Improving fatigue properties of normal direction ultrasonic vibration assisted face grinding Inconel 718 by regulating machined surface integrity

Fatigue properties is crucial for critical aero-engine components in extreme environments, and it is greatly affected by surface integrity (SI) indexes (especially surface topography, residual stress σres, and microhardness) after machining processing. Normal-direction ultrasonic vibration-assisted face grinding (ND-UVAFG) has the advantage in solving poor machinability of Inconel 718, but there is a competitive between higher compressive residual stress and higher surface roughness in the influence of fatigue strength. Lack of quantitative relationship between multiple SI indexes and fatigue strength makes it difficult to determine the control strategy for improving fatigue properties. In present work, a model of fatigue strength (σf)sur considering multiple SI indexes was developed. Then, high cycle fatigue tests of Inconel 718 samples with different SI characteristics were carried out, and the influence of ND-UVAFG process parameters on SI was analyzed. Based on SI indexes data, the (σf)sur of ND-UVAFG Inconel 718 with different SI characteristics was calculated by the developed model, and fatigue crack initiation (FCI) sites was predicted. The predicted FCI sites were consistent with the experimental results, which verified this model. A strategy for improving the fatigue life was developed in this work, which was to transfer the fatigue source from the machined surface to the bulk material by controlling the SI indexes. Finally, a critical condition of SI indexes that FCI sites appeared in the surface or bulk material was given by fitting the predicted results. According to the critical condition, a SI field where FCI sites appeared in the bulk material could be obtained. In this field, the fatigue life of Inconel 718 samples could be improved by about 140%.