Plasma transfer wire arc thermal spray system

Abstract

In one or more embodiments, the invention relates to a plasma transfer wire arc thermal spray system, comprising a section for feeding a wire acting as a first electrode, a source of plasma gas providing plasma gas, a nozzle directing the plasma gas stream from the source of plasma gas to a free end of the wire, and a second electrode located in the plasma gas stream towards the nozzle. In certain instances, the nozzle is made at least partially of electrically insulating material. The thermal spray apparatus with the inventive spray gun may have a simplified and faster starting procedure and the spray nozzle can be more durable.

Claims

1. A plasma transferred wire arc thermal spray apparatus comprising: a nozzle having an orifice with an upstream and downstream side with respect to a plasma gas jet path and a secondary gas orifice proximate the orifice configured to supply a secondary gas downstream of the orifice to surround the plasma gas jet path, a consumable feed wire electrode perpendicular to the plasma gas jet path and having a free end within the plasma gas jet path on the downstream side provided by a wire guide, and a non-consumable second electrode on the upstream side, the non-consumable second electrode generating a transferred arc from the non-consumable electrode, through the orifice, to the free end of the consumable feed wire electrode, wherein the nozzle has an inner portion adjacent the orifice oriented toward the second electrode, and an outer portion adjacent the orifice oriented toward the feed wire electrode, and wherein the nozzle comprises electrically insulative material at the inner portion and extending along the upstream side within the nozzle to inhibit a non-transferred arc between the nozzle and the second electrode.

2. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle includes a nozzle body at least partially covered with electrically insulative material, wherein the electrically insulative material is arranged at a front side of the nozzle, in the nozzle orifice, or at a back side of the nozzle.

3. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle comprises electrically insulating material at the outer portion.

4. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle comprises electrically conductive material at its back side, the nozzle orifice, or both, and the conductive material is connected electrically to the second electrode or is acting as the second electrode.

5. The plasma transferred wire arc thermal spray apparatus of claim 4, wherein a nozzle body or an inner part of the nozzle body is made of the conductive material.

6. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle introduces a secondary gas around the plasma jet.

7. The plasma transferred wire arc thermal spray apparatus of claim 6, wherein the nozzle includes a plurality of spaced converging secondary gas orifices surrounding the nozzle orifice.

8. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle orifice is formed as a Laval nozzle.

9. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the nozzle is made at least partially from an insulating material selected from the group consisting of SiN, Al.sub.2O.sub.3, ceramics, glass ceramics and SiC.

10. The plasma transferred wire arc thermal spray apparatus of claim 1, wherein the apparatus includes a high voltage power source connected to first and second electrode generating direct current, alternating current, or high-frequency current.

11. A plasma transferred wire arc thermal spray apparatus for applying a coating to a surface, the apparatus comprising: a nozzle having an upstream side and a downstream side with respect to an orifice, the nozzle configured to direct a gas flow through the orifice and defining at least one secondary gas orifice proximate the orifice for directing a secondary gas to surround the gas flow downstream of the orifice, the nozzle comprising an electrically insulating material at the orifice; the insulating material being configured to directly surround the gas flow along an entire length of the gas flow's path within the upstream side of the nozzle; a first electrode comprising a feed wire fed by a roller perpendicular to the gas flow, the feed wire having a continuously fed free end in the gas flow downstream of the orifice and being configured to melt in response to an electrical arc; and a second electrode located on the upstream of the orifice and terminating at a terminal end upstream of the orifice, the first and second electrodes being configured to transfer an electrical are from the terminal end of the second electrode to the first electrode, the are passing through the nozzle orifice, and terminating on the free end of the first electrode to melt the feed wire creating a liquid coating for depositing onto a surface by the gas flow; wherein the electrically insulating material inhibits a non-transferred arc between the nozzle and the second electrode.

12. The plasma transferred wire are thermal spray apparatus of claim 11, wherein the nozzle is coated at least partially with an insulative material.

13. The plasma transferred wire arc thermal spray apparatus of claim 11, wherein an inner part of the nozzle toward the second electrode comprises an electrically insulative material, and an outer part of the nozzle radially outward of the inner part with respect to a center of the orifice, oriented toward the feed wire, on the downstream side of the nozzle orifice comprises an electrically non-insulative material.

