WO1999004607A1

WO1999004607A1 - Apparatus for the application of protective coatings using the plasma technique

Info

Publication number: WO1999004607A1
Application number: PCT/EP1998/004960
Authority: WO
Inventors: Edoardo Prosperini
Original assignee: Flame Spray Spa
Priority date: 1997-07-18
Filing date: 1998-07-13
Publication date: 1999-01-28
Also published as: IT1293736B1; ITMI971714A1

Abstract

The invention relates to an apparatus for applying protective coatings onto surfaces intended to operate at high temperatures or in corrosive environments, such as gas turbine componenets. The apparatus, that comprises a plasma torch for forming an electric arc by means of which the coating material in the form of a powder is applied, is supplied with an alternating current rectified and converted into direct current to be supplied to the torch, by an inverter with solid-state devices. In accordance with a preferred embodiment, the solid-state devices are of the IGBT type and the inverter is water-cooled.

Description

APPARATUS FOR THE APPLICATION OF PROTECTIVE COATINGS USING THE PLASMA TECHNIQUE

The present invention relates to an apparatus for the application of protective coatings, designed to be supplied electrically with an alternating current, comprising a plasma torch for deposition of coating material and means for converting the alternating electric current into direct current to be applied to the torch.

The abovementioned apparatus are already known and used in an increasingly widespread manner for the application of coatings or linings in general, intended to protect surfaces operating in highly corrosive environments or which, on account of their particular mechanical functions, may be subject to wear; an example of possible uses of the coatings referred to, consists in the application of special alloys onto the blades of gas turbines or onto the various parts of jet engines used in the aeronautical sector. This technique for the deposition of protective coatings is commonly known with the term "plasma spraying" and the coating materials that are used, may vary in each particular case depending on different requirements; therefore, they may range from the alloys mentioned above in connection with turbine blades to ceramic materials and the like. In short, with this technology the application of a coating onto a surface to be treated is obtained by projecting with the torch, the particles of the corresponding material which, for this purpose, is generally in the form of a powder; this is carried out at very high speeds and supplying in the same time a large quantity of thermal energy, by means of an electric arc inside the torch. The apparatuses herein considered are usually powered with the normal mains alternating current and are provided with means for converting said current into a direct current suitable for generating, and subsequently maintaining, the arc required.

In order to rectify the electric energy supplied by the mains, as well as for subsequently processing it to obtain the voltage and current values suitable for powering the torch, conventional circuits are provided; in particular, these circuits are formed by components such as thyristors.

From a technological point of view, however, it must be emphasised that for carrying out the correct application cycle of the coatings considered here, it is essential that the electric arc inside the torch have stable characteristics.

Consequently, in view of the intrinsic nature of operation of the thyristors which are not able to ensure, downstream thereof, voltage and current levels particularly uniform and constant over time, the apparatuses currently available on the market are generally overdimensioned: this means that voltage and current levels higher than those theoretically necessary for generating and subsequently maintaining the electric arc, are usually applied to the terminals of the torch so that the fluctuations over time of these parameters do not result in instability of the arc.

In this connection it is pointed out that the current values normally used are the order of 300-400 A instead of about 180 A, which is a current value more or less sufficient for the stability of the arc.

The outcome of all this is that the power levels involved during coating treatments performed with the apparatuses of the known art, are fairly high: this also results in a relevant production of thermal energy in the form of heat transmitted to the surfaces to be coated. In the light of this premise it is possible to understand why the application of coatings using the plasma technique examined here, cannot be adopted in the case of objects with a small diameter, i.e. less than 40 mm or in the case where thicknesses of less than 3 mm are involved; it is indeed obvious that in such circumstances the thermal flows and the temperatures present during the coating process could seriously damage the parts to be processed.

The object of the present invention is therefore that of finding a remedy to the situation described heretofore, by providing an apparatus for the application of protective coatings with the plasma technique, having such structural and operational features as to overcome the drawbacks mentioned above in connection with the state of the art. This object is achieved by an apparatus of the type considered at the beginning of this description, whose characterising features are set out in the claims that follow later.

The invention will be better understood in the light of the description of a preferred and non-exclusive embodiment thereof which is illustrated in the accompanying drawings wherein:

Figure 1 is a diagramatic representation of the apparatus according to this invention;

Figure 2 shows a detail of the apparatus according to the invention. With reference to the diagram of Figure 1, the apparatus for the application of protective coatings according to the invention is connected to a 380 V, three- phase, mains power supply such as that commonly available in an industrial environment; it comprises a converter for the alternating electric supply current, intended to provide at its output a direct current together with an associated voltage to be applied to a plasma torch of the type with a non-transferred arc, known per se and which is therefore not considered further here.

The abovementioned converter comprises some components arranged in series with one another and which are briefly described hereafter; it should be noted that in Figure 1 , in order to facilitate the understanding of the apparatus, further to some reference numbers and written indications, those symbols conventionally used in the art to indicate the electronic components have also been shown in various blocks.

The converter has at its input a diodes group 10 by means of which the alternating power supply current is rectified in a direct current; downstream there is a capacitors group 12 for stabilizing the current output from the group 10.

