WO2013162978A1

WO2013162978A1 - A METHOD FOR PRODUCING TiAL3, AND AL-TiAL3, Ti-TiAL3 COMPOSITES

Info

Publication number: WO2013162978A1
Application number: PCT/US2013/037052
Authority: WO
Inventors: Gopal Subray REVANKAR
Original assignee: Ni Industries, Inc.
Priority date: 2012-04-23
Filing date: 2013-04-18
Publication date: 2013-10-31

Abstract

An apparatus and process are described which are used in the production of a composite material which is used in the production of armor plate. By suspending a Ti form in a volume of molten Al, the formation of a single thickness of intermetallic compound titanium aluminide is formed into the Ti form.

Description

A METHOD FOR PRODUCING TiAl₃, and AI-T1AI₃, Ti-TiAl₃ COMPOSITES

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. Provisional Application No. 61/636,777 filed April 23, 2012, which is hereby incorporated by reference.

BACKGROUND

Metal intermetallic laminate (MIL) composites, such as Ti-Ti aluminide, have been fabricated using, as a starting composition, alternating metal foil layers. One example of a MIL composite is the MIL composite which consists of alternating foil layers of Ti and Al. In the processing and fabrication of this type of MIL composite, depending on the starting compositions and thermodynamic conditions, several intermetallics are possible. For the Al-Ti system, atomic diffusion at the interface results in the formation of a layer of titanium aluminide (T1AI₃) on the Ti metal.

Currently available technologies and related methodologies for producing the types of MIL composites which are mentioned above are considered too time consuming and expensive and hence make commercial applications cost prohibiting. There is also a quality control consideration during production and a particular quality level or standard may be difficult to maintain due to the difficulty in maintaining uniform temperature and pressure in the production chamber. The present disclosure sets forth a simpler and less complex approach and the apparatus and method which are described herein are intended to help produce the same MIL composites at a lower cost and in less time while still providing improved quality control. Although mention is made of comparable quality in terms of the resultant MIL composite, the disclosed apparatus and method are envisioned to make the resulting products more commercially acceptable, and applicable to a variety of shapes and thicknesses. SUMMARY

The disclosed process (or method) and apparatus include the use of a heated crucible containing molten aluminum (Al). Titanium (Ti) forms, such as sheets and rods, are suspended in the molten Al. As such, the heated crucible becomes a containment vessel for the molten Al and for the Ti forms which are suspended in the molten Al.

A metallurgical reaction between Al and Ti is allowed to take place over a period of time. The metallurgical reaction is allowed to continue until the Al or Ti is consumed, either partially or completely, in order to obtain the desired product. The desired product is based on the intended application and the process can be controlled to produce one (1) of two (2) MIL composites Al + T1AI3 and Ti +

The prior art relating Al-Ti composites contemplates the use of very thin sheets (foils) of Al and Ti, or their alloys, which are stacked in alternating sequence, in order to form a thicker stack. This stack of alternating foil layers is heated in air in order to melt the Al. The individual Al foil layers are intended to react with their adjacent Ti foil layers in order to form the T1AI3 intermetallic compound. The reaction, according to the prior art, continues until the Al is consumed and the final product is a composite consisting of alternating layers of Ti metal and T1AI3 intermetallic compound. This fabricated stack would form a panel which was intended to be used for armor plates.

As noted in the Background, the prior art processes for those AI-T1AI3 MIL composites are believed to have quality control issues (e.g. uniformity of temperature in the stack) and are time consuming. Each of these "issues" contributes to making these prior art processes and methodologies expensive and less attractive for commercial applications as a result. The only required temperature control for the process of the present disclosure is that for the molten Al. This type of temperature control is simpler and easier to maintain. This simpler and easier temperature control results in better quality control for the disclosed process as compared to prior art processes.

