Objective Ti6Al4V alloys are used extensively in the aerospace industry. However, with the ongoing development of aerospace technology, higher demands are being placed on load- bearing capacity, service life, and reliability. Laser shock peening (LSP) is a novel and ecofriendly surface modification technique. However, its effectiveness for strengthening the surfaces of metals with high hardness and strength, such as Ti alloys, is limited. Dual- pulse laser shock peening (DPLSP) enhances the strengthening effect by using delayed dual- laser pulses to increase the pressure and duration of laser- induced shock waves. In this study, a Ti6Al4V titanium alloy is subjected to DPLSP. The microstructure evolution and tensile property changes of the specimens after single- pulse laser shock peening (SPLSP) and DPLSP treatments are compared and analyzed using methods such as microscopic structure observation, phase analysis, microhardness testing, tensile testing, and fracture morphology observation. Furthermore, the mechanism of the DPLSP process is elucidated. Methods Laser- processed samples are cut into standard tensile test samples via wire cutting (Fig. 1), and they are subjected to dual- sided DPLSP using a delayed dual- pulse laser with a time delay of 13.5 ns that is achieved through polarization spectroscopy and a mirror array (Fig. 2). First, the microstructures of the surface layers of the samples are observed using a transmission electron microscope. Second, the phase structures of the samples are examined using an X-ray diffractometer. Furthermore, the microhardness of the surface and cross-section of the samples after different treatments are measured using a microhardness tester. Room- temperature tensile tests are performed using an electronic universal testing machine. Finally, the fracture morphology is observed using a scanning electron microscope. Results and Discussions The results of the microstructural analysis of the surface layer show that after the DPLSP treatment, the dislocation density and complexity in the sample surface layer significantly increase and the grains are noticeably refined (Figs. 4-6). The X-ray diffraction results indicate that after SPLSP treatment, the (101) diffraction peak position of the sample shifts from 40.44 degrees to 40.5 degrees, and the full width at half maximum (FWHM) widens from 0.12 degrees to 0.21 degrees. After the DPLSP treatment, the (101) diffraction peak position shifts farther to the right, reaching 40.64 degrees, and the FWHM widens to 0.25 degrees (Fig. 7). The microhardness results show that after the SPLSP and DPLSP treatments, the surface microhardness values are 376.2 HV and 414.2 HV, respectively, and the affected layer depths are 400 mu m and 550 mu m, respectively. Tensile performance tests reveal that after the SPLSP and DPLSP treatments, the tensile strengths are 956.31 MPa and 1002.33 MPa, respectively, and the elongations are 14.61 % and 13.48 %, respectively. The fracture morphology results show that river pattern cleavage planes appear in the DPLSPed sample fractures. In the near- surface layer, the overall number and size of the dimples are further reduced. However, there is an increase in the localized density of the honeycomb- like dimples as well as an increased number of cleavage planes. Conclusions In this study, the surface of a Ti6Al4V alloy is strengthened by using DPLSP. A systematic study on the microstructure, phase composition, microhardness, tensile properties, and fracture morphology is conducted. This study compares and analyzes variations in the tensile properties of untreated, SPLSPed, and DPLSPed samples to reveal the mechanism of DPLSP on a Ti6Al4V titanium alloy. The main conclusions are as follows. 1) Because of the higher peak pressure and duration of the shockwave, DPLSP induces an increase in the dislocation density compared with SPLSP and dislocation walls, dislocation cells, and sub- boundaries appear. This process, combined with mechanical twinning, leads to the formation of many complex microstructures and results in the significant refinement of the original coarse grains. Additionally, owing to the dual- sided peening, a composite structure of fine- coarse- fine grains is formed in the depth direction. 2) The high- density dislocations and refined grains induced by DPLSP increase the surface microhardness of the Ti6Al4V titanium alloy samples to 414.2 HV, which is an improvement of 10.1 %, compared with that by SPLSP. The depth of the hardened layer is approximately 550 mu m, which is a 37.5 % increase over that by SPLSP. 3) The average tensile strength and elongation of the DPLSPed samples are 1002.33 MPa and 13.48 %, respectively. The former shows a 4.81 % increase over that of the SPLSPed samples, whereas the latter shows a 1.79% decrease. During the tensile process, the strain gradient occurs near the interface between the coarse- grained and fine-grained layers, leading to a significant increase in the tensile strength of the material while maintaining a good elongation rate.
