The TA15 titanium alloy is an important aerospace material that is extensively used for the manufacturing of aircraft structural parts. As aircraft missions over the sea have become more frequent in China, airframe components have become prone to electrochemical corrosion owing to factors related to the humid marine atmosphere. Simultaneously, the surfaces of the aircraft components are susceptible to damage from external objects during operation, leading to failure. Therefore, improving the surface properties of materials is crucial for extending the service life of critical parts. Currently, surface modification of materials can be achieved using surface-strengthening technologies. Laser shock peening (LSP) offers advantages such as high efficiency, controllability, and precision. It can introduce a larger compressive residual stress (CRS), has smaller impact on the surface roughness, and does not compromise the surface integrity of a material. Hence, we perform surface strengthening on the TA15 alloy using LSP, and conduct in-depth research and comparative analyses of the microstructural evolution of TA15 titanium alloys with different microstructures after surface modification. This can provide a theoretical basis and technical support for improving the corrosion resistance of TA15 titanium alloys and extending the service life of aircraft structural components.

First, TA15 titanium alloy was subjected to heat treatment to obtain equiaxed and basketweave microstructures. Subsequently, the specimens were subjected to LSP. A phase analysis was performed using X-ray diffraction (XRD). The microstructure of the TA15 titanium alloy was observed using optical microscopy (OM) and transmission electron microscopy (TEM). The surface roughness of the specimens was measured using a 3D profilometer. The hardness and residual stress were measured using a microhardness tester and a residual stress analyzer, respectively. Electrochemical performance tests were performed using an electrochemical workstation, and the corrosion rate of the material was determined through static immersion experiments. The corrosion morphology was observed using scanning electron microscopy (SEM) to reveal the corrosion mechanism.

After LSP, a severe plastic deformation (SPD) layer with a certain thickness is formed on the TA15 titanium alloy specimens (Fig. 3). Numerous crystal defects, such as dislocations, are generated in the microstructure (Fig. 4). The surface grains are refined into nanocrystals via a dislocation segmentation mechanism (Fig. 5). After LSP, the surface microhardness of the specimens with equiaxed and basketweave microstructures increases by 16.7% and 15.7%, respectively (Fig. 7). Along the depth direction, the microhardness and CRS exhibit gradient changes owing to the gradual decrease in the shock wave energy, with the maximum values occurring at the specimen's surface. After LSP, the corrosion current of the TA15 titanium alloy decreases (Table 3), corrosion rate decreases (Fig. 13), and the corrosion resistance improves. Grain refinement leads to a faster rate of formation of the surface passivation film that enhances its ability to resist corrosive media. The high-amplitude CRS on the surface inhibits corrosion.

In this study, LSP was performed on a TA15 titanium alloy with equiaxed and basketweave microstructures. The microstructural changes before and after LSP, as well as the corrosion performance in 3.5% NaCl and 5 mol/L HCl solutions, were comparatively studied. LSP does not change the microstructural composition of the material; however, the material undergoes SPD. As the energy of the laser shock wave energy gradually attenuates with increasing depths from the surface, a gradient microstructure is formed along the depth direction, and the microhardness and residual compressive stress exhibit decreasing gradient characteristics. The corrosion resistance of the TA15 titanium alloy follows the order of equiaxed microstructure > basketweave microstructure. After LSP, the thicknesses of the SPD layer thicknesses of the specimens with equiaxed and basketweave microstructures are 41 and 33 μm, respectively, and the surface grains transform into nanocrystals. Grain refinement and increased grain boundaries provide more nucleation sites for the passive film, accelerating its formation and enhancing the corrosion resistance of the TA15 titanium alloy. The reduction in surface roughness decreases the corrosion rate, whereas the high-amplitude CRS generated on the surface delays the initiation and propagation of corrosion cracks, further improving the corrosion resistance of the alloy.