Microstructure and mechanical properties of the near surface of titanium alloy Ti-6Al-4V machined by micro-textured tools

Cutting tools that incorporate texture on selected active cutting tool surfaces can improve the machinability of Ti-6Al-4V. In the current investigation, the rake face of a polycrystalline diamond (PCD) cutting insert was irradiated with laser light to create four different texture patterns. Outside turning was subsequently performed on Ti-6Al-4V with these inserts to assess their performance. The near surface microstructure of the machined surfaces was investigated using different microscopic techniques. In general, the near surface consisted of a white interface layer (WIL), a plastic deformation zone (PDZ), and then finally the unaffected bulk material. The grains in the PDZ were clearly deformed and rotated towards the cutting direction. The results showed that the extent of the visible Widmanstätten pattern in the PDZ was substantially reduced, accompanied by a concomitant increase in the presence of α-phase grains due to machining. TEM micrographs revealed a bimodal microstructure and a fully developed lamellar structure depending upon the texture pattern utilized. Machining with the textured tools produced lower peak cutting temperatures at the cutting interface that resulted in a reduction of the α grain size. In general, all the inserts utilized produced an increase in the microhardness of the surface due to the plastic deformation and phase transformation induced by the machining. The textured tools did however demonstrate reduced surface hardness, when compared to the plain tool, due to a reduction in the cutting force and subsequent plastic deformation and a reduction in the lamellar nature of the substructure.