Metallurgical analysis and nanoindentation characterization of Ti–6Al–4V workpiece and chips in high-throughput drilling

The metallurgical analyses, including scanning electron microscopy (SEM), X-ray diffraction (XRD), electron microprobe, and nanoindentation characterization are conducted to study the Ti–6Al–4V hole surface and subsurface and the chips in high-throughput drilling tests. The influence of high temperature, large strain, and high strain rate deformation on the β → α phase transformation and mechanical properties is investigated. Diffusionless β → α phase transformation in the subsurface layer adjacent to the hole surface can be observed in dry drilling, but not in other drilling conditions with the supply of cutting fluid. Nanoindentation tests identify a 15–20 μm high hardness subsurface layer with peak hardness over 9 GPa, relative to the 4–5 GPa bulk material hardness, adjacent to the hole surface in dry drilling. For drilling chips, the β phase is retained under all conditions tested due to rapid cooling. On the chips, the saw-tooth feature and narrow shear bands are only formed at the outmost edge and no significant change of hardness across the shear bands can be found in nanoindentation.