INTRODUCTION Active development of bulk product laser synthesis technology has become especially noticeable in recent years. This technology has a number of advantages, one of which is reduction of term of designing and layout of new equipment. These technologies can be of high demand by metallurgy, aerospace, automotive and other industries [1–5]. Experimental studies confirm the efficiency of the method of layered selective laser sintering (SLS) of bulk products made of powders. When determining the method, the selectivity refers to the ability to scan the free surface of powder material with a laser beam along a preset contour considering laser radiation dispensing at each point, or a direct supply of powdered material to the laser beam processing area (Fig.1–3). With a wide range of powder materials, one can manufacture products with high service properties. The SLS technologies have been actively developed in recent years in the technologically advanced countries. Despite the considerable experience in theoretical and experimental studies, there is practically no mass production of industrial equipment in the SLS field in Russia. A unit cost of purchased foreign equipment exceeds 800 thousand euro.
Selective laser sintering of powders allows for manufacturing of products of a sufficiently wide range of materials (metals, ceramics, plastics and powder, etc.). EXPERIMENTAL COMPLEX FOR ADDITIVE TECHNOLOGIES BASED ON WAVEGUIDE CO2-LASER WITH AN AVERAGE POWER OF RADIATION UP TO 2.5 kW Relying on extensive experience in engineering works on the modification of the surface, hardening, building-up and doping, in recent years the specialists of ILIT RAS have been developing a laser complex of additive technologies (selective laser sintering – SLS of materials and products from metal powders). The complex is based on a multi-channel waveguide CO2 – laser "Hybrid" (Fig.4) with an average power of up to 2.5 kW and 3-coordinate table manipulator [6–8]. Specifications of "Hybrid" laser – Industrial waveguide CO2-laser are shown in Table 1. Industrial laser "Hybrid-1" has a single-mode distribution, and allows you to focus the radiation within a spot diameter of less than 100 microns. With the appropriate hardware and software, it provides for the ability to grow a part with high spatial resolution. Laser "Hybrid-2" differs from the previous one only in the replacement of resonator mirrors. It provides for a unique uniform power density distribution on the processed field with a diameter of 0.5 to 10 mm. Figure 5 shows the measured distribution of the radiation power density on the target. In focus we have a Gaussian distribution (line 1), but at some distance out of focus we have a "super-Gaussian" distribution, i. e., we obtain a uniform shelf on a power density distribution pattern. EXPERIMENTAL RESULTS The aim of research was to study the possibilities of growing workpieces with a uniform fine-grained structure in laser melting of metal powders of heat-resistant nickel-based alloys, designed for figurine-shaped parts by layered laser fusion. The experiment involved building-up of materials made of powders VKNA-1VR, VZHL12U and VZH159. Chemical composition of the powders as a percentage (%) is given below: • Ni-based VKNA 1VR; Al: 8–9; Cr: 5–6; Hf: 0,4–0,6; Mc: 2,5–4,5; Ti: 1–2; W: 2–4; Si: ≤0,4; Fe: ≤0,5 • Ni-based VZHL12U; Co: 5–14,5; Cr: 9–10; Al: 5,1–5,7; Ti: 4,2–4,7; L10: 2,7–3,4; V: 0,5–1; Nb: 0,5–1; W: 1–1,8 • Ni-based VZH159: Al: 1.25–1.55; Mo: 7–8; Nb: 2,5–3,5; Si: ≤0,8; B: ≤0,005; Fe: ≤3,0; P: ≤0,013; Mn: ≤0,5; Mg: ≤0,03 Metal powders of heat-resistant nickel-based alloys VKNA-1VR, VZHL12U and VZH159 are designed for figurine-shaped parts of gas turbine engines (GTE) by layered laser fusion and for repair of GTE figurine-shaped parts by laser gas-powder building-up. According to RRDI, a workpiece with a uniform fine-grained structure can be grown through powder laser melting, which is not possible with conventional casting technology. Macrosection analysis shows that a satisfactory formation of the individual weld beads (Fig.6) is observed under the specified modes. As an example, Fig.7 shows cross-sections of beads obtained by melting of VKNA-1VR powder. With the increase of building-up rate, the width and height of weld beads decreases, the depth of the foundation submelting increases and the length of the heat affected zone (HAZ) decreases (Table 2). The second, and each subsequent, weld bead is formed due to mixing of the molten metal of the preceding bead with the metal powder obtained by melting. Therefore, as the number of beads increases, their chemical composition is stabilized. For laser welding, the structure alignment occurs after 3–5 layers. Based on these considerations, microstructure analysis was carried out in the 3rd bead. The cast weld metal demonstrates cellular or cellular-dendritic transgranular structure (Fig. 8). The crystal growth occurs in the direction from the substrate to the surface. The cast metal microstructure of multi-pass layer does not demonstrate significant differences as compared to the single bead. The solid phase growth starts from the molten structure elements of the previous built-up layer. The growth of the solid phase from molten elements of the primary structure between the individual weld beads along fusion line is observed, which indicates the establishment of a solid metal connection between the weld metal of the previous and subsequent beads. There is no reduction in etchability of fusion zones of the previous and subsequent weld beads under optimal conditions in