Plasma torch head for internal coating and method for manufacturing the torch head

Abstract

The present invention relates to a plasma torch head for the internal coating of rotationally symmetric, asymmetric, or free-form surfaces in concave or convex cavities of components with a distance of 40 mm or more between the surfaces to be coated using a plasma spray process. The plasma torch head includes a cathode (217) with a tip (305) having a surface roughness of Ra 0.2 μm or less, preferably with a groove extending in the cathode's axial direction, and is secured by a self-locking screw thread in the cathode holder (213). Gas-tightness against plasma gas is ensured by one or more high-temperature resistant O-rings (215), one of which prevents gas leakage between the cathode holder (213) and the insulating ring (207), and the other O-ring prevents gas leakage between the insulating ring (207) and the anode (205). The cathode holder (213) is designed with respect to the plasma gas flow such that the plasma gas exit velocity at the holes above the center of the cathode (303) is lower than that at the holes below the center. A plasma torch head constructed as disclosed reduces voltage fluctuations over the torch head's entire operating life, extending its potential useful life.

Description

[Technical Field]

[0001] The present invention relates to a plasma torch head for internal coating of rotationally symmetric, asymmetric, or free-form surfaces in concave or convex cavities using a plasma spray process. In internal coating, a coating lance with a torch head attached to the lance is inserted into the component to be coated to coat the rotationally symmetric, asymmetric, or free-form surface within the component's cavity. To coat those surfaces of non-rotationally symmetric components, the lance is rotated with its attached torch head, while to coat those surfaces of rotationally symmetric components, either only the component itself or both the lance and the component to be coated are rotated. [Background technology]

[0002] Plasma torch heads for applying arc wire or plasma internal coatings are well known and are used in the industry, for example, to coat the bores of piston engines. A typical coating lance consists of a rotating coating lance with a torch head attached or integrated at one end. Within the torch head, a plasma jet is generated by an arc between an anode and a cathode and plasma gas. The plasma jet melts injected powder particles or wire. The molten particles are accelerated by the plasma jet and impinge on the surface to be coated (substrate), forming a layer on the substrate. In an internal coating process, the plasma jet moves radially through the bore or cavity from the torch head toward the substrate. The coating lance rotates concentrically or eccentrically with respect to the bore axis during coating, and moves continuously up and down along the axis of the bore. This applies the coating to the substrate in a spiral motion. When coating cylinder bores in piston engines, the torch head must be inserted into bores with diameters ranging from 40 to 110 mm and must therefore be constructed in a compact manner to ensure sufficient spray distance between the torch head and the bore surface. This is particularly true for plasma torches used in plasma spray processes that use powder as the coating material, as these torches are more complex and therefore longer relative to the cathode axis than torches used in arc wire spraying.

[0003] The prior art approach outlined in U.S. Pat. No. 4,970,364 solves this problem by generating a plasma jet along the axial direction of the bore and then deflecting it 45° from the bore axis using a nozzle. This axial configuration of the torch head relative to the bore axis provides a longer length for plasma generation, resulting in a larger volume available for the torch head and allowing the use of a longer cathode. Media supply to such a torch head is easy to implement, and cooling is simplified due to the length of the structure. A drawback of this design is that the plasma jet can only be deflected at a limited angle relative to the bore axis. In the solution described in U.S. Pat. No. 4,970,364, the angle between the axis of the bore to be coated and the plasma jet is limited to less than 45°. Layers produced using this type of plasma spray torch exhibit undesirable characteristics, such as increased porosity and unfused particles embedded in the applied coating. These undesirable characteristics are caused by turbulence within the plasma jet due to the plasma jet's deflection within the nozzle. Furthermore, the large angle between the bore surface and the plasma jet causes a higher percentage of coating particles to bounce off the bore surface rather than adhere to it, reducing the efficiency of the coating process. Deflection of the plasma jet through the nozzle also causes significant wear on the torch head components, resulting in a shorter lifespan of the torch head nozzle.

