METHOD OF BONDING TITANIUM ARTICLES USING A SUPERPLASTIC INTERLAYER

Abstract: A method of bonding a titanium article to another metal article using a superplastic interlayer is disclosed. A fine-grain titanium alloy interlayer is provided between faying surfaces of the articles to be bonded. The interlayer has an alpha-beta microstructure and an average grain size of less than about 1-3 microns (.001-.003 mm) in at least one plane. The articles to joined and the interlayer are heated to between 1000 DEG F (538 DEG C) and 1500 DEG F (816 DEG C). A sufficiently large compressive force is then applied to the articles to cause superplastic deformation of the interlayer and bonding of the articles.

Description: METHOD OF BONDING TITANIUM ARTICLES USING A SUPERPLASTIC INTERLAYER Background of the Invention 1. Technical Field The invention relates to the solid state bonding of titanium alloy articles to each other and to other metal articles using a superplastic titanium alloy interlayer. 2. Background Information Titanium alloys are widely used in high-performance aerospace applications. Titanium and titanium alloys have historically been regarded as difficult to hot work. The difficulty results first from the high strength, at high temperatures, ofthe materials and second from the affinity of titanium alloys to absorb undesirable quantities of oxygen which change the chemical composition ofthe materials and compromise mechanical properties. Recently, methods have been developed to hot work titanium under conditions which produce in the titanium a usable degree of superplasticity. Such forging under superplastic conditions requires the material to be processed at elevated temperatures on the order of 1550°F (843°C)-1650°F (899°C). At such temperatures the titanium materials are extraordinarily susceptible to oxygen absoφtion and, consequently, such titanium superplastic forging processes must be performed under vacuum or inert conditions. Consequently, although progress has been made in the field of working titanium, there is still room for improvement. There are also problems which may be solved by producing titanium articles which have different properties in different regions ofthe article. For example, one portion of an article might require a high fatigue strength while another portion ofthe article might require a high tensile strength or a superior resistance to fatigue. Such articles can now be produced principally by welding titanium components together. The welding of titanium is a difficult task and it is commonly found that the reduction in mechanical properties in the weld zone is sufficient to make welded construction undesirable. There is therefore an outstanding need for a method to produce titanium articles which have different properties in different portions thereof. Titanium articles have successfully been bonded together using diffusion bonding. The diffusion bonding process requires heating the articles to an elevated temperature at which the flow stress ofthe articles being joined is low, so that upon the application of pressure between the surfaces to be bonded local deformation occurs in the bond region thereby promoting interdiffusion and bonding. Such processes are not desirable because they alter the microstructure and the mechanical properties ofthe titanium because the entire article must be heated to the temperature at which supeφlasticity is obtained, namely on the order of about 1600°F (871°C). Again, heating the articles to this elevated temperature requires careful attention to vacuum conditions to prevent oxygen absoφtion. Thus, there remains an outstanding need for a bonding method which can be performed under conditions which will not unduly affect the mechanical properties ofthe article being bonded. In previously filed co-pending U.S. patent application, U.S. serial number 08/149,437, assigned to the assignee ofthe present invention, methods have been described for producing superplastic titanium material using vacuum vapor deposition. The material is unique in that the grain size ofthe material is sufficiently small such that superplastic behavior is obtained at relatively low temperatures on the order of 1000°F (538°C)-1100°F (593°C). This low temperature permits the supeφlastic deformation ofthe ultra-fine grain titanium material at temperatures below which oxygen absoφtion is a significant problem. Disclosure of Invention According to the invention, a thin layer of ultra-fine grain titanium or titanium alloy is inteφosed between the faying surfaces of articles to be bonded. The interlayer material exhibits supeφlasticity and the ability to undergo substantial deformation, at temperatures on the order of about 1000°F (538°C) to about 1100°F (593°C). These supeφlastic deformation temperatures are below the temperatures which impair the surface and mechanical properties ofthe articles being joined. In addition, the interlayer and the articles to be joined do not absorb significant amounts of oxygen under the conditions at which the interlayer exhibits superplasticity. Consequently, the articles to be joined and the interlayer separating them can be forced together with suitable compressive force to permit the interlayer to accommodate the invariable differences in the surface flatness and surface roughness ofthe articles to be joined. Plastic deformation is confined to the superplastic interlayer. After bonding is accomplished the bonded article may optionally be briefly heated to an elevated temperature to coarsen any residual fine-grain material found in the interface. When the interface material composition is properly selected and the grain coarsening step is performed, there will be no discernible difference in composition or microstructure at the bond area and the bonded articles will exhibit substantially the properties ofthe parent articles which were joined. Detailed Description of the Invention