DE3119289A1 - "RESERVAGE MATERIAL FOR VACUUM WELDING" - Google Patents

Description

Patent attorney DR. RICHARD KNElSSL Widonm ^ yerstr. 46

D-8000 MUNICH Tel 089/295125

Munich, May 14, 1981

DE 19 / Dr. K-by

United Technologies Corp. in Hartford, Ct./V.St.A.

Reservage material for vacuum welding

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description

The invention relates to the field of reserve materials, especially those that are suitable for diffusion welding and brazing in vacuum.

When diffusion welding metals, especially superalloys, that have to be processed at high temperatures, there is often a need for a reserve material, to connect abutting material surfaces at certain Places or material surfaces with the printing tools to prevent. Furthermore, reserve materials are used in brazing used to prevent an undesirable flow of brazing material and hence bonding to certain Make or prevent unwanted loss of brazing material from the joint.

Reservoirs are generally made from coverings made of a ceramic or other material that does not chemically react with or bond to the metal to be protected. The reserve materials must be easy to apply, must adhere well during handling and connection, and must not adversely affect the material to be connected. Particularly strict requirements are met when parts made of superalloys are connected in a vacuum at high temperatures, such as 1200 to 125O ⁰ C, for example. There are many methods and materials for creating a reserve. However, they are not particularly suitable for complex shapes, high temperatures and especially vacuum. For example, ceramic materials form thick deposits and uneven surfaces, while papers and felts made from aluminum silicate fibers contain organic binders or, for other reasons, decompose at high temperatures, which leads to a breakdown of the metal surface.

surfaces under vacuum. Commercially available reserve materials consist of aluminum oxide, titanium oxide, magnesium oxide and calcium compounds in powder form, which are dispersed in a liquid carrier. They can be painted or sprayed onto a metal surface will. However, if the carrier is made of water, a volatile hydrocarbon, or the like, they will adhere Reservage materials are not on the surface and are easily removed during handling. Most of the reserve materials contain binders with a peculiar and unexplained nature. Mostly they are silicates, Phosphates and the like binders such as sodium silicate can be mixed with superalloys unfold harmful effects at high temperatures. Both sodium and silicon are great for such alloys harmful. The same can be said of phosphorus. Of course, the commercially available reserve materials were used created for brazing, which is carried out at atmospheric pressure. But if vacuum and a high Temperature prevail, then there is a tendency for residues to volatilize from the commercially available materials, what makes it difficult to set and maintain the desired vacuum.

So there is a need for improved reserve material, which is easy to apply, which adheres well and resists damage during handling, but which does not interfere with a vacuum or react harmful to a metal at high temperatures.

The invention was therefore based on the object of preventing the adhesion of metals at high temperatures in a vacuum .zu, without any harmful effect being exerted on the metals.

To solve this problem, an improved one is provided according to the invention Reservage material proposed, which consists of a grain

combination of fine ceramic particles and a polymer Binder consists, the latter turning into an easily evaporable one at low temperatures in a vacuum Monomer depolymerized. The binder is thus removed from the workpiece by the evacuation system, with only the ceramic Particles remain on the metal surfaces to be covered, so that the possibility of interactions and reactions is reduced. The ceramic particles adhere even during processing at high temperatures, so that they fulfill their function. The ceramic particles are selected from a group of materials, such as refractory metal oxides, which at high temperatures with the Metal are thermodynamically stable.

The binder has the relatively unique property that it depolymerizes in the absence of oxygen. Most polymers do not enter into such a reaction, but remain as a relatively non-volatile material back and thus react at a high temperature with the metal or build up into a residue away. Examples of materials that will suitably depolymerize are polystyrene and polymethyl acrylate. In order to disperse the binder evenly in the ceramic particles, the binder is preferably in a solvent, such as an aromatic or halogenated hydrocarbon. To the combination of binder, solvent and ceramic ,. Material sufficient processability, such as fluidity, a diluent such as ethyl acetate (paint thinner) can be used as needed be admitted. The ceramic particulate material is preferably composed of a finely divided metal oxide, with it making it easier to apply a uniform, thin coating becomes, and thus after the evaporation of the binder, during the processing of the metals a low, but sufficient functional adhesion between the particles is produced.

