Microstructure- and cooling/lubrication environment-dependent machining responses in side milling of direct metal laser-sintered and rolled Ti6Al4V alloys

Ti6Al4V manufactured by direct metal laser sintering (DMLS) is a difficult-to-cut material and has a different microstructure to conventional Ti6Al4V. Therefore, this work aims to investigate the influence of microstructure characteristic and sustainable cooling/lubrication method on machining responses of Ti6Al4V. Side milling tests were performed under dry and supercritical CO2-based minimum quantity lubrication (scCO2-MQL) conditions. Experimental materials were DMLS-produced Ti6Al4V and conventional rolled one. Milling force, subsurface microstructure evolution, crystallographic texture, and residual stress were evaluated. For DMLS Ti6Al4V, subsurface plastic deformation depth increased with increasing cutting speed vc. Only under the most drastic condition (highest vc = 180 m/min, dry milling), a white layer with grain refinement down to 0.4 μm was formed, which resulted from α→β phase transformation and twinning-induced dynamic recrystallization. The activated twinning within the grain refinement layer was 101¯21¯011 and 101¯11¯012 twinning. For conventional Ti6Al4V, in contrast, no subsurface microstructure evolutions were identified after milling. Milled surface of rolled Ti6Al4V exhibited residual tensile stresses while DMLS Ti6Al4V exhibited residual compressive stresses. Milling force of rolled alloy was 15 % higher than DMLS-produced one. The different cutting responses of the two alloys are attributed to the fact that rolled Ti6Al4V has fine-grained microstructures while DMLS alloy has coarse-grained microstructures. The proposed scCO2-MQL method can improve machinability and surface integrity, and suppress the activation of high order twinning of DMLS Ti6Al4V. This study's findings may be useful in developing finite element or analytical models for predicting the microstructural variations of DMLS Ti6Al4V during cutting.