Friction stir welding tool and friction stir welding method

A friction stir welding method characterized in that

• plate members are friction stir welded by using a friction stir welding tool including
• a shoulder portion, and
• a probe portion provided on a bottom surface of the shoulder portion, the probe portion having a length of 5.5 mm or more, the probe portion decreasing in diameter continuously from the bottom surface toward a distal end of the probe portion, and grooving and/or chamfering is not performed on the probe portion such that the probe portion has a smooth surface,
• wherein a ceramic material whose main phase is silicon nitride or sialon is used as a base material;
• the shoulder portion of the friction stir welding tool is brought into contact with the plate members, while the probe portion of the friction stir welding tool is inserted into the plate members by an amount of 5.5 mm or more;
• monitoring reaction forces applied from the plate members to the friction stir welding tool;
• holding the position of the friction stir welding tool for a predetermined period of time after the probe portion has been inserted into the plate members;
• and
• laterally moving the friction stir welding tool after the reaction forces applied from the plate members to the friction stir welding tool have decreased below a threshold value.