Influence of microstructure and hardness on machinability of heat-treated titanium alloy Ti-6Al-4V in end milling with polycrystalline diamond tools

The main objective of this paper is to develop the relationship between microstructure and machinability of titanium alloy Ti-6Al-4V which is given heat treatment beyond beta transus temperature, with the help of milling experiments using polycrystalline diamond (PCD) tools. Two of the specimens were solution treated at 1050 A degrees C/1 h. After the treatment, one of the specimen was cooled in water while the other specimen was air-cooled, followed by aging treatment at 550 A degrees C/4 h. The third specimen was used in as-received condition (casted). Cutting speed, feed rate, and depth of cut were varied, and their effects have been analyzed on cutting forces, vibration amplitude, temperature, and tool wear for all the three specimens. The results have shown improved and better machinability for the specimen which was cooled in air after the solution treatment with PCD tools. This behavior of the alloy is mainly attributed to formation of homogeneous lamellar alpha + beta Ti-6Al-4V microstructure. Although, the air-cooled specimen has higher hardness than the as-received specimen. But, the former alloy's fine homogeneous and lamellar alpha + beta Ti-6Al-4V microstructure as compared to the microstructure of the latter specimen (irregular alpha + beta Ti-6Al-4V structure) and absence of alpha' secondary precipitate (found in water-cooled specimen) has helped in giving improved performance. Thus, from this study, it can be concluded that machinability of titanium alloy Ti-6Al-4V can be further improved with controlled heat treatment process using PCD tools.