EP2622111B1 - Method and device for thermal spraying - Google Patents

Description

The invention relates to a method and apparatus for thermal spraying, in particular for arc spraying with filler material, as it is, for example, the wire arc spraying, according to the preamble of the first and eighth claim, wherein a filler material, usually two electrically conductive wires, melted by an arc and means a compressed gas (also called atomizing gas) are sprayed at high speeds in the range of 1 Mach and larger on a prepared surface.
For this purpose, a burner with a corresponding nozzle with a preferably centric flow opening for the compressed gas is used. The wires are fed through current-carrying sleeves with different polarity, with a wire feed device being used for this purpose.
For example, when wire arc spraying a short arc is generated, which partially melts the wire-shaped spray additive due to its high temperature (greater 5000K). One or more nebulizer gas nozzles (direct the nebulizer gas to the molten material phase.) As a result of the dynamic pressure of the nebulizer gas flow, the material phase is nebulized and accelerated by momentum transfer of the gaseous phase in the resulting free jet To massive recirculation areas behind the wire-shaped spray additive, with the result that molten spray material dwells on the vortex-shaped trajectories in the area of the wire ends.At this time, the material is oxidized due to the atmospheric oxygen of the ambient or compressed air.On the continuous melting of the wire increases the recirculating amount of the molten phase, with the result of inhomogeneous atomization critically grown melting droplets. The free jet propagating around the wire ends builds a so-called "K Armanian vortex street ". This has the consequence that the particle-laden free jet extremely dilates (diverges). Due to the high divergence, part of the particle phase enters slow outer areas of the atomizing gas flow and thus opposes a material-efficient coating (especially in the case of small components).

From the publication EP 0 879 645 A2 Although a solution is known by which the life of the device is to be extended. For this purpose, a so-called purge gas is introduced laterally into the current-carrying sleeve surrounding the wire, which purge gas is applied to the Surface of the wire located loose contaminants in the opposite direction of the feed direction of the wire removed. Preferably, a part of the purge gas of the Zerstäubergasstromes is branched off. However, this does not eliminate the aforementioned disadvantages of undesirable settling of molten material on areas of the nozzle or wire.

With one in the publication DE 433 46 10 A1 described methods for thermal spraying relatively rough wear layers are to be created by the particles impinging on the surface are greater than 0.2 mm and get to the surface by the action of gravity. It is possible to trigger the particles by pulses, in particular by compressed gas pulses, from the molten filler material. The velocities generated by the compressed gas are thus very low. This method is not suitable for the production of thin uniform and firmly adhering layers. Furthermore, this solution does not prevent the settling of molten particles on the nozzle or on the wire. Due to the system, it is only possible with this method to work with a burner pointing downwards, whereby no coatings are possible in other spatial directions.

A method of operating a plasma spray gun is disclosed in the document CH 578 622 known. In this case, the working gas before its entry into the nozzle has a pressure of greater than or equal to 7 atmospheres and the fuel flow of the arc to a value of greater than or equal to 1000 amps. The gas flow of the working gas and the fuel flow are pulsed synchronously, whereby the gas flow through the nozzle and the fuel flow between the pulses are brought to values close to or equal to zero. The plasma spraying burner has a pin-shaped cathode. This is followed in the direction of work by an anodically connected Laval nozzle, which widens in the working direction. A DC voltage generates an arc between the anode and the cathode and the working gas flowing through the plasma torch, which contains particulate spray material, is passed through the arc and ionized thereby, producing a plasma jet flowing through the nozzle above the nozzle inlet and thus within the plasma spray torch and exits from this at a speed of a few 1000 m / s, bouncing on the substrate to be coated and this coated with the molten particles. For pulsing the gas flow, the cathode is moved axially over cams up and down, so that the nozzle inlet is opened and closed in a pulsed manner. In this case, the nozzle inlet of the anodically connected nozzle is not complete be closed, as this would lead to a short circuit. There is thus always a flow of the working gas.
This solution is intended to be used to produce adherent layers, assuming that plasma jet speeds of a few 1000 m / s (subsonic flow of the plasma) can be generated when large values of the arc fuel stream and high gas pressures are provided. Since this leads to a very high thermal load on the nozzle, a pulsed mode of operation is proposed in this prior art.
The invention has for its object to develop a device for high-speed thermal spraying with filler, which avoids unwanted settling of the molten material at the nozzle or the additional material and ensures high-quality layer at high particle speeds in a homogeneous layer structure.

