CA1280341C - Low temperature underaging of lithium bearing alloys - Google Patents

Abstract

LOW TEMPERATURE UNDERAGING OF
LITHIUM BEARING ALLOYS
Abstract The combination of strength and fracture toughness properties of aluminum-lithium alloys are significantly enhanced by underaging the alloys at temperatures ranging from 200°F to below 300°F for relatively long periods of time.

Description

33~

LOW TEMPERATURE UNDEI~AGING O~
LITHIUM BEARING ALLOYS

. .
S Technicul Field The invent}on relutes ta ~luminum Qlloys containing lithium as an ulloyin~ element, and pQrticulQrly to a process for ilnproving the fr~cture toughness of sluminum-lithium nlloys without detracting from their strength.
uckground Or the I_vention It hns been estim~ted th~t some current large commercial tr~nsport aircr~ft m~y be ~ble to sQve from 15 to 20 gallons of fuel per yeur for cvery pound of welght thut csn be saved when building the aircraft. Over the pro~ccted ao yeur life of an airplane, this saving~ amounts to 300 to ~00 gallons of fuel. At current fuel costs, ~ significunt in~restment to reduce the structural weight of the ~ircr~ft c~n be made to improve overall economic efficiency of the aircraft.
The need for improved performance in aircraft of various types eun be sstisfied by the use of improved engines, improvad ~irframe design, or by theusc of new or improved structural mnterials. Improvements in engines and eircraft design have been vlgorously pursued~ but only recorltly h~s the dovelo~~nent of new nnd improved structurnl m~teri~ls recelvecl com mensurQte attention, and thelr implementation In new elrcr~ft designs is ~xpl~cted to yield significunt gains in performance.
Materials have always played ~n importQnt role in dictating air-cr~ft structurul concepts. Since the early lg3û's, structurQl materials for large uircraft hQve relnained remur1;ubly consistent, with uluminurn being the primaryrnateriul oî constluction in tt1e VJin~ ody ~nd e1npennuge, ~nd with steel bein~utilized for lunding~ ge~rs und certain other speciulity up[)licutions requiring very hig11 slrengtl1. CJver the [)!ISt sever~l yeurs, however, seYeral important neu materials concepts hnve been under dcvelopment for incorporntion into aircraft structures. These include new metallic mnterials, metlll matrix composites nnd resin m~trix com[)osite~. It is believed l~y muny th~t improved nlurninum alloysand cllrbon fiher resin mutrix will dornlnnte aircrllft structurnl materiuls in thc coming clecaàes. While composites will be used in incr~EIsed percentllges as ircrllft structllllll materials, new lighlweight aluminllm alloys, and especi~lly aluminum-lithi~m alloys show great promise for ext~nding the usefulness of mnteri~ls of this type.
~leretofore, Rl~lminum-litllium alloys have been used only sparsely lO in aircraft structllres. The low use hns been caused by their relatively low frHcture toughness and by casting difeiculties associated with lithium-bearing ~luminum alloys compared to other moré conventional aluminum alloys. I.ithium additions to aluminum, howéver, provide Q substantial lowering of the density which h~s been determined to be very important in decreasing the overhll 15 structural weight of aircraft. While substantial strides have been made in improving the aluminum-lithium processing technology, a major challenge is stillto obtain a good blend of fracture toughness nnd high strength in these alloys.
Summary of the Invention The present invention provides a method for aging aluminum-20 lithium alloys of various compositions at relatively low temperatures to develop R high nnd improved fracture toughness without reducing the strength of the alloy. Simply, after the alloy is formed into an article, solution heat treated and quenched, the alloy is aged at a relatively low temperature for a relatively long time. This process may be generally referred to as low temperature under~ging.
25 More specifically, the alloy can be aged at temperatures rnnging ~rom 200F to below 300F for n period of time ranging from 1 up to 80 or morc hours. This low temperature aging regimen will result in an alloy having a greater ~rRcture toughness, often on the order of 150 to 200 percent, than that of materinls ngednt conventional higher temperatures while maintaining an equivalent strength.
Brief Description of the D awin~
A better understanding of the present invention cnn be derived by reading the ensuing specification in conjunction with the accompanying drawing wherein:
FIGURE l is a graph showing fracture toughness/strength combina-35 tions of sever~l specimens of an aluminum-lithium alloy aged at vsrious times ~nd various temperatures as described in the Example.

