Patent Application
20110229649
This invention relates in general to flame spray apparatus and to methods of deposition of coatings and bulk materials with thermal spray techniques. More specifically, the invention relates to high-velocity oxidizer-fuel spraying apparatus and methods.
Thermal spraying is widely used to apply metals and ceramics in a form of coating or bulk materials on different types of substrates. A majority of thermal spray methods utilize energy of hot gaseous jets to heat and accelerate particles of spraying material. When impinging the substrate, the particles form a coating.
High-Velocity Oxygen-Fuel (HVOF) spraying apparatus and techniques, which use oxygen as an oxidizer gas, or High-Velocity Air-Fuel (HVAF) spraying apparatus and techniques, which use air as an oxidizer gas, generate a jet of hot gases due to combustion of a fuel and oxidizer in an internal burner at elevated pressure, usually several bars. The fuel can be gaseous (e.g., propane, methane, propylene, MAPP gas (i.e., liquefied petroleum gas (LPG) mixed with methylacetylene-propadiene), hydrogen, etc.) or liquefied fuel (e.g., kerosene). From the burner, the gas expands into an exhaust nozzle, reaching sonic velocity if the nozzle is straight. If the nozzle is convergent-divergent, further expansion into wider section of the nozzle results in formation of a supersonic velocity jet. This allows entrained particles of sprayed material to reach higher velocities and form coatings with better mechanical properties, compared to those achieved with straight nozzles.
In spite of the technological advantages of higher particle velocities generated by supersonic convergent-divergent nozzles, there are some disadvantages caused by gaseous flow temperature drop in the divergent portion of the nozzle due to gas expansion. For typical industrial HVOF torches, where combustion chamber temperature typically reaches about 2600° C., the exit gas temperature is only about 1900° C. at stagnation pressures 7-8 bar. So, lowered heating capacity of a supersonic gas flow further reduces particle temperature that, in turn, leads to lower deposition efficiency (DE) of sprayed material, compromises coating quality, and applies limitations for spraying materials with high melting points. This cooling effect of expanded gas flow is even more detrimental for HVAF torches, since maximal temperature of combustion of air-fuel mixture in a combustion chamber is only about 1900° C., and exit gas temperature is about 1200° C. for typical stagnation pressure 6-7 bar, e.g. Mach numbers around 2. This temperature is lower than melting points of most commercial hard facing alloys and cermets, such as the most popular tungsten carbide (WC) based and chrome carbide (Cr3C2) based composite powders. So, the use of convergent-divergent nozzles with HVAF torches significantly compromises coating quality and lowers DE of such materials due to lack of heat, in spite of higher particle velocities achievable with supersonic nozzles. For this reason supersonic nozzles have not found use for HVAF torches so far.
Another disadvantage of a convergent-divergent nozzle is the difficulty of powder injection. For example, the powder cannot be fed axially through the combustion chamber, since being heated and partially melted in the burner it would plug the nozzle at the throat, where the cross sectional area is minimal and powder particles come in physical contact with nozzle bore walls. Though clogging can be prevented by significant increase of nozzle bore diameter, this would simultaneously increase flows of both oxidizer and fuel, which reduces economical effectiveness of the process. So, in practice, the radial powder injection into divergent part of the nozzle is usually used. However, this type of powder injection also causes problems, such as lack of heat available for particle heating in the divergent portion of a nozzle, and nozzle clogging caused by radially injected powder.
Advancement in the HVAF apparatus and technique included a secondary fuel flow, which is added to an oxidizing flame jet in the divergent part of a nozzle such that any free oxygen within the flame jet is consumed by the secondary fuel to increase the static temperature of the jet in the divergent part of the nozzle. In order to combust effectively in the relatively cold supersonic jet, where fuel contact time with oxygen is too short, a very reactive secondary fuel of high flame temperature is used. The secondary fuel may be selected from the class consisting of acetylene, methylacetylene and its compounds, and hydrogen.
Disadvantage of said technique and apparatus is the complexity of the process due to the need in highly reactive high flame temperature secondary fuel, different from primary fuel. At the same time there is still a problem with injection of powder axially due to the plugging of the nozzle throat.
The present invention is related to a method of forming a coating by depositing a material onto a substrate with high-velocity thermal spray apparatus, wherein the apparatus comprises a mixing unit, a combustion chamber, and a non-clogging supersonic accelerating nozzle. The method comprises the steps of mixing of an oxidizer gas and a gaseous fuel in the mixing unit, igniting and combusting the oxidizer and gaseous fuel mixture in the combustion chamber, feeding products of combustion to the accelerating nozzle, introducing selected spraying material into accelerating nozzle to form a supersonic stream of hot combustion product gases with entrained particles of spray material, and spraying at high velocity onto a surface positioned in the path of the stream at the discharge end of the nozzle. The method further includes a step of forming a non-clogging convergent-divergent gas dynamic virtual nozzle (GDVN) in the accelerating nozzle by annularly introducing a coaxial gas flow, through a narrow continuous slot of circumferential ring geometry in the vicinity of the entrance to the diverging outlet bore of the accelerating nozzle. Thus, the hot combustion product gases discharged from a combustion chamber are compressed in diameter through gas dynamic forces exerted by a coaxially co-flowing gas, obviating the need for a solid nozzle to form a convergent-divergent flow and thereby alleviating the clogging problems that plague conventional solid nozzle, especially in its minimal diameter that creates choked flow condition needed to form a supersonic gas flow in the divergent part of the nozzle. Of particular advantage is the use of compressed air or air-fuel mixture for creating a coaxial gas flow.
