EP1552728B1

EP1552728B1 - A thermal spraying device

Info

Publication number: EP1552728B1
Application number: EP03797778A
Authority: EP
Inventors: Per Nylen; Alice Boussagol; Roger Svensson; Gabriel Mora; Mats-Olov Hansson; Jan Wigren; Jimmy Johansson
Original assignee: Volvo Aero AB
Current assignee: GKN Aerospace Sweden AB
Priority date: 2002-09-18
Filing date: 2003-09-17
Publication date: 2006-03-22
Anticipated expiration: 2023-09-17
Also published as: ES2260689T3; SE0202765D0; SE0202765L; JP2005539144A; SE525927C2; EP1552728A1; AU2003263709A1; DE60304224D1; WO2004028222A1; ATE321437T1; RU2314878C2; JP4481167B2; DE60304224T2; RU2005111602A

Abstract

A frame element (6) of the powder injection unit projects in the flame injection direction from the end piece. The frame element partly surrounds a flame zone extending from the end piece.

Description

TECHNICAL FIELD

The present invention relates to a thermal spraying device, comprising a means for generating a flame and a means for injecting a powder into the flame, said flame-generating means comprising an end piece out of which the flame is directed towards a substrate subjected to spraying.
"Thermal spraying device" is referred to as any device for generating a flame that can be used for the purpose of depositing a coating of metal or ceramic onto a substrate, and may include plasma spraying guns of different kinds, flame jet devices, HVOF devices, et cetera.
The technical field of the invention is particularly that of applying coatings, such as thermal barrier coatings of metal or ceramics, onto substrates, in particular onto substrates such as constructional elements in aero space constructions, in particular motor parts thereof. However, the invention is not restricted to such applications, but could find a number of applications outside this relatively narrow field.

BACKGROUND OF THE INVENTION

Prior art devices for plasma spraying a powder onto a substrate comprises a plasma jet-generating means and one or more powder injection ports via which a powder is injected to the plasma jet.
A conventional such plasma jet gun, for example the widely used F4 Sulzer Metco gun, comprises an end piece through which the plasma jet is directed out of the gun and towards the substrate that is to be coated. A shoulder or knob provided with a nozzle for injection of a powder towards and into the plasma jet is attached to the end piece.
During operation, when the powder is injected into the plasma jet, melted and deposited onto a substrate, characteristic flow patterns are generated as the powder reaches the jet. Often, during normal operation conditions, a back-stream of powder may return to the nozzle, resulting in the clogging thereof. Larger particles of aggregated powder clogged in the nozzle or the end piece will sooner or later get loose and ejected into the jet, thereby causing disturbances in the spraying process, resulting in blisters and lumps being generated in the coating.
US 5,406,046 discloses a plasma spray apparatus having a flame generating means and powder injection ports. The powder injection ports are provided as radial through holes in a frame projecting from said flame generating means. The nozzle is cooled by means of a water jacket surrounding the outlet area.
US 5,854,470 shows a plasma arc torch, in which control air is discharged parallel to the flame from nozzles arranged on a frame projecting from the flame generating means. The air nozzles are used to shape and stabilize the flame.

THE OBJECT OF THE INVENTION

It is an object of the present invention to present a thermal spraying device with an improved powder yield, that is an improved efficiency, in comparison to comparable devices according to prior art. In other words, the device according to the invention should guarantee an equal or better result than prior art devices with less powder consumption.
It is also an object of the invention to present a thermal spraying device for which the tendency of having unfavourable back-streams of powder with a resulting clogging of nozzles is reduced or even eliminated.
It is a further object of the invention to obtain an improved spray-rate, that is a reduced spray time for a given amount of powder used, with a maintained satisfactory quality of the applied coating.

