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

Abstract

The hardness and orientations of the primary α grains (αp) and the transformed β (βt) 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 αp 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 βt were about 20% lower. In the machined chips, αp grains showed hardness values similar to the as-received material while βt 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 μm and ∼2 μm 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 αp 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 <c+a> slip, the αp 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 α to β phase transformation. A transformation to soft β phase during shearing accelerates plastic instabilities. These observations and comparisons with finite element simulations indicate that temperatures reached the α+β phase field locally, and higher cutting speeds led to more β phase that is detrimental to the tool life.