High Velocity Air Fuel Research Articles

The Effect of Barrel Configurations on the Porosity During HVAF Thermal Spray Ni-Based Amorphous Coatings: Numerical Simulation and Experiment

The relationship between barrel configurations and porosity of a Ni-based amorphous coating (AM) that is fabricated using a high-velocity air fuel (HVAF) process was revealed by both numerical and experimental methods. A computational fluid dynamics model was applied to investigate the gas-flow field and the behavior of in-flight particles with various barrel configurations. It is found that barrel length obviously affects the particle velocity and temperature while it has a slight influence on the particle velocity and temperature. The longer the barrel length (diameter), the higher the flame (particle) velocity and temperature. By analyzing both particle velocity and temperature, the optimal barrel configuration (4E) to achieve low-porosity coatings was predicted. These calculations were experimentally verified by the production of a low-porosity (2.09%) Ni-based AM that was fabricated by HVAF using the predicted optimal barrel configuration.

Effect of aged treatment on improving the performance of HVAF WC–Cr3C2–Ni coating on crystallization roller

The thermal shock resistance and high-temperature performance of WC–Cr3C2–Ni coatings on the surface of the crystallization roll after aged treatment were investigated in this study. The aging treatment at 320 ℃ for 2.5 h facilitated the diffusion of Cu element from the C17200 beryllium copper substrate toward the coating side of WC–Cr3C2–Ni coating, thereby enhancing the bonding between the coatings and substrates. After the aged treatment, the internal structure of the C17200 substrate changed from a dendritic structure to an equiaxed crystal structure, the hardness increased from 225.71 Hv0.5 to 406.52 Hv0.5 (80.4 % increased), and the wear resistance increased.Although the aging treatment reduced the hardness of the coating slightly (from 1257.58 Hv0.5 to 1155.16 Hv0.5, a decrease of 8.1%) and wear resistance (the coefficient of friction, from 0.335 to 0.312, a decrease of 6.9%). However, in the thermal shock test, the thermal shock resistance of the coating after aging treatment was increased from 19 times to 20 times (5.3% increased). However, the thermal shock failure cracks between the coatings and the substrates were transferred from the substrate part to the joint part of the two, which effectively avoided the waste of the crystallization roller substrate in the maintenance process. This research effectively reduces the purchase and maintenance costs of steel mills in actual production and provides theoretical support for cost reduction and efficiency increase in other HVAF (high-velocity air fuel) coating applications.

Research on flame characteristics and coating growth process of titanium alloy in HVAF spraying

AbstractIn this work, a flow field model of WC–12Co powder sprayed by high‐velocity air fuel (HVAF) was established based on the computational fluid dynamics (CFD) method. The macro–micro coupled thermodynamic model of coating multilayer growth process was established based on the birth and death element method. The temperature and velocity of the particles in the combustion flame were applied to the coating growth model, and the transient evolution of the coating temperature and thermal stress was revealed. The results show the temperature and thermal stress of coatings increase with the increasing the number of spraying layers, and their incremental gradient gradually decreases with the increasing the number of deposition layers. The peak temperature of the coating surface reaches 1494 K, and the peak thermal stress of the first coating reaches 2693 MPa. The maximum stress of spraying particles appears at the contact edge of particles. Further preparation of WC coating, through the hardness, friction, and wear, corrosion tests verify that WC–12Co coating can effectively provide the TC18 substrate performance. The microhardness of WC–12Co coating was approximately three times that of TC18 substrate. The average friction coefficient and corrosion rate of the coating are 0.15 and 0.06 mg/cm2/h lower than that of the substrate, respectively.

