JPH0512431B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD This invention relates to a method of manufacturing porous metal structures by thermal spraying, such as plasma arc spraying. BACKGROUND ART Porous metal structures have been manufactured by various methods and used for various purposes. Porous metal structures have been found to be particularly useful in abradable seals, which wear quickly and easily when contacted by members moving at high speeds; A structure that remains intact when not contacted by moving members. Polishable seals are particularly used in turbomachinery. In this regard, for example, U.S. Pat.
See 4049428 and 3111396. One preferred method of manufacturing porous metal structures is to form a compression compact of metal and fugitive material, and then to efface the fugitive material, thereby creating a metal structure with a density of less than 100%. It is to form. Conventional powder metallurgy methods are conventionally used, which involve forming a mixture, compacting it, and sintering it. No. 3,864,124 in this regard;
Please refer to the same No. 3897221 and the same No. 3350178. Alloys made from sintered metal powders are less preferred because they undergo substantial shrinkage during sintering, and plasma arc spraying has traditionally been preferred for producing porous metal structures. According to plasma arc spraying, metal particles are bonded together during plasma arc spraying of a metal-polymer mixture. Thus, subsequent sintering is not required or used without causing excessive shrinkage. Composites are conventionally first formed, generally in accordance with the disclosure of US Pat. No. 3,723,165, in which a metal powder such as nichrome is thermally sprayed with a high temperature polyester powder such as poly(paraoxybenzoyl) having a high melting point. Polyester softens during thermal spraying but does not melt. Although the specific process of producing porous metal structures by subsequent oxidation of the polymer is not mentioned in the aforementioned U.S. Pat. No. 3,723,165, experimental porous structures for gas turbine engines are The above-mentioned steps are conventionally performed in the manufacture of seal structures. Since the polyester used is a high temperature material, it is necessary to heat the structure to a temperature of 540° C. in order to oxidize the polyester and make it disappear as a fugitive material. However, while the polishable seals thus produced are useful for gas turbine engine applications, the cost of the polyester resin particles is high. Accordingly, there is a continuing need to reduce the cost and improve the performance of abradable seals comprising porous metal structures by altering the physical and chemical properties of the porous metal structures. . DISCLOSURE OF THE INVENTION It is an object of the present invention to provide an improved method for manufacturing porous metal structures, especially abrasive seals for turbine engines that have improved performance and are low cost. An object of the present invention is to provide an improved method for manufacturing seals. According to the present invention, the present invention provides a method for manufacturing a porous metal structure with high polishability, which involves an endothermic reaction with the metal from which the porous metal structure is to be formed at a temperature below 540°C. A mixture of the metal powder and the polymer powder is thermally sprayed onto a substrate so that the polymer powder is melted and welded in a cohesive state to a desired size, and The polymer contained in the weld deposit is removed from the weld deposit formed by thermal spraying at a temperature of less than 540° C. for a substantial period of time, and the metal oxide content in the weld deposit is reduced to 30 wt% or less. This is achieved by a method for producing a porous metal structure characterized by suppressing heating. According to this configuration, in the thermal spraying process, at least a portion of the metals melts due to their high thermal conductivity, and the metals are welded onto the base body in a state where they are joined to each other. Polymers have lower melting and boiling points than metals, but due to their low thermal conductivity, they do not evaporate during thermal spraying and are melted into droplets, where multiple polymer droplets in close proximity coalesce into larger dimensions. It is welded onto the substrate in an aggregated state. Immediately after thermal spraying, the temperature drops so that the polymer in the deposit becomes solid again, forming a deposit containing polymer of larger dimensions than the polymer powder before spraying. Then, in the process of heating the weld at a relatively low temperature of less than 540° C., the polymer in the weld undergoes an endothermic reaction, converts into a gas, and is removed from the weld.
By heating and removing the polymer through an endothermic reaction at a relatively low temperature, the temperature of the metal portion in the remaining weld is kept relatively low, and the oxide content thereof is suppressed to 30 wt% or less. Reducing the oxide content in an abradable seal material improves its performance, particularly as it relates to avoiding glazing during interaction with compressor blades. The porous metal structures produced in this manner also have relatively large pore sizes, which improves their polishability due to their structure. As for the thermal spraying method, plasma arc spraying or other thermal spraying methods that avoid excessive oxidation of the particles are preferably used to form the initial deposit of the polymer and metal mixture on the substrate. The polymer may be heated to relatively low temperatures, depolymerized and vaporized, or chemically combined and vaporized, or melted, or by other processes that do not involve excessive heat generation. In this case, depolymerization is most preferred. This is because depolymerization is an endothermic reaction and is therefore preferable to oxidation reactions, which are exothermic reactions that raise the temperature and cause undesirable oxidation and other reactions. If the invention is applied to a nichrome alloy, the oxide content in the structure will be 4-30 wt%, which is 26-40% of the density of the solid metal. Such levels are above the typical oxide content of nichrome porous metals produced by conventional methods using high temperature polymers.