BRIEF DESCRIPTION OF THE INVENTION

The object of the invention is achieved by means of the initially defined thermal spraying device, which is characterised in that the powder-injection means comprises a frame element that projects in the flame ejection direction from the end piece and that the frame element at least partly surrounds a flame zone extending from the end piece. At least 1 /4, that is 90°, of the circumference around the flame zone is surrounded by the frame portion.
Thanks to the surrounding, at least partly annular shape of the frame element, an improved flow pattern is obtained, resulting in a remarkably reduced back-stream tendency. Normally, the shape and/or the measures of the inner periphery of the part of the frame element projecting in the flame direction correspond to those of the end piece of the flame-generating means. The nozzle(s), or powder port(s) is (are) located in the projecting part of the frame element, thereby directing power jets from the inner periphery of the frame element in a radial direction towards the central flame, perpendicularly to the length direction of the flame or obliquely, partly in the length direction of the flame.
According to a preferred embodiment the frame element covers at least 180°, preferably at least 270°, or, most preferably, 360° of a circumference around a flame zone extending from the end piece.
Preferably, the frame element defines a ring-shaped element and is designed as a continuous ring with a continuous inner periphery extending over and covering 360°. It should however be understood that, as an alternative, it may be constituted by two or more discrete ring parts, each defining a sector of the frame element. The discrete ring parts need not form a frame element that has a continuous inner periphery, but could as well define a discontinuous, broken ring, thereby extending over and covering at least 180°, preferably at least 270° in the peripheral direction thereof. One or more of the nozzles or powder ports may be arranged between individual such ring parts or ring segments.
Preferably, the frame element has an inner periphery, the cross section of which corresponds to the geometry of the cross section of the inner periphery of the end piece. The cross section should present rotational symmetry.
According to the invention the part of the frame element that projects beyond the end piece in the flame ejection direction comprises at least one radial, open through hole. Such holes provide-air cooling of the flame to stabilise the flow in the powder injection area. Apart from the openings defined by the holes on the inner periphery of the frame element and possible powder injection nozzles, the inner peripheral surface of the projecting part thereof is generally even, presenting no projections or the like that would negatively disturb the flow pattern of the flame and injected powder.
Preferably the frame element comprises a plurality of radial, open through holes, normally at least 6, preferably more than 10 radial, open through holes. The holes should be evenly distributed around the periphery of the frame element. Thereby, uniform flow conditions can be achieved all around the central flame or jet.
According to one embodiment, the end piece has an inner width or inner diameter d and the frame element projects a distance p. The relationship between d and p is: 0,5d < p < 2d. This has proven to be a preferred relationship at least for end pieces with an inner diameter of 6 or 8 mm, and is likely to be the preferred one also for most other diameters used in practice.
When the end piece has an inner width of or inner diameter d and the projecting part of the frame element has an inner corresponding width or diameter D. D is equal to or larger than d, and preferably D < 1,2 d. This relationship has been proven suitable at least for end pieces with an inner diameter d of 6 or 8 mm.
According to a further preferred embodiment of the invention two or more powder injection ports for directing a powder towards the flame are distributed around the inner periphery of the frame element. Thereby an improved and more even powder distribution within the plasma jet is achieved. Since the injected powder is distributed via a number of nozzles, a larger amount of powder per time unit can be injected into the plasma without the same problem with instability that would occur if only one nozzle or port was used.
Preferably, the powder injection ports are evenly distributed around the periphery of the frame element. Thereby, an even distribution of the powder in the plasma is promoted. Preferably, the device comprises or is connected to any means for distributing the powder evenly among the powder injectors.
According to one embodiment each powder injection port comprises a nozzle that is inserted in a radial hole or opening through the frame element, and at least one or more of the open through holes are adapted for accommodation of such a nozzle therein. Accordingly, the frame element is equipped with a plurality of radial through holes, extending from the outside to the inside of the frame element and permitting any medium such as air to pass through them. At least some of the holes are adapted to accommodate a nozzle or the like therein. For example some holes might be provided with a thread for engagement with a nozzle, resulting in a more versatile device.
The frame element should be removably attached to the end piece. For example, a part of the frame element could be adapted to be pulled onto the outer periphery of the end piece, that part of the frame element being provided with fastening screws that penetrate its wall. Any kind of clamp or the like could also be used in order to fix the frame element in relation to the end piece.
According to the invention the flame generated by the flame-generating means is a plasma jet, formed by letting a gas flow in an annular path between a cathode and an anode. Typically, the temperature of such a jet can reach 15 000 °C and the powder introduced into the plasma can obtain a speed of up to 500 m/s as it is melted and accelerated by the plasma jet before hitting a substrate.
Further features and advantages of the present invention will be presented in the following detailed description of a preferred embodiment of the inventive device.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the present invention will now be described with reference to the annexed drawings on which:

