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(11) | EP 0 149 194 A2 |
| (12) | EUROPEAN PATENT APPLICATION |
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| (54) | Aluminum-lithium alloy (2) |
| (57) An aluminum-lithium alloy exhibiting a good blend of fracture toughness and high
strength has a nominal composition of 2.45 percent lithium, 1.4 percent copper, and
0.12 percent zirconium with the balance being aluminum and trace elements. |
Background of the Invention
[0001] The present invention relates to aluminum-lithium alloys and more particularly to an aluminum-lithium alloy composition with good fracture toughness and high strength.
[0002] It has been estimated that current large commercial transport aircraft may be able to save from 15 to 20 gallons of fuel per year for every pound of weight that can be saved when building the aircraft. Over the projected 20 year life of an airplane, this savings amounts to 300 to 400 gallons of fuel At current fuel costs, a significant investment to reduce the structure weight of the aircraft can be made to improve overall economic efficiency of the aircraft.
[0003] The need for improved performance in aircraft of various types can be satisfied by the use of improved engines, improved airframe design, and improved or new structural materials in the aircraft. Improvements in engines and aircraft design have pushed the limits of these technologies. However, the development of new and improved structural materials is now receiving increased attention, and is expected to yield further gains in performance .
[0004] Materials have always played an important role in dictating aircraft structural concepts. In the early part of this century, aircraft structure was composed of wood, primarily spruce, and fabric. Because shortages of spruce developed in the early part of the century, lightweight metal alloys began to be used as aircraft structural materials. At about the same time, improvements in design brought about the development of the all metal cantilevered wing. It was not until the 1930's, however, that the metal skin wing design became standard, and firmly established metals, primarily aluminum alloys, as the major airframe structural material. Since that time, aircraft structural materials have remained remarkably consistent with aluminum structural materials being used primarily in the wing, body and empennage, and with steel comprising the material for the landing gear and certain other speciality applications requiring very high strength materials.
[0005] Several new materials are currently being developed for incorporation into aircraft structure. These include new metallic materials, metal matrix composites and resin matrix composites. It is believed that improved aluminum alloys and carbon fiber composites will dominate aircraft structural materials in the coming decades. While composites will be used in increased percentages as aircraft structural materials, new lightweight aluminum alloys, and especially aluminum-lithium alloys show great promise for extending the usefulness of alumium alloys.
[0006] Heretofore, aluminum-lithium alloys have been used only sparsely in aircraft structure. The relatively low use has been caused by casting difficulties associated with aluminum-lithium alloys and by their relatively low fracture toughness compared to other more conventional aluminum alloys. Aluminum-lithium alloys, however, provide a substantial lowering of the density of alumium alloys (as well as a relatively high strength to weight ratio), which has been found to be very important in decreasing the overall weight of structural materials used in an aircraft. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge still outstanding is an ability to obtain a good blend of fracture toughness and high strength in an aluminum-lithium alloy.
Summary of the Invention
[0007] The present invention provides a novel aluminum-lithium alloy composition with high strength, good fracture toughness, and relatively low density compared to conventional 2XXX aluminum alloys that it is intended to replace. An alloy prepared in accordance with the present invention has a nominal composition on the order of 2.45 weight percent lithium, 1.4 percent copper and 0.12 percent zirconium. The alloy is aged at a low temperature to near peak strength to provide a good blend of fracture toughness with high strength characteristics.
Detailed Description of the Invention
[0008] An aluminum-lithium alloy formulated in accordance with the present invention can contain from about 2.2 to about 2.8 percent lithium, 1.0 to 1.6 percent copper and a maximum of 0.15 percent zirconium as a grain refiner. Preferably from 0.1 to 0.15 percent zirconium is incorporated. All percentages herein are by weight percent based on the total weight of the alloy unless otherwise indicated. The copper adds strength to the alloy.
[0009] Iron and silicon can each be present in maximums up to a total of 0.3 percent. It is preferred that these elements be present only in trace amounts, limiting the iron to a maximum of 0.15 percent and the silicon to a maximum of 0.12 percent, and preferably to a maxiumu of 0.10 and 0.10, respectively. Certain trace elements such as zinc may be present in the amounts up to, but not to exceed, 0.25 percent of the total. Other elements such as chromium and manganese must be held to levels of 0.05 percent or below. If the maximums of these trace elements are exceeded, the desired properties of the aluminum-lithium alloy will tend to deteriorate. The trace elements sodium and hydrogen are also thought to be harmful to the properties (fracture toughness in particular) of aluminum-lithium alloys and should be held to the lowest levels practically attainable, for example on the order of 15 to 30 ppm (0.0015-0.0030 wt. %) for the sodium and less than 15 ppm (0.0015. wt. %) and preferably less than 1.0 ppm (0.0001 wt. %) for the hydrogen. The balance of the alloy, of course, comprises aluminum.
