Patent Application
20090250442
The present invention relates to a process for joining materials and, in particular, to a process for joining difficult-to-weld materials.
The manufacture of electrical power plants, petrochemical refineries, and other industrial facilities requires joining of various components. Joining of such components can be performed using welding, adhesives, threaded joints, flanges that can be bolted together, and the like. In many instances, the welding of components provides a sound engineering and economical method for joining said components, and in fact, the ability of a material to be welded can have a large impact on the material's commercial viability.
With demands for increasing the efficiency of electrical power plants, gas turbine engines and the like, the need for the use of materials that can withstand ever-increasing high temperatures continues. For example, dispersion-strengthened alloys are known to exhibit excellent high-temperature properties and have shown potential for use in many high-temperature applications. Likewise, nickel-based alloys strengthened by internal precipitants, such as gamma prime, are currently used in the hot sections of gas turbines. However, alloys such as these can present problems with respect to traditional fusion welding techniques since the melting of the base material results in the destruction of the microstructure which provides the excellent high-temperature properties.
Heretofore, joining techniques for such alloys have included diffusion bonding, friction welding, and other solid state welding processes. Diffusion bonding is a process wherein two nominally flat interfaces are joined at an elevated temperature using an applied pressure upon the interfaces to be joined. The diffusion bonding process affords the joining of dissimilar materials and/or similar materials wherein the melting of the base material has detrimental effects. However, the presence of oxide layers at the joining surfaces can affect the quality of the joint and thereby make sound, reproducible joints difficult to obtain.
A modified form of diffusion bonding is known as transient liquid phase (TLP) diffusion bonding wherein liquid-state diffusion bonding relies on the formation of a liquid phase provided by a bonding film that is inserted between the interfaces to be joined during an isothermal bonding cycle. The liquid phase subsequently diffuses into the base material and eventually solidifies as a consequence of continued diffusion into the bulk material at the isothermal temperature. The liquid phase enhances dissolution and/or disruption of any oxide layer that may be present on the interfaces to be joined and, thereby, promotes intimate contact between said interfaces. As such, the presence of the bonding film and thus the liquid phase reduces pressure and time that may be required for diffusion bonding. However, methods to TLP diffusion bond dispersion-strengthened high-temperature alloys and gamma prime nickel-based alloys have met with limited success. Therefore, a process for bonding of such alloys and/or a bond foil having a composition that affords for improved bond joints would be desirable. In addition, a clamping device that affords the application of applied stress to the components to be joined would be desirable.
The present invention discloses a process for joining materials. The process can include providing a first component with a first joint face and a second component with a second joint face. The first joint face and the second joint face can be prepared for bonding, and a bonding layer can be provided. The first component, second component, and bonding layer can be assembled such that the first joint face is oppositely disposed from the second joint face with the bonding layer located at least partially therebetween. In addition, a force can be applied to the assembly of the first component, second component, and bonding layer such that the first joint face is compressed against the second joint face with the bonding layer therebetween. In some instances, the bonding layer can be a bonding foil, and the bonding foil may or may not be a zinc foil.
A thermal treatment can be applied to the first joint face and the second joint face with the bonding layer therebetween, thereby affording for at least part of the bonding layer material to melt, the first joint face coming into intimate contact with the second joint face and forming a bond interface, and the first component being bonded to the second component across the bond interface, with at least part of the bonding layer vaporizing during the process. In addition, an atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be controlled before, during, and/or after the thermal treatment. In some instances, the thermal treatment can be a multiple step thermal treatment to the first joint face and the second joint face with the bonding layer therebetween.
The present invention discloses a process for joining materials using diffusion bonding, transient liquid phase (TLP) diffusion bonding, and modifications thereof. As such, the process has utility as a process for joining materials, and in particular, for joining difficult-to-weld materials.
The process includes providing components to be joined, for example, a first component having a first joint face and a second component having a second joint face. The first joint face and/or the second joint face can be prepared for bonding to each other. In some instances, the first joint face and/or the second joint face is machined. Optionally, the first joint face and/or the second joint face can be polished in addition to, or in place of, the machining.
