sectional height Rt of the shoulder portion are measured by operating a measurement needle along the shoulder portion, and

wherein both the friction surface and the shoulder portion are subjected to polishing processing, the surface roughness Ra of the friction surface is an average value, the surface roughness Ra of the shoulder portion is an average value, the maximum cross-sectional height Rt of the friction surface is an absolute value, and the maximum cross-sectional height Rt of the shoulder portion is an absolute value.

2. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein the additive component includes four or more elements selected from Y, Al, Mg, Si, Ti, Hf, Mo and C.

3. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein a fracture toughness value of the silicon nitride sintered body is 6.5 MPa·m 1/2 or more.

4. A friction stir welding apparatus using the welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 .

5. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein the welding tool member is used in a high temperature environment of 800° C. or higher produced by friction heat at the friction surface.

6. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein the friction stir welding tool member is used with an indentation load of 5 kN or more.

7. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein the friction stir welding tool member is rotatable at a rotation speed of 800 rpm or more.

8. The welding tool member for friction stir welding, made of a silicon nitride sintered body, according to claim 1 , wherein the welding tool member is used under an environment such that a welding temperature of the friction surface is 800° C. or more.

9. The friction stir welding apparatus according to claim 4 , wherein a welding temperature of the friction surface is 800° C. or more.

10. The friction stir welding apparatus according to claim 4 , wherein a member to be welded is a cold rolled steel plate.