Vol 20, No 4 (2018)

Introduction. In recent years, the fundamentals of the laser-plasma methods for surface modification and micropowder coating application have been developed in the Institute of Laser Physics. The methods are based on the use of optical pulsed discharge plasma. The discharge is ignited by the repetitive laser pulses focused on the surface of the workpiece in a gas or a gas-powder stream. The high pulse repetition frequency of 10-120 kHz is achieved using the generator-amplifier CO2-laser system with the half-height pulse duration set τ = 150-200 ns. The search for other timely applications of these methods is currently ongoing. An interest in the obtaining of amorphous metallic coatings on the surface of structural materials is yet to wane after the decades of intense research done by scientists all over the world due to the outstanding physical, chemical and mechanical properties of such coatings. The purpose of this work is to obtain the hardening coatings based on the Fe-Cr-Si-B-C alloys with high glass-forming ability and to investigate the possibility of obtaining a coating with an amorphous structure using laser-plasma methods. Theory. As the surface of the metallic alloys was under the intense thermal influence of the pulsed laser plasma, the numerical modeling was applied to determine the dependence of amorphized layer’;s thickness on the material properties, as well as in relation to the parameters of the laser irradiation and the laser plasma. Experimental methods. The experiments are carried out in two stages using the installation designed at the Institute: (1) at first, the uniform coatings were prepared on the surface of steel substrates using the laser-plasma application method incorporating the powders of the AP-FeCr4Mn2Si2B4V1 (Fe71.75Cr3.33Si3.54B14.10C4.81Mn1.74V0.73) and AP-FeCr11Mn4SiB (Fe66.8Cr10.79Si5.3B11.42C2.85Mn2.84) grades; (2) then, the surface coatings underwent the rapid laser-plasma modification to ensure the remelting of the thin surface layer. Results and discussion. The numerical methods applied have proven the theoretical possibility of obtaining an amorphous layer of about 3-5 μm thick, based on the Fe-Si-B alloys. As a result, the parameter range required for the successful laser-plasma modification is determined. The hardness of the obtained coatings is measured and its thickness has been determined in dependence on the application parameters. The hardness is measured using the nanoindentation method and equals 12 ± 1 GPa in regards to the coating incorporating the powder AP-FeCr4Mn2Si2B4V1 and 8.5 ± 0.7 GPa in case of the powder AP-FeCr11Mn4SiB; the thickness of the coatings is up to 0.1-0.4 mm. Using optical microscopy, SEM and X-ray diffraction the structure of the coatings is investigated. It is demonstrated that the laser-plasma modification of the coatings on the surface leads to the structure refinement of the surface layer. The characteristic size of the crystallites is 0.5-1 μm. In addition, the hardness of the remelted layer is increased up to 13.8 ± 0.7 GPa for the AP-FeCr4Mn2Si2B4V1 alloy and up to 10.5 ± 0.5 GPa for the AP-FeCr11Mn4SiB alloy. Using SEM and X-ray diffraction the structure of the coatings is investigated. The amorphous phase in the remelted coating layer is not detected, which might be due to an increase in the critical cooling rate during the laser amorphization as compared to the traditional methods of melt quenching.

Introduction. Surface hardening of a large nomenclature of machine-building parts is performed by surfacing with iron-chromium-based powder wires, which ensures the production of metal coatings with high strength and corrosion resistance. At the same time, the resistance of coatings on an iron-chromium base is insufficient, when operating under abrasive wear, due to the small number of strengthening phases in the structure of the surfaced metal. The high operational properties of the surfaced metal can be obtained by combining solid-solution hardening and hardening by second-phase particles in an iron-based matrix. One of such effective method of hardening the metal is surfacing with a flux-cored wire alloyed with boron compounds. However, all the studies performed refer only to the metal coatings in the state after surfacing. The hardness of such coatings is high, which makes it difficult for machining. Purpose of the work: selection of rational parameters for thermal treatment of surfaced coatings based on chromium steel with carbide-boride-nitride alloying. The effect of heat treatment regimes on the microhardness, microstructure and phase composition of the coating metal surfaced by the high-chromium flux cored wire alloyed with complex boride compounds is studied. The composition was the following: 15% Cr + 0.5% B4C + 0.5% BN + 2.5% + TiB2 + 1.0% ZrB2. The methods of investigation are metallography; measurements of microhardness; X-ray phase analysis and transmission electron microscopy. Results and discussion. It is shown that tempering at 800 °C with a 2-hour equalizing ensures the hardness of the surfaced metal within the range of 32-37 HRC, which is acceptable for machining. The microstructure of the metal coatings after tempering is characterized by the structural components decay; the amount of boride eutectic and strengthening phases decreases and its size increases. It is found that to restore the high hardness of the metal after tempering with subsequent machining, it is advisable to conduct quenching from 1020 °C, providing a hardness within the range of 53-58 HRC. This heat treatment leads to the stabilization of the microhardness values at a high level, even higher than the level of the metal coatings microhardness after surfacing. It is shown that this is due to the formation of a composite structure with a martensitic matrix, an eutectic component based on chromium and iron borides Fe1,1Cr0,9B0,9, and dispersed inclusions of carbonitride, carbide and nitride particles for the most part Ti2CN and Cr7С3 and intermetallic compounds Cr4TiZr in the size from 0.4 to 6.5 µm. The established rational parameters of heat treatment can be used in the technology of wear-resistant coatings surfaced with powdered wires alloyed with boride compounds.

Introduction. The development of instrument and mechanical engineering is based on the achievement of high quality indicators and precision dimensional processing of modern structural materials. An important direction of improving the manufacturing technology of critical products is the use of materials with improved physical and mechanical properties and structure. Most often used methods of severe plastic deformation (SPD) are used to obtain such materials. As a result of SPD exposure, an ultrafine-grained (UFG) material structure is formed and its strength increases. The preservation of structural integrity and mechanical properties is an important task in the manufacture of parts from UFG materials that have a low temperature of the onset of recrystallization processes. During dimensional processing, the material is subject to significant deformation and thermal effects, which can affect its structural integrity and mechanical properties. Milling is one of the most common methods for producing high-quality parts from aluminum alloys. This method is preferred for dimensional processing of aluminum alloys with a UFG structure, since it is characterized by a local effect on thin surface layers of the material, in which no substantial heating of the entire volume of the workpiece occurs. The deformation of the surface layer of the material under the action of the cutting blade of the cutter forms the micro-relief of the surface of the part. The difference in the deformation behavior of coarse-grained (CG) and UFG materials can significantly affect the quality of mechanical processing of the latter. As a result, the known optimal machining conditions may not be applicable to UMP materials. The purpose of the work: to study the effect of structural changes in aluminum alloy 7075 on the quality of its machining during milling. In this work, samples of aluminum alloy 7075 in the as-delivered condition and after structure formation are investigated using modern metal-cutting tools and equipment, as well as recommended cutting conditions. The methods of investigation are mechanical tests for compression and tension, optical metallography, transmission electron microscopy, laser scanning microscopy. Results and discussion. Based on the obtained experimental results, it can be concluded that ECAP is an effective way to improve the quality of surface machining when milling 7075 aluminum alloy. At the same time, to ensure the optimum ratio of processing quality and high mechanical strength, two ECAP passages are sufficient, under the selected conditions for the process of structure formation. The obtained results indicate a great potential for the use of products from bulk UFG materials in industry due to the possibility of combining high mechanical properties and quality of dimensional machining in them. The data obtained can be applied in the design of technological processes for the machining of aluminum alloy 7075 with an ultrafine-grained structure under conditions of mass production engineering.