Nozzle for a thermal spray gun and method of thermal spraying

[0001] The present invention relates to a nozzle for a thermal spray gun and a method of thermal spraying, and also relates particularly to a nozzle for a thermal spray gun for spraying by the HVOF method (high speed flame spraying) and a method of thermal spraying by the HVOF method.

[0002] Thermal spray technologies where a coating of heated or molten material is sprayed onto a surface are well known. One such technique is high speed flame spraying in which a powdered material, for example a tungsten carbide and cobalt (WC-Co) composite, is fed into the exhaust gas flow generated by the spray gun and the heated particles are accelerated towards the substrate, which is to be coated. The powder is heated by burning fuel and oxygen mixture and accelerated by a converging and diverging nozzle (Laval nozzle).

[0003] Examples of HVOF thermal spray guns are disclosed in GD, Power, EB Smith, TJ Barber, LM. Chiapetta UTRC Report No. 91-8, UTRC, East Hartford, CT, 1991, in Kamnis S and Gu S Chem. Eng. Sci. 61 5427-5439, 2006 and in document S. Kamnis and

S. Gu Chem. Eng. Processing. 45 246-253, 2006. The nozzles of two such spray guns are shown in Fig. 1. The nozzle 10 of the HVOF spray gun has a combustion chamber 12 into which a mixture of oxygen and fuel is injected through the inlet 14 together with the powder, which is to coat the substrate (not shown). The combustion of the fuel takes place in the combustion chamber, while the exhaust gases expand and pass through the tapering and diverging constriction 16 and through the sleeve 18 before escaping through the outlet 20.

[0004] Similarly, the nozzle 22 has a combustion chamber 24 with different fuel and oxygen inlets 26 and a convergent divergent nozzle 28 with an expanded divergent sleeve forming portion that includes an outlet 30. The coating powder is introduced into the sleeve, where divergence begins.

[0005] To avoid oxidation of the powdered material, heating must be carried out smoothly over a certain temperature range, without exceeding the critical value. The temperature at which oxidation begins for most sprayed materials is well below the maximum flame temperature of about 3300 K. For example, oxidation of a tungsten carbide and cobalt composite begins at a surface temperature of about 1500 K. As a result, powder injection into the combustion chamber is not suitable for this material and for non-ceramic materials in general, and therefore powdered matter! must be injected into the stream of supersonic gases. However, this gives the particles momentum in a radial direction, which means that they will probably leave the gas stream with some probability before hitting the coating object. What's more, larger and heavier particles follow different trajectories compared to smaller and lighter particles. In practice, the spread of particles reduces spraying accuracy and reduces deposition efficiency because the impact of the particle is not normal relative to the surface being coated.

[0006] Furthermore, injection of powder into the nozzle results in damage to the nozzle, in particular erosion of the tee wall, and as a result the nozzle, or at least the sleeve part, typically needs to be replaced every ten hours of operation, powdered particles accelerate to supersonic speeds, followed by a series of expansion and compression inside the sleeve. The gas stream inside expands and cools, and is compressed and heated as it passes through the Mach mesh. Wave Mach mesh results in temperature loss, while expansion at the sleeve outlet increases temperature loss. The general decrease in static temperature (from about 3000 K to about 2000 K) and the general increase in speed (from about 200 m / s to about 1800 m / s) after compression and expansion in the convergent-divergent area of ​​the nozzle, causes such action inside the sleeve. When the powder is injected into the high speed gas stream, its residence time is shortened due to the increase in the acceleration factor. Therefore, in order to ensure sufficient heating of the particle, a long sleeve is necessary to maintain high gas temperatures. This long sleeve, typically 350 mm, limits applications to those where such a thermal atomizer can be used, for example, it is not possible to spray internal surfaces, even quite long components.

[0008] Small particles below 10 pm cannot practically be used because such finely powdered material disperses in the gas field and therefore reflects off the article being sprayed or never reaches it. As a result, small particles will never reach the flow centerline and therefore cannot use the high speed / temperature flow areas. Instead, they follow the path at the free-flow boundary and when they mix with the ambient air, outside the starting point of the sleeve, they disperse in all directions. Light particles, therefore, follow the flow direction and are thus blown off the ground.

