Hole is widely considered as a kind of geometrical–discontinuous structure with high stress or strain concentration.The effect of direct hole cold expansion (HCE) using the mandrel on the fatigue behaviour of centre holes of FGH95 superalloy was investigated. The fatigue fracture and the main parameters of surface integrity of hole wall were characterized by scanning electron microscopy (SEM)

roughmeter

X–ray diffraction (XRD) instrument and microhardness tester. The mechanism of the HCE on the fatigue life was also investigated. The results show that the median fatigue life of the HCE specimens increased by above 0.9 times and 10.3 times under room temperature/650MPa and 527℃/575MPa conditions compared to the specimen without HCE

respectively. It is found that the roughnesss of hole wall decreased sharply

while the deep surface strengthened layer with high hardness and residual compressive stress were formed around the hole after HCE. These changes are of great benefit to the enhancement of the fatigue life under room and high temperature. In addition

the exit of grain boundary and the difference of crystallographic orientation of neighbor grains have an evident effect on the fatigue crack growth path.