Theoretical study and prediction of the inner hole reduction and critical mandrel diameter in cross wedge rolling of hollow shaft

Abstract

Abstract The design of mandrel is crucial in the Cross-wedge rolling technology (CWR) of hollow shafts with mandrel. The critical mandrel diameter is a mandrel diameter that the inner hole just contacts the mandrel during the rolling process. The effective mandrel diameter should be between the critical mandrel diameter and the inner diameter of hollow-billet. The prediction of critical mandrel diameter can be finished by predicting the reduction process of inner hole. The reduction process of inner hole is related to metal flow law, we create a new theoretical model to predict the reduction process of inner hole based on the volume consistency and the hypothetical conditions of ring deformation. However, the theoretical model must be solved by numerical method because it is not an explicit analytic expression. For the convenience of engineering application, we also create a semi-empirical model to predict the reduction process of inner hole and the critical mandrel diameter, and verify its accuracy by contrasting the theoretical values with the simulation and experimental values. The prediction model is based on the relationship of radial and axial metal flow that considers the effect of the CWR rollers and hollow-bar parameters. The results show that the critical mandrel diameter increases with increasing the forming angle, and is negatively correlated with the stretching angle, reduction ratio, and initial wall thickness. Finally, the reference range of critical mandrel diameter was calculated and drew to guide the mandrel design. The results show that the prediction model can provide theoretical design reference for mandrel geometry in the hollow shaft CWR practical production.