Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium

Definitions

• the present invention relates to aluminum-lithium alloys and more particularly to an aluminum-lithium alloy composition with good fracture toughness and high strength.
• 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 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.
• 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.
• 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.
• 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 comprises aluminum.
• 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.
• the alloy is converted into a usable article by conventional mechanical formation techniques such as rolling, extrusion or the like.
• 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 150 0 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.
• 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 150 0 F.
• 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.
• 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.
• 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 975 0 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 275 0 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.
• 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 / in 2 .

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• Chemical & Material Sciences (AREA)
• Engineering & Computer Science (AREA)
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• Mechanical Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Powder Metallurgy (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)
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Cited By (1)

• 1984
  • 1984-12-20 EP EP84115929A patent/EP0149194A3/fr not_active Withdrawn
  • 1984-12-28 JP JP28208984A patent/JPS60211036A/ja active Pending

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