Formation and process-based adjustment of residual stresses in cold cup forward extrusion

Abstract Cold cup forward extrusion (CFE) enables near-net-shape production of thin-walled steel components, but the resulting residual stresses can affect dimensional stability during subsequent machining. This study analyzes the formation and adjustment of axial and tangential residual stresses in CFE of low-carbon steel using X-ray diffraction, depth-resolved layer removal, microhardness mapping and finite element simulation. The model is parameterized by experimentally determined flow curves and friction data and validated against surface and depth-resolved residual stress measurements. The results show that surface data alone are insufficient for model selection, since different modelling assumptions can reproduce selected surface trends while deviating below the surface. A rigid die model with temperature-dependent flow curves provides the most consistent agreement with measured stress gradients and hardening patterns, indicating that process-induced heating must be considered even in cold forming. The residual stress topology is governed by inhomogeneous plastic deformation during material redirection from bottom to wall and by subsequent redistribution during unloading and ejection. The bottom-to-wall transition is identified as the primary zone of strain heterogeneity. Parameter variations show that preform height has only minor influence on the dominant stress topology, whereas stroke-controlled bottom height controls the coupling between bottom and wall. Punch-side parameters act locally: the punch face angle mainly affects the bottom and transition region, while the punch corner radius modifies stresses at the lower inner wall. The results demonstrate that residual stress control in cup forward extrusion requires a region-specific process design strategy.