[0010] An aluminum-lithium alloy formulated in the proportions set forth in the foregoing paragraph is processed into an article utilizing known techniques. The alloy is formulated in molten form and cast into an ingot. The ingot is then homogenized at temperatures ranging from 925° F to 1000°F. Thereafter, the alloy is converted into a usable article by conventional mechanical formation techniques such as rolling, extrusion or the like. Once an article is formed, the alloy is normally subjected to a solution treatment at temperatures ranging from 950° F to 1000°F, quenched in a quenching medium such as water that is maintained at a temperature on the order of 70°F to 1500 F. If the alloy has been rolled or extruded, it is generally stretched on the order of 1 to 3 percent of its original length to relieve internal stresses.
[0011] The alumium alloy can then be further worked and formed into the various shapes for its final application. Additional heat treatments such as solution heat treatment can be employed if desired. For example, an extruded product after being cut to desired length are generally solution heat treated at temperatures on the order of 975° F for 1 to 4 hours. The product is then quenched in a quenching medium held at temperatures ranging from about 70° F to 1500 F.
[0012] Thereafter, in accordance with the present invention, the article is preferably subjected to an aging treatment that will increase the strength of the article, while maintaining its fracture toughness at a relatively high level. The article is preferably aged low temperatures ranging from about 200° F to about 300° F, and under some circumstances at higher temperatures, but generally less than 350° F. It is preferred that the alloy be aged at temperatures in the range of from about 250° F to 275° F.
[0013] It is preferred that when the alloy of the present invention is aged at the lower temperatures that it be aged for a period of time that will carry it to 92 to 99 percent of peak strength, and preferably to 98 to 99 percent of peak strength. At temperatures on the order of 250 to 275° F, the alloy of the present composition will achieve the desired strength level in from 4 to 100 hours.
Example
[0014] The following example is presented to illustrate the superior characteristics of an aluminum-lithium alloy aged in accordance with the present invention and to assist one of ordinary skill in making and using the present invention. Moreover, it is intended to illustrate the signifeantly improved and unexpected characteristics of an aluminum-lithium alloy formulated and manufactured in accordance with the paramters of the present invention. The following example is not intended in any way to otherwise limit the scope of this disclosure or the protection granted by Letters Patent hereon.
[0015] An aluminum alloy containing 2.45 lithium, 1.4 percent copper, 0.12 percent zirconium with the balance being aluminum was formulated. The trace elements present in the formulation constituted less than 0.25 percent of the total. The iron and silicon present in the formulation constituted less than 0.08 percent of the formulation. The alloy was cast and homogenized at about 9750 F. Thereafter, the alloy was hot rolled to a thickness of4l.2 inches. The resulting sheet was then solution treated at about 975° F for about 1 hour. It was then quenched in water maintained at about 70° F. Thereafter, the sheet was subjected to a stretch of 1 1/2 percent of its initial length. The material was then cut into specimens. The specimens were cut to a size of 0.5 inch by 2 1/2 inch by 0.2 inch for the precrack Charpy impact tests, which measure fracture toughness. The specimens prepared for the tensile strength tests were 1 inch by 4 inches by 0.2 inches. A plurality of specimens were then aged for 72 hours at about 2750 F. Each of the specimens aged at each of the temperatures and times were then subjected to the tensile strength and precrack Charpy impact tests in accordance with standard ASTM testing procedures.
[0016] The specimens aged at 275° F exhibit an ultimate strength ranging from about 65 ksi to about 75 ksi with the toughness on the order of 800 to 1400 in-lbs/in2.
[0017] The present invention has been described in relation to various embodiments, including the preferred formulation and processing parameters. One of ordinary skill after reading the foregoing specification will be able to effect various changes, substitutions, other equivalents and other alterations without departing from the broad concepts departed herein. It is therefore intended that the scope of the Letters Patent granter hereon will be limited only by the definition contained in the appended claims and equivalents thereof.
1. An aluminum-lithium alloy exhibiting good fracture toughness consisting essentially
of
2. The alloy of Claim 1 wherein said zirconium is present in amounts up to about 0.10
percent.
3. The alloy of Claim 1 having a nominal composition of 2.45 percent lithium, 0.6
percent magnesium, 1.8 percent copper, and 0.12 percent zirconium.
4. The alloy of Claim 1 wherein said alloy is aged at a relatively low temperature
to near peak strength.
5. The alloy of Claim 1 wherein said alloy has been aged at a temperature in the range
of from 200° F to 300° F.
6. The alloy of Claim 5 wherein said alloy has been aged for a period of at least
4 hours.