Microstructure/Stress Evolution and Notch Vibration Fatigue Property of Laser Shock Peened TC17 Titanium Alloy Blades

With the development of new generation aero-engines, the high weight reduction requirement leads to the thin design of blades. Trickily, these blades are vulnerable to complex loads such as rotating centrifugal force, air flow excitation force as well as foreign objects, resulting in deformation, fatigue, and fracture failures, which seriously affect the safety and reliability of aero-engine. In this paper, laser shock peening, an advanced surface modification technique, was employed to treat TC17 titanium alloy manufactured blisk simulated blades. The leading edge, trailing edge, and blade tip of TC17 blade specimens were peened once with a power density of 8.33 GW/cm2 and an overlapping rate of 15%. After peening, a femtosecond laser was employed to introduce the artificial pre-crack on the leading edge of both as-received and laser shock peened TC17 blade specimens. Surface microstructures before and after laser shock peening were characterized by a scanning electron microscope and a transmission electron microscope. Residual stress and macro plastic deformation during the laser shock peening process were measured by X-ray diffraction and coordinate measuring instruments, respectively. The effect of laser shock peening on the first-order bending vibration fatigue was evaluated on an electromagnetic vibration tester. Results showed that dislocation structures, such as dislocation line, dislocation tangle, dislocation wall, and dislocation cell, were introduced into the surface of the TC17 titanium alloy blade even though no obvious grain refinement was detected due to the lower peening times. In addition to the microstructure changes, residual stresses, and macro plastic deformations were also studied. After conducting laser shock peening on the blade basin surface, the surface presented a compressive stress state with an averaged residual stress value of 330.5 MPa, while the other side (blade back) surface exhibited tensile stress with an averaged residual stress value of 55.5 MPa. The subsequent peening on the blade back surface indicated that the former tensile residual stress on the surface turned into compressive stress with a value of 267.0 MPa, and the compressive residual stress on the blade basin surface decreased from 330.5 MPa to 261.9 MPa. Similar changes were found in the macro plastic deformation. The displacement at the intersection of the leading edge and the blade tip was 0.119 1 mm and 0.129 1 mm measured on two separate peened blades after peening on the blade basin surface, and then the corresponding displacement decreased to 0.071 08 mm and 0.099 mm after peening on the blade back surface. Besides, the first-order bending vibration fatigue life illustrated a significant improvement after laser shock peening, which underwent 199 515 fatigue cycles, 2.52 times higher than that of the as-received blades (56 696 cycles). The subsequent scanning electron microscope observations carried out on the fracture surface indicated an obvious feature of crack closure. The current work indicates that laser shock peening is an effective method in introducing high-density dislocation structure and double-sided through-type compressive residual stress on the surface of TC17 titanium alloy manufactured blades, and can control the macro plastic deformation within 0.1 mm off the initial position. Our work seeks to fundamentally understand of shape and property synchronous control of laser shock peened thin wall structures, and provides a theoretical basis and data support for the laser shock peening engineering application in aero-engine blade manufacturing. © 2023 Chongqing Wujiu Periodicals Press. All rights reserved.