Titanium alloy, referred to as marine metal, is widely used in the field of marine engineering owing to its high specific strength, corrosion resistance, high pressure resistance, weldability, and other excellent characteristics. However, the surface of welded titanium alloy components is prone to corrosion and premature failure because of long-term use in harsh marine environments. Therefore, improving the corrosion performance of welded titanium-alloy components is particularly important to increase service life. Laser shock peening (LSP) is a new and environmentally friendly surface strengthening technology that can produce a deep residual compressive stress layer (CRS) on metal surfaces, thereby effectively enhancing the microhardness, strength, and corrosion resistance of metals. In this study, the surfaces of TC4 ELI alloy welded joints are treated by LSP, and the microstructure evolution and corrosion properties of the welded joints before and after LSP treatment are compared and analyzed using microstructure observation, phase analysis, microhardness testing, and electrochemical tests. The purpose is to provide a theoretical basis and experimental support for applications in marine engineering.

First, the surface strengthening treatment of TC4 ELI alloy welded joints is carried out using laser shock peening technology, and the sample size required for the experiment is obtained using a wire cutting process. The phase compositions of the welded joints before and after LSP treatment are studied using an X-ray diffractometer. The cross-sectional morphology of the welded joints is observed using optical microscope (OM) and scanning electron microscope (SEM). Transmission samples (10 mm×5 mm×1 mm) are mechanically milled and then punched out into discs of Φ3 mm in diameter. An ion thinning instrument is used to decrease the thickness of the perforation with the help of a transmission electron microscope to observe the microstructures of the surface and subsurface layers. Electron backscatter diffraction (EBSD) samples (20 mm×8 mm×1 mm) are prepared by argon-ion polishing. Electron backscatter diffraction (EBSD) tests are performed using an EBSD detection system integrated with a field-emission scanning electron microscope, to analyze the degree of plastic deformation, kernel-averaged misorientation (KAM), and high- and low-angle grain boundary distributions of the sample cross-sections. Finally, the surface microhardness of the welded joints before and after LSP treatment is measured using a microhardness tester, and the corrosion performance is tested using scanning Kelvin probes (SKP) and an electrochemical workstation.

Under the effect of “dislocation intersection,” the grain size of the surface layer of the welded joints is refined to nanoscale, and grain sizes of base metal (BM) and weld metal (WM) are refined to approximately 11 nm and 23 nm, respectively. The subsurface layer dislocation density increases significantly, and a large number of dislocation lines pile up, leading to substructures such as dislocation tangles and walls. After LSP treatment, the α(101ˉ1) diffraction peak is left-shifted and full width at half maximum (FWHM) increases in all regions of welded joints. Overall micro-hardness of welded joints from the weld center to base metal increases by about 10%. Self-corrosion potentials of the WM and BM are positively shifted with LSP treatment, and self-corrosion current densities are reduced by an order of magnitude, from the original 1.154×10^-6 A·cm^-2 and 1.010×10^-6 A·cm^-2 to 4.859×10^-7 A·cm^-2 and 4.500×10^-7 A·cm^-2, respectively. The potential difference between BM and WM surfaces decreases from 212 mV to 187 mV, and surface metal activity is reduced. In addition, corrosion tendency is weakened, and corrosion resistance is significantly improved.

In this study, LSP is used to strengthen the surface of TC4 ELI alloy welded joints. A systematic study is conducted on the microstructure, phase composition, microhardness, and corrosion performance of welded joints before and after LSP treatment, followed by a comparative analysis of microstructure evolution. The main conclusions are as follows: 1) After LSP treatment, the surfaces of welded joints form a gradient nanostructure and a layer of intense plastic deformation of 10?20 μm. The surface layer grain is refined to nano-scale, with dislocation tangles, dislocation cells, and other sub-structures forming at the sub-surface layer. 2) The overall microhardness of the welded joints increases by up to 10% as a result of work hardening and grain refinement. 3) Both WM and BM self-corrosion potentials are positively shifted with LSP treatment, and self-corrosion current densities decrease by an order of magnitude, from 1.154×10^-6 A·cm^-2 and 1.010×10^-6 A·cm^-2 before LSP to 4.859×10^-7 A·cm^-2 and 4.500×10^-7 A·cm^-2 after LSP, respectively. 4) The potential difference between the surfaces of the welded joints from BM to WM decreases from 212 mV (before LSP) to 187 mV (after LSP). In addition, the surface metal activity decreases; corrosion tendency is lower; and corrosion resistance is remarkably improved.