New processes to prevent a flow defect in the combined forward–backward cold extrusion of a piston-pin

A flow defect of a piston-pin for automobile parts are investigated in this study. In the combined cold extrusion of a piston-pin, a lapping defect, which is a kind of flow defect, appears by the dead metal zone. This defect is evident in products with a small thickness to be pierced and is detrimental to dimensional accuracy and decrease of material loss. The flow defect that occurs in the piston-pin has bad effects on the strength and the fatigue life of the piston-pin. Therefore, it is important to predict and prevent the defect in the early stage of process design. The best method that can prevent the flow defect is removing or reducing dead metal zone through the control of material flow. Finite element simulations are applied to analyze the flow defect. This study proposes new processes which can prevent the flow defect by removing the dead metal zone. Then the results are compared with the results of experiments for verification. These FE simulation results are in good agreement with the experimental results.