Metal fusion bonding – Process – Specific mode of heating or applying pressure
Utility Patent
1998-10-21
2001-01-02
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Specific mode of heating or applying pressure
C228S193000, C228S115000, C075S746000, C148S513000, C148S516000, C419S048000
Utility Patent
active
06168072
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to diffusion bonding, and more particularly to the use of the thermally activated expansion agents for diffusion bonding of porous articles.
Methods for manufacturing porous articles using thermal expansion of embedded agents have previously been developed. For example, Kearns U.S. Pat. No. 4,659,546 describes a method of manufacturing a porous body using thermal expansion of entrapped gas to create discrete internal porosity and to reduce the overall density of the body. Another example is described in Allen et al. U.S. Pat. No. 3,087,807, which teaches a method of producing a porous metal body using a thermally activated gas-emitting powder to create porosity. Porous articles manufactured using such prior art methods are typically processed first into an intermediate non-porous form, and the porosity is introduced into the article by subsequently annealing the article at an appropriate temperature. The annealing process causes the expansion agent to expand and form porosity throughout the article, which further causes the dimensions of the porous article to increase.
In the manufacture of porous articles, it is frequently necessary to attach the porous article to other articles or parts (e.g., stiffeners, stringers, sleeves, panels, sheets, rods, blocks, spheres, tees, bolts, lugs, hinges, lands, struts, coverings, hardware, etc.) to a parent porous article. Heretofore, such composite articles have been manufactured utilizing at least a two-step process. First, the parent porous article is manufactured as described above using an annealing process. The part is then bonded to the parent using conventional welding and bonding techniques. This two-step process typically involves at least two heating steps, one in conjunction with the annealing required to create the porous article, and a second heating of the bonding surfaces during conventional fusion or diffusion bonding processes. Thus, the conventional manufacturing process is inefficient in that it requires two separate manufacturing steps, and requires the expense and delay of two separate heat treatments.
Moreover, many conventional fusion bonding techniques such as brazing and tungsten-inert gas (TIG) or oxyacetylene welding are inappropriate for applications utilizing porous metallic articles, such as aerospace manufacturing, which require high strength bonds with minimal deformation to component articles. Fusion bonding methods such as these involve localized melting, which causes deformation and microstructural changes that weaken porous metallic articles. Unlike conventional methods of fusion welding, diffusion bonding is a process that produces solid state coalescence between two materials. In diffusion bonding, joining occurs at a temperature below the melting point of the materials to be joined. Coalescence is produced with loads below those that would cause macroscopic defonnation of the article. Thus, diffusion bonding is often preferred in applications such as aerospace manufacturing because it produces components with no abrupt discontinuity in the microstructure and with minimal deformation.
A disadvantage of conventional methods of diffusion bonding is that they are relatively complex and typically require expensive equipment to create pressure required for bonding. For example, bonding pressure is often created by metal or rubber bladders inflated with either a gas or liquid. This technique requires sophisticated plumbing and connections to avoid leakage. Bonding pressure is also conventionally created through use of a hydraulic hot press, where the press applies pressure to a die containing the parts to be bonded. The primary disadvantage of this method is that hydraulic presses are generally very expensive (multimillion dollar) tools. Hence, there is a need for a method of diffusion bonding that produces the bond without expensive pressurizing equipment such as inflatable bladders and the hot presses.
Another disadvantage of conventional methods of diffusion bonding is that they typically require exacting surface preparation and expensive tools and dies to ensure the proper amount of pressure and intimate contact between the articles to be joined. Hence, there is a need for a method of diffusion bonding that produces proper bonding pressure while correcting imperfections at the faying surfaces.
For the foregoing reasons, there is a strong need for an improved method of diffusion bonding of parts to porous articles that 1) produces bonds with no abrupt discontinuity in the microstructure and minimal deformation, 2) does not require an additional bonding or heating step, 3) obviates the need for expensive pressurizing equipment, and 4) reduces the amount of surface preparation by correcting imperfections at the faying surfaces. Such a method would substantially reduce the cost of manufacturing composite porous articles.
SUMMARY OF THE INVENTION
The method of diffusion bonding of the present invention preferably comprises the principal steps of placing at least a first article, containing a thermally activated expansion agent, adjacent to at least a second article at a bonding area; constraining the articles at the bonding area such that contact is maintained between the articles at the bonding area; and heating the articles at a temperature that causes at least the first article to expand, thereby creating pressure between the articles at the bonding area.
Under the method of the present invention, articles are preferably joined by diffusion bonding at temperatures below that which is required for fusion welding techniques, in which the bond region is melted. As a result, the bond can be made with relatively little detrimental effect upon the base microstructure of the porous article. One of the many advantages of the method of the present invention is that it can preferably be accomplished concurrently with the annealing step required to create porosity in an article containing a thermally activated expansion agent. Therefore, the cost of bonding the articles is reduced because fewer manufacturing steps are required and only a single heating is needed. Another advantage of the preferred method described herein is that it utilizes pressure or force resulting from the expansion of the expansion agent, thereby eliminating the need for expensive pressurizing equipment required in conventional diffusion bonding techniques. Additionally, under the method of the present invention, expansion of the porous article preferably fills initial areas of non-contact between the articles at the bonding area due to imperfect mating. As such, the method of the present invention reduces or may eliminate the need for surface preparation.
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Deuser Donald A.
Schwartz Daniel S.
Bryan Cave LLP
Cooke Colleen
Ryan Patrick
The Boeing Company
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