The capacitors group 12 is followed by an inverter with solid-state devices that in a preferred embodiment of the invention are of the IGBT (Insulated Gate Bipolar Transistor) type; the inverter 14 has the function of converting the direct current output from the capacitors group 12 into an alternating current with high- frequency (10-15 KHz) square wave, suitable for subsequently being brought to the desired voltage (30-80 V) for the purposes required by the apparatus, through a transformer 16 located downstream the inverter 14 itself.

At the output of the transformer 16 the square-wave alternating current is definitively rectified by a second diodes block 18. In accordance with the invention, the inverter 14 is cooled as schematically shown in Figure 1 by the corresponding cooling block coupled to it and shown in more detail in Figure 2.

The latter Figure is a front view of the inverter 14 which essentially consists of a box-shaped body 30, housing the IGBT solid-state devices mentioned and having, on one side, the electrical contacts 31, 32, while cooling fins 33 are provided on the other side. Moreover, ducts 35 for the circulation of a cooling fluid, normally water, are housed between the adjacent fins. In this case the ducts 35 are simply various sections of a single coil and, in order to enhance heat exchange between the cooling fluid and the fms 33, the ducts are embedded in a copper and silver based metal brazing alloy 36.

For completion of the description made hitherto, the apparatus according to the invention is also provided with a control console (see Fig. 1) by means of which an operator may set and/or regulate operation of the apparatus, an interface and control block devised to allow communication between the control console and the various components present in the apparatus; the latter components, in addition obviously to the converter considered above, also include a high- frequency generator, known per se, which is connected to the input of the plasma torch and which has the function of triggering the electric arc when the apparatus is started or whenever this is required. Another component connected to the control and interface block consists of a power supply block containing a series of transformers, the outputs thereof lead directly to the inputs of the alternating current converter.

Finally, the apparatus according to the invention may also be provided with means for stabilizing the current output from the converter, consisting essentially of an LC filter with capacitors and inductances. For obvious safety reasons, the present apparatus is also provided with suitable means such as emergency switches, designed to interrupt the power supply in the event of short-circuits or the like, so as to comply with the safety regulations; these means are schematically shown in Figure 1 in the form of an emergency services block active upstream the inverter power supply.

From the description of the various components of the apparatus given hitherto, it is possible to understand how the initial object thereof is achieved.

Indeed the use of the inverter with solid-state devices and in particular with IGBTs, given the performance of these electronic components, allows the coating processes to be carried out with the plasma technique, using power levels which are substantially lower than those required by thyristors conventional apparatuses.

In practice it has been possible to establish that, assuming the same voltage, the current needed to power the torch is half of the 300-400 A normally used in the known art; these results have been obtained by making the inverter and the associated IGBT devices to work at temperatures lower than 70°C.

In this connection it should be emphasized how the water-cooling system for the IGBT components helps to ensure stable maintenance of the optimum temperature conditions (<70°) required for operation thereof and, consequently, the reliability of the entire apparatus. Obviously, however, any other liquid suitable for obtaining the same results could be used instead of water.

The lower values of the current used result in lower levels of power involved in the coating process and therefore also the thermal flows arising therefrom are considerably smaller than those occurring in the known apparatuses.

On account of these effects the heat transmitted to the surfaces coated is sufficiently small, thereby allowing to use the apparatus according to the invention also for the small diameters and thicknesses referred to above.

In other words it can be said that, owing to the performance characteristics of the solid-state inverters and in particular of those of the IGBT type, greater control of the direct current to be transmitted to the plasma torch is achieved; as a consequence of this effect it is no longer necessary to overdimension the apparatus in order to be sure that the current supplied to the torch is not affected by fluctuations which make the electric arc unstable, since the inverter is now able to produce a direct current with a voltage characterized by minimum ripple (less than 5%) which is more or less negligible. The apparatus according to the invention, moreover, has particularly small dimensions compared to the known ones since the smaller amount of energy required for its powering also allows easier cooling. In this connection it is necessary to mention the effectiveness of liquid cooling of the inverter which allows the removal of significant flows of heat produced by the inverter, helping to keep the structure of the apparatus compact.

Finally, although the example of the apparatus described herein envisages the use of a plasma torch of the type with a non-transferred arc, the provision of a torch with a transferred arc should not be excluded.

Claims

1. Apparatus for the application of protective coatings, of the type electrically supplied with alternating current, which comprises a plasma torch, means (10, 12, 14, 16) for converting the alternating electric power supply current into a direct current to be applied to the abovementioned torch, characterized in that these current converting means comprise an inverter provided with solid-state devices.

2. Apparatus according to Claim 1, wherein the solid-state devices are of the IGBT type.

3. Apparatus according to Claims 1 or 2, wherein the operating temperature of the solid-state devices of the inverter is less than 70┬░.

4. Apparatus according to any of Claims 1 to 3, wherein the solid- state devices are housed in a box-shaped body (30) cooled with a cooling liquid.

5. Apparatus according to Claim 4, wherein the box-shaped body (30) is provided externally with cooling fins (33) and wherein the cooling liquid flows inside ducts (35) housed between said fins.

6. Apparatus according to any of the preceding claims, wherein a diodes group (10) and a capacitors group (12) in series with one another are arranged upstream the inverter.

7. Apparatus according to any of the preceding claims, wherein a transformer (16) and a second diodes group (18) connected in series with one another is present downstream the inverter.

8. Apparatus according to any of the preceding claims, wherein the plasma torch is of the non-transferred arc type.