This object is achieved with the features of the first and seventh patent claim.
Advantageous embodiments emerge from the subclaims.
In the method for high-speed thermal spraying, in particular for arc spraying with filler material, which is melted by an arc generated by electrodes, a compressed gas flows through a nozzle at high speed and injects the molten material of the filler material on a mostly prepared surface, said Pressure gas produced according to the invention with a modulated / pulsating gas flow of high velocity and thereby the formation of a quasi-stationary vortex field of the molten material in the region of the ends of the filler material is limited or prevented.
By reducing or avoiding the constituent vortex area of melted particles of the filler material, which is caused in particular in the area behind the ends of the filler material (seen in the flow direction), settling or residence of particles / drops of the molten material on the burner or components the burner of the nozzle and / or the filler material are avoided or be solved settled particles / drops. This undesirable settling of particles / drops of molten filler material at the burner / nozzle and / or on the Zusatzstuff even avoided, whereby a more uniform application of the filler material is given on the base material.
The modulated / pulsating gas flow is generated with a time variation of the gas volume flow of the compressed gas using a nebulizer gas nozzle.
The high-pressure gas flow expands preferably in the nozzle to speeds greater than Mach 1. These high speeds of the atomizing gas ensure Bescheunigung the molten material phase and their adhesion to the base material.
Advantageously, the modulation of the gas flow of the compressed gas takes place as a function of the control behavior of the current-voltage source. However, it is also possible to control the frequency / modulation of the compressed gas regardless of the frequency of the current-voltage source or regulate.
The pulsed gas flow, which leads to avoid the recirculation of the atomizing gas behind the wire ends and behind the arc and / or the instantaneous detachment of the molten droplets or particles of the filler materials, is preferably in dependence on the electrical parameters (current, voltage or current Voltage characteristic) of the current-voltage source and / or adjusted as a function of the fluid mechanical conditions of the respective coating task and can be determined by a few experiments.

The change in the frequency of the pulse of the gas flow of the atomizing gas is preferably carried out with a frequency from 20 Hz, wherein it is also possible, if necessary, to change the frequency or the pulse duration of the gas flow during the coating process.

The device according to the invention has a device for generating a compressed gas, which flows out at high speed through a nozzle and injects the molten material of the filler material onto a surface, wherein means are provided by which a modulated / pulsating gas flow of high velocity of the compressed gas can be generated through which the formation of a quasi-stationary vortex field of the behind the spray filler material and behind the short arc prevented or at least restricted. As a result, the recirculation of the particle phase or of the molten material on the trajectories (trajectories) of these vortex regions is minimized or extinguished, which leads to a shorter residence time and a sudden atomization of the material phase at the nozzle exit. At the nozzle and / or at the ends of the filler material thereby a rapid detachment of particles / drops is ensured.

Most (eg thermal spraying) of the filler material in the form of two electrically conductive wires is formed, the ends of which are arranged in the region of the outlet of the nozzle or between the nozzle outlet and the surface to be coated, wherein in the region of the ends of the filler, through the means arranged in the device, a modulated / pulsating gas flow of the compressed gas with a time variation of the atomizing gas volumetric flow, the restriction or prevention of the quasi-stationary vortex field.

The means may be formed for example in the form of the gas flow of the pressure gas changing valves or in the form of mechanical elements and according to the required pulses interrupt the gas flow and greatly reduce and release again.

The mechanical elements are preferably arranged in a pressure line leading to the nozzle for the compressed gas and may be formed, for example, in the form of an angle to the pressure line through this rotatable shaft having one or more transverse bores in the region of the pressure line and by rotation the flow of the pressurized gas locks or greatly reduces or releases.

By this means, in particular a modulated / pulsating gas flow in the required frequency with a temporal change of the Zerstäubergasvolumenstroms can be generated.

The means are designed in particular in the form of valves which change the gas flow, which may be, for example, high-performance valves which ensure a fast switching frequency.

In order to ensure an expansion / acceleration of the gas flow, the nozzle is designed in the manner of a nebulizer gas nozzle, in particular a Laval nozzle, which extends in Direction of flow of the gas first tapers and then expanded again, whereby the gas flow is first braked and then expanded, whereby its speed increases again.
In this case, the filler material is inclined with respect to the longitudinal axis of the nozzle in a shallow angle of attack, which is a maximum of 90 ° to 0 °, preferably about 25 ° ± 5 °, with its ends in the region of the nozzle outlet or between the nozzle outlet and the to be coated surface.

