EP1847629A2 - Method for determining process parameters in a thermal spraying process - Google Patents

Abstract

Method for determining process parameters in a thermal spraying process comprises using an operating model for the thermal spraying process consisting of an electromagnetic model and a fluidic model connected together. Independent claims are also included for the following: (1) Thermal spraying device used in the above process; and (2) Computer program product for implementing the process.

Description

The invention relates to a method for determining process parameters in a thermal spraying process.

Thermal spraying processes such as plasma spraying are used today for a wide variety of coatings on very different substrates. For this purpose, in a plasma spraying device, such as a plasma torch, an arc is generated between an anode and a cathode. A gas is ionized between the electrodes, creating a plasma. The material required for the coating to be produced is usually blown into the hot plasma in powder form, melted there or at least made plastic and applied by the gas flow at high speed to the substrate to be coated.

Since the coatings to be produced are often quite different in nature, the thermal spraying process usually has to be adapted to the particular application. Often, the result to be achieved is predetermined, such as the deposition rate, the layer thickness, the layer structure or other layer properties such as porosity, adhesion, surface roughness, electrical conductivity, thermal conductivity, viscosity, wear resistance, the proportion of unmelted Particles or chemical properties such as the degree of oxidation of the layer.

In addition, it is very important for industrial applications in particular that the injection process per se has a high stability, ie provides reproducible results, and that it involves a high process and deposition efficiency.

In order to adapt the thermal spray devices to the particular application under these exemplified aspects, it is desirable, if not necessary, to have information about the process parameters such as gas velocity and temperature, particle velocity and temperature. Although such quantities are in principle detectable by measurement, for example with the aid of high-speed cameras, such measurements are very complex.

Also with regard to the further or new development of thermal spraying devices, which today is mostly empirical, it is desirable to have more information about the process parameters or to obtain such information as simply as possible.

It is therefore an object of the invention to propose a method with which the simplest and yet reliable determination of process parameters under different operating conditions in a thermal spraying process is made possible.

The problem solving method is characterized by the features of independent claim 1.

According to the invention, therefore, a method is proposed for determining process parameters in a thermal spraying process in which particles are melted or plastified or vaporized by means of a thermal spraying device and transported by a fluid flow to a substrate, in which process for the thermal spraying process or for the thermal Sprayer an operational model is built, with which a simulation of the thermal spraying process is feasible, and which includes a fluid mechanical model and an electromagnetic model, wherein the fluid mechanical model and the electromagnetic model coupled to each other and at least one process parameter is determined by means of the operating model.

As a result of the fact that the thermal spraying device or the thermal spraying process is described by an operating model, the process parameters can be determined without the need for meso-technological detection of the respective operating parameter. Since the operating model comprises the coupling of a fluid mechanical model with an electromagnetic model, ie the mechanical-electromagnetic interactions e.g. taken into account between the fluid flow and the arc, a reliable determination of the process parameters is made possible.

This operating model also makes it possible to make statements about the process parameters in thermal spray devices when operating under extreme operating conditions. Thus, for example, load limits for thermal spray devices can be investigated.

Furthermore, the inventive method is particularly advantageous for further and new developments of thermal spray devices used. Namely, by determining the process parameters according to the invention, the entire injection process or the spraying device can be simulated. This allows a much simpler and faster optimization of the design of the spray device or parts thereof, for. B. the nozzle.

Preferably, the operational model comprises the interaction between the particles and the fluid stream. By taking into account the particles introduced into the fluid flow in the operating model, the process parameters can be determined even more accurately. In addition, this makes statements such as the trajectory of the particles or their speed possible.

The particles are considered as extended bodies during modeling. In one embodiment of the method, the process parameters which relate to a particle, for. B. the temperature of Particles are assumed to be constant over the extent or the total volume of the respective particle. This means, for example, it is assumed that the particle has a homogeneous or uniform temperature, which of course changes with its position in the gas stream. In another embodiment, during modeling, variations of the process parameters over the extent of a particle are allowed, that is, for example, the temperature is no longer assumed to be constant over the extent of the particle.

