PSI - Issue 74

Kipkurui Ronoh et al. / Procedia Structural Integrity 74 (2025) 77–84 Kipkurui Ronoh / St ructural Integrity Procedia 00 (202 5 ) 000 – 000

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long, while at 10 J/cm 2 appeared shorter, less smooth and included shallow trench-like structures perpendicular to the LIPSS. Micro-ripples also began forming along the scanning direction (Ronoh et al., 2024b). The morphological changes are associated with the increasing fluence, and peak fluence F 0 depends on pulse energy E pulse through (1); 0 = 2 = 2 0 2 (1) where A is the laser focus area and ω o is the beam waist radius at 1/e 2 value. LIPSS are generally almost equal to or smaller than the laser spot size and oriented perpendicularly to the scan direction (Biffi et al., 2023; Ronoh et al., 2024b). Absence of debris or resolidified melt droplets on the ablated surfaces in Fig. 1 suggests that the equilibrium vaporisation occur (Zhao et al., 2015). 3.2. Topographies of the surfaces of the samples Fig. 2 show the surface topographies of the textures of the laser-ablated 699 XA under various laser fluences and the identical scanning parameters of scanning velocity, hatching distance and scanning pass of 100 mm/s, 5 µm and 1 scanning pass, respectively. The resulting textures are rough, densely packed and feature non-uniform peaks. Similar topographies were observed for the other alloys. The lack of prominent structures is attributed to extensive material removal caused by high pulse (93%) and scan line (80%) overlap.

Fig. 2: Topographical images of the laser-ablated surfaces of 699 XA as a function of laser fluence of a) 1, b) 4, c) 8 and d) 10 J/cm 2 .

Fig. 3 show the surface topographies of the laser-ablated MONEL® alloy 400 using a constant fluence of 8 J/cm 2 and varying hatching distances of 5, 20, 50 and 100 µm. At a hatching distance of 5 µm, the surface structures are small, sharp and densely packed. At a 20 µm, sharp, conical-like structures are formed. With increasing hatching distance to 50 µm and 100 µm, the structures change to small, blunt conical and then to flat-topped structures, respectively. These areas represent regions less affected by laser radiation or unablated areas. The micro-grooves observed in Fig. 3c, d) are formed in regions where the laser beam was focused, leading to localized material removal. The surface roughness of the laser-ablated alloys under various fluences is tabulated in Table 3. The ablated surfaces for the three alloys have low surface roughness, as noted in Table 3. It can be noted that as laser fluence increases, Sa increases too for the three alloys. This is attributed to the increase in pulse energy per unit area, which intensifies thermal effects on the material surface, resulting in rougher and deeper surfaces (Biffi et al., 2023; Ronoh et al., 2024a). Overall, all samples exhibited nanoscale surface roughness across the entire range of laser fluences.