TITANIUM ALLOY (10V-2Fe-3Al) FOR AIRCRAFT DATA RECORDER

Abstract: An aircraft flight data recorder housing (12) comprising a titanium alloy having a nominal composition of 10 weight percent vanadium, 2 weight percent iron, and 3 weight percent aluminum with the balance being titanium and, within limitations, certain trace elements. The alloy is preferably forged into a recorder housing (12) by a two-step isothermal forging process. Working during the first step occurs above the beta transus, while working during the second step occurs near, but below, the beta transus. It is preferred that about 70 to 90 percent of the work introduced into the housing (12) during the isothermal forging be introduced during the second working step. Subsequent heat treatment procedures combined with the above processing result in a fine grained recrystallized microstructure having discontinuous grain boundary alpha which surprisingly provide the alloy with very high penetration resistance.

Description: TΪTANIUM ALLOY (10V-2Fe-3Al) FOR AIHCRA Γ DATA RECORDER Background of the Invention The present invention relates to titanium alloys and more particularly to a novel and unique titanium alloy that exhibits high strength and fracture toughness for use in aircraft flight data recorders. Although there are numerous situations in which it is necessary or desirable to protect a device from deleterious exposure to a crash environment, shielding the memory device of an aircraft flight recorder system during crash presents extremely demanding design constraints. In this regard, in order to preserve flight data supplied to the memory unit of a flight data recorder during a predetermined time interval immediately prior to an aircraft crash, the memory unit must be constructed to endure crushing and penetration forces experienced either as the aircraft impacts the ground or as a result of secondary impacts with other portions or pieces of the aircraft. Furthermore, the memory unit of a flight data recorder system is subject to additional design constraints imposed by considerations generally applicable to aircraft equipment and systems, including constraints relating to size, weight, cost, serviceability and reliability. Prior flight data recorders have been encased in stainless steel housings that are capable of withstanding very high crushing and penetration forces. Stainless steel, however, is relatively heavy when compared to other lightweight metals. Heretofore, other lightweight materials such as titanium alloys used on flight data recorders have not achieved desired weight reductions because they lacked the required blend of strength, ductility and fracture toughness to resist penetration during a crash. Summary of the Invention In accordance with the present invention, a unique titanium alloy is provided for use in housing of a flight data recorder. The titanium alloy formulated and processed in accordance with the present invention provides a flight data recorder housing that is not only as resistant to penetration as prior art stainless steel, but also exhibits sufficient fracture toughness to resist shattering or extensive cracking during a crash. In its preferred form, the titanium alloy consists essentially of: Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.6 to 2.5 Al 2.5 to 3.5 °2 0.13 max N 2 0.05 max C 0.05 max H 0.015 max Y 0.005 max Residual elements, each 0.10 max Residual elements, total 0.30 max Ti Balance. The alloy is conventionally formulated and processed into forging blanks. An article such as the flight data recorder housing is forged in a two-step isothermal forging process. The first step is performed at a temperature at which the alloy is in its beta phase. The second step is performed at a lower temperature at which the alloy is a mixture of both the alpha and beta phases. It is preferred that a majority of the work introduced into the article during forging be introduced during the second step. More preferably, 70 to 90 percent of the work introduced into the article is introduced during the second step. Formulating and processing the titanium alloy in accordance with the present invention provides an alloy that is characterized by a fine grained, recrystallized mirerostructure having discontinuous grain boundary alpha particles, which surprisingly provides the alloy with a very high fracture toughness and penetration resistance. Detailed Description of the Invention A thermally protected flight data recorder system memory unit configured in accordance with this invention is illustrated in FIGURE 1 and is generally denoted herein by the reference numeral 10. As is known in the art, such a memory unit is configured to provide a record of various important aircraft performance parameters over a predetermined time interval that occurs immediately prior to each time the flight data recorder is deactivated (including deactivation which occurs if the aircraft crashes). In operation, the information stored within the memory unit is ongoingly supplied by additional components of the flight data recorder system such as a data acquisition unit that receives input signals from various aircraft sensors