This is comparable to ~40wt%. The polymer and metal deposits are preferably heated in air to reduce costs. For nichrome alloys, this means carrying out the polymer removal process at relatively low temperatures below 540°C, preferably between 250 and 430°C. In one preferred embodiment of the invention,
14wt% polymethyl methacrylate powder is 86wt
% 80Ni-20Cr powder. The deposit thus formed is heated to about 315°C to convert the polymer into volatile monomers. The porous metal structure thus obtained has an apparent specific gravity of 2.7 g/cc, or approximately 32% of the theoretical density of the metal. The use of low temperature polymers that can be melted during thermal spraying with virtually no evaporation improves the properties of porous metal structures compared to the use of high temperature polymers that only soften due to heat during thermal spraying. Improved. Such porous metal structures have excellent performance as polishable seals due to their low oxide content. DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention will be explained in detail below by way of example embodiments with reference to the accompanying figures. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will now be described with respect to manufacturing a nickel-chromium alloy (80Ni-20Cr) polishable seal structure. Thermal spraying of mixtures of metals and polymers is disclosed in US Pat. Nos. 3,179,784 and 3,723,165. In a preferred embodiment of the invention, a polymer powder having a depolymerization temperature of 430° C. or less is used. The polymer powder was Lucite Grade 4FNC−99 powder (Dupont
Preferred is polymethyl methacrylate, such as Polymethyl methacrylate, such as A. 14wt% Lucite and 86wt%
A powder mixture of nichrome is sprayed. Nichrome is -250+500Tyler Sieve Series mesh size (hereinafter referred to as "-250 Nichrome"),
Lucite is -80 + 400 meshes. Approximately 0.2 wt% of sub-1 micron silica particles, such as Cab-o-Sil powder (Cabot Corporation), are optionally added to improve the flow properties of the powder mixture. The powder mixture is passed through a plasma arc torch and deposited to a thickness of about 2 mm onto a suitably prepared substrate, such as a nickel superalloy workpiece, by procedures generally known for plasma arc spraying. After the desired thickness of the deposit is obtained, the deposit is heated in air, vacuum, or an inert atmosphere. Air is preferably used for simplicity. The article is heated to a temperature of about 315° C. for about 2 hours to depolymerize the polymer and form monomers that volatilize from the article, thereby forming a porous metal structure on the substrate. Figure 1 is a micrograph showing the cross-sectional structure of the nichrome metal layer thus formed, and the metal layer has an apparent density of 2.7 g/cc, which is about 32% of the theoretical density of nichrome (the first The structure shown in Figures 1 and 2 is substantially the same when viewed perpendicular to the cross section of the figures). Density is determined in the conventional manner by dividing the weight of the specimen by the volume enclosed by the outer surface of the specimen.
However, the porous structure of Nichrome, as described above, is comprised of metal and about 25% metal oxide. Ni-
The specific gravity of Cr oxide (the characteristics of which are not precisely known) is lower than that of metal Ni-Cr.
The actual porosity of the porous structure produced according to the invention is thus less than the value indicated by the ratio of the specific gravity of the porous structure to the specific gravity of a completely dense metal. For example, it has an apparent density of 32%
A porous structure of 2.7 g/cc of 8.4 g/cc nichrome is
Somewhat less than 68% porosity. To appreciate the advantages of the present invention, it is important to note that the desired function of an abradable material is that it remains integral with the substrate even under conditions of particle erosion and other mechanical stresses. You have to understand that. Moreover, the abradable material must be easily disintegrated into pieces when it is contacted by a member moving at high speed, such as the tip of a blade. Such characteristics are called abradability. If such easy collapse behavior were not present, the blade tip would be overheated and degraded. In performance tests that simulate operation within a gas turbine engine, porous structures made using polymethyl methacrylate resin performed surprisingly better than porous structures made using polyester resin. I found it to be different and better. Most significantly, the abradable material of the present invention has less tendency to glaze when contacted with the blade tip than similar structures made using polyester resins. . Glazing is an indicator of poor abrasiveness. The inventors have discovered that performance can be improved by using meltable low temperature polymer powders such as polymethyl methacrylate. First, the oxide content of the seal made with polyester resin is approximately 35 wt%, whereas the oxide content of the improved polishable seal is -250 wt% when nichrome is used. is about 25wt%
It was found that Relatively high oxide contents appear to be desirable in that relatively high oxide contents generally lead to embrittlement of metals based on relevant experience; It has been found that relatively low content seals actually improve performance by reducing the tendency to glaze as described above. Third
