EP0461259A1

EP0461259A1 - Plasmatron

Info

Publication number: EP0461259A1
Application number: EP90905345A
Authority: EP
Inventors: David Grigorievich Bykhovsky; Vadim Vasilievich Volchkov; Mikhail Valentinovich Karasev; Nikolai Alexandrovich Kelin; Valery Stepanovich Klubnikin; Alexandr Yakovlevich Medvedev
Original assignee: Leningradsky Politekhnichesky Institut
Current assignee: Leningradsky Politekhnichesky Institut
Priority date: 1989-12-26
Filing date: 1989-12-26
Publication date: 1991-12-18
Also published as: WO1991009702A1; EP0461259A4

Abstract

In a plasmatron, between the electrodes (1, 4), are mounted a inlet nozzle (2) and a interelectrode insert (3). The nozzle (2) is provided with additional gas channels (6) connected to the arc channel (5). The internal diameter of the interelectrode insert (3) is not less than the diameter of a circumference (8) circumscribed around the outlet opening of additional channels (6), and the ratio between the diameter of this circumference (8) and the length of the insert (3)is within 0.6-1.7. With tangential feeding of the plasma-forming gas (9) a non-cooled axial plasma jet (15) with uniform temperature and speed profiles (14) is formed behind the anode (4).

Description

Field of the Invention

This invention generally relates to plasma technology and more particularly, to air plasma spray guns. It may be employed for coating deposition, particle spheroidization, facing and etc. mainly when refractory powders with low heat conductivity and low density are used (among them aluminium oxide, circonium oxide, chromium oxide and the like).

Background of the Invention

Known in the art is an air plasma spray gun ("Gazotermicheskie pokrytia iz poroshkovykh materialov". Reference Book, Ju.S.Borisov et al. 1987, Naukova Dumka, Kiev, p.76) comprising a cathode and a nozzle-anode. Plasma-forming gas is fed tangentially into a cathode region of an electric arc and sprayed powder is supplied into the de-energized region of the plasma jet behind the anode.
Tangential feeding of the plasma-forming gas into the plasma spray gun is applied to prolong the life of the electrodes in case the gas is chemically active (air, oxygen etc.) When tangential feeding of the plasma-forming gas is employed eddy flow of plasma jet is observed along the whole of the arc channel as well as in the air plasma spray gun output in the region of powder input. As the result the powder is scattered by centrifugal forces of plasma jet from its axis region, where jet temperature and velocity are maximum into the peripheral region. Eddy jet flow results in the nonuniformity of its temperature and velocity profiles of the cross-section, i.e. to the nonuniformity of heating and of powder particle movement along the plasma jet cross-section. All this lowers the powder usage factor and impairs the properties of the sprayed coatings.
Also known is a plasma spray gun (SU, A, 430801) wherein interelectrode inserts are installed between the cathode and the anode. In this case some reduction of the tangential component of the plasma jet velocity is achieved by increasing the electric arc length. However, the above problems are not completely solved as the length of the interelectrode inserts is limited by the considerations of gaining high thermal efficiency (over 60%) of the plasma spray gun.
To do away with the eddy plasma jet flow in the region of powder input, the plasma jet flow is divided in the arc channel into two zones, eddy and axial ones. Thus, also known is a plasma spray gun (FR, A, 2600229) comprising a cathode and a nozzle-anode in which supplementary cylindrical gas channels are made around the arc channel. The gas channels are made parallel to the nozzle axis and communicate from one side with a part of the nozzle space into which the plasma-forming gas is fed, and from the other side, with the nozzle outlet region. Channels for feeding powder into the plasma jet are also made in the nozzle-anode, with outlet holes of these channels being disposed between the outlet holes of the supplementary gas channels and the outlet hole of the nozzle-anode. When electric arc is burning, the region of higher pressure is created in the arc channel. Due to that a considerable part of the plasma-forming gas fed from the cathode side does not flow through the arc channel, but flows through the supplementary gas channels. Hence, even when tangential feeding of the plasma-forming gas is employed the transfer of eddy plasma flow to the axial one will take place in the region of the plasma jet powder input. Therefore, supplementary channels provide plasma flow division into the zone of eddy flow in the cathode region of the arc channel and the zone of axial flow in the region of the arc column where the powder is fed. Axial plasma flow in the technological zone of the plasma spray gun contributes to the powder entering the axial region of the plasma jet having maximum temperature and velocity. Feeding powder into the zone of the axial plasma jet flow excludes ejection of powder particles from its axial region into a peripheral one.
The disadvantage of the above plasma spray gun (FR, A, 2600229) is that the cold, i.e. non-ionized gas zone is available between the anode wall and plasma jet and the zone size is considerable. It is connected with the fact that the plasma-forming gas fed through the supplementary channels, cones into the powder input region without interacting with the electric arc, i.e. with low temperature. In order to be fed into the high temperature axial plasma jet region, the powder of the sprayed material must at first pass through this "cold" zone thus necessitating a fairly large amount of gas transporting the powder. In its turn, it may lead to plasma jet cooling, thus lowering the sprayed material utilization factor and impairing the coatings quality.
Also known is a plasma spray gun (FR, B, 2054127) comprising a cathode, an inlet nozzle, an interelectrode insert and an anode serving as the outlet nozzle, which are installed one after another along the plasma jet propagation. The powder is introduced into the de-energized region of the plasma jet behind the outlet anode opening. In the interelectrode insert around the arc channel there are provided cylindrical gas channels which are arranged at an angle to the plasma spray gun axis. These channels communicate with the arc channel of the plasma spray gun for additional gas feeding into this channel limited by the internal surfaces of the inlet nozzle, interelectrode insert and the anode.
When tangential plasma-forming gas feeding is employed from the cathode side on the arc channel part limited by the interelectrode insert, eddy plasma flow interacts with the straight jet gas flow emerging out of the supplementary channels of this insert. As the result eddy plasma flow is destroyed and the axial plasma jet is formed at the anode outlet. It should be noted that for efficient destruction of the eddy plasma flow, the flow rate of the gas supplied through the supplementary channels is to be comparable with the flow rate of the gas passing through the arc channel from the cathode side.
The disadvantage of the above plasma spray gun is that when the gas is supplied into plasma from the supplementary channels, with the gas flow rate being rather high, the plasma is cooled. As the supplementary gas channels are made in the interelectrode insert right before the anode, the distance between the zone of the eddy plasma flow destruction and the technological zone of the plasma spray gun is small and the cooled plasma flow, when interacting with the electric arc, have not enough time to get heated up to the tempera ure required and to provide hereby uniform profiles of the plasma jet temperature and velocity along the jet cross-section in the technological zone of the plasma spray gun. As the result, powder particles movement and heating are nonuniform, particles penetration is not complete and eventually sprayed coating properties become worse and powder utilization factor is reduced.

