OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 26 No. 2 2024 and low thermal conductivity, while LENS allows the formation of complex-profiled parts with minimal mechanical processing. Moreover, diminishing mechanical processing helps reduce the amount of expensive alloy waste. It is known that the quality of the objects manufactured by LENS depends on its technological parameters. Quality in this case implies the absence of macrodefects like cracks and pores that can form during laser remelting of the powder material. In [6–13] authors optimize the LENS technological parameters for manufacturing parts from titanium alloys by varying laser power, scanning speed, powder feed rate, laser spot diameter, distance between laser tracks and laser operating mode (pulse/continuous) [5]. However, the practical application of modes developed by other researchers is mainly hindered by two reasons. The first reason is that most researches have been conducted on the titanium alloy Ti-6Al-4V [4–9] and significantly less studies have been devoted to the development of LENS technology for titanium alloys of different compositions (Ti-Al-Sn-Zr-Mo [10], Ti-Al-Sn-Zr-Mo-Cr [11], Ti-Al-Mo-Zr-Si [12]), and there are almost no studies on LENS of the Ti-5Al-4V-2.5Mo-1Cr-0.7Fe-0.1Si alloy (VT23). However, it is known that changes in the physical properties of the alloy affect the powder material remelting processes, and the alloy composition have to be taken into account. Moreover, LENS is a multi-parameter process, while most papers only provide values for basic parameters. Thermal conditions of powder remelting significantly depend on the working setup, the initial state of the powder material and even slight changes in re-melting conditions can result in shift of the optimal parameter range. Thus, developing modes of manufacturing high-quality objects of a given composition is an integral part of LENS technology. The purpose of this work was to develop LENS modes on the InssTek MX-Grande printer for the formation of high-quality products from a titanium alloy (VT23). To achieve this goal, the following tasks were solved during the research: - determination of intervals of the LENS technological parameters (laser radiation power, scanning speed and distance between laser tracks), which make it possible to form a structure without cracks, with minimal porosity and surface roughness, with penetration into the lower layer less than 40 %; - metallographic study of the structure formed in the alloy during LENS; - determination of the phase composition of the alloy after LENS; - study of the influence of the LENS technological parameters on the microhardness of the obtained material. Methods The objects of the study were specimens manufactured by LENS from titanium alloy VT23. X-ray fluorescence spectral analysis was used to assess the elemental composition of the initial titanium powder, and the concentrations of gas impurities in the powder were determined by the reduction burning method (Table 1). The chemical composition of the VT23 powder, including the concentrations of gas impurities, corresponded to OST 1-90013-81 [14]. The dispersion of the powder ranged from 40 to 100 µm. The phase composition of the initial powder material was determined by X-ray diffraction method and consisted of a α-Ti solid solution with a cubic crystal lattice and titanium oxide TiO2 (rutile) phase with a tetragonal crystal lattice while β phase was not detected (Figure 1). The presence of the TiO2 phase in the diffraction pattern is likely associated with a high proportion of the surface oxide layer in the irradiated volume of a dispersed (less than 100 µm) powder material during X-ray analysis. LENS process was carried out on the InssTek MX-Grande laser system in an Ar protective atmosphere, and the LENS modes are provided in Table 2. The selection of LENS modes that allow forming parts of satisfactory quality was carried out in several steps. In the first step, the selection is based on the geometric parameters of the cross-section of a single laser track. In the second step, the characteristics of monolayers, i.e., objects with a height of one deposited layer, are considered, and in the final step, the quality of the volumetric specimen is analyzed.
RkJQdWJsaXNoZXIy MTk0ODM1