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Homogenized alumina and platelet-shaped particles are particularly suitable according to the invention.

A usable reserve material can be obtained by cold mixing getting produced. It can be applied to the workpiece, for example, by painting or spraying. The diluent, which is selected from the point of view of high volatility, disappears in the generally at room temperature by evaporation together with part of the solvent. When the work pieces are put together and'in a vacuum and heated to a low temperature, then the remaining Solvent removed. Transformed with increasing time and temperature when heated in the range of 150-425 ° C the binder is almost completely absorbed by the monomer and is also removed. Because during this process the temperature is low, there is no adverse reaction between the binder and any metals. The ceramic particles that remain on the surface are caused by attraction held between the particles. At high processing temperatures the stability of the ceramic material prevents any adhesion of those in contact with one another Metal surfaces. In the same way, the non-wettability of the ceramic material can be used to Avoid leakage of brazing materials from the connection and thereby wetting and distribution of the brazing material to prevent outside the junction.

The reserve material according to the invention can easily with relatively low cost and made from readily available materials. It has good workability and easy to apply. The reserve material is compatible with a wide variety of metals and processing temperatures. While the invention is particular to the processing of nickel superalloys in vacuum

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tet, but it can also be used for other purposes.

The accompanying drawing shows the use of a reserve in joining two airfoil halves using a mold and an intermediate layer to create hollow air vanes to manufacture.

The invention is particularly suitable for use with nickel superalloys that are produced by a diffusion welding process, especially by TLP ^ -Binding ^ at temperatures from 1200-1250 C in vacuum. The invention but has also proven useful in other methods where joining one metal surface to another should be prevented.

The invention is particularly suitable for use in processing superalloys. However, it is in It will subsequently be shown, that the invention is also suitable for the processing of other materials. The processing temperatures of superalloys are generally in the range of 950-1300 ° C. Most of the others Metals of interest, such as iron, nickel, copper, titanium and other base alloys, include the temperature which diffusion welding or brazing is carried out, generally above 550 C. Brazing is generally called Process defined that takes place above 425 ° C. If so When speaking of "high temperatures" here, temperatures above 425 ° C are meant. If here from "lower Temperatures "is spoken, then it means temperatures generally from 150-425 ° C.

As already mentioned, the attached illustration shows the use of the invention in connecting two halves a turbine blade, so that a hollow component is created. For a better understanding of the invention, reference is made to US Pat

4,089,456 "Controlled Pressure Diffusion Bonding and Fixture Therefor" and US Pat. No. 3,678,570, which describes the TLP bonding process, should be referred to. According to the invention, two blade halves 12, 12 ′ are located between two tools 14, 14 ¹ . A braze interlayer 16 lies between the opposing surfaces of the blade halves where bonding is desired. Since it is desirable to prevent the metal of the tools from adhering to the superalloy blade halves, a first reserve coating 18 is applied to the surfaces 17 of the tools where they contact the blade halves. Alternatively, the coating can be applied to the blade surface or also to both metal surfaces that come into contact. Similarly, a second covering of reserve material 20 is applied to the interior of the cavity of the part in the vicinity of the joints. So when the intermediate layer melts, the reserve material prevents the braze from flowing by capillary action and wetting the metal. The reserve material prevents the braze from coming into contact with the metal and thus prevents it from flowing out of the joint. Often, however, the second coating 20 is not required for a precision connection. The desired general properties of a good reserve material can be understood from the drawing. They are also known to those skilled in the brazing arts. The reserve material must adhere sufficiently to the metal surfaces as well as to itself so that small particles do not peel off, damaging the reserve and entering the joint area. Once the parts are in the heating device, the adhesion requirements are greatly reduced since there is little mechanical failure. In many cases, the reserve material is also trapped between the desired surfaces between which binding is to be prevented. The reserve material is such that