Disclosure of the Invention

The principal object of the present invention is to provide an air plasma spray gun in which supplementary gas channels are so located with respect to the interelectrode insert, and the latter hare such relative dimensions that when tangential plasma-forming gas feeding into the air plasma spray gun is employed, the axial plasma jet flow would be created in the region of the powder input with the jet temperature required for the technological process and with the uniform temperature and velocity profiles along the plasma jet cross-section, thus increasing the powder utilization factor and improving the properties of the processed products.
With this principal object in view, there is provided an air plasma spray gun comprising a first electrode, an inlet nozzle, an interelectrode insert, a second electrode, and supplementary gas channels located around the arc channel defined by the internal surfaces of the inlet nozzle, the interelectrode insert and the second electrode, the supplementary gas channels communicating with the arc channel, wherein according to the invention, the supplementary gas channels are made in the inlet nozzle, with the internal diameter of the interelectrode insert being not less than the diameter of the circle circumscribed around the outlet holes of the supplementary gas channels and the ratio of the diameter of this circle to the length of the interelectrode insert being within the range of from 0.6 to 1.7.
The above relations between the diameter of the circle circumscribed around the outlet holes of the supplementary gas channels and the interelectrode insert dimensions were obtained by the inventors experimentally. When following these relations the interaction and mixing of the eddy plasma flow and direct jet gas flows are taking place in the region of the arc channel defined by the interelectrode insert. Therefore, at the outlet from the interelectrode insert in the region of the inlet hole of the second electrode, axial flow of plasma is already formed having uniform temperature and velocity profiles virtually without cooling of this flow.
It should be noted that the problem of smoothing down temperature and velocity profiles of the plasma jet cross section at the outlet of the anode (second electrode) is solved by installing the interelectrode insert only when purely eddy plasma flow or purely axial one is considered in the arc channel, that is when the supplementary gas channels in air plasma spray gun are not available (Zhukov M.F. et al., "Elektrodugovye generatory s mezhelektrodnymi vstavkami", 1981, Nauka (Novosibirsk), pp.25-37). As for the case when the transfer from the eddy to axial plasma flows is taking place in the arc channel of the air plasma spray gun, the principles of the arc channel construction providing efficient use of the air plasma spray gun are presently unknown.
In the series of experiments the inventors have established that if the stated relative size of the interelektrode insert is made less than 0.6, that is if the insert length is comparatively large, then the reliability and stability of the air plasma spray gun operation are affected and erosion wear of the insert arc channel is observed (double arc formation is taking place). If, on the other hand, the insert length is too small, that is its relative size is more than 1.7, then such insert length turns out to be unsufficient for providing turbulent mixing of the straight jet gas flow and eddy plasma flow on the section of the arc channel limited by the interelectrode insert and it is unsufficient either for the smoothing down of the temperature and velocity profiles of the plasma and for its heating up to the temperature required.
The internal diameter of the interelectrode insert should not be less than the diameter of the circle circumscribed around the outlet holes of the supplementary gas channels so that direct jet gas flows on the arc channel section limited by the interelectrode insert should be directed along the stream of the plasma flow. It increases the intensity of the process of smoothing down plasma temperature and velocity profiles and improves reliability of the air plasma spray gun operation.
The invention is hereinafter explained by the detailed description of its preferred embodiments taken in conjunction with the accompanying drawings.