[0004] WO 2018 / 219497(A1) addresses this problem by using a known torch head that generates a plasma jet that is nearly perpendicular to the bore surface. The plasma jet is generated radially relative to the bore axis and exits from the front face of the torch head toward the bore surface. The torch head is attached to a lance by a curved intermediate piece, which is guided concentrically with the bore axis. To maintain a constant spray distance between the front face of the torch head and the surface to be coated for different bore sizes, the eccentricity of the torch head relative to the lance can be affected by using different intermediate pieces, each of which creates a different offset between the axis of the lance, and therefore the axis of the bore, and the front face of the torch head. In the above disclosure, the optimal spray distance between the front face of the torch head and the bore surface for different bore diameters is achieved by applying the appropriate intermediate piece. In the case of WO 2018 / 219497(A1), the plasma jet is nearly perpendicular to the bore surface, improving layer quality and coating process efficiency and reducing nozzle wear compared to the solution presented in U.S. Pat. No. 4,970,364. However, generating the plasma jet radially relative to the bore axis within the torch head poses design constraints. Because the plasma jet is centered on the axis of the rotationally symmetric cathode, which extends radially relative to the bore axis, the cathode must be constructed accordingly short to ensure optimal spray distance even in small bores. This is particularly true when the torch head needs to be inserted into bores with diameters of 40 to 45 mm. With the torch head's structure, thermal stress and repeated on-off cycles can lead to voltage fluctuations and gas leakage between the torch head components. These wear phenomena are caused by loosening of the cathode, which is fastened to the cathode holder using a screw thread, and gas leakage between the insulating ring and insulating plate and / or between the cathode holder and the cathode. In addition to the effects described above, the inventors have observed that the surface roughness and orientation of structures present on the surface tip affect voltage fluctuations during the plasma spray process.The cathodes used in this prior art approach exhibit grooves transverse to the cathode's axis of rotational symmetry and a surface roughness of Ra 0.4 or greater due to the manufacturing process. The cathode surface roughness, especially the transverse grooves, leads to voltage fluctuations that increase over the cathode's operational life. The increased voltage fluctuations result in undesirable layer characteristics, such as increased porosity. Gas leaks cause undesirable turbulence in the plasma that can be detected in the generated layer.

[0005] Object of the invention The object of the present invention is to create a torch head for plasma coating lances used for internal coating, which exhibits reduced voltage fluctuations and a longer lifespan at 200-800 rpm compared to the solution presented in WO 2018 / 219497 A1. Summary of the Invention

[0006] The object of the present invention is achieved by a torch head having the features of independent claim 1. The dependent claims each relate to particularly advantageous embodiments of the invention. The torch head of the present invention features a cathode, the tip of which has a surface roughness of Ra 0.2 μm or less and preferably has an axial groove relative to the cathode axis. The cathode is fixed in a cathode holder with a rounded inlet channel for the plasma gas using a self-locking Spiralock screw thread. Gas-tightness between the cathode holder and the cathode is achieved by an insulator with an O-ring.

[0007] The above features reduce voltage fluctuations over the life of the torch head, extending its life. The low surface roughness of the cathode tip, the axial grooves in the cathode tip, and the prevention of cathode loosening reduce voltage fluctuations and extend the life of the torch head. Furthermore, the gas seal between the insulator and the anode and the optimized flow of plasma gas result in increased torch head life and reduced plasma turbulence. [Brief explanation of the drawings]

[0008] [Figure 1]1 illustrates a process for internal coating using a plasma torch head. [Figure 2] 1 shows a cross section of a plasma torch head of the present invention for internal coating. [Figure 3] 1 shows a cathode used in the plasma torch head of the present invention. [Figure 4] 1 shows a cathode holder of the present invention optimized with respect to plasma gas flow. [Figure 5] 1 shows the voltage profile of a known plasma torch head for internal coating. [Figure 6] 1 shows the voltage profile of the plasma torch head of the present invention for an internal coating. DETAILED DESCRIPTION OF THE INVENTION

[0009] The invention and its positive influence on voltage fluctuations and therefore layer quality will now be illustrated by way of a non-limiting example on the basis of the drawings.

[0010] As shown in Figure 1, a plasma torch head (103) attached to a lance (101) rotates within a cavity that can be rotationally symmetric, asymmetric, or comprise a free-form surface with a concave or convex shape. In the plasma torch head, a plasma jet (107) is generated using an electric arc with the aid of a plasma gas. Powder particles (109) transported by a carrier gas are injected into the plasma jet, melted, and accelerated toward a substrate (111) where they are deposited to form a layer. If the cavity to be coated is a bore, the spraying distance (105) can be adjusted to a value between 20 mm and 65 mm by offsetting the axis of the lance from the axis of the bore. If a non-rotationally symmetric inner surface is coated with the plasma torch head, the spraying distance can be adjusted by the path of travel of the plasma torch head.

