A computational fluid dynamics model for understanding the HVAF process and the influence of the process parameters on the particle flight properties is investigated. Achieving this objective involves a novel approach to modeling the HVAF process with pressure inlet boundary conditions and integration of the mixing chamber. The study comprises the prediction of the flow fields described by a set of equations consisting of continuity, momentum, energy, and species transport. These equations are then solved with realizable k-ε turbulence model, a two-step chemistry model and eddy dissipation model to simulate the combustion reaction. Consequently, the interaction between the CoNiCrAlY alloy particles and the flow is modeled using a Lagrangian approach considering the forces acting on the particles and the heat transfer. The results show that the combustion chamber pressure is mainly affected by the compressed air and propane parameters. Furthermore, the flight behavior of the smaller particles is significantly influenced by the gas flow, while the larger particles tend to maintain their momentum and energy. Through the simulation model, an in-depth process understanding of the HVAF process can be achieved. More importantly, the model can be used as a tool for efficient process development.

Activated Combustion HVAF (AC-HVAF) spraying provides efficient deposition of metallic and carbide coatings using solid particle spray technology. Oxidation and thermal deterioration of sprayed materials is significantly reduced, resulting in improved quality of coatings. Resistance of different WC-Co and WC-Co-Cr AC-HVAF coatings to abrasive wear was investigated using ASTM G-65 test. It was found that the AC-HVAF hardware setup, type of fuel gas and spray parameters affected deposition efficiency but not wear resistance of coatings. Herewith, the method of powder manufacturing revealed significant influence on coating wear resistance. The AC-HVAF sprayed coatings were compared to HVOF-sprayed counterparts, as well as to hard surfacing and chrome plating. The AC-HVAF sprayed coatings were efficient in competing with modern surfacing technologies in many industrial applications.

Activated Combustion HVAF Spraying (AC-HVAF) involves a jet of air-fuel combustion products to deposit coatings of metallic and carbide powders. In the process, spray particles are heated below their melting temperature while accelerated to velocity typically 700-850 m/s, forming a coating upon impact with a substrate. Extremely low oxygen content and high density are distinguished features of the AC-HVAF coatings, resulting in their excellent performance under conditions of severe wear and corrosion. Besides new level of coating quality, the AC-HVAF process demonstrates outstanding technological efficiency and spray rates 5-10 times exceeding those of the HVOF counterparts. The paper presents results on characterization of selected metallic and carbide coatings and describes their applications.

In Activated Combustion HVAF process, coatings are formed of powder particles, heated and accelerated by high-velocity jet of air and gaseous fuel combustion products. A distinguished feature of the process is that spray particles are heated below their melting point while accelerated to velocity well above 700 m/s. Such spray scheme appeared beneficial for deposition of cemented carbides, in particular, WC-based composites. Dense, practically non-oxidized neither heat-deteriorated coatings were formed. Spray rates from 1 to 25 kg/hr were achieved without decline of coating quality or deposition efficiency. Specific coating structure resulted in noticeably improved resistance to fatigue at high level of stresses.

According to aerodynamics and thermodynamics, High Velocity Oxygen/Air Fuel Spray system was successfully developed. The system introduced stream atomization, high-pressure combustion chamber, converging/diverging nozzle, spark plug ignition, radial powder injection with reliable operation. The system has the function of both of HVOF and HVAF. It can not only use air and oxygen to spray respectively, but also the mixture of air and oxygen, so the velocity and temperature of the flame can be changed by adjusting the flux of air and oxygen. The system can produce high quality cermets, metal and alloy coatings for the adjustable of flame velocity and temperature.

Recent developments of High-Velocity Air-Fuel (HVAF) spraying and blasting focused on a substantial increase of spray particles velocity. The efforts further improved coating quality, allowing deposition of metallic and carbide-base coatings non-permeable to gas at thickness as low as 40-50 micron. The coatings demonstrate low dissolved oxygen content, a favorable combination of high hardness and toughness. Coupled with the enhanced technological efficiency of modern HVAF equipment, this initiated not only the acceptance of HVAF technologies in established thermal spray markets in the oil and gas industry, but also the development and successful implementation of new coating applications. The examples are wear and corrosion resistant tungsten carbide-based coatings on hydraulics rods of dock cranes, corrosion resistant Ni-Cr-Mo-type coatings on vessels of sulfur removal equipment, tungsten carbide coatings on restriction grid plates and slide gates of catalyst towers, high-temperature erosion resistant chromium carbide- based coatings on thermowells and valve stems, wear and cavitation resistant Co-Cr-W-C-type and carbide coatings on housing wear rings and impeller hubs of high-temperature pumps.

