Fundamental study of central crack mechanism and criterion in cross wedge rolling

Abstract

Cross wedge rolling (CWR), a novel metal forming process for manufacturing axisymmetric stepped shafts, is widely applied in transport industries. Central crack, the cavity formed in the product centre, is a critical problem, preventing its development in safety-critical industries. However, the understanding of the central crack mechanism is insufficient, and there is not yet a robust fracture criterion to predict its occurrence. This study aims to establish a fundamental understanding of the central crack mechanism and build a robust physically-based fracture criterion. An innovative CWR physical model with plasticine billets was built in house, which allowed the dies to be rapidly 3D printed and the workpiece with specific mechanical properties to be efficiently manufactured. The effects of the stress variables and initial material properties (ductility) on central cracking were investigated by varying the die geometries and billet material compositions, respectively. It is found that the maximum shear stress plays a dominant role in the central crack formation, and with the increase of the material ductility, the central crack transitions from brittle fracture to ductile fracture. A robust physically-based damage model set was proposed, along with a novel material constant calibration method. The reliability of the proposed model was validated quantitively by 60 groups of CWR tests with different materials and die geometries. The proposed calibration method will significantly benefit the industry due to the extremely simplified die geometries. To further understand the central crack mechanism in the practical industry, the microstructural characteristics (e.g., inclusion, grain size and phase composition) of two high-strength steel CWR billets (with/without high possibility to crack) were quantitatively analysed and compared. It is found that central cracking can be effectively avoided by controlling the inclusion content in the CWR billets.