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It is investigated to what extent the evolution of ductile damage in cold forging can be controlled without changing the geometry of the produced part. Besides the effects of strain hardening and residual stresses, damage, which is the nucleation, growth and coalescence of voids on microscopic level, affects product properties of the manufactured components such as fatigue strength, impact strength or elastic stiffness. Former investigations have shown that the load path dependent damage evolution in forward rod extrusion and thus the performance of produced parts can be controlled by the process parameters extrusion strain and shoulder opening angle. As these parameters also affect the geometry of extruded parts, design requirements of components might be violated by varying these. Thus counter pressure is used to superpose purely hydrostatic stresses to forward rod extrusion in order to decrease triaxiality in the forming zone without causing geometric variations in the produced parts. The counter pressure is either introduced by a counter punch or by modified process routes. The achieved improvements in product performance are in agreement with results obtained by variation of extrusion strain and shoulder opening angle as described in literature. In addition, it is observed in tensile tests, that damage in cold extruded parts does not significantly affect flow stress. All advancements in product performance are realized without affecting the products’ geometries.

In this paper a process sequence, that uses forward rod extrusion with cold forged C15 steel cup billets to produce lightweight shafts, is presented. The steel cup billets feature either a lightweight magnesium alloy core or a granular medium core that is removed after forming to obtain hollow shafts without the need of complex tools and highly loaded mandrels. It is shown that composite shafts featuring magnesium cores can be produced for a wide range of extrusion strains. Due to high hydrostic pressures in forward rod extrusion, the forming limit of magnesium at room temperature can be expanded. The observed bond strength between core and sheath is below the shear yield strength of utilized magnesium AZ31 alloy. Hollow shafts are successfully produced with the presented process route by utilizing zirconium oxide beads or quartz sand as a lost core. As the law of constant volume in metal forming is violated by compressible granular media, a simulation approach using a modified Drucker-Prager yield surface to model these materials is validated to provide a tool for efficient process design. Granular cores and magnesium alloy cores offer new possibilities in production of lightweight shafts by means of composite cold forging. Both process variants allow for higher weight savings than composite shafts based on aluminum cores.

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