Preferably, at least one of the following process parameters is determined: velocity of the particles, temperature of the particles on the surface of the particles, temperature inside the particles, state of aggregation of the particles, trace of the particles, impact point of the particles. Another advantageous measure is to create a temperature profile for the particles.

By knowing the temperature in the interior of the particles or the temperature profile, it is possible, for example, to detect whether the particles are also melted or plastic in the interior. Such information is useful to control the properties of the coating to be formed.

For the same reasons, it is advantageous to create a velocity profile or a temperature profile for the fluid flow.

The inventive method is also suitable for the optimization of injection processes and devices in an advantageous manner. Thus, for at least one process parameter, a desired value can be preset and the thermal spraying device or the thermal spraying process can be optimized by means of the operating model until the desired value is reached within predefinable limits. This allows much faster optimization than a purely empirical approach.

In a preferred application, namely if the thermal spray device comprises a nozzle through which the fluid flow exits, the operating model is used to optimize the nozzle.

The method according to the invention is particularly suitable when the thermal spraying device is a plasma spraying device in which an arc is generated between an anode and a cathode. In particular, the method according to the invention is also suitable for multi-cathode plasma torches.

An advantageous measure is to determine the shape and / or the contact points of the arc by means of the operating model. As a result, for example, extend the life of the sprayer. In addition, the stability of the arc under different operating conditions can be examined.

The invention further proposes a thermal spraying device, in particular a plasma spraying device, which is operated by means of a method according to the invention.

In a particularly preferred embodiment, the fluid mechanical model is a CFD model and the electro-magnetic model is a model based on Maxwell's equations, which is suitable for quantitatively describing the interacting electrical and magnetic effects.

The invention also proposes a computer program product for implementing a method according to the invention in a data processing system.

Further advantageous measures and preferred embodiments of the invention will become apparent from the dependent claims.

Fig. 1:

eine schematische Darstelllung eines Ausführungsbeispiels einer thermischen Spritzvorrichtung, die als Plasmaspritzvorrichtung ausgestaltet ist.

In the following the invention will be explained in more detail by means of embodiments and with reference to the drawing. In the schematic drawing show:

Fig. 1:

a schematic representation of an embodiment of a thermal spray device, which is designed as a plasma spraying device.

The invention proposes a method for determining process parameters in a thermal spraying process in which particles are melted or made plastic by means of a thermal spraying device and transported by a fluid stream, for example a gas stream, to a substrate. By the term "process parameters" are meant all quantities which serve in any form for the characterization of the operating state of a thermal spraying device or the characterization of the thermal spraying process. Such process parameters are, for example: velocity or velocity field of the fluid or gas, temperature or temperature profile of the fluid or gas, velocity of the particles (at different locations), temperature of the particles on the surface or inside the particles, state of aggregation of the particles , Position or trace of the particles, disintegration or breakup of particles, erosion, contact points between an arc and the electrodes, shape and shape of the arc, characteristic properties of the fluid or gas such as specific heat capacity, ionisationsgrad. This list is not exhaustive.

In the following, reference will be made to the application, which is particularly important in practice, in that the thermal spraying process is a plasma spraying process and the spraying apparatus is a plasma spraying apparatus. Of course, the invention is not limited to such applications, but is also suitable for other thermal spraying methods such as Radio Frequency (RF) plasma spraying or arc wire spraying.

FIG. 1 shows in a highly schematic representation an embodiment of a plasma spraying device, which is designated overall by the reference numeral 1. The plasma spraying apparatus 1 comprises a housing 2 in which a cathode arrangement 3 and an anode 4 which is electrically insulated on the other hand are provided. The anode 4 is designed here as a ring anode, which has in its center an outlet opening 42 which is provided with a nozzle 41. During operation, a gas is blown through the plasma spray device 1 in the axial direction as indicated by the two arrows indicated by the reference symbol G. In the flow direction seen behind the annular anode 4, a powder feeder 5 is provided which has one or more feed channels 51, which extend substantially in the radial direction. Of course, it is also possible for the feed channels 51 for the powder or the particles to extend in the axial direction or obliquely, ie between the axial and radial direction.