[0009] An example of the prior art has been disclosed in RU 2160640. The device disclosed in this document applies a decorative or anticorrosive protective coating to various substrates and includes a combustion chamber and an axial pipe for supplying the sprayed material. A conical grille is installed at the outlet of the combustion chamber.

[0010] Preferred embodiments of the present invention aim to eliminate the above-described disadvantages of the prior art.

[0011] According to one aspect of the present invention, there is provided a nozzle for a thermal spray gun for working with the HVOF method, said nozzle comprising:

at least one combustion chamber having at least one fuel inlet for receiving at least one fuel, at least one combustion zone in which said at least one fuel is burned to produce a bonding gas stream, and at least one outlet , for discharging said combustion gas stream; and also separating elements positioned at least partly in said combustion chamber to create discrepancies in said combustion gas stream and thereby create a number of streams or annular stream before combining them into a single stream, wherein said separating elements further comprise at least one inlet of a coating material for introducing at least one coating material into said stream of said combustion gases, at the farthest point relative to the direction of flow, the point of said separating elements in

- said stream, characterized in that said separation elements extend at least partly outwardly of said combustion chamber through said outlet.

[0012] By creating divergences in the combustion gas stream, which then merge into a single stream, a number of advantageous features are provided. First, the nozzle of the present invention produces a more stable supersonic jet, which achieves higher axial velocities (about 2 mach) and is maintained longer than in the prior art apparatus, under the same conditions of the oxy-fuel mixture and mass flow rate. The device according to the present invention also reduces the shock flow waves (faie of the shock flow in the Mach grid, observed in the prior art stream) and thus reduces the energy / temperature losses of the powdered particles. This results in a single flow expansion, just after the separation elements are completed, which reduces energy losses. As a result of this increased jet stability, the nozzle sleeve portion is not necessary and can be eliminated. The overall length of the nozzle is therefore reduced, which allows spraying previously unreachable surfaces, e.g., internal surfaces of the components.

[0013] Furthermore, since the divergence is formed in the combustion gas stream, creating two or more linear gas streams with separating elements between them, or an annular stream with separating elements in the middle, the coating material can be introduced inside crevices or discrepancies formed in the stream by the separating elements. As a result, the coating material is never in contact with the fuel and the oxygen mixture, but is only in contact with the combustion gases when the combustion is complete. As a result, the risk of oxidation of the coating material is reduced. This risk of oxidation is further reduced by flame stability, which increases the likelihood of oxidation by mixing the ambient air with the stream of burned gases and the coating material.

[0014] Another factor that allows the sleeve to be eliminated is that the introduction of powder immediately after the separating elements allows the coating material to be introduced into the relatively slow moving but hot part of the gas stream. As a result, the transit time that a particle of coating material experiences, i.e. the time from entering the gas stream to settling on the coated product, increases, while ensuring that each particle is properly heated. In some prior art nozzles where the particles are introduced into a fast flowing gas stream, there is little time for the particles to be heated sufficiently and the sleeve is used to keep the gas stream warm before it begins to mix with the ambient air to provide sufficient heating the particles.

[0015] In a preferred embodiment, the separating means further comprise at least one inlet of a coating material for introducing at least one coating material into said stream of said combustion gases.

[0016] In another preferred embodiment, the inlet of the coating material comprises at least one opening in said separating elements at the furthest point in the flow direction of said separating elements in said stream.

[0017] By introducing the coating material on the distal side of the flow side of the separating elements, the advantage is provided that the coating particles do not pass through the nozzle and thus do not come in contact with any part of the nozzle, such as a sleeve. As a result of,

-4 heated particles do not damage the nozzle, which extends the service life of the nozzle. Moreover, due to the fact that the coating material particles are introduced into the center of the stable combustion gas stream, the particles no longer have such radial deflections as much, and this means that they are more likely to remain in the gas stream. This in turn means that smaller particles of coating material (<10 pm) can be used for coating. Furthermore, the introduction of the coating material into the center of the stable and converging stream results in a reduction of losses from a larger particle moving radially and missing the target.

[0018] In a preferred embodiment, the outlet comprises a substantially annular opening extending between said combustion chamber and said separation elements.

[0019] In another preferred embodiment, the outlet comprises a number of substantially linear openings extending between said combustion chamber and said separation elements. [0020] In a further preferred embodiment, the separation elements extend, at least partly, to the outside of said combustion chamber through said outlet.

