Acoustic wave detection of laser shock peening

Laser shock peeing or laser shock processing (LSP) is a new type of surface modification technology that can improve the fatigue life of materials by using laser-induced plasma shock waves. Laser is irradiated on metal surface under confinement layer such as glass or water to generate the high pressure near material surface by laser ablation in LSP process .The LSP utilizes the laser-induced plasma shock wave, and the shock wave pressure is increased through the constraining layer, which is formed to the principle structure of the absorption layer and the constraining layer. And the principle structure is representative and has the characteristics of high pressure, high energy and super high strain rate. The surface residual stress of material is generally used to evaluate the degree of LSP process. The measurement of the surface residual stress is mainly carried out by the offline methods such as the hole drilling strain method or the X-ray stress measurement. However, these methods are inefficient, and unable to change the process of LSP in time. In order to achieve the large-scale industrial application and actual production of LSP technology, it is necessary to develop non-destructive online detection technology of LSP.

Schematic diagram of acoustic wave detection of laser shock peening

    The research group of Prof. Jibin Zhao from Manufacturing Technology Department, Shenyang Institute of Automation, Chinese Academy of Sciences, aimed at exploring the phenomenon of plasma acoustic wave, and proposed an online laser shock peeing detection method based on acoustic wave energy. TiAl alloy, which is an emerging metal compound structural material, was used as a sample in the research. Laser is absorbed by the ablative layer on the surface of sample, and plasma of high temperature and pressure is formed under the tamping layer and propagated outward in the form of shock wave, which is also called plasma acoustic wave. The acoustic wave is sampled, stored, digitally filtered and analyzed by the acoustic wave online detection system, and the system gets the acoustic wave signal energy. The surface residual stress after the LSP treatment was measured by an X-ray stress analyzer. The experimental results show that the residual compressive stress is generated on the surface of material after the treatment of LSP. Both the surface residual stress and the acoustic wave signal energy increase as the laser pulse energy is increased from 5 J to 7 J, and their growth trends are consistent. Finally, the empirical formula between the surface residual compressive stress and the acoustic wave signal energy is obtained by polynomial fitting, which provides a theoretical reference for the development of laser shock peening online detection technology.

About Team
The research group of Prof. Jibin Zhao from Manufacturing Technology Department, Shenyang Institute of Automation, Chinese Academy of Sciences, has been committing to the research on advanced manufacturing technologies and technologies for high-end equipment. And the research group has achieved rich achievements in the fields of laser shock peening, precision measurement technology, complex surface numerical control machining, lapping and polishing, and additive manufacturing. Nowadays, the research group has 36 staffs and 34 postgraduate students. The research group combines the national strategy of developing equipment manufacturing, and adhere to the combination of "application-based research and high-tech development, high-tech development and the promotion and application of results". The research group undertook and completed a number of projects, including National 973 Programs, National Major Projects, National 863 Program, the National Natural Science Foundation, the Chinese Academy of Sciences Knowledge Innovation Projects, the National SME Technology Innovation Fund and the provincial and municipal science and technology projects. In 2011, the research group successfully developed the overall blade disk laser shock peening system, which is the first Chinese industrial applicable laser shock peening equipment. In recent years, the research group published nearly 200 papers, including more than 100 EI journal articles , more than 60 SCI journal articles . At the same time, the research group has applied for more than 50 invention patents.

Wu J J, Zhao J B, Qiao H C, Liu X J, Zhang Y N et al. Acoustic wave detection of laser shock peening. Opto-Electronic Advances 1, 180016 (2018).