Friction element welding method

1 . A friction element welding method for welding a sheet set that includes two or more overlapping metal sheets by pressing a rotating element having a tapered portion at a tip into the sheet set, the method comprising:

a first step of removing a film of oxide on an upper surface of a lower sheet disposed as a lowermost layer of the sheet set, wherein in the first step;

the oxide on the upper surface of the lower sheet is removed while L/t, R 1 , and P 1 are controlled such that 0.5≤L/t<1.3, 4000≤R 1 ≤8000, and 4000≤P 1 ≤8000,

where P 1 (N) is a pressurizing force in the first step, R 1 (rpm) is a rotation speed in the first step, t (mm) is a height of the tapered portion, L (mm) is a cylinder displacement at an end of the first step, with a cylinder position when the tip of the rotating element comes into contact with the upper surface of the lower sheet set to 0, and L/t is a penetration ratio obtained by dividing the cylinder displacement by the height of the tapered portion; and

a second step of pressing the rotating element into the lower sheet to form a welded portion, wherein in the second step:

a pressurizing force in the second step is denoted as P 2 (N), and an increase in the pressurizing force in the second step with respect to the pressurizing force in the first step is denoted as ΔP, and

the pressurizing force in the second step is increased such that the increase satisfies ΔP≥2×(2.5−L)×((12000−P 1 )/100)) when the penetration ratio L/t at the end of the first step is 0.5 or more and less than 0.7,

the increase in the pressuring force satisfies ΔP≥(3.6−L)×((12000−P 1 )/100) when the penetration ratio L/t at the end of the first step is 0.7 or more and less than 0.9, and

the increase in the pressuring force satisfies ΔP≥0.3×(9.9−L)×((12000−P 1 )/100) when the penetration ratio L/t at the end of the first step is 0.9 or more and less than 1.3.

2 . The friction element welding method according to claim 1 , wherein the two or more overlapping metal sheets are steel sheets, and the steel sheets have a tensile strength of 590 MPa or more.

3 . The friction element welding method according to claim 1 , wherein the height of the tapered portion of the rotating element is more than 0 mm and 1.0 mm or less, and the tapered portion has an elevation angle of 10 to 45°.

4 . The friction element welding method according to claim 2 , wherein the height of the tapered portion of the rotating element is more than 0 mm and 1.0 mm or less, and the tapered portion has an elevation angle of 10 to 45°.