An advanced process of mandrel forging and necking (MFN) was proposed for a hollow shaft with an inner stepped hole. The conventional mandrel forging process with an equal-diameter mandrel was used to form the outer stepped preform, and then the preform was formed into the hollow shaft with an inner stepped hole using the MFN process. A numerical simulation model was established to study the effect of the pressing reduction and the rotation angle on the MFN process. A preforming design method based on the isometric radius difference was given according to the principle of the equal volume, and the parameter relationships between the outer and inner stepped shapes were clarified. The experimental deformation laws of the MFN process were consistent with those obtained by the simulation. The MFN process and its preforming design method provide a new free forging approach for large hollow forgings with inner stepped holes.
Keywords: free forging; hollow shaft with inner stepped hole; large forgings; mandrel forging and necking process; preforming design method.