Erosion Wear Behavior of HVAF-Sprayed WC/Cr3C2-Based Cermet and Martensitic Stainless Steel Coatings on AlSi7Mg0.3 Alloy: A Comparative Study

The paper presents a comparative study of the erosion wear resistance of WC-10Co4Cr, Cr3C2-25NiCr and martensitic stainless steel (SS) coatings deposited onto an AlSi7Mg0.3 (Al) alloy substrate by high-velocity air‒fuel (HVAF) spraying. The influence of the abrasive type (quartz sand or granite gravel), erodent attack angle, thickness, and microhardness of the coatings on their and Al substrate’s wear resistance was comprehensively investigated under dry erosion conditions typical for fan blades. The HVAF-spraying process did not affect the Al substrate’s structure, except for when the near-surface layer was 20‒40 μm thick. This was attributed to the formation of a modified Al-Si eutectic with enhanced microhardness and strength in the near-substrate area. Mechanical characterization revealed significantly higher microhardness values for the cermet WC-10Co4Cr (~12 GPa) and Cr3C2-25NiCr (~9 GPa) coatings, while for the SS coating, the value was ~5.7 GPa. Erosion wear tests established that while Cr3C2-25NiCr and SS coatings were more sensitive to abrasive type, the WC-10Co4Cr coating exhibited significantly higher wear resistance, outperforming the alternatives by 2‒17 times under high abrasive intensity. These findings highlight the potential of HVAF-sprayed WC-10Co4Cr coatings for extending the service life of AlSi7Mg0.3-based fan blades exposed to erosion wear at normal temperatures.

An Assessment of Coating Thickness on the Microstructure and Mechanical Behavior of IN625 Coating on Ni-Based Superalloy Substrate Deposited by High Velocity Air Fuel Technique

An Assessment of Coating Thickness on the Microstructure and Mechanical Behavior of IN625 Coating on Ni-Based Superalloy Substrate Deposited by High Velocity Air Fuel Technique

Improving thermal shock and oxidation resistance of Cr3C2/WC-NiCr cermet coating by embedding large NiCrAlY superalloy particles

Cr3C2/WC-based cermet coating is widely used on the surface of mechanical equipment to strengthen its wear and corrosion resistance. Improving the high-temperature performance of cermet coating is the key to its industrial application. In this study, a novel type of bimodal microstructure cermet coating with the design of functional sites was fabricated via high-velocity air fuel (HVAF) spray technology. The toughness NiCrAlY superalloy particles are introduced into the traditional Cr3C2/WC-NiCr cermet structure to construct the thermal stress release site, which significantly improved the thermal shock resistance and thermal oxidation properties of the coating. The addition of NiCrAlY superalloy particle reduces the porosity of Cr3C2/WC-NiCr coating to 0.29 % and creates high-quality interface bonding and surface passivation layer. The newly structured cermet coating exhibits outstanding high-temperature performance. In a 1000 °C oxidation environment, the new coating remained intact after 120 h of exposure, while the traditional Cr3C2/WC-NiCr cermet coating exfoliated completely after 48 h of exposure. Further, in the thermal shock condition at 1000 °C, the lifetime of the new coating is 50 times greater than that of the traditional cermet coating, displaying excellent ability to relieve thermal stress. Additionally, the new coating shows good wear resistance and stability during high-temperature thermal exposure. After thermal exposure for 24 h and 72 h, the wear rates of the coating are 5.28 and 5.83 × 10−5 mm3 N−1 m−1, respectively, which is slightly lower than that of the as-sprayed coating. This study provides guidance for the improvement of high temperature resistance of thermally sprayed Cr3C2/WC-based cermet coatings.

Preparation process, surface properties and mechanistic study of HVAF-sprayed CoCrFeNiMo0.2 high-entropy alloy coatings

The objective of this study is to develop infrared reflective coatings with excellent service performance in extreme environments, aimed at reducing system energy consumption and enhancing service life. In this work, CoCrFeNiMo0.2 high-entropy alloy (HEA) powder was deposited on the 316 L stainless steel (SS) substrate using High-Velocity Air Fuel (HVAF) thermal spray technology. The microstructure, mechanical properties, infrared reflectivity, and corrosion resistance of the coatings under different spraying parameters were investigated and compared with those of the substrate. Computational Fluid Dynamics (CFD) simulation was employed to analyze the influence of HVAF spraying parameters on flame characteristics and particle flight behavior, providing guidance for optimizing the spraying process and explaining underlying mechanisms. Results demonstrated that adjusting parameters such as propane and air pressure, spraying distance, and powder feed rate could produce dense coatings with <1 % porosity. The microhardness of the coatings was approximately 1.8 times that of the substrate, and the bonding strength between the coating and the substrate is increased to 39.4 MPa. Additionally, the near-infrared reflectivity of the coatings was significantly higher than that of the substrate, with an enhancement of 18.7 %. Coatings sprayed under different parameters exhibited superior passivation and pitting corrosion resistance compared to the substrate in 3.5 wt% NaCl and 1 mol/L HCl solutions. Optimization of spraying parameters further enhanced the protective capability of the passivation film and corrosion resistance. Consequently, CoCrFeNiMo0.2 HEA coatings prepared by HVAF spraying technology demonstrate excellent comprehensive performance, holding promising potential for industrial applications.

