| Warm laser peening (WLP) can modify the material surface properties by combining light/mechanics/thermotics muti-energy fields. WLP processing can manipulate and optimize the mechanical and fatigue properties of materials by improving the microstructures induced by laser shock wave, which significantly enhances the stability and reliability of mechanical properties generated by laser peening. Aiming at the vibration fatigue failure of aviation parts made by aerial light alloy, the surface modification induced by thermal-mechanical coupling effects and its effects on the vibration fatigue life and fracture were researched. Moreover,2024-T351 aluminum alloy was used to investigate the effects of WLP on the microstructure evolution, residual stress and modal parameters through theoretical, simulation and experimental methods. Finally, the fatigue improvement and fracture mechanisms were researched and the technological criterion of WLP was established. The main contents of this paper include the following aspects.Firstly, the induced and relaxed mechanism of compressive residual stress was explored.An X-ray diffraction method was used to measure the residual stress induced by WLP and its relaxation during cyclic loading. The physical model of compressive residual stress and plastic strain were deduced in terms of the microstructures generated under elevated temperature and high strain rate. Based on the dynamic precipitation in aerial light alloys, the Johnson-cook model was corrected and thus the simulations of residual stress induced by WLP were explored. Moreover, the relaxation mechanism of residual stress generated by WLP was researched and its finite element analysis was established.Secondly, the dynamic precipitation, dislocation evolution and dynamic recry-stallization during thermal-mechanical processing were investigated. The optical microscope,XRD diffraction equipment and transmission electron microscopy (TEM) were adequately used to measure the morphology of precipitates, dislocations and grains/sub-grains of 2024-T351 aluminum alloy treated by WLP. The dynamic recrystallization mechanism during WLP was put forward based on the analysis of dislocation evolution. The molecular dynamic model of Al-Cu alloy containing dynamic precipitates was built based on the lattice parameters of S-phases. The effects of dynamic precipitates on laser shock wave and its induced dislocation morphology and density were researched, which provided physical basis for dislocation evolution and dynamic recrystallization during WLP.Thirdly, the improvement of natural frequency and damping ratio induced by WLP was studied. The elastic module and nano-hardness of 2024-T351 aluminum alloy were measured by nanoindentation tests. The mathematical model about the relationship between elastic module and natural frequency was deduced and thus the simulations of modal analysis for structures made by aerial light alloys were put forward. The damping ratio of cantilever beam treated by WLP was measured by experimental modal analysis. Based on the G-L damping theory, the effects of dynamic precipitation and dislocations induced by WLP on damping ratio were analyzed. The mathematical model of vibration stress considering damping ratio and compressive residual stress was deduced on basis of the stress state on the WLPed surface. Moreover,frequency response analysis was conducted in this study to investigate the effects of mechanical properties including damping ratio on the vibration stress in dangerous sections.Fourthly, the estimation of vibration fatigue life induced by WLP was studied. The measurement of crack initiation life and crack growth life were both researched. The mathematical models of vibration fatigue life during crack initiation and crack growth period were respectively deduced considering yield stress, damping ratio and compressive residual stress, which laid the foundations for the mechanism of lifetime extension and its estimation.Aiming at 2024-T351 aluminum alloy, the fatigue damage rate during vibration were measured by decreasing rate of natural frequency which was used to analyze the crack initiation life and crack growth life generated by WLP. The finite element simulations was conducted on WLPed samples by ABAQUS and MSC.Fatigue software based on the technological process of "Microstructures - compressive residual stress & natural frequency& damping ratio - vibration stress - fatigue life".Finally, vibration fatigue fractures of WLPed 2024-T351 aluminum alloy was tested and discussed in the paper. Based on vibration fatigue damage, the composition and characteristics of vibration fatigue fractures were studied. The effects of WLP on vibration fractures were analyzed considering vibration stress and material's strength. The vibration fatigue fracture mechanism of WLPed aerial light alloy was investigated.Moreover,Ti6A14V titanium alloy was used in this study to investigate the effects of WLP on the phase morphology and dislocations of muti-phase alloys, which laid the foundation for the application of WLP to muti-phase alloys. Based on the results of theoretical analysis,numerical simulation and experimental research,WLP shows an advantage in stability of microstructures and compressive residual stress. Therefore, WLP can significantly increase the vibration fatigue life and effectively delay the fracture failure of aerial light alloy which provides a stable and reliable technology to extend the service life of the aircraft components. |
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