Modelling ultrafast laser structuring/texturing of freeform surfaces

Laser structuring/texturing of freeform surfaces is attracting the attention of researchers and industry because it can enable high impact applications and also the technology can offer important advantages compared to alternative/conventional processes. So far, laser structuring/texturing has been applied mostly on planar surfaces, while employing it for 3D processing it introduces some disturbances that affect the processing conditions. In particular, Beam Incident Angle (BIA) and Focal Offset Distance (FOD) variations are two important processing disturbances that impact the resulting structures/textures on freeform surfaces and also their functional responses. Furthermore, those disturbances should be considered as constraints in planning the laser processing operations, i.e. when pre-processing 3D models by partitioning into laser processing fields, and also in designing the processing strategies. However, such constraints are always material specific for a given parameters’ domain and can be time-consuming to determine empirically. In this research, a model for calculating the accumulated fluence for generating Laser Induced Periodic Surface Structures (LIPSS) throughout the processed freeform surfaces is proposed. It considers the actual spatial intensity distribution of a Gaussian beam when processing 3D surfaces in the presence of varying FOD and BIA. It was demonstrated that the 3D surface processing leads to variations in their processing conditions, in particular changes of beam spot size that affect local fluence thresholds. The comparison of simulation and experimental results has shown that LIPSS main characteristics, i.e. their amplitudes and periodicity, can be predicted with acceptable accuracy. Also, changes in processing conditions caused by disturbances that affect LIPSS performance can be identified. The results of this research can be used to determine the BIA and FOD limits/tolerances within which the LIPSS functional response on freeform surfaces can be maintained within acceptable levels for any given application.