On the presentation of other known per se components of the plasma spray device 1, such as, cooling, power and control devices was omitted for the sake of clarity.

In particular, the plasma spraying apparatus 1 may also be a multi-cathode burner, such as the burner marketed by the applicant under the tradename TriplexPro. In this burner, the cathode assembly 3 comprises a total of three cathodes. In operation, then arise three arcs.

During operation, the gas G flowing in the axial direction through the plasma spraying device 1 is ionized, and at least one arc is generated between the cathode assembly 3 and the anode 4. The gas H heated by the plasma exits the anode through the nozzle 41 at high speed and high temperature. Directly behind the anode 4 (seen in the direction of flow of the gas) 5 particles are blown in the form of a powder in the hot gas stream through the feed channels 51 of the powder feeder. The particles are melted or at least plasticized in the gas stream, accelerated by the gas stream and thrown onto a substrate 6, where they form a coating 7. The laden with the particles gas stream is shown schematically in Figure 1 as a coating beam B.

It is often the case in the application that the result to be achieved - ie the coating 7 on the substrate 6 and their properties are given and the thermal spraying process is adjusted so that the desired result as well as possible, efficient, inexpensive and reproducible realized. For this it is important to know process parameters.

The invention now proposes a method for determining process parameters in which an operating model is set up which comprises a fluid mechanical model and an electromagnetic model coupled thereto and one or more process parameters are determined by means of this operating model.

It has been shown that by taking into account both the fluid mechanical effects and the electromagnetic or electrodynamic effects a reliable determination of the process parameters is made possible.

Since a metrological detection of the process parameters is no longer necessary, but the injection process can be simulated, the behavior of the thermal spray device can now be analyzed even in those operating conditions that have not yet been investigated. In addition, it is possible to determine process parameters that were hitherto difficult or impossible to determine by measurement, for example those of particles (especially in their interior).

In the following an embodiment of the inventive method will now be explained.

The fluid mechanical modeling is preferably carried out by means of numerical fluid dynamics simulation (CFD - Computational Fluid Dynamics). The CFD method has become a very efficient tool for studying flows in recent years. The CFD and its fundamentals per se are known to the person skilled in the art and therefore need not be explained in more detail here.

For each flow, the three fundamental principles of conservation of mass, momentum and energy apply. However, the resulting physical relationships and equations (the Navier-Stokes equations) are no longer analytically solvable in their general form. It's the Subject of the CFD to determine numerical solutions for such equations, so as to describe a flow field as realistic as possible. The Navier-Stokes equations contain the variables describing the flow, such as velocity, pressure, density, viscosity, and temperature as a function of location and time.

For the purposes of this application, CFD is understood as the method of calculating both smooth and frictional flows of single- or multi-phase fluids (continuous phase), optionally with simultaneous consideration of the movement of liquid droplets or solid particles (disperse phase). The fluids may be compressible or incompressible. The interaction or interaction of the continuous phase with the disperse phase can be described both with the Lagrange-Euler and with the Euler-Euler models. The exchange of mass, momentum and energy can be considered either in one direction (from continuous to discrete phase or one-way coupling or vice versa) or in both directions (full coupling or two-way coupling).

Thus, it is meant both CFD methods in which the disperse phase is included in the model and CFD methods in which the disperse phase is not included in the model. This means that the particles do not necessarily have to be considered in the model. Preferably, however, the operational model also includes the particles and the interaction between the particles and the gas stream.

Both the continuous phase and the discrete phase may each contain multiple components (multi-component phase). For example, when plasma spraying a mixture of argon and helium can be used, then the continuous gas phase comprises the two components argon and helium. The discrete phase may also contain several components, for example if a powder mixture of different substances is used as particles in plasma spraying, or if already molten and still solid particles form two components of the discrete phase.

