雷射衝擊強化(Laser shock peening, LSP)處理是用超快短脈衝雷射能量直接衝擊於材料表面，雷射光與被加工件的接觸、作用時間極短，一般為3到5奈秒，高能量衝擊能使材料表面產生細小塑形變形，從而長久產生殘留應力。在衝擊作用下，材料表層組織更細密，降低表面粗糙度，雷射衝擊產生的殘留應力深度可達1～2 mm，殘留應力可以有效地抑制表面裂紋的擴展，因此LSP可有效提升材料的抗疲勞性能。本文以SKD61(或稱AISI-H13)工具剛當作基材，以純水為約束層，使用脈衝式Nd:YAG雷射綠光雷射為光源(脈衝寬度5 ns、重複頻率10 HZ)，進行LSP。
本文第一部分探討LSP對表SKD61表面硬度提升與表面粗糙度改善之能力，結果顯示在未經雷射照射之前SKD61硬度約為200 Hv，以功率200 mW到450 mW的雷射進行LSP處理後，其硬度會隨雷射功率的提升逐漸升高至300 Hv左右，但在功率超過450 mW後，一直到700 mW，都無法再進一步地提升SKD61的硬度。LSP處理也可明顯地降低SKD61的表面粗糙度，結果顯示平均粗糙度(Ra)平均能下降25%，最大粗糙度高度(Rmax)則平均下降47%。
LSP之表面硬化機制來自雷射引發等離子體(Laser induced plasma)產生之高壓衝擊波(Shock wave)對材料表面之衝擊效應，故衝擊波的壓力大小是改質效果的關鍵。本研究第二部份旨在獲得LSP處理時所引發衝擊波之壓力，方法是經由聲光效應(Photoaccoustic effect)量得衝擊波之波速，再轉換成壓力，藉以直接連結衝擊波壓力與LSP材料處理的效果。結果顯示在雷射功率450 mW時衝擊波的波速為3.99 km/s，對應之等離子體壓力為0.484 GPa。

Laser shock peening (LSP) is the use the high pressure, resulting from a momentum impulse generated by material vaporization at the target surface when rapidly heated by an ultrashort laser pulse, to impart long term, beneficial residual stresses in materials. To generate an instantaneously high pressure shock wave, the laser peak power should be very high, which is usually achieved by an ultrashort pulsed laser with pulse durations less than several nanoseconds. After the LSP processing, the surface is finer and roughness becomes smaller. The residual stress generated by LSP can reach 1 ~ 2 mm in depth that can effectively restrain the propagation of surface crack. In this study, the target is SKD 61 (AISI-H13) tool steel immersed in deionized water and the light source is from a pulsed Nd:YAG laser, with pulse duration of 5 ns and repetition rate of 10 HZ.
The first part of this thesis discusses the improvements in the surface hardness and roughness of the target by LSP at various laser operation parameters. Surface hardness of the as-received SKD 61 was measured to be 200 HV. It can be apparently enhanced to 250 HV after the LSP processing using laser power of 200 mW and scan speed of 1 mm/s. It is noted, when the scan speed is 1 mm/s, as the laser power is enhanced from 200 to 450 mW, the hardness is enhanced monotonically with the laser power. It reaches a maximum of 300 HV at the power of 450 mW. But, there is no more recognizable increase if the power is larger than 450 mW, and this trend remains even the power is up to 700 mW. LSP treatment can also significantly reduce the surface roughness, the average roughness (Ra) showed an average decrease of 25% and the maximum roughness (Rmax) decreased by an average of 47%.
The hardening mechanism of LSP is the impulse pressure from shock waves, generated by laser induced plasma. Thus, the magnitude of shock wave pressure is the key to the result of the surface modification. Therefore, the second part of this study aims to obtain the plasma pressure in the LSP process. This is accomplished by directly measuring the shock speeds based on the photo-acoustic effect, then convert them to the corresponding plasma pressures. Therefore, we can correlate the LSP treatment effect with the shock wave pressure. The results show that the speed of the shock wave is 3.99 km /s at a laser power of 450 mW, and the corresponding plasma pressure is 0.484 GPa.

[1.] Rubio-González, C., Felix-Martinez, C., Gomez-Rosas, G., Ocana, J. L., Morales, M., & Porro, J. A. (2011). Effect of laser shock processing on fatigue crack growth of duplex stainless steel. Materials Science and Engineering: A, 528(3), 914-919.
[2.] Guo, Y. B., & Caslaru, R. (2011). Fabrication and characterization of micro dent arrays produced by laser shock peening on titanium Ti–6Al–4V surfaces. Journal of Materials Processing Technology, 211(4), 729-736.
[3.] Lim, H., Kim, P., Jeong, H., & Jeong, S. (2012). Enhancement of abrasion and corrosion resistance of duplex stainless steel by laser shock peening. Journal of Materials Processing Technology, 212(6), 1347-1354.