A further advantageous embodiment of the invention is that the wire feed device, with which the filler material is tracked, not realized as usual a continuous tracking of the wire, but the wire feed discontinuously, or gradually regulates, such that the tracking of the wire in particular Dependence / correlation with the pulses of the pulsating gas flow of the compressed gas takes place.

The conceptual approach of the invention is based on a targeted, "high velocity" 'modulated' gas flow adapted to the process (or a temporal change in sputtering gas volumetric flow) to prevent settling of the molten phase (s) behind the wire ends and beyond the arc (preferably outside the nozzle).

This "pulsation" is preferably realized by high-performance solenoid valves, which relate their control signal as a function of the time interval of melting or melting a corresponding material fraction of the wire-shaped spray additive. The high pressure gas flow is directed through suitable nebulizer gas nozzles (Laval similar) directed to speeds greater than Mach 1. The impulsive impact of the gas phase leads to a fine and homogeneous atomization of the material phases. The resulting quasi-stationary vortex field (behind the wire ends) is greatly reduced due to the pulsed gas flow and minimizes beam divergence. The recirculation areas in the region of the wire ends are also greatly reduced, so that the melt film or drops do not or only partially recirculate and oxidize in these fluidized areas. The control signal of the solenoid valves is synchronized for better process control with the control circuit of the current-voltage source.

The nebulizer gas nozzles are designed mass flow-specific for this process.

1. 1. homogen zerstäubte Materialphase (homogener Schichtaufbau),
2. 2. verringerte Oxidation des Spritzzusatzwerkstoffes,
3. 3. Schichten hoher Güte,
4. 4. Partikelgeschwindigkeiten größer 300 m/s,
5. 5. Materialeffiziente Beschichtung (Erhöhung des Auftragwirkungsgrades).

Achievable results include:

1. 1. homogeneously atomized material phase (homogeneous layer structure),
2. 2. reduced oxidation of the spray additive,
3. 3. layers of high quality,
4. 4. Particle velocities greater than 300 m / s,
5. 5. Material-efficient coating (increase of the application efficiency).

In order to improve the quality of arc-sprayed coatings, consistent adaptation of the process control under flow-dynamic optimized conditions is absolutely necessary. It can be assumed that even minor changes to marketable burner technology are sufficient to achieve significant improvements. Since the area of high-speed spraying with arc systems has so far only been insufficiently developed, the particle-laden free jet is to be influenced in such a way that higher particle velocities and lower spray jet divergences are established by the targeted design of the expansion nozzles necessary for generating supersonic gas flows. The basis for the invention is the exploitation of a pulsed high-velocity gas flow of the atomizing gas.

The gas flow pulsed by valves is adapted to the respective coating task, depending on the electrical parameters (current, voltage - including their frequencies and current-voltage characteristic) and the flow-mechanical conditions that lead to droplet detachment.
By using a pulsed atomizing gas flow, a more homogeneous and finer particle phase is expected. The expression of strong turbulence bales in the area of the wire tips, which are responsible for the massive expansion of the particle-laden free jet, should be reduced. In the context of a deliberately expanding (prestressed) gas flow, particle velocities of more than 300 m / s are achieved.

Coming as compressed gas, homogeneously flowing, partly expanding, hot or cold, prestressed process gases (of various types - for example, combustible, inert gases, combustion products, etc.) are used.

Figur 1

die Prinzipdarstellung einer erfindungsgemäßen Einrichtung mit einem Hochleistungsventil zum Pulsen des Druckgases,

Figur 2

eine Prinzipdarstellung einer Variante zum mechanischen Pulsen des Druckgases.

The invention will be explained in more detail with reference to an embodiment and associated drawings. Show it:

FIG. 1

the schematic representation of a device according to the invention with a high-performance valve for pulsing the compressed gas,

FIG. 2

a schematic diagram of a variant of the mechanical pulses of the compressed gas.

In FIG. 1 a device for coating the surface 1a of a base material 1 is shown, whereby a spray coating 2 was produced by means of wire arc spraying.