-9L~8~

Detailed Description of the Invention An aluminum-lithium alloy formulQted in accord~nce with the present invention can contain from about 1.0 to ubout 3.2 percent lithium. The current data indicstes that the benefits of the low temperature underaging are 5 most apparent at lithium levels of 2.7 percent and below. A11 percent~ges herein are by weight percent (wt~6) based on the total weight of the alloy llnless otherwise indicQted. Addition~l alloying Qgents such as magnesium, copper and mangQnese cnn also be included in the alloy. Alloying addit~ons function to improve the general engineering properties but also flffect density somewhat.
10 Zirconium is ~lso present in these dlloys QS a grain refiner at levels between 0.08 to ().l5 percent. Zirconium is essential to the devclopment of the desired combination of engineering properties in uluminum-lithium Qlloys, including those subjected to our low tempel uture underaging treatment.
The impurity elements iron flnd silicon can be present in amounts 15 up to 0.3 and 0.5 percent, respectively. It i9 preferred, however, thQt theseelements be present only in trace amounts of less thQn 0.10 percent. Certain tr~ce elements such as zinc and titanium may be present in amounts up to but not to exceed n.25 percent and 0.15 percent, respectively. Certain other trace elements such as cadmium and chromium must each be held to levels of 0.05 20 percent or less. If these mRximums are exceeded, the desired proper-ties of the aluminum-lithium Qlloy will tend to deterior~te. The trace elements sodium and hydrogen are also thought to be hQrmful to the properties of nluminum-lithium alloys and should be held to the lowest levels practically attainable, for exQmple on the order of 15 to ~0 ppm (0.0015-0.0030 wt96) mQximum for the sodium and 25 less than 15 ppm (0.0015 wt96) and preferably less than 1.0 ppm (0.0001 wt96) for the hydrogen. The balance of the Qlloy, of course, comprises aluminum.
The following Table represents the proportions in which the alloying and trace elements may be present. The broadest ranges are acceptable under some circumstances, while the preferred rnnges provide a better balance 30 of fracture toughness and strength. The most preferred ranges yield alloys that presently provide the best set of overall properties for use in aircrQft structures.

~2~3~3~

T~BLE

Element Amount (wt%) .. . .. _ _ . . _ Acceptable Preferred Most Preferred .... __ Li l.0 to 3.2 1.5 t~ 3.0 1.8 to 2.7 Mg 0 to 5.5 0 to 4.2 0 to 3.2 Cu 0to~.5 Oto3.7 0.~to3.0 Zr 0.08 to 0.lS 0.08 to 0.15 0.08 to 0.15 Mn 0 to 1.2 0 to 0.8 0 to 0.6 Fe 0.3 max 0.15 max 0.lO max Si û.5 max 0.12 max 0.10 max Zn 0.25 max 0.lO max 0.t0 max Ti 0.t5 max 0.10 max 0.10 max Na 0.()030 max n.û015 max 0.0015 max ~I n.~)nl5 max 0.0005 max 0.0001 max Other trace elements each o.ns max n.n5 max 0.05 max total n.25 max ~.25 max 0.2S max Al 13alance Ba1ance Balnnce An aluminum-lithium alloy formulated in the proportions set forth in the foregoing paragraphs and Table i~s processed into an article utilizing known techniques. The alloy is formulated in molten form and cast into nn ingot. The 25 ingot is then homogenized at temperat~lres ranging from 925F to approximately 1000F. Thereaîter, the alloy is converted into a usable article by conventionalmechanical forming 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 l010F, followed by quenching into a 30 medium such ns water that is maintained at a temperature on the order of 70Fto 150F. If the alloy has been rolled or extruded, it is generally stretched onthe order of 1 to 3 percent of its original length to relieve internal stresses and improve engineering properties. The aluminum alloy may then be further worked and formed into the various shapes for its final application. Additional heat 35 treatments such as those outlined above may then be employed if desired.
Thereafter, in accordance with the present invention, the article is subjected to an ~ging treatment that will increase the strength of the material while maintaining its fracture toughness and other engineering properties at , ~Z~3~