The figure shows how glazing depends on oxide content over a certain useful range of densities for nichrome. The data in Figure 3 shows the friction of a porous metal structure contacted by six simulated AMS 4928 titanium alloy blade tips at a friction velocity of approximately 290 m/s at a temperature of approximately 24°C. This is based on the results of visual observation of the exposed surface. Glazing is evident if the abraded surface of the sprayed porous metal structure exhibits a shiny, metallic appearance after the abrasion test, rather than a dull appearance. Furthermore, the occurrence of glazing is significantly lower than under no glazing conditions, i.e., the total volume of titanium lost from the blade during the friction test is 0.5-2% of the total volume of material removed from the seal structure. This is obvious from the fact that the wear of the friction blade is more pronounced than when the friction force is small. The oxide content of the seal structure can be calculated using conventional dipping methods. For nichrome, hot methanol-5% bromine is used and the insoluble residue is considered the oxide. In conventional processes that utilize the high temperature properties of polyester resin to remove polyester resin from deposits, a furnace set at approximately 540°C must be used. It has been found that oxidation of the polymer during such resin removal steps increases the temperature of the porous metal structure to about 620°C due to an exothermic reaction. Due to the large surface area of the porous metal structure, nichrome is a highly oxidation resistant alloy, but a relatively high degree of oxidation occurs. The polymer removal process is thus important and requires the use of fugitive materials that depolymerize or otherwise disappear at low temperatures. Oxides are generally dispersed throughout the metal of the porous metal structure and are not concentrated on the surface surrounding visually observable pores. Table 1 below shows the oxide content of 80Ni-20Cr porous material of 2.7 g/cc obtained by exposure for 100 hours to various calcination temperatures used to remove various fugitive materials. ing. The data in Table 1 shows that the oxidation resistance of weldments made with polymethyl methacrylate (Lucite) is unexpectedly superior to similar weldments made with polyester (Ekonol). It shows that there is. This is believed to be due to the more favorable pore structure provided by the meltable polymer. According to the method of the present invention, 35% or less
Desired oxide contents typically in the range 20-30% can be achieved. For metals with oxide properties similar to 80Ni−20Cr, generally around 540°C
At this temperature, the polymer contained in the weld is vaporized and removed, for example by depolymerizing into a monomer. In the case of metals with low oxidation resistance, lower temperatures than the above-mentioned values are required.

[Table] Second, as far as abrasion resistance is concerned, porous metals manufactured using meltable low-temperature polymers are superior to porous metals manufactured using polymers that are softened by heat at relatively high temperatures. The physical structure of the quality metal is improved. This can be seen by comparing FIG. 1 with FIG. 2. Figure 2 shows 75wt% Ni−
Figure 2 is a micrograph showing the cross-sectional structure of a seal formed by thermal spraying a mixture of Cr and -150+325 mesh of 25 wt% Econol polyparaoxybenzoyl polyester resin (Carborundum Co. and Metco Inc.). The specimen shown in Figure 2 has a density of 2.8 g/cc, which is virtually the same as the density of the specimen shown in Figure 1 (the same porosity is obtained due to the relatively lower welding efficiency). In order to achieve this, it is necessary to make the volume fraction of Ekonol higher than that of Lucite). Furthermore, it can be seen that the porosity of the structure shown in FIG. 1 is relatively high. This seems to be due to the behavior of Lucite particles. This is because Lucite particles melt and aggregate in the process of transferring to the substrate, whereas Ekonol does not cause such a phenomenon.
The melting type of polymer produces a wide range of pore sizes, and the relatively large amount of large pores gives the structure a more porous appearance. Figures 5 and 6 illustrate the difference in behavior between particles of Lucite and Ekonol resins when they are plasma arc sprayed into free air and collected. The Ekonol resin shown in FIG. 5 does not show any signs of melting and remains as separate particles, whereas the Lucite resin melts and aggregates into spherical shapes. Generally, the invention involves the use of polymers that melt and volatilize at temperatures below those that generate 30% or more oxides in the metal. A variety of polymers are suitable for nichrome, including those selected from the general group consisting of polystyrene, polyethylene, polypropylene, and polyacrylate;
Disappears under temperature conditions below 540℃. For example, polymethyl methacrylate decomposes at about 250℃,
For convenience and speed purposes, it is heated to about 350°C. Another reason why polymethyl methacrylate is preferred is that it is readily available as particles at low cost. Of course, the primary source of oxidation of the porous structure in the method of the present invention involves the step to remove the fugitive polymer. However, care must also be taken to prevent oxidation during the thermal spray process. For nichrome-polymer, approx.
