Influence of pulsed electron beam parameters on the thickness of modified layer


Completed orders:

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 mathematical simulation of the interaction between the high-energy electron beam and the material shows that a mere increase in specific power of the loaded beam by increasing its current density does not contribute significantly to the thickness of the molten metal layer. Only by increasing the accelerating voltage of the electron accelerator the molten layer about 100 *m thick can be obtained.

Pulse duration in the present work is corresponded to adiabatic condition of material heating only. The calculations have revealed that an increase in the accelerating beam voltage is the only effective method for attainment of the required thickness of the thermally hardened surface layer of treated material. This conclusion has been confirmed by not presented here additional calculations and analysis for a wide range of metals characterized by various thermophysical properties, e.g. copper, titanium, nickel, chromium, aluminum, etc.

As the subsequent analysis of the calculation results has shown, the main reason for this phenomenon is an intensive evaporation of the sample material and, hence, an entrainment of material and energy from the treated surface and the shielding effect of the evaporated cloud absorbing the beam energy and moderating the fraction of energy absorbed by the condensed phase. Further increase of the energy flux density, with the accelerating voltage remaining unchanged, results in the so-called automodel regime at when the boundary of the evaporating material front on the sample surface and the melting front boundary within the material thickness move with the same velocity without increasing the resulting depth of the molten layer. This conclusion is confirmed by the results of calculation of the simplified model problem (not taking into consideration the temperature dependence of the material properties, energy exchange of the evaporated material cloud with the condensed phase surface, surface heat load) given in [5]. This fact was verified experimentally by the results of heat treatment of metal surfaces on the GESA I (100 keV) and GESA II (400 keV) electron-beam facilities.

This study also shows that taking into account the energetics of the material melting processes has a marked effect on the calculation results: for the parameters of the considered problem the energy absorption for melting phase transition can decrease the molten layer by 50%. Therefore, in most cases to obtain correct results it is necessary to take into account latent melting heat of material.



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