A bonding layer can be provided. In some instances, the bonding layer is a bonding foil. The bonding foil can be a metallic foil such as a zinc foil, the term “zinc foil” for the purposes of the present invention including foil made from high-purity zinc, commercial pure zinc, zinc alloys, and the like. For example, the zinc foil can be made from zinc alloyed with aluminum, copper, lead, magnesium, nickel, iron, and/or tin. It is appreciated that the bonding layer can be a paste that is applied to the first joint face and/or second joint face or a coating that has been applied to one of the joint faces. The coating can be applied by any method known to those skilled in the art, illustratively including sputtering, chemical vapor deposition, physical vapor deposition, and the like.
The first component, second component, and bonding layer can be assembled such that the first joint face is oppositely disposed from the second joint face and at least part of the bonding layer is located therebetween. A force can be applied to the assembly of the first component and the second component with the bonding layer therebetween and afford for the first joint face to be compressed against the second joint face and the bonding layer.
The first joint face and the second joint face with the bonding layer located therebetween can be subjected to a thermal treatment, the thermal treatment affording for at least part of the bonding layer material to melt and the first joint face bonding to the second joint face, with at least part of the bonding layer vaporizing. In some instances, the thermal treatment can be a multiple-step thermal treatment, or in the alternative, a single-step thermal treatment where the temperature of the joint region is continuously increased to a final temperature. In the instance of a multiple-step thermal treatment, the thermal treatment can include a first step that includes heating the first joint face and the second joint face with the bonding layer therebetween to a first temperature, followed by holding at the first temperature for a predetermined amount of time, and a second step that includes heating to a second higher temperature followed by holding at the second temperature for a predetermined amount of time.
The first temperature may or may not be higher than the melting point or solidus temperature of the bonding layer, and the second temperature may or may not be higher than a recrystallization temperature of the first component and/or the second component. In this manner, the first temperature may result in the melting of the bonding layer, and the second temperature may result in grain growth across a bond interface between the first and second components. It is appreciated that melting of the bonding layer can afford for wetting of the first and/or second joint face and/or disrupting of any surface oxide on the first joint face, second joint face, and/or bonding layer.
It is appreciated that grain growth across the bond interface can result in improved bond joint quality and strength. It is further appreciated that cold-working of the first and/or second component proximate the first joint face and/or second joint face, respectively, can enhance grain growth across the bond interface. In the alternative, the bonding of components having different compositions can afford for one or more concentration gradients across the bond interface, the concentration gradient(s) enhancing grain growth across the bond interface and cold-working of the first and/or second component not being required.
In some instances, an atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be controlled. The atmosphere can be controlled by purging with an inert gas and/or by pulling or drawing a vacuum on a chamber in which the first joint face and the second joint face with the bonding layer therebetween is contained within. The inert gas can include a reducing gas such as hydrogen, for example argon with 5 volume percent hydrogen. It is appreciated that terms such as “draw,” “drawing,” “pull,” “pulled” and “pulling” are terms of art when used in the context of a vacuum and refer to the removal of atoms and/or molecules from an enclosed container, i.e. a chamber, and the establishment of a pressure that is less than atmospheric pressure therewithin.
Control of the atmosphere surrounding the first joint face and the second joint face with the bonding layer therebetween can be combined with the multiple-step thermal treatment. For example and for illustrative purposes only, a chamber containing the first joint face and the second joint face with the bonding layer therebetween can be purged with an inert gas before and/or during the first step, followed by establishing a vacuum before and/or during the second step.
During at least part of the thermal treatment, with or without the atmosphere control, contact between the interfaces to be bonded is sufficient such that diffusion takes place therebetween, and a sound metallurgical bond is provided. As stated above, the thermal treatment can include a step that affords grain growth across the bond interface, the bond interface being defined herein as an interface between two components to be joined, across which diffusion occurs to form a bonded joint. In this manner, a process wherein joints having acceptable room-temperature and/or high-temperature properties is provided.
Components that can be joined using the process disclosed herein range from typical metals and alloys used for fabricating structures to difficult-to-weld metals and alloys. For example and for illustrative purposes only, materials such as the commercial alloys MA956, PM2000, CM247LC, APMT, and the like can be joined to themselves and/or to other materials. It is appreciated that the MA956 alloy is an oxide dispersion-strengthened (ODS) alloy having a nominal chemical compositions of Fe-20Cr-4.5Al-0.5Ti-0.5Y2O3 (wt %); the PM2000 alloy is also an ODS alloy having a nominal chemical compositions of Fe-20Cr-5.5Al-0.5Ti-0.5Y2O3 (wt %); the CM247LC alloy is a gamma prime-strengthened alloy having a nominal composition of Ni-8.1Cr-9.2Co-0.5Mo-9.5W-3.2Ta-0.7Ti-5.6Al-0.01Zr-0.01B-0.07C-1.4Hf (wt %); and the APMT alloy is a powder metallurgy alloy having a nominal composition of Fe-21Cr-3Mo-5Al (wt %). It is further appreciated that these alloys, and other alloys joined by the process disclosed herein, can have other incidental impurities and additional alloying elements.