Brief Description of the Drawings

FIG.1: is a sectional view taken longitudinally and schematically illustrating an air plasma spray gun.
FIG.2: is a sectional view taken along the line II-II of FIG.1.

Best Mode to Carry out the Invention

The proposed air plasma spray gun comprises installed coaxially a cathode face 1 (FIG.1) , an inlet nozzle 2, an interelectrode insert 3 and an anode 4. An arc channel 5 of the air plasma spray gun is defined by the internal surfaces of the inlet nozzle 2, interelectrode insert 3 and anode 4. Provided in the inlet nozzle 2 are supplementary gas channels 6 of cylindrical shape, in this embodiment the axes of said channels being parallel to the axis of the nozzle 2. These channels 6 connect the cathode section of the arc channel 5 with its section limited by the interelectrode insert 3. Mounted below the anode 4 along the stream of the plasma jet is a tube 7 serving for feeding powder material into the plasma jet.
The channels 6 are uniformly distributed along the circumference arranged concentrically to the arc channels 5 as shown in FIG.2. The number of the supplementary gas channels 6 and their diameter are selected so that the total area of their cross sections would not be less than the area of the cross section of the arc channel 5 on the section of the inlet nozzle 2.
According to the present invention, the internal diameter of the interelectrode insert 3 is not less than the diameter of the circle 8 circumscribed around the outlet holes of the supplementary gas channels 6 tangentially with respect to these outlet holes. This condition is necessary for the gas flows at the outlet of the supplementary channels 6 not to run on the face surface of the interelectrode insert 3 and flow along its wall in the direction of the plasma flow in the arc channel 5, which provides stability of the air plasma spray gun operation. The ratio of the diameter D of the circle 8 to the length L of the interelectrode insert 3 ranges from 0.6 to 1.7. The choice of the exact value of the D/L ratio is determined by the power of the air plasma spray gun which, in its turn, is determined by the required production rate of the plasma treatment process. The less is the D/L ratio, the more is the power of the air plasma spray gun.
It is evident that for increasing the power level of the air plasma spray gun supplementary interelectrode inserts can be installed between the interelectrode insert 3 and the anode 4 (FIG.1). In this case their geometric sizes are selected taking into account dependencies known to those skilled in the art of plasma technology, as at the inlet of these supplementary interelectrode insert plasma flow will have axial direction and uniform profiles of temperature and velocity.
The air plasma spray gun operates as follows. Cooling water and plasma-forming gas 9 are fed beforehand into the air plasma spray gun. An electric arc 10 is produced between the electrodes 1 and 4. The plasma-forming gas 9 supplied tangentially through the swirler (not shown in the drawings) from the side of the cathode 1 is uniformly distributed in the gap provided between the cathode 1 and the internal cone surface of the inlet nozzle 2. The electric arc 10 burning between the cathode 1 and the anode 4, creates the region of higher pressure in the arc channel 5 in the section limited by the inlet nozzle 2. Therefore most of the plasma forming gas 9 does not flow through this section of the arc channel 5, but flows through the supplementary gas channels 6 in the form of straight jet flows 11. Only a negligible part of the plasma forming gas flows through the arc channel 5 in the section of the inlet nozzle 2, thus creating eddy plasma flow 12. In the section of the arc channel 5 limited by the interelectrode insert 3, interaction and turbulent mixing of eddy plasma flow 12 and straight jet gas flows 11 are taking place. As the result an axial plasma flow 13 is formed in the section of the outlet hole of the interelectrode insert 3. This axial plasma flow is substantially uncooled and has uniform transverse profiles of temperature and velocity. Temperature and velocity profiles of the plasma flow 13 are known to be similar and are shown in FIG.1 with a line 14.
Respectively, a plasma jet 15 with uniform temperature and velocity profiles is formed in the technological zone of the air plasma spray gun, behind the nozzle-anode 4. Powder for plasma treatment is fed into the plasma jet through the tube 7. The powder is fed substantially immediately into the high temperature region of the plasma jet thus differing from the air plasma spray gun described in the FR, A, 2600229. Unlike the air plasma spray gun disclosed in FR, B, 2054127, in the proposed invention movement and heating of powder particles take place in the uniform high temperature axial plasma jet whose parametres are close to the laminar one. Powder particles are thus subjected to the effective and uniform melting and acceleration.
The invention will be further described with reference to the following illustrative examples delineating more clearly the choice of the given range of the D/L ratio.

