Effect of shearing on production stability and die life in automatic multi-stage cold forging of an automobile wheel nut

Abstract

In this study, the effects of earing in a sheared billet of S25C steel formed during rod shearing in an automatic multi-stage cold forging (AMSCF) process for an automobile wheel nut were experimentally and numerically investigated. Precision forging simulations with an emphasis on rod shearing were conducted using the implicit elastoplastic finite element method with a tetrahedral MINI-element scheme. AMSCF of an automobile wheel nut involves the use of a converging or near-conical die that shows a gradual reduction in its cross-sectional area. Our results showed that the shearing-induced earing plays the role of a pivot during balancing of the material inside the die cavity in the early stroke, before significant material contact. Earing formation resulted in a wobble motion of the material that accompanied the rigid-body motion. This ultimately led to a change in the center of mass of the material and unbalanced loading. Earing had a significant influence on the local high-cycle fatigue (HCF) fracture of die insets, especially in the cold forging of nut-like short products. The stress concentration on the die was attributed to asymmetric localized contact between the material and critical die corner, leading to changes in the mass center. Taken together, our results indicate that sheared billet characterization should account for earings with roll-overs and inclined surfaces to not only better predict the HCF life of die parts but also to optimize the forging outcome, particularly for safety products such as automobile wheel nuts.