DE102008022719A1 - Plasma burner for plasma applications e.g. powder coatings and welding and cutting, comprises toroidal cathode, plasma nozzle with connection to mounting plate comprising cooler, gas intake ports, and electrode acting as toroidal anode - Google Patents

Abstract

The plasma burner for plasma applications e.g. powder coatings and welding and cutting, comprises a toroidal cathode (1), a plasma nozzle with a connection to a mounting plate (10) comprising a cooler, gas intake ports, and an electrode acting as toroidal anode (6), which has a surface turned to the plasma (4) with uniform high temperature, where a plasma projection (8) takes place diffusely at the surface. The current density is smaller than 5 A/mm 2>in the plasma projection. The plasma at the anode is temporally stable at a surface area. The anode consists of two parts. The plasma burner for plasma applications e.g. powder coatings and welding and cutting, comprises a toroidal cathode (1), a plasma nozzle with a connection to a mounting plate (10) comprising a cooler, gas intake ports, and an electrode acting as toroidal anode (6), which has a surface turned to the plasma (4) with uniform high temperature, where a plasma projection (8) takes place diffusely at the surface. The current density is smaller than 5 A/mm 2>in the plasma projection. The plasma at the anode is temporally stable at a surface area. The anode consists of two parts. The transition of the concentrated plasma projection on the anode to the projection diffusing in a large surface takes place in the temperature window of 1000-2000[deg] C. The anode temperature in the plasma projection is 100[deg] C smaller than the melting temperature of the anode material. The connection of the anode surface to the cooler has a thermal resistance of 1[deg] C per Watt and is made of metal foam. The anode is structurally stabilized and is formed as cylinders made of sheet metal. A working gas and a powder are supplied parallel to the symmetry axis by the anode. The laser radiation overlaid to the plasma for the welding and cutting. The plasma projection of the anode takes place at a region of the inner surface to the cathode, where the region has an angular range relating to the symmetry axis of 360[deg] . The plasma projection in the channel takes place at regions of the inner surface to the cathode, where each region has an angular range relating to the symmetry axis of 15[deg] . The anode at side turned away from the plasma is coated with a thermally insulating material.

Description

The Invention relates to plasma torches for material processing such as welding, soldering, cutting, surface cleaning or spray coating comprising a first acting as a cathode Electrode, acting as an anode second electrode, holding body for both Electrodes and connections and openings to the feeder of process gases, process media or cooling media.

The The invention further relates to a method of operating such plasma torches.

State of the art

plasma torch The prior art has a cathode for electron emission with holding body a nozzle-shaped anode, from a body made of a good thermally conductive material, preferably copper, in which the actual anode is made of high temperature resistant Material, preferably tungsten or a tungsten compound used is. The main body is in good thermal contact with active cooling, in general, water cooling or is with cooling fins for air cooling Mistake. Due to the intensive cooling is the anodic plasma approach of the plasma arc in a very small area on the anode surface concentrated. This leads to a high local temperature in the plasma spot and a lower one mean temperature of the remaining anode. In the plasma spot is going through the high current density and the high temperature make the anode surface strong loaded. It can melt and evaporate locally. In extreme cases melts the anode at this point until the water cooling through. In general Forms a deformation of the surface.

There the total power for the anodic approach as heat in the approach area is implemented, the temperature is in the concentrated plasma approach, compared to the mean temperature of the anode, much higher.

Of the anodic base conventional burner of the concentrated plasma approach may be due to the design by gas-dynamic and eigenmagnetic forces azimuthally or axially move the anode inner wall.

azimuthal Fußpunktänderungen but lead to a changing one Symmetry of the plasma jet and a negative result.

longitudinal Displacements of the arch at the anode change the arc length of the plasma and with it the voltage requirement of the arc with constantly impressed current. There may be a so-called Restriking effect, in which with increasing arc length the tension on the arc increases so much that the arc between the cathode and the cathode-near part of the anode is re-ignited.

Both Fluctuations lead to noise emissions and fluctuations in electrical power and thus to negative fluctuations in the work process and process quality.

A another solution The prior art divides the anode, for example, into three Partial anodes, which are arranged symmetrically. The fluctuations on the part anodes cancel in part. The anodes will be but still heavily polluted.

task

Of the The invention is therefore based on the object, the anode for plasma applications to improve so that the the anode erosion is reduced, so that a longer life and stability achieved becomes, on the other disturbing Fluctuations of the bow attachment position omitted.

solution

These Task is inventively characterized solved, that he from the previous intense cooling the plasma-facing anode surface is ignored by heat conduction. Depending on the conditions for heat and cooling the anode is formed on the plasma-facing surface so that a greater area on even higher Temperature is and sets a diffuse plasma. additionally for cooling via heat conduction the anode, in contrast to previous anodes, essential more cooled by radiation.

