Preform, hydroforming method, and hydroformed product

A hydroforming method comprising:

• a) disposing a perform inside forming dies having cavity surfaces that correspond to outer surfaces of a hydroformed product, said perform having edges overlapped and jointed, two outer members for forming the outer surfaces of the hydroformed product, and reinforcement members for forming reinforcement ribs that divide a hollow cross section of the hydroformed product, and comprising a joint formed by, when more than three sheet materials for forming said outer members and said reinforcement members are overlapped together, welding a first sheet material on a surface of said overlapped sheet materials to a second sheet material positioned inside said first sheet material, and a space disposed and aligned with a joint plane between said second sheet material and a third sheet material located inside said second sheet material during said welding process;
• b) applying a hydraulic pressure to the inside of said perform to expand and deform said perform;
• and
• c) forming the outer surfaces of said hydroformed product and reinforcing ribs that divide the hollow cross section of said hydroformed product using the outer members and the reinforcement members.