The Ti forms for the disclosed process are suspended in molten Al. As a result of this process feature, it is expected that the diffusion of Al into Ti and hence the T1AI3 formation rate will be high as the Al concentration is close to 100% at the Ti-Al interface throughout the process. This aspect is expected to reduce the process time required for the formation of T1AI3. The prior art processes are rather long, approximately 40 hours, due to decreasing Al concentration at the Ti or T1AI3 surface as the T1AI3 formation process continues. One of the features of the disclosed process is the stirring of the molten Al bath which is considered unique and which results in an enhanced formation rate of T1AI3 due to a nearly constant 100% Al concentration at the interface.

Additionally, the disclosed process allows several Ti forms, either sheets, rods, cylindrical tubes or other shapes to be simultaneously suspended in the molten Al bath. Further, the size and shape of those various Ti forms are not a limiting factor in the disclosed process. As would be clearly evident from the alternating foil layers of the prior art, the presently disclosed process as well as the corresponding apparatus has the advantage of allowing the choice of random shapes of parts to be submerged in the molten Al.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a front elevational view in partial section of the apparatus used for the disclosed process.

FIG. 2 is a perspective view of a Ti form which may be submerged in the molten Al of the FIG. 1 apparatus.

FIG. 3 shows an alternate Ti form which may be suspended in the molten Al of the FIG. 1 apparatus.

FIG. 4 shows an alternate Ti form which may be suspended in the molten Al of the FIG. 1 apparatus.

DESCRIPTION OF THE SELECTED EMBODIMENTS

For the purpose of promoting an understanding of the principles of the invention, reference will now be made to the embodiments illustrated in the drawings and specific language will be used to describe the same. It will nevertheless be understood that no limitation of the scope of the invention is thereby intended. Any alterations and further modifications in the described embodiments, and any further applications of the principles of the invention as described herein are contemplated as would normally occur to one skilled in the art to which the invention relates. One embodiment of the invention is shown in great detail, although it will be apparent to those skilled in the relevant art that some features that are not relevant to the present invention may not be shown for the sake of clarity.

Referring to FIG. 1 there is illustrated a suitable apparatus 20 for performing the process which is described herein. This process pertains to producing Al-TiAl₃ and Ti-TiAl₃ composites and TiAl₃. TiAl₃ is an intermetallic compound which is one (1) of several possible titanium aluminide compounds. One contemplated and suitable commercial application for these composites is in the production of armor plates. These armor plates are constructed and arranged to be suitable as armor plates for military vehicles, as one example.

As used herein, an "intermetallic compound" refers to a material composed of two (2) or more types of metal atoms, which exist as homogeneous, composite substances and differ in structure from that of the constituent metals. Alternative terminology uses the phrase intermetallic phases. The properties of intermetallic compounds are distinct from those of the constituent elements and cannot be graded into those of the elements. These compounds form distinct crystalline species separated by phase boundaries from their metallic components.

As used herein, "titanium aluminide" refers to an intermetallic compound which is based on a composition of the metal atoms of titanium (Ti) and aluminum (Al). There are four (4) main intermetallic compounds which are generically referred to as titanium aluminide. These four (4) intermetallic compounds include TiAl₃, Ti₃Al, TiAl₂ and TiAl. Apparatus 20 includes a heated crucible 22 which functions as a containment vessel and contains a volume 24 of molten aluminum (Al). There are no particular size or shape restrictions on crucible 22 so long as a sufficient volume of Al is able to be retained therein and so long as the upper opening 26 is sized and shaped to enable the introduction of the desired titanium (Ti) forms 28. With regard to the requisite size and shape of the upper opening 26, and the desired volume 24 of Al, these variables are influenced by the size and shape of the Ti forms 28 to be submerged in the molten Al volume 24 as well as by the number of such Ti forms to be submerged at a time.