The process can also include the use of a fixture device for holding the components to be joined in an appropriate orientation with a desired stress applied thereon.
Turning now to
Included in the process 5 is a bonding layer at step 20. The bonding layer can be a zinc foil or, in the alternative, made from a material not containing zinc so long as the material has a tendency to vaporize during thermal treatment of a joint region as taught below. For example, zinc has a vapor pressure of 0.13 kilopascal (kPa) (1 torr) at 487° C., 101.3 kPa (760 torr) at 907° C. and 10,132 kPa (7600 torr) at 1180° C. In addition, zinc has a melting point of 420° C., which is less than, or about the same as, other iron-zinc or nickel-zinc alloy or intermetallic melting temperatures. As such, a zinc foil will melt before or at the same temperature as other possible zinc-containing compounds in an iron-based or nickel-based component, and vaporization of at least part of the zinc foil will reduce or eliminate the diffusion of zinc into the adjacent components.
It is appreciated that other low boiling point elements can be used for the bonding layer. For example and for illustrative purposes only, foils can be made primarily from elements such as arsenic (Tb=610° C.), cadmium (Tb=765° C., cesium (Tb=690° C.), magnesium (Tb=1110° C.), mercury (Tb=357° C.), phosphorus (Tb=283° C.), polonium (Tb=960° C.), potassium (Tb=770° C.), rubidium (Tb=700° C.), selenium (Tb=685° C.), sodium (Tb=890° C.), sulfur (Tb=445° C.), and/or tellurium (Tb=962° C.) where Tb is the boiling point of the given element at atmospheric pressure. It is appreciated that some of these materials are considered poisonous, fire hazardous and/or radioactive, and thus may limit their use as a bonding layer, but have in common with zinc a relatively low vaporization temperature.
After the components and the bonding layer have been provided, the components and said bonding layer are assembled at step 30. It is appreciated that the components to be joined can have joint faces that have been properly prepared, for example, by machining and/or polishing, and the bonding layer can be dimensioned to fit between the joint faces. The bonding layer can have a thickness between 1 micron and 1 millimeter, inclusive. In some instances, the bonding layer has a thickness between 5 microns and 200 microns and in other instances can be between 20 microns and 50 microns. Assembly of the components with the bonding layer at step 30 includes bringing the joint faces to be bonded into intimate contact with the bonding layer, the joint faces being oppositely disposed from each other with the bonding layer therebetween. In addition, pressure or an applied stress can be applied to the components such that the interfaces to be bonded and the bonding layer are under compression.
The pressure can assist in the breaking up or disruption of any oxide scale that is present on the first joint face, second joint face, and/or bonding layer. It is appreciated that the pressure can be applied with a fully articulated press or assembly device that affords for the first joint face and the second joint face to be easily aligned with each other and thus provide for intimate contact therebetween once the bonding layer has melted and diffused into the components and/or been vaporized away from the joint region.
An assembly device shown generally at reference numeral 80 in
Proximate to the top portion 110 is a pressure application member 112. In some instances, the pressure application member 112 can be a threaded bolt, screw, and the like. The pressure application member 112 can have a pressure end 114 that can be moved in a back and forth direction 1. In addition to the body 100 and the pressure application member 112, a hemispherically shaped cap 140 can be placed between the pressure end 114 of the pressure application member 112 and the first component 210 to be joined. Likewise, a second hemispherical cap 150 can be placed between the member 100 and the second component 220 to be joined. It is appreciated from
The hemispherical caps 140 and 150 can be made from any material known to those skilled in the art, illustratively including high-temperature alloys, alumina, silica, and the like. In some instances, the hemispherical cap 140 and 150 has a hemisphere diameter that is generally equivalent to the diameter of a rod, tube, and the like that is to be joined; however, this is not required. If the components to be joined have a cross-sectional polygon shape such as a square, rectangle, and the like, the cap 140 and cap 150 can be manufactured such that one end is complementary to the pressure end 114 and/or machined region 122 and the other end is complementary to the components to be joined.
