|Abstract: ||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.