GB2445926A

GB2445926A - Powder injection apparatus with shroud arrangement

Info

Publication number: GB2445926A
Application number: GB0701305A
Authority: GB
Inventors: Michael Bernard Coupland Quigley
Original assignee: Individual
Current assignee: Individual
Priority date: 2007-01-24
Filing date: 2007-01-24
Publication date: 2008-07-30
Also published as: GB0701305D0

Abstract

Powder injection apparatus (Fig 1) includes the control of the injection (14, Fig 1) of the powder particles (15) into a plasma, flame, gas jet or shock front (13). Included is a shroud arrangement 21 with at least one slit or hole 34 cut into it and positioned such air or other externally applied gas is sucked in through the slit or holes 34 so as to cause any particles that have passed through the spray jet to be forced back into the spray jet for further heating. Tubes (44, Fig 4) can be fitted around the shroud 21 so as to control the amount and direction of the ingress of gas into the shroud 21. A powder inlet tube 33 is also included. Shroud 21 and tube arrangement (44) may have circular or non-circular sections and may be made from metals or non-metals such as heat resistant 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 called 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 particles 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.

We use the 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 bamer coating or similar, or to have a double skin so that cooling fluid can be used to prevent the shroud from overheating.

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.

Referring to Figure 2 there is shown the proposed design of a new shroud 21 with a slit 22 positioned opposite to the powder feed inlet tubes 23.

Figure 3 shows the shroud 21, the powder inlet tube 33 from a single injector system, and a holes 34 positioned either side of the line of the powder injection but angled so that the air/gas injected through the shroud wall 21 will cause any powder passing through the jet 13 to be pushed back into the jet 13.

Figure 4 shows a number of tubes that can be placed around the shroud and designed so as to vary the amount of the opening through which the air or other gas passes into the jet. The fitment 41 at the torch end of the shroud 21 is shown together with a slot 42 opposite the powder port orifice 43. 44 is an outer tube with a slot 47. 48 is an outer tube with no slots or holes and is designed to block any ingress of air or gas from slots or holes.

Claims

What we claim is I Powder injection apparatus in which a shroud is

applied to the outside of a spraying torch and in which a slit or slits or a number of holes is made in the shroud and positioned so as to allow 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 Powder injection apparatus as in claim 1 in which tubes are fitted around the shroud so as to control the amount and direction of the ingress of gas into the shroud.
3. Powder injection apparatus as in claims 1 and 2 in which the shroud and the surrounding tubes are either circular in section or non-circular.
4. Powder injection apparatus as in claims 1, 2 and 3 in which the shroud and the surrounding tubes are made from metals or non-metals such as heat resistant ceramics.