PSI - Issue 23

Anton Zhukov et al. / Procedia Structural Integrity 23 (2019) 305–309 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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The revealed features indicated that the short-term and repetitive processes of melting and crystallization of the metal were localized and occurred under conditions of unsteady heat exchange. In the used powder raw materials, the maximum particle size is 8 0 μm. Since the identified and listed structural features of the SLM metal were detected in the field of view 120x150 μm, it can be assumed that laser heating in the powder layers is sufficient for melting and subsequent crystallization and phase γ → α trans formations to reach depths of ~ 120 μm. With the used speed V of the laser motion, the complete melt of even large particles D is possible during the time determined by the formula (1). t=D/V ~2•10 -7 sec (1) The obtained estimate is comparable with the data on the time of formation of structures in the formation of permanent joints using the energy of an explosion (Grinberg et al. (2017)) laser surface treatment and welding (Sadovsky et al. (1989)). Data on structural changes during laser processing of technical iron powder does not contradict the conclusions of work Sadovsky et al. (1989), where it says that all the structural features encountered after laser processing of steel can be understood and explained taking into account the unusually high heating and cooling rates, which is the main specificity use of laser heating. The data obtained indicate fast kinetic processes occurring in unsteady heat flows not only in the melting zones, but also in the crystallization of the metal in closely spaced microvolumes. It is necessary to emphasize that the problem of structural changes for the times of 10 -5 ÷ 10 -7 sec is relevant and has not been resolved so far. In the present work, traces of mass localized microplastic flow in the form of extended (up to 350x40 µm) and curved microregions with unequal-sized grains bordering microregions filled with equiaxial recrystallized grains (Fig. 4) were recorded. Note that similar structures were recorded in SLM samples of alloyed steels of different chemical composition in Kuznetcov et al. (2018).

Fig. 4. Traces of localized microplastic flow in SLM samples of iron powder.

In the systematic work on growing single crystals from molten metals, which were carried out under the direction of A.V. Stepanov, by theoretical calculations and experimentally, a causal relationship was established between temperature conditions near the mold and convection of heat fluxes in growing crystals with a subsequent vortex distribution of the components of the tensor of elastic stresses in the volume of the metal being crystallized. In Antonov et al. (1979), it was established that a gradient thermal field can cause the generation of dislocations responsible for local deformation and structural stresses even at low crystal growth rates. Since diffusion processes are capable of transferring a material medium to distances of only a few nanometers, it can be argued that the recorded traces of a microplastic flow are due to the action of inhomogeneous thermal fields.