Deformation mode and wall thickness variation in conventional spinning of metal sheets

In the conventional spinning of metal sheets, the deformation mode and wall thickness variation have a critical effect on forming stability, quality, and accuracy. Although it is widely accepted there is near-constant wall thickness during conventional spinning, there is actually a certain degree of variation in wall thickness during this process. The nature of this variation has yet to be fully understood, which makes it difficult to precisely control wall thickness during conventional spinning. In this study, we investigated and predicted the deformation mode and wall thickness variation in the conventional spinning of a 1060 aluminium alloy plate. It is found that there are three deformation modes that change sequentially in conventional spinning: shear deformation, compression-shear deformation, and tension-shear deformation. These modes correspond, respectively, to the wall thickness variation of (a) sine-law reduction, (b) a reduction intermediate between sine-law reduction and no reduction, and (c) wall thickening. These dynamic changes in deformation mode and wall thickness variation are caused by a decrease in stress triaxiality in the forming region, which is induced by a decrease in the constraint from the flange region to the forming region. We quantified this constraint by calculating the bending rigidity of the flange region, which represents the resistance to the elastic bending deformation that occurs once the flange loses its stability. Then, we developed a formula with the bending rigidity of the flange region as an independent variable to predict the deformation mode and wall thickness variation that occurs during the conventional spinning of 1060 aluminium alloy. Moreover, we determined the effects of the processing parameters on the deformation, and then used a process-related correction factor to incorporate these effects into the above predictive model. By this model, the effects of processing parameters on the change in the deformation mode during spinning were revealed. The model was also used to modify the traditional flange wrinkling model, in which the wall thickness is assumed to be constant. This modified wrinkling model considers the actual variation in wall thickness, which enabled us to accurately determine the maximum circumferential compressive stress on the flange. Thus, the predictive accuracy and applicability of the modified wrinkling model for characterizing the conventional spinning of 1060 aluminium alloy was far superior to that of the traditional wrinkling model. These results increase the understanding of deformation behavior in conventional spinning, thereby providing important guidance for improved processing design and forming accuracy.