Effect Tool Wear During End Milling on the Surface Integrity and Fatigue Life of Inconel 718

Abstract

Inconel 718 is known to be among the most difficult-to-cut materials due to its high strength even at high temperatures, low thermal conductivity, and especially rapid work hardening. Machining Inconel 718 is a challenging task since tool wear adversely affects surface integrity and product performance of machined components. In this paper, the effect of tool wear on surface integrity and its impact on fatigue performance of Inconel 718 alloy (45 ± 1 HRC) by end milling using PVD coated tools are studied. The evolutions of surface integrity including surface roughness, microstructure, and microhardness were characterized at three levels of tool flank wear (VB = 0, 0.1mm, 0.2mm). At each level of tool flank wear, the effects of cutting speed, feed, and radial depth-of-cut on surface integrity were investigated respectively. End milling can produce surface finish between 0.1μm and 0.3μm under most of the conditions. Roughness is generally higher in step-over direction than feed direction. No obvious white layer is observed in subsurface microstructure. The machined surface is significantly work-hardened due to the dominant mechanical loading. Four-point bending fatigue test shows that none of the milled samples failed within four million cycles. Fatigue endurance limits of the machined samples at different reliability levels were calculated and correlated with the experimentally determined fatigue life.