Metal fusion bonding – Process – Preplacing solid filler
Reexamination Certificate
1998-09-16
2001-05-01
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
Preplacing solid filler
C228S181000, C228S194000, C228S262610, C228S262720
Reexamination Certificate
active
06223976
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a process for the assembly of titanium aluminide intermetallic articles by diffusion brazing. The process is also of use for repairing or refacing such articles.
2. Summary of the Prior Art
A variety of intermetallic materials have been developed which have interesting properties and which have advantages for use in particular applications, such as aviation applications, over conventional metal alloys and superalloys. This class of materials includes TiAl or Ti
3
Al titanium aluminides. This invention is particularly though not exclusively concerned with these two materials. These materials are typically shaped by casting or forging. For example, U.S. Pat. No. 4,294,615 describes a TiAl material of this kind. These materials may also include different constituents, such as Nb, Mo, Si, Zr, V, Ta, etc., depending upon the required application.
However, assembling the shaped articles has hitherto proved extremely difficult.
The known assembly techniques used for conventional metal alloy and superalloy materials are unsuitable for these novel intermetallic materials. In particular, assembly processes involving fusion (with or without a filler metal) such as energy beam, electron beam, or laser beam welding or TIG electric arc or plasma processes have the disadvantage of greatly modifying the metallurgical structure of the material. This causes difficulties when fragile materials like intermetallic compounds are assembled using such fusion processes. For example, titanium aluminide articles often experience cracking or crazing because of the severe heat stresses associated with the use of these conventional processes and because of the poor ductility of the materials. In other processes such as diffusion welding and friction welding, assembly is effected in the solid state. However, diffusion welding needs mating surfaces of precise geometry and very close control over cleanliness and heat cycle conditions. Friction welding requires special rheological qualities and is unsatisfactory for some assembly configurations.
Laboratory experiments have shown that since diffusion brazing produces zero thermal stressing, assembling titanium aluminide articles by diffusion brazing avoids cracking and crazing problems. The filler metals used in this case were those conventionally used in the assembly of titanium alloy articles and are, for example, Cu, TiCu, TiNi or 15% of TiCu and 15% of Ni. However, when diffusion is used with these known parameters, this process requires strict limitations to be observed if the results are to be of acceptable quality. In particular, the assembly gap is limited to 0.1-0.2 mm and is often necessary to apply pressure during assembly. Also, it is impossible to reface any surface of significant thickness.
SUMMARY OF THE INVENTION
One of the objects of the invention is to provide a process for assembling or refacing titanium aluminide articles by diffusion brazing which is free from the disadvantages of the previous known processes and can provide a satisfactory quality standard, more particularly such as is required in aviation.
Accordingly, in a first aspect of the invention, a process for assembly titanium aluminide articles by diffusion brazing comprises the following steps:
(a) preparation of a homogenous mixture of powder A and B, powder A consisting of a titanium aluminide alloy representing a proportion of from 40 to 90 weight % of the total and powder B consisting of a titanium-based or copper-based alloy which has a melting point appreciably below the incipient melting point of powder A, which can wet powder A chemically represent a proportion of from 10 to 60 weight % of the total, said powder proportions being determined according to process performance parameters, temperature and powder grain sizes;
(b) preparation of a paste by mixing an organic binder known per se in brazing technologies with the powder mixture obtained in step (a);
(c) depositing the paste plumb on the assembly gap of the articles, and
(d) heating the assembly obtained in step (c) in a vacuum furnace at a temperature between 1000° C. and 1300° C. for between a few minutes and 6 hours.
In a second aspect of the invention, a process for repairing a titanium aluminide article by diffusion brazing comprises the performing steps (a), (b) and (d) of the process just described and in the intermediate step (c), depositing the paste on the fissure of the article to be repaired.
According to a third aspect of the invention, a process for refacing at least one surface of a titanium aluminide article comprises following steps (a), (b) and (d) of the process hereinbefore described and in the intermediate step (c) depositing the paste on the surface of the article to be refaced.
An alternative refacing method is based on self-brazable compacts. According to a fourth aspect of the invention a process for making a compact element of predetermined shape of titanium aluminide comprises performing step (a) previously described, whereafter the powder mixture is placed in a mould which is homothetic to the shape of the compact end product, and which is preferably chemically inert. Step (d) is then preferably performed for a period reduced to a few minutes to avoid complete diffusion of the elements of powder B into powder A. The compact remains brazable and can be used to reface a surface.
If step (d) is carried out for a longer period of one hour, this method of making self-brazable compacts can be used to make dense shapes.
Advantageously, a powder A having a composition in atomic percentages of Al 46 to 50, Cr2, Nb2 and Ti as the remainder is used with a powder B which has a composition in weight percentages of Cu15, Ni15 and Ti as the remainder or a powder B having a composition in weight percentages of Ti30 and Cu as the remainder.
Alternatively, powder A having a composition in atomic percentages of Al 46 to 50, Mn2, Nb2 and Ti as the remainder may be used with powder B having a composition in weight percentages of Cu15, Ni15 and Ti as the remainder or a powder B having a composition in weight percentages of Ti30 and Cu as the remainder.
Depending upon uses, the predetermined grain size of the powders A and B used is either less than 63 &mgr; or less than 150 &mgr;m.
Other characteristics and advantages of the invention will be better understood from the following description of some examples of the application of a diffusion brazing process in accordance with the invention, to titanium aluminide articles, reference being made to the accompanying drawings.
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Clement Jean-François Didier
Ferte Jean-Pierre
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Pittman Zidia T.
Ryan Patrick
Societe Nationale d'etude et de Construction de Moteurs d&a
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