WO2014012797A1

WO2014012797A1 - Cold gas spraying method using a carrier gas

Info

Publication number: WO2014012797A1
Application number: PCT/EP2013/064156
Authority: WO
Inventors: Oliver Stier
Original assignee: Siemens Aktiengesellschaft
Priority date: 2012-07-19
Filing date: 2013-07-04
Publication date: 2014-01-23
Also published as: EP2859133B1; DE102012212682A1; EP2859133A1

Abstract

The invention relates to a cold gas spraying method. In said method, particles are accelerated onto a substrate (18) using a cold gas stream (17) and remain adhered to said substrate. According to the invention, forming gas 95/5 with a nitrogen content of 95 mol.% and a hydrogen content of 5 mol.% is used as the carrier gas for the cold gas stream (17). Using said carrier gas, a high gas speed can be advantageously achieved in the cold gas stream (17), at any rate a higher speed than with air or nitrogen, the conventional gases for the carrier gas. The use of helium can also be advantageously omitted for reasons of cost. In addition, the mixture ratio of the forming gas advantageously allows the forming gas to be used without separate safety precautions which would be necessary if hydrogen were selected as the carrier gas without the addition of other gases. According to the invention, gas speeds of over 1400 m/s can be advantageously achieved in the cold gas stream (17) using the forming gas, thus exceeding the achievable speeds using air or nitrogen.

Description

description

The invention relates to a method for cold gas spraying, in which particles are accelerated with a carrier gas in a convergent-divergent nozzle directed onto a substrate to be coated and remain adhered to a substrate. Cold gas spraying is a process known per se, in which particles intended for coating are preferably accelerated to supersonic speed by means of a convergent-divergent nozzle, so that they adhere to the surface to be coated on account of their impressed kinetic energy. In this case, the kinetic energy of the particles is used, which leads to a plastic deformation of the same, wherein the coating particles are melted on impact only on their surface. Therefore, this method is referred to as cold gas spraying in comparison with other thermal spraying methods, because it is carried out at comparatively low temperatures at which the coating particles remain substantially fixed. Preferably, for cold gas spraying, which is also referred to as kinetic spraying, a cold gas spray system is used, which has a gas heater for heating a gas. To the gas heater a stagnation chamber is connected, which is connected on the output side with the convergent-divergent nozzle, preferably a Laval nozzle. Convergent-divergent nozzles have a merging section and a flared section connected by a nozzle throat. On the output side, the convergent-divergent nozzle generates a jet of powder in the form of a gas stream with particles in it at high speed, preferably supersonic speed.

The cold gas jet is generated in the cold gas spraying with a carrier gas, which in the front of the convergent-divergent Stagnationkämmer lying nozzle under high pressure (the stagnation pressure) stands. The carrier gas is expanded and accelerated by the nozzle and thus forms the cold gas jet. This also contains the particles intended for coating. As the carrier gas, different gases are used.

According to EP 484 533 Bl, it is stated that, due to the gas-dynamic properties, basically air is suitable as the carrier gas, which can additionally be admixed with helium. Other sources, such as EP 1 152 067 Bl introduce other gases such as nitrogen, argon, hydrogen, oxygen, water vapor or mixtures of these substances. In addition, the special properties of helium are highlighted. The special importance of inert gases is also according to the US

6,759,085 B2, wherein argon and helium are listed and are mentioned as the miscible gases nitrogen and hydrogen. According to US 2004/0037954 AI nitrogen and helium and mixtures of these two gases are listed as particularly advantageous as well as according to US 2010/0143700 Al, as these mixtures behave due to the gas-dynamic properties for the cold gas spraying even more favorable than Helium alone. Helium or

Helium mixtures are necessary in any case according to the prior art, when gas velocities of more than

1400 m / s to be achieved. According to US 2004/0037954 AI is also mentioned that hydrogen can also be used as gas, which has even more favorable gas-dynamic properties. However, it should be noted that hydrogen gas is an explosive substance and must therefore be handled with particular care. According to US 2009/0282832 Al, it is stated that in principle hydrogen, nitrogen or mixtures of these gases can be used as the carrier gas. However, detailed information on the possible mixing ratio is not provided according to this document. In the US 2010/0015467 AI hydrogen is listed among other gases such as argon, neon, helium and nitrogen and it is noted that these gases can also be mixed together NEN. In addition, according to the US 2011/0303535 AI pointed out the possibility of being able to achieve the highest gas velocities with hydrogen can. This also applies with restrictions for mixtures of hydrogen with other gases.

