rus
Issue #6/2023
V. P. Biryukov
Optimization of Laser Surfacing Technology and Its Effect on Coating Properties
Optimization of Laser Surfacing Technology and Its Effect on Coating Properties
DOI: 10.22184/1993-7296.FRos.2023.17.6.442.452
Optimization of Laser Surfacing Technology and Its Effect on Coating Properties
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Mosscow, Russia
The paper considers the results of metallographic and tribotechnical tests of the zones of laser surfacing of the sublayer with a powder containing Fe-Co-Cr-Mo and a composite charge Ni-Cr-B-Si+WC on steel samples 40Kh. It is shown that processing using transverse beam vibrations normal to the scanning velocity vector increases the productivity of the surfacing process. The application of a sublayer not prone to cracking improves the quality of deposited coatings with the addition of a carbide phase, eliminates the formation of cracks in the deposited charge with carbides. An increase in the energy density above the optimal values leads to partial dissolution of carbides, evaporation of carbon, a decrease in the thickness of the sublayer and mixing with the charge with the carbide phase and a decrease in the microhardness of coatings. Laser surfacing at optimal conditions allowed to increase the abrasive wear resistance when tested with loose grains by 11 times compared to the base steel.
Keywords: laser surfacing, microhardness, abrasive wear resistance
Article received: 30.08.2023
Article accepted: 18.09.2023
Introduction
The desire to improve the operational parameters of machine parts and tools and increase their durability has led to the development of surface treatment and coating methods. One of the advanced modern methods of coating is laser surfacing, which has a number of advantages over other traditional methods of surfacing and coating [1–3]. Laser surfacing technology can be used for the manufacture of wear-resistant or corrosion-resistant metal coatings and composite materials with a metal matrix. Typically, Ni or Co-based alloys are used as a metal matrix for a composite coating, since they are highly resistant to oxidation, including at high temperature. It has been found that if 6.0% Re is added to Ni-Cr alloys, the resistance to corrosion and the formation of oxide coatings operated at elevated temperatures can be significantly increased [4]. Medium carbon steels are widely used for the manufacture of connecting rods, gears, bearings and other important structural components [5, 6]. However, the wear resistance of these steels is limited, and when used in harsh conditions, these parts often fail due to severe surface wear, which significantly increases the cost of operating costs in the industrial use of machines and aggregates [7–9]. Laser surfacing is widely used to increase the wear resistance of industrial parts [10, 11]. In laser surfacing, two methods are used with preliminary application of slip coatings and synchronous powder feeding [12, 13].
Coatings with NiCrBSi powders with a particle size of 13–63 µm with additives of WC–Co tungsten carbides were applied to a substrate consisting of low-carbon mild steel [14]. A WC 50 optical head with coaxial powder feeding was used for laser surfacing. Single and partially overlapping roads were applied to steel samples. As a variable parameter, a different speed of beam movement was used. The experiments were carried out using a Coherent F1000 diode laser (Coherent, Santa Clara, California, USA) equipped with a seven-axis robot CLOOS 7. An AT‑1200HPHV Termach feeder was used to transport the powder, and argon was used as a protective gas and carrier gas. Laser surfacing was carried out at a radiation power of 720 W, a travel speed of 0.45–0.85 m/min and a powder consumption of 4 g/min. A TR‑20 tribometer (Ducom Instruments, Bangalore, India) was used to assess the wear resistance of the applied coatings. Before testing, the samples were ground and polished to obtain a mirror surface. A WC ball with a diameter of 6 mm was used as a counter-sample with an applied load of 15 N. The diameter of the wear track was 12 mm, the test time was 132 min, and the friction path was 2 000 m. The friction coefficients varied in the range of 0.463–0.695, and the wear rate of 0.125–0.735 mm3 / N ∙ km. Crack-free coatings with the desired carbide distribution can be obtained by optimizing the laser deposition rate. In addition, an increase in hardness was achieved by reducing the melt zone between the coating and the substrate.
