Ablation Behaviour and Surface Characteristics of Conventional and Additively Manufactured Alloy 602 CA under Picosecond Laser Irradiation

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Abstract
Ultrafast laser ablation provides a powerful approach for tailoring the surface properties of nickel-based alloys. In this study, picosecond laser ablation of three Alloy 602 CA variants, namely conventionally manufactured (C-602 CA), additively manufactured (SAC-UN-602 CA), and nitrided additively manufactured (SAC-ISIN-602 CA), were systematically investigated under ambient conditions. The influence of laser fluence and scanning velocity on ablation behaviour, surface morphology, surface chemistry, and wettability was examined. Picosecond laser processing produced well-defined laser-induced periodic surface structures (LIPSS) on all variants, with increasing periodicity and surface roughness at higher fluence. Ablation depth and rate increased with fluence but decreased with increasing scanning velocity, while ablation efficiency decreased at higher fluence. Nanoindentation revealed slightly lower hardness and reduced modulus for the additively manufactured alloys compared to the conventional alloy. X-ray diffraction confirmed that all variants retained a single-phase austenitic FCC Ni–Cr–Fe solid solution after processing, with no laser-induced phase transformation, indicating structural stability. SEM–EDX analysis confirmed that ultrafast laser ablation did not significantly alter the overall elemental composition of the alloys. X-ray photoelectron spectroscopy revealed laser-induced surface oxidation and hydroxylation, with Ni predominantly present as Ni(OH)2 and Cr as Cr2O3/NiCr2O4. Wettability measurements demonstrated a transition from hydrophilic polished surfaces to strongly hydrophobic laser-ablated surfaces. Despite differences in manufacturing route and microstructure, all variants exhibited comparable behaviour, indicating that surface modification is governed predominantly by the laser processing parameters. These findings demonstrate that picosecond laser processing enables controlled and reproducible surface modification of Alloy 602 CA for advanced functional applications.

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