and systems and processes those signals to produce signals compatible with the recording or storage medium employed by the flight data recorder memory unit. In the case of the disclosed embodiment of the invention, which utilizes a solid state electronic device such as electronically erasable programmable read only memory circuits as the information " storage medium, the data acquisition unit periodically supplies digital signals which are sequentially written into the semiconductor memory circuits so that the memory circuits store a sequence of digital words that is a time sampled data representation of the history for each parameter being monitored. Typically, with respect to currently employed techniques, data compression is generally employed to allow storing digital signals representative of a 15-30 minute time history for each monitored parameter. As is shown in FIGURE 1, the present embodiment of the flight data recorder system memory unit of this invention includes an outer housing 12 that is substantially rectangular in cross section when viewed perpendicular to each of its major axes. Flanges 14 extend orthogonally from oppositely disposed edges of the base of outer housing 12 to facilitate mounting memory unit 10 at a convenient location within the aircraft by means of bolts or other conventional fasteners. A substantially rectangular cavity 16 extends inwardly from one face of outer housing 12 toward the base of memory unit 10 so that the major portion of outer housing 12 is configured as a substantially rectangular shell. Outer housing 12 is constructed of a titanium alloy that exhibits relatively low density and very high resistance to crushing and penetration with the wall regions that are defined between cavity 16 and the outer surfaces of outer housing 12 being dimensioned to withstand crushing and penetration should the aircraft crash. The titanium alloy utilized in accordance with the present invention is formulated and processed so as to provide it with unique and superior character¬ istics that make it especially adaptable for use as a flight recorder housing. An intumescent coating or paint is applied to the outer surfaces of outer housing 12 to provide thermal insulation during the initial phases of a fire. A shell-like thermal liner 18, nested within cavity 16 of outer shell 12, provides a first thermal barrier for shielding components that are located in the interior regions of memory unit 10 from high temperature fires that may occur during such an aircraft crash. Thermal Ipier 18 is substantially rectangular in cross-sectional geometry relative to each of its major axes and forms an inwardly extending cavity 20 that is coaxially positioned within cavity 16 of outer housing 12. Thermal liner 18 is preferably a unitary structure that is formed of a solid material that is a good thermal insulator. As is illustrated by FIGURE 1, a relatively thin walled central shell 22 that nests within cavity 20 receives and contains one or more printed circuit boards 24 that provide physical support for and electrical innerconnection for a number of solid state memory devices 26. Although the arrangement of FIGURE 1 depicts a conventional printed circuit arrangement wherein each solid state memory device is encapsulated to form what is known as a dual in-line package, other configurations can be employed. For example, in some realizations of the invention it may be advantageous to bond semiconductor chips that contain circuitry for a number of electronically erasable programmable read only memories directly to a ceramic substrate or other carrier that includes electrical innerconnections that are vacuum deposited or otherwise formed thereon. In any case, central shell 22 is preferably formed from a material such as stainless steel or another metal that presents a reasonable density-heat capacity trade off (i.e., the product of material density times heat capacity is relatively high) and that also is easily worked or formed. Further, each printed circuit board 24 is mounted within central shell 22 so that each solid state memory device 26 is spaced apart from the inner surfaces of central shell 22. To provide a high degree of heat shielding, the open regions between the inner walls of central shell 22, printed circuit board 24 and adjacent solid state memories 26 are filled by a meltable insulator that exhibits a solid- liquid phase transition at or below the desired temperature limit for solid state memories 26. Such a material is characterized by a first temperature range wherein heat energy supplied to the material first results in a linear increase in material temperature and is followed by a relatively constant temperature region wherein the heat energy supplied is consumed by the heat of fusion of the material, causing the material to melt. A continued supply of heat energy to such a material after it reaches the molten state generally will cause the material temperature to rise to the boiling point of the material. When additional heat energy is supplied, no further temperature increase is experienced until the material is vaporized. This latter characteristic is of importance only in that