Alloys or metallic compositions – Titanium base – Aluminum containing
Reexamination Certificate
1999-03-16
2001-01-16
Yee, Deborah (Department: 1742)
Alloys or metallic compositions
Titanium base
Aluminum containing
C148S421000, C148S669000
Reexamination Certificate
active
06174495
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention generally relates to titanium aluminide for precision casting, and more particularly to titanium aluminide which is not heat treated after a precision casting process but results in a cast with high creep strength.
2. Description of the Related Art
Titanium aluminide (TiAl alloy) possesses various advantages such as being lightweight, demonstrating satisfactory strength at elevated temperature and having decent rigidity. Therefore, the titanium aluminide is considered as a new favorable material for rotating parts of an aircraft engine and vehicle engine or the like, and there is an increasing tendency to put it to practical use.
Conventionally, as taught for example in Japanese Patent Application Laid-Open Publication No. 8-311585, Fe and/or V is added to TiAl alloy as a third element to improve castability and B is added to TiAl alloy to provide fine crystal grains. By adding these third elements, it has become possible to fabricate a complicated product by precision casting. It is also known from the above mentioned Japanese publication that TiAl alloy having improved room temperature ductility and/or processability is obtainable by optimizing heat treatment. TiAl alloy disclosed in this Japanese publication is referred to as the conventional TiAl alloy or titanium aluminide according to the prior art hereinafter.
However, studies of TiAl alloys are primarily focused on improvements of room temperature ductility so that developed TiAl alloys have relatively low creep strength. Particularly, satisfactory creep strength is not demonstrated beyond 700° C.
In order to raise the creep strength of TiAl alloys, there is known a method of adding a third element (Mo, Cr, W, Nb, Ta, etc.) into a TiAl mother alloy. This method, however, considerably degrades the precision castability of TiAl alloy so that a product with a complicated shape cannot be fabricated.
To overcome the above problems, the inventor proposed a novel TiAl alloy and casting method using the same in a copending U.S. patent application Ser. No. 09/217,673, entitled “TITANIUM ALUMINIDE FOR PRECISION CASTING AND METHOD OF CASTING USING TITANIUM ALUMINIDE” filed Dec. 21, 1998. The entire disclosure thereof is incorporated herein by reference and this TiAl alloy is referred to as TiAl alloy or titanium aluminide of earlier invention. The inventor disclosed how to heat treat the TiAl alloy in order to have a desired (or controlled) structure. The creep characteristic and precision castability are both improved according to this teaching. In particular, the improved creep strength demonstrates a value ten times (or more) greater than the conventional TiAl alloy without deteriorating the precision castability.
However, this TiAl alloy includes a trace amount of &bgr; phase precipitated in the structure in an as-cast condition. The &bgr; phase has an adverse effect on the room temperature tensile strength so that a particular heat treatment is required to disperse the &bgr; phase. This raises the manufacturing cost. If this TiAl alloy is used to fabricate rotating parts of an aircraft engine which are not generally manufactured on a mass production basis, the resulting products are satisfactory both in terms of mechanical property and cost, but if it is used as a material for rotating parts of an automobile engine which are manufactured on a mass production basis, the products have desired mechanical characteristics but entail a high manufacturing cost.
SUMMARY OF THE INVENTION
One object of the present invention is to provide titanium aluminide for precision casting which can eliminate the above described problems of the prior art and earlier invention. Specifically, the present invention intends to provide titanium aluminide for precision casting which has decent creep strength, castability and manufacturing cost.
According to one aspect of the present invention, there is provided titanium aluminide for precision casting, having the following chemical composition:
Al: 31.3 to 32.0 wt %,
Fe: 0.5 to 1.0 wt %,
V: 1.0 to 1.5 wt %, and
B: 0.03 to 0.06 wt %, with the remainder being Ti and inevitable impurities. If a melt of this titanium aluminide is poured into a die and cooled at a general speed (15-150° C./sec, preferably 30-100° C./sec), a product (cast) will have a fully lamellar structure almost entirely in an as-cast condition. This titanium aluminide does not have precipitation of &bgr; phase in a colony grain boundary of the lamellar structure. It is therefore possible to obtain a higher degree of grain boundary serration in the as-cast condition. As a result, the titanium aluminide product has an excellent creep property. The titanium aluminide may be used as a material for a rotor of a turbocharger which is a rotating part of an automobile engine. The die may have a complicated shape for precision casting.
According to another aspect of the present invention, there is provided a method of casting comprising:
providing a melt of titanium aluminide which possesses the following chemical composition:
Al: 31.3 to 32.0 wt %,
Fe: 0.5 to 1.0 wt %,
V: 1.0 to 1.5 wt %, and
B: 0.03 to 0.06 wt %, with the remainder being Ti and inevitable impurities;
pouring the titanium aluminide melt into a mold; and
cooling the titanium aluminide melt to obtain a cast.
The cast (product) has a higher degree of grain boundary serration even in the as-cast condition and therefore demonstrates improved creep strength. This method does not include any heat treatment steps to control a structure of the alloy. The mold may have a complicated shape for precision casting. The cast may be a turbocharger rotor for an automobile engine.
REFERENCES:
patent: 469525 (1991-07-01), None
patent: 620287 (1991-07-01), None
patent: 560070 (1993-02-01), None
patent: 7-18392 (1995-01-01), None
patent: 8-311585 (1996-11-01), None
Ishikawajima-Harima Heavy Industries Co. Ltd.
McCormick Paulding & Huber LLP
Yee Deborah
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