Fig. 1 is an enlarged view of an end piece of a thermal spraying device provided with a powder injection means according to prior art,
Fig. 2 is an end view of the device shown in fig. 1,
Fig. 3 shows a first embodiment of the device according to fig. 1, provided with a powder injection means that comprises two opposite nozzles arranged on a frame element formed by two discrete ring parts,
Fig. 4 is an end view of the device shown in fig. 3,
Fig. 5 shows a preferred second embodiment of the thermal spraying device according to the invention,
Fig. 6 is an end view of the device shown in fig. 5,
Fig. 7 is a cross section of the frame element shown in figs. 5 and 6, and
Fig. 8 is a cross section of atypical plasma spraying device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Fig. 1 and 2 shows an end piece 1 of a thermal spraying device, more precisely a plasma spraying device, according to prior art. The device comprises means 2 for generating a flame, here a plasma jet. Such means includes a cathode and an anode, as shown in fig. 8, arranged in a way known per se and defining an annular path between them. It also comprises a means 3 for injecting a powder into the plasma jet.
The end piece 1 comprises a tube with circular cross section and may also include the anode. The powder injection means 3 comprises a shoulder or knob 4 attached to an the end piece 1. The shoulder or knob 4 comprises a radial hole penetrated by a powder injection nozzle 5 that defines a port for powder injection towards the flame.
Fig. 2 indicates how only a small part of the flame is actually taken advantage of upon injection from the single nozzle 5. Due to the small angular sector covered by the shoulder or knob 4 a back-stream of returning partly melted powder will be generated, resulting in unwanted build up on the nozzle 5.
Fig. 3 and 4 show a first embodiment of the thermal spraying device according to the invention. A flame, or plasma jet, is generated by the same means as described in figs. 1 and 2. A frame element 6 formed by two discrete ring parts 7,8 covers approximately 180° of a circumference around-the flame. In other words, it covers 50% of the circumference that a corresponding continuous ring would have covered; Here each ring part 7,8 defines a sector that covers at least 90° of said circumference.
The frame element 6 projects and prolongs the end piece 1 in the longitudinal direction thereof, which is the same as the flame direction. Each ring part 7,8 is provided with one or more radial holes, at least one of which is penetrated by a powder injection nozzle 5. Each nozzle 5 may be arranged and directed as described earlier for the prior art nozzle in figs. 1 and 2. Thanks to the double nozzle arrangement and the presence of the frame element 6 the tendency of having powder back-flow is suppressed and a more stable and better used plasma jet is achieved. Accordingly a higher powder yield is achieved as compared to prior art.
In figs. 5-7 a preferred second embodiment of a device according to the invention is presented. The device comprises means for generating a flame, preferably as described earlier with regard to figs. 1-4. It differs from the embodiment shown in figs. 3 and 4 in that it comprises a frame element 6 formed by one single, continuous ring.
The ring 6 is detachably attached to and projects a distance p beyond the end of the end piece 1 in the plasma jet direction. The end piece 1 has an inner diameter d, whereby 0,5d<p<2d, and preferably d ≅ p.
The ring 6 has a circular inner periphery with a diameter D which is approximately equal to the inner diameter d of the end peace 1. More precisely, as in this case, the inner diameter D corresponds to the outer diameter d of the end piece 1 plus the thickness of the wall of the end piece 1.
The frame element 6 further comprises a plurality of radial through holes 9 evenly distributed around the periphery of the projecting part thereof. At least some of the holes 9 are provided with a thread for engagement with a powder injection nozzle 5 that may be accommodated therein. Alternatively a separate set of holes, in line with or not in line with the holes 9, could be arranged to act as nozzle accommodation holes or powder ports.
The holes 9 are generally in line with each other around the inner periphery of the ring 6. Those of the holes that do not accommodate a powder injection nozzle 5 contribute to a radial communication between the interior an exterior sides of the ring. Normally, the exterior comprises air atmosphere, and the holes 9 act as air cooling holes that further stabilise the jet and also counteract powder back-flow towards the nozzles 5.
Preferably, the nozzles (or powder ports) are evenly distributed (at the same angular distance from each other) around the inner periphery of the frame element 6. The number of nozzles 5 may vary, but it has been shown in computer simulations that three nozzles is often preferred, resulting in a advantageous powder yield (low loss of powder) and stable flow conditions.
In order to be easily attached to and detached from the end piece 1, the powder injection means 3, here the frame element 6, is adapted to be pulled onto the end of the end piece 1 and fixed in position by means of fixation screws 16. Other connection means, such as clamps or the like, could be used as an alternative.
A plasma spraying device according to the invention is schematically shown in fig. 8. The device comprises an anode 10 surrounding a central cathode 11 and forming a nozzle or annular passage for gases, this kind of device being well known in the state of art and needing no further detailed explanation. An electric arc or plasma jet 12 is generated by means of controlling the voltage difference between the anode 10, cathode 11 and letting gases flow through the nozzle. The device further comprises a means 3 according the invention for introducing a stream of powder particles 13 into the plasma jet12. The jet 10 is directed towards a substrate 15 and will transport the powder particles 13 towards the substrate 15 while at the same time at least partly melting said particles 13.
A particular advantage of the invention is that a frame element 6 as described above could be used to replace the single shoulder and nozzle arrangement of prior art on widely used plasma jet guns available on the market today, such as the F4 gun, without extensive work, resulting in improved powder yield, plasma jet efficiency and stability, and less risk of powder port clogging.
It should be realised that the above presentation of the invention has been made by way of example, and that alternative embodiments will be obvious for a man skilled in the art. However, the scope of protection claimed is defined in the patent claims supported by the description and the annexed drawings.