[0011] FIG. 2 shows a plasma torch head according to the present invention. As shown in FIG. 2, the torch head is attached to a lance or intermediate piece (231), and the injector (201) is connected to a powder-gas hose (233) through which a powder-gas mixture is supplied. An electric arc is formed between the cathode (217) and the anode (205) by an ignition spark. Plasma gas (217) is supplied through the cathode's gas channel, and the electric arc is converted into plasma (203). The plasma melts powder particles (227) transported through the injector (201) using a carrier gas. The plasma is surrounded by a shroud gas, which reduces oxidation of the coating material during the coating process. The shroud gas is supplied through the gas channel (225). The cathode (217) and anode (205) are electrically insulated except for the cathode tip (305). In a preferred embodiment of the torch head, electrical insulation is achieved using an insulating plate (223) and an insulating ring (207) concentrically attached to the cathode (217). In some embodiments, the insulating plate is equipped with cooling gas channels opening toward the side of the torch head and / or toward the opposite side of the plasma jet to cool the substrate during the coating process. Due to thermal stresses, the insulating ring (207), consisting of two surfaces extending concentrically with respect to the cathode axis and which may be concave or convex relative to the cathode axis, is made of a ceramic material. In a preferred embodiment, the insulating plate (223) has two parallel surfaces for mounting the anode and cathode holder (213), as well as holes for accommodating the insulating ring and for passing cooling water between the anode and cathode holder. To enable replacement of individual worn components, the cathode (217) and cathode holder (213) are preferably separate components, with the cathode screwed into the cathode holder. The cathode holder contains an inlet, not shown in FIG. 2, for plasma gas and a plasma gas channel (221) through which the plasma gas is transported to the cathode (217).To ensure that the plasma gas exits only through the gas channel (219) of the cathode (217), in a preferred embodiment, seal rings (215) are disposed axially of the cathode between the cathode holder (213) and the insulating ring (207) on one side, and between the insulating ring (207) and the anode (205) on the other side. In a preferred embodiment, the seal rings (215) are made of a heat-resistant material that allows for operating temperatures up to 300°C. The cathode (217) is secured within the cathode holder (213) using self-locking fasteners, preferably with self-locking Spiralock threads. The cathode holder (213) and anode (205) are cooled by water supplied through the channel (209). The cooling water is supplied to the anode (205) from a lance using a water inlet (not shown in FIG. 2) and exits the torch head through an outlet in the insulating plate (223), also not shown in FIG. 2. In the embodiment shown in Figure 2, the anode (205) includes an insert (229) made of a copper alloy. In some embodiments, the anode may include cooling gas channels that open toward the substrate. In some embodiments, an electrically insulating cover, not shown in Figure 4, is applied to the torch head opposite the end of the lance and / or to the cathode holder to prevent discharge in the event of impact with the surface to be coated.

[0012] The cathode shown in FIG. 3 comprises a cathode body (307) and a cathode tip (305) inserted into the cathode body. The cathode body (307) has a thread (301) for mounting within a cathode holder, which is elongated compared to prior art approaches such as those described in WO 2018 / 219497 A1. In a preferred embodiment, the length of the thread (301) on the cathode, measured along the axis of rotational symmetry of the cathode, is 2.5 to 5 mm, preferably 3 to 4 mm, and even more preferably 3 to 3.5 mm. Plasma gas is supplied through a bore (303) disposed radially relative to the axis of the cathode. In a preferred embodiment, the cross section of the bore is circular. However, the cross section may be non-circular, such as oval or elliptical. In a preferred embodiment, an axial cross section through the cathode tip (305) has a conical or elliptical paraboloid shape. The surface roughness of the cathode tip (305) is achieved using a post-turning or milling process to a maximum value of Ra 0.2 μm. The post-treatment of the cathode tip is preferably performed by lapping, but can also be performed by abrasive flow machining or polishing. During the polishing process, grooves introduced radially relative to the cathode axis by the mechanical pre-treatment are removed. In an alternative embodiment of the post-treatment, targeted surface structures such as grooves or ridges can also be achieved axially relative to the cathode axis, further reducing voltage fluctuations.

[0013] As shown in Figure 4, the cathode threads into the cathode holder bore (401), which is equipped with a self-locking fastener, in this embodiment, a self-locking Spiralock thread. The self-locking threads can be provided on either the cathode holder, the cathode, or both the cathode holder and the cathode. In a preferred embodiment, the length of the thread (301) applied to the cathode, measured along the cathode's axis of rotational symmetry, is 2.5 to 5 mm, preferably 3 to 4 mm, and even more preferably 3 to 3.5 mm. The self-locking threads prevent the cathode from loosening over the course of operation due to thermally induced movement of the cathode holder. Preventing the cathode from loosening, especially after a period of operation, can reduce voltage fluctuations, thereby extending the torch head's lifespan. Plasma gas is supplied to the cathode through a CFD-optimized channel (405), which leads to an annular recess (403). From this recess, the plasma gas flows through the cathode bore (303). CFD simulations showed that the plasma gas exit velocity through the cathode bore (303), located above the cathode's central axis with respect to the lance's axial direction, should be lower than the plasma gas exit velocity through the cathode's lower half bore. The above-mentioned distribution of the plasma gas flow reduces turbulence. The reduction in turbulence of the incoming plasma gas results in improved layer quality. To manufacture the complex shape of the channel (405) in the cathode, additive manufacturing processes can be used to create the components layer by layer.