the meltable insulator 28 that is employed in the practice of the invention is selected so that little or no vaporization occurs when memory unit 10 of FIGURE 1 is subjected to a high temperature environment associated with a burning aircraft Referring again to FIGURE 1 and the physical configuration of memory unit 10, electrical connection to printed circuit board 24 is provided by means of a flexible, flat cable assembly 30 that is constructed of a polyimide ribbon or other such material that includes a series of spaced apart conductive strips. When printed circuit board 24 is placed in central shell 22, cable 30 extends through a rectangular notch 32 that is formed in one boundary edge of central shell 22. Thermal isolation for the face of central shell 22 that is defined by cover assembly 34 is provided by a substantially rectangular thermal insulator 40 that is constructed of the same material utilized in forming thermal liner 16. As is shown in FIGURE 1, thermal insulator 40 preferably is covered with a fiberglass reinforced resin 42 (or other durable material) to protect thermal insulator 40. A second substantially rectangular cover plate 44 that is constructed of the same titanium alloy as outer housing 12 covers the open face of outer housing 12 so as to fully enclose memory unit 10 in a manner that substantially seals the unit and provides substantially identical thermal conductance relative to heat energy that is coupled through each rectangular face of memory unit 10. With particular reference to FIGURE 1, electrical cable 30 exits cavity 16 of outer housing 12 through a rectangular slot 46 that is formed in one wall of outer housing 12. A connector 48 at the outward terminus of cable 30 mates with a connector 50 that is located on a printed circuit board 52. In the depicted embodiment, printed circuit board 52 is mounted substantially parallel to the face of outer housing 12 that includes slot 46 and contains conventional electronic interface or control circuitry (not shown in FIGURE 1) for sequentially addressing solid state memory devices 26 during operation of the flight data recorder system. Although this control circuitry need not survive a fire in order to preserve the data stored in solid state memory devices 26, it is preferably mounted within memory unit 10 in order to eliminate data errors that might otherwise be caused by electromagnetic interference and various other signal transients encountered in aircraft electrical systems. To complete memory unit 10 and provide electrical connection between the system data acquisition unit and printed circuit board 52, memory unit 10 includes an electrical connector 54 that passes through the major face of a substantially U-shaped flange 56. As is indicated in FIGURE 1, flange 56 is mounted to outer housing 12 with connector 54 spaced apart from circuit board 52. A suitably configured ribbon-type cable assembly 58 provides electrical interconnection between connector 54 and printed circuit board 52. The titanium alloy formulated in accordance with the present invention has a nominal composition comprising 10 percent vanadium, 2 percent iron, 3 percent aluminum, with the balance being titanium and various trace elements and impurities. In its broadest embodiments, the vanadium content can vary between 9.0 and 11.0 percent, the iron between 1.0 and 3.0 percent and the aluminum between 2.0 and 4.0 percent. All percentages utilized herein are weight percent based on the total alloy. In its most preferred form, the titanium alloy has the following composition: Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.6 to 2.5 Al 2.5 to 3.5 °2 0.13 max N 2 0.05 max C 0.05 max H 0.015 max • Y 0.005 max Residual elements, each 0.10 max Residual elements, total 0.30 max Ti Balance. It is especially important that the trace elements oxygen, nitrogen, carbon and hydrogen be maintained at or below the maximums indicated in the foregoing table. An alloy of the foregoing composition is available from Timet, 400 Rouser Road, Pittsburgh, PA 15230. Prior to processing in accordance with the present invention, the manufacturer of the alloy will formulate an alloy in accordance with the nominal composition, keeping in mind the maximums listed above for the various trace elements. The alloy is usually cast into an ingot having a length of about 6 feet and a diameter of about 28 inches. This ingot is then forged to a diameter of about 16 inches round corner squares at temperatures above the beta transis temperature, that is, at temperatures on the order of 2000° down to about 1500° F. It is important that the alloy not be maintained at these temperatures for too long, or a desirable reduction in grain size will not ultimately occur. The alloy is then reduced to a round or square bar having a diameter or side equal to about 4 and 1/2 inches. This bar is then cut into forging blanks or multiples (sometimes called "mults"), which are employed as the starting stock for processing in accordance with the present invention. Thereafter, in accordance with the present invention, the alloy is hot worked to form an article such as a flight data recorder enclosure at