14. The plasma transferred wire are thermal spray apparatus of claim 11, wherein the nozzle comprises an electrically insulating material and is made at least partially from SiN, Al.sub.2O.sub.3, yttrium oxide, ceramics, glass ceramics, or SiC.

15. The plasma transferred wire are thermal spray apparatus of claim 11, wherein the nozzle is a de Laval nozzle.

16. A plasma transferred wire arc thermal spray system comprising: a nozzle defining an orifice and having a gas flow path therethrough with an upstream side and a downstream side with respect to the orifice and defining a secondary gas orifice proximate the orifice having a secondary gas flow path therethrough for surrounding the gas flow path downstream of the orifice; the nozzle having a plasma gas source in fluid communication with the upstream side of the nozzle; a first electrode downstream of the orifice, the first electrode comprising a continuously fed feed wire perpendicular to the gas flow path, the feed wire having a melting point and a free end in the gas flow path downstream of the orifice, the first electrode being insulated from the nozzle with an electrically insulating material at the orifice; a second electrode upstream of the orifice, the second electrode being electrically insulated from the nozzle, wherein the electrically insulating material is disposed along the entire gas flow path within the nozzle and at the nozzle orifice on an inner part of the nozzle between the first electrode and the second electrode to inhibit a non-transferred arc or pilot arc between the nozzle and the second electrode; and a high voltage power source electrically coupled to the first and second electrodes and configured to generate an electric arc from the second electrode to the first electrode during start-up by providing up to 200 A of current without a pilot arc, the electric are passing through the orifice and having a temperature above the melting point of the feed wire to melt the feed wire at the free end.

17. The plasma transferred wire arc thermal spray system of claim 16, wherein the nozzle includes a nozzle body made entirely of insulative material.

18. The plasma transferred wire arc thermal spray system of claim 16, wherein the nozzle is a de Laval nozzle.

Description

(1) Below, the invention will be described in detail with reference to the drawing, in which

(2) FIG. 1 is a schematic of a PTWA gun of the state of the art showing schematically relevant components of a thermal spraying gun;

(3) FIG. 2 is a part of a spray gun according to the invention in cross-section;

(4) FIG. 3 is a part of a spray gun according to FIG. 2 having a two-part nozzle in cross-section;

(5) FIG. 4 is a part of another embodiment of a spray gun according to the invention in cross-section;

(6) FIG. 5 is a part of the spray gun according to FIG. 4 having a two-part nozzle in cross-section;

(7) FIG. 6 is an enlarged cross section of a spray gun with a nozzle comprising a non-conductive cover;

(8) FIG. 7 is an enlarged cross section of a spray gun with a nozzle comprising a non-conductive cover and acting as second electrode;

(9) FIG. 8 is an enlarged cross section of a spray gun with an insulating nozzle comprising a conductive cover acting as second electrode; and

(10) FIG. 9 is a flow sheet of the PTWA steps according to the invention.

(11) Reference will now be made in detail to presently preferred compositions or embodiments and methods of the invention, which constitute the best modes of practicing the invention presently known to the inventors. In one embodiment of the present invention, an improved PTWA spray gun is proved. The spray gun of the present invention is a component in a plasma transferred wire arc thermal spray apparatus that may be used to coat a surface with a dense metallic coating. The spray gun of the present invention includes an assembly that has a wire feed guide section for introducing wire into a plasma torch, a secondary gas section for introducing a secondary gas around the plasma formed by the plasma torch, and a nozzle section for confining a plasma formed by the plasma torch.

(12) With reference to FIG. 1, a schematic drawing of a thermal spraying process is shown. In thermal spraying using wire a wire 20 is continuously fed into the heat source, where the material is at least partially molten. The electrically provided heat source thereof is a plasma or arc. The PTWA has a plasma generator or gun head comprising a nozzle 10 with a nozzle orifice 11, an electrically conductive consumable wire 20 connected as first electrode and a second electrode 30. The second electrode 30 is insulated to the nozzle 10 by an insulating body 32. Electric power is applied as indicated by the power source U as a direct current, whereas the positive potential is connected to the wire 20 and the negative potential is connected to the second electrode 30.

