Sheet metal spinning is a common method for manufacturing thin-walled components. Its benefits include high flexibility, low cost, low forming force and high material utilization. There are two types of sheet metal spinning, namely, conventional and shear spinning. Unlike shear spinning, conventional spinning is more susceptible to flange wrinkle defects because the thin-walled flange is normally in a free state subjected to circumferential shrinkage induced deformation. The flange wrinkling defects greatly impact the forming quality of workpieces. Moreover, predicting the occurrences of wrinkling defects is challenging because it is affected by the workpiece’s geometric, strain, and transient states.
Most studies have related the flange wrinkling associated with conventional spinning to excessive circumferential flange shrinkage, laying a theoretical underpinning for establishing the wrinkling mechanism. Currently, different methods used to predict the initiation of the flange wrinkling are either detection of the wrinkling-induced changes in the formed part or based on theoretical analysis of the wrinkling mechanism. Nevertheless, the first method is mainly based on subsequent flange wrinkling-induced changes that can be overpredicted. Additionally, this method fail to provide an in-depth understanding of the flange wrinkling mechanism.
The recently proposed prediction model based on the energy method is a promising theoretical tool. Compared with the first method, it not only provides a better theoretical understanding of the flange wrinkling mechanism but also provides a more accurate prediction of the flange wrinkling. However, this model does not adequately account for instantaneous geometry states that may reduce the prediction accuracy. Furthermore, the instantaneous mechanical state extraction bias caused by the non-uniform distribution of the strain and stress on the flange could reduce the robustness of this model for wrinkling prediction. Therefore, developing more accurate and robust theoretical models for predicting flange wrinkling in conventional spinning is highly desirable.
To this end, S.W. Chen (PhD candidate), Professor Mei Zhan, Associate Professor Pengfei Gao and Dr. H.R. Zhang from Northwestern Polytechnical University in collaboration with Dr. F. Ma from Sichuan Aerospace Long March Equipment Manufacturing Co., Ltd proposed a new theoretical model for analyzing flange wrinkling mechanism and predicting the occurrence of flange wrinkling in conventional spinning. This method was used to analyze the effects of material parameters on the occurrence of flange wrinkling. The robustness and prediction accuracy of the proposed model was validated by comparing it to those of the energy method. The work is currently published in the research Journal of Material Processing Technology.
The researchers reported a close relationship between the initiation of flange wrinkling and excessive circumferential compressive stress only in the zone affected by the roller action and not the whole flange because the wrinkling defects are localized behaviors. Under the combined effects of the circumferential stress and roller action-induced bending moment, circumferential compressive stress was observed in the affected zone and flange wrinkling appeared when the stress exceeded the critical value. By comparing the computed critical stress with the maximum circumferential stress in the affected zone using the proposed theoretical model, it was possible to predict the occurrence of the flange wrinkling accurately.
The proposed model presented better robustness and prediction accuracy than the energy method-based model. This was attributed to two main advantages. First, it accounted for the displacement amplitude of the defection mode that was ignored in the energy method. Second, this model considered the blank as a rigid-plastic material which did not only reduce the computation complexity but also reduced the effects of the mechanical states on the wrinkling prediction, thus improving prediction accuracy. Additionally, the new model was used to analyze the influence of different parameters on the initiation of flange wrinkling under different forming conditions. The results revealed that the initiation of flange wrinkling of significantly influenced by the strengthening coefficient of the material, followed by elastic modulus and, finally, hardening index.
In summary, the flange wrinkling mechanism during conventional spinning was analyzed and a new theoretical model for predicting flange wrinkling was proposed and successfully validated. The present theoretical model overcame most of the limitations of the energy method, and it thus exhibited superior prediction accuracy and robustness. It was demonstrated that blank sheets with a larger strengthening coefficient of material and smaller elastic modulus could delay the initiation of flange wrinkling. In a statement to Advances in Engineering, the authors said their findings provide a better understanding of the underlying mechanism and prediction of flange wrinkling in conventional spinning.
Chen, S., Zhan, M., Gao, P., Ma, F., & Zhang, H. (2021). A new robust theoretical prediction model for flange wrinkling in conventional spinning. Journal of Materials Processing Technology, 288, 116849.