sufficiently high temperatures so that the characteristic microstructure described in more detail below is achieved. Forging the article at temperatures on the order of 1400° F will generally result in the characteristic microstructure. The resulting alloy will exhibit a fine grained, reerystallized microstructure. The microstructure will be characterized by the presence of both discontinuous grain boundary alpha particles as well as a scattering of acicular alpha particles. It was heretofore thought that a titanium alloy having high fracture toughness required the presence of a substantial number of acicular alpha particles. Surprisingly, however, it has been found that with an alloy formulated and processed in accordance with the present invention, few acicular particles need be present to achieve high strength and fracture toughness. Instead, a substantial proportion of discontinuous grain boundary alpha particles are present, which lead to the relatively high fracture toughness and penetration resistance of an article produced in accordance with the present invention. It has been found, however, that the discontinuous boundary alpha particles must be present in combination with a fine grained microstructure. It is preferred that the grain size be on the order of less than about 8 ASTM grain size units and preferably finer than 10 ASTM grain size units. It has been found, however, that an alloy having an average grain size of 8 to about 10 ASTM grain size units will have sufficiently high strength, fracture toughness and penetration resistance to function acceptably as a flight data recorder housing. It is most preferred in accordance with the present invention, that an article be formed from the forging blanks or mults via a two-step isothermal forging process. The first step of the forging process is carried out at a temperature at which the titanium alloy is in its beta phase. Forging at these temperatures is commonly referred to as beta working. Typical beta working temperatures for a titanium alloy having a nominal composition as set forth above are in the range of from about 1500° F to about 1600° F. It is preferred that the alloy be beta worked at a temperature of about 1550° F. The second step of the two-step isothermal forging process in which the alloy is forged into its final shape is carried out at lower temperatures at which the alloy is a mixture of both the alpha and beta phases. Working at these temperatures is sometimes referred to as alpha-beta working. The alpha-beta working temperatures range from about 1250° F to about 1450° F. It may be possible to work the material at higher temperatures up to the beta transis temperatures. However, in accordance with the present invention, a most preferred alpha-beta working temperature is about 1400° F. Moreover, it is most important to achieving the desired blend of strength and fracture toughness that a majority of the work introduced into the article during the isothermal forging occur during the second step, that is, during the alpha-beta working. It is preferred that at least 50 percent of the work be introduced during the second step and more preferably from 70 to about 90 percent of the work be introduced during the second step of the isothermal forging. While not absolutely necessary, it is most preferred that the article be subjected to a duplex heat treatment after isothermal forging. The duplex solution treatment comprises first maintaining the alloy at a temperature of from 10 to 30 degrees less than the beta fleck temperature for a period on the order of 1 to 2 hours. The beta fleck temperature is a temperature conventionally determined by the manufacturer during inspection and evaluation of the forged bar. It is currently preferred that the beta fleck temperature be no less than 60° F less than the beta transis temperature, and most preferably no less than 30° F less than the beta transis temperature. It is most preferred that the first step of the solution heat treatment occur at a temperature of about 20 degrees less than the beta fleck temperature. For a titanium alloy having a nominal composition formulated in. accordance with the invention, this temperature is on the order of 1440° F. The alloy is maintained at this temperature for a period generally on the order of from 1 to 2 hours and is then air cooled to ambient temperature. The second step of the duplex solution heat treatment occurs at a temperature of about 50 to 70 degrees less than the beta fleck temperature and preferably at a temperature of 60 degrees less than the beta fleck temperature. For an alloy having the nominal composition set forth above, this temperature is on the order of 1400° F. The alloy is maintained at this temperature for a period of 1 to 3 hours, after which it is cooled rapidly to room temperature by air cooling, fan air cooling, or water quenching. It is then preferred that the alloy be aged for a period of from 6 to 10 hours, preferably 8 hours at temperatures on the order of from 900 to 980° F. This will result in a titanium alloy having a strength on the order of 175 ksi. It is important that the alloy only be aged to strength no greater than 