Mechanics of the Selective Laser Raster-Scanning Surface Interaction
In recent years, the use of a high power laser beam actuated by fast speed scanning mirrors has opened up novel selective laser raster-scan processing venues as extremely rapid motion and high overlapping of the beam can be attained. This permits distribution of laser energy precisely over geometric patterns such as rectangles, circles, triangles etc. The surface thermal history at any given point under such processing was estimated using an analytical solution for the 1D, semi-infinite, surface flux boundary condition heat conduction problem together with linear superposition theory. Presented here is the comparison of the thermal histories of different selective laser surface processes previously implemented, namely: laser surface polishing of flat surfaces, laser induced cementation of cylindrical surfaces and direct laser single layer masked deposition. It was possible to verify that in laser induced cementation, long-width and short-length scanned regions provided low average temperature and low heating rate with spaced out temperature peaks, whereas for direct laser single layer deposition in which a narrow-width – long-length region is scanned, the heating rate and peak temperature are higher and the peaks are squeezed. The analysis also provided ways to estimate the Andrew’s number associated with a raster-scan process for the sake of comparison with single-beam processes having a given number value. Understanding the influence of scan geometry and overlapping on the selective raster-scan processing provides a method to tailor the surface peak temperature as well as the heating and cooling rates, affecting the solidification or sintering conditions and therefore the mechanical properties of the parts obtained.