According to the invention, at least one titanium alloy article is bonded to another metallic or intermetallic article using a supeφlastic titanium alloy interlayer. In one embodiment, the titanium interlayer is formed by condensing titanium alloy vapor on one surface ofthe articles which are to be joined. In another embodiment, the supeφlastic material is deposited on all the surfaces to be joined. It is also within the scope ofthe invention to provide the interlayer in the form of a separate sheet of thin material. It is also within the scope ofthe invention to bond a titanium article to another alloy of nickel, iron, cobalt, zirconium or a different titanium alloy or an intermetallic ofthe same. The primary required characteristic ofthe interlayer material is that it exhibits supeφlasticity, meaning that the material must have at least about 300% deformation when tested at a temperature of less than about 1350°F (732°C). We currently know of only one way to obtain such a supeφlastic interlayer and that is by vacuum vapor deposition onto a cooled substrate. This is described in our co-pending U.S. patent application, U.S. serial number 08/149,437 filed on November 8, 1993. According to this prior invention, it is possible to produce titanium alloy materials having an average grain size in at least one plane of less than about 1-3 microns (.001 mm-.003 mm). A grain size in the range of about 0.3 microns (.0003 mm) to about 1-3 microns (.001 mm-.003 mm) will produce the desired supeφlastic interlayer properties. Preferably the grain size is from about .1 micron (.0001 mm to about 3 microns (.003 mm). Such material can be deposited using vacuum vapor deposition onto a substrate cooled to a temperature of less than about 1300°F (704°C). Preferably the substrate temperature ranges from about 1050°F (566°C) to about 1300°F (704°C) because at temperatures of less than about 1050°F (566°C) a porous deposit is likely to result. U.S. Patent No.: 5,242,479 illustrates one form of apparatus which can be used to provide the vapor deposited interlayer material ofthe appropriate grain size. According to the invention described in this patent, high-rate evaporation of a titanium alloy is performed by melting the titanium alloy to be deposited and causing the molten alloy to flow onto a heated refractory metal surface selected, for example, from the group consisting of tungsten and tantalum. The refractory metal substrate is heated using an electron beam and the titanium alloy forms a molten alloy with the refractory metal. The titanium-refractory metal alloy serves as a source of titanium atoms for vapor deposition. The use ofthe refractory metal substrate permits the titanium material to be heated to an effectively higher temperature than would otherwise be possible without any significant spattering. The low vapor pressure of the refractory metal effectively keeps it from evaporating. The process is performed under high vacuum conditions less than about 10 "3 torr (1.33 Pa). The invention illustrated in U.S. Patent No.: 5,242,479 includes an extra source of material for deposition consisting essentially of carbon so that the resultant deposit in the material is a carbide compound. Obviously, the carbon codeposition feature is not employed in the present process. We have found that we can deposit titanium having the required fine grain size (preferably the grain size is between about .1 microns (.0001 mm) and about 3 microns (.003 mm)) at about 10 microns (.01 mm) per minute to about 100 microns (.1 mm) per minute. Preferably, the interlayer is produced using a vacuum vapor deposition process with substrate temperature control. We prefer to deposit the interlayer material on both ofthe surfaces which are to be joined together. The reason for this is that these fine-grain titanium interlayers can absorb oxygen which might otherwise contaminate the bond. The fine-grain material can absorb a greater degree of oxygen than the coarse grain substrate because of its higher grain boundary area and more rapid diffusion. By coating both the surfaces to be bonded, we believe that the chances for an oxide contaminated bond will be reduced. Of course, as previously stated, it is possible to apply the superplastic titanium material to only one ofthe surfaces to be bonded or even to provide it as a separate interlayer sheet. The thickness ofthe interlayer required depends largely upon the conditions ofthe surfaces to be joined. In general, rougher surfaces will require a thicker interlayer than smoother surfaces to ensure that there is enough volume of material to fill any voids. For bonding together two planar surfaces having extremely good surface finishes, flatness and smoothness on the order of those required for optical components could be accomplished with a total interlayer thickness on the order of as little as 1,000 angstroms. In practice, for commercial applications it will be found necessary to employ total interlayer thickness of preferably at least about 20 microns (.02 mm) and most preferably at least about 10 microns (.01 mm). There is no particular upper limit for interlayer thickness, however, if the interlayer chemistry differs substantially from that ofthe articles to be bonded and if it desired to achieve interlayer chemistry homogeneity by way of diffusion, it will be appreciated that it will be desirable to employ the minimal total interlayer thickness which will permit full surface contact between the articles to be joined. In addition to grain size, it is highly desired that the titanium alloy interlayer material consists of a mixture ofthe alpha and beta phases. The alpha phase is desirable because it has a high oxygen diffusivity. This means that oxygen contamination at the bonding surface can be absorbed though diffusion into the alpha phase. However, the beta phase has a considerably higher degree of superplasticity than the alpha phase under