Cracking resistance behavior of HVAF-sprayed monolayer and hierarchical Fe-based amorphous coatings under bending stress

The bending strength and fracture toughness are two key indicators that determine the service performance of thermally sprayed Fe-based amorphous coatings (AMCs) in harsh environments. Unfortunately, due to the inherent heterogeneous porosity and limited thickness (typically <1 mm) of the coatings, obtaining these properties is constrained by traditional fracture analysis methods. To address this, a three-point bending combined with the digital image correlation (DIC) method, without prefabricating notch or removing substrate, was proposed in the current work, and the effect of hierarchical structures on the fracture performance of the coatings was successfully revealed. Excitingly, the results show that compared to monolayer coating, hierarchical structures comprising two layers with different porosities can realize the synergistic improvement in the fracture strength and fracture strain of the coatings by alleviating local stress concentration and inducing crack deflection. Further, a finite element method (FEM) was employed to gain insight into the intrinsic relationship between the coating microstructures and the crack evolution behavior. This work not only proposes a simple method for evaluating the fracture performance of Fe-based AMCs sprayed with high velocity air fuel (HVAF), but also presents an innovative idea for optimizing the coating's properties.

Microstructure, multi-scale mechanical and tribological performance of HVAF sprayed AlCoCrFeNi high-entropy alloy coating

Thermal spray high-entropy alloy (HEA) coatings have demonstrated potential for improving the wear resistance of conventional materials used in extreme engineering environments. In the present work, an equiatomic AlCoCrFeNi HEA coating was manufactured using the high velocity air fuel (HVAF) process. The phase and microstructural transformations in gas-atomized (GA) powder during HVAF spraying were analyzed using SEM, EDS and EBSD techniques. The tribological properties of this HEA coating sliding against an Al2O3 ball at both room temperature (RT) and 600 °C were also evaluated. The GA powder was composed of Body Centred Cubic (BCC) + ordered BCC (B2) phases, which transformed to BCC + B2 + minor Face Centred Cubic (FCC) phases during the HVAF coating process, validating the thermodynamic phase prediction projected by the Scheil simulation for non-equilibrium processing conditions. The rapid solidification and high velocity impact-assisted deformation of GA powder resulted in significant grain refinement in the HVAF coating, which ultimately improved the mechanical properties at both micro and nanoscale levels. The wear resistance of the HEA coating at RT was severely impacted by the relatively brittle BCC/B2 phase structure, leading to susceptibility to abrasive wear and surface fatigue. The wear resistance at 600 °C was slightly lower at RT due to the formation of a brittle oxide layer on the worn surface, which induced surface fatigue and aggravated mass loss of the coating.

Thermal shock resistance of WC–Cr3C2–Ni coatings deposited via high-velocity air fuel spraying

In this study, the high-velocity air fuel (HVAF) spraying technique was used to deposit WC–Cr3C2–Ni wear-resistant coatings on the surface of crystallization rollers to overcome the wear problem of the substrate of thin-strip continuous-casting crystallization rollers and thus improve their efficiency. The performance and durability of the WC–Cr3C2–Ni coatings on the surface of the roller body during the actual continuous casting process were investigated by performing thermal shock tests. It was found that the thermal shock resistance of the closed-curved surface coatings increased with their thickness. The coatings exhibited the best thermal shock resistance at a thickness of 300 μm, and the number of cycles they could withstand at 800 °C reached 15. The results of the high-temperature wear tests showed that the WC–Cr3C2–Ni coatings deposited on the surface of the crystallization roller via HVAF reduced the friction coefficient of the surface of the roller body (from 0.425 to 0.362), and greatly reduced the wear loss volume during the wear process (reduced by 83 %). The WC-Cr3C2-Ni coating showed excellent resistance to crack initiation and propagation during wear and can be stabilized in long-cycle production and protect the substrate of the crystallization roller effectively. This study provides a theoretical basis for the selection and preparation of surface coatings for thin-strip continuous-casting crystallization rollers.

