THERMAL SPRAYING EQUIPMENT

Abstract: Thermal spraying apparatus comprises a spray head defining a nozzle with a throat (132) through which gas under pressure can be supplied. First and second guides are provided to guide feedstock wires (124) towards a point of intersection in or adjacent an end of the throat, preferably at an angle of between 45 and 90 degrees. The gas is forced through the throat at high pressure, preferably at a pressure sufficient to cause supersonic flow, thereby generating a finely atomised spray of molten feedstock material. The feedstock guides can take the form of drilled tubular inserts, or inserts with grooves which are received in cavities in body halves of the spray head.

Description: THERMAL SPRAYING EQUIPMENT BACKGROUND OF THE INVENTION THIS invention relates to thermal spraying equipment and to a thermal spraying method for producing corrosion resistant and/or hard coatings on a substrate. International patent application no. WO 98/00574 describes a thermal spraying method and apparatus of the above general kind. It is an object of the invention to provide a method and apparatus which is a development of the known method and apparatus. SUMMARY OF THE INVENTION According to the invention there is provided thermal spraying apparatus comprising a nozzle defining a throat having an inlet and an outlet and a gas flow path which is aligned with the axis of the throat, so that gas under pressure can be supplied to the inlet, at least first and second guides arranged to guide respective feedstock wires via the inlet towards a point of intersection in or adjacent an end of the throat, a power supply arranged to be connected to the feedstock wires to cause an arc in the throat between the wires, and a supply of compressed air arranged to supply air to the throat, the guides being arranged to direct the feedstock wires to the point of intersection so that they define an angle of between 45° and 90° between them Preferably, the guides are arranged so that the angle defined between the feedstock wires is approximately 60° The nozzle may be formed from first and second body halves, each defining a portion of the throat Typically the throat is tapered between the inlet and the outlet The guides may comprise respective bores formed in the body halves and each intersecting the portion of the body half defining a respective portion of the throat, each bore being inclined relative to the axis of the throat The guides may include inserts receivable in the respective bores, each insert having an aperture therein through which a feedstock wire can pass, and having an inclined end face shaped complementally to the shape of the throat Alternatively, the respective bores may intersect locating cavities, inserts being receivable in the locating cavities so that they abut feedstock wires passing through the respective bores The inserts may be polygonal in section and define planar faces each having a locating formation for engagement with a feedstock wire in use Preferably, the inserts are square in section and define planar, rectangular faces with a groove formed in at least one face for engagement with a feedstock wire in use The inserts may comprise copper or copper/tungsten The body halves are preferably conductive, with a terminal or contact on each body half for connection to the power supply The body halves are preferably mounted on a non-conductive head which holds the body halves in a spaced-apart condition BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is an exploded view of a first embodiment of a spray head of thermal spraying apparatus according to the invention, Figure 2 is a pictorial view of the apparatus of Figure 1 , Figure 3 is a rear end view of the apparatus of Figure 1 , Figure 4 is a section on the line 4-4 in Figure 3, Figure 5 is a partial exploded view of a second embodiment of a spray head of the apparatus, and Figure 6 is a sectional view of the second embodiment of the spray head DESCRIPTION OF EMBODIMENTS The apparatus illustrated in Figures 1 to 4 comprises a thermal spraying head which is connected in use to a source of compressed air or nitrogen by means of a hose 10, and to a high current power supply 12 in use The general principle of operation of the apparatus is substantially similar to that described in the abovementioned international patent application no WO 98/00574, the contents of which are incorporated herein by reference The spray head of the apparatus comprises first and second conductive body halves 14 and 16 which are machined from aluminium alloy and which are mounted on a non-conductive head 18 which can, for example, be moulded or machined from a suitable plastics material such as PTFE As shown in the exploded view of Figure 1 , the body halves 14 and 16 are attached to the head 18 hmgedly by means of pins, which assist in assembly of the head and in correct location of the body halves with respect to one another An outer sleeve 56 of tough plastics material such as nylon holds the body halves in position and is located positively by a pin 58 The front face of the head has a circular central portion 20, at the centre of which is an outlet 22 of a nozzle which has an inlet 24, and a cylindrical throat 26 extending between the inlet and the outlet The throat is tapered from the inlet to the outlet such that it has an inlet diameter of 8mm and an exit diameter of 8 5mm Cavities 28 and 30 are machined into the respective body halves 14 and 16, and define respective inclined bores 32 and 34 which intersect with respective grooved portions 36 and 38 of the body halves which define the throat 26, at an angle of approximately 30° to the axis X-X of the throat, so that the included angle between the two bores 34 is 60° Locatable