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 The microstructure of the samples has a shape characteristic of coarse-grained cast austenite steels. It consists of columnar grains growing along the height of the wire layers (Fig. 3a). The average size of the austenitic grain is d = 150...200 μ m (Fig. 3b). During consistent layer-by-layer welding of the wire, each consequent layer was formed due to fusion of the metal with a previous layer due to partial heating of metal up to the melting temperature. Such fusion mode revealed no evident interfaces between the layers. However, different parts of the blank were characterized by nonuniformity of the phase composition and different mechanical properties. Fig. 3a. Columnar grains in the longitudinal section of the specimen Fig. 3b. Grain structure in the cross-section of the specimen Two-phase structure was revealed inside the columnar grains both in longitudinal and transverse direc- tions. The two-phase nature is characterized by the γ - Fe -based austenitic matrix with FCC -lattice (bright color) and inclusions from  - Fe -based high-temperature ferrite with BCC lattice of various shape (dark color). The microstructure analysis allowed distinguishing three typical morphological types of ferrite in- clusions: needle-like, vermicular and granular (Fig. 5). The specimens have a columnar large-grain microstructure oriented along the growth direction. Epitax- ial growth leads to the formation of grains with a width from 70 to 230 μ m and length from 180 to 630 μ m. The microstructure of the formed grains forms an austenitic matrix, including needle-like, granular and vermicular forms of δ -ferrite. There are no macro-scale nonuniformities in the form of boundaries between deposited metal layers. The studies of the surface at a different scale by the method of scanning electron microscopy allowed for unveiling defects in the form of residual gas pores formed during printing of the blank. The dimensions of gas pores in the metal structure varied between 0.5 and 5.2 μ m (Figs. 4, a and b ). The shape and size of the  - Fe inclusions are different in different grains. (Fig. 5, a ). There are austenite grains with needle-like ferrite and negligible amount of grain ferrite (Fig. 5, b ). In addition, austenite grains with vermicular ferrite and a large amount of grain ferrite were identi fi ed (Fig. 5, d ). Low carbon content in austenitic steel of the WAAM -specimens under study did not lead to the formation of metal carbides; however, it contributed to the formation of grain ferrite with grain size d = 1 μ m. According to [15, 16], such chemical composition of austenitic steel (Table 1) provides crystallization starting from  - Fe -ferrite formation from liquid melt in line with the mechanism of peritectic transformation. The amount and form of ferrite inclusions is determined by different cooling rate of the deposited wire layers. Under high cooling rate of a new wire layer, the diffusion of the major alloying elements ( Ni , Cr ) that induce phase transforma- tion of  - Fe into austenite is almost absent; the size and amount of ferrite increases, while its major share has vermicular or needle-like form. The decrease in the previous layer cooling rate and high temperature al- lows diffusion of the alloying elements ( Ni , Cr ). The diffusion leads to the dissolution of  - Fe . That is why the boundaries of formed columnar grains are austenitic having a low amount of granular  - Fe (Fig. 5, c ). The XRD analysis (Fig. 6) has shown the following phase composition of the specimens: the main phase is austenite ( γ - Fe , FCC ) with a unit cell parameter of 3.5807 Å, while the second phase is low-