Friction element welding method

A friction element welding method of joining a sheet assembly including two or more stacked metal sheets by pressing an element provided with a tip angle into the sheet assembly while rotating the element, the method comprising, before a step of starting frictional heat welding:

• an oxide film removal step of bringing a lower part of the element into contact with a top surface of a lowermost metal sheet arranged at the bottom of the sheet assembly, and removing an oxide film,
• the element used in the oxide film removal step being such that an elevation angle of a tip of the element satisfies:
• expression (1) when 5.0>D≥3.0,
• expression (2) when 7.0>D≥5.0, and
• expression (3) when D≥7.0,
• (−0.016×D+0.13)×(P×R/TS)≤θ≤60−D (1)
• (−0.004×D+0.071)×(P×R/TS)≤θ≤60−D (2)
• (−0.0004×D+0.045)×(P×R/TS)≤θ≤60−D (3)
• wherein D is an element diameter (mm), θ is the elevation angle (°), P is a pressurizing force (kN), R is a rotational speed (rpm), and TS is a tensile strength (MPa) of the lowermost metal sheet.