For experiments [15], a powder consisting of 60% WC mixed with 40% NiCrBSi particles with particle sizes of 45–106 µm was used. Laser surfacing was performed using a Trumpf TruDisk 8002 disk laser equipped with a Precitec YC52 head with coaxial powder feeding. Two series of experiments were conducted. In the first series of experiments, the beam velocity (S) changed, while the power (P) and the powder consumption (F) remained constant. In the second series, the powder consumption remained constant and the speed of the beam movement, and the radiation power was variable. The coefficient of overlap of the surfacing tracks was 50%. The coatings were applied to carbon steel plates (EN10083 2: C45) with dimensions of 100 × 100 × 20 mm to ensure sufficient heat dissipation. The substrates were preheated to 350 °C before application to avoid cracking of the coating due to the high temperature gradient. After laser surfacing, the samples were left to cool at room temperature. The test for abrasive wear with loose grain was carried out according to the scheme “flat sample-forming surface of a rubber disc” in accordance with ASTM G‑65. The test parameters were as follows: load 22 N; abrasive medium quartz sand Al2O3 with a grain size of 200–300 µm; total friction path 718 m. The hardest coating has reached a hardness value above 800 HV1. The hardness of the coatings decreased with a decrease in the P/F·S parameter to 600–700 HV1. In areas with a higher content of WC particles, the measured hardness exceeded 1 000 HV1, and in places where the tracks overlap, the hardness varied from 500 to 700 HV1. The values of the wear coefficient varied from 2.2 · 10–4 to 1.1 · 10–3 mm3/Nm, depending on the laser processing parameters. The deposited coatings with a higher concentration of spherical carbides resisted abrasive wear better than those samples in which carbides dissolved in the coating. The material of the previous laser surfacing track was re-melted, the carbide particles were dissolved and re-deposited again. The melting of the surface of the sample based on iron, and the dissolution of carbides led to a deterioration in the hardness of the coatings and wear resistance.
The laser surfacing system [16] consisted of an industrial Nd-YAG laser with a power of 2 kW (Rofin-Sinar DY 22), an optical head, a powder feeder (Sulzer-Metco Twin 10 c). The laser beam was defocused to a diameter of 3.5 mm on the work surface. The surfacing was carried out on samples with a thickness of 5 mm made of low-carbon steel C25 preheated to 400°C. The surfaced sections with a size of 30 × 30 mm were obtained at track overlap coefficients of 35–40%. Technolase nickel-chromium-based tungsten carbide powders T60 (700 HV), T40 (400 HV) and T30 (300 HV) were used for laser surfacing experiments. Tribotechnical tests were carried out on surfaces with applied coatings during dry sliding using the MT4002 tribometer according to the friction scheme “ball (Al2O3 with a diameter of 4 mm, 1 500 HV) – disk (deposited sample)”, in accordance with the ASTM G99-05 standard. The tests were performed at a sliding speed of 100 mm/s, a normal load of 20 N, and a friction path of 500 m. After the tests, the weight loss of coated samples and counter-tiles was estimated. Cracking of the coatings could not be avoided in any of the treated T60 samples. The average number of cracks was 10 at a length of 40 mm, perpendicular to the trajectory of the laser tracks. Cracking has noticeably decreased on T40 coatings and it was avoided on samples deposited with T30 alloy. The wear rates for T30 and T40 were 0.0176 and 0.0242 ∙ 10–9 Kg / N ∙ m and were lower than those obtained for T60, 0.0375 ∙ 10–9 Kg / N ∙ m.