In summary, a wealth of different possible carrier gases is disclosed in the literature, with nitrogen, air, helium, and hydrogen being commonly named. However, because of the risk of explosion, hydrogen can only be used with high safety precautions, which is why its use is uneconomical. With ordinary cold spray systems, therefore, helium is more likely to be processed, with which also very high gas velocities can be realized. However, helium is very expensive to procure and is about two orders of magnitude higher in procurement costs than the standard gas for the nitrogen spraying process. Due to the increasing global demand for raw materials, rising prices are even expected, which makes the use of helium for cold gas spraying even more unattractive in the future, if economic arguments play a role. Nevertheless, 0. Stier et al. in its article "Cost Analysis of Cold-Sprayed MCrAlY Coatings for Industrial Power Genera- tion Gas Turbine Blades" at the North American Cold Spray Conference from 25.-27.10.2011 as main carrier gases nitrogen, air, helium or mixtures of nitrogen with helium or air with helium.

The object of the invention is to provide a carrier gas for cold gas spraying, which is economical to use and yet technically enables gas velocities of more than 1400 m / s.

This object is achieved according to the invention with the method given in the introduction by using forming gas 95/5 with a nitrogen content of 95 mol% and a hydrogen content of 5 mol% as the carrier gas. This is a commercial product, which among others by the company Linde AG is offered. The small proportion of 5 mol% hydrogen has the advantage that the ignition limit for hydrogen-nitrogen mixtures of about 5.7 mol% is exceeded and therefore the forming gas can be safely processed. The gas mixture also does not separate so that no concentration of hydrogen can occur during processing. Additional safety measures beyond those of ordinary cold gas injection are therefore advantageously not necessary. On the other hand, gas forming velocities of more than 1400 m / s can be achieved with forming gas. The proportion of

Hydrogen should be as high as possible, which may not exceed 5.7 mol%, taking into account tolerances, to ensure the processing safety. Becomes

Forming gas in the stagnation chamber of the cold spray system, for example, heated to 1000 ° C stagnation and relaxed by a stagnation pressure in the stagnation of 50 bar against atmospheric pressure, for example, flow velocities of about 1460 m / s can be achieved. This represents a clear violation of the applicable for example nitrogen nitrogen limit of 1400 m / s, where

Forming gas advantageous in relation to nitrogen is only slightly more expensive and thus the high costs associated with the use of helium as the carrier gas can be advantageously avoided.

According to an advantageous embodiment of the invention, it is provided that the forming gas 95/5 is fed to a system used for cold gas spraying as a mixture. This has the advantage that the mixing ratio can be adjusted with comparatively high accuracy by gas suppliers and can already be stored as a mixture in a dedicated memory in the vicinity of the plant.

Alternatively, however, it is also conceivable that a system used for cold gas spraying is supplied with hydrogen and nitrogen and these gases are mixed in the system for forming gas 95/5. Here, hydrogen gas and nitrogen gas are stored in separate memories and stored in a separate suitable mixing device mixed in the desired ratio. The unmixed gases are advantageously inexpensive to purchase. The safety precautions for handling hydrogen gas are advantageous in the mixture before the plant only to the mixing device, so that the system itself can already be operated with the forming gas 95/5 and the safety precautions are therefore limited to the general necessary for cold gas spraying measure , It is advantageous if the permissible tolerance in setting the mixing ratio for the hydrogen content is +/- 0.5 mol%. This ensures that a safety margin of 0.2 mol% to the ignition limit of hydrogen-nitrogen mixtures is still maintained. According to another embodiment of the invention it is provided that the forming gas 95/5 is also used as a powder conveying gas for feeding the particles into the carrier gas stream. Usually, the particles to be processed are in fact supplied to the cold spray system in that they are conveyed in a gas. In this they can be finely dispersed, so that clumping is counteracted. In addition, the powder conveying gas can be present at a pressure with which the back pressure of the carrier gas at the feed point can be overcome. If the powder conveying gas also consists of forming gas, then the carrier gas of the carrier gas stream is not changed by feeding the particles together with the powder conveying gas in its desired concentration. This has the advantage that the cold gas jet when leaving the convergent-divergent nozzle still has the optimum composition already explained above.