Claims

A thermal spraying device, comprising a means (1,2) for generating a flame and a means (3) for injecting a powder into the flame, said flame-generating means (1,2) comprising an end piece (1) out of which the flame is directed towards a substrate subjected to spraying, said powder-injection means (3) comprising a frame element (6) that projects in the flame ejection direction from the end piece (1), and said frame element at least partly surrounding a flame zone extending from the end piece (1), characterised in that at least the part of the frame element (6) that projects beyond the end piece (1) in the flame ejection direction comprises at least one radial, open through hole (9) for providing air cooling of the flame to stabilise the flow in the powder injection area.
A thermal spraying device according to claim 1, characterised in that the frame element (6) covers at least 90°, preferably 180°, of a circumference around a flame zone extending from the end piece (1).
A thermal spraying device according to claim 1, characterised in that the frame element (6) covers at least 270° of a circumference around a flame zone extending from the end piece (1).
A thermal spraying device according to any one of claims 1-3, characterised in that the frame element (6) has an inner periphery, the cross section of which corresponds to the geometry of the cross section of the inner periphery of the end piece (1).
A thermal spraying device according to any one of claims 1-4, characterised in that the frame element (6) defines a ring-shaped element.
A thermal spraying device according to any preceding claim, characterised in that it comprises a plurality of radial, open through holes (9).
A thermal spraying device according to claim 6, characterised in that it comprises at least 6, preferably more than 10 radial, open through holes (9).
A thermal spraying device according to any one of claims 6-7, characterised in that the open through holes (9) are evenly distributed around the periphery of the frame element (6).
A thermal spraying device according to any one of claims 1-8, characterised in that the end piece (1) has an inner width or inner diameter d and that the frame element (6) projects a distance p, and that 0,5d < p < 6d, preferably 0,5d < p < 2d.
A thermal spraying device according to any one of claims 1-9, characterised in that the end piece (1) has an inner width of or inner diameter d and that the projecting part of the frame element (6) has an inner corresponding width or diameter D, and that D is equal to or larger than d, and that preferably D < 1,2 d.
A thermal spraying device according to any one of claims 1-10, characterised in that two or more powder injection ports (5) for directing a powder towards the flame are distributed around the inner periphery of the frame element (6).
A thermal spraying device according to claim 11, characterised in that the powder injection ports (5) are evenly distributed around the periphery of the frame element (6).
A thermal spraying device according to claim 11 or 12, characterised in that each powder injection port (5) comprises a nozzle that is inserted in a radial hole or opening through the frame element, and that at least one or more of the open through holes (9) are adapted for accommodation of such a nozzle (5) therein.
A thermal spraying device according to any one of claims 1-13, characterised in that the frame element (6) is detachably attached to the end piece (1).
A thermal spraying device according to any one of claims 1-14, characterised in that the flame generated by the flame-generating means is a plasma jet.