it can be easily applied evenly so as to interfere with the fit of the tools with the external surfaces of the shovels ^! should be avoided. In addition, the reserve material must consist of a material that can withstand high Temperatures performs its functions, but does not react with the workpieces. As the parts in a heating device Generally used in the cold state, any residue or other volatile material will be removed implemented or driven off during the heating stages, depending on their nature. A desired one Reservage material does not create products that react with the workpieces or tools or excessively vacuum disturb.

As discussed below, applying a fine ceramic particulate material to the workpiece will be best achieved by spraying a combination of binder, solvent and thinner. All components up to on the fine ceramic particles are removed from the workpiece before the actual hot working is performed will. Since the ceramic particles are selected from a group of thermodynamically stable compounds, the neither react with the workpieces nor with the fastenings and tools or wet them, there are no harmful ones Influences at high temperatures.

The preferred reserve material consists of homogenized aluminum oxide and a polystyrene binder. To the binder and to distribute the alumina into one another, the polystyrene is dissolved in a solvent such as an aromatic one Hydrocarbon, dissolved. In order to make the resulting mixture suitable for most application methods, a volatile diluent which is compatible with both the binder and the solvent is added. For the production of a reserve material for application by spraying, the following composition is preferred: un-

about 1250 g of homogenized alumina with a particle size of about 8 µ (available from Norton Co., Worcester, Mass.), 100 g of polystyrene beads, 600 ml of toluene solvent and 1800 ml of ethyl acetate lacquer thinner.

The preferred reserve material is at room temperature made in the following manner.

1. 100 g of polystyrene are dissolved in 600 ml of toluene. The dissolution must be over before the next stage.

2. The mixture from step 1 is diluted with 1800 ml of ethyl acetate mixed.

3. Homogenized aluminum oxide is dispersed in the mixture of stage 2, corresponding to a ratio of 200 g of aluminum oxide to each 400 ml mixture of level 2.

The constituents can of course be varied. Areas that are in a preferred reservage material · good 80-100 g of polystyrene, 500-700 ml of toluene, 160-240 g of aluminum oxide and 900-3600 ml of ethyl acetate can be used.

The well-mixed preferred formulation that emerges from the The above steps is applied to a metal surface by spraying, which is achieved by conventional metal cleaning methods has been cleaned, such as sandblasting, steam degreasing, etc. If spraying with a propellant, such as air, then the propellant must be clean and free of impurities such as moisture, oie and inorganic parts. After applying if the reserve material is allowed to air dry, with most of the diluent and most likely some of the solvent evaporating. The thickness of the reserve material is between 0.05 and -0.5 mm, preferably between about 0.13 and 0.25 mm. Then will

the parts to be joined are placed in the heating chamber and heated to a low temperature, preferably under vacuum, with any residual solvent and diluent evaporating.

On further heating in vacuum it can be observed that the polystyrene binder when heated in the absence of air the shows a relatively unusual property of chain depolymerization. Polystyrene transforms in the range of 250-350 ° C practically completely into the styrene monomer and others volatile 'low molecular weight oligomers. Styrene, an aromatic vinyl compound, is volatile (boiling point approx 145 C). Because of the volatile properties of the depolymerization products and the low evaporation temperature there will be no reaction with the metal parts before they are removed from the evacuation system. So all that remains is the aluminum oxide on the surface, which takes over the reservation function. The behavior of the polystyrene binder is in place in contrast to many other organic polymers, which have binding properties. These require less than 425 ° C oxygen for a complete breakdown, or they decompose gradually or by arbitrary chain breakage. When these or other types of polymer binders on the metal surfaces remain while the temperature is ramped up, they can form charcoal or otherwise decompose and thereby adversely affect the metal. In the phenomenon of chain depolymerization takes place a gradual one Release of predominantly monomer units from one chain end instead. The volatile products additionally include to the monomer the dimer, the trimer and other higher fragments. Other materials that have the properties you want can also be used. Polymethyl methacrylate is another suitable material. The properties the selected polymers in vacuum and the method for determining suitable polymers for the invention

