With the increasing lightweight requirements of aviation equipment, thin-walled structural parts have become an indispensable key part of aerospace. These parts are generally made of light alloys such as aluminum alloy. However, the structural strength of aluminum alloy aviation thin-walled parts is low and the fatigue crack propagation resistance is weak. The fatigue life is the key to restricting the service life of aerospace vehicles. The fatigue crack sources are generally concentrated on the surface of parts, so many surface deformation strengthening technologies including laser shock peening have attracted much attention in recent years. However, the action time of laser shock wave is very short, the induced microstructure is mainly a dislocation structure, the stabilities of the microstructure and stress are poor, and the surface plastic deformation is serious, which all reduce the quality of laser shock peening. Therefore, it is necessary to explore new processes to improve the surface quality of machined parts, except to use the advantages of laser shock peening. Because the ultrasonic impact peening technology uses high-frequency vibration to impact the material surface, produce an ultrasonic shock wave, refine the grains on the material surface, and form residual compressive stress field, it can obtain more stable microstructures and residual stress than laser shock peening. However, the shock wave pressure is low in the process of ultrasonic impact peening, and thus a long time is generally needed to obtain a better strengthening effect, which results in a low strengthening efficiency. The laser ultrasonic composite shock peening technology uses the composite effect of an ultrasonic shock wave and a laser shock wave to regulate the microstructure and surface residual stress, so as to improve the surface integrity and mechanical properties of metal materials. However, there are only few reports on the exploration of the laser ultrasonic composite shock peening technology at present. Taking the 2024-T351 aluminum alloy as the research object, this paper experimentally studies the influence of laser ultrasonic composite shock peening (LSP+ UIP) on the surface morphologies and microstructures of materials, analyzes the relationship between microstructures and mechanical properties, and explores the evolution mechanism of microstructures and the change law of mechanical properties under the action of LSP+ UIP.

The 2024-T351 aviation aluminum alloy plate with a thickness of 2 mm is selected as the experimental material. The rectangular sample is cut and polished. The sample surface is strengthened by laser ultrasonic composite shock peening. The surface morphology and surface linear roughness in the treatment area of the strengthened sample are measured. First, the surface micro-hardness of the sample is tested by the micro-hardness tester and the surface residual stress of the sample is tested by the residual stress tester. Second, the sample is cut along its section direction by wire cutting, the cross section is ground and polished, the sample's section is soaked with the Keller's corrosive agent for corrosion, and the sample's section is observed by the scanning electron microscope after treatment. Third, the surface layer of the sample is subject to wire cutting, grinding, and ion thinning until the thickness is ≤200 nm, and the microstructure of the sample's section is observed by the transmission electron microscope.

Through surface morphology observation and roughness measurement, it can be seen that the surface quality of the LSP+ UIP area is significantly improved compared with that of the LSP area (Fig. 3). Compared with LSP, LSP+ UIP reduces the surface roughness of the sample by 34.19% (Fig. 4). The scanning electron microscope shows that the surface hardened layer depth of the LSP+ UIP sample reaches 20 μm. The depth of the grain refinement layer is about 50 μm. The average grain size is about 7 μm. Compared with those of the LSP and UIP samples, the average grain size of the LSP+ UIP sample is reduced by 53.3% and 50.0% (Fig. 5), respectively. It is observed by the transmission electron microscope that many nanometer-sized sub-crystals are formed on the surface of the LSP+ UIP sample, and the dislocations with a certain density are gathered around some sub-crystals, indicating that the LSP+ UIP strengthened structure effectively combines the structural response characteristics induced by plastic deformation of LSP at a high strain rate and UIP at a low strain rate and the microstructure of "nanocrystalline+ stacking dislocation" is formed (Fig. 6). The surface micro-hardness of the LSP+ UIP strengthened sample is up to 189.0 HV, increased by 12.9% and 6.4% compared with those of the LSP and UIP samples, respectively (Fig. 7). Because LSP+ UIP can obtain higher plastic deformation than LSP or UIP, the induced residual compressive stress is significantly higher than those for LSP and UIP. Compared with those by LSP and UIP, the residual compressive stress induced by LSP+ UIP is increased by 25.8% and 18.2%, respectively (Fig. 8).

The effects of laser ultrasonic composite shock peening on the microstructure and surface properties of the 2024-T351 aluminum alloy are studied, and the evolution and strengthening mechanism of the microstructure under laser ultrasonic composite shock peening are analyzed. Laser ultrasonic composite shock peening can effectively solve the phenomenon of surface quality reduction after the LSP treatment, reduce the material surface roughness, and improve the surface flatness of the material. In terms of microstructure, LSP+ UIP can further refine the grains and form the microstructure of "nanocrystalline+ stacking dislocation" . In terms of micro-hardness, LSP+ UIP significantly increases the surface hardness of the material, increased by 12.9% and 6.4% respectively compared with those by LSP and UIP. In terms of residual stress, LSP+ UIP introduces a large amplitude of residual compressive stress on the material surface. Compared with those by LSP and UIP, the residual compressive stress induced by LSP+ UIP is increased by 25.8% and 18.2%, respectively. Our research shows that with a reasonable experimental design, laser ultrasonic composite shock peening can make 2024-T351 aluminum alloy obtain better surface properties.