Effect of thermal convection in the subsurface molten layer on its thickness


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Efremov Institute (St. Petersburg, Russia) - Electron accelerator for surface modification GESA-I
Efremov Institute (St. Petersburg, Russia) - Electron accelerator for surface modification GESA-II


The calculation was made for a double-layer sample (2.5 *m Ni layer deposited on 1mm steel С60 substrate). The beam energy in the experiment amounted to 80 keV and 140 keV. The calculations were made in the non-linear and non-steady-state formulation of the thermal conduction problem the mathematical model and the details of which were reported in our previous paper [6]. So attention here is focused only on the basic physical processes which are the main features of the developed model:

- non-equilibrium evaporation of Ni or steel from the sample surface producing a vapour cloud;
- shielding effect by evaporated material;
- material melting/crystallization inside the depth of the condensed material phase;
- temperature dependence of the thermophysical properties of the sample material;
- real distribution of thermal load penetrating in depth of the subsurface layer of a sample.

Heat load distribution in penetration depth (obtained by Monte-Karlo method) is shown in Fig. 1. The distribution was obtained for a beam accelerating voltage of 80keV and a current density of 1 A/cm2. For other values of current density, with the accelerating voltage remaining constant (80 keV), the heat release distribution in depth remains unchanged, with the absolute value scaled to the ratio of the real beam current density to the value taken as the tabulated one (1 A/cm2).

The analysis of the presented experimental and calculated data allows the general conclusion to be made that the convective mechanism of the thermal energy transfer into the sample material is absent in case of short pulses which are required to provide the quasi-adiabatic conditions for melt zone heating necessary to maintain high-temperature gradients on the boundary between the melt and the solid phase. The depth of the recrystallized zone is determined only by the electron penetration depth.



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