Effect of CoCrMo Die and Tool Surface Nano-Texture on Micro Backward Extrusion Formability of AA6063-T6

Abstract To manufacture microparts used in medical and electronic devices, the machining scale must be reduced to the microscale. However, when applying existing plastic forming processes to the machining of microscale parts, the size effect caused by material properties and friction results in variations in product accuracy. To suppress the size effect, tool materials and tool surface treatments suitable for microscale machining must be considered. Using AA6063-T6 billets as test specimens, this study investigated the effects of tool surface properties, such as die surface nanotexture, on micro-extrudability such as extrusion force, product shape, and crystal structure of the product. A cobalt-chromium-molybdenum (CoCrMo) die was used as a new die material suitable for micro-extrusion. To investigate the effects of the die material and die surface nanotexture, AISI H13, CoCrMo, and nanotextured CoCrMo dies were used. The extrusion force increased rapidly with the progression of the stroke for both dies. Compared with the AISI H13 die, the CoCrMo die with nanotexture exhibited considerably lower extrusion force, longer extrusion length, and less adhesion on the die surface. The results of material analysis using electron backscatter diffraction indicated that the nanotextured CoCrMo die improved material flowability and facilitated the application of greater strain. In contrast, the AISI H13 die exhibited lower material flowability and nonuniform strain. Therefore, the tribology between the tool and material was controlled by changing the surface properties of the die to improve the formability.