GB2461747A - A powder injection apparatus with a shroud having a gas port opposing a powder port - Google Patents

Abstract

This invention relates to thermal spraying processes and control of the injection of the powder particles into a plasma, flame, gas jet or shock front. The powder injection apparatus comprises spraying torch 12, a shroud 20 surrounding the spraying jet 13, the shroud having a powder injection port 14 and an opposing gas port 24, the gas issuing from the gas port serving to push powder particles that have passed through the spray jet back into the spray jet for further heating. Preferably, an inert gas, eg. Argon, is used as the gas so that it does not cause oxidation of the powder. Typically the heavier / larger particles (see figure 1, path 18) pass the jet and are blown back by the gas jet. A valve 22 and gas nozzle 24 may be provided to control the amount and direction of the opposing gas entering the shroud. The shroud / shield may be made from metals or ceramics.

Description

This invention relates to thermal spraying processes and is most particularly concerned with the control of the injection of the powder particles into the plasma, flame, gas jet or shock front.

Thermal spraying processes typically generate a high temperature plasma, flame, gas jet or shock front, hereinafter to be caNed the jet, in order to heat and propel powder particles towards the substrate to be sprayed. These particles then impact on the surface and form a specialised coating which will protect the substrate from corrosion, erosion or both or to impart special properties such as low friction. In most spraying processes powder is injected into the plasma or flame from a side port. It is thus imperative to arrange that the powder particles are injected into the jet at a speed such that they reach the centre of the jet and are carried along axially towards the workpiece.

As the particles typically have a range of sizes of 3:1 the mass of the individual particles will vary by a factor of 27:1. As the particles are injected into the jet the lighter particles will tend to bounce off the jet whereas the heavier ones will tend to penetrate through the jet. In both cases the particies will not be propelled axially through the plasma; the lighter ones will be overheated, oxidise and tend to form a "fog" above the jet and will create oxide on the workpiece and in the coating. The heavier particles will not be heated sufficiently to enable melting to take place and will result in a number of unfused (i.e. loose) particles being present in the coating. Current "older type" spray systems have this type of powder injection and are renowned for their "spray droop" problems. With this phenomenon the spray pattern tends to be below that of the aiming point. In practical terms the spray stream does not go where it is aimed resulting in a loss of efficiency and productivity and an increase in environmental issues such as powder recovery and handling. Many of the powders involved in these processes are hazardous and it is essential that both the fume and the powder residue are kept to a minimum.

In UK Patent GB 2300649B Quigley Associates disclosed a device that would separate the different sized particles into different positions for injection into the jet. In this way the heavier particles would be injected into the jet at a position close to the end of the nozzle and thus be in the jet for the longest period. They would thus be melted and accelerated to much higher velocities than normally. Conversely the smaller particles will be injected into the plasma at a position much further away from the torch nozzle and therefore receive less heating than before and oxidation will be prevented. To achieve this separation it was necessary to design an injector that automatically arranges the particles into the appropriate sizes.

This application discloses the invention of a shrouding arrangement that is added to a spraying torch whether the standard or new injector designs are used. This new shroud will have two major effects that are well known but in the following specific embodiments a third effect is disclosed.

The first effect is that the shroud will prevent ingress of air into the proximity of the injector and, more importantly, the plasma jet. Because of this the plasma jet is longer than normal and therefore has longer to heat up the particles injected into it.

The second effect is that the reduction of air, and therefore oxygen, ingress into the plasma jet reduces the oxidation of particles that normally takes place.

Shrouds have been used previously and are not, in themselves, new. In this embodiment we are using a shroud in a novel way to produce the third effect.