[4.] Fan, Y. J., Zhou, J. Z., Huang, S., Wang, W., Wei, D. H., Fan, J. R., & Gao, B. (2013). Study on 2-D shock wave pressure model in micro scale laser shock peening. Rev. Adv. Mater. Sci, 33, 111-118.
[5.] Shukla, P. P., Swanson, P. T., & Page, C. J. (2014). Laser shock peening and mechanical shot peening processes applicable for the surface treatment of technical grade ceramics: A review. Proceedings of the Institution of Mechanical Engineers, Part B: Journal of Engineering Manufacture, 228(5), 639-652.
[6.] Cao, Y., Xie, X., Antonaglia, J., Winiarski, B., Wang, G., Shin, Y. C., ... & Liaw, P. K. (2015). Laser shock peening on Zr-based bulk metallic glass and its effect on plasticity: experiment and modeling. Scientific reports, 5, 10789.
[7.] Ebrahimi, M., Amini, S., & Mahdavi, S. M. (2016). The investigation of laser shock peening effects on corrosion and hardness properties of ANSI 316L stainless steel. The International Journal of Advanced Manufacturing Technology, 1-9.
[8.] Park, H. K., Kim, D., Grigoropoulos, C. P., & Tam, A. C. (1996). Pressure generation and measurement in the rapid vaporization of water on a pulsed‐laser‐heated surface. Journal of applied Physics, 80(7), 4072-4081.
[9.] Braisted, W., & Brockman, R. (1999). Finite element simulation of laser shock peening. International Journal of Fatigue, 21(7), 719-724.
[10.] Sano, Y., Akita, K., Masaki, K., Ochi, Y., Altenberger, I., & Scholtes, B. (2006). Laser peening without coating as a surface enhancement technology. Pulse, 100(40), 250mJ.
[11.] Vukelic, S., Wang, Y., Kysar, J. W., & Yao, Y. L. (2009). Dynamic material response of aluminum single crystal under microscale laser shock peening. Journal of Manufacturing Science and Engineering, 131(3), 031015.
[12.] Jae-Ho, L. E. E., Jeong-Hwan, J. A. N. G., Byeong-Don, J. O. O., Young-Myung, S. O. N., & Young-Hoon, M. O. O. N. (2009). Laser surface hardening of AISI H13 tool steel. Transactions of Nonferrous Metals Society of China, 19(4), 917-920.
[13.] Zhang, X. C., Zhang, Y. K., Lu, J. Z., Xuan, F. Z., Wang, Z. D., & Tu, S. T. (2010). Improvement of fatigue life of Ti–6Al–4V alloy by laser shock peening. Materials Science and Engineering: A, 527(15), 3411-3415.
[14.] Doukas, A. G., Zweig, A. D., Frisoli, J. K., Birngruber, R., & Deutsch, T. F. (1991). Non-invasive determination of shock wave pressure generated by optical breakdown. Applied Physics B, 53(4), 237-245.
[15.] Berthe, L., Fabbro, R., Peyre, P., Tollier, L., & Bartnicki, E. (1997). Shock waves from a water-confined laser-generated plasma. Journal of Applied Physics, 82(6), 2826-2832.
[16.] Zhu, S., Lu, Y. F., Hong, M. H., & Chen, X. Y. (2001). Laser ablation of solid substrates in water and ambient air. Journal of Applied Physics, 89(4), 2400-2403.
[17.] Kruusing, A. (2004). Underwater and water-assisted laser processing: part 1—general features, steam cleaning and shock processing. Optics and Lasers in Engineering, 41(2), 307-327.
[18.] Yavas, O., Schilling, A., Bischof, J., Boneberg, J., & Leiderer, P. (1997). Bubble nucleation and pressure generation during laser cleaning of surfaces. Applied Physics A: Materials Science & Processing, 64(4), 331-339.
[19.] Nath, A., & Khare, A. (2011). Transient evolution of multiple bubbles in laser induced breakdown in water. Laser and Particle Beams, 29(01), 1-9.
[20.] Hutchins, D. A. (1988). Ultrasonic generation by pulsed lasers. Physical Acoustics, 18, 21-123.
[21.] Fujimoto, J. G., Lin, W. Z., Ippen, E. P., Puliafito, C. A., & Steinert, R. F. (1985). Time-resolved studies of Nd: YAG laser-induced breakdown. Plasma formation, acoustic wave generation, and cavitation. Investigative ophthalmology & visual science, 26(12), 1771-1777
[22.] Hickling, R., & Plesset, M. S. (1964). Collapse and rebound of a spherical bubble in water. Physics of Fluids (1958-1988), 7(1), 7-14.
[23.] Yang, J. M., Her, Y. C., Han, N., & Clauer, A. (2001). Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes. Materials Science and Engineering: A, 298(1), 296-299.
[24.] Rubio-González, C., Ocana, J. L., Gomez-Rosas, G., Molpeceres, C., Paredes, M., Banderas, A., ... & Morales, M. (2004). Effect of laser shock processing on fatigue crack growth and fracture toughness of 6061-T6 aluminum alloy. Materials Science and Engineering: A, 386(1), 291-295.