Furthermore, it is advantageous if the process is operated at a stagnation temperature of 800 ° to 1200 ° C., preferably 1000 ° C., and a stagnation pressure of 30 to 60 bar, preferably 50 bar. As a result, the already mentioned gas velocities of over 1400 m / s can be achieved. Further details of the invention are described below with reference to the drawing. Identical or corresponding drawing elements are each provided with the same reference numerals in the individual figures and will only be explained several times to the extent that differences arise between the individual figures. Show it

Figure 1 shows an embodiment of the method according to the invention, shown with reference to a schematic system structure for the cold gas spraying using premixed forming gas and

Figure 2 shows an alternative embodiment for the mixing of forming gas from its individual components.

According to FIG. 1, a plant 11 suitable for cold gas spraying is used for the process according to the invention. The centerpiece of this system 11 is a convergent-divergent nozzle 12, wherein in FIG. 1 a convergent part 13 and a divergent part 14 can be seen, which are connected to one another by a nozzle neck 15, the narrowest point in the nozzle 12. Before the convergent part 13 is a stagnation chamber 16, in which the carrier gas is under high pressure and flows relatively slowly.

The carrier gas leaves the nozzle 12 in the form of a cold gas jet 17 in the direction of a substrate 18, wherein the particles not shown in detail in the cold gas jet 17 adhere to the substrate 18 and form a layer 19. The substrate 18 is moved in the direction of the indicated arrow 20 for the purpose of forming a layer.

The carrier gas comes from a pressure vessel 21 for the

Forming gas 95/5. By a first line 22, the Trä gergas passes through a first compressor 23 in the stagnation chamber 16, which is pre-heated on the way with a heater 24. Via a second line 25 and a second compressor 26, a powder reservoir 27 with the in this Case used as a powder feed gas used forming gas, which can be fed in the embodiment of Figure 1 in the convergent part 13 of the nozzle. According to FIG. 2, two pressure vessels 21a and 21b are used. There is nitrogen in one, hydrogen in the other. Via a suitable mixing device 28, these two gases are mixed via throttle valves 29 and leave the mixing device 28 as a finished forming gas. The outlet 30 of the mixing device can optionally be connected to the first line 22 and / or the second line 25 of the cold spray system 11 according to FIG.

Claims

claims

1. A method for cold gas spraying, in which particles are accelerated with a carrier gas in a convergent-divergent nozzle (11) directed onto a substrate to be coated and remain adhered to a substrate (12),

characterized ,

Forming gas 95/5 with a nitrogen content of 95 mol% and a hydrogen content of 5 mol% is used as the carrier gas.

2. The method according to claim 1,

characterized ,

the forming gas 95/5 is supplied as a mixture to a plant (13) used for cold gas spraying.

3. The method according to claim 1,

characterized ,

that a system (13) used for cold gas spraying is supplied with hydrogen and nitrogen and that these gases are mixed in the system for forming gas 95/5.

4. The method according to claim 3,

characterized ,

that the allowable tolerance in adjusting the mixing ratio for the hydrogen content is plus / minus 0.5 mol%.

5. The method according to any one of the preceding claims,

characterized ,

that the forming gas 95/5 is also used as a powder feed gas for feeding the particles into the carrier gas stream.

6. The method according to any one of the preceding claims,

characterized ,

that the process at a stagnation temperature of 800 to 1200 ° C, preferably 1000 ° C, and a stagnation pressure of 30 to 60 bar, preferably 50 bar, operated.