There are numerous well-known and commercially available computer program products and algorithms for CFD that are well known to those skilled in the art, so will not be discussed further here.

In fluid mechanical modeling, the flow space to be calculated is first defined as a three-dimensional volume body, for example by means of a CAD model of the spray device. Then small finite sub-volumes are defined, into which the volume body is divided. These sub-volumes make up the numerical computational grid. The boundary conditions are defined which define the physical operating conditions, for example mass flow or inlet flow rate, inlet gas temperature, wall temperature, amperage or the like. Now, with numerical procedures known per se, the flow quantities such as pressure, velocity or temperature in each sub-volume are determined. The results lead to a three-dimensional flow field, which is then quantitatively and qualitatively evaluated.

Due to the extremely high temperatures in the plasma spraying - the plasma can reach temperatures of up to 19,000 Kelvin, for example - the temperature dependence and / or the pressure dependence of the material properties is taken into account. With regard to the continuous phase, in this case the gas phase, the temperature and / or pressure dependence, in particular of the following variables can be taken into account: electrical conductivity, thermal conductivity, viscosity, specific heat capacity, electron density, molar mass, ion concentration for the various ionization stages, speed of sound. These dependencies are known or can be determined in a manner known per se.

For some process variables or qualitative statements or first approximations, for example in optimization processes, it may be sufficient to ignore the particles blown into the gas stream during plasma spraying during fluid mechanical modeling. Preferably, however, the particles are taken into account as a disperse phase in the operating model.

The fluid mechanical model is coupled according to the invention with an electromagnetic model. The at least one arc generated in the plasma spray device 1 between the cathode assembly 3 and the anode 4 heats and accelerates the gas G. The coupling between the fluid mechanical and electromagnetic model allows the description of the arc (s). The arc or the plasma in turn cause electromagnetic effects such as electrical potentials, magnetic fields, etc., whose influence is taken into account by the electromagnetic model or its coupling to the fluid mechanical model.

Boundary conditions are also defined for the electromagnetic properties, in particular for the electrical potential and for the magnetic vector potential. For the electrical potential, it can be assumed, for example, that the cathode arrangement 3 is at ground potential, that is to say zero volts, and the potential of the anode is controlled in such a way that the predetermined current flows.

The electromagnetic model is based on the Maxwell equations and the material properties for the polarization (dielectric constant), the magnetization (permeability) and the conductivity.

The coupling between the fluid mechanical model and the electromagnetic model takes place via Ohm's law, the Lorentz force (force on moving charge carriers in the magnetic field) and the resistance heating. The Lorentz force couples the electromagnetic effects with the fluid dynamics while the resistance heating couples the electromagnetic effects with the thermodynamic energy equations.

The solution of the resulting equations usually takes place numerically. The person skilled in the art is sufficiently well-known how to create and calculate such electromagnetic models per se. Computer program products are also known for this, so that no further explanations are necessary in this regard.

Programmatically, the consideration of the electromagnetic model in the form of a program module (plug-in) done, which is inserted or integrated into the CFD program for fluid dynamics modeling.

With the help of the operating model, a complete simulation of the thermal spraying process is possible. This means in particular that each process parameter can be determined by means of the operating model.

In the following, some application examples will now be explained.

Due to the fact that the entire thermal spraying process can be simulated by the operating model, it becomes possible to adapt the thermal spraying device much more quickly and efficiently to the respective application or to optimize it for the respective application. This is an important advantage, especially with regard to the new and further development of thermal spray devices. Namely, time-consuming and costly experimental series are no longer necessary for the adaptation and optimization, in which empirically motivated modifications are tested, but the influence of changes on the process parameters can be examined on the basis of the operating model without any experimental effort.

For industrial applications of thermal spraying, in particular the stability of the process is of great importance, that is to say over a longer period of time the same coating with the same properties is to be produced over and over again. Here, the inventive method can be used to identify the process variables essential process variables and to analyze the impact of their operational fluctuations.

