Forming Quality Indices for Tube Roll Forming: Definition and Modelling

Cold roll forming is traditionally widely used to incrementally manufacture cost-effective welded tubes, where the metal strip is progressively deformed by multiple rollers to the final round cross-section following the so-called flower pattern. However, different flower patterns may allow obtaining the same final round cross-section with a variable number of forming steps and intermediate geometries. The deformation steps are obtained by combining vertical and horizontal rollers, with specific geometrical features, but, in the case of inappropriate design, a non-homogeneous distribution of the mechanical properties along the tube cross-section may result and affect the tube behavior in subsequent processes (i.e. bending) or even in its service-life. The paper aims at proposing reliable quality indices to evaluate the homogeneity of the mechanical properties as a function of the flower pattern design and of the main process parameters. To this purpose, a numerical model of the tube roll forming was developed in LS-Dyna and validated on an industrial case for welded tubes. Then, the validated model was used to investigate the effects of three different flower patterns, named respectively W-shaped, P-shaped and C-shaped, on the introduced indices. The results show the influence of the patterns on the strain distribution and workpiece final characteristics.