Referring to the drawing, a better understanding of the principles of this invention may be gauged by inspection of
Thus, reactants including a fuel as indicated by arrow F1 and an oxidizer as indicated by arrow P1 are fed into the mixing chamber 23 where they form a combustible mixture, which is fed through the orifices 16 into the distribution chamber 24 and further, through the plurality of orifices 13 in the permeable burner block 12, into the combustion chamber 27 with ignition and combustion taking place within the chamber 27 and hot combustion product gases pass through the accelerating nozzle piece 1. The ignition means is not shown, but it is usually a regular spark plug placed in the combustion chamber. High melting point particles indicated schematically by arrow G may be introduced axially into burning gases within combustion chamber 27 through the tube 21 and powder injector 22 and further accelerated in the supersonic GDVN 31 formed within the bore of accelerating nozzle piece 1. A heated powder stream 29 forms a coating 32 upon impact against a substrate 33.
In one aspect the present invention is directed to a method and apparatus for eliminating clogging of the throat of a supersonic nozzle by utilizing GDVN instead of actual solid convergent-divergent nozzle. A coaxial gas flow as indicated by arrow P2 is fed into the circular cavity 10 through the secondary gas supply tube 8 and orifice 9. A supersonic GDVN 31 is defined as an inner boundary of a coaxially co-flowing gas 30 through a narrow continuous slot of circumferential ring geometry 11 under pressure that is higher than the static pressure in the main flow of hot combustion product gases H. Formed this way GDVN 31 has a supersonic convergent-divergent shape having convergent 31a and divergent 31b portions, with a virtual throat 28 having flow area at sonic point A*, and exit 37 having exit flow area A. The ratio A/A* is determined by Mach number at which the spray torch is supposed to operate, and can be adjusted by changing the flow rate of a coaxial gas flow 30 forming GDVN. Thus, the main high velocity stream of hot combustion product gases, as indicated by arrows H, discharged from the combustion chamber 27 and flowing through the inlet bore 5 is further compressed in diameter through gas dynamic forces exerted by gas 30 coaxially co-flowing through a narrow continuous slot of circumferential ring geometry 11 and forming convergent portion 31a of GDVN 31. The main high velocity hot gas stream H with entrained powder particles is further accelerated to supersonic velocity in the divergent portion 31b of GDVN 31 forming a supersonic flame jet indicated generally at 36, characterized by oblique shock waves 7, Mach disks 34, and expansion fans 35. Therefore, a supersonic GDVN 31 obviates the need for a solid nozzle to form a convergent-divergent flow and at the same time alleviates a possible build-up 38, as shown in
In another aspect the present invention is directed to a method and apparatus for increasing the jet temperature by adding a reactive fuel to the gases in the coaxial gas flow 30 forming a supersonic GDVN 31. The secondary fuel as indicated by arrow F2 may be pre-mixed with air, oxygen or other gas forming a coaxial gas flow 30 and a supersonic GDVN 31, and fed through tube 8 and hole 9. Alternatively, the secondary fuel may be fed at least through one additional circular series of orifices (not shown), or narrow continuous slot of circumferential ring geometry (not shown), or a permeable portion of the nozzle wall of circumferential ring geometry (not shown), located in the vicinity of the narrow continuous slot of circumferential ring geometry 11. The secondary fuel may be low reactive gaseous fuel, selected from the group consisting of propane, propylene, methane, ethane, butane, or liquid fuel which may in the form of mist, vapor, or liquid. The secondary fuel is pre-heated by the stream of hot combustion product gases discharged from the combustion chamber 27, reaching auto ignition temperature, and burns in the divergent portion of the coaxial gas flow 30 that forms a supersonic GDVN 31. This burning gas expands inwards the core of the stream of hot combustion product gases, which is supersonic due to expansion in a supersonic GDVN 31, until essentially complete mixing takes place. Therefore, the combustion of the secondary fuel increases the static temperature of a supersonic flow, which in turn increases velocity of main stream of hot combustion product gases, as well as temperature and velocity of entrained particles. Greater particle velocity and temperature are of extreme importance for low combustion temperature HVAF thermal spray process, and allow to significantly improve coating quality. When even higher particle temperature is needed, the secondary fuel may be a highly reactive gaseous fuel, selected from the group consisting of methyl-acetylene and its compounds, and hydrogen.
In accordance with an exemplary embodiment, a coating is sprayed with an HVAF apparatus 25 comprising an accelerating nozzle piece 1 with means of forming a supersonic GDVN 31 (as described with reference to the
Alternatively, for comparison, a regular HVAF apparatus, without supersonic GDVN, but instead having regular straight accelerating nozzle of the same length, and diameter similar to the diameter of the throat 3 of the apparatus with supersonic GDVN, was used to apply a coating with the same material: 5-30 μm particle size tungsten carbide-cobalt-chrome 86% WC-10% Co-4% Cr agglomerated-sintered powder. The apparatus operates with air flow of about 85 liters per second, an inlet pressure of about 6.3 bar, and propane flow of about 3.4 liters per second under a pressure of about 5.3 bar, thus generating 1,050,00 Btu/hr, e.g. the same total amount of heat energy as apparatus with supersonic GDVN according to the exemplary embodiment. The mean coating hardness is measured at about 1,040 HV300. The particle velocity of about 664 msec and particle temperature of about 1,690° C. have been measured with AccuraSpray sensor.
Thus, the use of supersonic GDVN combined with feeding of secondary fuel to the coaxial gas flow forming supersonic GDVN, provides non-clogging operation of HVAF or HVOF apparatus, and when compared to typical HVAF apparatus with a straight cylindrical nozzle, allows for a nearly 2 fold increase in the particle velocity without lowering particle temperature, which significantly improves coating properties.
In accordance with the provisions of the patent statutes, the principle and mode of operation of this invention have been explained and illustrated in its preferred embodiment. However, it must be understood that this invention may be practiced otherwise than as specifically explained and illustrated without departing from its spirit or scope.