At the Ignite of the plasma arises first at low power a concentrated plasma approach. With the adjusted operating current, depending on the cooling and Geometry conditions, reached a certain average temperature of the anode. Let it through reduced cooling a higher one mean temperature increases, so in the temperature window of about 1000 ° C up to 2000 ° C with suitable geometry and cooling the desired one diffuse plasma approach. The mean temperature of the anode is then higher as in previous versions, the local temperature in the plasma approach, however, lower.

The temperature for the desired diffuse approach is, in addition to the anode geometry, the cooling conditions for the anode and the gas stream, also the anode material, the dopants and the diffus depending on the material.

About radiation, Heat conduction, gas flow and work function of the anode material is the heat of Anode removed. With a known current can be over the Geometry, dependent from the material data, the desired Estimate temperature and temperature distribution and set approximately.

The Anode is inventively designed so that they are between the plasma set surface on the anode and its attachment a high thermal resistance and at the same time a large approach surface on the anode for the plasma allowed.

The Anode points for the diffuse plasma approach on a recognizable larger plasma projection surface as in the well-cooled Operation that becomes a punctiform Approach of the plasma leads. It is particularly advantageous for the formation of a diffused plasma approach when a large anode surface area a consistently high Temperature has.

Cheap is it, albeit a part of the volume below the surface area a substantially uniform temperature having. Due to the substantially uniform temperature are unfavorable influences and changes the material composition, for. B. by diffusion prevented or at least delayed.

Around the formation of a diffuse plasma approach as possible all operating states ensure is the surface area preferably gradual and edge-free - or more generally - without discontinuities train. Such a smooth execution suppresses the Tendency of the gas discharge to stay in spot mode.

The surface does not have to necessarily a flat cylindrical surface but may be convex, concave or generally continuously curved.

Preferably has the surface area, on the diffuse gas discharge begins, in relation to the cross-sectional area, the the anode with the cooling connects a significant big Expansion. At least the surface area is equal to the smallest Cross sectional area the anode.

Advantageous for applications the plasma burner is that the diffuse plasma approach to the anode takes place stable over time.

There flow through the anode current must, is about that electrical connection and the attachment forcibly given a heat conduction. The anode is designed so that the heat conduction for cooling the anode over Attachment and connection strong is reduced and the cross section to the current transport and mechanical stability is sufficient. The proportion of cooling for the anode by radiation is larger than in anodes of plasma torches according to the prior art. Essentially you design the anode so that, the thermal resistance between the plasma surface and cooling about 10 ° C per watt.

Around the anode mechanically additional To stabilize the electrical connection, you can use the anode in addition with thermal insulators Fasten. This can be done, for example, by a high-temperature resistant ceramic respectively.

As well can the thermal resistance and the electrical connection between anode surface and cooling be made by a suitable metal foam. By pore size and distribution the pores in the foam will be the desired one thermal resistance set.

There the anode in addition through the inflowing Gas cooled becomes, chooses cross section and position of the mechanical connection of the anode to the cooling, so that under desired Operating parameters one possible constant temperature along the anode surface established. This can be with Help of finite element calculations simulate and determine.

It has proved to be particularly favorable proven when the anode material is made of a refractory metal, z. B. tungsten, in addition to a material doped, which leads to a reduction of the work function and thus leads to a reduction of the anode heating. As doping materials can the most diverse materials come into question. These materials can for example, oxides of rare earths.

When It has proven particularly suitable when the electrode material doped with lanthanum between 0.1% by weight and 5% by weight.

Especially is appropriate when the working temperature of the anode is lower than its melting temperature is. Deformation, evaporation and sputtering are thereby reduced or completely prevented.

Also is advantageous in addition to a doping to reduce the work function an additional Doping to stabilize the crystal structure.

The anode may also be composed of two parts, wherein the plasma supplied turned part of a high temperature resistant material, eg. B. tungsten and the other part to hold, fasten or adjust the thermal resistance of another electrically conductive material.

As well it is advantageous if the diffused plasma approach at the anode simultaneously combined with a diffused approach at the cathode.

in the Difference to solutions with multiple anodes requires an annular or toroidal anode with uniform diffuse Plasma approach only one power supply and provides the desired rotational symmetry for the plasma automatically and completely.

execution

In all versions the heat flow becomes vertical to the anode surface in the direction of cooling across from solutions reduced according to the prior art. At the same time the execution is a good one heat conduction parallel to the anode surface.

The Connection of the anode to the electrical connection and the cooling is designed to be the wished having thermal resistance. This can be for a cylindrical example Anode be a ring with a suitable cross section.

The anode surface is preferably made of a high temperature resistant conductive material, preferably Tungsten, in addition can still be endowed. The heat distribution under the anode surface is made by an intimate connection with a good heat-conducting Material, preferably copper, achieved.