[25.] Rubio-González, C., Felix-Martinez, C., Gomez-Rosas, G., Ocana, J. L., Morales, M., & Porro, J. A. (2011). Effect of laser shock processing on fatigue crack growth of duplex stainless steel. Materials Science and Engineering: A, 528(3), 914-919.
[26.] Ruiz, A., Ortiz, N., Carreón, H., & Rubio, C. (2009). Utilization of ultrasonic measurements for determining the variations in microstructure of thermally degraded 2205 duplex stainless steel. Journal of Nondestructive Evaluation, 28(3-4), 131-139.
[27.] Zhang, Y. K., Hu, C. L., Cai, L., Yang, J. C., & Zhang, X. R. (2001). Mechanism of improvement on fatigue life of metal by laser-excited shock waves. Applied Physics A, 72(1), 113-116.
[28.] Ding, K., & Ye, L. (2006). Laser shock peening: performance and process simulation. Woodhead Publishing.
[29.] Dorman, M., Toparli, M. B., Smyth, N., Cini, A., Fitzpatrick, M. E., & Irving, P. E. (2012). Effect of laser shock peening on residual stress and fatigue life of clad 2024 aluminium sheet containing scribe defects. Materials Science and Engineering: A, 548, 142-151.
[30.] Zhang, L., Zhang, Y. K., Lu, J. Z., Dai, F. Z., Feng, A. X., Luo, K. Y., ... & Qi, H. (2013). Effects of laser shock processing on electrochemical corrosion resistance of ANSI 304 stainless steel weldments after cavitation erosion. Corrosion Science, 66, 5-13.
[31.] Yang, J. M., Her, Y. C., Han, N., & Clauer, A. (2001). Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes. Materials Science and Engineering: A, 298(1), 296-299.
[32.] Rubio-González, C., Gomez-Rosas, G., Ocaña, J. L., Molpeceres, C., Banderas, A., Porro, J., & Morales, M. (2006). Effect of an absorbent overlay on the residual stress field induced by laser shock processing on aluminum samples. Applied Surface Science, 252(18), 6201-6205.
[33.] Sanchez-Santana, U., Rubio-Gonzalez, C., Gomez-Rosas, G., Ocana, J. L., Molpeceres, C., Porro, J., & Morales, M. (2006). Wear and friction of 6061-T6 aluminum alloy treated by laser shock processing. Wear, 260(7), 847-854.
[34.] Real, E., Rodríguez, C., Belzunce, F. J., Sanjurjo, P., Canteli, A. F., & Pariente, I. F. (2009). Fatigue behaviour of duplex stainless steel reinforcing bars subjected to shot peening. Fatigue & Fracture of Engineering Materials & Structures, 32(7), 567-572.
[35.] Sano, Y., Kimura, M., Mukai, N., Yoda, M., Obata, M., & Ogisu, T. (2000, February). Process and application of shock compression by nanosecond pulses of frequency-doubled Nd: YAG laser. In Advanced High-Power Lasers and Applications (pp. 294-306). International Society for Optics and Photonics.
[36.] Fabbro, R., Peyre, P., Berthe, L., & Scherpereel, X. (1998). Physics and applications of laser-shock processing. Journal of laser applications, 10(6), 265-279.
[37.] Fabbro, R., Fournier, J., Ballard, P., Devaux, D., & Virmont, J. (1990). Physical study of laser‐produced plasma in confined geometry. Journal of applied physics, 68(2), 775-784.
[38.] Hammersley, G., Hackel, L. A., & Harris, F. (2000). Surface prestressing to improve fatigue strength of components by laser shot peening. Optics and Lasers in Engineering, 34(4), 327-337.
[39.] Peyre, P., Fabbro, R., Merrien, P., & Lieurade, H. P. (1996). Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour. Materials Science and Engineering: A, 210(1), 102-113.
[40.] Fox, J. A. (1974). Effect of water and paint coatings on laser‐irradiated targets. Applied Physics Letters, 24(10), 461-464.
[41.] Gujba, A. K., & Medraj, M. (2014). Laser peening process and its impact on materials properties in comparison with shot peening and ultrasonic impact peening. Materials, 7(12), 7925-7974.
[42.] Fairand, B. P., & Clauer, A. H. (1979). Laser generation of high‐amplitude stress waves in materials. Journal of Applied Physics, 50(3), 1497-1502.
[43.] Kruusing, A. (2010). Handbook of liquids-assisted laser processing. Elsevier.
[44.] US data for solids [http://traktoria.org/files/sonar/passive_materials/acoustic_impedace_of_some_solids.htm]
[45.] Clauer, A. H., Walters, C. T., & Lahrman, D. F. (2003). U.S. Patent No. 6,539,773. Washington, DC: U.S. Patent and Trademark Office.
[46.] Chou, P. C., & Hopkins, A. K. (1972). Dynamic response of materials to intense impulsive loading.
[47.] Tenaglia, R. D., & Lahrman, D. F. (2009). Surface treatment: Shock tactics. Nature Photonics, 3(5), 267-270.