which is located in S.I. Madorsky in "Rates of Thermal Degradation of Polystyrene and Polyethylene in a Vacuum", Journal of Polymer Science, Vol. IX, No. 2, pp. 133-156 (1952). See also generally "Polymer Science", A.D. Jenkins, Ed., Cape. 22, 25, North-Holland Publishing Co. (1972). From these references it can be seen that the binders (a) are decomposed like a zipper, (b) form volatile decomposition products and c) volatilization rates of more than 0.1% / min at 35O ° C. If the polymer has too slow a rate of volatilization, then too long heating times are required. Polymers which meet the criteria according to the invention allow continuous heating from room temperature to a high processing temperature, for example at one rate of 35 ° C / min while maintaining the vacuum at approximately 0.0133 Pa or better.

The amount of binder can be selected according to the temporary adhesion requirements and the properties of the workpiece and the particulate ceramic material. A workpiece that has a relatively poorly receptive surface requires more binder (and possibly the addition of a surfactant). The preferred homogenized alumina has particularly desirable flaky particles and a high surface area. Other less desirable ceramics require more binders than stated. The useful proportions of binder and particulate matter cannot readily be stated since the proportion of binder necessarily decreases with a high density ceramic material and increases with a smaller particle size (larger surface area). However, it can be said that the least amount is that which coats the particles sufficiently to allow them to adhere to one another and to the workpiece and to hold the coating in place prior to heating. It

it can also be said that the maximum amount must be smaller than that which has such a high particle distance creates a continuous barrier layer in intimate contact with the workpiece after evaporation of the Binder does not arise, which means that too much binder results in a low concentration of particles and that they fall off when the tie is removed.

The polystyrene binder is dissolved in the toluene solvent, to make a liquid so that the alumina particles can be completely dispersed in the binder. Accordingly, the amount of solvent can be varied from that indicated above, both for convenience and as a function of the properties of the ceramic Particle. If too little solvent is used, the solution will have too high a viscosity, what. one Distribution of ceramic particles makes it difficult, though If too much solvent is used, undesirably long drying times for the covering can result. In the preferred In the formulation, approximately 100 ml of polystyrene per 300 ml of alumina or a volume ratio of 1: 3 are used. If ceramic material with a higher density is used, then the ratio will change according to the density change. If a very fine material with a large surface area is used, the amount of binder is increased accordingly. Here, however, the shape of the particles and the factors of attraction have to be taken into account. . .

The diluent is used in an amount sufficient to to give the mixture of binder and ceramic particles an increased flowability, so that the application is easier will. The desired properties for the diluent are low viscosity and preferably high Volatility. With high volatility, there is a boiling point of about 70-75 ° C or lower and a vapor pressure of no more

as 90-100 mm Hg at 25 ° C, - both at standard pressure, meant. However, lower volatilities are acceptable when dry time is not important or when the pavement is wet is introduced into the vacuum. In this regard, the solvent can also act as a diluent in an increased amount works. No diluent may be needed when the mixture of solvent, binder and ceramic material has sufficient processability, for example when the reserve material is applied as a paste.

Instead of the above components, other materials can be used can be used. The preferred particulate homogenized alumina is a fine particulate one Material. A fine particle size is desirable because uniform deposits in a relatively small thickness are easier can be achieved, but larger particles can also be used. The ceramic particulate material should be in vacuum at the high processing temperatures in contact with the workpiece or the tool metals chemically be stable. Refractory metal oxides or relatively refractory materials such as aluminum oxide, Zirconia and magnesia, preferred for their thermodynamic properties and low cost, but other compounds can also be selected on the basis of known properties or on the basis of experimentation will.

