| dc.description.abstract | 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.
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