Effect of cutting speed on shear band formation and chip morphology of Ti-6Al-4V alloy using nanoindentation and EBSD mapping

The hardness and orientations of the primary & alpha; grains (& alpha;p) and the transformed & beta; (& beta;t) grains in segmented chips obtained by turning an as-received solution treated-and-aged bar of Ti-6Al-4V (wt.%) at cutting speeds of 1, 1.5 and 2 m/s (61, 91, and 122 m/min) were analyzed using nanoindentation mapping and electron backscattered diffraction (EBSD) maps. The hardness of each & alpha;p grain in the as-received Ti-6Al-4V bar highly depends on the crystal orientation, varying from 4.5 GPa to 6.7 GPa, and the hardness values of & beta;t were about 20% lower. In the machined chips, & alpha;p grains showed hardness values similar to the as-received material while & beta;t became slightly harder. The homogeneous shear strain within the segments (between shear bands) was estimated to be around 0.4-0.7 at all three cutting speeds. The width of the shear bands in the 1 and 1.5 m/s chips were -1.3 & mu;m and -2 & mu;m in the 2 m/s chips, which are 10-40% of thicknesses predicted by Molinari's isotropic continuum model, respectively. A smaller homogeneous shear strain was correlated with larger shear cracks at each cutting speed. Grain orientations favored by prism (a) slip in & alpha;p when cutting at 1 m/s made the localized shear band less adiabatic since both the shear stress and strain were lower when these orientations were dominant. In contrast, in orientations that favored pyramidal slip, the & alpha;p grains experienced much higher shear stress and more localized shear strain when cutting at 2 m/s, leading to higher temperatures that were much more likely to induce the & alpha; to & beta; phase transformation. A transformation to soft & beta; phase during shearing accelerates plastic instabilities. These observations and comparisons with finite element simulations indicate that temperatures reached the & alpha;+& beta; phase field locally, and higher cutting speeds led to more & beta; phase that is detrimental to the tool life.

Automated summary

The hardness and orientations of the primary αp and transformed βt grains in segmented chips obtained by turning Ti-6Al-4V bar were analyzed. The hardness of αp grains highly depends on the crystal orientation, varying from 4.5 GPa to 6.7 GPa. In the machined chips, αp grains showed similar hardness values while βt grains became slightly harder. The width of shear bands in the chips varied and smaller shear strain was correlated with larger shear cracks.