NiCrBSi powder was deposited on a steel substrate [17] using a CO2 laser at a power of 1.4–1.6 kW, a scanning speed of 160–180 mm/min, a spot of 6.0 × 1.5 mm, with a powder consumption of 2.9–4.9 g / min. The deposited samples were further processed by heating in the range of 200–1050 °C, followed by cooling in air and in a vacuum furnace. Samples from Cu-Cr-Zr S18150 ASTM alloy with dimensions of 100 × 100 × 40 mm were deposited with a NiBSi-WC coating on a HighLight 10000D diode laser (Coherent, USA), at a power of 5 kW, with a spot of 6 × 2 mm, with a step of 6 mm and a powder consumption of 36 g / min. Coatings with a thickness of 0.6 and 1.6 mm were applied at a scanning speed of 10 and 2.5 mm/s, respectively. NiCrBSi coated samples were subjected to abrasive wear on a fixed abrasive grain of corundum Al2O3 with a specific load of 1 MPa and an average sliding speed of 0.175 m / s. according to the “pin – disc (steel Kh12M, 61.5 HRC)” scheme at a pressure of 2 MPa, sliding speeds of 3.1, 4.7, 6.1 and 9.3 m / s, test time 9.5–30 min. Tribotechnical tests of the NiBSi-WC coating were carried out according to the scheme “pin (surfaced sample)-plate (steel X12M)” with reciprocating motion with a pressure of 6 MPa, with a sliding speed of 0.08 m/s, and a double stroke length of 60 mm. High-temperature (1 025 °C) annealing of samples with NiCrBSi laser surfacing led to the formation of large phases of carbides and borides allowing to maintain wear resistance at high temperatures. NiBSi-WC coatings on a Cu-Cr-Zr alloy substrate had large carboborides in the structure, which increased the wear resistance of the deposited layers in thicker coatings by 20% compared to thin coatings.
Laser surfacing [18] was carried out on 42CrMo steel samples with dimensions of 70 × 15 × 10 mm with powders of high-entropy alloys (HES) in accordance with the molar ratio FeCoNiCrNb0.5Mox (x = 0.00, 0.25, 0.50, 0.75, 1.00). The treatment was carried out on a fiber laser FL020 in an argon atmosphere at a radiation power of 1 400 W, scanning speed of 3 mm / s, spot diameter of 4 mm. The coatings Mo0.00 and Mo0.25 had a pre-eutectic structure, while the coatings Mo0.50, Mo0.75 and Mo1.00 had a completely eutectic structure. The coating of the Mo0.75 wind farm during corrosion tests had the lowest current density and had the best corrosion resistance.
Equipment and research methods
To optimize the laser surfacing technology, 40Kh steel samples with dimensions of 15 × 20 × 70 mm were used. The samples were processed on the automated system of IMASH RAN. Iron-based powder Fe-Co-Cr-Mo (50–150 µm) was used as a sublayer, and nickel-based powder with the addition of tungsten carbide 40 wt. % (WC-W2C) (40–100 µm) + 60 wt. % (Ni-Cr-B-Si) was used for surfacing the main coating with particle sizes of 40–125 microns. To obtain various parameters of the deposited zones, the laser radiation power density was changed in the range of 28–45 J / mm2 (Fe-Co-Cr-Mo) and 32–86 J / mm2 (Ni-Cr-B-Si+WC). The speed of application of laser tracks varied between 5–10 mm / s and 5–7 mm / s, and the spot diameter was 2.5–3.5 mm, respectively. Surfacing was performed with a defocused and oscillating beam to equalize the energy density along the cross-section of the tracks with a frequency of 218 Hz normal to the processing speed vector. The thickness of the slip coatings with a water-based binder was 0.6 and 0.85 mm. After applying the coating to the samples, they were dried at a temperature of 80 °C for 2 hours. The surfacing of the laser tracks was performed with an overlap of 35%. After applying the sublayer, the samples were ground to a size of 12.3–0.1 mm. A digital microscope, the OMOS M1000 metallographic system and the PMT‑3 microhardness meter were used in the conduct of metallographic studies. Determination of the elemental composition of coatings was carried out on a scanning electronic complex SEC SNE 4500M Plus, Korea, equipped with an energy dispersion analyzer from Bruker, Germany, in reflected electrons.
Tribotechnical tests for abrasive wear with loose abrasive grain were carried out according to the scheme “the wide side of the sample with a surfaced coating, the base is the annular surface of a flat rubber disc”. Quartz sand with a particle size of 0.2–0.6 mm was used as an abrasive.
