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
  • C22C21/12—Alloys based on aluminium with copper as the next major constituent
• • 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 rotates 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 aluminum 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 alloy composition that can be worked and heat treated so as to provide an aluminum-lithium alloy with high strength, good fracture toughness, and relatively low density compared to conventional 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 2.2 weight percent lithium, 0.7 percent magnesium, 2.5 percent copper and 0.12 percent zirconium. By underaging the alloy at a low temperature, an excellent combination of fracture toughness and high strength results.
• An aluminum-lithium alloy formulated in accordance with the present invention can contain from about 2.0 to about 2.4 percent lithium, 0 to 0.9 percent magnesium, 2.3 to 2.7 percent copper and a maximum of 0.15 percent zirconium as a grain refiner. Preferably from 0.10 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. While no magnesium need be employed in the alloy, it is preferred that magnesium be included to increase strength without increasing density. Magnesium also provides solid solution strengthening. Preferred amounts of magnesium range from 0.5 to 0.9 percent, with 0.7 percent being more preferred. 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 maximums of 0.10 and 0.10 percent 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.
• the desired properties of the aluminum-lithium alloy will tend to deteriorate.
• the trace elements sodium and hydrogen tire 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° 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 0 F to 150° F.
• the article is preferably subjected to an aging treatment at relatively low temperatures on the order of from 200 to 300° F. Since this alloy is intended to replace conventional 7XXX series type alloys, it is preferred that the alloy be aged for a period of time that will allow it to achieve at least about 95 percent of its peak strength. It is preferred that the alloy be aged for a period of time allowing it to achieve 95 to 97 percent of its peak strength. Preferred aging temperatures range from 250 to 275° F. Within these temperature ranges, 95 to 97 percent peak age can be achieved by aging from about 4 to 120 hours.
• An aluminum alloy containing 2.2 lithium, 0.5 percent magnesium, 2.5 percent copper, 0.1 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 0.07 percent of the formulation.
• the alloy was cast and homogenized at about 975° F. Thereafter, the alloy was hot rolled to a thickness of 0.2 inches.
• the resulting sheet was then solution treated at about 975 0 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 120 hours at 275° 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 underaged at 275° F exhibit an ultimate strength ranging from about 865 ksi to about 95 ksi with a toughness on the order of 220 to 280 in-lbs/in 2 .

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• Powder Metallurgy (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)

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Citations (1)

• 1984
  • 1984-12-20 EP EP19840115928 patent/EP0156995B1/de not_active Expired - Lifetime
  • 1984-12-20 DE DE19843486352 patent/DE3486352T2/de not_active Expired - Fee Related
  • 1984-12-28 JP JP28208884A patent/JPS60211035A/ja active Pending

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