Effect of Phase Transformation upon Hole Making Accuracy of Ti6Al4V by Orbital Drilling

Abstract Ti6Al4V, which is one of difficult-to-cut metals, is widely used in an aircraft structure, parts of a gas turbine and medical equipment, and a hole making operation of Ti6Al4V is needed to fasten the parts. When a high speed drilling by a conventional twist drill is applied to hole making of Ti6Al4V, it is very difficult to obtain highly accurate hole in diameter, roundness and inlet - outlet edge quality due to a rise of cutting temperature caused by a small heat conductivity. Incidentally, it is well-known that Ti6Al4V transiently causes phase transformation from α phase (close-packed hexagonal lattice) to β phase (body-centered cubic lattice) as soon as it reaches the transformation temperature of 883 ℃ (1621 F). One of the authors newly developed a hole making machine to enable orbital drilling based on Double Eccentric Mechanism, in which an endmill driven by a built-in AC motor can rotate clockwise on its own axis at high speed and simultaneously can revolve counter-clockwise on eccentric axis at low speed [1]. In dry cut orbital drilling tests of Ti6Al4V by the machine, it was ascertained that a hole diameter decreased from 15.02 mm at the first hole to 14.43 mm at the 40th hole although wear of endmill was maintained to be tiny value. However, by supplying oil mist from oil hole in the endmill the hole diameter was almost constant from 15.00 mm at the first hole to 14.96 mm at the 40th hole [2]. Based on the fact ascertained by temper color of Ti6Al4V’s chip that the temperature of Ti6Al4V being machined exceeded the transformation temperature of 883 ℃ (1621 F) after the 16th hole drilling, this paper discloses that the phase transformation from α phase to β phase may be a prime cause of the decrease in hole diameter by applying an image data processing to microstructures obtained optically from the specimen of Ti6Al4V around the inner surface of hole.