multi-bead building-up. Moreover, at the contact point of the weld beads, the microhardness remains unchanged (Fig. 9). This indicates that diffusion processes do not have time to develop in the heat-affected zone of the subsequent bead due to high heating and cooling rates during laser exposure. With the increase of building-up rate, there is grinding of elements of the primary structure and the emergence of cellular dendrites (Fig. 10). This indicates an increase in thermal concentrating supercooling, which is fully consistent with the modern concepts of primary structure formation under non-equilibrium conditions. Analysis of the cast structure suggests the improvement of structure-sensitive properties with the increase of building-up rate associated with grinding and "dendritization" of primary structure. Reduction of powder consumption causes its certain coarsening and deviation from the cellular structure towards cellular dendrites (Fig. 11). There is no significant pore formation in the metallographic polished sections. Weld metal primary structure is a cellular-dendritic one. The structure study of metal obtained by VZHL12U powder melting, as well as analysis of bead macrosection shows that, similarly to VKNA 1VR, weld samples have three clearly defined zones. With the increase of building-up rate, the width and height of beads decreases and the depth of the foundation submelting is increased. When a single bead is built-up to the substrate, there is a coarsening of the structure in the HAZ. With the increase of building-up rate, the length of the HAZ decreases (Table 3). The structure study of the metal obtained by VZH159 powder melting showed the following. The trends similar to VKNA-1VR and VZHL12U powders are observed when the building-up rate using VZH159 powder is changed. Similarly, the built-up samples have three clearly defined zones: the cast zone, the metal exposed to thermal influence (HAZ), the base metal not exposed to heat. With the increase of building-up rate, the width and height of beads decreases and the depth of the foundation submelting is increased. When a single bead is built-up to the substrate, there is a coarsening of the structure in the HAZ and with the increase of building-up rate, the length of the HAZ decreases. The laser melting of powders made of heat-resistant nickel-based alloys such as VKNA-1VR, VZHL12U and VZH159 results in a composite structure allowing for high structure-sensitive properties. The cast metal has no pores and crack-like defects. The cast metal microstructure with multi-pass layer shows no significant differences in comparison with the single bead. In microhardness distribution through the thickness of the weld bead, there is a failure of ductility in the HAZ. This may indicate a relatively powerful thermal effect on the substrate during building-up that should be taken into account in the assignment of laser processing modes in the event of bulk product growing. Therefore, a research to clarify the parameters of laser radiation for the future work appears to be necessary. When selecting a building-up mode, one should optimize the parameters of laser building-up process, as there is some discrepancy between the amount of built-up metal and, therefore, performance of the process, and the alleged properties of cast metal. GAP WELDING Along with building-up technologies for the restoration of worn parts, creation and prototyping of various parts already implemented in the industry, the possibility of gap welding additive technologies with metal powder laser welding has been considered recently . This technology has a number of significant advantages as compared to laser welding with deep penetration. For example, it does not have strict requirements to the size of the gap between the surfaces to be welded, and laser layered building-up of metal powder in the gap can provide welding of large thicknesses. The modeling and optimization of basic process parameters for gap welding with laser building-up of metal powders has been performed. The dependence of the absorption coefficient of the powder particles on their radius has been analyzed for application of the powder through the dispenser. The optimal particle speed and dispenser mass speed values have been identified. Figure 12 shows preliminary experimental results. CONCLUSION The development of additive technologies has been launched in Russia. According to Dmitry Rogozin, Deputy Prime Minister of the RF, the additive technologies are, in fact, a new industrial revolution of the sixth technological order, extensively discussed by the experts. To use the vast opportunities of additive manufacturing, it is necessary to conduct fundamental and fundamentally-oriented research. It should be emphasized that the success of the implementation of the results of work on the modification of the surface, hardening, building-up and doping is in direct dependence on the materials used. RAS academician, Evgeniy Kablov, at the opening of the International conference "Additive Technologies: Their Present and Future", has noted that the world leaders in the field of additive technologies are the US, EU and Japan. Furthermore, 22 countries have already established national associations of additive technologies, joined in GARPA alliance, and he has added that Boeing Corporation produces more than 22 thousand parts of 300 styles for 10 brands of commercial and military aircraft thanks to 3D-printing. To exclude a critical dependence on technologies and industrial products in these countries, there is a need to implement own projects.