PSI - Issue 81

Valeriy Lazaryuk et al. / Procedia Structural Integrity 81 (2026) 529–535

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Fig. 5. Transverse thermal cracks.

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Fig. 6. Longitudinal thermal cracks.

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Fig. 7. Additional defects (cavities, interference rings, micro-craters, cellular cracking).

After measuring the groove width and the width of the HAZ on microscopy, graphical dependencies were constructed to show how scanning speed and laser power influence the geometry of the processed track (Fig. 8). The groove width was defined as the maximum transverse extent of the molten region, and the HAZ width as the boundary marked by a change in optical contrast. The plots indicate that both parameters decrease with increasing scanning speed, which reflects a lower energy input per unit length. The coefficients of determination ( R 2 ≈ 0.98 – 0.99) indicate a consistent correlation between the process parameters and the resulting groove geometry. The analysis shows that the groove and HAZ widths depend clearly on both speed and power. A critical speed of about 150 mm/s was identified, below which the process enters a regime of uncontrolled crack formation. In this low-speed region, high thermal input leads to a rapid increase in both groove and HAZ width, and the trends become nonlinear. When the speed exceeds approximately 100-150 mm/s, the behaviour stabilises, the curves approach linearity, and crack formation becomes controlled. A comparison of regression slopes for different power levels shows that although higher speed and power reduce overheating, powers above 60 – 70 W increase sensitivity to parameter variation, Fig. 8h. For further experiments, the 40 – 50 W range is the most practical, as it produces smaller regression slopes and more consistent groove and HAZ dimensions. It was found that the density of thermally induced transverse cracks per unit length does not depend on the pulse repetition frequency but shows a clear correlation with the width of the HAZ. The CO 2 laser system employed in this study operates in continuous-wave mode with superimposed short high-power spikes. Due to the nonuniform ignition of the gas discharge, these spikes are distributed irregularly along the groove, resulting in a non-periodic sequence of thermal shock events. To quantify the evolution of the fracture with increasing scanning speed, an effective crack density per unit length was introduced. This parameter represents the number of all cracks of comparable length occurring within 1 mm, taking the length of a transverse crack as the reference unit, as shown in Fig. 9. This metric provides a consistent and comparable measure of groove fragmentation across different scanning speeds and laser power levels. As the scanning speed increases, the energy delivered to the material per unit length decreases, reducing the magnitude of thermally induced stresses along the groove. Consequently, the number of transverse cracks diminishes sharply. At high scanning speeds of 250 – 300 mm/s, only a few isolated transverse cracks are formed, and the effective crack density falls to approximately 2 – 2.5 cracks · mm -1 , in agreement with the observed narrowing of the HAZ and the attenuation of local thermal stresses.