Influence of Deformed Surface Layer When Machining Wrought Alloy 718 in an Annealed and in a Pre-Strained Condition

Abstract

Superalloys are difficult to cut, mainly due to their preserved strength at elevated temperatures and their exceptional work hardening, where the latter results in a deformed layer on the machined surface. The thickness and degree of deformation of this layer depends on several parameters, such as cutting speed, uncut chip thickness, tool material and edge geometry. This layer enters the cutting zone of subsequent revolutions in turning or in milling. Under certain circumstances, as when the uncut chip thickness is small, all deformation at subsequent machining takes place within the deformed layer. The same phenomenon may occur at the outer edge of the cutting zone where the unrestricted material flow leads to burr formation. The deformed layer at the outer edge is suggested as the cause for notch wear often found when machining Superalloys. In this paper the effect of deformed layer on machining is examined through controlled experiments on the material in to different states of strain; in the annealed state without any remaining strain and with a substantial amount introduced by tensile straining. The results showed that cutting forces were reduced when pre-straining the material. Tool wear was similar in both materials, with large scatter due to built-up edge formation.