[0021] In accordance with another aspect of the present invention, there is provided a thermal spray gun comprising:

at least one nozzle, substantially as described above;

fuel supply means for supplying fuel to at least one fuel inlet; and also coating material delivery means for feeding coating material to said coating material inlet.

[0022] The spray gun is a spray gun for spraying a mixture of oxygen and high speed fuel.

[0023] In accordance with another aspect of the present invention, there is provided a method of applying a coating material to an article, comprising the steps of:

introducing at least one fuel into the combustion chamber of the thermal gun nozzle and burning said fuel to produce combustion gases that form a gas stream in said combustion chamber towards the outlet;

dispersing said stream around the at least one separating device and thereby producing a number of streams in a plurality of streams or an annular stream before joining said streams into a single stream;

introducing at least one coating material into said stream and spraying said material onto the object.

[0024] In a preferred embodiment, the at least one coating material is introduced into said streams in the space between a number of divergent streams or into the center of the annular stream. [0025] In another preferred embodiment, the fuel is thiene and at least one liquid fuel.

[0026] Preferred embodiments of the present invention will now be described by way of example only and not in a limiting sense, with reference to the accompanying drawings, in which:

Fig. 1 is a perspective view of two prior art nozzles;

Fig. 2 is a cut away perspective view of the nozzle of the present invention;

Fig. 3 is a perspective view with the cut-out of the front part of the nozzle Fig. 2;

Fig. 4 is a schematic illustration of the front nozzle of Fig. 3;

Fig. 5 is a schematic illustration of the spray gun according to the present invention;

Fig. 6 is a schematic illustration of the front part of the nozzle, according to another embodiment of the present invention;

Fig. 7 is a schematic illustration of the front part of the nozzle, according to a further embodiment of the present invention;

Fig. 8 is a graph showing a comparison between gas velocity flow fields according to the present invention and an example of the prior art;

Fig. 9 is a graph showing a comparison between the temperature flow fields according to the present invention and an example of the prior art;

Fig. 10 is a graph showing a comparison of the particle velocity between the present invention and an example of the prior art;

Fig. 11 is a graph showing a particle temperature comparison between the present invention and an example of the prior art;

Fig. 12 is a graph showing the path of a particle in 2D, comparing the present invention and prior art;

Fig. 13 is a graph showing a comparison of surface oxidation in the present invention and in the prior art;

Fig. 14 is an outline of the oxygen mole fraction profile for the external region, comparing the present invention and the prior art.

[0027] Referring to Figs. 2 to 5, the nozzle 100 to the thermal spray gun 102 has a combustion chamber 104. The inlet 106 introduces fuel into the combustion chamber from the fuel supply pipe 108. The fuel is burned in the combustion zone 110 and a stream of combustion gases that they leave the combustion chamber 104 through the outlets 114. The nozzle 100 also includes separating elements in the form of an air cone 116, which is located partly in the combustion chamber. This air cone 116, in combination with the edges 118 of the curved upper and lower walls 120 and 122 and the side walls 124 with the edge 126, form outlets 114. It should be noted that the side wall, opposite the side wall 124 shown in Fig. 3, has not been illustrated neither in Fig. 2 nor in Fig. 5, but is partly present in Fig. 3.

[0028] The presence of an air cone 116 between the outlets 114 causes the combustion gas stream 112 to be dispersed, as indicated by 128, and also converges as indicated by 130.

[0029] The nozzle 100 also has inlets 132 of the coating material in the form of holes at the feed end of the coating material 134. The inlets 132 are preferably located at the lowest edge 136 of the air cone 116 and on a short flat surface that is perpendicular to the direction of the stream

112.

[0030] The operation of the thermal spray gun 102 will now be described with continuous reference to Figs. 2 to 5. The fuel is pumped into the combustion chamber 104 of the thermal spray gun 102 through the fuel inlet 106 from the fuel supply pipe 108. A typical fuel is a gas mixture, example of propane and oxygen, Fuel is supplied at 68 l / min, with oxygen supplied at 220 l / min. Propane and oxygen are mixed with air (flowing at 471 i / min) and carrier gas, e.g. nitrogen or argon, flowing at 14.5 l / min. still

However, this nozzle may also be used with other fuels, including, but not limited to, kerosene, propane, propylene and hydrogen. When liquid fuel such as kerosene is used, a sprayer is necessary to ensure efficient combustion, however, this increases the length of the nozzle. In the case of propane, the fuel is ignited by a spark at the front of the nozzle, outside the main body of the gun. Initially, the mix flow rate is very low so that the mix ignites outside the gun body and the flame moves backwards in the chamber. By increasing the flow rate slowly and in small increments, the turbulent flame stabilizes in the chamber. For liquid fuels such as kerosene, a spark ignition system from inside the chamber is required.

