Effect of Coolant upon Hole Making Accuracy of Ti6Al4V by Drilling - Consideration of Hole Diameter in the Depth Direction -

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 so that a hole making operation of Ti6Al4V is indispensable to fasten the parts. When a highspeed 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 its small heat conductivity. Moreover, it is well-known that Ti6Al4V causes transiently phase transformation from α phase (close-packed hexagonal lattice) to β phase (body-centered cubic lattice) as soon as it reaches the phase transformation temperature of 883 ℃ (1621°F).When coolant is spouted from two coolant holes at the tip of twist drill to decrease a rise of cutting temperature, the temperature rise of twist drill and chip will be restrained thoroughly. However, it is thinkable that the temperature of Ti6Al4V neighboring the hole’s wall drilled would not be reduced so much since heat entering the workpiece is scarcely carried away by supplying coolant. In a machining site a deep hole shape of Ti6Al4V drilled often faces to be smaller in the depth direction although it is well-known that those of steel and aluminum alloy are smaller at the top than near the bottom such as a bell shape.To make clear the effect of coolant upon hole diameter in drilling of Ti6Al4V, a lot of holes having ϕ 8 mm × 24 mm depth for Ti6Al4V were machined by dry drilling and wet drilling on the condition that a feed rate is fixed at 0.05 mm/rev and a rotational speed is varied at 12 steps from 500 rpm to 2500 rpm. After hole diameter and roundness were measured in relation to depth of hole, effect of temperature heated and phase transformation of workpiece upon hole diameter is considered by using the temperature distribution in Ti6Al4V neighboring the hole’s wall drilled, which is estimated by referring the temperatures of workpiece, machined surface and drill during drilling verified by previous researchers, and metallurgy of phase transformation.Consequently, it was ascertained that the hole shape of Ti6Al4V, which is larger at the top than near the bottom, is caused by contraction and phase transformation of Ti6Al4V due to heat entering workpiece, and coolant is not effective to correct the hole shape in the depth direction although it decreases hole diameter uniformly from the top to the bottom due to reduction of drill’s heat expansion.