GB2240942A

GB2240942A - Melting and casting of beta titanium alloys.

Info

Publication number: GB2240942A
Application number: GB8713689A
Authority: GB
Inventors: Douglas Michael Berczik
Original assignee: United Technologies Corp
Current assignee: Raytheon Technologies Corp
Priority date: 1986-01-02
Filing date: 1987-06-11
Publication date: 1991-08-21
Anticipated expiration: 2007-06-11
Also published as: GB2240942B; GB8713689D0

Description

Melting and Casting of Beta Titanium Alloys Technical Field The invention relates to the melting and casting of beta titanium alloys in low reactivity crucibles and molds.

Background Art Conventional titanium alloys are highly reactive, particularly when molten. The extreme reactivity of molten titanium alloys has required that the melting and casting of such alloys be carried out using skull techniques. In melting titanium using a skull technique a water cooled copper container is provided and the melting of the titanium alloy takes place under conditions which provide for solidification of an initial layer of the titanium composition on the water cooled copper chill surfaces so that the molten titanium alloy contacts only solid titanium rather than the copper container itself. Such techniques are necessary because of the reactivity of titanium but are also desirable because the molten product is free from contamination. Skull melting techniques have drawbacks including the limitation on the amount of superheat which is a consequence of the necessity of maintaining a solid skull between the molten material and the copper shell plate. In practice this leads to the requirement that the superheat in the molten titanium be not greater than about 40 F. This limitation on superheat in turn can lead to casting problems relating to a lack of fluidity in the molten titanium with such a low superheat.

The limitation to low superheat means that complex titanium castings are very difficult to produce so that most complex titanium shapes are produced by forging, an expensive process.

The reactivity between pure titanium and commercial titanium alloys and carbon is extremely high as a consequence of the high energy of formation of titanium carbides. In practice this high reactivity and the detrimental effect of carbon contamination on the mechanical properties of the resultant alloys have required that carbon be excluded from contact with molten titanium.

Recently a new class of Beta titanium alloys has been developed. These alloys are described in U. K. patent application No. (Ref: N453/14/RTT) filed on even date herewith and based on USSN 815506 and are comprised of major constituents titanium, vanadium and chromium with an example alloy being Ti-35% vanadium-15% chromium.

Despite being formed from alloy constituents which all are energetic carbide formers it is a surprising observation that alloys of the approximate composition described above are relatively nonreactive with carbon.

Disclosure of Invention This invention relates to the melting and casting of beta titanium alloys of a particular class of compositions using melting and casting apparatus having molten metal contacting surfaces which are formed essentially of carbon. It has been found that a certain class of beta titanium alloys is relatively nonreactive with carbon and so can be advantageously processed in contact with carbon.

Further, it has been determined that amounts of carbon which are dissolved by the alloy are not deleterious to the material properties and in fact under some circumstances may be advantageous.

The foregoing and other features and advantages of the present invention will become more apparent from the following description of the preferred embodiments.

Best Mode for Carrying Out-the Invention The invention relates to the technology for melting and casting beta titanium alloys which consists of more than 10% chromium, more than 20% vanadium, and at least 40% titanium. Such alloys are the subject of U. K. Patent application No.

(Ref: N453/14/RTT) filed on even date herewith, the contents of which are incorporated by reference.

These alloys have a notable combination of strength and incombustibility under the moderately severe conditions which are encountered in the turbine section of gas turbine engines.

It has been found that such materials can readily be contacted with carbon in various forms while the alloy material is molten without undue adverse reactions. Thus, for example, the alloy may be melted in a graphite crucible and the crucible can be inductively heated using the well-known properties in graphite as a susceptor without undue reaction with the graphite. Use of a carbon base crucible with the previously described beta titanium alloys can eliminate the necessity for and disadvantages of the skull melting techniques used heretofore.

In fact it has been observe that the beta alloy material appears to reach an equilibrium carbon content which is related to the degree of superheat of the material. Thus, for an example, material with a negligible amount of superheat (i. e., very close to the freezing point) will contain an equilibrium amount of carbon on the order of. 1-. 3%. At 100 superheat the material will contain an equilibrium amount of carbon on the order from. 4 to. 6 weight percent. At 200 it is estimated that the material will contain an amount of carbon from. 6 to 1. 2% by weight.

The implications of the present invention are particularly apparent in the casting process.

Whereas in the prior art it has been difficult if not impossible to cast to size complex titanium articles having close geometry because of mold metal reactions, and low superheat with the present invention it is possible to form a complex carbon mold, for example by machining graphite by coating a ceramic mold with carbon (e. g., pyrolytic graphite) or by using investment shell mold techniques but wherein the inner metal contacting stucco and slurries are comprised essentially of carbon. or by using investment casting techniques wherein the metal contacting surfaces are formed from carbon particles bonded with colloidal silica or colloidal alumina or other titanium shell system. This will permit the casting of complex shapes such as gas turbine engine components having a casting surface free from mold metal attack and a highly precise geometry which will minimize the necessity for further machining.

Claims

CLAIMS 1. A method of casting true beta titanium alloy articles based on Ti-V-Cr and containing more than about 10% Cri more than about 20% V and more than about 40% Ti which comprises a. melting the alloy in a crucible having a carbon metal contacting surface, and without formation of a titanium skull; b. applying sufficient energy to heat the molten beta alloy to the desired superheat ; c. casting said controlled superheat beta titanium alloy into a mold.
2. A method as in claim 1-in which the metal contacting mold surfaces are essentially carbon.
3. A method of casting according to claim 1 in which the alloy articles contain at least 35% vanadium, at least 15% chromium and the balance essentially titanium.

3. In the melting and casting of alloys which con tain more than about 10% Cr, more than about 20% V and more than about 40% Ti, and are comprised essentially of beta titanium, the improvement which comprises providing at least a surface coating of essentially carbon on all surfaces which contact the molten alloy.

AMENDMENTS TO THE CLAIMS HAVE BEEN FILED AS FOLLOWS.

1. A method of casting beta titanium alloy articles based on Ti-V-Cr and conta. n. ng more than 10% Cr, more than 20% V and more than 40% Ti which comprises a. melting the alloy in a crucible having a carbon metal contacting surface, and without formation of a titanium skull; b. applying sufficient energy to heat the molten beta alloy to the desired superheat; c. casting said superheated beta titanium alloy into a mold having its surface which contacts the molten metal essentially of carbon.

2. A method of melting and casting alloys which contain more than 10% Cr, more than 20% V and more than 40% Ti, and are comprised essentially of beta titanium, which comprises providing at least a surface copting of essentially carbon on all surfaces which contact the molten alloy.