The paper analyzed microstructure and property of WC-17Co coatings sprayed by High Velocity Oxygen/Air Fuel Spray under three kinds of spray conditions, which are HVOF, HVO-AF and HVAF. Coatings bond well with the substrate. The average bonding strength exceeds 70Mpa. Coatings are dense and hard, and the average porosity is about 1%. Microhardness of coatings is between HV1000 0.2 and HV1200 0.2 . Coatings are mainly composed of WC with little W 2 C and Co 3 W 3 C. With the increasing of Nitrogen, decarburization of WC was reduced.

Owing to gas velocities in the super-sonic regimen in combination with moderate flame temperatures, the HVOF processes are preferred for the deposition of wear and/or corrosion resistant carbides as well as Hastelloy, Triaballoy and Inconel alloys. The resulting coatings have usually very high bond strengths, fine as-sprayed surface finishes and low oxide levels. However, the generation of a supersonic flow of combustion products supposes the implementation of relatively high gas flow rates and high energetic gas mixtures, which are intrinsically associated with high production costs, limiting the application of this technology in some industrial fields. This work summarises the first results in the development of a prototype aimed to show the potential of a new thermal spray technology named Oxy-Fuel Ionisation spraying for the development of high quality carbide base coatings. The OFI process is a supersonic combustion process as well, enhanced by the addition of ionised gas specimens. The arising combustion process is characterised by its stability within a broader range of the “fuel/oxidant” correlation in comparison to conventional HVOF systems, because of the presence of ionised gas specimens which are acting as a catalyst. It has been proved that this developed prototype allows the thermal spray deposition of carbide based materials with relatively low oxygen flow rates. For comparison two different coating materials were investigated, WC-17Co and Cr 3 C 2 -NiCr. The process parameters were optimised in terms of the micro hardness, the porosity and the decarburization of the resulting coatings.

HVAF Arc process deposited coatings of dual wire stock, fused by an electric arc and atomized by a high-velocity jet of air and gaseous fuel combustion products. The HVAF jet was generated in toroidal chamber, where a hot catalytic insert activated the combustion process. Such atomization produced fine, 5-20 micron, spray particles accelerated to over 200 m/s. Excess of fuel in the arc zone prevented rapid oxidation of fused material. The process is specifically beneficial for deposition of high-quality coatings of aluminum, corrosion resistant nickel-base alloys and hardface Fe-Cr-B-C cored wires.

This paper presents the development of a new thermal spray gun for the so-called warm spraying process in which powder particles are not melted but heated to temperatures much higher than those typically found in a cold spray process. The increased heating leads to a reduction in the particle impact velocity required to deposit the coating and hence reduces operating cost. The new gun utilizes methane-oxygen combustion for particle heating and features a swirl-type combustion chamber to create a turbulent mixture of the fuel and oxidizer for efficient combustion. Powder can be fed axially or radially into the gun. To control particle temperature independently, combustion gases are diluted by adding nitrogen gas through axial or radial ports provided in the gun. A converging-diverging nozzle with a downstream cylindrical barrel accelerates the burnt gases to supersonic velocities. The design of the nozzle and barrel was optimized using numerical simulations. Mass flow rates of methane, oxygen, and nitrogen were calculated using energy balance, stoichiometric combustion, and nozzle flow rate equations. The gun is designed to operate up to 200 kW and is water-cooled. Experiments were conducted to test the performance of the new gun in which tungsten carbide coatings were deposited on aluminum substrates. Coatings were analyzed using standard methods and showed promising results.

