Abstract
Laser peening without coating (LPwC) involves irradiating materials covered with water with intense laser pulses to induce compressive residual stress (RS) on a surface. This results in favorable effects, such as fatigue enhancement; however, the mechanism underlying formation of the compressive RS is not fully understood. In general, tensile RS is imparted on the surface of the material due to shrinkage after heating by laser irradiation. In this study, we assessed the thermo-mechanical effect of single laser pulse irradiation and introduce a phenomenological model to predict the outcome of LPwC. To validate this model, RS distribution across the laser-irradiated spot was analyzed using X-ray diffraction with synchrotron radiation. In addition, the RS was evaluated across a line and over an area, following irradiation by multiple laser pulses with partial overlapping. Large tensile RSs were found in the spot irradiated by the single pulse; however, compressive RSs appeared around the spot. In addition, the surface RS state shifted to the compressive side due to an increase in overlap between neighboring laser pulses on the line and over the area of irradiation. The compressive RSs around a subsequent laser spot effectively compensated the tensile component on the previous spot by controlling the overlap, which may result in compressive RSs on the surface after LPwC.
Highlights
Laser peening without coating (LPwC) is a well-established technique to introduce compressive residual stress (RS) in the near-surface layer of metallic components [1,2,3,4,5,6,7,8]
If an overlay is applied to block a thermal effect on the top surface [27,28,29], the mechanism is easy to understand; sudden generation of high-pressure plasma due to the ablation produces a shock wave, which propagates into the material to cause a permanent strain; a compressive RS field remains on the surface due to the elastic constraint of the surrounding area [30,31,32,33]
In the case of LPwC, this mechanical effect competes with the thermal effect during each laser pulse, which can last for nanoseconds or tens of nanoseconds
Summary
Division of Research Innovation and Collaboration, Institute for Molecular Science, National Institutes of Natural Sciences, 38 Nishigo-Naka, Myodaiji, Okazaki 444-8585, Japan
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