A novel cold expansion process for improving the surface integrity and fatigue life of small-deep holes in Inconel 718 superalloys

Abstract

Hole cold expansion is an effective anti-fatigue manufacturing technology, which is widely used in hole strengthening of aviation components. In this paper, a novel small-deep hole (with the diameter less than 2 mm and the depth greater than 10 mm) cold expansion process, called the multi-spherical bump rotating cold expansion process (MBR-CEP), was proposed and verified by experiments on the Inconel 718 superalloy. The MBR-CEP tool with multi-spherical bumps produced by the laser texturing process (LTP) was first designed, and the MBR-CEP with different expansion degrees (δ = 2.1%, 2.6%, and 3.1%) was conducted on small-deep hole specimens. The surface integrity of the small-deep holes before and after the MBR-CEP in the inlet, middle, and outlet areas was studied. Results showed that compared with the small-deep hole after reaming, the hole treated by the MBR-CEP with different δ formed a plastic deformation layer with a maximum depth of 32.73–85.54 μm, a minimum surface roughness of 0.115–0.138 μm in Ra, and a compressive residual stress (CRS) layer with a depth of 0.39–1.077 mm. In addition, the CRS increased by up to 84%, and the microhardness increased by up to 13%. The microstructure evolution on the top surface and sub-surface of the small-deep hole wall was observed by transmission electron microscopy (TEM), which showed that nanocrystals with the size of 48–90 nm and a large number of dislocations and deformation twins formed. Finally, the effectiveness of the proposed process was verified by fatigue tests with different δ at 400 °C. The average fatigue life increased by 3.66 and 8.05 times under δ = 2.1% and 2.6%, respectively. The fatigue fracture surface was examined with scanning electron microscopy (SEM), which revealed that low surface roughness and existence of a plastic deformation layer could effectively reduce the probability of crack initiation.