within each bore is a copper insert 40, the inserts 40 having ends 42 which are shaped complementally to the grooved surfaces 36 and 38 defining the throat of the nozzle, so that when the inserts are in position in the bores 34, their ends 42 are flush with the surface of the throat A bore 44 is formed in each insert which is sized to receive a metal feedstock wire 46 and to make electrical contact with the wire as the wire passes through the insert and into the throat 26 In the prototype apparatus, the feedstock wires 46 were directed to a point of intersection at the outer end of the throat 26 essentially coinciding with the outlet 22 and the central axis X-X of the throat In some applications, it may be desirable to move the point of intersection outside the throat, although it has been found in practice that it is generally preferred for the point of intersection to be within the throat Electrical terminals 48 and 50 on the respective body halves 14 and 16 are provided for connection of heavy duty conductors 52 and 54 which connect the body halves to the power supply 12, and which thus pass current to the feedstock wires 46 via the body halves and the copper inserts, causing an arc at the point of intersection of the feedstock wires in the throat In the prototype apparatus of the invention, the power supply 12 was a constant voltage source operating at approximately 42 to 48 volts (compared with approximately 32 to 36 volts in the case of conventional apparatus) The arc current was between 150 Amperes and 300 Amperes and the gas pressure at the entrance 2 to 5 bar (g), typically 4 bar Figures 5 and 6 illustrate a second embodiment of a thermal spraying head according to the invention The second embodiment is to a large extent similar to the first, and Figure 5 therefore illustrates only a single conductive body half 114, which corresponds to the body half 14 of Figure 1 The body half 114 mates with a complemental body half 116 (see Figure 6) and the halves are assembled to a non-conductive body in the same way as in the first embodiment The mam difference between the first and second embodiments is that in the latter case, each body half defines a rectangular slot or cavity 118 which receives a contact tip in the form of a removable insert 120 A relatively small bore 122, which is larger in diameter than the diameter of the feedstock wires 124, extends from an upstanding head portion 126 of the body half 114, exiting the head portion 126 at the base thereof, just above the bottom of the cavity 118, and extending further through an end portion 128 of the body half as best seen in the sectional view of Figure 6 The insert 120 has a central groove 134 machined in each of its four adjacent faces, so that the grooves effectively run around the body of the insert The function of the insert is described in more detail below As in the first embodiment, the alignment of the bores 122 and the central groove 134 in the insert 120 is such that the feedstock wires 124 intersect at a point just within or just beyond the end 130 of a throat 132 defined between the two body halves The throat 132 in this embodiment is a tapered square/rectangular section channel, rather than a conical bore, since this is easier to machine and a round orifice for the outlet is not a prerequisite The entrance to the throat is 6 mm by 6 mm, with the exit being 6 mm by 7 mm (rectangular) The insert 120 serves the same purpose as the copper inserts 40 of the first embodiment, and typically comprises copper or copper/tungsten A through bore 136 extends through the insert from side to side, and is provided to facilitate storage of the inserts In the prototype, the insert was approximately 20 mm in length in the direction of the through bore 136, and the faces thereof were 12 mm square with the grooves 134 being approximately 1 mm deep Dimples 138 are provided on each face on either side of the central groove, and serve as locators for locking balls or grub screws 140 as shown in Figures 5 and 6 As mentioned above, the feedstock wires 124 he within the grooves 134 in use (see Figure 6), with the insert providing the force which directs the feedstock wire to its point of intersection, thus providing sufficiently good electrical contact between the feedstock wire and the insert The above described inserts have a number of advantages over the tubular inserts 40 of the first embodiment Firstly, each insert has four wear surfaces, so that it need only be rotated through 90 degrees to present a fresh surface This means that the life of the insert is relatively long It has been found that this insert does not clog as easily as is the case with drilled- hole inserts The insert is easy to manufacture, and is also easily fitted into the spray gun head from the front thereof As described in the abovementioned International patent application, the pressure and volume of the gas supplied to the interior of the nozzle are preferably adjusted to cause sonic or choked gas flow within the throat prior to generation of an arc, so that the generation of an arc within the throat has the effect of generating supersonic flow within or just beyond the exhaust end of the nozzle, which would otherwise not be attainable The resultant high flow velocity results in very fine atomisation of the molten feedstock particles and very high particle speeds as the particles are emitted from the nozzle towards a substrate It has been found that by using a flow of gas which is sonic or supersonic and positioning the arc in the area of supersonic or sonic flow, the most effective results are obtained Due to the very high temperature of the arc (typically 4000°C) the arc rapidly heats and drastically expands the gas entering the arc This rapid expansion