Results of experimental studies
Fig. 1 shows the microslips of deposited coatings with powder for the Fe-Co-Cr-Mo sublayer (Fig.1, a) and the main powder (Ni-Cr-B-Si+WC) (Fig. 1, b). The height and width of the surfacing zones during processing with a defocused and oscillating beam along the normal to the vector, the scanning speeds of the beam were 0.48–0.86, 0.45–0.79 mm and 1.9–2.9 mm and 3.2–5.8 mm, respectively. The depth and width of the quenching zones of the base, 40Kh steel, was 0.49–0.86, 0.35–0.89 mm and 1.8–2.85 and 3.1–5.7 mm, respectively. The use of transverse oscillations of the laser beam led to an increase in the surfacing performance by 1.6–2.2 times compared to the treatment with a defocused beam. Fig. 2 shows the microstructures of laser tracks at the boundary with the base material. With a higher laser surfacing speed and lower power, the thickness of the sublayer was 200–300 microns, and with an increase in the radiation power to 1 000 W and a decrease in the scanning speed to 5 mm/s, the thickness of the sublayer decreased and was in the range of 25–50 microns. In fact, the elements of the Fe-Co-Cr-Mo sublayer diluted the melt bath of the main coating Ni-Cr-B-Si+WC. In addition, the number of tungsten carbide particles at an increased energy density of the laser beam sharply decreased, saturation of the nickel matrix with carbon and tungsten occurred during the dissolution of carbides and partial carbon burnout.
Fig. 3 shows graphs of changes in microhardness, obtained as a result of its measurement, from the surface of the deposited track deep into the base material with a step of 100 µm. The curve (Fig. 3, a) was obtained by measuring a sample processed at a speed of 7 mm/s and a radiation power of 1 000 watts. The deposited layer with Ni-Cr-B-Si+WC powder had a high microhardness of 9 000–11 000 MPa. The sublayer zone had a lower microhardness of 6 500–7 000 MPa with a depth of up to 300 µm. As a result of heating above 1 250 °C of the liquid melt bath and the sublayer above 1 000 °C, a complete hardening of the main material of 40Kh steel to a microhardness of 6 500–7 000 MPa at 300 µm and below occurred, an incomplete hardening section with a microhardness of 2 900–6 000 MPa with a depth of 250 µm was observed. The curve in Fig. 2, b is obtained at a radiation power of 1000 W, a travel speed of 5 mm / s with a coating thickness of 0.6 mm. In the surfacing zone of the main charge Ni-Cr-B-Si+WC, a decrease in microhardness to 7 000–8 000 MPa was observed, which is associated with a large dissolution of the carbide phase and partial mixing with the sublayer.
Figure 4 shows the study area of the elemental composition of the main coating Ni-Cr-B-Si+WC and the distribution of elements W, Ni, Fe, Cr, C. The quantitative composition of the elements in the coating is shown in Table.
Analysis of the results of abrasive wear (Fig. 5) with loose grain showed that the wear resistance of Ni-Cr-B-Si+WC coatings increases by 11 times at a radiation energy density of 48 J/mm2 compared with the base material.
Discussion of the results
The obtained research results have shown that the application of coatings with an increased content of the carbide phase is accompanied by the appearance of defects in the form of cracks and pores in the case of incorrectly selected processing modes. To reduce stresses at the boundary of the base material and coatings with carbides, it is proposed to apply a sublayer that is not prone to cracking and has a damping ability due to the high vanadium content in the initial charge. A decrease in the microhardness of coatings with a hardening phase at an increased energy density of laser radiation indicates the dissociation of carbides and partial carbon burnout. The use of optimal laser surfacing modes made it possible to obtain coatings with the highest possible microhardness and wear resistance when worn by loose abrasive grain.
Conclusion
The technology of laser surfacing with an intermediate sublayer Fe-Co-Cr-Mo and the main coating Fe-Co-Cr-Mo on 40Kh steel samples with the application of transverse oscillations of the laser beam to the processing speed vector has been developed. An increase in the radiation energy density significantly above the optimal values led to the dissolution of carbides and a decrease in microhardness. The wear resistance of coatings with a carbide phase obtained at an energy density of 48 J/mm2 is 11 times higher than that of the base material.