Regarding The shape of the anode were in connection with the previous embodiments no closer Information provided. Thus, in principle, any, in particular axial symmetrical shapes conceivable. Advantageously, cylindrically symmetric Torusformen. A particularly, because of their simplicity, cheap anode shape provides that the Anode is designed as a cylindrical tube.

Around the heat dissipation to compensate for the passing gas, the anode becomes preferably attached to the, the cathode side facing away. Around To ensure sufficient mechanical stability, the anode can additionally with a be attached to thermal insulation. The material for the thermal Isolation can z. As a high temperature resistant ceramic or quartz be.

The Attachment of the anode can be used to set a uniform surface temperature The anode also structured and distributed done. So can several electrically and thermally conductive Connections can be used alternately with insulators. A different possibility is the connection of the anode to cooling and electrical connection via a Metal foam. The design of the metal foam in terms of porosity, pore distribution and geometry allows the adaptation of the thermal resistance and the resistance curve between anode and cooling.

From Advantage is when the anode for the gas flow rate, is optimized in the form, for. B. as a Laval nozzle.

Becomes the anode as a cylinder-like Body executed, can be the process gas, the powder for coating, a laser beam or Insert combinations of them rotationally symmetric through the anode. The editing process becomes thus largely independent from the relative direction of movement between the plasma processing head and the material. The approach surface of the anodic plasma can then be full or in angular ranges in terms of the axis of symmetry of the anode facing on one of the cathodes area respectively.

For all embodiments it is advantageous if the diffuse plasma approach at the anode with a diffused plasma approach is combined at the cathode.

Especially It is advantageous if in addition to Anode also the cathode has an axially symmetrical shape.

drawings

Further Features and advantages of the solution according to the invention are the subject of the following Descriptions and the drawings.

In show the drawings:

1 a plasma torch with rotationally symmetrical anode conventional design. The anode is cooled well and the plasma arc starts locally concentrated in the so-called spot.

2 an embodiment of a plasma torch with nozzle-shaped anode, which is thermally formed according to the invention. The plasma arc is applied over a large area to the inner surface of the anode.

3 an inventive embodiment Example of a plasma torch with nozzle-shaped anode. The anode consists of a high-temperature-resistant metallic surface with a good heat-conducting core.

4 an embodiment of a plasma torch with anode, designed as a torus, Kreisringfäche.

5 an embodiment of a plasma nozzle, a so-called plasma needle

6 an embodiment of a plasma torch with anode, which is connected via metal foam to the cooling.

7 an embodiment of a plasma torch with anode, in which the plasma approach is carried out at the anode in full.

8th an embodiment of an anode plasma torch in which the plasma approach to the anode takes place on a surface which comprises an angular range.

Description of the drawings

In 1 become electrons from the cathode 1 emit and ionize the incoming gas. The cathode is located in the cooled holding body 2 , The anode 6 made of high-temperature-resistant material, a conventional plasma torch is with good thermal contact of a good heat-conducting body 5 Normally it is cooled directly with water. The cold anode laces the plasma 4 on a relatively small area on the anode to the anode extension 7 together. This leads to a local overload in operation, the associated corrosion phenomena and the jumping of the bow approach.

As in 2 there is shown an advantageous embodiment of an anode with diffuse plasma approach in the simplest case of a cylindrical torus 6 the thermally with a ring 9 on a bracket 10 is attached. The holder is cooled. The ring 9 has a suitable thermal resistance for the anode over its cross-section and width. The fastening ring 9 does not have to have a continuous cross section on the entire circumference but can be interrupted azimuthally, z. B. with holes to adjust the thermal resistance. The plasma approach 8th extends to a much larger anode area than in previous solutions according to the prior art. The mean temperature of this anode 6 is compared to the mean temperature of the anode 6 in 1 , much higher but the local temperature in the plasma approach is lower.

3 shows an exemplary embodiment of an anode with hochtemperturfester surface 12 that have a core 13 made of good heat-conducting material, so that the temperature differences on the largest possible anode surface for the plasma approach are very low.

Another exemplary embodiment of an anode for diffuse approach is a torus 14 z. B. with ring or elliptical cross section 4 that have a ring 9 attached to the heat sink. With cross section and extension of the ring 9 the desired thermal resistance is set. Since the plasma approach takes place on the entire ring surface, the approach surface is much larger than normal anodes. The electrically non-conductive 15 Nozzle forms the gas stream.

A torus anode 14 in 5 with fastening ring 9 for the thermal resistance is for the execution of a plasma nozzle, a so-called plasma needle also beneficial. The cathode 17 is needle-shaped.

6 shows an embodiment of a plasma torch, wherein the thermal resistance and the axial resistance distribution for the anode via a metal foam 18 is set. The metal foam establishes the electrical connection to the anode.

