Machining conditions impact on the fatigue life of Waspaloy: impact of grain size

To assess the severity of a machining process and its impact on the integrity of engine turbine disk, near-surface microstructure (NSM) is usually an indicator. Waspaloy was machined by broaching with different severities (mild, aggressive and very aggressive), tested and compared with low-stress ground material. This work has shown that for Waspaloy, even though the NSM is a good indicator of the severity of the broaching process, it is not necessarily a good predictor of the resultant low –cycle fatigue (LCF) life. Waspaloy displayed a fatigue life dependent on grain size. ASTM4 and ASTM7 material showed different fatigue behavior. Fatigue life of large grain (LG) Waspaloy improves when the material is mildly broached compared to low-stress grinding, while fine grain (FG) material does not see improvement unless the broaching is more aggressive. For LG, suppression of supergrains by mild broaching may be the cause of increasing life, while for FG aggressively broached; surface compressive residual stresses may be the driver. When the broaching is very aggressive, causing the formation of a highly distorted surface layer, none of these phenomena seems to occur and the fatigue life of the material decreases significantly. (AU) © 2010 by The Materials, Metals, & Materials Society. All rights reserved.