Vol 20, No 3 (2018)

Introduction. The performance characteristics of the surfaces of technical system parts are provided at the finishing operations of the manufacturing process by surface strengthening methods. Despite the fact that there are a sufficiently large number of surface strengthening methods, most of it have limited manufacturing application and require specific and expensive equipment for its implementation, others have not been put into practice on a large scale or have exhausted its technological potential. In this regard, the development of innovative methods of finishing and strengthening treatment of machine part surfaces is an issue of immediate relevance. The aim of the research is to increase the efficiency of strengthening based on the complex energy deposition in the surface layer of ferromagnetic parts by a rotating magnetic field and dynamic surface plastic deformation. The research hypothesis is as follows: the combined magnetic force has an effect on a ferromagnetic part surface, helps to refine the grains of the deformed metal to a nanoscale dimension and increases the depth of the modified (altered) surface layer. Methods and approaches. The paper presents a method of finishing and strengthening treatment, in which a concentrated energy flux of a rotating magnetic field and oscillating deforming balls, which perform multiple pulse-impact deformation, act simultaneously on the surface of a ferromagnetic part. In this case, the induction of the rotating magnetic field acting on the surface of the part is selected in the range from 0.10 to 1.20 T. In order to implement the method of finishing and strengthening treatment, a combined tool has been developed, which contains the following parts: a body; deforming balls freely installed in the annular chamber; magnetic system based on cylindrical permanent magnets made from rare-earth materials. The magnetic system of the tool is designed to create rotating magnetic field acting on the surface of the ferromagnetic part and transmit working oscillating motions to the deforming balls. The paper deals with characteristics of dislocation structures formed in the surface layer of steel and cast iron workpieces after strengthening by magnetic-dynamic rolling (MDR), combined treatment by MDR and a rotating constant magnetic field, combined treatment by MDR and a rotating alternating magnetic field. The research methods are as follows: X-ray diffraction studies of the surface layer; microstructural examination; X-ray microanalysis of the surface layer of strengthened workpieces made of steel and cast iron. Results and discussion. Analysis of the research findings allows establishing that a combined strengthening treatment by MDR and a rotating magnetic field makes it possible to form subgrain structure with nanoscale dimensions in the surface layer of steel and cast iron workpieces to a depth of up to 3.0 μm with a block size of up to 100 nm. In this case, there is an increase in the depth of the modified surface layer, in the dislocation density and in the lattice constant of the ferromagnetic materials being treated. Besides, compressive residual stresses in the strengthened surface layer of the samples are formed. It follows from the physical model of obtaining subgrain structure with nanoscale dimensions in the surface layer of ferromagnetic parts, which is presented in this paper, that the degree of grain crushing (grinding) of the material being strengthened is determined by the number of received force pulses from the deforming balls of the tool. The particles formed as a result of multiple crushing of grains and subgrains have an irregular asymmetric shape and a magnetic moment that does not coincide with the direction of the external magnetic field. As a consequence, the particles formed due to grain and subgrain crushing, trying to orient in the direction of the external magnetic field, turn in space and additionally smooth out the boundaries heated by local eddy currents in the area of their contact with the adjacent fragments of particles, characterized by accumulation of imperfections in the form of dislocations. The developed method of combined MDR belongs to nanotechnologies of surface modification and is recommended for implementation at machine building enterprises to improve operational properties of technical system parts.

Introduction. The paper describes the research results of technological modes for high-speed processing of billets with the aim of obtaining juvenile surfaces and ultrafine powders. Methods of research. As technological factors, the presence/absence of liquid nitrogen in the treatment zone, the rate of revolution of grinding disk, the longitudinal feed, the characteristics of the abrasive tool, and the physical and mechanical characteristics of the materials being processed are taken. As response functions, when considering the influence of technological factors, foreign impurities are taken on the treated surface, the particle size of the powder and the wear of the abrasive tool. All the studies were carried out on the following materials: sintered-hard alloy VK-8, tool steel R-18, brass L63, aluminum alloy D16, ferromagnet M2500NMC1 and neodymium magnet N45M. A scanning electronic microscope Jeol JSM-5700 was used in the studies. The method of planning a two-factor experiment was used to obtain the ratio connecting the size of powder particles with technological factors. Results and discussion. The presence of liquid nitrogen in the processing area allows keeping the surface clean, preventing its oxidation and the appearance of abrasive wear products on it. The processing of viscous materials becomes possible only with the use of liquid nitrogen. The dispersion of the billet at grinding disc rate of revolution higher than 100 m/s leads to a sharp decrease in the particle size of the resulting powder. The use of a feed of less than 1 mm/min in the processing of billets is optimal in terms of the particle size of the powder obtained and the wear of the abrasive tool. The tensile strength of materials is the only parameter considered by physicomechanical characteristics of materials that affects the particle size.

