Structural and mechanical properties of stainless steel formed under conditions of layer-by-layer fusion of a wire by an electron beam

OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 23 No. 4 2021 Materials and Methods The specimens were printed on the electron-beam setup developed at Tomsk Polytechnic University [15] using a stainless steel wire. The accelerating voltage was 30 kV and the beam current varied from 15 to 20 mA (depending on the distance from the substrate). Thus, the power input varied from 450 to 600 W. The fi rst three layers were printed at an accelerating voltage of 30 kV and beam current of 20 mA. High current was used to heat up the substrate. The next three layers were formed at 30 kV and 17 mA to reduce the specimen heat up rate. Consequent layers were formed at 30 kV and 15 mA to reject generated heat. Focused beam (with a diameter of 150 μ m) was moving along circular sweep with a diameter of 4 mm. The beam movement frequency was 1,000 Hz. The wire was fed into the sweep zone, while the specimen geometry was formed by moving the bed along three axes. The hatching distance was 4 mm, while the layer height was 0.8 mm. The specimen was moved in a zig-zag pattern. The appearance of the specimen and the build-up scheme are presented in Figure 1. Fig. 1. Appearance of the specimen (a), specimen cutting area (b) and specimen build-up scheme (c) a b c The raw material for the blank to be produced by electron-beam-assisted WAAM was AISI 308LSi steel wire. The chemical composition of the wire is presented in Table 1. Ta b l e 1 Chemical composition of the raw material Fe [%] (balance) С [%] Mn [%] Si [%] Cr [%] Ni [%] P [%] S [%] 64.82…69.37 0.03 1.4…2.1 0.65…1.0 19.5…21.0 9.0…11.0 0.03 0.02 The specimens for consequent studies were cut by the electrospark method from the blank to form geometrically valid parallelepiped specimens with dimensions of 5×5×10 mm. Tomographic investigation to control micro- and macro-defects of the specimens was carried out on an Orel-MT X-ray computer microtomograph. The device was equipped with an XWT 160-TC X-ray tube and PaxScan-2520V detector panel with a positioning control system. The specimens were scanned with the following parameters: accelerating voltage – 130 kV, current – 27 μ A, resolution – 11.3 μ m, number of projections – 1,200, scanning step – 0.3, copper fi lter – 2mm. The tomographic reconstruction was performed using NRecon Reconstruction Software ( Bruker Micro-CT ). After reconstruction, the tomograms were segmented to obtain two models: the specimen material and internal porosity. In addition, morphological properties of single pores were studied: volume, typical diameter and sphericity. The segmentation and analysis were carried out in CTanalyser software ( Bruker Micro-CT ). The microstructure was studied on an Axio Observer metallographic microscope ( Carl Zeiss ) with 1,000x magni fi cation. The microscope comes with software for quantitative analysis of the phase and structural composition of alloys. The transverse and longitudinal metallographic sections were prepared by grinding on polishing papers with different grit sizes. The sections were fi nished on a fabric with an