Since the efficiency of the sprayer plays an essential role in economic terms, there is an effort to operate such spraying devices at the limit of their capacity. Here, the method according to the invention is suitable for more precisely determining these limits.

Other important aspects, in particular for industrial applications, are a high application rate (how fast can the coating be produced?), High application efficiency (how much energy is needed to apply a certain mass of coating material?) And a long service life of the device and its components , Again, the inventive method is to improve the performance of the spray device in an efficient manner significantly.

Also with regard to new and further developments of spray devices, the inventive method forms a very useful tool for optimizing the design.

It is often the case that the coating to be produced is predetermined, for example, by a customer, and the spray device or the spray method has to be adapted to these specifications.

Such constraints may be, for example, the type and degree of adhesion or adhesion of the coating to the substrate, or other properties of the coating 7 such as e.g. the structure, the crystallinity, the texture, the thickness, the porosity, the roughness, the electrical or thermal conductivity, the viscosity, the wear resistance or the degree of oxidation, to know only a few properties. In order to intentionally and controllably adjust such properties of the coating, suitable process parameters must be known.

This will be illustrated by an example: In order to produce a given coating, it is necessary, for example, for the particles of a given size to strike the substrate 6 with a setpoint temperature and a setpoint speed. Now, with the aid of the operating model, an optimization can be carried out in which the adjustable process parameters, such as current or gas flow rate are varied until the desired setpoint for the temperature and the velocity of the particles is reached within predeterminable limits when hitting the substrate 6.

Alternatively, it is also possible to first determine which speed and temperature profile the gas flow must have, so that the particles are heated to the desired temperature and accelerated to the desired speed. Subsequently, the controllable parameters are then varied until these profiles for the gas flow result. In this case, it is also possible in particular in new and further developments of spray devices to vary the geometry of the spray device as a parameter and to optimize.

If the optimization takes place via the determination of the profiles of the gas flow, it can be advantageous for reasons of efficiency to first determine some possible optimal variants, for example for the geometry of the spray device, by not taking into account the particles in the operating model. If then some possible designs or parameter combinations are determined, the refinement and finally the optimization takes place by means of an operating model in which the particles and possibly also the substrate are taken into account.

Further process parameters, which can be determined by the method according to the invention and whose knowledge is advantageous, are the state of aggregation of the particles, the trace, that is to say the trajectory of the particles, the point of impact of the particles on the substrate.

It is also advantageous to determine the velocity field or the velocity profile of the gas stream. Based on this, an optimization of the flow conditions in the spray device can be achieved.

Furthermore, it is advantageous to know the temperature profile of the gas stream. For example, unevenness in the temperature distribution, so-called hot spots, can be localized.

For example, it is possible to generate a thermal or a thermodynamic image of the spray device. With this, then the course of the cooling channels can be optimized so that just so much heat is dissipated that the temperature of the inner surfaces within preset limits to avoid erosion and other undesirable effects.

Knowing the trace or the trajectory of the particles, for example, allows targeted influencing the quality of the coating (porosity, adhesion, etc.), because it is known that these properties of the coating depend on the angle at which the particles hit the substrate.

The shape and the course of the arc or of the arcs and the associated starting points on the electrodes 3, 4 can also be determined by the method according to the invention. By means of this knowledge, the stability of the arc or of the arcs can be optimized so that, for example, a uniform and predictable heating of the gas flow results.

If the particles are considered not only as expanded structures with a specific diameter or a certain extent but as expanded bodies with varying process variables in the operating model, both the temperature at the surface and the temperature in the interior of the particles can be determined with the method according to the invention , It is also possible to create a temperature profile for the particles. It is precisely the temperature inside the particles that represents a quantity whose knowledge is of great interest, but which can not yet be measured today. The metrological determination is limited to the temperature at the surface of the particles. However, in order to influence the coating in a targeted manner, it is advantageous to know the temperature inside the particles, because it is quite possible that the particles are melted on their surface but are still solid and "cold" in their interior. This results in a high proportion of unmelted areas in the coating, which are usually undesirable.

