Advanced aircraft has put forward the requirements of lightweight, high strength and high reliability for the materials used in its key structural parts. Near-β titanium alloy has the characteristics of high strength, good fracture toughness, excellent fatigue resistance, large quenching depth and so on, is widely used in the manufacture of key bearing parts such as aircraft landing gear, helicopter rotor system central parts and connectors. TB6 (nominal composition Ti-10V-2Fe-3Al) and Ti55531 (Ti-5Al-5Mo-5V-3CR-1Zr) are two typical near-β titanium alloys. Laser shock strengthening (LSP) is a surface treatment that can improve the fatigue life by inducing residual compressive stress (CRS), which cause change of microstructure and increase of dislocation density on the surface of the material, thus reduce the probability of fatigue crack initiation and propagation at surface. In this study, laser shock processing (LSP) was carried out on the surface of as-forged TB6 and Ti5553 titanium alloy after solution aging treatment in two-phase region. X-ray diffractometer and Vickers hardness tester were used to measure the residual stress and Vickers micro-hardness on the surface and along the depth of the sample. The results display that surface residual stress of TB6 sample is ‒639.06 MPa, and that of Ti55531 sample is ‒588.24 MPa. The surface hardness of Ti55531 and TB6 after LSP reaches 452HV0.1/10 and 400HV0.1/10, respectively, the value increases by 11.3% and 12.1% respectively compared with that before LSP. The hardness of the TB6-LSP sample and Ti55531-LSP sample at the depth of 900 μm and 700 μm respectively is consistent with the hardness of the matrix material. Tensile tests at room temperature and high cycle fatigue tests were carried out on the specimens before and after laser shock processing. The tensile and high cycle fatigue fractures were observed and analyzed by scanning electron microscopy (SEM). Compared with the sample without LSP, the tensile strength (Rm) of LSP’ed TB6 and Ti55531 titanium alloys increased by 25 MPa (2.26%) and 25 MPa (2.02%), respectively. The yield strength (Rp0.2) decreased by 48 MPa (4.58%) and 30 MPa (2.54%), respectively. The percentage reduction of area (A), elongation (Z) and elastic modulus (E) were slightly improved. Fatigue life of Ti55531 alloy after LSP is higher than that of TB6 alloy at low stress level, while the fatigue life of TB6 alloy is slightly higher than that of Ti55531 alloy at high stress level. The increase of micro-hardness can be attributed to the severe plastic deformation of the material caused by the high-intensity shock wave in the process of laser surface shot peening, and there are a lot of high-density dislocations in the plastic deformation region, which leads to the increase of the hardness of the material. The tensile fracture modes of TB6 and Ti55531 titanium alloys with and without LSP are microporous aggregated ductile fracture and inter-granular brittle fracture. Surface laser shock processing does not change the tensile fracture mode of the alloys in this study. The difference of fatigue life of near β titanium alloy under different stress states is related to the difference of fatigue crack initiation and propagation rate caused by the difference of material microstructure. © 2022, Chongqing Wujiu Periodicals Press. All rights reserved.
