Design of a novel cold expansion tool for deep small holes based on FEM simulations and experimental study

Cold hole expansion is a crucial technology for enhancing the fatigue life of hole structures. This paper presents the design and optimization of a novel multi-convex hulls expansion tool for deep small holes in Inconel 718 superalloy using the finite element method (FEM) and experimental techniques. The results from the FEM model, which was used to investigate the compressive residual stress (CRS) distribution and contact force during the expansion process, indicate that the curved single convex hull (CSCH) structure is more suitable for deep small hole expansion (DSHE). However, accounting for the strength limitations of the actual expansion tool, a multi-convex hulls expansion tool structure was proposed. The FEM model was employed to compare the effects of various tool structures on the hole wall CRS distribution and the contact force during the expansion process. The obtained optimal parameters for enhancing deep small holes in Inconel 718 superalloy under high temperature and high load conditions are as follows: expansion section hull spacing l1 = 0.4 mm, taper ratio C = 0.008, number of convex hulls in the sizing section n = 10, and sizing section hull spacing l2 = 1 mm. Moreover, an optimized convex hull expansion amount allocation scheme was obtained through an experimental study. The results of the FEM simulations and experimental study demonstrate that the optimized multi-convex hulls expansion tool (with an expansion rate of 1.92%) can induce the formation of a high CRS layer and a refined surface without causing significant microcracks in the hole walls.