Effects of cutting speed on surface integrity evolution and fatigue behavior of shot-peened Inconel 718 in sequential turning and shot peening

The fatigue performance of conventionally shot-peened components is highly sensitive to the surface conditions produced by prior machining, due to variations in surface integrity. This study investigates the influence of cutting speed during turning (ranging from 30 m/min to 120 m/min) on surface characteristics and how these affect the subsequent shot peening response and fatigue behavior of Inconel 718. A comprehensive surface integrity evaluation was conducted before and after shot peening, including residual stress profiling, grain refinement analysis, work hardening assessment, and subsurface microcrack detection. The results reveal that cutting speed significantly alters the initial surface condition, leading to varied responses to shot peening and, consequently, to distinct fatigue performance. In particular, the T₂SP process achieved a fatigue life 11.79 times greater than the T₄ process, primarily due to more favorable residual compressive stresses, finer grains, and enhanced work hardening, while suppressing microcrack formation. Conversely, excessive surface hardening in the presence of subsurface microcracks was found to increase stress concentration, leading to premature fatigue failure. These findings highlight the critical role of pre-treatment conditions in governing failure resistance and demonstrate that optimizing machining parameters is essential for maximizing the effectiveness of surface treatments like shot peening.