For this purpose, a filler material 3, here two wires, is guided in contact tubes 4. Both contact tubes 4 are connected to a voltage source 5, so that they are poled differently. The filler material 3 can be tracked by a respective wire feed device 6. The ends of the filler material 3 are between the unspecified outlet of the nozzle 7 and the surface 1 a of the base material 1. Its melting takes place via the voltage source 5, by their application between the facing ends of the filler material 3, an arc is formed, whereby the ends of the By means of the centrally arranged between the contact tubes 5 nozzle 7, which has a longitudinal axis A, a compressed gas 8 (shown by an arrow, also referred to as Zerstäubergas) pulsed and at high speed in the direction of the base material 1, whereby the molten material of the filler material 3 on the surface 1 a splashes and the sprayed layer 2 is generated. In this case, a mist 3a forms from the ablated filler material, with the risk that it forms a quasi-stationary vortex field, ie massive recirculation areas behind the wire-shaped spray additive material of the molten material before the nozzle in the region of the ends of the filler material, which can lead to that melted sprayed material dwells in the region of the wire ends on the vortex-shaped trajectories and oxidized in this time due to the atmospheric oxygen of the ambient or compressed air.
This is restricted or prevented according to the invention by the fact that the droplets of the plasma mist can not settle on the nozzle 7 or the contact tubes 4 or the additional material 3 due to the pulsed compressed gas. Should melt drops nevertheless settle, they will be released by the pulsating gas flow dissolved again and transported in the direction of the surface 1a of the base material. The pulsed gas flow of the compressed gas thus reduces the recirculating amount of the molten phase, or the recirculation is completely prevented, thus ensuring a material-efficient coating.

By the voltage source 5, an alternating current or a pulsed direct current is preferably generated, wherein the pressurized gas 8 is pulsed by a corresponding device 9, which has a high-performance valve not designated, in particular in dependence on the frequency of the voltage source (in particular synchronously thereto). Alternatively, it is also possible to set the frequency with which the compressed gas is pulsed, regardless of the frequency of the voltage source.
If DC power is provided by the voltage source, only the pressurized gas pulsates.
The required frequency of the pulsation of the compressed gas, which is required for preventing the deposition of drops on the system or their removal, can be determined by a few experiments and is preferably ≥20 Hz.
The speed of the gas flow of the compressed gas is greater than 1 after the nozzle at Mach numbers.

The Drahtanstellwinkel α of the filler material 3 is very flat and is preferably in the range of about 10 ° relative to the longitudinal axis A of the nozzle. 7
The flow cross-section of the nozzle 7 decreases in the flow direction in the manner of a diffuser and then widens again in the manner of a confuser, whereby the compressed gas 8 is accelerated. Its speed is after the nozzle Mach 1 and more, whereby an excellent homogeneity of the spray layer 2 and a good adhesion to the base material 1 is ensured.

A schematic diagram of a variant for the mechanical pulsing of the compressed gas is shown in FIG FIG. 2 shown. The means for generating the pulsating gas flow are integrated into the pressure line 10 for the compressed gas and can interrupt and release the gas flow. For this purpose, the pressure line 10 passes through a housing 11 in which at an angle of 90 ° and thus transversely to the pressure line 10 a this interrupting and is arranged by this rotatable shaft 12 is arranged. The rotatable shaft 12 has in the region of the pressure line 10 has a transverse bore 13 in the form of a through hole. By rotating the shaft 12, the compressed gas flows through the transverse bore 13 in the direction of arrow to the nozzle, not shown here, when the transverse bore 13 is in a position in which the two openings 13.1, 13.2 have a connection to the pressure line 10. If the shaft 12 continues to rotate, the through-bore 13 is rotated in such a way that the pressure line 10 is interrupted. The shaft 12 is rotatably supported by bearings 14 in the housing 11.
By rotation of the shaft 12, the flow of the pressurized gas is alternately locked or released and thereby generates the pulsating gas flow. The frequency with which the gas flow pulsates can be determined in a simple manner by the rotational speed of the shaft, which is driven by a motor, not shown here.
According to a variant, not shown, several through holes can pass through the shaft.

The solution according to the invention as a whole creates a simple and practical possibility for reducing or avoiding the developing vortex region of melted particles of the filler material and thereby undesirable settling or residence of particles / drops of the melted filler material on the burner / nozzle and / or on the filler material even avoided. As a result, a high-quality coating result can be realized.