Good results are obtained by plasma arc spraying in air with a 1:1 argon-helium atmosphere gas at an enthalpy of 7 kWhr/m ³ . Plasma arc spraying in air is generally useful because it involves the use of non-oxidizing gases. However, other thermal spray methods, such as combustion spray and explosive gun processes, are equally useful if they can form relatively small deposits, such as those with an oxide content of about 25% or less. . Also, of course, the oxide content of the deposit will vary depending on the size of the metal powder used, with the oxide content being lower when relatively coarse powders are used. −
For nichrome 250, the oxide content after the polymer is removed is in the range of 4-10%. The as-sprayed deposits formed by spraying relatively coarse metal powders not only contain small amounts of oxides;
It has been found that the oxidation rate at a constant temperature in the range of 200-650°C is low (due to differences in the characteristics of the sprayed structures). -325 mesh +500
mesh powder (-325 mesh 80Ni−20Cr)
Refer again to Table 1 above, which shows the higher oxide content compared to the -240 mesh 80Ni-20Cr. If abradable seals to be used at high temperatures are manufactured, the weld may be made of, for example, IN718 alloy or
Sprayed onto curved parts of nichrome superalloys and iron superalloys such as AISI 140 alloy. After the fugitive material is removed, the porous metal structure can be successfully bonded to the superalloy substrate. This means that the porous metal structure is fixed within the engine. To produce such structures of nichrome, 80-90 wt% nichrome is sprayed with 20-10 wt% polymethyl methacrylate. This mixture forms a deposit containing 35-45 vol% polymer, preferably 37-43 vol% polymer. The metal weld thus obtained has a pore content of 50 to 70 vol% after the polymer has disappeared;
Its apparent density is therefore 30-50%. As one particular example, the nichrome specimen described above, formed by spraying 86 wt% nichrome with 14 wt% polymethyl methacrylate, had a density of about 2.7 g/cc and an oxide content of 7%. are doing. Weight loss measurements during removal of the fugitive polymer indicate that 43 vol% of the deposit was polymer. The apparent density is 32% and therefore the apparent pore content is 68%, unless a small correction is made for the volume of metal oxide compared to the pure alloy. The pore content of the finished weld is thus higher than that provided by the polymer. In general, it is well known that coatings formed by plasma spraying are already porous in the as-sprayed state, and both the generation of inherent pores and the generation of pores due to the above-mentioned process work together. and provide the finished product. In order to obtain similarly good results with the other polymers mentioned above, appropriate corrections must be made in the composition of the deposit to compensate for density changes and obtain the desired porosity. Further corrections must also be made to the composition of the sprayed mixture to compensate for differences in the deposition efficiency of the polymeric and metallic components. The present invention is particularly useful for porous abradable seal structures formed from nichrome alloys since nichrome alloys are commonly known in a variety of compositions. This is because such materials have previously shown better performance in seals made by traditional methods. However, the invention is equally applicable to other materials such as nickel-containing alloys and alloys based on iron, cobalt, and aluminum. Although the present invention has been described in detail with respect to specific embodiments above, the present invention is not limited to such embodiments, and it is understood that various embodiments are possible within the scope of the present invention. It will be clear to those skilled in the art.

[Brief explanation of drawings]

FIG. 1 is a micrograph showing the cross-sectional structure of a nichrome porous metal structure manufactured using polymethyl methacrylate according to the present invention. FIG. 2 is a microscope similar to FIG. 1 showing the structure of a nichrome porous metal structure manufactured using polyester according to the prior art and having different porosity characteristics than the structure shown in FIG. It's a photo. Figure 3 is
Figure 2 is a graph showing how oxide content affects the glazing properties of a 80Ni-20Cr nichrome structure (preferably no glazing). FIG. 4 is a micrograph showing the shape of polymethyl methacrylate (Lucite) particles after being thermally sprayed into free air. FIG. 5 is a photomicrograph similar to FIG. 4 showing the shape of polyparaoxybenzoyl (Ekonol) particles after being thermally sprayed into free air.

Claims (1)

[Claims] 1. A method for producing a porous metal structure with high polishability, which transforms into a gas through an endothermic reaction with the metal from which the porous metal structure is to be formed at a temperature below 540°C. A mixture of the metal powder and the polymer powder is thermally sprayed onto a substrate so that the polymer powder is melted and welded to a desired size, and the thermal spraying heating the formed weld at a temperature below 540° C. for a substantial period of time to remove the polymer contained in the weld and keep the oxidation content of metal in the weld to 30 wt% or less; A method for manufacturing a porous metal structure, characterized by: 2. The method for manufacturing a porous metal structure according to claim 1, wherein a nickel alloy is used as the metal, a polyacrylate polymer is used as the polymer, and 35 to 45 vol% of the polymer is applied by the thermal spraying. A method of manufacturing a porous metal structure for use as a seal in a gas turbine engine, the method comprising forming a weld comprising: heating the weld in air to a temperature below 540°C.