GB2281488A

GB2281488A - Improvements in or relating to thermal spraying

Info

Publication number: GB2281488A
Application number: GB9317427A
Authority: GB
Inventors: Roy Peter Mcintyre; Michael Bernard Coupla Quigley
Original assignee: PLASMA TECHNIK Ltd
Current assignee: PLASMA TECHNIK Ltd
Priority date: 1993-08-21
Filing date: 1993-08-21
Publication date: 1995-03-01
Also published as: GB9317427D0

Abstract

A shroud 20 for thermal spraying torches has a baffle 22 which separates out from the spray stream the underheated or overheated particles 24 and 25 respectively thus providing a stream of particles 26 that are in the correct condition to adhere well to the substrate 23. In this way oxide levels are reduced together with the number of unfused and loosely adhered particles. There is further provided means 31 of removing most of the fume that builds up in shrouds by injecting gas transverse to the spray stream so "blowing away" the fume particles. <IMAGE>

Description

DRAFT PATENT SPECIFICATION IMPROVEMENTS IN OR RELATING TO THERMAL SPRAYING We PLASMA TECHNIK LIMITED, a British Body Corporate, of Maesglas Industrial Estate, Newport, Gwent NP9 2NN, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement: This invention relates to Thermal Spraying Processes and is most particularly concerned with the control of oxide and removal of oversized particles in the coating being Reposited.

Thermal Spraying Processes typically generate a high temperature plasma flame 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 thermal shock resistance or low friction.

In most of the above processes powder is injected into the plasma flame or shock front, hereinafter to be called the jet, from a side port. It is thus imperative to arrange that the powder partiles 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. The particles are propelled into the J et by means of a carrier gas and the velocity of this gas will clearly be important. The other factors are the velocity and temperature of the jet and the size range of the particles. It is impractical to have particles with a very small size distribution and so if the average size particles reach the centre of the jet then the smaller ones will only reach the upper periphery and the larger ones will penetrate through the jet. The significance is with the smaller particles; they will fly off axis and may become heavily oxidised.

In many coatings used for corrosion protection and other applications unwanted oxide of this sort can be harmful. It may reduce the effectiveness of the coating.

Over many years attempts have been made to reduce air entrainment into the jet and so reduce oxidation of the particles. Many shrouds have been invented, the purpose of which is to exclude air from the jet as far as possible. However it is always the finer particles which will create the problem because of their greater surface area for a given volume compared with larger particles.

The principle of this invention, therefore, is to use a shroud to reduce air entrainment, described in numerous papers, but now to add internal baffle(s) to separate the under and overmelted particles from the main spray stream thus preventing them reaching the substrate.

In the following description reference will be made to the accompanying drawings in which: Figure 1 is a drawing of normal powder injection into a plasma or flame showing the divergence of the spray and with the smaller particles being convected upwards and the larger particles being transported through the jet.

Figure 2 is a schematic drawing of the proposed shroud with baffle(s).

Referring to figure 1 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 size particles will form the useful spray 17.

Referring to figure 2 there is shown the proposed shroud design. The outer skin 20 of the shroud provides protection against much of the oxidation of particles. Argon or other protective shield gas is injected into the shroud through holes in the ring 21.

The internal baffle 22 provides a physical barrier separating those particles which have diverged significantly from the axis of the torch and shroud and is mounted close to the workpiece being sprayed.

The light particles 24 will "bounce" off the jet, will not enter the main stream 26 and will be deflected by the baffle 22. They will then form effluent 27 and be ejected away from the shroud. Similarly the heavier particles 25 will pass through the stream 26, will be deflected by the baffle 22 and will then form effluent 28 and be ejected away from the shroud.

Those particles in the correct size range will form the main stream 26 and will impact on the prepared substrate 23 to form the desired coating.

In shrouded plasma and other spraying systems a problem generally arises of fume build up within the shroud or within a recess being sprayed. The fume particles are very much smaller and slower moving than the particles being sprayed and it is possible to remove most of them by blowing them away with a directed gas jet.

A further principle of this invention is to provide means of directing a gas jet to carry out this task. This can be achieved in a number of ways, all dependent on how the specific shroud has to be designed to fit around the item to be sprayed.

One particular embodiment is to provide a stream of argon or other gas which may be non-oxidising or not depending on the powder being sprayed. The stream of gas is provided from an injector 31 which could be of any configuration and size in order to provide a sufficient flow of gas across the spray stream and/or into the recess significantly to reduce the fume level.

In general the direction of the gas stream should be normal to the direction of the spray stream and along the surface being sprayed. For example in the particular embodiment shown in Figure 3 the injector 31 has a rectangular cross-section designed to provide a flow of gas of width greater than the spray stream and normal to it.

The flow must also be such that the gas flows along the slots providing the maximum possible fume removal.

In this particular example the shroud has a flat end for sprayed onto flat components and the injector 31 has a simple rectangular cross-section. For spraying onto components of circular cross-section the end of the shroud would be of the same curvature or similar to that of the component and it would be preferable for the injector 31 a to have a similar shape

Claims

WHAT WE CLAIM IS:1. A shroud for thermal spraying torches wherein there is provided a baffle for separating out from the spray stream the underheated or overheated particles.
2. A shroud as claimed in claim 1 where the baffle is made from a ceramic, cermet, metallic or composite material.
3. A shroud as claimed in claim 1 where the surface of the baffle is coated with a non stick sprayed coating.
4. A shroud as claimed in claim 1 where the surface of the baffle is coated with "anti-bond" material typically a graphite based compound.
5. A Shroud as claimed in claim 1 where the baffle is made from any material in claim 4 and with a polished finish.
6. An injector for directing gas through the spray stream to remove fume.
7. An injector as in claim 6 combined with a shroud system without a baffle.
8. An injector as in claim 6 combined with the shrouds as in claims 1 to 5.