AUTHOR
Biryukov V. P., Cand. of Scin.(Eng.), Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Mosscow, Russia.
ORCID: 0000-0001-9278-6925
V. P. Biryukov
Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Mosscow, Russia
The paper considers the results of metallographic and tribotechnical tests of the zones of laser surfacing of the sublayer with a powder containing Fe-Co-Cr-Mo and a composite charge Ni-Cr-B-Si+WC on steel samples 40Kh. It is shown that processing using transverse beam vibrations normal to the scanning velocity vector increases the productivity of the surfacing process. The application of a sublayer not prone to cracking improves the quality of deposited coatings with the addition of a carbide phase, eliminates the formation of cracks in the deposited charge with carbides. An increase in the energy density above the optimal values leads to partial dissolution of carbides, evaporation of carbon, a decrease in the thickness of the sublayer and mixing with the charge with the carbide phase and a decrease in the microhardness of coatings. Laser surfacing at optimal conditions allowed to increase the abrasive wear resistance when tested with loose grains by 11 times compared to the base steel.
Keywords: laser surfacing, microhardness, abrasive wear resistance
Article received: 30.08.2023
Article accepted: 18.09.2023
Introduction
The desire to improve the operational parameters of machine parts and tools and increase their durability has led to the development of surface treatment and coating methods. One of the advanced modern methods of coating is laser surfacing, which has a number of advantages over other traditional methods of surfacing and coating [1–3]. Laser surfacing technology can be used for the manufacture of wear-resistant or corrosion-resistant metal coatings and composite materials with a metal matrix. Typically, Ni or Co-based alloys are used as a metal matrix for a composite coating, since they are highly resistant to oxidation, including at high temperature. It has been found that if 6.0% Re is added to Ni-Cr alloys, the resistance to corrosion and the formation of oxide coatings operated at elevated temperatures can be significantly increased [4]. Medium carbon steels are widely used for the manufacture of connecting rods, gears, bearings and other important structural components [5, 6]. However, the wear resistance of these steels is limited, and when used in harsh conditions, these parts often fail due to severe surface wear, which significantly increases the cost of operating costs in the industrial use of machines and aggregates [7–9]. Laser surfacing is widely used to increase the wear resistance of industrial parts [10, 11]. In laser surfacing, two methods are used with preliminary application of slip coatings and synchronous powder feeding [12, 13].
Coatings with NiCrBSi powders with a particle size of 13–63 µm with additives of WC–Co tungsten carbides were applied to a substrate consisting of low-carbon mild steel [14]. A WC 50 optical head with coaxial powder feeding was used for laser surfacing. Single and partially overlapping roads were applied to steel samples. As a variable parameter, a different speed of beam movement was used. The experiments were carried out using a Coherent F1000 diode laser (Coherent, Santa Clara, California, USA) equipped with a seven-axis robot CLOOS 7. An AT‑1200HPHV Termach feeder was used to transport the powder, and argon was used as a protective gas and carrier gas. Laser surfacing was carried out at a radiation power of 720 W, a travel speed of 0.45–0.85 m/min and a powder consumption of 4 g/min. A TR‑20 tribometer (Ducom Instruments, Bangalore, India) was used to assess the wear resistance of the applied coatings. Before testing, the samples were ground and polished to obtain a mirror surface. A WC ball with a diameter of 6 mm was used as a counter-sample with an applied load of 15 N. The diameter of the wear track was 12 mm, the test time was 132 min, and the friction path was 2 000 m. The friction coefficients varied in the range of 0.463–0.695, and the wear rate of 0.125–0.735 mm3 / N ∙ km. Crack-free coatings with the desired carbide distribution can be obtained by optimizing the laser deposition rate. In addition, an increase in hardness was achieved by reducing the melt zone between the coating and the substrate.
