Prediction of cold forging tool-life based on finite element analyses

Abstract

The present investigation is an attempt to estimate the cold forging tool life, The cold forging tool life is usually limited by high and low cycle fatigue, wear, geometries of tools, defects in tool material, properties of tool and workpiece materials, friction conditions on the interface of tools and workpiece and the surface finish of tools. Since processing conditions like friction and material defects are immeasurable and tool material and geometry are usually considered as constant after designing stage, therefore fatigue would be the most dominant variable in limiting cold forging tool life. High strength and hardness of tool material along with the requirement of net shape manufacturing limits the chances of plastic deformation in tool, therefore, high cycle fatigue phenomenon usually takes place. In the present study, four different workpiece materials were considered and their material properties were determined through the compression test. Finite element analysis of multistage bolt forming process was carried out by employing rigid viscoplastic program and then the elastic analysis of tool without and with stress ring was also carried out. Due to very high computation time required for 3D simulations, 2D axi-symmetric approximation was introduced and then computational results were compared between 2D axi-symmetric and 3D hexagonal bolt simulations. It was found out that the stress conditions on lower dies for both cases showed an acceptable difference. Therefore, 2D approximation results were used in the rest of the investigation. The life cycles of already in use tools were obtained from industry. The fatigue strength coefficient and fatigue strength exponent was inversely calculated by plotting stress amplitude against the number of lifecycles. These fatigue properties contain the effect of all the immeasurable service life limiting parameters like friction, temperature change, non-uniform loading and unloading of tools and other unknown processi...