In patent application number GBO7OI 305.5 Quigley Associates describe the use of a shroud, which may or may not be cylindrical, not only to prevent ingress of air over the whole volume of the plasma jet in the normal manner but also to allow a jet of air, or other externally applied gas typically non-oxidising, to help to propel particles that may have passed through the jet, after injection, back into the jet so that they can be reheated. This is achieved by the use of a thin slit or slits, or one or more holes, cut into the shroud body typically, but not exclusively, opposite to the injector; the natural vacuum that is achieved with a shroud ensures that air will rush in through any hole or slit. In this case we position the slit or slits or one or more holes, hereinafter collectively called the ingress slit, opposite or nearly opposite to the particle injection point so that particles that pass through the jet will encounter a jet of air to push them back so that they are further heated. The ingress slit in the shroud body can be arranged to have different lengths and widths and can be of any shape to suit different applications. If one or more holes are used then the positioning and size of the holes can be varied according to the specific application. The holes do not need to be circular and can be of any shape. For some applications it may be appropriate to have more than one slit or hole and sited other than opposite to the powder injection position. It may be appropriate to have holes shaped to provide very specific flow patterns, the shaping referring not only to the pattern of the hole but also to the variation of pattern through the wall of the shroud. A hole shaped in this manner might be shaped like a Venturi nozzle for example.

It may be appropriate to have a number of holes in the shroud with some holes blocked off and others open as appropriate for any specific powder and plasma combination. For example when powders are of a lighter mix than normal the particles that pass through the plasma might pass through later in the plasma jet than when heavier particles are used. Thus the holes applicable to the heavier particles will be blocked off and those applicable to the lighter particles open. Further variations in the ingress of air or other gas can be made by the use of rings placed judiciously around the shroud body so as to allow some ingress of gas through a part of the slit or hole but not through other parts.

These tubes, which may or may not be cylindrical, are designed to fit closely to the shroud shape and can be so arranged that they can vary the size of the opening through which the air or other gas passes into the jet A number of such tubes can be employed so that a wide combination of slot or hole sizes can be achieved. In this way a variable quantity and direction of ingress gas can be achieved.

It may further be appropriate to have the shroud made from a temperature resistant material, or material coated with a thermal barrier coating or similar, or to have a double skin so that cooling fluid can be used to prevent the shroud from overheating.

A new embodiment is to use a variable gas supply and nozzle provided opposite to the powder inlet so that the powder coming through the plasma jet can be pushed back into the flame. This has the particular advantage that the gas pushing the particles back can be non-oxidising such as argon instead of air being sucked in as described in GB0701305.5.

Referring to Figure 1 as an example of standard spraying systems there is shown an electrode 11 in a plasma torch 12 and the plasma jet generated is shown as 13. The powder injector 14 injects the powder 15 into the jet 13. The lighter particles will be convected upwards as 16, the heavy particles will pass through the jet as 18 and the medium sized particles will form the useful spray 17.

Figure 2 shows how an opposing gas jet would be sited on the shroud. The exact position will need to be determined for the materials being sprayed but it is likely to be approximately opposite to the powder inlet port and may be at right angles to the shroud or inclined so that the gas impinges on the plasma or other jet further down the shroud.

The pressure and gas flow can be varied to suit by means of a valve. This valve can be adjusted either directly or remotely from a distance when the torch has been started and the flow so arranged that any spray deviation can be corrected easily. Various nozzles can be fitted to the end of the gas inlet tube so as to arrange a suitable gas flow distribution.

In Figure 2 the spraying torch 12 is shown fitting into the shroud 20. The normal powder inlet port 14 enters the shroud 20 and feeds powder into the plasma or other jet 13. The opposing gas flow from pipe 21 is controlled by a valve 22 which may be operated remotely and the resultant gas flow passes through pipe 23 and into the nozzle 24 before entering the jet 13. Pipe 23 can be positioned at any angle as appropriate.

Claims (5)

1. What we claim is 1. Powder injection apparatus in which a shroud is applied to the outside of a spraying torch and in which an opposing gas jet is provided so as to provide the ingress of air or other gas into the shrouded volume as a jet, this jet pushing particles that have passed through the spray jet back into the spray jet for further heating.
2. 2. Powder injection apparatus as in claim I in which a valve is fitted so as to control the amount of the ingress of gas into the shroud.
3. 3. Powder injection apparatus as in claims 1 and 2 in which a nozzle is provided to arrange the opposing gas flow in the best direction and pattern for controlling the powder passing through the jet.
4. 4. Powder injection apparatus as in claims 1, 2 and 3 in which the shroud is either circular in section or non-circular.
5. 5. Powder injection apparatus as in claims 1, 2, 3 and 4 in which the shroud is made from metals or non-metals such as heat resistant ceramics.