Finite element modelling and experimental validation of two-roller vertical forward flow forming process of EN36B steel

Flow forming is a high-precision metal forming process used to produce thin-walled, rotationally symmetric components with enhanced mechanical properties. This study investigates the two-roller vertical forward flow forming process for EN36B steel through Finite Element Analysis (FEA) using FORGE® NxT 4.0, complemented by experimental validation. Material properties of EN36B steel, including elasticity, thermal, physical, and plasticity characteristics, were modelled with JmatPro software to ensure accurate simulations. Experimental trials included microstructural characterisation, hardness testing, surface roughness evaluation, and twist measurements to validate the numerical model. The FEA simulations provided critical insights into key process parameters such as Von Mises stress, strain, Latham-Cockroft damage, and force dynamics. Defects such as bulging and material build-up were effectively predicted and modelled. Dimensional accuracy was assessed using 3D GOM scanning, revealing a maximum thickness error of 0.3 mm. Discrepancies in force measurements between simulations and experiments were minimal, with deviations of 6.5 % for radial forces and 2.5 % for axial forces. Surface roughness improved significantly, with values decreasing from 2.1 μm Ra to 0.7 μm Ra after vertical forward flow forming. Furthermore, the hardness increased from 186 HV to 260 MPa (around 40 %) after the forming due to the work hardening process with plasticity. Tensile stress of the workpiece increased from 620 MPa to 880 MPa without an additional heat treatment process. Due to the roller's high force on the workpiece's outer surface, the hardness testing revealed a maximum value of 279 HV on the outer surface, reducing to a minimum of 236 HV closer to the inner surface. The hardness error between FEA and experimental results is around 2 %. Electron Backscatter Diffraction (EBSD) analysis indicated higher grain deformation at the outside surface compared to the middle and inner surface of the flow-formed tube. The vertical forward flow forming process reached a maximum temperature of approximately 200 °C, which was efficiently managed through water cooling. The study highlights the utility of Arbitrary Lagrangian-Eulerian (ALE) formulations and remeshing techniques in simulating complex deformation patterns. These methods provide critical insights for optimising the flow forming process and advancing the manufacture of EN36B steel components.