Traditional metal spraying techniques, which have been used in industry for decades, such as Wire Flame and Twin-Wire Arc are classified as low velocity processes because the sprayed material is conveyed by compressed air having subsonic velocity. In order to improve the bond strength, HVAF was applied for thermal spraying for anticorrosion protection. In this paper, zinc-aluminium (Zn-Al) coatings thermal sprayed using the HVAF method are analysed. The thermal sprayed coatings were characterized by the standard techniques, such as light microscopy, scanning electron microscopy with energy-dispersive spectroscopy, X-ray diffraction, salt spray and bond strength tests. The results show that thermal sprayed coatings have a dense structure, a high bonding strength, low presence of oxides and high resistance to corrosion. This is attributed to high flow/particle velocities and relatively low combustion temperatures of HVAF in comparison with other thermal spraying technologies. High spray rate and good coating quality make the HVAF thermal spray method a viable alternative to the conventional Wire Flame and Twin-Wire Arc methods for thermal spraying of Zn-Al coatings.

The general method of process maps to understand and control thermal spray processes has been applied to monitor the deposition of WC-Co by high velocity oxygen fuel (HVOF). A selected number of particle state conditions (velocity and temperature) has been performed to produce a variety of coatings. Microstructure, mechanical properties, and wear resistance were evaluated and compared. A second order process map for sliding wear, impacting particle erosion and abrasive wear control can be constructed from the process map to provide the limits within which the particle state can be changed to achieve a predefined coating specification. The mechanisms behind the wear resistance are discussed within the framework of wear maps –third order process map-in the context of analysis of inter splat de-bonding, mechanical properties of the coating, and delamination failure.

A significant group of steel-making process parts is exposed to high contact pressure, shock abrasive wear and elevated temperature. High productivity repair techniques are necessary because of the large size of the parts. Analysis of coating metallographic investigations, wear and corrosion test results, full-scale tests shows that restoration of base share of these parts is possible by High Velocity Oxygen Fuel / High Velocity Air Fuel (HVOF/HVAF) process. Comparison of manufacture's data has showed that HVAF excels HVOF alternatives noticeably at productivity. At the same time production costs are 2-2.5 times less. With regard to typical steel-making process parts some investigations results and examples of HVAF restoration at Joint Stock Company "Mashprom" are represented.

The paper is devoted to gaseous detonation spraying and presents the state of current knowledge, as well as the following research and development needs: gaseous detonation as spraying heat source; operation cycle of detonation guns and its possible versions; operating frequency; impulse jet formation; basic detonation guns design concepts, as fuel and oxidant types, features of barrel design, predetonation distance control, valved and valveless detonation guns concepts, etc.; gas dynamic characteristics of detonation spraying.

The Oxy-Fuel Ionization system (OFI) is a new thermal spray process which consists basically on a high velocity combustion process enhanced by a low energy plasma source. The system is characterized by its stability over a relatively large range of fuel/oxidant conditions, the possibility to use poor fuels like natural one (with low gas consumption) and the high deposition rates that can be achieved in comparison to conventional HVOF guns. The OFI gun has been designed following a modular concept, which in combination with the high flexibility of the system is expected to allow the deposition of coating materials with the most different physical and chemical natures. This work deals with the experimental analysis of the process using methane as fuel gas and its correlation with the deposition of WC-base materials. Two in-flight particle diagnostic systems were used: the Spray Watch diagnostic system (from OSEIR) and the Spray and Deposit Control (SDC) system (developed by the SPCTS laboratory of the University of Limoges). Results are presented for the most representative properties of the optimized coatings (micro hardness distributions on the coating cross section and crystallographic analysis).

The fluid and particle dynamics of a High-Velocity Oxygen-Fuel Thermal Spray torch are analyzed using computational and experimental techniques. Three-dimensional Computational Fluid Dynamics (CFD) results are presented for a curved aircap used for coating interior surfaces such as engine cylinder bores. The device analyzed is similar to the Metco Diamond Jet Rotating Wire (DJRW) torch. The feed gases are injected through an axisymmetric nozzle into the curved aircap. Premixed propylene and oxygen are introduced from an annulus in the nozzle, while cooling air is injected between the nozzle and the interior wall of the aircap. The combustion process is modeled using a single-step finite- rate chemistry model with a total of 9 gas species which includes dissociation of combustion products. A continually-fed steel wire passes through the center of the nozzle and melting occurs at a conical tip near the exit of the aircap. Wire melting is simulated computationally by injecting liquid steel particles into the flow field near the tip of the wire. Experimental particle velocity measurements during wire feed were also taken using a Laser Two-Focus (L2F) velocimeter system. Flow fields inside and outside the aircap are presented and particle velocity predictions are compared with experimental measurements outside of the aircap.

