A finite-element model to analyse forming defects in rolling-extrusion of finned tubes

Finned tubes improve the efficiency of air-cooled heat exchangers by increasing the transmission surface area. The rolling-extrusion process can be employed in their manufacturing by plastic deforming single- or multi-material tubes. The forming steps are usually performed by rotating specific heads composed of a series of disks, whose dimensions are customized to achieve the desired fins’ sizes in terms of height, thickness and pitch. Process parameters, such as the temperature of the disks and the processed material or the lubricant conditions mainly affect the quality of the obtained products in terms of performance, but also in terms of integrity of both manufactured parts and employed equipment. The rolling-extrusion was analysed by a two-dimensional finite-element model to detect stress and strain distributions due to the forming phases. The numerical model focuses on bimetallic finned tubes utilizing an axisymmetric tube geometry with a Norton–Hoff viscoplastic material model. The executed simulations were set to accurately predict the fin shapes using as reference experimental evidence. Effects of specific process variables, including disk temperature, tube preheating and lubrication conditions were monitored. The results highlighted how changes in both disk and tube temperatures and lubrication conditions influence the risk of defects, including fin ruptures, pitch irregularities and tool failures. Specifically, tube preheating increased disk temperatures and minimized friction conditions reducing stresses and deflections in the critical zones. The results offer practical guidelines to improve product quality and production efficiency in industrial settings.