For experiments [15], a powder consisting of 60% WC mixed with 40% NiCrBSi particles with particle sizes of 45–106 µm was used. Laser surfacing was performed using a Trumpf TruDisk 8002 disk laser equipped with a Precitec YC52 head with coaxial powder feeding. Two series of experiments were conducted. In the first series of experiments, the beam velocity (S) changed, while the power (P) and the powder consumption (F) remained constant. In the second series, the powder consumption remained constant and the speed of the beam movement, and the radiation power was variable. The coefficient of overlap of the surfacing tracks was 50%. The coatings were applied to carbon steel plates (EN10083 2: C45) with dimensions of 100 × 100 × 20 mm to ensure sufficient heat dissipation. The substrates were preheated to 350 °C before application to avoid cracking of the coating due to the high temperature gradient. After laser surfacing, the samples were left to cool at room temperature. The test for abrasive wear with loose grain was carried out according to the scheme “flat sample-forming surface of a rubber disc” in accordance with ASTM G‑65. The test parameters were as follows: load 22 N; abrasive medium quartz sand Al2O3 with a grain size of 200–300 µm; total friction path 718 m. The hardest coating has reached a hardness value above 800 HV1. The hardness of the coatings decreased with a decrease in the P/F·S parameter to 600–700 HV1. In areas with a higher content of WC particles, the measured hardness exceeded 1 000 HV1, and in places where the tracks overlap, the hardness varied from 500 to 700 HV1. The values of the wear coefficient varied from 2.2 · 10–4 to 1.1 · 10–3 mm3/Nm, depending on the laser processing parameters. The deposited coatings with a higher concentration of spherical carbides resisted abrasive wear better than those samples in which carbides dissolved in the coating. The material of the previous laser surfacing track was re-melted, the carbide particles were dissolved and re-deposited again. The melting of the surface of the sample based on iron, and the dissolution of carbides led to a deterioration in the hardness of the coatings and wear resistance.
The laser surfacing system [16] consisted of an industrial Nd-YAG laser with a power of 2 kW (Rofin-Sinar DY 22), an optical head, a powder feeder (Sulzer-Metco Twin 10 c). The laser beam was defocused to a diameter of 3.5 mm on the work surface. The surfacing was carried out on samples with a thickness of 5 mm made of low-carbon steel C25 preheated to 400°C. The surfaced sections with a size of 30 × 30 mm were obtained at track overlap coefficients of 35–40%. Technolase nickel-chromium-based tungsten carbide powders T60 (700 HV), T40 (400 HV) and T30 (300 HV) were used for laser surfacing experiments. Tribotechnical tests were carried out on surfaces with applied coatings during dry sliding using the MT4002 tribometer according to the friction scheme “ball (Al2O3 with a diameter of 4 mm, 1 500 HV) – disk (deposited sample)”, in accordance with the ASTM G99-05 standard. The tests were performed at a sliding speed of 100 mm/s, a normal load of 20 N, and a friction path of 500 m. After the tests, the weight loss of coated samples and counter-tiles was estimated. Cracking of the coatings could not be avoided in any of the treated T60 samples. The average number of cracks was 10 at a length of 40 mm, perpendicular to the trajectory of the laser tracks. Cracking has noticeably decreased on T40 coatings and it was avoided on samples deposited with T30 alloy. The wear rates for T30 and T40 were 0.0176 and 0.0242 ∙ 10–9 Kg / N ∙ m and were lower than those obtained for T60, 0.0375 ∙ 10–9 Kg / N ∙ m.
