The purpose of this study is to explore the healing mechanism and repair effect of shot peening on sub-surface cracks in 7075-T651 aluminum alloy. Numerical simulation and experimentation were combined to establish a simplified crack model and ANSYS was used to perform numerical simulation on shot peening repair of cracks to preliminarily verify the feasibility of shot peening repair of cracks. The main factors such as diameter and velocity of the shot during shot peening were considered, and the shot peening parameters were optimized. The experiment followed the simulated crack model and obtained uniform standard samples after 0.3 MPa shot peening pretreatment. Cracks with a width of 0.05 mm and depths of 0.10 mm, 0.15 mm, and 0.20 mm were created with a wire cutting machine, and the cracks were located at the chamfer of the sample. The optimized process according to the simulation results was used for shot peen repair of cracks, and the crack repair effect was analyzed and evaluated from residual stress, fatigue strength, and microstructure. For this research of crack model with a crack width of 0.05 mm and a depth of 0.15 mm, simulation results showed that the best shot peening repair effect was achieved with a shot diameter D=0.5 mm and a shot velocity v=100 m/s. The optimized shot peening process parameters in the simulation corresponded to an experimental shot peening pressure of 0.56 MPa, which was used to carry out experiments on repairing surface cracks in aluminum alloy by shot peening. The experiment showed that in terms of residual stress, X-ray diffraction was used to measure the residual stress in the crack area and non-crack area of the sample. After the crack repair experiment, the residual stress at the crack was 83.17% of that in the non-crack area, which was close to the simulation result of 81.97%. At the same time, the overall deviation between experiment and simulation residual stress was about 20 MPa, which was due to the fact that the experimental sample underwent 0.3 MPa shot peening pretreatment, but the change trend between experiment and simulation was the same, indicating that shot peening had a good effect on repairing cracks. In terms of fatigue strength, experiments showed that as crack depth increased, sample fatigue strength decreased and sample damage became more severe. For shot-peened samples, the ratio of fatigue strength between samples and standard samples without cracks could represent the degree of crack repair. Experiments showed that cracks with depths of 0.1 mm and 0.15 mm had similar repair effects at 71.18% and 70.32%, respectively; while cracks with depths of 0.20 mm had a repair degree of only 44.36%, indicating poor repair effect. Shot peening could largely repair surface cracks while also having the effect of strengthening materials to improve their fatigue strength. However, fatigue testing of samples showed that samples still broke along repaired cracks after shot peening repair, indicating that cracks after shot peening repair were still weak points for sample fatigue and that shot peening repair had limited effect. In terms of microstructure, metallographic microscope and scanning electron microscope were used to observe the morphology of repaired cracks in samples by shot peening. Microstructure studies showed that severe impact from shot peening caused sub-surface material around cracks to generate large micro-deformation heat conducive to deformation organization formation, promoting closure extrusion between grains on both sides of cracks at macro level manifested as tight closure under compressive stress organization. This closure played a role in repairing cracks overall belonging to physical repair but was still unable to completely eliminate negative impact from cracks on materials. The study shows that shot peening repairs sub-surface material cracks through pressure action, and restores the material fatigue strength was well. It has positive meaning for early repair and emergency repair for aluminum alloy structural components. © 2024 Chongqing Wujiu Periodicals Press. All rights reserved.