Improving wear and corrosion resistance of HVAF sprayed 316L stainless steel coating by adding TiB2 ceramic particles and CeO2

316L stainless steel is used in a wide variety of corrosion-prone locations. However, its low hardness and poor tribological properties limit its application under heavy wear conditions. In this study, 316L Fe-based composite coatings co-doped with TB2 hard phases and rare-earth CeO2 exhibited excellent wear and corrosion resistance. Firstly, 316L-TiB2 cermet powder and (316L-TiB2)/CeO2 composite powder with high sphericity and density were prepared by high energy ball milling (HEBM), spraying drying granulation (SD), plasma spheroidization (PS) process. Then, 316L coatings, 316L-TiB2 and (316L-TiB2)/CeO2 cermet coatings were deposited by high velocity air fuel (HVAF) spraying. Finally, the effects of TiB2 and CeO2 additions on the microstructure, wear and corrosion behavior of the coatings were investigated. The results showed that TiB2 hard particles and CeO2 were uniformly distributed in the HVAF sprayed 316L-TiB2 and (316L-TiB2)/CeO2 coatings. The wear resistance of the coating prepared by 316L Fe-based composite powder with TiB2 and CeO2 is significantly improved. The hardness of the (316L-TiB2)/CeO2 composite coating was increased from 269 Hv0.3 to 826 Hv0.3 for the pure 316L coating, while the volumetric wear rate was only 3.6 % of that of the pure 316L coating (110 × 10−5 down to 4 × 10−5 mm3/N). Electrochemical tests show that the corrosion resistance of the coating decreases with the addition of TiB2 alone, but it can be improved by doping CeO2. The 316L Fe-based coating prepared by adding TiB2 and CeO2 can significantly improve the wear resistance of the coating, while the corrosion resistance decreases only slightly.

Microstructure, property and high-temperature wear performance of Cr3C2-based coatings deposited by conventional and ID-HVAF spray torches: A comparative study

The durability of industrial components exposed to high temperatures can be increased by depositing high-velocity air-fuel (HVAF) sprayed Cr3C2-based coatings. The uniform embedding of wear-resistant Cr3C2 within an oxidation-resistant binder matrix allows HVAF-sprayed Cr3C2-based coatings to be widely used in high-temperature environments. However, consolidating these coatings demands an optimum particle velocity and temperature combination, primarily dependent on the spray torch design. Understanding the role of spray torch variants with altered combustion chamber and nozzle designs is essential, which might influence the coating (splat) formation mechanism and the resultant wear performance. Accordingly, Cr3C2–25NiCr and Cr3C2–30NiCrMoNb were sprayed using different HVAF torches (high-power AK06 and low-power AK05-ID (inner diameter): generate different energy (thermal + kinetic) output) to understand the role of the binder as well as the torch design. The deposited coatings were comprehensively compared based on splat morphology, microstructure, mechanical properties, room and high-temperature erosion and sliding wear behavior. Subsequently, a detailed post-wear analysis was performed to decipher the associated wear mechanism. As inferred from the results, Cr3C2–25NiCr coatings deposited through a high-power AK06 spray torch displayed preferable microstructure, properties and superior wear resistance. The study also offered new insights into HVAF-ID torch-deposited coatings.

Prediction of HVAF thermal spraying parameters and coating properties based on improved WOA-ANN method

In this study, high-velocity air fuel (HVAF) thermal spray technology was utilized to deposit Hastelloy C276 powder onto 316 L stainless steel substrates. The effects of various spray process parameters such as propane pressure, air pressure, spray distance, and powder feeding rate on the microstructure, microhardness, and corrosion resistance of the coatings were investigated. In addition, the coupling effects among multiple spray parameters were considered by employing a combined approach with the Improved Whale Optimization Algorithm (IWOA) and Artificial Neural Network (ANN) model, and the mapping relationships between the spray process parameters and coating porosity, microhardness, and corrosion current density was established. The results revealed that the prepared C276 coating exhibits lower porosity, higher microhardness, and exceptional corrosion resistance. And the IWOA-ANN model outperforms the ANN model and WOA-ANN model in the prediction accuracy and stability for all three performance measures, particularly for porosity and microhardness. Furthermore, the comparison between experimental results and predicted results validates the reliability and accuracy of the trained IWOA-ANN model, demonstrating its efficacy in predicting HVAF spray parameters and coating properties.