equivalent conditions. Consequently, it is desired to have a substantial amount of beta phase present to maximize the superplastic effect. Preferably, the alpha-beta ratio ranges from 5:95 to 95: 10 and most preferably from 30:70 to 70:30 by volume percent. The process is performed as follows: The article surfaces to be joined are provided with the appropriate geometry in terms of flatness and roughness. The surfaces to be joined are cleaned using chemical or physical means or a mixture or combination thereof. Chemical means include acid baths, for example, while physical means include mechanical abrasion. Immediately after such chemical and physical cleaning steps, the articles are preferably preserved under conditions of low oxygen and low moisture to minimize the formation of surface oxides of detrimental thickness. The superplastic interlayer is preferably deposited on one or both ofthe surfaces to be joined. Such deposition is performed under conditions of high vacuum (i.e., less than 10 '3 torr (1.33 Pa)). It may be found desirable to perform further cleaning step after the pieces are in the vacuum chamber, but prior to the deposition ofthe supeφlastic interlayer. Cleaning under high vacuum conditions may be performed by either using the sputtering process to remove any layers of surface contamination and/or by using a process known as reverse transferred arc. Thereafter, the supeφlastic interlayer is deposited by vacuum vapor deposition. Preferably the substrate interlayer grain substrate temperatures are maintained from 1050°F (566°C) to about 1300°F (704°C) and a deposition rate of from 1 μm/min to about 100 μm/min is employed. This will give an interlayer grain size from about 0.3 μm to about 3 μm and a time to produce a 10 μm interlayer of about 0.1 minutes to about 10 minutes. As previously mentioned, we prefer to use a heated substrate to evaporate the molten titanium as described for example in U.S. Patent No.: 5,242,479. When the interlayer thickness reaches the necessary thickness the parts are ready for bonding. Bonding may be performed in air, but more consistently favorable results may be obtained through bonding under vacuum conditions to minimize surface contamination. Bonding may be performed in a temperature range from about 1000°F (538°C) to about 1400°F (760°C) with an applied stress of about 2000 newtons per centimeter squared. The time required will be on the order from about 20 minutes to about 60 minutes. At the conclusion ofthe bonding process, the bond may be heated from about 1500°F (816°C) to about 1700°F (927°C) to cause coarsening ofthe microstructure in the interlayer and diffusional homogenization. Although the invention has been described with respect to detailed embodiments thereof, it will be understood by those skilled in the art that various changes in foπn and detail thereof may be made without departing from the spirit and scope ofthe claimed invention. What is claimed is:

Claims: Claims: 1. A method of bonding a titanium article to another metal article at a temperature between about 1000°F (538°C) and about 1500°F (538°C) including steps of: a. providing a fine-grain titanium alloy interlayer between faying surfaces ofthe articles to be bonded, said interlayer having an alpha- beta microstructure and an average grain size of less than about 1-3 microns (.001 mm-.003 mm) in at least one plane; and b. heating the articles to be joined and the interlayer to between 1000°F (538°C) and 1500°F (538°C) and applying a sufficiently large compressive force to the articles to be joined to cause supeφlastic deformation ofthe interlayer and bonding ofthe articles to be joined. 2. A method as in claim 1 wherein the interlayer is provided as a coating on at least one ofthe faying surfaces, said coating having a thickness of about 0.5 mils (.013 mm) to 2 mils (.051 mm). 3. A method as in claim 2 wherein the interlayer is provided as a coating on both ofthe faying surfaces, each of said coating having a thickness of at least .5 mils (.013 mm). 4. A method as in claim 1 in which the interlayer is provided in the form of a thin foil. 5. A method as in claim 1 wherein the bonding is performed in air. 6. A method as in claim 1 wherein both ofthe articles to be joined are titanium alloy articles. 7. A method as in claim 1 wherein the interlayer is at least 10 microns (.01 mm) thick. 8. A method as in claim 1, wherein the interlayer contains from 80:20 to 20:80 by volume ofthe alpha and beta phases. 9. A method as in claim 1, wherein the interlayer contains from 70:30 to 30:70 by volume ofthe alpha and beta phases. 10. A titanium article suited for bonding to another metallic article wherein the titanium article has, on its faying surface, a thin adherent coating of fine¬ grained alpha-beta titanium alloy having an average grain size in at least one plane of less than about 1 -3 μm. 11. A metallic article suited for bonding to a titanium alloy article wherein the metallic article has, on its faying surface, a thin adherent coating of fine grained alpha-beta titanium alloy. 12. An article as in claim 1 1 wherein the fine grained alpha-beta titanium coating has a thickness for 10-15 μm. 13. An article as in claim 12 wherein the interlayer contains from 80:20 to 20:80 by volume ofthe alpha and beta phases. 14. A composite metallic article having different properties in different portions thereof produced according to the method of claim 1.

Inventors: KORZH ALEXANDER V, MOVCHAN BORIS B, NEAL JAMES W, TOPAL VALERY, KORZH, ALEXANDER V., MOVCHAN, BORIS B., NEAL, JAMES, W., TOPAL, VALERY
Applicants: INT CENTER FOR PATON INST, KORZH ALEXANDER V, MOVCHAN BORIS B, NEAL JAMES W, TOPAL VALERY, UNITED TECHNOLOGIES CORP, INTERNATIONAL CENTER FOR PATON INSTITUTE, KORZH, ALEXANDER V., MOVCHAN, BORIS B., NEAL, JAMES, W., TOPAL, VALERY, UNITED TECHNOLOGIES CORPORATION
Application year: 1997
Main classification: C22C
IPC codes: B23K20, B23K35