Self-organization of vertex filament structures generated in pulsed-laser melting of rough metal surfaces

摘要

Laser melting of rough metal surfaces, in particular in those containing parallel scratch lines of micron and submicron size, differs from melting of smooth planar surfaces. It is associated with the reach of the spectra of self-organized structures, which consist of vortex filaments as basic entities. Self-organized structures of vortex filaments on rough metal surfaces were generated by short laser pulses and studied by optical and scanning electron microscopy. Their formation starts with melting of the surface that generates a shear layer with radially oriented flow in the laser spot. Parallel scratch lines represent the flow perturbation which is spanwise, streamwise or oblique in different zones of the spot giving, rise to the self-organized flow structures. The flow structures are permanently frozen by ultra-fast cooling after laser pulse termination, thus enabling a posteriori analysis. Long vortex filaments organized into very complex structures ranging from parallel Kelvin-Helmholtz rollers, to 'helically paired' counter-rotating filaments, to the braided vortex filaments (which become broken by the shock wave at higher pulse energy), and finally to the 'hairpin' or the Omega-shaped vortices (as the channel structure between two scratch lines) have been observed. The spectrum of surface self-organized hydrodynamic structure was found to depend on laser parameters: the beam energy, beam wavelength, pulse duration and the beam profile (Gaussian or 'top hat' type). Motivation for these studies is twofold: first theoretical, directed to elucidating the conditions of the hydrodynamic self-organized structure formation, and, second, technological, directed to elucidating and eventually opening up new possibilities in laser surface alloying, cladding, etc., with respect to the dynamics of the mixing layer.