GB2300649A - Powder injection apparatus - Google Patents

Abstract

A powder injection apparatus for application to thermal spraying systems has a curved body 21 in which are disposed baffles to separate out particles of different sizes from the original powder stream 15 and as propelled by the gas stream 22. The exit orifices are disposed along the length of the spraying jet so as to arrange that the heating of the particles is equalised. The sizes of the exit orifices are also arranged to equalise the penetration of the particles into the jet.

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 in order to heat and propel powder particles towards a substrate. These particles will then impact on the surface and form a coating which will protect the substrate from corrosion or erosion or to impart special properties such as low friction. In most of the above processes powder is injected into the plasma,flame,gas jet or shock front, hereinafter to be called the jet, from a side port whether internal or external to the nozzle. 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 centre of the workpiece.

The particles typically have a range of sizes in a particular specification with a ratio of diameters of the largest to the smallest perhaps being 3:1 or more. In those circumstances the mass of the individual particles will vary by a factor of at least 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 particles 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 particles being present in the coating.

The principle of this invention is to segregate 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 closest to the end of the torch nozzle and thus be in the jet for the longest period. They would thus be melted and be 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 much less heating than before and oxidation will be prevented. Those particles of median size will be heated and accelerated normally.

To achieve this segregation it is necessary to design an injector nozzle which automatically arranges the particles into the appropriate sizes.

Referring to Figure 1 as an example 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 powder injector nozzle. The particles of mixed sizes 15 are injected into the injector tube ?1 by means of powder feed gas 22. The injector tube 21 is typically of curved shape which csuses the different sized particles to separate around the curves. Internal baffles are provided which allows that separation to be defined and provides a classification mechanism.

There will be provided a number of exit points through which will pass particles of different sizes.

A typical embodiment might be to have 3 exits (although any number could be used) for the classified powder which would then have smaller, average and larger sized particles emanating from them. These would then be disposed such that the larger particles are injected into the jet at a point closer to the nozzle than normal. The average sized particles would be injected in the normal position and the smaller sized particles would be injected at a point further away from the nozzle. In this way the larger particles would be in the plasma for a longer period and the smaller particles would have a shorter time available for being heated.

A further sophistication will be to arrange that the exit ports for the larger particles will be larger than those for the smaller particles. In this way the gas velocities at the exit of the port issuing the larger particles will be less than that for the smaller particles meaning that the larger particles will receive a smaller transfer of momentum from the gas than will the smaller particles. In this way the penetration of particles into the jet can be balanced between all the various sizes of particles.

Claims (6)

What we claim is:

1. powder njection apparatus, the apparatus having an inlet orifice and a number of outlet orifices.

2. powder injection apparatus as specified in claim 1 in which there are internal baffles 50 disposed as to separate the heavy particles from the light particles.

3. powder injection apparatus as claimed in claim 2 in which the outlet orifices are of different sizes such that the heavy particles will enter the jet a similar distance to the lighter particles.

4. powder injection apparatus as claimed in claim 3 in which the outlet orifices are positioned such that the heavier particles will enter the jet upstream of the lighter particles.

5. powder injection apparatus as claimed in claim 3 in which a number of outlet orifices are used.

6. powder injection apparatus as claimed in claim 3 in which a number of outlet orifices are disposed along the jet.