The arcuate surface of the hemispherical cap 140 and/or 150 affords for the components to fully articulate, or move, independently from the body 100 and the pressure applied by the pressure application device 112. This articulation or movement can be critical since the joint diffusion zone that results in the bond can be relatively thin and the interfaces to be joined are preferably in intimate contact along the complete surfaces of the joint. Without the articulation, the joints can become cocked, misaligned, etc., with force on one portion being greater than another portion and the interfaces to be joined not being parallel with each other.
It is appreciated that the member 100 and the pressure application device 112 can be made from any material known to those skilled in the art for use at generally high temperatures, such as molybdenum, niobium, other metals having high-temperature strength, high-temperature nickel-based alloys, high-temperature iron-based alloys, high-temperature cobalt-based alloys, ceramics, metal matrix composites, and the like.
Returning to
An oxygen getter can be placed proximate to the joint region and/or within an enclosed chamber that contains the joint region such that excess oxygen within the atmosphere is reduced. Any oxygen getter known to those skilled in the art can be used, illustratively including an oxygen getter made from zirconium, aluminum, tantalum, titanium, and the like. In some instances, the oxygen getter is in the form of a sponge or some other high-surface-area structure. In addition, the joint region can be wrapped with oxygen getter foils such as aluminum, zirconium, tantalum, titanium, and the like.
A thermal treatment of the joint region can be provided at step 50. The thermal treatment can result in the heating of the joint region, and the heating can be provided by thermal resistance, thermal resistance furnaces, induction heating, radiant heating, and the like. The thermal treatment can include a series of time-temperature steps, such as a ramp up to a first temperature, holding the first temperature for a predetermined amount of time, ramp up or down to a second temperature, holding at the second temperature for a predetermined amount of time, ramp up or down to a third temperature, holding at the third temperature at a predetermined amount of time, and so on.
For example and for illustrative purposes only, a first component 210 made from the CM247LC alloy can be joined to a second component 220 made from the APMT alloy using a zinc foil. A chamber surrounding the first joint face 212 and the second joint face 222 with the zinc foil 230 therebetween can be purged with an argon+5% hydrogen gas and held at a pressure of between 10 to 304 kPa (0.1 to 3 atmospheres) while the joint is heated to 700° C. and held for 1 hour. This initial step can result in the melting of the zinc foil and disruption or dissolving of oxide surfaces at the first joint face 212 and/or second joint face 222. Thereafter, a high vacuum, for example a vacuum of 10−7 kPa (10−6 millibar), can be pulled around the joint region and the temperature increased to 1214° C. and held for 24 hours. During this second thermal step, grain growth and interdiffusion across the joint interface can be promoted and at least part of the zinc from the zinc foil vaporized.
It is appreciated that the second thermal processing step can include holding the joint region at the second temperature for a shorter amount of time, for example, 1 hour and thereafter reducing the vacuum and providing an Ar+5% H2 gas. Such an alternative thermal treatment can reduce vaporization losses from assembly devices, furnace tubes, clamps, joint rods, and the like. It is appreciated that with any of these thermal treatment steps, the atmosphere can be further controlled by the introduction of oxygen getter materials therein.
Additional thermal treatment steps can be included, such as additional heating steps and subsequent cooling steps. Heat treatment, stress relief, and/or aging thermal treatment steps can be included along with the joining steps and still fall within the scope of the invention.
In this manner, a first component can be joined to a second component using a bonding layer that melts at a temperature that is lower than the melting temperature of the first component and the second component. In addition, at least part of the melted bond layer can vaporize after it wets and dissolves at least a portion of a first joint face and/or a second joint face.
The foregoing drawings, discussion, and description are illustrative of specific embodiments of the present invention, but they are not meant to be limitations upon the practice thereof. Numerous modifications and variations of the invention will be readily apparent to those of skill in the art in view of the teaching presented herein. It is the following claims, including all equivalents, which define the scope of the invention.
This application claims the benefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent Application No. 60/991,966 entitled “Joining of Difficult-to-Weld Materials,” filed Dec. 3, 2007, the disclosure of which is hereby incorporated herein by reference.
This invention was made with Government support under Cooperative Agreement Nos. DE-FC26-98FT40320 and DE-FC26-05NT42465 awarded by the United States Department of Energy. In addition, this invention was made with Government support under Subcontract No. 4000028064. The Government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 60991966 | Dec 2007 | US |
