Influence of surface integrity induced by multiple machining processes upon the fatigue performance of a nickel based superalloy

Machining operations are of key importance to the fatigue performance of nickel based superalloys due to the high thermal/mechanical loadings yielded on the machined workpiece which can significantly alter the surface integrity of the components. Therefore, understanding the influence mechanisms of machining induced surface integrity upon fatigue response is vital to determine their manufacturing processes and applications. In this respect, this paper investigates the surface integrity of nickel based superalloy subject to different mechanical and thermal loadings induced by various machining processes including conventional machining (e.g. finish and rough milling) and nonconventional machining (e.g. laser assisted milling and abrasive waterjet cutting) methods, as well as their influences upon fatigue performance and failure mechanisms. In-depth surface metallurgical and crystallographic analysis has been conducted to reveal the surface damage mechanisms, which allows the description of the machining induced mechanical and thermal alterations on the machined workpiece. Furthermore, the examination of the fractography from the fatigue specimen has been conducted, which enables the understanding of the influence mechanism of the corresponding surface defects on the fatigue crack initiation and propagation, subject to a four points bending fatigue test. While the resulted S-N curves indicate that the high cycle fatigue of machined nickel based superalloy is mainly dominated by the machining induced residual stress conditions, the surface defects from different machining processes can particularly influence fatigue crack initiation and propagation mechanisms in both the low and high cycle regimes.