Colloquia

Summary

Flow forming is a specialised cold forming process used to produce high-quality tubes. Residual stress formation in this process is inevitable due to significant non-uniform plastic deformation. This research examines the impact of process parameters on residual stress formation during flow forming, focusing on thickness reduction and feed ratio. The research involves an experimental study, aided by a theoretical analysis and simulation. The theoretical analysis shows how non-uniform deformation leads to residual stress formation in the flow formed tubes. In flow forming, the non-uniform stress distribution during deformation results in the release of an internal moment (elastic springback) at the tube ends after deformation (tube flaring). In the simulation, an axisymmetric model is used to study the effect of thickness reduction and roller attack angle on residual stress formation. The simulation results align with cold forming theory: residual stress is low if the die ratio is low, which is the ratio between the mean thickness and contact area. This occurs when thickness reduction is large and attack angle is low. In the experiment, strain gauges are used to measure the elastic springback of the tube after releasing the residual stress by cutting it. The experiment shows that the elastic springback is low if thickness reduction is large and feed ratio is high. To reduce residual stress, thickness reduction and feed ratio should be increased. More generally, the contact area should be large relative to the mean thickness to reduce residual stress.