Dry, Cold Forging of Oxygen-Free Copper by Massively Nitrogen-Supersaturated CoCrMo Dies

Abstract

With the aim of achieving a sustainable society with green manufacturing, every metal-forming process has been changed to a dry process with the use of limited lubricants via regulation. In parallel, die materials must have sufficient wear resistance to prolong the die life even when forming active metals. A massively nitrogen-supersaturated (MNSed) superalloy was selected as a galling-free die substrate to forge oxygen-free copper wires and bars in dry conditions. A plasma immersion nitriding system was utilized to induce nitrogen supersaturation in CoCrMo, forging dies at 723 K for 21.6 ks with a high nitrogen solute content. Microstructure analyses and microhardness testing proved that the MNSed CoCrMo die had a multilayered structure from the top surface to the depth and that the surface hardness increased up to 1300 HV. Dry, cold forging experiments demonstrated that the oxygen-free copper bar was upset, with a reduction in thickness of 70% in a single stroke under low friction. No fresh copper work debris adhered onto the MNSed CoCrMo die surface. The loading–stroke relationship was used to describe the forging behavior, with low friction and without galling.