Study On the Residual Stress Distribution of Bi-directional Cold Expansion Process Performed On Open Holes

Traditional unidirectional cold expansion technology usually generates non-uniform distribution of residual stress in the thickness direction of holes, which is harmful to the improvement of fatigue life of holes. The present work proposed a bi-directional cold expansion procedure to realize the homogenization of residual stress in the thickness direction of the cold expanded hole, thereby further improved anti-fatigue performance of cold expanded hole. For this aim, a series of finite element (FE) simulations were carried out to investigate the effectiveness of the bi-directional cold expansion procedure and optimize the process parameters. The results showed that the optimized bi-directional cold expansion process generated a more uniform distribution of residual circumferential compressive stress in the thickness direction comparing to the simplified bi-directional cold expansion process using a single mandrel. For the Aluminium alloy 7050-T7451, when the first interference level I1=1.8%, the largest and the most uniform residual circumferential compressive stress was achieved, which suggested the best anti-fatigue performance.