Metallic sheet hydroforming method, forming die, and formed part

A metallic sheet hydroforming method comprising:

• clamping two stacked metallic sheets between holding surfaces of a pair of upper and lower dies respectively having die cavities of the same inner contour shape as an outer contour shape of product;
• forming a thru-hole for introducing a fluid in one of said dies, said thru-hole leading to the holding surface of the one die;
• positioning a pierced hole for introducing the fluid with said thru-hole, said pierced hole being formed in one of said metallic sheets in a portion of the one metallic sheet which portion is in contact with the holding surface of the one die;
• and
• introducing said fluid in a pressurized state between mating surfaces of said two stacked metallic sheets through the pierced hole from the thru-hole, thereby causing the metallic sheets to be stretch formed into an internal space defined by said die cavities;
• wherein an O-ring is recessed into a circular groove around the thru-hole positioned with the holding surface, the O-ring being elastically deformed with the pressing force which works between the holding surface and one of the metallic sheets thereby preventing the pressurized fluid between said holding surface and said one metallic sheet from leaking.