effectively acts as a gas source in the flow field, effectively blocking the flow of gas through the arc Due to this blockage effect, the gas tends to flow around the "obstruction", similarly to the way water flows around a concrete pillar of a bridge When a supersonic gas flow exists, the gas flowing towards the "obstruction" (the arc) is not "aware" of the arc until it actually enters the arc zone and the gas is thereby forced into the arc region The resulting high pressure, high flow situation results in very fine atomisation of the molten feedstock Thus, where a supersonic basic gas flow is used, the arc can be positioned anywhere along the throat, up to and including the exit region of the throat and just beyond it In the case of a supersonic output spray, the exhausting flow has a diamond shaped supersonic flow field structure The feedstock wires 46 can be conventional solid wires, but it has been found that the use of cored wires comprising a tubular metallic body containing a cermet powder, together with the use of nitrogen or another suitable inert gas, provides excellent results The cermet material, typically an agglomerated and sintered mix of metal and carbide, tends to improve the deposition of carbide material from the thermal spray method, as the hard carbide material is contained within a metal binder Use of pure nitrogen or another suitable inert gas protects the atomised metal feedstock particles from oxidation, significantly improving the quality of the coating produced It has been found that utilising relatively short spray distances between the outlet of the nozzle and the substrate to be coated, preferably less than 100 mm and down to as little as 5 mm, further reduces the formation of oxides which detrimentally effect the quality of the coating In prior art devices, short spraying distances can lead to overheating of the substrate, but due to the very high gas flow through the spray device of the invention, the gas jet has a cooling effect, preventing overheating It is believed that the use of a relatively steep angle of intersection between the feedstock wires of at least 45°, and typically 60°, compared with the conventional angle of intersection of approximately 30°, ensures that the ends of the feedstock wires at the point of generation of the arc are more stable in the high velocity gas stream in the throat, and that the atomisation of the molten metal resulting from the arc is more uniform due to the smaller exposed surface cross section of the feedstock wires In this regard, the design of the wire guides so as not to protrude into the throat and therefore not to disturb the gas flow in the throat is also important

Claims: CLAIMS: Thermal spraying apparatus comprising a nozzle defining a throat having an inlet and an outlet and a gas flow path which is aligned with the axis of the throat, so that gas under pressure can be supplied to the inlet; at least first and second guides arranged to guide respective feedstock wires via the inlet towards a point of intersection in or adjacent an end of the throat; a power supply arranged to be connected to the feedstock wires to cause an arc in the throat between the wires; and a supply of compressed air arranged to supply air to the throat, the guides being arranged to direct the feedstock wires to the point of intersection so that they define an angle of between 45° and 90° between them. Thermal spraying apparatus according to claim 1 wherein the guides are arranged so that the angle defined between the feedstock wires is approximately 60°. Thermal spraying apparatus according to claim 1 or claim 2 wherein the nozzle may be formed from first and second body halves, each defining a portion of the throat. Thermal spraying apparatus according to claim 3 wherein the guides comprise respective bores formed in the body halves and each intersecting the portion of the body half defining a respective portion of the throat, each bore being inclined relative to the axis of the throat. Thermal spraying apparatus according to claim 4 wherein the guides include inserts receivable in the respective bores, each insert having an aperture therein through which a feedstock wire can pass, and having an inclined end face shaped complementally to the shape of the throat. 6. Thermal spraying apparatus according to claim 4 wherein the respective bores intersect locating cavities, inserts being receivable in the locating cavities so that they abut feedstock wires passing through the respective bores. 7. Thermal spraying apparatus according to claim 6 wherein the inserts are polygonal in section and define planar faces each having a locating formation for engagement with a feedstock wire in use. 8. Thermal spraying apparatus according to claim 7 wherein the inserts are square in section and define planar, rectangular faces with a groove formed in at least one face for engagement with a feedstock wire in use. 9. Thermal spraying apparatus according to claims 5 to 8 wherein the inserts comprise copper or copper/tungsten. 10. Thermal spraying apparatus according to any one of claims 3 to 9 wherein the body halves are preferably conductive, with a terminal or contact on each body half for connection to the power supply. 11. Thermal spraying apparatus according to claim 10 wherein the body halves are mounted on a non-conductive head which holds the body halves in a spaced-apart condition. 12. Thermal spraying apparatus according to any one of the preceding claims wherein the throat is rectangular in cross section.

Inventors: SEITZ MICHAEL WALTER, SEITZ, MICHAEL, WALTER
Applicants: METALSPRAY INTERNAT LC, SEITZ MICHAEL WALTER, METALSPRAY INTERNATIONAL LC, SEITZ, MICHAEL, WALTER
Application year: 2000
Main classification: B05B7
IPC codes: B05B7, C23C4