PSI - Issue 72

E.A. Gachegova et al. / Procedia Structural Integrity 72 (2025) 260–264

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To preserve the integrity of the surface of processed samples, researchers have used opaque coatings to protect them from laser radiation since the very beginning of laser shock peening technology development. These coatings are designed to ensure that the surfaces of the samples are protected during cold treatment processes,Clauer (1996). During the ionization of these coatings, a plasma is formed that propagates a shock wave through the coating into the material, while maintaining the surface quality of the sample. Sometimes, applying protective coatings is impossible or difficult, such as when processing nuclear reactor parts or using low energy levels during processing. In these cases, the effect of the shockwave on the sample material is simply not noticed. This method of treatment is known as laser shock peening without coating (LSPwC) or direct ablation treatment, Peyre et al. (2007). In this case, plasma is formed due to ionization of the surface of the sample. As the wave develops, heat is transferred to the material, causing a thermomechanical process. The treatment zone is typically several microns thick near the surface being treated. During irradiation of a surface without insulation, peak temperatures of the plasma can reach 10,000 K in few microseconds. Thermal effects usually occur within first 50 μm, while additional tensile residual stresses caused by mechanical component affect this depth, Trdan et al. (2024), Clauer (2019). Figure 1a shows the surfaces after LSP using an ablative coating. Clear indentations can be seen where the surface has been hardened. The clarity of the spot depends on the laser parameters and material properties. There are no hints of surface changes associated with ablation. Thisconfirms that the coated LSP isa purely mechanical cold treatment process. Direct irradiation of the sample surface leads to visible surface changes due to direct ablation. Figure 1b shows an example of surface changes caused by this direct ablation during treatment without a protective coating. The dark areas on the sample surface areexplained by the formation of an oxide layer caused by a reaction between the surface and oxygen.

Fig. 1. Samples after LSP: (a) with coating; (b) without coating

When using this treatment mode, multiple shots or a large overlap are often required to obtain a uniform residual stress profile. An increase in the number of impacts leads to increased surface ablation and subsurface (>5 microns from the treated surface) damage to the material structure, Trdan et al. (2024). In all cases of LSPwC, a suitable depth of compressive stress can be achieved. However, it is necessary to take into account the changes in the surface. As for the materials used for absorbent coatings, there are three main types: metal foil (aluminum or steel), , Mironov et al. (2022), Ziwen et al. (2013), PVC tape, Wang et al. (2024), Ramesh and Maharjan (2021), and organic black paint, Rozmus-Gornikowska et al. (2021), Stransky et al. (2024). When choosing a specific material, it's important to consider the energy density of the processing that will be done, as not all coatings can withstand heavy loads. For instance, paint has been shown in research to not withstand processing with an energy power density greater

than 10 GW/cm². 2. Experiments

A pulsed Nd:YAG laser with a wavelength of 1064 nm and pulse duration of 10 ns was used to irradiate samples. The laser beam was focused using a special optical system, which provides a laser beam spot size of 1 mm at the sample surface. The samples were square plates with a side of 40 mm and a thickness of 3 mm. The material of these samples is titanium alloy Ti64, one of the most popular alloys, which is actively used in aerospace engineering. Its chemical composition is presented in Table 1.