190 ksi. If it is aged to a strength higher than that limit, fracture toughness and other desirable properties will tend to fall off. Similarly, if the material is aged to strength levels below 165 ksi, penetration resistance will tend to fall off. Examples The following examples are intended to illustrate to one of ordinary skill how to make and use the present invention. The examples are not intended in any way to limit the scope of Letters Patent granted hereon. For both of the ensuing examples, titanium alloy blanks having the nominal composition set forth above were processed by the manufacturer in accordance with the procedure set forth above. In both examples, a processing blank or mult having a diameter of about 4.5 inches was utilized. Example I A first mult was subjected to isothermal forging first at a temperature of about 1550° F, followed by a second step isothermal forging at about 1400° F. Only about 25 percent of the total work introduced in the article was conducted at the 1400° F forging temperature. The mult was worked down to a specimen plate thickness of about 0.15 inches from an original mult thickness of about 1 inch. The isothermal forging was conducted in a flat hot die. Thereafter, the forged specimen plate was subjected to a duplex solution heat treatment. In the first step, the material was maintained at about 1425° F for about two hours and then air cooled to ambient temperature. In the second step, the plate was heated to a temperature of about 1400° F where it was maintained for an additional 2 hours, after which it was water quenched to room temperature. The plate was then aged for 8 hours at 950° F and air cooled to ambient. The specimen plate, which was machined to a thickness of 0.122 inch and which has a length and width of about 6 inches by 8 inches, was then bolted in a steel rectangular frame exposing a central portion of the plate having a dimension of about 4 inches by 6 inches. The plate and frame were then placed on a bed of sand having a depth of 18 inches and length and width of 48 inches. A 500 pound penetration weight was then elevated 10 feet above the plate. The bottom of the penetration weight carries a vertically oriented steel pin about 2- 1/2 inches long having a 1/4 inch diameter and rockwell hardness of about C 30. When the penetration weight was released and allowed to gravitate toward the specimen plate, the steel pin penetrated through the titanium alloy plate, indicating that the plate had insufficient penetration resistance to acceptably function as a flight data recorder housing. Example II A titanium alloy plate having the nominal composition set forth above was prepared as in Example I with the exception that the two-step isothermal forging was carried out so that about 80 percent of the work -iϋ- introdueed during the isothermal forging was introduced during the second step, that is, during the alpha-beta work at about 1400° F. Again, a specimen plate having a thickness of about 0.150 inches was produced and machined to a thickness of 0.122 inch. When subjected to the penetration test, the plate dented slightly. The penetration weight was dropped an additional four times with no penetration of the test specimen. Lack of penetration of the penetration pin indicates that the titanium alloy has an appropriate blend of strength, fracture toughness and penetration resistance to function as a flight data recorder housing. The present invention has been described broadly and in relation to its preferred embodiments. One of ordinary skill will be able to effect various changes, substitutions of equivalents, and other alterations without departing from the broad concepts .disclosed herein. It is, therefore, intended that the protection granted by Letters Patent hereon be limited only by the definition contained in the appended claims and equivalents thereof.

Claims: The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 1. A flight data recorder housing comprising an alpha-beta titanium alloy having a nominal composition of Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.0 to 3.0 Al 2.0 to 4.0 Ti Balance, said alloy being characterized by a fine grained, recrystallized microstructure, and the presence of discontinuous grain boundary alpha particles. 2. The flight data recorder of Claim 1 wherein said alloy consists essentially of Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.6 to 2.5 Al 2.5 to 3.5 °2 0.13 max N 2 0.05 max C 0.05 max H 0.015 max Y 0.005 max Residual elements, each 0.01 max Residual elements, total 0.30 max Ti Balance. 3. The flight data recorder of Claim 1 wherein said housing is formed by a two-step forging, a first step being conducted at temperatures in which said alloy is a beta phase, and a second step being conducted at temperatures in which a mixture of said alloy is alpha and beta phases, and wherein at least 50 percent of the work introduced to the alloy during the isothermal forging occurs during said second step. 4. The flight data recorder of Claim 3 wherein from about 70 percent to about 90 percent of the work introduced during isothermal forging occurs during said second step. ι 5. An article comprising a titanium alloy consisting essentially of Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.0 to 3.0 Al 2.0 to 4.0 Ti Balance, said alloy being characterized by a fine grained, recrystallized microstructure, and the presence of discontinuous grain boundary alpha particles. 