|
A physical model of formation of stratified structures of components in a stiffening melt after the action of a penetrating laser beam on the heterogeneous system is considered. The model explains the formation of layers by centrifugal running or tightening of microparticles in size about 10-4 - 10-2 cm of an impurity (either heavier or easier in relation to the basic material). This centrifugal effect should be shown in vortex melt flows, with speeds in the range of several cm/s, generated by hydrodynamic instabilities of the certain nature. In the case of continuous scanning of a CW beam along the surface of a material, there can be generated non-stationary regular three-dimensional vortex melt flows explained by injection of flooded melt jets from a front wall keyhole (KH). After solidification of the heterogeneous melt, behind of traveling KH the strip-looking layers of an insoluble impurity are observed in the direction of beam scanning velocity. Our experimental data do not contradict to this model. In the case of cylindrical-symmetric layer melt, surrounding the KH in the absence of scanning of the laser beam, the vortex flows rise due to capillary collapse of the KH. After deenergizing the laser and solidification of the melt, periodic layers of the insoluble impurity are fixed which look like curved disks located transverse to the keyhole axis.
|
