RU142250U1 - PLASMOTRON FOR SPRAYING - Google Patents

Abstract

A plasma torch for sputtering comprising a cathode and anode nodes separated by an insulating insert, a plasma gas supply system into the anode arc channel and a swirler installed in the central channel of the air-cooled cathode nozzle-cathode holder, characterized in that a second swirler is made at the end of the cathode, made in the form of a truncated cone with helical grooves on the side for passing and twisting the air-powder mixture.

Description

The utility model relates to the field of engineering, in particular to arc plasmatrons with axial powder input for the manufacture of products and coatings by plasma spraying.

Various plasmatron designs are known, which differ in various ways, for example, at the place where the powder enters the plasma jet. Currently, there are four main schemes for supplying powder for plasma spraying: in the column of the arc discharge (in the arc gap); to the anode spot using a portion of the arc discharge column; behind the anode spot inside the channel of the anode nozzle; behind the cut of the anode nozzle into the external freely expanding part of the plasma jet [1, p. 170; 2, p. 61].

In most plasmatrons existing at the moment, there is a radial flow of powder behind the anode spot into the channel of the anode nozzle or a radial flow of powder under the cut of the anode nozzle, but they have the lowest powder efficiency (about 50% of particles hitting the nozzle wall do not fall into the high-temperature region plasma arc and fly out of the nozzle before it has time to melt).

The most effective plasma spraying process is realized in plasmatrons when powder is introduced into an arc discharge column or into the region of the anode spot. In this case, the highest efficiency in powder is achieved (up to 70%).

However, the disadvantage of these plasmatron designs is that they are almost very difficult to implement due to the complexity of the design solutions for introducing the powder into the region of the arc discharge, which would avoid destabilizing the arc and forming accretions on the inner wall of the anode nozzle channel.

On the other hand, three main methods are used to stabilize the plasma arc itself: axial or tangential gas flow and magnetic swirl of the arc column [3, p. 8]. Moreover, the tangential gas injection not only stabilizes the plasma arc most effectively, but if the powder particles fall into its high-temperature region it provides better conditions for their heating due to the longer residence time of the powder particles in the plasma jet, since the length of the powder particle’s path during helical motion is longer than straightforward.

Known plasmatron [4] with the axial introduction of powder into the arc column, in which the gas-powder mixture is supplied to the axial channel of the cathode assembly, exits through the central hole of the cathode and enters the arc channel of the anode nozzle. In this case, the cathode material (tungsten) is located coaxially to the flow of the gas-powder mixture and is a thick-walled tube with a small hole. The plasma torch contains cathode and anode nodes separated by an insulating insert. The cathode is hollow of tungsten, and an inert gas, argon, is used as the plasma-forming gas.

However, this form of the cathode involves the formation of slats in the zone of exit of the gas-powder mixture from the cathode, the instability of the arc discharge burning due to uneven and intense erosion of the cathode material during arc burning, and, consequently, the low life of the plasma torch.

The closest technical solution to the claimed invention is a plasma torch for spraying [5], containing the cathode and anode nodes separated by an insulating insert, and at the end of the air-cooled cathode at an acute angle α to its axis there are powder channels of diameter d in the amount of n, which are located around the thermochemical cathode insert with the possibility of passing through them, swirled by a swirler, an air-powder mixture for axially introducing it into an arc discharge column in an arc channel of a water-cooled of the anode with the simultaneous entry of the main plasma-forming gas-air, while the swirler is installed in the Central channel of the nozzle-cathode holder of the air-cooled cathode.