[0014] The voltage profile shown in Figure 5 for a known plasma torch head used for internal coating shows increased voltage fluctuations after a 20-hour operating period. Starting with an acceptable voltage fluctuation of ±2 volts around the nominal voltage of 40 volts, after 20 hours, it can be observed that the low-voltage measurement falls below the acceptable low-voltage level of 38 volts. Therefore, the end of life of the known plasma torch head is reached after 20 hours of operation. If the acceptable voltage fluctuation of ±2 volts is exceeded, the torch head must be modified, and the anode and / or cathode must be replaced. The short-term voltage spikes above 44 volts shown in Figures 5 and 6 correspond to the higher ignition voltage applied during the ignition process. The short-term voltage drop from below 36 volts to 0 volts indicates that a single operating cycle beginning with a short-term voltage spike above 44 volts ends with the gun shutting down.

[0015] Figure 6 shows that the use of the plasma torch head of the present invention for internal coating significantly reduces voltage fluctuations during a single operating cycle. The voltage fluctuations of the plasma torch head of the present invention are limited to less than ±1 V within a single operating cycle, over a period of more than 100 operating hours. A comparison of the number of operating cycles in Figures 5 and 6 also shows that the number of operating cycles can be increased using the plasma torch head of the present invention compared to using a plasma torch head of the prior art. This indicates that the use of the plasma torch of the present invention not only reduces voltage fluctuations compared to known plasma torch heads for internal coating, but also extends the lifespan to more than 100 operating hours.

[0016] A torch head for the internal coating of rotationally symmetric, asymmetric, or free-form surfaces within concave or convex cavities of components, where the distance between the surfaces to be coated is 40 mm or greater, using a plasma spray process, is disclosed. The torch head includes an axisymmetric, preferably rotationally symmetric, anode (205) and cathode (217). The anode (205) and cathode (217) are electrically isolated by an insulating plate (223) and insulating ring (207), except for the cathode tip (305). The cathode tip (305) has a surface finish of Ra 0.2 μm or less and, in a preferred embodiment, features a groove in the axial direction of the cathode. The cathode is secured within a cathode holder (213) using a self-locking fastener, preferably a self-locking Spiralock thread. In an embodiment, the self-locking thread can be applied to the cathode and / or the cathode holder. The channels for supplying plasma gas to the plasma in the disclosed torch head are designed so that the plasma gas exit velocity in the cathode hole (303) located above the cathode's midpoint in the lance axial direction is lower than in the hole below the midpoint. Airtightness between the cathode (217) and the anode (205) is ensured by an O-ring, preferably multiple O-rings, particularly preferably two O-rings (215).

[0017] A method for manufacturing a torch head is disclosed in which a surface roughness of Ra 0.2 or less is achieved on the cathode tip (305), and preferably grooves are introduced in the axial direction of the cathode tip.

[0018] Additionally, a method is disclosed for manufacturing the cathode holder (213) using an additive manufacturing process, preferably by layer-by-layer construction.

Claims

1. 1. A torch head for the internal coating of concave or convex cavities of rotationally symmetric, asymmetric or free-form surfaces of components using a plasma spray process, the distance between the surfaces to be coated being 40 mm or more, comprising: an anode (205), and an axially symmetric, preferably rotationally symmetric cathode (217); The anode (205) and the cathode (217) are electrically isolated from each other by an insulating plate (223) and an insulating ring (207) except for a cathode tip (305), the cathode tip (305) has a surface roughness Ra of 0.2 μm or less, and the cathode (217) is fixed to a cathode holder (213) using a self-locking fastener.

2. 2. The torch head of claim 1, wherein the cathode holder (213) and / or the cathode (217) are provided with a self-locking screw thread.

3. 3. The torch head according to claim 1, wherein the plasma gas exit velocity in the bore (303) of the cathode above the axis of rotational symmetry of the cathode in the lance axial direction is lower than in the bore below the center.

4. The torch head of claim 1, wherein the cathode tip (305) includes a groove in the axial direction of the cathode.

5. 2. The torch head according to claim 1, wherein the gas-tightness between the cathode (217) and the anode (205) is ensured by one O-ring, preferably by several O-rings, particularly preferably by two O-rings.

6. 2. The method of manufacturing a torch head according to claim 1, wherein a surface roughness of Ra 0.2 or less is achieved during the manufacture of the cathode tip (305).

7. 7. A method for manufacturing a torch head according to claim 4 or 6, characterized in that during the manufacture of the cathode tip (305) a groove is introduced extending in the axial direction of the cathode tip.

8. 2. The method for manufacturing a torch head according to claim 1, characterized in that the cathode holder (213) is manufactured by an additive manufacturing process, preferably by layer-by-layer construction.

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