(13) This head is normally mounted onto a rotating spindle (not shown). The wire 20 is fed perpendicularly to the center nozzle orifice 11 of the nozzle 10. The second electrode 30 is circulated by an ionized gas mixture also called gas plasma 16, provided by a plasma gas source 15. The plasma gas 16 exits the nozzle orifice 11 as a plasma jet 12 at high, preferably supersonic velocity and completes the electrical circuit when meeting the consumable wire 20 as first electrode.

(14) Transport secondary gas 14 is added through secondary gas orifices 24 in the nozzle 10 surrounding the plasma jet 12. The secondary gas 14 works as secondary atomizer of the molten droplets formed from the wire 20 and support transferring the droplets as a metal spray 18 onto the target surface. Preferably the secondary gas 14 is compressed air.

(15) Plasma transferred wire arc thermal spray apparatus is shown to include the plasma torch gun. During operation as set forth below, plasma jet 12 and metal spray 18 emerge from plasma torch gun. The assembly includes a nozzle 10 which has a cup-shaped form with a nozzle orifice 11 located at the center of the cup-shaped form. Second electrode 30, which may be constructed from any material known to the expert for this purpose, like 2% thoriated tungsten, copper, zirconium, hafnium or thorium for easy electron exit, is located coaxial with the nozzle orifice 11 and has second electrode free end. The second electrode 30 is electrically insulated from nozzle orifice 11 and an annular plasma gas chamber is provided by the nozzle internally between the second electrode 30 and the inner walls of the nozzle 10 and insulating body. In addition, a separate secondary gas inlet 26 for the secondary gas is formed within the outer section of the nozzle 10. Secondary gas inlet 26 leads to secondary gas orifices 14 in the nozzle section to provide an enveloping secondary gas stream around the plasma jet 12.

(16) Wire feed section 22 is mechanically connected to nozzle 10 and formed within the assembly. Wire feed section 22 made of isolating or non-isolating material holds the consumable wire 20. In operation of the apparatus wire 20 is constantly fed by means known in the art, like wire feed rolls through feed guide. A free wire end 21 emerges from wire feed section 22 and contacts the plasma jet 12 opposite to the nozzle orifice 11 to form a metal spray 18. In operation, metal spray 18 is directed towards a surface 40 to be coated.

(17) The positive terminal of the power supply is connected to the wire 20 and the negative terminal is connected to the second electrode 30. For certain conditions a high-frequency current can be added to the direct current during the start-up phase, but is not necessarily required. Simultaneously, the high voltage power supply is pulsed on for sufficient time to strike a high voltage arc between the second electrode 30 and the wire tip 21. The high voltage arc thus formed provides a conductive path for the DC current from the plasma power supply to flow from the second electrode 30 to the wire 20. As a result of this electrical energy, the plasma gas is intensely heated which causes the gas, which is in a vortex flow regime, to exit the nozzle orifice 11 at very high velocity, generally forming a supersonic plasma jet 12 extending from the nozzle orifice 11. The plasma arc thus formed is an extended plasma arc which initially extends from the second electrode 30 through the core of the vortex flowing plasma jet 16 to the maximum extension point. The high velocity plasma jet 12, extending beyond the maximum arc extension point provides an electrically conductive path between the second electrode 30 and free end 21 of the wire 20.

(18) A plasma is formed between second electrode 30 to wire 20 causing the wire tip to melt as it is being continuously fed into the plasma jet 12. A secondary gas 14 entering through openings 24 in the nozzle 10, such as air, is introduced under high pressure through peripheral openings 26 in the nozzle 10. This secondary gas is distributed to the series of spaced bores. The flow of this secondary gas 14 provides a means of cooling the wire feed section 22, nozzle 10, as well as providing an essentially conically shaped flow of gas surrounding extended plasma jet 12. This conically shaped flow of high velocity secondary gas intersects with the extended plasma jet 12 downstream of the free end 21 of wire 20, thus providing addition means of atomizing and accelerating the molten particles formed by the melting of wire 20 and creating the metal spray 18.