NiCrBSi powder was deposited on a steel substrate [17] using a CO2 laser at a power of 1.4–1.6 kW, a scanning speed of 160–180 mm/min, a spot of 6.0 × 1.5 mm, with a powder consumption of 2.9–4.9 g / min. The deposited samples were further processed by heating in the range of 200–1050 °C, followed by cooling in air and in a vacuum furnace. Samples from Cu-Cr-Zr S18150 ASTM alloy with dimensions of 100 × 100 × 40 mm were deposited with a NiBSi-WC coating on a HighLight 10000D diode laser (Coherent, USA), at a power of 5 kW, with a spot of 6 × 2 mm, with a step of 6 mm and a powder consumption of 36 g / min. Coatings with a thickness of 0.6 and 1.6 mm were applied at a scanning speed of 10 and 2.5 mm/s, respectively. NiCrBSi coated samples were subjected to abrasive wear on a fixed abrasive grain of corundum Al2O3 with a specific load of 1 MPa and an average sliding speed of 0.175 m / s. according to the “pin – disc (steel Kh12M, 61.5 HRC)” scheme at a pressure of 2 MPa, sliding speeds of 3.1, 4.7, 6.1 and 9.3 m / s, test time 9.5–30 min. Tribotechnical tests of the NiBSi-WC coating were carried out according to the scheme “pin (surfaced sample)-plate (steel X12M)” with reciprocating motion with a pressure of 6 MPa, with a sliding speed of 0.08 m/s, and a double stroke length of 60 mm. High-temperature (1 025 °C) annealing of samples with NiCrBSi laser surfacing led to the formation of large phases of carbides and borides allowing to maintain wear resistance at high temperatures. NiBSi-WC coatings on a Cu-Cr-Zr alloy substrate had large carboborides in the structure, which increased the wear resistance of the deposited layers in thicker coatings by 20% compared to thin coatings.
Laser surfacing [18] was carried out on 42CrMo steel samples with dimensions of 70 × 15 × 10 mm with powders of high-entropy alloys (HES) in accordance with the molar ratio FeCoNiCrNb0.5Mox (x = 0.00, 0.25, 0.50, 0.75, 1.00). The treatment was carried out on a fiber laser FL020 in an argon atmosphere at a radiation power of 1 400 W, scanning speed of 3 mm / s, spot diameter of 4 mm. The coatings Mo0.00 and Mo0.25 had a pre-eutectic structure, while the coatings Mo0.50, Mo0.75 and Mo1.00 had a completely eutectic structure. The coating of the Mo0.75 wind farm during corrosion tests had the lowest current density and had the best corrosion resistance.
Equipment and research methods
To optimize the laser surfacing technology, 40Kh steel samples with dimensions of 15 × 20 × 70 mm were used. The samples were processed on the automated system of IMASH RAN. Iron-based powder Fe-Co-Cr-Mo (50–150 µm) was used as a sublayer, and nickel-based powder with the addition of tungsten carbide 40 wt. % (WC-W2C) (40–100 µm) + 60 wt. % (Ni-Cr-B-Si) was used for surfacing the main coating with particle sizes of 40–125 microns. To obtain various parameters of the deposited zones, the laser radiation power density was changed in the range of 28–45 J / mm2 (Fe-Co-Cr-Mo) and 32–86 J / mm2 (Ni-Cr-B-Si+WC). The speed of application of laser tracks varied between 5–10 mm / s and 5–7 mm / s, and the spot diameter was 2.5–3.5 mm, respectively. Surfacing was performed with a defocused and oscillating beam to equalize the energy density along the cross-section of the tracks with a frequency of 218 Hz normal to the processing speed vector. The thickness of the slip coatings with a water-based binder was 0.6 and 0.85 mm. After applying the coating to the samples, they were dried at a temperature of 80 °C for 2 hours. The surfacing of the laser tracks was performed with an overlap of 35%. After applying the sublayer, the samples were ground to a size of 12.3–0.1 mm. A digital microscope, the OMOS M1000 metallographic system and the PMT‑3 microhardness meter were used in the conduct of metallographic studies. Determination of the elemental composition of coatings was carried out on a scanning electronic complex SEC SNE 4500M Plus, Korea, equipped with an energy dispersion analyzer from Bruker, Germany, in reflected electrons.
Tribotechnical tests for abrasive wear with loose abrasive grain were carried out according to the scheme “the wide side of the sample with a surfaced coating, the base is the annular surface of a flat rubber disc”. Quartz sand with a particle size of 0.2–0.6 mm was used as an abrasive.
