PLASMA TRANSFER WIRE ARC THERMAL SPRAY SYSTEM

Abstract

A plasma transfer wire arc thermal spray system includes 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 method of starting a plasma transferred wire arc thermal spray apparatus, comprising: directing a plasma gas stream into a nozzle passing a second electrode and exiting a nozzle orifice as plasma gas jet; switching on electrical power to form a plasma arc between a wire free end of a wire and the second electrode thereby melting the wire free end; and atomizing a molten wire by the plasma gas jet and propelling an atomized metal spray onto a surface for forming a metal coating thereon.

2. A method of claim 1, wherein certain spray parameters, in particular wire feed rate, voltage or current of power supply, flow rate and chemical composition of the plasma gas stream, are the same during start of the spray process and during the spray process.

3. A surface coated with a method of claim 1, wherein the surface is of a cylinder bore of a combustion engine.

Description

[0035] Below, the invention will be described in detail with reference to the drawing, in which

[0036] FIG. 1 is a schematic of a PTWA gun of the state of the art showing schematically relevant components of a thermal spraying gun;

[0037] FIG. 2 is a part of a spray gun according to the invention in cross-section;

[0038] FIG. 3 is a part of a spray gun according to FIG. 2 having a two-part nozzle in cross-section;

[0039] FIG. 4 is a part of another embodiment of a spray gun according to the invention in cross-section;

[0040] FIG. 5 is a part of the spray gun according to FIG. 4 having a two-part nozzle in cross-section;

[0041] FIG. 6 is an enlarged cross section of a spray gun with a nozzle comprising a non-conductive cover;

[0042] FIG. 7 is an enlarged cross section of a spray gun with a nozzle comprising a non-conductive cover and acting as second electrode;

[0043] FIG. 8 is an enlarged cross section of a spray gun with an insulating nozzle comprising a conductive cover acting as second electrode; and

[0044] FIG. 9 is a flow sheet of the PTWA steps according to the invention.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] 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.

[0053] 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.

[0054] 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 H.sub.2 or He molecules are necessary for heat transfer. Hydrogen is considered useful for heat transfer, but may have limitations. 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.

[0055] 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.

[0056] 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.

[0057] 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, ZrO.sub.2, Al.sub.2O.sub.3 or the like.

[0058] 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 ZrO.sub.2, Al.sub.2O.sub.3, SiC, B etc. The inner part 10b is supported by the nozzle carrier 31, which is made of an non-conductive material.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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.

[0063] 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: [0064] 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. [0065] 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. [0066] 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.

[0067] 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.

[0068] 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.

[0069] 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

[0070] 10 Nozzle [0071] 10a Outer part of nozzle 10 [0072] 10b Inner part of nozzle 10 [0073] 10c Nozzle body [0074] 11 Nozzle orifice [0075] 12 Plasma jet [0076] 13 Laval nozzle [0077] 14 Secondary gas [0078] 15 Plasma gas source [0079] 16 Plasma gas stream [0080] 18 Metal spray [0081] 20 Wire (first electrode) [0082] 21 Wire free end [0083] 22 Wire guide [0084] 24 Secondary gas orifice [0085] 26 Secondary gas inlet [0086] 30 Second electrode [0087] 30a Second center electrode [0088] 30b Second nozzle electrode [0089] 31 Nozzle carrier [0090] 32 Insulating body [0091] 33 Insulating cover [0092] 34 Front side of nozzle [0093] 35 Back side of nozzle [0094] 36 Center part [0095] 37 Conductive layer [0096] 40 Surface