Investigation on tribological properties of high-velocity air fuel spraying WC–Cr3C2–Ni coating on the surface of continuous casting crystallization rollers

To overcome the wear problem of the substrate of thin-strip continuous casting crystallization rollers and improve economic efficiency, high-velocity air fuel (HVAF) technology was used to deposit WC–Cr3C2–Ni wear-resistant coatings on the surface of the crystallization rollers. The existing performance and wear condition of curved WC–Cr3C2–Ni coatings during continuous casting were investigated by the ring-block wear system. The experimental results showed that the WC–Cr3C2–Ni coating with a thickness of 300 μm effectively reduced the coefficient of friction (by 16.4 %) and the wear rate (by 82.5 %) on the surface of the crystallization roller and significantly reduced the roughness of the wear interface (by 71.2 %), which provided reliable protection for the substrate. The results of high-temperature (200 °C) wear tests showed that the WC–Cr3C2–Ni coating deposited via HVAF protected the substrate in actual long-cycle production and exhibited an excellent resistance to the emergence and expansion of failure cracks. This study provided a theoretical basis for the selection and preparation of surface coatings for thin strip continuous casting crystallization rollers.

Performance of Atmospheric Plasma-Sprayed Thermal Barrier Coatings on Additively Manufactured Super Alloy Substrates

This work represents a preliminary study of atmospheric plasma-sprayed (APS) Yttria-Stabilized Zirconia (YSZ)-based thermal barrier coatings (TBCs) deposited on forged and additive manufactured (AM) HAYNES®282® (H282) superalloy substrates. The effect of different feedstock morphologies and spray gun designs with radial and axial injection on APS-deposited YSZ layer characteristics such as microstructure, porosity content, roughness, etc., has been investigated. The performance of TBCs in terms of thermal cycling fatigue (TCF) lifetime and erosion behaviour were also comprehensively investigated. In view of the high surface roughness of as-built AM surfaces compared to forged substrates, two different types of NiCoCrAlY bond coats were examined: one involved high-velocity air fuel (HVAF) spraying of a finer powder, and the other involved APS deposition of a coarser feedstock. Despite the process and feedstock differences, the above two routes yielded comparable bond coat surface roughness on both types of substrates. Variation in porosity level in the APS topcoat was observed when deposited using different YSZ feedstock powders employing axial or radial injection. However, the resultant TBCs on AM-derived substrates were observed to possess similar microstructures and functional properties as TBCs deposited on reference (forged) substrates for any given YSZ deposition process and feedstock.

High-velocity oxy-fuel and high-velocity air fuel sprayed WC-Co-Cr coatings on solution-treated 21-4N steel for slurry and corrosion resistance

This study aims to fabricate WC-Co-Cr coatings on the solution-treated 21-4N steel utilizing high-velocity oxy-fuel (HVOF) and high-velocity air fuel (HVAF) techniques. The microstructure, hardness, surface quality, porosity, slurry erosion, and corrosion resistance of HVOF and HVAF coatings on solution-treated 21-4N steel were investigated and compared. The HVAF sprayed WC-Co-Cr coating exhibited dense structure, more hardness (1582 HV), greater fracture toughness (5.69 MPa m1/2), less decarburization, and lower porosity (0.97 %) as compared to HVOF sprayed coating. Further, slurry jet erosion tests and electrochemical corrosion tests provide a comprehensive evaluation of the coating’s performance under erosive and corrosive conditions. Eventually, the conclusive results of the study affirm the exceptional performance of the high-velocity air fuel sprayed WC-Co-Cr coating, demonstrating its superiority in both erosion and corrosion resistance compared to the high-velocity oxygen fuel sprayed coating.

