Technology of obtaining composite conglomerate powders for plasma spraying of high-temperature protective coatings

OBRABOTKAMETALLOV Vol. 23 No. 1 2021 TECHNOLOGY destruction during classi fi cation after sputtering and which have greater fl uidity. Fourth, the sintering surface of the subparticles in conglomerates increases, and as a result, their strength increases as well. With optimal sintering of the subparticles, the maximum density of conglomerated particles is achieved. By adjusting the size of the subparticles, the density of the powder can be controlled within the accepted limits. The formation of composite particles (conglomerates) occurs in the process of suspensions spray drying. The size of the forming particles depends on the subsequent sintering. The granulometric composition, fl uidity of the resulting powder, and its mechanical strength depend on the solvent and binder used, their concentration, the size of the initial powders, the air fl ow rate and the sprayed suspension, the temperature of the spraying air, and the type of nozzle used. According to [26], the average diameter of the sprayed particles can be determined from the empirical equation                      0.45 1.5 1000 585 507 p p G d m U V , (1) where U is the relative velocity between the solution and the gas, m/s; G and V are the fl ow rate of the solution and gas, m 3 /s, respectively; η is the viscosity coef fi cient of the solution, P; σ is the surface tension of the solution, dyn/cm; γ p is the density of the solution, g / cm 3 . Equation (1) demonstrates that obtaining smaller powders particles necessary for plasma spraying in a dynamic vacuum requires increasing the speed of the spraying air and the density of the suspension used, as well as reducing the surface tension and viscosity of the solution. The decrease in the viscosity of the suspension, in turn, is achieved by reducing the concentration of the binder and increasing the amount of the solvent. To isolate the necessary fraction of the powder, the latter was classi fi ed in sieves. The waste of small and large fractions is returned to the process at the stage of the suspension preparation. The fractionated pow- ders were sintered in a free back fi ll in vacuum or argon under the following conditions: the binder (PVA) was distilled at 573...723 K, after which the temperature was increased to such values when the subparticles sintered inside the conglomerates, while the conglomerates did not sinter with each other. The temperature and sintering time for each powder composition were determined experimentally. The results of the research have shown that, on average, the sintering temperature of conglomerated powders should be set at 100 ... 250 K below the sintering temperature of compact materials. The optimal conditions for sintering conglomerated powders are achieved when the sintered mass of the powder is a briquette that easily collapses when crushed. After the control sieving, which is necessary to remove fi ne particles formed during the destruction of individual granules and large sintered conglomerates, the pow- der is ready for spraying. The shape of the integrated powders particles after spray drying conglomeration is close to spherical (Fig. 5). According to the data of X-ray microanalysis, the composition of the ob- tained integral complexes was as follows: 1) 71 wt. % Ni, 17 wt. % Cr, 10 wt. % Al, 1 wt. % Y and 2) 61 wt. % Ni, 22 wt. % Cr, 16 wt. % Al, 1 wt. % Y. If large batches of powders are conglomerated according to the full technological scheme (see Fig. 1) with the return of small and large fractions, the yield of fractions allocated for spraying can be close to 100 %. Thus, obtaining clean-cut fraction powder materials for gas-thermal spraying becomes highly ef fi cient. The spray drying process was optimized to obtain a conglomerate powder of the composition Ni-22Cr- 16Al-1Ywith a grain size of 0 ... 100 μ m. A complete 2 3 factorial experiment was used. Factors: the amount of solvent per 1 kg of mixed powder V ml/kg; excess pressure in the feeder with the suspension P , atm. ; the height of the nozzle cut above the place where the suspension is injected into the gas stream, X , mm. The spray air consumption was 0.5 m3 /min. Insuf fi cient air fl ow leads to the powder sticking to the walls of the chamber. The powder that falls off the chamber walls as it dries has poor sphericity and reduces the quality of the fi nal product. The air pressure inside the pipes of the aerodynamic classi fi er was selected in such a way that the powder with a grain size of more than 100 μ m was deposited in the central pipe, and the