In the field of metal forming, the cold extrusion process is widely used for producing prismatic products due to various merits such as high strength, good surface quality, high accuracy and reduction in material waste. In this process the design of extrusion die sets is most vital to obtain high quality extruded products. In general, the die insert is subjected to very high contact pressure at the workpiece/die interface during cold extrusion. This high contact pressure leads to reduction in tool life caused by fatigue fracture due to repeated operations or overload fracture due to excessive hoop stress. A typical method of improving tool life is the use of shrink fitting, which generates compressive residual stress in the circumferential direction of the die.
In this study, an optimal design technique was used for determining appropriate dimensions of components of the die set for shrink fitting. For this, an axisymmetric elastic FEM program for the analysis of deformation of the shrink fit die set was developed with the Lagrange multiplier method. In order to examine the accuracy of the developed program, the numerical results for a simple shrink fit die set were compared with the exact solutions provided by Lame's equation. Then, an optimal design system was developed with three components. A thermo-rigid- viscoplastic FEM program, CAMPform, was used for the forming analysis of extrusion process. The developed elastic FEM program was used for stress analysis of the die set. And an optimization program, DOT, was used for the optimization process. With the constructed system, optimal design was carried out for various single stress ring and multi-stress ring cases. It is construed that the proposed design method can be beneficial for improving the tool life of cold extrusion die set.