7 shows an embodiment of a rotationally symmetric plasma torch, in which the plasma approach to the anode at a region on the, the cathode facing inner surface and this region includes the full angular range of 360 °.

8th shows an embodiment of a plasma torch, wherein the plasma approach to the anode at the, the cathode-facing inner surface takes place in a limited angular range.

Claims (26)

Plasma torch for plasma applications comprising a cathode, with a connection to a holder with cooling, a plasma nozzle, one or more gas supplies and an anode acting as an anode, characterized in that the anode in plasma operation has a plasma-facing surface with uniformly high temperature and that At this surface the plasma approach takes place diffusely. Plasma torch for plasma applications according to claim 1, characterized in that the current density in the plasma mixture is less than 5 A / mm ² . Plasma torch for plasma applications according to Claims 1 and 2, characterized in that the plasma attaches to the anode stable in time to a surface area. Plasma torch for Plasma applications according to Claims 1 to 3, characterized that the transition from the concentrated plasma approach on the anode to the large-area diffuse Approach in the temperature window of 1000 ° C to 2000 ° C takes place. Plasma torch for Plasma applications according to Claims 1 to 4, characterized that the anode temperature in the plasma approach at least 100 ° C is less than the melting temperature of the anode material. Plasma torch for Plasma applications according to Claims 1 to 5, characterized that the connection the anode surface to the cooling has a thermal resistance of at least 1 ° C per watt. Plasma torch for Plasma applications according to Claims 1 to 6, characterized that the connection the anode surface to the cooling and electrical connection via metal foam he follows. Plasma torch for Plasma applications according to Claims 1 to 7, characterized that the anode made of refractory material is preferably tungsten, the doped with oxides having a lower work function as pure tungsten. Plasma torch for Plasma applications according to Claims 1 to 8, characterized that the refractory Material consists of tungsten, which is doped with lanthanum oxide. Plasma torch for Plasma applications according to Claims 1 to 9, characterized that the Anode of refractory material, preferably tungsten, doped to the fabric stabilized is. Plasma torch for Plasma applications according to claims 1 to 10, characterized in that the anode torus is trained. Plasma torch for Plasma applications, in particular for welding and cutting, according to claim 11, characterized in that the Plasma torch a toroidal Anode and a torusfömige Cathode covers. Plasma torch for Plasma applications according to claim 11 and 12, characterized in that the anode as a cylinder formed of sheet metal is bent. Plasma torch for Plasma applications according to claims 11 to 13, characterized in that the anode the working gas about is fed parallel to the axis of symmetry. Plasma torch for Plasma applications, in particular powder coatings, according to claim 11 14 characterized in that the powder passes through the anode supplied becomes. Plasma torch for Plasma applications, in particular for welding and cutting, according to claim 11 to 15, characterized in that through the anode Laser radiation superimposed on the plasma becomes. Plasma torch for Plasma applications according to Claims 11 to 16, characterized in that the plasma mixture the anode is at an area on the inner surface facing the cathode and this range at least the angular range with respect to Symmetry axis of 30 ° includes. Plasma torch for Plasma applications according to Claims 11 to 16, characterized in that the plasma mixture the anode is at an area on the inner surface facing the cathode and this range at least the angular range with respect to Symmetry axis of 60 ° includes. Plasma torch for Plasma applications according to Claims 11 to 16, characterized in that the plasma mixture the anode is at an area on the inner surface facing the cathode and this range at least the angular range with respect to Symmetry axis of 120 ° includes. Plasma torch for Plasma applications according to Claims 11 to 16, characterized in that the plasma mixture the anode is at an area on the inner surface facing the cathode and this range at least the angular range with respect to Symmetry axis of 240 ° includes. Plasma torch for Plasma applications according to Claims 11 to 16, characterized in that the plasma mixture the anode is at an area on the inner surface facing the cathode and this range the full angle range with respect to the symmetry axis of 360 ° included. Plasma torch for Plasma applications according to Claims 11 to 21, characterized in that the plasma mixture in the channel at two areas on the, the cathode facing inner surface, each region having at least one angular range with respect to Symmetry axis of 15 ° includes. Plasma torch for Plasma applications according to Claims 11 to 22, characterized in that the plasma mixture in the channel at several areas on the, the cathode facing Inside surface takes place, each region having at least one angular range with respect to Symmetry axis of 15 ° includes. Plasma torch for Plasma applications according to Claims 1 to 23, characterized in that the anode is composed of two parts, wherein the plasma facing Part of refractory material, preferably tungsten, consists, which is doped with lanthanum oxide. Plasma torch for Plasma applications according to claims 1 to 24, characterized by two Parts is composed, wherein the plasma remote part from a good heat-conducting Material, preferably copper, consists. Plasma torch for Plasma applications according to Claims 1 to 25, characterized in that the anode on the side facing away from the plasma with a high temperature resistant, thermally insulating material is wrapped.