Metal treatment – Process of modifying or maintaining internal physical... – Heating or cooling of solid metal
Reexamination Certificate
2003-11-07
2004-10-19
Jones, Deborah (Department: 1775)
Metal treatment
Process of modifying or maintaining internal physical...
Heating or cooling of solid metal
C148S670000, C148S421000, C420S418000, C420S590000, C428S546000, C428S651000, C428S660000
Reexamination Certificate
active
06805759
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a shaped part consisting of an intermetallic gamma TiAl material (&ggr;-TiAl, gamma titanium aluminide alloy) with 41-49 atom % Al. The invention also relates to a process for producing the part.
Gamma TiAl materials are frequently referred to as “near-gamma-titanium aluminides”. The metal structure in these materials consists primarily of a TiAl phase (gamma phase) and a small proportion of a Ti
3
Al (&agr;
2
phase). In some multi-component alloys, a small proportion of a beta phase may also be present. This phase is stabilized by such elements as chromium, tungsten, or molybdenum.
According to J. W. Kim (
J. Met.
41 (7), pp. 24-30, 1989
, J. Met.
46 (7), pp. 30-39, 1994), individual groups of advantageous alloy elements in gamma TiAl alloys can be described as follows (in atom %):
Ti—Al
45-48
—(Cr, Mn, V)
0-3
—(Nb, Ta, Mo, W)
0-5
—(Si, B)
0-1
. Niobium, tungsten, molybdenum and, to a lesser degree, tantalum improve oxidation resistance, while chromium, manganese and vanadium have a ductilizing effect.
Due to their high strength/density ratio, their high specific Young's Modulus, their oxidation resistance, and their creep resistance, intermetallic gamma TiAl materials present interesting possibilities for a wide range of different applications. These include, for example, turbine components and automotive engine or transmission parts.
The prerequisite for the use of gamma TiAl on an industrial scale is the availability of a technically reliable forming process which facilitates the cost-effective production of shaped parts with properties that meet the specific requirements of a given application.
Based on experience with the processing of titanium in casting operations, considerable effort has been made in recent years to develop a fine casting process for gamma TiAl materials.
It has been demonstrated that the coarse casting structure ordinarily achieved is highly disadvantageous with regard to the mechanical properties of gamma TiAl. Molded parts made of intermetallic gamma TiAl materials based on Ti—45 atom % Al—5 atom % Nb, produced using fine casting methods, exhibit an unacceptable coarse structure with a mean grain size of >500 &mgr;m, whereby minimum and maximum grain sizes are distributed over a very broad range.
A molded part produced using fine casting methods with an alloy composition of 44 atom % Al—1 atom % V—5 atom % Nb—1 atom % B, remainder Ti (an alloy in conformity with European patent publication EP 0 634 496 and U.S. Pat. No. 5,514,333) exhibits a mean grain size in the range of 550 &mgr;m and also has a broad grain-size range.
The following attempts to achieve a fine grain structure using different alloy compositions and production processes are described as representative of the many such experiments conducted in recent years.
U.S. Pat. No. 5,429,796 describes a cast article made of a titanium aluminide material consisting of 44-52 atom % aluminum, 0.05-8 atom % of one or more elements from the group chromium, carbon, gallium, molybdenum, manganese, niobium, silicon, tantalum, vanadium and tungsten and at least 0.5 vol. % of boride dispersoids with a yield strength of 55 ksi and a ductility of at least 0.5%. The achievable mean grain sizes in the preferred alloys produced using the processes cited in the patent, Ti—47.7 atom % Al—2 atom % Nb—2 atom % Mn—1 vol. % TiB
2
Ti—44.2 atom % Al—2 atom % Nb—1.4 atom % Mn—2 vol. % TiB
2
and Ti—45.4 atom % Al—1.9 atom % Nb—1.6 atom % Mn—4.6 vol. %, TiB
2
, ranged between 50 and 150 &mgr;m, i.e. the structure was relatively fine. With an alloy composition of Ti—45.4 atom % Al—1.9 atom % Nb—1.4 atom % Mn—0.1 vol. %, TiB
2
, the mean grain size was 1000 &mgr;m, i.e. the structure was relatively coarse.
The two alloys with a high proportion of TiB
2
dispersoids tend to form coarse boride excretions at the grain boundaries during slow cooling following the casting process. These have a highly disadvantageous effect on the mechanical properties of the article. It is not possible to increase the cooling speed, as this induces thermal tensions which cause cracks to appear. The borides are added to the pre-alloy in a molten state. In order to reduce the unavoidable coarsening of the borides in the melt to the lowest possible level, the time interval between casting and the beginning of the hardening process must be kept short, which presents a further difficulty in the manufacturing process. In addition to these problems affecting the production process, high boride concentrations, which appear to be helpful in achieving effective grain size reduction, have a negative effect on the mechanical characteristics of the alloy.
The use of heat treatment to achieve a fine grain structure in intermetallic gamma TiAl materials is well known; see for example U.S. Pat. Nos. 5,634,992; 5,226,985; 5,204,058; and 5,653,828. With the aid of the heat treatments described in these patents, a degree of fineness is achieved in which the grain size of the cast structure is the most favorable that can be achieved through heat treatment. Ultimately, a degree of fineness that meets all the requirements of users cannot be achieved in a matrix structure produced in a casting process.
In addition to the coarse matrix structure, casting pores and blowholes have a disadvantageous effect on the mechanical properties of cast gamma TiAl articles. Consequently, recompression processes such as hot isostatic pressing or reforming processes must be applied in order to produce technically viable cast articles.
Due to the difficulties described above, the manufacture of shaped parts made of intermetallic gamma titanium aluminides using conventional casting processes such as fine casting has not been realized on an industrial scale.
As an alternative to casting, shaped parts with near-final form, shaped parts with final form and pre-material for further form processing are produced using standard powder-metallurgic processes such as hot isostatic pressing (see, for example, U.S. Pat. Nos. 4,917,858; 5,015,534; and 5,424,027). In those cases, powders produced using standard spray processes are used. Shaped parts produced using powder-metallurgy processes are significantly more fine-grained that those produced by casting. However, material produced using powder-metallurgy processes exhibits gas-filled pores—usually argon gas used in spray powder production. The pores have a negative effect on both creep deformation and fatigue resistance.
A satisfactory degree of grain fineness can be achieved in cast articles made of gamma TiAl with specially developed refining processes such as extrusion, forging, rolling and combinations of these processes. Thus industrial-scale production of gamma TiAl alloys ordinarily involves the use of VAR (vacuum arc remelting) base material which is converted to a fine-grained state through deformation and heat treatment. The actual forming of such products is effected following heat treatment in time-consuming mechanical processing which usually involves machining operations.
The entire manufacturing process for such shaped parts is thus expensive and restricts the range of possible applications due to cost considerations.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide an intermetallic gamma titanium aluminide alloy article, which overcomes the above-mentioned disadvantages of the heretofore-known devices and methods of this general type and which, measured against the current state of the art as described above, provides a fine-grained shaped part that is as pore-free and ductile as possible on the basis of intermetallic gamma TiAl using comparatively economical production technology.
With the foregoing and other objects in view there is provided, in accordance with the invention, a shaped part formed of an intermetallic gamma TiAl alloy with 41-49 atom % Al, which exhibits a grain size of d
95
<300 &mgr;m and a pore volume of <0.2 vol. %. The manufacture of the article comprises at least the following pr
Hoffmann Andreas
Kestler Heinrich
Plansee Aktiengesellschaft
Savage Jason L
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