High-temperature fatigue life improvement of small-deep holes by using a novel cold expansion process in a nickel-based superalloy

Cold expansion of holes is a typical strengthening technology widely used in aviation parts and can effectively improve the fatigue life of hole structures. In this paper, a novel hole strengthening technology called the multiannular convex hull rotating cold expansion process (MCR-CEP) for small-deep holes (diameter 1.72 mm, depth 10 mm) is proposed. After application of MCR-CEP at different expansion degrees (delta= 20-50 mu m), the hole walls exhibit a lower surface roughness (Ra from 0.152 mu m to 0.296 mu m), higher microhardness (HV from 465 to 510), an obvious plastic deformation layer (depth of 15-35 mu m) and a compressive residual stress layer. Under the optimal expansion degree (delta= 40 mu m), the average fatigue life of the MCR-CEP specimens increased by 1.06-3.18 times at different thermal loads (300-700 degrees C) and mechanical loads (700-1000 MPa). The results of scanning electron microscopy (SEM) examination of fatigue fractures show that the possibility of crack initiation can be effectively reduced by the existence of a low surface roughness and plastic deformation layer in the top layer of the hole wall when the tested temperature is lower than 600 degrees C. The higher the mechanical loads are, the closer the crack initiating source is to the surface of the hole wall.