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 structure of the samples was studied by metallography using light (Axio Vert.A1) and scanning electron (Tescan Vega 3) microscopes. On thin sections prepared on longitudinal sections of cylinders, the metal structure was revealed by chemical etching in a 4% alcoholic solution of nitric acid. 3. Experimental results and discussion When inspecting non-etched thin sections in a SLM metal, many small (<2 μm) oxidized particles and pores were detected, as well as a single number of spherical particles with traces of surface oxidation in the form of captivity (Fig. 2). Oxidation of the metal could be caused by gases that were in numerous pores in the feedstock.

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Fig. 2. (a) the original porous powder before sieving; (b) structure of the SLM metal on the non-etched section, gray areas – oxides.

The detected features indicated that the laser effect did not completely melt the raw material, and partially melted particles could be present in the SLM metal microvolumes. The subsequent chemical etching of thin sections confirmed the heterogeneous structure of the SLM metal. For example, on Fig. 3 we can see equiaxed recrystallized grai ns up to 20 μm in size, randomly distributed particles in the form of fragments up to 5 μm in size and equally oriented non-axial crystallites with an axial ratio of ~ 1:5, forming groups in the form of arcuate layers with front width up to 800 microns and a depth of 10 to 30 microns.

Fig. 3. Diffusion in the SLM metal microvolume. Alternating arrows indicate suspected thermal fronts.

Between the groups of crystallites in the form of layers, narrow zones of increased chemical content were found, which are interpreted as probable traces of stopping of thermal fronts.