Calibration of the residual stresses with an active die during the ejection phase of cold extrusion

The resulting tensile residual stresses on the surface of cold full-forward extruded parts are unfavorable for the fatigue life of these parts. The final stress state is determined by the combination of two process stages: forming and ejection. This is due to the fact that the workpiece undergoes a second plastic deformation after forming during the ejection from the die. So far, literature is focusing mainly on the identification of the parameters affecting the residual stresses during the first stage. In the present paper, the attention is drawn to the ejection phase during cold extrusion of workpieces made out of the austenitic stainless steel AISI 316 L. First of all, a system consisting of an active die is presented. This technology allows the control of the applied pre-stress on the die during the process. It is experimentally and numerically demonstrated that a significant shift of the residual stress state in the near-surface region can be achieved. Even compressive axial and tangential residual stresses can be induced in this area. Also the limits of this system are numerically investigated. It is observed that a different deformation mechanism occurs above a certain pre-stress level. Finally, an analytical model is created and observations are presented relatively to the mechanisms that influence the plastic deformation during ejection.