(19) FIG. 2 shows schematically a section through a torch head according to the invention used in the spraying process according to the invention. Here, the whole nozzle 10 is made of a non-conductive material such as ceramics. This results in an insulating of the whole nozzle 10 against the wire 20 respectively the first electrode. In operation, plasma gas enters into the internal chamber formed by nozzle 10 and insulating body 32 surrounding the second electrode 30. The plasma gases flow into chamber and form a vortex flow being forced through the nozzle orifice 11.

(20) An example of a suitable plasma gas can be a gas mixture consisting of 88% argon and 12% hydrogen. The heavier gas molecules, like Argon, are necessary for the kinetic energy of the plasma, whereas the light H2 or He molecules are necessary for heat transfer. Hydrogen is considered useful for heat transfer, but is dangerous due to explosion risks. So it could be replaced by He. Other gases have also been used, such as nitrogen, argon/nitrogen mixtures, noble gases and mixtures thereof, nitrogen/hydrogen mixtures as they are known to the expert in the field. The gases depend inter alia on the metal to be sprayed and on the geometry of the apparatus.

(21) Different to the prior art process, no pilot plasma is required. Power supply can be activated with full power, which leads immediately to an electric arc between wire 20 as first electrode and second electrode 30. Because of the insulated nozzle 10 there is no pilot arc between nozzle 10 and second electrode 20, which results in an significant reduction of wear of the nozzle 10. Further the start-up procedure of the process is accelerated, because no pilot phase is required. That means the spray process can start immediately without delay. Thus the spray process can start each time when the spray torch is positioned on a new surface for coating. No idling process is necessary during positioning of the torch in different bores of an engine block for example. The process can start in each bore. This reduces power consumption, wire feed and gas consumption.

(22) In FIG. 3 another embodiment of the plasma torch assembly according to the invention is shown wherein the nozzle part 10 is made of two parts 10a, 10b, whereas the outer part 10a is made of ceramics and is located between the wire 20 and the inner part 10b, thus insulating the nozzle 10 against the wire 20. The inner part 10b comprises the nozzle orifice 11. To ensure insulation of the inner part 10b towards the torch support the nozzle carrier is made of a non-conductive material, too.

(23) FIG. 4 shows another embodiment of a nozzle 10 in a plasma torch according to the invention. Nozzle 10 is formed as a Laval nozzle 13 and has a rather small diameter behind the nozzle orifice 11. Thus the plasma stream 16 will accelerate to supersonic speeds in plasma jet 12 without requiring high pressures in the plasma gas source. In this embodiment the whole body of the nozzle 10 is made from one single ceramic material, e.g. SiC, ZrO2, Al2O3 or the like.

(24) In FIG. 5 the Laval nozzle 14 from FIG. 4 is made of two parts, whereas the primary part of the Laval nozzle 13 is incorporated in the insulated ceramic outer part 10a, while the nozzle orifice 11 is located in the inner part 10b. The inner part 10b is made from copper, whereas the outer part 10a is made from insulating material as ZrO2, Al2O3, SiC, B etc. The inner part 10b is supported by the nozzle carrier 31, which is made of an non-conductive material.

(25) Due to the Laval nozzle 13 the embodiments of FIGS. 4 and 5 have a different gas management. The primary gas is ejected in a more concentrated plasma jet 12 and enveloped by a secondary gas stream, thereby leading to higher spray velocities and less overspray when compared to the geometry of FIGS. 2 and 3.

(26) FIG. 6 shows schematically a section through a torch head according to the invention similar to FIG. 2. While in FIG. 2 the nozzle 10 is made of a non-conductive material, the nozzle 10 in FIG. 6 comprises an insulating cover 33 as the electric insulation. The body of the nozzle 10c is made of a conductive material like copper or brass. The surfaces of the front side 34, of the back side 35 and in the nozzle orifice 11, i.e. all surfaces directed to the electrode 30, the wire 20 or the nozzle orifice 11 are covered with the insulating cover 33 made from a non-conductive material, preferably ceramic. This electrically insulates the plasma gas stream from the conductive nozzle body 10c and ensures that the pilot arc will not contact the nozzle 10. The nozzle body 10c is supported by the nozzle carrier 31, which preferably is made of non-conductive material.

