PROBLEM TO BE SOLVED: To provide a hydroforming method capable of swelling a part of a hollow work to be hydroformed to be large outwardly without being affected by the thickness of the workpiece, and preventing occurrence of any wrinkles or thickness reduction, and favorable in the aspect of the equipment cost.
SOLUTION: In the hydroforming method, a hollow workpiece 1 is arranged in a hydroforming die K, and a part of the workpiece 1 is swollen outwardly by applying the axial compressive force while applying the hydraulic pressure inside the workpiece 1. The hydroforming die K is split into partial dies 10b, 11b in contact with the workpiece 1, and permanent dies 10a, 11a other than the partial dies. The hydroforming is performed while oscillating the partial dies 10b, 11b in the axial direction of the workpiece 1.
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