Example 1

Corundum powder with the average particle size of 28 µm was sprayed. Diameter D of the circle circumscribed around the outlet holes of the supplementary gas channels was 10 mm. Internal diameter of the interelectrode insert was 10 mm and its length L 6.2 mm, that is the ratio D/L was equal to 1.612 thus being within the range of 0.6..1.7. Arc current was maintained equal to 200 A, plasma forming gas (air) flow rate was equal to 0.8 g/sec. Arc voltage was 150 V. The following parametres of the spraying process were obtained: output 2.7 kg/h; powder utilization factor 0.78; adhesion strength of the coating 27 MPa; porosity 2%.

Example 2.

The procedure used to prepare the corundum coating was the same as described in Example 1. The difference was in the length L of the interelectrode insert being equal to 5.8 mm, that is the ratio D/L being 1.724 i.e. beyond the 0.6...1.7 range. The following parametres of the spraying process were obtained: output is 0.9 kg/h; powder utilization factor 0.25; adhesion strength of the coating 12 MPa; porosity 15%. These parametres are much lower than those in the Example 1.

Example 3.

The procedure used to prepare the corundum coating was the same as described in Example 1. The difference was in the length L of the interelectrode insert being equal to 15.8 mm, i.e. the D/L ration being 0.633 that is within the 0.6...1.7 range. The same spraying process parametres were obtained as those in the Example 1.

Example 4

The procedure used to prepare the corundum coating was the same as described in Example 1. The difference was in the length L of the interelectrode insert being equal to 18.18 mm, i.e. the D/L ratio being 0.55, that is below the 0.6...1.7 range. It was found out that the plasma flow parameters were unstable, double arc formation occurs.
The present invention is not limited to the air plasma spray gun shown in FIGS.1 and 2. Among other things, in the present design of the air plasma spray gun the supplementary gas channels may be autonomous, i.e. the gas in them may not be fed from the cathode section of the arc channel as shown in FIG.1, but from the outside source. In this case the composition of this gas may differ from that of the plasma-forming gas. Furthermore, the axes of the supplementary gas channels may not be parallel to the axis of the arc channel but at an angle to the axis thereof, and the supplementary channels themselves may be of the shape differing from cylindrical one.

Industrial Applicability

The invention may be employed for coat spraying, particle spheroidization, facing and etc., mainly when refractory powders with low heat conductivity and low density are used (among them aluminium oxide, circonium oxide, chromium oxid and the like).

Claims

An air plasma spray gun comprising a first electrode (1), an inlet nozzle (2), an interelectrode insert (3) a second electrode (4) and supplementary gas channels (6) located around an arc channel (5) defined by the internal surfaces of the inlet nozzle (2) of the interelectrode insert (3) and of the second electrode (4), the supplementary gas channels (6) being in communication with this arc channel (5), characterized in that the supplementary gas channels (6) are provided in the inlet nozzle (2), with the internal diameter of the interelectrode insert (3) being not less than the diameter of the circle (8) circumscribed around the outlet holes of the supplementary gas channels (6), and the ratio of the diameter of this circle (8) to the length of the interelectrode insert (3) being within the range of from 0.6 to 1.7.