(27) FIG. 7 shows schematically a section through a torch head similar to FIG. 6. The nozzle 10 comprises an insulating cover 33 as the electric insulation on the front side 34 and in the nozzle orifice 11. The nozzle body 10c, made of a conductive material like copper or brass, is electrically connected to the power source and is acting at its back side 35 as the second electrode 30. The center part 36 in the plasma source 15 is build as a swirl generator to obtain the swirl in the plasma stream. The nozzle body 10c is supported by the nozzle carrier 31, which preferably is made of non-conductive material. Preferably the secondary gas inlets 26 are covered with a non-conductive layer.

(28) FIG. 8 shows schematically a section through a torch head with a nozzle 10 similar to FIG. 7, but the conductivity in the nozzle 10 is the other way round. The nozzle body 10d itself is made of a non-conductive material. At its back side 35 the nozzle 10 comprises a conductive layer 37, which is electrically connected to the second center electrode 30a and therefore the conductive layer 37 is acting as a second nozzle electrode 30b. Which such nozzle 10 it is also possible to have no center electrode 30a at all.

(29) FIG. 9 describes a method of the present invention, utilizing the plasma spray torch as described above. Accordingly, the method of the present invention comprises the following: A plasma gas stream 16 is directed into the nozzle 10, passing the second electrode 30 and exiting the nozzle orifice 11 as plasma gas jet 12. Switching on the power forms immediately a plasma arc between the free end 21 of the wire 20 and the second electrode 30, thus melting the free wire end 21. The molten metal of wire 20 is atomized by the plasma gas jet 12 and propelled as atomized metal spray 18 onto the surface 40 for forming the metal coating thereon.

(30) This start-up process does not require any regulation of the process parameters. The process can start with the wire feed rate, the voltage or current of the power supply, the flow rate and the chemical composition of the plasma gas stream 16 as they are required during the spray process. This allows a significant reduction in the control effort of the start-up process, accelerates the start-up because the spray process starts immediately, and it saves wire material, gas and electrical power.

(31) In general it is preferred to introduce a plasma gas under pressure tangentially into the nozzle and creating a vortex flow around the second electrode and exiting the restricted nozzle orifice. Furthermore, the method optionally includes directing a secondary gas stream towards the wire free end in the form of an annular conical gas stream passing by the wire free end and having a point of intersection spaced downstream of the wire free end. When an interior concave surface such as a cylinder bore of a piston of a combustion engine is to be coated, the method will include rotating and translating the nozzle and the second electrode as an assembly about a longitudinal axis of the wire while maintaining an electrical connection and an electrical potential between the wire and the second electrode, thereby directing the atomized molten feedstock rotationally and coating an internal arcuate surface with the dense metal layer. Moreover, the assembly and method of the present invention are able to coat bores of diameter equal to or greater than about 3 cm. More preferably, the torch assembly of the present invention is useful in coating bores having a diameter from about 3 cm to about 20 cm.

(32) While embodiments of the invention have been illustrated and described, it is not intended that these embodiments illustrate and describe all possible forms of the invention. Rather, the words used in the specification are words of description rather than limitation, and it is understood that various changes may be made without departing from the spirit and scope of the invention.

REFERENCES

(33) 10 Nozzle 10a Outer part of nozzle 10 10b Inner part of nozzle 10 10c Nozzle body 11 Nozzle orifice 12 Plasma jet 13 Laval nozzle 14 Secondary gas 15 Plasma gas source 16 Plasma gas stream 18 Metal spray 20 Wire (first electrode) 21 Wire free end 22 Wire guide 24 Secondary gas orifice 26 Secondary gas inlet 30 Second electrode 30a Second center electrode 30b Second nozzle electrode 31 Nozzle carrier 32 Insulating body 33 Insulating cover 34 Front side of nozzle 35 Back side of nozzle 36 Center part 37 Conductive layer 40 Surface