Element, friction element welding method, and method for producing friction element welded joint

An element for performing friction element welding on a sheet stack of two or more stacked metal sheets by pressing the element into the sheet stack while the element is rotated, the element comprising:

• a circular columnar mandrel configured to enter the sheet stack;
• a circular disk-shaped collar disposed at an upper end portion of the mandrel, a diameter of the collar being larger than a diameter of the mandrel;
• a first conical body extending from a lower end surface of the mandrel;
• and
• a second conical body disposed in contact with a lower side of the first conical body, a bottom surface of the second conical body being smaller in diameter than a bottom surface of the first conical body,
• wherein an outer circumferential portion of the collar has a downward inclined or curved shape,
• a center axis of the first conical body coincides with a center axis of the mandrel,
• a center axis of the second conical body coincides with the center axis of the mandrel,
• the bottom surface of the second conical body has an apex that terminates with a pointed tip,
• a vertical angle β(°) of the second conical body and a vertical angle α(°) of the first conical body satisfy the following relation:
• β<α,
• the vertical angle α(°) of the first conical body satisfies 140≤α<180, and
• the vertical angle β(°) of the second conical body satisfies 90≤β<140.