Date of Award
Doctor of Philosophy (PhD)
Dr. Xin Zhao
Dr. Huijuan Zhao
Dr. Hai Xiao
Dr. Hongseok Choi
Laser shock peening (LSP) is an important material surface strengthening technique that uses laserinduced shock waves to cause severe plastic deformation near material surfaces. It can significantly improve material properties like hardness, strength, fatigue life, and corrosion resistance and is widely used in industrial sectors such as automotive, aerospace, nuclear, and medical areas. The conventional nanosecond laser shock peening (ns-LSP) has demonstrated effectiveness but faces challenges such as complex setup, low throughput, high energy consumption, and inflexibility for complex geometries. Femtosecond laser shock peening (fs-LSP) offers a promising alternative to overcome these issues, thanks to its ultra-high laser intensity and limited heat-affected zone resulting from its ultrashort pulse duration. However, compared to ns-LSP, fs-LSP has received limited attention, and its effectiveness in enhancing surface properties remains unclear. Moreover, the nature of fs laser-induced shock waves is not well understood. This dissertation aims to demonstrate the effectiveness of fs-LSP in ambient air without additional setups, investigate its impact on surface mechanical property enhancement and microstructural change, and understand the fundamental physics of fs laser-induced shock waves. Integrated with experimental study, a hybrid numerical model combining a two-temperature model and a hydrodynamic model has been developed to study the generation and propagation of fs laser-induced shock waves. For the first time, it is demonstrated that fs-LSP is most effective in the air environment without any confining medium or protective coating, which are essential for ns-LSP. The surface hardness of stainless steel samples is increased by over 45% by fs-LSP without any confining medium or protective coating due to its extremely high laser-induced shock wave (over 600 GPa). In the confining medium of water, the peening effect is lessened since the strong ionization of water blocks the laser energy. Also, with the protective coating, the shock wave is dramatically weakened before arriving at the sample surface, and therefore the peening effect is reduced. To demonstrate how fs-LSP changes the microstructure and the hardness, experimental and numerical studies have been conducted on fs-LSP of copper. The surface hardness increases by 18.7%, and the increase in depth hardness can reach 90 µm, which is much smaller than the affected layer depth by nsLSP. Even though the fs laser can generate shock pressure that is up to over two orders higher than that of the ns laser, its shock pressure attenuation process is much faster. Another unique signature given in fs-LSP is that no severe grain refinement to nanograin level is observed. We proposed that it is because the plastic deformation region width following the shock wave is only around 2 μm, which is much smaller than the initial grain size of 40 µm. Also, under a super-high strain rate of 108 s-1, there was not enough time for the grains to develop into finer ones. Understanding the generation and propagation dynamics of laser-induced shock waves is critical to elaborating on the application of fs-LSP. In the fs laser, the ultra-high surface pressure can reach 5700 GPa during laser irradiation coming from the electron temperature of over 100 eV, and within the same period, the thermal conduction controls the pressure attenuation at the interface and the shock wave propagation into deeper material. Within its propagation into material, the super-strong shock pressure keeps decaying exponentially, reducing by over 97% at 1 ns, and even when it reaches 3 GPa, similar to the ns laser, its attenuation is still much faster than the one by the ns laser. By studying the fs laser-induced shock waves in different materials (copper, iron, and aluminum), it is elucidated that the electron thermal conductivity and heat capacity are critical factors affecting the shock wave intensity and attenuation speed. With the lowest electron thermal conductivity and electron heat capacity in iron, the maximum pressure of 13000 GPa is identified with a laser fluence of 132 J/cm2 in air, and these also cause the steepest pressure gradient in iron than the shock waves in aluminum and copper. Therefore, the compression effects of the shock wave in iron are the smallest, and it can only reach a 74-µm depth, which is consistent with the experimental results. iii Finally, an unprecedented approach is presented: fs laser burst shock peening at GHz repetition rates. It has been observed that when a pulse is divided into multiple ones using GHz repetition rates, the shock waves generated by consecutive pulses merge with one another and ultimately surpass the intensity of the shock wave generated by the original single pulse. Furthermore, a substantial decrease in the attention rate facilitates the propagation into a considerably deeper region. This significant finding not only establishes a novel approach to enhance the efficiency of fs-LSP but also, for the first time, reveals a novel phenomenon: shock accumulation. This discovery has the potential to yield benefits for various other fields, including solid mechanics, materials science, and plasma science.
Li, Yuxin, "Femtosecond Laser Shock Peening and the Dynamics of Femtosecond Laser-Induced Shock Waves" (2023). All Dissertations. 3542.
Available for download on Tuesday, December 31, 2024