FIG. 2 shows one option for a Ti form which may be submerged in the molten Al volume of FIG. 1 and this Ti form is in the shape of a generally rectangular, flat plate 28. Another Ti form is illustrated in FIG. 3 in the shape of a solid rod 28a. Another Ti form is illustrated in FIG. 4 in the shape of a generally cylindrical hollow tube 28b, wherein opening 29 may extend through the entire length (L) of tube 28b or only part way. Other Ti forms 28 are contemplated including irregular shapes and complex geometries. Reference number 28 is being used generically for all styles of Ti forms.

Apparatus 20 further includes hollow tube 30 which is constructed and arranged for the introduction of a gas into the molten Al for stirring of the molten Al. Thermocouple 32 may be inserted into the molten Al in order to monitor and maintain a desired temperature for the molten Al. The desired temperature may be maintained via a feedback connection from thermocouple 32 to the crucible heating element 34 which is represented by block 34 in FIG. 1. A support 36 is used to suspend the Ti forms 28 into the volume 24 of molten Al. A flux layer 38 may be maintained on the upper surface 40 of volume 24 of molten Al.

With regard to the use of support 36 and the manner in which the Ti forms 28 are submerged down into the volume 24 of molten Al, it will be understood that any portion of any Ti form 28 which is not completely submerged into the molten Al will not have the same metallurgical properties which are added to the submerged part of the Ti form according to the disclosed process. Consequently, those portions of the Ti forms which are not submerged in the molten Al would typically be machined off prior to final processing. Another option would be to modify the design of the support 36 so as to include a longer extension means so that the entirety of each Ti form could be completely submerged in the molten Al beneath the flux layer 38.

The process, as described herein, produces Ti-TiAl₃ composite layers so as to form a thick plate without resorting to the use of the alternating foils of Al and Ti or their alloys, as disclosed in the prior art. One intended benefit of the disclosed process is to be able to produce the desired thick plate faster, as compared to the prior art. As will be described, the metallurgical modifications to the starting Ti forms 28 result in an intermetallic compound layer as part of the Ti form based on the infusion of Al. While any dimensional build up of the starting Ti form is minimal, there is a metallurgical modification of the portions of the Ti form which are exposed to the Al and all Ti form surfaces which are submerged will have a relatively uniform intermetallic compound layer of T1AI₃. If, however, any dimensional changes do occur during the process they can be managed by selecting the starting dimensions such that they would produce the final desired dimensions.

The disclosed process begins with the creation or fabrication of apparatus 20 including a suitable volume 24 of molten Al and the process "controls" including stirring tube 30, thermocouple 32, heating element 34 and flux layer 38. The next step in the described process is to select the number and style or shape of Ti forms 28 to be submerged into the volume 24 of molten Al. These Ti forms 28 are attached to support 36 and submerged into the volume 24 of molten Al. As noted, virtually any type of Ti form 28 can be selected and submerged, including irregular shapes and more complex geometries, including the examples of FIGS. 3 and 4 which show a rod and a tube. In the exemplary embodiment of FIG. 1, the two (2) illustrated Ti forms 28 are rectangular plates (see FIG. 2). These generally rectangular plates are considered to be "thick" in the comparative context relative to the prior art which uses alternating "thin" foil layers for the Al and Ti metals. While a "foil" is typically thought of as being of a thickness which is less than 0.2mm, the "thick" plate 28 of FIG. 2 has a thickness (t) of approximately 2.0mm or greater without imposing limitations on the process results, though this exemplary thickness is not limiting. The temperature of the molten Al which is contained within crucible 22 is monitored and closely controlled by means of thermocouple 32 and heating element 34. The molten Al may be protected from direct contact with ambient air by the use of a suitable flux layer 38. An alternative to the use of flux layer 38 is to use a non-reacting gas shield. Whichever approach is used, either a suitable flux layer or a non-reacting gas shield, the objective is to protect the molten Al from direct access to ambient air, mainly oxygen, in order to reduce Al oxidation.