Results of experimental studies
Fig. 1 shows the microslips of deposited coatings with powder for the Fe-Co-Cr-Mo sublayer (Fig.1, a) and the main powder (Ni-Cr-B-Si+WC) (Fig. 1, b). The height and width of the surfacing zones during processing with a defocused and oscillating beam along the normal to the vector, the scanning speeds of the beam were 0.48–0.86, 0.45–0.79 mm and 1.9–2.9 mm and 3.2–5.8 mm, respectively. The depth and width of the quenching zones of the base, 40Kh steel, was 0.49–0.86, 0.35–0.89 mm and 1.8–2.85 and 3.1–5.7 mm, respectively. The use of transverse oscillations of the laser beam led to an increase in the surfacing performance by 1.6–2.2 times compared to the treatment with a defocused beam. Fig. 2 shows the microstructures of laser tracks at the boundary with the base material. With a higher laser surfacing speed and lower power, the thickness of the sublayer was 200–300 microns, and with an increase in the radiation power to 1 000 W and a decrease in the scanning speed to 5 mm/s, the thickness of the sublayer decreased and was in the range of 25–50 microns. In fact, the elements of the Fe-Co-Cr-Mo sublayer diluted the melt bath of the main coating Ni-Cr-B-Si+WC. In addition, the number of tungsten carbide particles at an increased energy density of the laser beam sharply decreased, saturation of the nickel matrix with carbon and tungsten occurred during the dissolution of carbides and partial carbon burnout.
Fig. 3 shows graphs of changes in microhardness, obtained as a result of its measurement, from the surface of the deposited track deep into the base material with a step of 100 µm. The curve (Fig. 3, a) was obtained by measuring a sample processed at a speed of 7 mm/s and a radiation power of 1 000 watts. The deposited layer with Ni-Cr-B-Si+WC powder had a high microhardness of 9 000–11 000 MPa. The sublayer zone had a lower microhardness of 6 500–7 000 MPa with a depth of up to 300 µm. As a result of heating above 1 250 °C of the liquid melt bath and the sublayer above 1 000 °C, a complete hardening of the main material of 40Kh steel to a microhardness of 6 500–7 000 MPa at 300 µm and below occurred, an incomplete hardening section with a microhardness of 2 900–6 000 MPa with a depth of 250 µm was observed. The curve in Fig. 2, b is obtained at a radiation power of 1000 W, a travel speed of 5 mm / s with a coating thickness of 0.6 mm. In the surfacing zone of the main charge Ni-Cr-B-Si+WC, a decrease in microhardness to 7 000–8 000 MPa was observed, which is associated with a large dissolution of the carbide phase and partial mixing with the sublayer.
Figure 4 shows the study area of the elemental composition of the main coating Ni-Cr-B-Si+WC and the distribution of elements W, Ni, Fe, Cr, C. The quantitative composition of the elements in the coating is shown in Table.
Analysis of the results of abrasive wear (Fig. 5) with loose grain showed that the wear resistance of Ni-Cr-B-Si+WC coatings increases by 11 times at a radiation energy density of 48 J/mm2 compared with the base material.
Discussion of the results
The obtained research results have shown that the application of coatings with an increased content of the carbide phase is accompanied by the appearance of defects in the form of cracks and pores in the case of incorrectly selected processing modes. To reduce stresses at the boundary of the base material and coatings with carbides, it is proposed to apply a sublayer that is not prone to cracking and has a damping ability due to the high vanadium content in the initial charge. A decrease in the microhardness of coatings with a hardening phase at an increased energy density of laser radiation indicates the dissociation of carbides and partial carbon burnout. The use of optimal laser surfacing modes made it possible to obtain coatings with the highest possible microhardness and wear resistance when worn by loose abrasive grain.
Conclusion
The technology of laser surfacing with an intermediate sublayer Fe-Co-Cr-Mo and the main coating Fe-Co-Cr-Mo on 40Kh steel samples with the application of transverse oscillations of the laser beam to the processing speed vector has been developed. An increase in the radiation energy density significantly above the optimal values led to the dissolution of carbides and a decrease in microhardness. The wear resistance of coatings with a carbide phase obtained at an energy density of 48 J/mm2 is 11 times higher than that of the base material.
AUTHOR
Biryukov V. P., Cand. of Scin.(Eng.), Mechanical Engineering Research Institute of the Russian Academy of Sciences (IMASH RAN), Mosscow, Russia.
ORCID: 0000-0001-9278-6925
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