Other aromatic hydrocarbon solvents can also be used. For example, xylene has been found in of the preferred formulation has been found to be satisfactory. Halogenated hydrocarbons such as 1,1,1-trichloroethane also have the ability to dissolve polymer binders such as polystyrene, so they are also used can be. However, there is a tendency to avoid chlorine-containing compounds, as they may be unfavorable

Reactions with certain critical materials must be considered. Of course you can in certain cases the ignition properties of aromatic hydrocarbons may be undesirable. In such a case, they will be halogenated Hydrocarbons preferred. Other solvents can also be used, such as aliphatic Hydrocarbons. An example is ethylene glycol monoethyl ether acetate (Cellosolve acetate) suitable. Another useful solvent is N-methylpyrrolidone, which can be used with water as a diluent. There is flexibility in the choice of solvent. Other suitable solvents can certainly be found by simple experiments.

The diluent must, of course, be compatible with the solvent and binder and based on those mentioned above Properties can be selected. Factors such as flammability, toxicity, bad smell, etc., influence the choice of diluent, of course. Table 1 lists some compatible solvent / diluent combinations which can be used with homogenized alumina and polystyrene.

Table 1

Useful reserve systems for polystyrene binders

Solvent thinner

Toluene ethyl acetate

Xylene ethyl acetate

N-methylpyrrolidone water

Cellosolve acetate ethyl acetate

1,1,1-trichloroethane 1,1,1-trichloroethane

As mentioned above and as indicated in the table, excess amounts of solvent can partially or can be used entirely as a substitute for the diluent in order to give the reserve material sufficient workability admit.

Claims (8)

3119283 P at en tan s check 1. For a metal bonding process at high temperatures prepared article in which selected areas are coated with a reserve material, thereby characterized in that the reserve material consists of fine ceramic particles and a polymer binder, which at low temperatures in a vacuum with the formation of volatile components by chain depolymerization degrades. 2. Article according to claim 1, characterized in that that the binder is made of polystyrene or polymethyl methacrylate consists. 3. Article according to claim 1 or 2, characterized in that the ceramic particles of homogenized Alumina (levigated alumina) consist. 4. The object of claim 1, characterized by the Presence of a hydrocarbon solvent consisting of aromatic hydrocarbons and aliphatic Hydrocarbons is selected. 5. The article of claim 4, characterized in that the reserve material. such proportions of chemical Has substances that result from dissolving approximately 80-100 g of polystyrene binder in 500-700 ml an aromatic hydrocarbon solvent, mixing with 160-240 g of homogenized aluminum oxide part- . Chen and incorporation of a sufficient amount of one volatile hydrocarbon diluent to give the material a workable fluidity for the intended mode of application. 6. method of preventing adhesion between metals during a high temperature process, thereby characterized that one: Applying a material consisting of a fine ceramic to the surface of at least one metal particulate material, made of a polymeric binder, which is .at low temperatures in vacuum under Formation of volatile products degrades through chain depolymerization without any reaction products with the metals are formed from a solvent and from a diluent; Most of the diluent and some of the solvent evaporated before taking the metals down Vacuum ■ sets; heating the workpiece to a low temperature to drive off any residual solvent or diluent ; further heated in vacuum to a low temperature in order to degrade the binder, thereby turning the binder into volatile Transfer components so that the binder is removed, with only the ceramic particles as a layer left behind on the metal surface; and the workpiece is heated to a high temperature in order to achieve the to impart desired processing, wherein, the ceramic particulate prevents adhesion. 7. The method according to claim 6, characterized in that the ceramic material consists of a homogenized metal oxide compound (levigated metal oxide compound), which is stable at the high processing temperature. - y-3 8. The method according to claim 7, characterized in that the polymer binder consists of polystyrene and that the ceramic particulate consists of alumina.