The flux which is used to create flux layer 38 is provided in a sufficient volume based on the size of the exposed upper surface 40 so as to result in a layer thickness for the flux which does not break open due to moderate surface turbulence. Any gas bubbles from stirring tube 30 which may need to escape will break through the flux layer 38. However, if that flux layer has a sufficient thickness it will close back as the gas bubbles escape and any exposure of the molten Al to ambient air is kept to a minimum.

The molten Al may be stirred using a jet flow of a suitable gas via stirring tube 30. As the gas exits from the lower end of the stirring tube 30, a turbulence is created within the molten Al and this imparts a stirring motion to the molten Al so as to continuously expose the outer surface of each Ti form 28 which is submerged to a "fresh" concentration of molten Al which should be close to a 100% concentration. Another option for imparting a stirring motion to the molten Al is to use a ceramic stirrer which is powered by either compressed air or by electric power both of which can be arranged outside of the molten Al. With one or more Ti forms 28 submerged in the molten Al, according to the apparatus of FIG. 1, and while the molten Al is being stirred, a metallurgical reaction between Al and Ti occurs. This metallurgical reaction is allowed to take place over a period of time which is expected to be shorter than the period of time required for the prior art to form the same aluminide thickness due to improved aluminum concentration as explained above. The process time may also be shortened by using higher temperature of the molten Al than is used in the prior art, to increase the diffusion rate of the Al at the Ti-Al interface. According to the exemplary embodiment, this metallurgical reaction continues until the Al or Ti is consumed partially or completely in order to obtain the final product containing either:

(i) a composite of Al and T1AI₃;

(ii) a composite of Ti and T1AI₃; or

The decision and selection would be as desired for a given use or application. Since the only required temperature control is that for the molten Al, this control requirement is simpler and easier than what is required for the prior art which is based on alternating Al and Ti foil layers. Accordingly, when the exemplary embodiment is compared to the prior art, it is clear that the exemplary embodiment includes an improvement in quality control of the process and improved quality control for the final product.

As described and illustrated, each Ti form 28 which is submerged in the molten Al is held there during the metallurgical reaction. Due to the Al concentration which is constantly in contact with the Ti, it is recognized that the diffusion of Al into Ti and hence the T1AI₃ formation rate will be high. The comparatively high rate results in part from the Al concentration which is close to 100% at the Ti-Al interface throughout the process. As such, the process time for the formation of T1AI₃ is reduced as compared to the corresponding process time for the prior art using alternating Al and Ti foil layers. In the prior art, the Al is gradually depleted and thus the available Al for continued formation of T1AI₃ is reduced and the rate of formation slows as the prior art process proceeds. The prior process is rather long and takes approximately forty (40) hours to complete. Further, in the prior art, the Al and Ti foil layers are fixed in position and there is no option of exposing the Ti to a higher concentration of "fresh" Al. Due to a decreasing Al concentration in the prior art at the Ti or T1AI₃ surface, the T1AI₃ formation process slows, resulting in the longer process times as compared to the exemplary embodiment.

In contrast to this limitation of the prior art due to the fixed foil positions and fixed number of foil layers, the exemplary embodiment uses a stirred volume 24 (bath) of molten Al. The motion imparted to the volume 24 of molten Al enhances the formation rate of T1AI₃ due to a nearly constant Al concentration of approximately 100% at the Al to Ti interface or at the Al to T1AI₃ interface.

The described apparatus and process of the exemplary embodiment permits several Ti sheets, rods, tubes (see FIGS. 2-4) and other Ti forms 28 to be simultaneously suspended in the molten Al (see FIG. 1). The size, shape and number of Ti forms which can be suspended in a volume of molten Al is not limited and only requires a suitable crucible 22 and a sufficient volume of molten Al. The option of having and using various Ti forms of random shapes and sizes, some of which could be at or close to a final form, is not available in the prior art which is limited to alternating Al and Ti foil layers. Since the outer surface portions of the Ti form 28 which are actually submerged receive the infusion of Al, a single T1AI₃ layer exists as part of the Ti form 28 at the end of the process. This single layer covers all of the exposed surfaces of the Ti forms 28 which are submerged, thereby creating a uniform covering layer regardless of the shape of the Ti form 28.