Furthermore, it is possible to determine the temperature at the surface of the substrate 6 by means of the method according to the invention. This is advantageous because in some applications, the substrate may not be heated too much, or because a certain temperature range at the Substrate surface is necessary to achieve the predetermined properties of the coating.

In particular, the method according to the invention is also suitable for optimizing the nozzle 41 and in particular its geometry in a thermal spraying device 1, in particular in a plasma spraying device. The nozzle 41 is a replacement part, that is, depending on the application, different nozzles 41 with different geometries and correspondingly different flow properties are used. If, for example, a nozzle 41 with a large opening is used, the plasma is very hot and the speed of the exiting gas flow is lower. With a smaller nozzle opening, the gas flow is slightly cooler, but has a higher speed. To generate colder high-speed streams, for example, convergent-divergent nozzles are used, which are designed similarly to a Laval nozzle. The inventive method now makes it possible to optimize the design of the nozzle 41 so that it realizes as well as possible the predetermined process parameters for the exiting fluid or gas flow.

The method according to the invention is also suitable for operating a thermal spraying device, in particular a plasma spraying device 1. In this case, the operating model can serve to record and store given process parameters, which are not directly measurable, during operation, for example, or the operating model can be integrated into the control or regulation of the spraying device, for example, to one or more process parameters to a desired value regulate.

The inventive method is also suitable for the development and / or the implementation of hybrid processes, in which thermal spraying is combined with other processes, for example for a hybrid process cold gas spraying / plasma spraying.

The inventive method is preferably implemented in the form of a computer program product in a data processing system.

According to the invention, therefore, a method is proposed for determining process parameters in a thermal spraying process in which particles are melted or plasticized by means of a thermal spraying device (1) and transported by a fluid stream (G) to a substrate (6) the thermal spraying process or for the thermal spraying device, an operating model is built, which comprises a fluid mechanical model and an electromagnetic model, wherein the fluid mechanical model and the electromagnetic model are coupled together and at least one process parameter is determined by means of the operating model.

Claims (11)

Method for determining process parameters in a thermal spraying process, in which particles are melted or plasticized or vaporized by means of a thermal spraying device (1) and transported by a fluid flow (G) to a substrate (6), in which process for the thermal spraying process or for the thermal spraying device, an operating model is set up, with which a simulation of the thermal spraying process is feasible, and which comprises a fluid mechanical model and an electromagnetic model, wherein the fluid mechanical model and the electromagnetic model are coupled to each other and determines at least one process parameter by means of the operating model becomes. The method of claim 1, wherein the operating model comprises the interaction between the particles and the fluid stream (G). Method according to one of the preceding claims, wherein at least one of the following process parameters is determined: velocity of the particles, temperature of the particles on the surface of the particles, temperature inside the particles, state of aggregation of the particles, trace of the particles, impact point of the particles. Method according to one of the preceding claims, in which a temperature profile is established for the particles. Method according to one of the preceding claims, in which a velocity profile or a temperature profile for the fluid flow is created. Method according to one of the preceding claims, wherein for at least one process parameter, a desired value is specified and the thermal spraying device or the thermal spraying process is optimized by means of the operating model until the desired value is reached within predefinable limits. Method according to one of the preceding claims, in which the thermal spraying device comprises a nozzle (41) through which the fluid flow exits, the operating model being used to optimize the nozzle. Method according to one of the preceding claims, in which the thermal spraying device is a plasma spraying device in which at least one arc is produced between an anode (4) and a cathode arrangement (3). Method according to Claim 8, in which the shape and / or the contact points of the arc are determined by means of the operating model. Thermal spray device, in particular plasma spray device, which is operated by means of a method according to one of the preceding claims. Computer program product for implementing a method according to one of claims 1-9 in a data processing system.

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