| SHI Xiaobo,SU Zilong,XIE Enhan,FENG Shihe,LONG Yu,ZHOU Zhukun.Effect of Single/Double Pulse Femtosecond Laser Shock Peening on Surface Properties[J],54(1):228-241 |
| Effect of Single/Double Pulse Femtosecond Laser Shock Peening on Surface Properties |
| Received:February 07, 2024 Revised:April 10, 2024 |
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| DOI：10.16490/j.cnki.issn.1001-3660.2025.01.021 |
| KeyWord:femtosecond laser shock Ti single/double pulse surface quality surface microhardness |
| Author | Institution |
| SHI Xiaobo |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
| SU Zilong |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
| XIE Enhan |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
| FENG Shihe |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
| LONG Yu |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
| ZHOU Zhukun |
School of Mechanical Engineering, Guangxi University, Nanning , China;State Key Laboratory of Featured Metal Materials and Life-cycle Safety for Composite Structures, Nanning , China |
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| Abstract: |
| Constrained by surface quality, the pressure of shock waves induced during direct impact by low-power femtosecond laser is lower and shallower compared to that by traditional nanosecond laser shock peening (ns-LSP) techniques. However, the energy coupling characteristics of double pulse femtosecond laser can be used to significantly enhance the pressure of shock waves induced by low-power femtosecond laser. Titanium metal was chosen as the research subject, and femtosecond laser shock peening (fs-LSP) process was conducted with a 14 μm spacing grid pattern under conditions of unrestricted layers and the absence of absorbing layers. Through comprehensive analysis of experimental results, it was found that compared to single pulse femtosecond laser, double pulse femtosecond laser significantly increased surface hardness while maintaining high surface quality, even reaching the level of improvement achieved by traditional ns-LSP for titanium metal. This demonstrated the superiority of double pulse femtosecond laser in low-power fs-LSP. The experimental setup was used to investigate the effects of single/double pulse fs-LSP processes on the surface morphology, roughness, and microhardness of titanium metal by characterization methods such as depth-of-field microscopy, microhardness testing equipment, scanning electron microscopy (SEM), SEM energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), and acoustic emission signal detection under different laser parameters (repetition rate, laser power, and number of processing cycles). Additionally, the trends in full width at half maximum (FWHM) and residual stress distribution after femtosecond laser shock on titanium surfaces were analyzed, along with the intensity of vibration generated during impact. The energy of femtosecond laser and its interaction time with materials were the main factors affecting changes in surface morphology, depending on the combined effects of pulse number, laser power, repetition rate, and number of processing cycles. Within the range of engineering surface roughness standards, the surface of titanium after single pulse femtosecond laser shock peening mainly exhibited a fish scale-like structure, while the surface after double pulse treatment showed a pit-like structure. The surface roughness of samples processed with single pulse was significantly lower than that of samples processed with double pulse, due to the higher temperature and stronger impact generated during double pulse processing. Following low-power fs-LSP, the formation of laser-induced periodic surface structures (LIPSS) occurred, gradually disappearing with increased processing cycles, resulting in particle and cluster oxide structures under single/double pulse conditions, respectively. Surface roughness increased with laser power. Increasing repetition rate and processing cycles improved surface quality after femtosecond laser peening under low pulse energy conditions, compensating for insufficient thermal flux by extending the interaction time with materials, thereby reducing surface roughness and improving surface quality. The enhancement of surface hardness primarily stemmed from the coordinated effects of work hardening, grain refinement, and oxide layer hardening, with double pulse demonstrating significant advantages. Among samples with surface roughness below Ra 1.6 μm, three times of processing under suitable low-power laser parameters resulted in a maximum increase in surface microhardness up to 42% under double pulse femtosecond laser shock peening (fs-LSP), compared to a maximum increase of 32% for single pulse fs-LSP samples. The degree of oxidation increased significantly with the number of processing cycles, with significantly higher oxidation levels observed for double pulse treated samples compared to single pulse treated samples after one and three cycles, while oxidation levels were nearly identical for both single and double pulse treated samples after five cycles. At extremely high repetition rates and medium power (5 MHz, 8.1 W), the increase in surface microhardness reached 85%. As the number of processing cycles increased, FWHM slightly increased under single pulse, while double pulse treated samples showed a more pronounced increase. Residual stress decreased significantly due to thermal relaxation, reaching –25.6 MPa and –22 MPa after five cycles, respectively, from initial values of –67 MPa and –72.4 MPa after three cycles. Under the same pulse energy and repetition rate, the amplitude of impact vibration during single pulse fs-LSP was 7.377×10–4, while that during double pulse fs-LSP coupled the energy of two sub-pulses, enhancing the amplitude of impact vibration to 1.509×10–3. |
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