Friction-welded part and method of friction welding

A method of friction-welding to join a first member and a second member together, the first member being made of an aluminum alloy material and having a uniform cross-section portion in a cylindrical shape with a pipe-wall thickness “t” ranging from 2 to 5 mm and a length equal to or more than (6 mm+t/2), and the second member being made of an aluminum alloy material and having a uniform cross-section portion in a cylindrical shape with the same cross-section as the first member and a length equal to or more than (6 mm+t/2),

• the method of friction-welding comprising the steps of:
• (a) a friction process of butting the uniform cross-section portion of the first member with the uniform cross-section portion of the second member and applying a friction pressure to the first and the second members while the first and the second members are rotating relatively to each other, and
• (b) an upset process of applying an upset pressure greater than the friction pressure,
• wherein a friction upset length is set to be a length within t to 2×t,
• wherein the pressure between the first and the second members rises to the upset pressure within 0.5 seconds after the relative rotation starts to stop, and
• wherein a portion corresponding to each of the uniform cross-section portions of the first member and the second member has a length as long as or longer than 5 mm after the friction-welding is finished.