OPTIMIZATION OF MODIFYING MATERIAL DISTRIBUTION DURING LASER PROCESSING OF THE METAL SURFACE

The improvement of operational properties of metal parts during the laser surface treatment can be obtained by modifying the melt with the prepared nano-sized particles of refractory compounds (carbides, nitrides, and others.). It allowsthe number of crystallization centersto be increased, the structure to be grinded and the uniformity of the solidified metal to be raised. It is known that thermocapillary convection promotes the homogeneous distribution of materials penetrating into the molten metal. It is also known that the presence of surface-active substances in the melt influences the character of convection. There is evidence that the structure of flow in the melt depends on the amount of surface-active substances in the metal. Therefore, the studies were conducted to obtain data on the relationship of characteristics of metal processing by laser and the uniformity degree of modifying additives distribution. Using the numerical modeling the effects of characteristics of pulsing laser action on allocation of modifying particles are estimated at the presence of surface-active substance in metal. With the help of mathematical proposed model the following non-stationary processes are considered: heating and melting of the metal, heat transfer in the melt, fluid motion, nucleation and growth of the solid phase after termination of the pulse. During the numerical realization of the model the heat conduction equation was firstly solved. Upon appearance of molten metal the boundary of the liquid phase was determined. Further the coupled equations of convective heat transfer and motion of an incompressible fluid were solved. After the laser pulse termination, the calculations are continued until the complete solidification of the substrate material is achieved. It was supposed that modifying nano-sized particles under the influence of convective flows penetrate into depth of a melt from its surface. The movement and distribution of particles in a fluid was modeled using markers. The movement of the markers was determined by the local velocity of the melt. According to the results of numerical experiments, the effect of the parameters of pulsed laser action on the nature of the flow and distribution of the particles in the melt were determined. Optimum parametersfor the melting of the metalsubstrate by a laser pulse are proposed for various surface-active substances concentrations.

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