High-velocity air-fuel (HVAF) is a combustion process that allows solid-state deposition of metallic particles with minimum oxidation and decomposition. Although HVAF and cold spray are similar in terms of solid-state particle deposition, slightly higher temperature of HVAF may allow further particle softening and in turn more particle deformation upon impact. The present study aims to produce dense Ti-6Al-4V coatings by utilizing an inner-diameter (ID) HVAF gun. The ID gun is considered a scaled-down version of the standard HVAF with a narrower jet, beneficial for near-net-shape manufacturing. To explore the potential of the ID gun in the solid-state deposition process, an investigation was made into the effect of spraying parameters (i.e., spraying distance, fuel pressure, and nozzle length) on the characteristics of in-flight particles and the attributes of the as-fabricated coating such as porosity, oxygen content, and hardness. Using online diagnostics to monitor temperature and velocity of in-flight Ti-6Al-4V particles is challenging due to exothermic oxidation reaction of fine particles, while larger particles are too cold to be detected from their thermal emission. However, DPV diagnostic system was successfully employed to differentiate the non-emitting solid particles from the burning ones. It was found that increasing air and fuel pressure of the ID-HVAF jet led to an increase of the velocity of the in-flight particles, and resulted in improved density and hardness of the as-sprayed samples. However, increasing the spraying distance had a negative effect on the density and hardness of the deposits. It was also observed that the phases of the Ti-6Al-4V deposits were altered by producing vanadium oxide due to the high temperature of the spray jet.

This paper presents a new strategy for improving the quality of HVOF sprayed coatings as well as the deposition efficiency of the process. The highly turbulent expanding gas jet is stabilized and focused with the aid of a helical gas shroud. The effect of the design modification is demonstrated by numerical calculations and through the use of a prototype torch.

A study of thermal fluxes transferred during the HEATCOOL process is proposed. The concept of this process, specially designed to enhance the residual stresses relaxation, consists in the use of a consecutive three-step procedure during the coating elaboration (heating / spraying / cooling). The present study focuses on thermal exchanges occurring during the heating step. For this, the elaborated experimental equipment incorporates a series of ten holes aligned equidistantly with 5 mm separation. A burning gas mixture (premixed acetylene and oxygen) is injected through these holes and the burning gas jets impinge and heat the substrate. The stand-off distance between the heating device and the substrate may be adjusted between 30 and 90 millimeters. Concerning thermal fluxes transferred using this experimental device, a front work piece incorporating several thermocouples was used to perform heat flux measurements. In a first step, the case of a single hole was considered. Since this method is not able to provide the thermal flux directly, the corresponding thermal fluxes were deduced using an inverse heat conduction problem method that was specially developed. Results obtained using this inverse problem method based on experimental measurements are then compared with numerical predictions obtained using a computational fluid dynamic model representing the system. For this part, the PHOENICS software was used to perform the corresponding computations.

Within the High Velocity Oxygen Fuel Process (HVOF-Process) various fuels can be used to provide the needed thermal and kinetic energy such as ethene, propane, methane or kerosene. Modelling the combustion in a HVOF-System poses a challenge concerning chemical kinetics of the kerosene reaction process. In this work a reduced reaction mechanism and a model describing chemical reactions as well as governing fluid dynamics are presented to simulate kerosene driven HVOF-Process. The kerosene combustion process within a HVOF-System usually takes place above temperatures of 2000 K, where some species dissociate. Therefore, accruing species have to be included in the reaction mechanism. The combustion process is described with a reduced reaction mechanism. The reaction rate is described by a finite rate model in form of Arrhenius. The gas flow is considered as a first phase and the kerosene droplets injected into the combustion chamber become a second phase. Afterwards simulation results are presented and discussed.