While the invention has been illustrated and described in detail in the drawings and foregoing description, the same is to be considered as illustrative and not restrictive in character, it being understood that only the preferred embodiment has been shown and described and that all changes, equivalents, and modifications that come within the spirit of the inventions defined by following claims are desired to be protected. All publications, patents, and patent applications cited in this specification are herein incorporated by reference as if each individual publication, patent, or patent application were specifically and individually indicated to be incorporated by reference and set forth in its entirety herein.

Claims

1. A composite material which includes as part of its composition the intermetallic compound titanium aluminide, said composite material being prepared by a process comprising the steps of;

a) providing a containment vessel;

b) providing a volume of molten Al within said containment vessel; c) providing a Ti form;

d) suspending said Ti form within said volume of molten Al;

e) maintaining a molten condition for said molten Al; and f) enabling the formation of a single thickness of intermetallic compound titanium aluminide into said Ti form.

2. The composite material of claim 1 wherein said process includes the additional step of providing said containment vessel with a heating element for maintaining said molten condition of said molten Al.

3. The composite material of claim 2 wherein said process includes the additional step of inserting a thermocouple into said molten Al, said thermocouple being electrically connected to said heating element.

4. The composite material of any preceding claim wherein the process further includes the step of imparting a stirring motion to said molten Al.

5. The composite material of claim 4 wherein said imparting step is based on creating bubbles within the molten Al.

6. The composite material of claim 5 wherein the step of creating bubbles is accomplished by forcing a gas flow through a tube which is inserted into said molten Al.

7. The composite material of any preceding claim wherein the process further includes the step of providing a flux layer on an upper surface of said molten Al.

8. An apparatus for the production of a composite material, including titanium aluminide, said apparatus comprising:

a containment vessel;

a volume of molten Al in said containment vessel; a support structure which is constructed and arranged to suspend a Ti form from said support structure into said volume of molten Al;

a heating element; and

a temperature sensor cooperatively arranged with said heating element for maintaining said volume of molten Al in a molten condition.

9. The apparatus of claim 8 wherein said temperature sensor is a thermocouple which is positioned in said volume of molten Al.

10. The apparatus of claims 8 or 9 which further includes a stirring structure inserted into said volume of molten Al, said stirring structure being constructed and arranged for imparting a stirring motion to said volume of molten Al.

11. The apparatus of claim 10 wherein said stirring structure includes a tube constructed and arranged to allow a gas to be bubbled into said volume of molten Al.

12. The apparatus of claims 8, 9, 10 or 11 which further includes a flux layer on an upper surface of the volume of molten Al.

13. A process for the production of a composite material, including titanium aluminide, comprising the following steps:

a) providing a containment vessel; b) providing a volume of molten Al within said containment vessel;

c) providing a Ti form;

d) suspending said Ti form within said volume of molten Al; e) maintaining a molten condition for said molten Al; and f) enabling the formation of a single thickness of intermetallic compound titanium aluminide into said Ti form.

14. The process of claim 13 which includes the further step of providing said containment vessel with a heating structure for maintaining said molten condition of said molten Al.

15. The process of claim 14 which includes the additional step of inserting a temperature sensor into said molten Al, said temperature sensor being electrically connected to said heating structure.

16. The process of claims 13, 14 or 15 which includes the additional step of creating bubbles within said molten Al for imparting a stirring motion to said molten Al.

17. The process of claim 16 wherein said stirring motion results from the use of a tube and forcing a gas flow through said tube into said molten Al.

18. The process of claims 13, 14, 15, 16 or 17 wherein the process further includes the step of providing a flux layer on an upper surface of said volume of molten Al.

19. The process of claim 13 wherein the suspending step includes leaving a portion of the Ti form out of the molten Al.

20. The process of claim 19 which further includes the step of machining off said portion from the remainder of said Ti form.