In this design, the axial flow of the air-powder mixture and the axial stabilization of the plasma arc are realized, which provides a high efficiency of the plasma torch for the powder (up to 80%). At the same time, the swirling of the air-powder mixture is used only to increase the uniformity of the powder supply through the holes in the lower part of the cathode to the arc discharge column. The disadvantages of this design of the plasma torch are:

- Low productivity, because with an increase in the powder content in the air-powder mixture or the feed rate of the conveying gas, the channels become clogged with powder. At the same time, if at least one channel is clogged, then the plasma jet is bent and defects appear on the coating, and if several channels are clogged, the plasma torch completely fails due to overheating, if the installation is not equipped with an automatic pressure control system in the transport gas line . Moreover, neither an increase in the number of powder holes (even above the recommended four) nor an increase in their diameter helps. Since the introduction of a large amount of powder into the arc discharge column by directed concentrated air-powder jets causes destabilization of the arc due to the unevenness of the thermal regime in its different parts. In addition, with an increase in the diameter of the powder openings, part of the powder flies out of the nozzle before it has time to melt due to the high heat capacity of the “thick” air-powder jet, which leads to a decrease in the plasma torch efficiency over the powder.

The practical productivity of such a plasma torch with a plasma jet power of 9-10 kW (for example, when spraying aluminum oxide) does not exceed 1.2 kg / hour, which corresponds to an efficiency of a plasma jet of 11-12%, although the plasma torch efficiency in powder is 75-80%;

- A small service life (30-35) hours of the anode nozzle, due to its erosion from the action of the plasma jet, since it is stabilized exclusively by the axial feed of the air-powder mixture, since its rotation relative to the central axis of the plasma torch stops when passing through the powder channels. As a result, with a small efficiency of the plasma jet, most of its energy is spent not on heating and melting the powder, but on heating and ionizing the air and on heating and erosion of the nozzle of a water-cooled anode;

- The need to manufacture a new cathode with a complex configuration when a heat-resistant insert is burned out, since the end of the cathode is destroyed as it erodes.

The problem to which the claimed utility model is directed is to increase the performance of the plasma torch and increase its life.

The problem is solved by achieving rotational-translational motion of the air-powder mixture when it is fed into the arc discharge column.

This technical result is achieved by introducing at the bottom of the cathode a second swirler, made in the form of a truncated cone with helical grooves on the side surface, providing an axially tangential air-powder mixture into the arc discharge column.

The proposed plasma torch for spraying (Fig. 1) contains the cathode 1 and the anode 2 nodes, which are separated by an insulating insert 3. The anode node 2 contains a water-cooled anode with a nozzle 4, sealed with two rubber rings 5. The anode nozzle 4 has water cooling, since the main thermal the load falls on the region of the anode spot of the arc discharge. The cathode assembly 1 contains an air-cooled cathode 6, which is fixed at the end of the nozzle-cathode holder 7, in the central channel 8 of which a swirl 9 is installed, which allows the air-powder mixture to be twisted as it passes through channel 8.

For additional twisting of the air-powder mixture and entering it in a twisted state into the arc discharge column with the simultaneous supply of the main plasma-forming gas, a second swirler 10 with a heat-resistant insert 11 is installed at the end of the air-cooled cathode 6. If the first swirl 9 is made in the form of a spiral, then the second the swirl is made in the form of a truncated cone with helical grooves on the side. The geometric dimensions of the second swirler 10, as well as the size and number of helical grooves and the angle of inclination of the helix, depend on the power, the geometric dimensions of the plasmatron and the required performance. The second swirler 10 is mounted in the lower part of the cathode 6 along the instrumental conical fit (Morse cone), which on the one hand provides the necessary strength and electrical conductivity of the connection, and on the other hand facilitates the disassembly and repair of the cathode assembly 1 when the heat-resistant insert 11 is burned out.

In the process of operation of the plasma torch, a plasma-forming gas (air or air-propane mixture) passing through the cathode assembly 1, coaxially flows around the cathode 6, cooling its outer surface. Next, the heated plasma-forming gas enters the arc channel of the anode 2, where it is heated by the arc, ionized, accelerated, and exits the nozzle 4 in the form of a plasma jet. Simultaneously with the supply of the main plasma-forming gas, the air-powder mixture passing through the swirler 9 installed in the central channel 8 of the nozzle-cathode holder 7 cools the cathode 6 along its inner surface and twists, which ensures a uniform and continuous supply of heated gas-powder mixture to the second swirler. When passing through the helical grooves of the second swirler 10, the air-powder mixture further twists and passes through the arc discharge column into the arc channel of the anode nozzle 4, where the final heating and acceleration of the particles of the sprayed powder takes place.

