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 Introduction Additive Technologies ( AT ), or 3D-printing with metals, is the most rapidly growing sector of Additive Manufacturing ( AM ). ATs allow manufacturing the parts that cannot be produced by other methods; they are less material-consuming as compared to other technologies and reduce production costs [1–4]. It is important in 3D-printing with metals that the solid-state digital model of the part is combined with the software of the printing device itself. The production of parts involves layer-by-layer deposition welding using various heat sources and raw materials. Such parts are demanded by aerospace industry, medical, energy and transport engineering. Examples of such parts are custom-tailored medical implants, turbine blades, special coolers with internal channels, fasteners, reticulated structures and trusses with optimal weight-to-strength ratio for space equipment, etc. It is remarkable that with the help of ATs it is possible to manufacture parts with a speci fi c composition and properties. The methods of layer-by-layer deposition welding of metal powders on a substrate with direct melting of the material in the zone of high-energy beam have gained the widest application in additive manufacturing of metal products. The most often used high-energy sources are lasers and electron-beam guns [1–6]; in some cases they are arc discharges and gas-discharge plasmas [7, 8]. It should be noted that in recent years, along with selective methods of fusion of materials in additive technologies, methods of layer-by-layer melting are increasingly used in the direct deposition of a material in the zone of a high-energy laser or electron beam. The feeding of the material to be deposited (a wire or a rod) is used quite often [7–13]. In such cases, electron-beam or arc sources are used as heat sources [7, 10–13]. These sources allow to carry out the 3D-printing process with high productivity and give low porosity of the printed products. In ATs , various metals, alloys and compositions are used as raw materials [4, 14]. However, the mostly used are stainless steels and titanium alloys. The potential of such parts is outstanding. The implementation of titanium alloys is accompanied with a number of issues, such as the requirement of welding in vacuum. Welding in vacuum has higher ef fi ciency when electron beam is used [1, 5, 10–13]. Austenitic stainless steels are welded well by both laser sintering and electron-beam welding [1, 4, 12, 14–17]. The structure of parts from stainless steels and titanium alloys is in direct dependence on the method they are manufactured. The parameters of printing equipment have direct impact on the strength, hardness, heat resistance, heat stability and other properties [18]. When printing steel parts by layer-by-layer deposition welding, an ordered crystalline structure forms. In comparison with traditional methods of manufacturing (casting, shaping of metals) new phases and defects may occur, chemical composition and structure may change at different scales [4]. Changes in the structure of steel parts manufactured by AT methods give them changes in properties such as elasticity modulus, strength, viscosity, fatigue resistance, creep. As a result, such changes affect the corrosion resistance of steel and the part as a whole [15–17]. Despite the considerable number of works on 3D-printing with steels, there are few works investigating steel specimens at different scales, including those with implementation of non-destructive testing methods. The discontinuities and pores are typical defects in parts 3D-printed layer-by-layer. To remove such defects, a variety of post-processing treatment methods are used. Post-processing complicates the manufacturing process and increases the cost of the fi nal product. At the same time, the study of the pore formation process in different 3D-printing modes has shown that the porosity can be reduced without additional treatment just by selecting suitable modes [19]. In recent time, different non-destructive testing methods have been used to study porosity in 3D-printed parts [13, 20]. Printed titanium alloy parts are often tested using nanoindentation. This method is used to study the properties of titanium alloys formed using plasmas [21], electron-beam melting of powders [22] and electron-beam additive processing [23]. Nanoindentation is also used to study materials produced by contact welding of titanium and gold alloys [24]. Therefore, the development of new methods and technics for manufacturing parts from stainless steels is one of the key directions of modern manufacturing development. The development of proprietary setups and complexes for additive manufacturing and AT -based production are of high priority [12]. The work is aimed at manufacturing stainless-steel specimens possessing uniform structure and minimal amount of macro- and micro-defects using Wire Arc Additive Manufacturing ( WAAM ) method on an electron-beam setup developed at Tomsk Polytechnic University.