Study on the spiral microstructure defect of workpiece with a small area reduction formed via cross wedge rolling

Local deformation of workpieces is a main process characteristic of cross wedge rolling; thus, it is difficult to cross wedge rolling the workpiece with a small area reduction owing to the weak ability to flow in the axial direction and the significant deformation difference between the surface and interior. Besides the central damage of workpiece, a spiral microstructure defect in a certain depth of surface layer reduces the mechanical strength of products manufactured by cross wedge rolling. A rolling experiment is performed to reveal that the spiral microstructure defect appears macroscopically as a bright band along the stretching spiral line in a certain depth of the surface layer after machining and microscopically as a strip microstructure distribution that extends from the surface fold to the interior of the workpiece. The cause of the microstructure defect is investigated by a combination of rolling experiments with finite element analysis, and the metal in the forming zone flows along the negative stretching direction, resulting in the surface fold being distributed along the stretching spiral line and the small axial strain band on the formed surface. Meanwhile, large radial compression near the spiral band causes the directional distribution of the microstructure along the fold cracks from the surface to the interior of the workpiece. In addition, the fillet of the wedge tip is adopted to improve the local metal axial flow in the negative stretching direction, which can eliminate the spiral microstructure defects near the surface and avoid the risk of central damage to ensure that the forming quality in the center of workpiece meets the requirements. Finally, the optimal value of fillet on the wedge tip is determined via experimental verification.