SELF-ORGANIZATION OF VERTEX FILAMENT STRUCTURES GENERATED IN PULSED-LASER MELTING OF ROUGH METAL-SURFACES

Laser melting of rough metal surfaces, in particular in those containi ng parallel scratch lines of micron and submicron size, differs from m elting of smooth planar surfaces. It is associated with the reach of t he spectra of self-organized structures, which consist of vortex filam ents as basic entities. Self-organized structures of vortex filaments on rough metal surfaces were generated by short laser pulses and studi ed by optical and scanning electron microscopy. Their formation starts with melting of the surface that generates a shear layer with radiall y oriented flow in the laser spot. Parallel scratch lines represent th e flow perturbation which is spanwise, streamwise or oblique in differ ent zones of the spot giving, rise to the self-organized flow structur es. The flow structures are permanently frozen by ultra-fast cooling a fter laser pulse termination, thus enabling a posteriori analysis. Lon g vortex filaments organized into very complex structures ranging from parallel Kelvin-Helmholtz rollers, to 'helically paired' counter-rota ting filaments, to the braided vortex filaments (which become broken b y the shock wave at higher pulse energy), and finally to the 'hairpin' or the Omega-shaped vortices (as the channel structure between two sc ratch lines) have been observed. The spectrum of surface self-organize d hydrodynamic structure was found to depend on laser parameters: the beam energy, beam wavelength, pulse duration and the beam profile (Gau ssian or 'top hat' type). Motivation for these studies is twofold: fir st theoretical, directed to elucidating the conditions of the hydrodyn amic self-organized structure formation, and, second, technological, d irected to elucidating and eventually opening up new possibilities in laser surface alloying, cladding, etc., with respect to the dynamics o f the mixing layer.