[0031] Combustion takes place in the combustion zone 110 and a high pressure stream is created, usually above 5 bar, and high temperature, usually 3300 K, flue gas is produced. The high pressure flue gas stream 112 must leave the combustion chamber through the outlets 114 until, in doing so, the stream is split into a pair of streams by an air cone 116. The air cone 116 forms on the one hand a virtual bell, which has the shape of a cone (with at least 2 inflection points) of a pair of divided streams, forming a cone of air, on the other hand created by external air. The upper and lower curved surfaces of the wedge-shaped air blade 115 cause the two streams to converge, as indicated by reference number 130.

[0032] At the convergence point, the coating material, for example, a tungsten carbide and cobalt carbide powder composite, is added to the converging gas stream 112, at a rate of 50 g / min. At the powder injection point, the gas temperature is about 1500 K, while the axial gas speed is about 30 m / s. These values ​​increase sharply to 2,500 K and 1,700 m / s, respectively, before the powder particle hits the surface that is magnified. Nevertheless, the residence time of the particle in the gas stream is sufficient to allow gentle and better heating of the particle than observed in the prior art.

[0033] Linear outlets 114 are narrow elongated openings in the combustion chamber and are the result of the action of the linear air cone that is used here. This shape of the opening gives the advantage of producing an extended coating spray. As a result, the coating material is applied to the surface very efficiently and evenly in a spray pulse, similar to the use of a wide paint brush. However, other air cone shapes are equally good for use in this type of nozzle. When the nozzle shown in the figures is cut in a cross-section perpendicular to the axial flow of gases indicated by arrow 112, the cutting edges form a series of rectangles. An annular air cone motor in which the same cross section will produce a series of circular edges can also be used. In this case, the outlet will be a single circular annular outlet extending around a centrally located air cone. In addition, non-circular annular air cones such as squares, ovals or rectangles can be used.

[0034] It will be appreciated by those skilled in the art that the above embodiments have been described by way of example only and not in any limiting sense, while various changes and modifications are possible without departing from the scope of protection which is defined by the appended claims. For example, the material used for the coating may be in a form other than powder, such as a wire fed into a flame and a wire smelted coating. In addition, the nozzle of the present invention can be used in other thermal spraying techniques that require acceleration of gas, such as flame, tuk, plasma or even cold spraying.

[0035] For example, Fig. 6 shows a nozzle 100 adapted for use in a wire spray gun. In this example, the wire 140 is fed through a heated ceramic air cone 116 to the converging gas streams 112 at 130, where it is atomized in the atomization region 142. The resulting spray 144 hits the surface to be coated (not shown).

[0036] In another example, Fig. 7 shows a nozzle 100 adapted for use as a plasma gun. The arc gas passes through the nozzle in streams 112, with an air cone 116 forming a pair of tungsten cathodes 144 and surfaces 146 of the upper and lower walls 120 and 122, which form water-cooled anodes. The powder is introduced into the converging gas stream through the inlet pipe 148.

[0037] The nozzle of the present invention may also be used in cold spraying. In this case, the backfire combustion gases are displaced with ordinary cold spray gases, such as helium or nitrogen carrier gases, used at higher flow rates.

[0038] In the following, with reference to Figs. 8 to 14, examples are given of modular performance analysis of the embodiment of the present invention shown in Figs. 2 to 5 in comparison with the prior art example. The nozzle according to the present invention creates a stable supersonic jet which is strongly directed towards the spray line. Comparing this with an example of the prior art in which a convergent-divergent nozzle (CDN) is used, the nozzle according to the present invention achieves a higher axial speed (see Fig. 8) which is maintained longer than in the prior art. This increase in speed is due to the delayed mixing of the stream core with ambient air due to a narrower jet spread. Despite the fact that the results clearly show that the nozzle of the present invention creates a stronger and axially limited stream at the same operating conditions as in the prior art (e.g. the same oxy-fuel mass flow rate), it is not possible to completely eliminate the shock waves that are the result of a bevelled nozzle body. It should be noted that higher speed values ​​do not occur on the front nozzle base, but at a distance from it. The short low speed area works in favor of powder heating. More specifically, the residence time for the particle is increased when an increase in temperature occurs.