LIST OF REFERENCE NUMBERS

1

Parent material

1a

surface

2

spray layer

3

Additional material

3a

fog

4

contact tubes

5

voltage source

6

Wire feeder

7

jet

8th

compressed gas

9

High-performance valve

10

pressure line

11

casing

12

wave

13

cross hole

13.1, 13.2

openings

14

camp

α

Drahtanstellwinkel

A

longitudinal axis

Claims (15)

1. A method for thermal spraying, especially for arc spraying using a filler material (3) which is melted by an arc generated by means of electrodes, wherein a compressed gas (8) flows out of a nozzle (7) at high velocity and sprays the molten material of the filler material (3) onto a surface (1a), wherein

   - a compressed gas (8) is generated with a modulated/pulsating gas flow of high velocity and

   - the formation of a quasi-stationary vortex field of the molten material of the filler material (3) is thus limited or prevented, such that

   - particles/drops of the molten material are prevented from depositing or dwelling on the nozzle (7) and/or on the filler material (3), and/or particles/drops that have been deposited thereon are detached,

   characterized in that
   the limitation or prevention of the quasi-stationary vortex field in the region of the ends of the filler material (3) occurs by modulation/pulsation of the gas flow of the compressed gas (3) with a temporal change in the gas volume flow.
2. A method according to claim 1, characterized in that a modulated/pulsating gas flow with a temporal change in the gas volume flow of the compressed gas (3) is generated by using a nozzle (7) in form of an atomiser gas nozzle.
3. A method according to claim 1 or 2, characterized in that the high-pressure gas flow of the compressed gas (8) expands to velocities greater than Mach 1.
4. A method according to one of the claims 1 to 3, characterized in that the modulation of the gas flow of the compressed gas (8) is controlled or regulated depending on the modulation of the velocity of the compressed gas (8) independent of the frequency of the current-voltage source (5) of the electrodes.
5. A method according to one of the claims 1 to 4, characterized in that the pulsed gas flow is adjusted to the respective coating task depending on the electrical parameters of the electrodes and/or depending on the flow-mechanical conditions.
6. A method according to one of the claims 1 to 5, characterized in that the change in the frequency of the pulse of the gas flow of the compressed gas (8) occurs at a frequency from 20 Hz and that the frequency of the gas flow is selectively variable.
7. An apparatus for thermal spraying, especially for arc spraying using a filler material (3), which is melted by an arc generated by means of electrodes, wherein the apparatus comprises a device for generating a compressed gas (8) which flows out of a nozzle (7) at high velocity, and sprays the molten material of the filler material (3) onto a prepared surface (1a), characterized in that

   - the device for generating the compressed gas comprises means through which a modulated/pulsating gas flow of the compressed gas (8) can be generated,

   - by which the formation of a quasi-stationary vortex field of the molten material is limited or prevented, so that a depositing of particles/droplets of the molten material on the nozzle (7) and/or at the ends of the filler material (3) is prevented, and/or particles/drops that have been deposited thereon are detached, and that

   - in the region of the ends of the filler material (3)

   - the limitation or prevention of the quasi-stationary vortex field can be achieved by the means arranged in the apparatus in such a way

   - that a modulated/pulsating gas flow of the compressed gas (8) with a temporal change in the gas volume flow can be realised by the means.
8. An apparatus according to claim 7, characterized in that the means are arranged in form of valves changing the gas flow of the compressed gas (8) or in form of mechanical elements which interrupt or strongly reduce and release the gas flow according to the required pulses.
9. An apparatus according to claim 8, characterized in that the mechanical elements are arranged in a pressure conduit (10) for the compressed gas (8) leading to the nozzle (7), and block or strongly reduce and then release the flow of the compressed gas (8) through the pressure conduit (10).
10. An apparatus according to claim 9, characterized in that at an angle to the pressure conduit (10) a rotatable shaft (12) which leads through said pressure conduit is arranged, which shaft comprises one or several transverse boreholes (13) in the region of the pressure conduit (10), and blocks or strongly reduces or releases the flow of the compressed gas (8) by rotation.
11. An apparatus according to one of the claims 7 to 10, characterized in that the nozzle (7) is arranged in the manner of an atomiser gas nozzle, especially a laval nozzle.
12. An apparatus according to one of the claims 7 to 11, characterized in that the nozzle (7) tapers at first in the direction of flow of the compressed gas (8) and subsequently expands.
13. An apparatus according to one of the claims 7 to 12, characterized in that the nozzle (7) comprises a longitudinal axis (A) and the filler material is inclined at an angle (α) of 90° to 0° with respect to the longitudinal axis (A) of the nozzle.
14. An apparatus according to one of the claims 7 to 13, characterized in that the filler material (3) arranged in form of a wire can be resupplied by a wire feed device (6), wherein the resupply of the wire/filler material (3) occurs "step-by-step" in correlation with the pulses of the pulsating gas flow of the compressed gas (8).
15. An apparatus according to one of the claims 7 to 14, characterized in that the apparatus comprises means, by means of which the modulation of the gas flow of the compressed gas (8) can be realised depending on the frequency of the current-voltage source (5) or the melting behaviour of the filler material (3).

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