INFLUENCE OF SPRAY DISTANCE ON THE POROSITY OF Ni-BASED AMORPHOUS COATINGS: NUMERICAL SIMULATION AND EXPERIMENT

The influence of injection distance on the porosity of Ni-based amorphous coatings (AMCs) prepared with a high-velocity air fuel (HVAF) process is discussed based on a numerical analysis and experimental methods. A computational fluid dynamics model was established to demonstrate the gas flow field and behavior of particles in flight at different spraying distances during HVAF spraying. When analyzing the changes in the particle velocity and temperature, the spraying distance is less than 30 µm. The velocity and temperature changes of small particles have a significant impact, and the optimal spray distance (350 mm) for obtaining a low porosity coating is predicted. The calculation was validated experimentally by producing a Ni-based AMC with a low porosity (1.87 %) that was manufactured using the predicted HVAF optimal spraying distance.

High Velocity Air Fuel Spraying for Metal Additive Manufacturing - A Study on Copper

High Velocity Air Fuel Spraying for Metal Additive Manufacturing - A Study on Copper

Understanding the effect of bondcoat surface treatment on enhanced lifetime of suspension plasma sprayed thermal barrier coatings

Thermal barrier coatings (TBCs) are extensively used in gas turbine engines in power generation and aerospace applications. Improvements in TBC performance and lifetime are continuously pursued by the gas turbine industry. Suspension plasma spraying (SPS) has shown promising results in recent years as it could produce a porous columnar TBC microstructure exhibiting low thermal conductivity and high durability. Further improvements in lifetime and fundamental understanding of failure mechanisms would lead to their wider application.Lifetime of a TBC system is dependent not only on the topcoat microstructure and chemistry, but also on topcoat-bondcoat interface and bondcoat characteristics as they influence the growth rate of thermally grown oxide (TGO) layer as well as the mismatch stresses generated in the topcoat during operation. Moreover, in SPS TBCs, the column density and porosity in the topcoat are also affected by the bondcoat surface roughness which in turn affects TBC lifetime. In previous work, it was shown that bondcoat surface treatment by shot peening followed by light grit blasting can significantly enhance the cyclic lifetime of SPS TBCs.The objective of this work was to gain further insight into the lifetime improvements due to surface treatment of bondcoat. Commercial NiCoCrAlY powder was deposited by high velocity air fuel (HVAF) and high velocity oxy fuel (HVOF) spraying while SPS was used to deposit yttria stabilised zirconia topcoat. Before spraying the topcoat, the bondcoat samples were subjected to vacuum heat treatment, shot peening, and/or grit blasting. Residual stress measurements were performed by X-ray diffraction on samples with different variants of bondcoat treatments and compared with Almen strip tests. The lifetime was examined by thermal cyclic fatigue testing. The relationships between changes in bondcoat surface roughness, residual stress state and microstructure due to surface treatments, and resultant topcoat microstructure and lifetime were studied and discussed.

Influence of nano-sized WC addition on the microstructure, residual stress, and tribological properties of WC-Co HVAF-sprayed coatings

It has recently been proposed to modify conventional WC-Co coatings with nano-sized particles to improve their properties. Modification of the coatings with nano-sized carbides may provide higher wear resistance or phase composition stability. This work characterized the changes in the residual stresses induced by introducing WC nanoparticles into a WC-Co coating and the resulting change in their sliding wear behavior.A powder mixture consisting of agglomerated and sintered WC-17Co feedstock powder and 5 % nano-sized WC was deposited on a carbon steel substrate using the High Velocity Air Fuel (HVAF) process. Microstructural characterization using scanning electron microscopy and EBSD confirmed that nano-sized WC particles were embedded in the coatings after the spraying process, primarily along the boundaries of the WC-Co lamellae. This resulted in an enhancement of microhardness, indentation fracture toughness, and dry sliding wear resistance, evaluated via ball-on-disk tests against Al2O3 counterparts. Residual stresses in the coatings were also measured by XRD using the sin2ψ method. The normal stress along the torch movement direction decreased significantly when nano-sized WC was added. The addition of nanoparticles also caused a slight decrease in the normal stress perpendicular to the torch movement.