[0039] The comparison between the gas temperature for the nozzle according to the present invention and the prior art (Fig. 9) clearly demonstrates the possibilities of the present invention to generate a higher temperature flow field. The reason for such a large temperature difference between the nozzle according to the present invention and the nozzle according to the prior art is that in the prior art the static temperature decreases when the gas is compressed and then expands several times during the process. In the state of the art, the gas is compressed and accelerated at the outlet to the convergent-divergent nozzle and along the sleeve, with the accompanying direct drop in gas temperature of over 1000 K. Then, the flow expands again at the outlet of the sleeve, where the temperature further decreases. In contrast, the nozzle of the present invention is designed in such a way that the flow expands only once at the end of the nozzle. The upper and lower spray streams, which are further combined, provide enough energy by convection and radiation to heat the powder at the desired level. In addition, the nozzle of the present invention prevents direct contact between the powder and the flame, eliminating unwanted reactions on the powder surface. The gas temperature flow generated by the nozzle of the present invention has a configuration that is excellent for heating particles with a low level of surface reactions.

[0040] Improvements in gas flow characteristics are reflected in particle heating and acceleration. The powder material used for these simulations is a composite of tungsten carbide and cobalt (WC-12Co). The nozzle of the present invention is designed in such a way that the air cone provides a robust configuration for supplying maximum kinetic and thermal energy to the powder, by reducing aerodynamic losses and consequently losses of supplied energy. These simulations show in Figs. 10 and 11 that both key speed and temperature parameters have values ​​well above those obtainable in the prior art. For 20 pm particles, the surface temperature reaches 1200 K and the speed is 650 m / s. At this higher temperature, softening of the material begins to occur and, in combination with higher kinetic energy, increases the deposition rate, and thus the quality of the coating is expected.

[0041] Typical powder sizes currently used in industry according to the prior art do not fall below 10 [mu] m. The reason is that the powder material disperses in the gas field and then reflects or never reaches the ground.

[0042] In Fig. 11, a particle-path line in the radial direction is shown. Small particles (5 pm in diameter) never reach the flow centerline for prior art configurations. This means that they cannot use high speed-temperature flow areas and instead follow the path at the edge of the free stream. When turbulent mixing with ambient air begins to increase, the flow disperses in all directions. Lightweight particles pursue the flow direction and are then blown off the ground. However, the nozzle of the present invention is designed in such a way that it makes it even more suitable for the needs of spraying small particles. The air cone nozzle design allows axial powder injection, in which the dispersion of particles is limited, as shown in Fig. 12. The resulting velocity vector of particles in the radial direction is significantly lower than in the prior art, so the spray location on the substrate can be precisely controlled.

[0043] The high thermal profiles provided for the sprayed particles give rise to oxidation on the surface of the powders that have been determined in such as sprayed metallic coatings using a microscopic imaging technique. The metal oxides are brittle and have different coefficients of thermal expansion compared to the surrounding metals. Therefore, the oxides in the coating have a negative effect on the mechanical properties of the coating, which degrades the performance of the coated products. This increases the importance of limiting the development of oxides due to thermal spraying to achieve higher quality coatings. Oxidation on the surface of the particle will occur when a sufficiently large amount of oxygen is available in the surrounding gas flow. Based on the Mott-Cabrera theory, oxidation is controlled by the transfer of ions through the oxygen coating and in this way the build-up of oxide layers can be limited by reducing the proportion of oxygen that surrounds the particle. The oxygen mole fraction increases in the stream as it mixes with ambient air. Oxygen profile outline in Fig. 14 shows that the supersonic gas stream generated by the nozzle of the present invention can protect more than in the prior art, where excessive oxygen enters the core of the stream.

As a result, very little oxygen is available in the present invention and less oxidation is expected. The thickness of the oxygen layer is 5 times smaller than that produced in the prior art.