Numerical and experimental investigation of complex surface topography evolution during laser surface modification with raster scan

This work investigates the complex surface topography evolution of an area during laser surface modification with raster scan. A nonlinear three dimensional transient numerical model is developed to capture the melt pool dynamics, solidification and resulting surface evolution, during laser surface modification with raster scanning. Further laser surface modification of Ti6Al4V is performed with different combinations of scanning speed and beam overlap. Surface profiles from laser modified samples are captured using optical profilometer. In order to study the surface features quantitatively and compare against the numerical predictions, three surface parameters are defined: crest-trough area ratio, trough spacing along overlap direction and mean feature aspect ratio. A modified non-dimensional heat input for raster scan is also introduced, to which Marangoni number is more sensitive, as compared against the traditional non-dimensional heat input for laser based processes. Surface parameters from numerical predictions compare well against the experimental results. Irrespective of beam overlap, slower scanning speeds led to lower crest-trough area ratio and mean feature aspect ratio. This is attributed to wider melt pool (lower melt pool aspect ratio), owing to higher heat input at lower scanning speeds, which promotes the surface tension driven forces and melt pool circulation. Numerical results indicated improved fluid circulation at higher beam overlaps as well. However, crest-trough area ratio increased at higher beam overlaps, as surface features from previous passes undergo re-melting. Numerical results reveal that higher beam overlaps lead to downward movement of molten metal towards the re-melted region, which leads to asymmetric melt pool and filling of trough regions (from previous passes). This reduces trough area upon solidification (higher crest-trough area ratio) and lowers average surface feature height as beam overlap increases.