The deformation behavior of a hollow cylindrical workpiece was investigated in upsetting with torsional oscillation. The aluminum workpiece was simultaneously compressed in the axial direction and twisted in the circumferential direction between two conical dies at a torsion/compression speed of 0-18°/mm. The workpiece was deformed without lubrication at room temperature during sliding on the end face of the conical dies with a mirror-like finish surface. The conical dies with an apex angle of 160° were used for increasing the non-uniform deformation of the workpiece and the axial centering of the workpiece. Owing to the superposition of compression and torsion, the plastic flow of the workpiece was promoted in the outer radial direction on the die-workpiece interface. The barreling of the workpiece was reduced at a torsion/compression speed lower than 12°/mm in the superposition, whereas hourglass deformation occurred at higher than 12°/mm. The hourglass deformation was concluded to be a unique deformation in upsetting with torsion based on material mechanics theory and finite element simulation.

The material strength of cold-rolled steel sheets for car bodies is increasing considerably. It has been experientially known that die deflection affects springback, but there are only a few experimental verification cases. In this work, two bending dies with different rigidities but the same shape were manufactured. The springback of panels bent by each die was compared in detail. Using 590 MPa class and 980 MPa class steel sheets, the deformation of the dies due to bending also measured. The springback and deflection of the die were clearly significant, when the less rigid die was used. From these results, finite element analysis was carried out to investigate the effect of die deflection on springback. Material modeling for the Bauschinger effect was used for the FE analysis. The prediction results were reasonable and showed the same trend as the experimental results. Experiments and FE analysis clarified the mechanism of springback due to the effect of die deflection.