Effects of Punch and Die Surface Textures and CoCrMo Die Material on the Micro-Backward Extrudability of Pure Magnesium

Magnesium, a biomaterial, is crucial for medical applications. Conventional forming processes, such as extrusion, are applied to micromedical parts such as stents. Because conventional forming processes, size effects due to the crystalline structure and friction of the work material must be controlled. In this study, we analyzed the effects of tool surface properties, such as punch and die surface nanotextures, on the microextrudability (including the extrusion load), product shape, and product crystalline structure, using pure magnesium billets as test pieces. CoCrMo, which is suitable for microextrusion, was used as a new die material. The extrusion load increased rapidly as the stroke progressed under all tool conditions, and it was significantly reduced for nanotextured punches and dies. The nanotextured punches and dies exhibited less adhesion to the tool surface. Material analysis using electron backscatter diffraction revealed that the nanotextured tool promoted crystal refinement, improved material flowability, and uniformly introduced more strain. However, using a mirror tool resulted in lower material flowability and nonuniform strain. AISI H13, CoCrMo, and nanotextured CoCrMo dies were used to analyze the effects of the die material and surface nanotexture. Compared with the AISI H13 die, the CoCrMo die reduced the adhesion to the tool and the machining effect, which resulted in enhanced formability. The CoCrMo die with a nanotexture exhibited the lowest forming load and tool adhesion. These results indicate that the die surface properties can be optimized to reduce tool–material friction and adhesion compared with those of the punch.