| The destruction of the mechanical structure starts from material surface and the quality of the surface performance of the aircraft parts directly affect flight safety. Ultrasonic shot peening(USP) as a new, energy saving and high efficiency technology of material strengthening and forming can significantly improve the surface performance of forming parts, which has a widely application prospect in local forming and sizing, surface strengthening of the large plane wall plate structure.Therefore, the research on material surface performance with ultrasonic peening is extremely critical.Not only can it provide technology guidance to practical production, but also has a great significance for prolonging the life of flight.2024 aluminum alloy is used in the stiffened panel of C919 airliner, which was been processed by USP local forming. According to this engineering practice, this paper analyses the effect of different peening process parameters on surface performance of 2024 aluminum alloy. Based on USP forming theory, this paper analyzes the principle of sheet forming and the general raw for the change of surface morphology and microstructure after USP treatment, providing theoretical basis for the study of the surface performance.Target material are impacted by USP firing pin with high energy and the surface of bump area generates plastic deformation result from great tension exceed the elastic limit to prevent the rebound of lower compression grains and produce spherical pit with high downward compression force and uplift spherical edge. Numerous pits overlap with each other, which would produce the residual compressive stress layer, to make sheet forming. This paper study 2024 aluminum alloy plate under USP for various diameters of peening firing pin and a constant impact amplitude. Observations found that the original residual milling traces on the surface of the material replaced by USP forming impact craters and the residual stress, hardness and surface roughness of forming part were greatly increased.Effect of strengthening on residual compressive stress is not effective and surface hardness is obviously improved while roughness is slightly reduced.Results of fatigue test show that anti-fatigue damage ability of materials have positive correlation with dimension and depth of residual compressive and negative correlation with surface roughness and no correlation with surface hardness. Therefore, residual compressive is the critical factor to analysis and evaluate the surface properties of sheet metal forming under USP technology. The introduction of residual compressive leads to the crack originated from the surface of the material.The bigger residual stress result in the stronger anti-fatigue damage ability. It is concluded that carbonand oxidation are appeared in the fracture surface during the fatigue process.The erosion as exposing to air are replaced by the salt spray corrosion test. Research shows that USP can significantly improve the corrosion resistance of materials and the roughness and decrease hardness of the materials. The relatively short corrosion time has little effect on the fatigue life of the specimen. After prolonging the corrosion time, the residual stress can not restrain the crack propagation and the specimen is quickly broken. Therefore, in view of the seriousness of the problem,anti-corrosion measures should be taken for aircrafts which exposed to the air for a long time or flight in the serious salt fog environment. |
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