Increasing the Lightweight Potential of Composite Cold Forging by Utilizing Magnesium and Granular Cores

Abstract

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.