A novelty mandrel supported thin-wall tube bending cross-section quality analysis: a diameter-adjustable multi-point contact mandrel

The mandrel plays a vital role in metal tube bending limit improvement and high-quality achievement. For the bending forming process of metal tubes with different diameters, the corresponding mandrel needs to be customized, which has the disadvantages of a long design cycle, high manufacturing cost, and low reuse rate. Therefore, a novelty concept of diameter adjustable mandrel (DAM) based on the multi-point contact was proposed to adapt different diameter tube bending processes. Since the point-contact characteristics of the DAM can lead to irregular cross-section deformation, the cross-sectional full profile radial error (FP-RE) model was proposed. As a more comprehensive evaluation model, the FP-RE model is used to measure the deformation degree of the whole bending cross-section. A diameter-adjustable DAM was adopted in the AISI 304L tubes (the inner diameter = 40-56 mm) bending experiments. The maximum absolute error of FP-RE was 0.11% compared with the ones obtained from FEA in Abaqus/explicit. In addition, the effects of the DAM characteristic parameters, viz., the support blocks amount, the effective magnification, and the effective support diameter, on the tube forming quality were discussed. The results indicated that (a) an even number of support blocks could prevent the collapse of the outer convex tube wall and improve the quality of the inner concave tube wall. (b) When the DAM was used in the larger inner diameter tube bending, the larger non-contact area would cause an increase in the roundness section distortion. (c) A larger DAM diameter would lead to a more minor cross-section deformation degree for the tubes with the same diameter, while the wall thinning was just the opposite. Finally, the result also proved that the DAM (the minimum effective diameter is 40 mm with six support blocks) could be used for tubes with an inner diameter of about 40-56 mm, and the forming quality was acceptable.