Relevance. The titanium alloy Ti6Al4V is widely used in aerospace and medical industries, due to its high specific strength, ductility and corrosion resistance. However, the use of Ti6Al4V alloy in some important structural elements is limited due to its relatively low oxidation resistance and high viscosity during wear. The purpose of the study is to investigate the increase in the oxidation resistance of the titanium alloy Ti6Al4V at high temperatures and dry sliding wear, by applying Ti-Al-Si-C composite protective layers. The coatings obtained by the method of spark deposition using electrodes made in the form of rods by sintering Ti3Al aluminide powders with 5-15 wt% SiC carbide additives are investigated. Materials and methods. The phase composition of the coatings is studied by X-ray diffraction analysis. Corrosion tests of the coatings are presented by a study of oxidation resistance at a temperature of 900 ° C for ~ 62 hours and potentiodynamic tests in 3.5% NaCl solution. The microhardness of the deposited layers is determined by Vickers indentation at the load of 0.5 N. The wear resistance and coefficient of friction of the coatings are determined in the dry sliding mode versus to high-speed steel R6M5 at a sliding speed of 12 m / s and a load of 25 N. Results and discussion. Electrode materials with addition of Ti3Al intermetallide contained titanium carbide TiC, titanium silicide TiSi2 and complex carbide Ti4Al2C2. According to the analysis of kinetic mass transfer curves, the optimum deposition time of the spark Ti-Al-Si-C coatings for Ti6Al4V alloy is 4 min / cm2. It is established that the basis of coatings is composed of intermetallides Ti3Al and TiAl. In addition, it includes TiC carbide and Ti5Si3 titanium silicide, the content of which increases with increasing the concentration of the SiC additive in the initial composition of the powder mixture. The oxidation resistance of Ti6Al4V alloy with a composite coating obtained from Ti3Al with the addition of 5 wt% silicon carbide was 2.7 times higher than that without coating. According to the results of potentiodynamic tests, it was concluded that Ti3Al coating with a 15 wt% SiC additive had the best anticorrosion characteristics. This coating allows decreasing the wear rate of the titanium alloy Ti6Al4V from 1.9 × 10-4 to 1.2 × 10-6 mm3/(Nm). The hardness of coatings was in the range of 10 - 22 GPa.

Introduction. The refractory materials are of interest for high temperature applications in aerospace, nuclear and military industries, since they possess high melting temperature (> 2000 ?C). Molybdenum (Mo) is among these materials of high interest due to its excellent properties such as good thermal conductivity, high strength and toughness. The production of molybdenum is difficult due to its high melting point and the temperature of the ductile-brittle transition, therefore, in the production of this metal, powder metallurgy methods are mainly used. To implement these methods, it is necessary to have high-quality molybdenum powders, in particular, a high degree of purity and homogeneity of particle size distribution. One of the powder processing methods that is used to produce nano- and microsize powders, is the high energy kinetic milling. This cost-effective method is based on the friction and the high-energy collisions between the balls and the powder particles. And therefore, the purpose of the current work is to optimize the parameters of high energy kinetic milling for molybdenum powder. Optimization of processing parameters has a significant influence on the acceleration of the process of product formation, on subsequent sintering and achievement of the best mechanical properties of the final product. Optimization of milling parameters of Mo powder was achieved under different milling parameters including among others the rotation speed, the ball to powder weight ratio (BPR) and the milling time. Initially, the rotational speed was determined; it varied from 600 to 1200 rpm (where rpm are revolutions per minute). After this determination, milling parameters such as the milling time and the BPR were varied. The milling time ranged from 2 to 60 min and the BPR varied from 100:3 to 200:3. After that, influence of variable parameters on morphology and powder particles size distribution was investigated. The initial powder used in these experiments was Mo powder (particle size ~100 µm). The methods of investigation. Scanning electron microscopy and laser diffraction methods were used to estimate the particle size distribution. Results and Discussion. Particle size was decreased from 100 to 4 µm with increasing grinding time from 2 to 60 min. However, in each batch, a number of cold-welded particles measuring 200-400 μm was detected. These cold-welded particles were about 200-400 µm in size. As the result, the optimal milling parameters were: rotation speed of 900 rpm, BPR (200:3) and milling time of 60 minutes.