In this case, the air flow of the transporting gas swirled by the second swirler not only cools the working end of the cathode better, but also stabilizes the plasma jet better than with a simple axial flow of the air-powder mixture, since the plasma arc is partially stabilized by tangential air flow. And since there are no abrupt changes in the direction of movement along the entire path of motion of a rotation-stabilized air-powder mixture, the helical grooves do not clog the powder even at high feed rates and a higher powder content in it.

Thus, the proposed design of the plasma torch allows not only to improve the stabilization quality of the plasma arc and the performance of the plasma torch for the sprayed powder, but also to increase the life of the plasma torch, due to the improvement of the thermal regime of the water-cooled anode, since most of the plasma jet energy is spent on the melting and delivery of material particles to product.

Example:

A test of the known [5] and proposed design of plasmatrons was carried out on a plasma spraying unit UPN-350. The cathode of known design was made of copper, with four powder holes with a diameter of 1.6 mm with a thickness of the lower part of the cathode 12 mm The heat-resistant insert with a diameter of 2.5 mm was made of hafnium.

Spraying modes: current 100 A, voltage 100 V, plasma-forming and transporting gas-air.

The sprayed material is an electrocorundum powder of grade 25A according to GOST 28818-90 of fraction F 320 with an average particle size of 32 × 10 ^-6 m (32 μm). Material was sprayed onto a water-cooled steel mandrel for the manufacture of a high-precision heat-resistant ceramic pipe with a diameter of 300 mm and a length of 800 mm.

The maximum productivity of the plasma torch in the sprayed material was 1.18 kg / hr, the powder efficiency was 77.9%, and the plasma jet efficiency was 11.4%. The life of the anode nozzle of the plasma torch is 36 hours.

Testing of the proposed plasmatron design was carried out on the same setup and under similar spraying conditions (only the powder content in the air-powder mixture was changed).

The cathode of the proposed plasma torch design was made of copper with a thickness in the lower part of 12 mm with a conical hole along the Morse cone KM 2 in accordance with GOST 25557-2006 (taper ≈ 1:20). The swirl was made of a cylindrical copper billet with a diameter of 17.72 mm and a height of 12 mm. Previously, four helical grooves were cut on a cylindrical billet at an angle of 30 ° to the cylinder axis, the width of the grooves was 1.8 mm; height 1.2 mm. After receiving the Morse cone (KM 2) from the cylinder, the diameter of the small base of the body became 17.14 mm, and the depth of the grooves on the small base decreased to 0.9 mm. The heat-resistant insert with a diameter of 2.5 mm was also made of hafnium.

The maximum plasma torch productivity on the material was 5.15 kg / hour, the powder efficiency was 71.6%, the plasma jet efficiency was 49.8% with a plasma torch anode nozzle life of 67 hours.

As can be seen from the above results, the use of a plasma torch of the proposed design allows to increase its productivity in sprayed powder by almost 4.5 times while increasing the operating life by 1.8 times. In this case, a certain decrease in the efficiency of the plasma torch by powder (by 10%) is compensated by a more than fourfold increase in the efficiency of the plasma jet.

Claims (1)

A plasma torch for spraying, containing the cathode and anode nodes separated by an insulating insert, a plasma gas supply system to the anode arc channel and a swirler installed in the central channel of the air-cooled cathode nozzle-cathode holder, characterized in that a second swirler is made at the end of the cathode, made in the form of a truncated cone with helical grooves on the side for passing and twisting the air-powder mixture.

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