6. The article of Claim 5 wherein said alloy consists essentially of Element Weight percent (wt. %) V 9.0 to 11.0 Fe 1.6 to 2.5 Al 2.5 to 3.5 °2 0.13 max N 2 0.05 max C 0.05 max H 0.015 max Y 0.005 max Residual elements, each 0.10 max Residual elements, total 0.30 max Ti Balance. 7. The article of Claim 5 wherein said article is formed by a two-step isothermal forging, said first step being performed at a temperature at which said alloy is in said beta phase, said second step being performed at a temperature at which said alloy is a mixture of the alpha and beta phases, and wherein greater than 50 percent of the work introduced during said isothermal forging occurs during said second step. 8. The article of Claim 7 wherein from about 70 percent to about 90 percent of the work introduced by said isothermal forging occurs during said second step. 9. The article of Claim 7 wherein said first step is conducted at a temperature in the range of from about 1500° F to 1600° F. 10. The article of Claim 9 wherein said second step is conducted at a temperature in the range of from about 1250° F to about 1450° F. 11. The article of Claim 5 wherein the average grain size of said alloy is less than about 8 ASTM grain size units. 12. The article of Claim 11 wherein the average grain size of said alloy is less than about 10 ASTM grain size units. 13. A method for preparing a titanium alloy exhibiting high strength and fracture toughness wherein said alloy consists essentially of Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.0 to 3.0 Al 2.0 to 4.0 Ti Balance, said method comprising the steps of: a. formulating said alloy and forming an ingot, b. working said ingot and forming forging blanks therefrom, c. forging an article from said forging blanks at sufficiently high temperature to produce an alloy, said alloy being characterized by a fine grained, recrystallized microstructure, and the presence of discontinuous grain boundary alpha particles. 14. The method of Claim 13 wherein said alloy consists essentially of Element Weight percent (wt. 96) V 9.0 to 11.0 Fe 1.6 to 2.5 Al 2.5 to 3.5 °2 0.13 max N 2 0.05 max C 0.05 max H 0.015 max Y 0.005 max Residual elements, each 0.10 max Residual elements, total 0.30 max Ti Balance. 15. The method of Claim 13 wherein said forging comprises a two step isothermal forging, said first step being performed at a temperature at which said alloy is in the beta phase, said second step being performed at a temperature at which said alloy is a mixture of alpha and beta phases, and wherein greater than 50 percent of the work introduced into said article during said isothermal forging occurs during said second step. 16. The method of Claim 15 wherein from about 70 percent to about 90 percent of the work introduced into said article during said isothermal forging occurs during said second step. 17. The method of Claim 15 wherein said first step of said isothermal forging is conducted at a temperature at a range of from 1500° F to 1600 β F. 18. The method of Claim 17 wherein said second step is conducted at a temperature in the range of from 1250° F to 1450° F. 19. The method of Claim 13 wherein the average grain size of said alloy is less than about 8 ASTM grain size units. 20. The method of Claim 19 wherein the average grain size of said alloy is less than about 10 ASTM grain size units. 21. The method of Claim 13 wherein said isothermal forging is followed by a duplex solution treatment comprising the steps of: first maintaining the temperature of the article at from 10 to 30 degrees less than the beta fleck temperature for a predetermined period, thereafter air cooling the article, and thereafter maintaining the temperature of said article at a temperature of from 50 to 70 degrees less than the beta fleck temperature of said article, and thereafter rapidly cooling said article by air cooling, fan air cooling, or water quenching. 22. The method of Claim 21 wherein said first solution treatment is conducted at a temperature of about 20 degrees less than the beta fleck temperature and wherein said second solution treatment step is conducted at a temperature about 60 degrees less than the beta fleck temperature of the alloy. 23. The method of Claim 22 wherein the article is then aged at a temperature in the range of from 900 to 980° F for 8 hours and thereafter air cooled to obtain a strength level in the range of from 165 to 190 ksi. 24. The article of Claim 11 wherein the average grain size of said alloy is between about 8 and about 10 ASTM grain size units. 25. The method of Claim 19 wherein the average grain size of said alloy is between about 8 and about 10 ASTM grain size units.

Inventors: SPURR WILLIAM F, WHITE WILLIAM F, SPURR, WILLIAM, F., WHITE, WILLIAM, F.
Applicants: SUNDSTRAND DATA CONTROL, SUNDSTRAND DATA CONTROL, INC.
Application year: 1985
Main classification: C22C
IPC codes: C22C14, G12B17, H05K5