Role of Initial Surface Roughness in LIPSS Formation during Picosecond Laser Structuring of Titanium Alloy

This study investigates the influence of surface roughness on the LIPSS formation on Ti6Al4V processed by a 1064 nm picosecond laser. Surface roughness significantly affects LIPSS uniformity when the ablation depth is insufficient to eliminate the initial topography and surface defects such as scratches. Under high pulse repetition rates (≥ 500 kHz) and single-pulse fluence levels around 1 J cm⁻², laser processing can lead to microhole formation along surface defects due to increased heat accumulation in the interaction zone. Therefore, achieving homogeneous LIPSS across the entire surface requires an appropriate combination of single-pulse fluence F_s and effective fluence dose F_eff to ensure sufficient ablation depth. This result is particularly important for practical LIPSS applications, as it enables uniform and reproducible surface structuring without additional mechanical polishing.