relatively high levels. In accordnnce with the present invcntion, the article issubjected to ~ low temperature underuge heat treatment at temperntllres ran~ing from about 2001~ to less than 300F. Low tempcrntllrc ~Indera~in~ at temperatllres in the range of from about 250F to about 27S~ is considercd 5 preferred for most alloys, t~lcing into consideration the economic impetlls for minimizing the time spent in commercial he~t-treutment f~cilities. At thc higher temperatures, less time is needed to bring about the proper balance between strength and fracture tou~hness thaIl nt lower ~ging temper~tures, but the overall property mix will be slightly less desirable. For example, when the 10 aging is conducted Qt temperatures on the order of 275F to just below 300F, it is preferred that the product be subjected to the aging temperature for periods of from 1 to 40 hours. On tIle other hand, when aging is conducted at temperatures on the order of 250F or below, aging times from 2 to 80 hours or more are preferred to bring about the proper bQlance between fracture toughness 15 and strength. After the aging trentment, the aluminum lithium flrticle is cooled to room temper~ture.
When the low temperature underaging treatment is conducted in accordance with the pnrameters set forth abovet the trentment will result in an aluminum-lithium alloy hQving nn ultimute strength typic~lly on the order of 45 20 to 95 I<si, depending on the composition of the particul~r alloy. The fracture toughness of the Lllloy will be greater, often on the order of 1 1/2 to 2 times greater, than thuL of similar aluminum-lithium alloys ~lged to equivalent strength leve~s by conventional aging treatments nt temperflt~lres ~reater than 300F.
The following ExQmple is presented to illustrllte the superior 25 strength and tollghness combin~tion achieved by low temper~ture underaging nnnluminllm-lithium alloy in accord~nce Witll the present invention and to assist one of ordinary sl~ill in ma~;ingt nnd using the prcsent invcntion. The following Example Is not intended in uny way to otherwise limit the scope of this disclosure or the protection granted by Letters Patent hereon.
EXA MPLE
An aluminum a]loy containing 2.4 percent lithium, 1 percent mùgnesium, 1.3 percent copper, 0.15 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 in35 the formulation constituted less than 0.07 percent each of the formulation~ The alloy was cast and homogenized at 975F. Thereafter, the alloy WQS hot rolled to a thickness of 0.2 inches. The resulting sheet WQS then solution treated at 975F for about 1 hour. The sheet was then quenched in water maintained at , -6 ~2~3~)3~

about 70 F. ThereE~fter, tlle sheet w~s subjected to rl stretch of 1 1/2 percent of its initial length and was then cut into specimens. Some specimens were CUt to size of 0.5 inch by 2.5 inch by 0.2 inch for precrQck Chrlrpy imp~ct tests, r~
'<nown method of measuring fracture toughness. Other specirnens prep~red for t~nsile strength tests were l inch by 4 inches by 0.2 inchcs. A plurE~lity of specimens were then aged at 350~F for 4, 8, and lfi hours; at 325F for ~, l6, and 48 hours; at 305F for 8 hours; at 275~ for 16 and 40 hours; and at 250F
for 40 nnd 72 hours. Specimens aged at each of the temperature6 and times were then subjected to precrack Charpy impact and tensile strength tests in accordance with standard testing procedures. The test values of the specimens aged at a particular tempernture and time were then averaged. These average test values are set forth in the graph shown in FIGURE 1.
By examining FIGURE 1 it will be readily observed that specimens aged at temperatures greater than 300F exhibited a toughness on the order of 15 from 225 to 525 inch-pounds per square inch as measured by the Charpy impact test. In contrast, the specimens underaged at a low temperature in accordance with the present invention exhibited toughnesses on the order of 650 to almost 850 inch-pounds per square inch as indicated by the Charpy impact test. At the same time, the average strengths of the materials fell generally within the 64 to 20 71 ksi range, with the exception of the specimens aged at 350F for 16 hours.(The 350~ age specimens, however, exhibited the lowest toughness of any of the specimens.) Thus, the test results indicate that aging at a temperature less than 300F for a relatively long time will clearly provide a strength/toughness combination th~t is superior to that Oe specimens aged in ~ccordance with 25 conventionQl procedures at temperatures on the order of 325 to 350F or more for relatively short periods of time. 'rhe test results Ltlso show that there is a remt~rkal)le improvement in the strength--tollghness combination of properties as ~he ~ging temperlltllre is lowered below 3(J0~, i.e., a higher frilcture toughness ~or any given slrength level.

3~ The present invention has been described in relation to various embodiments, including the preferred processing parameters and formulations.
One of ordinary skill after reading the foregoing specification will be nble to effect various chnnges, substitl~tions of equivalents and other alterations without departing frorn the broad concepts disclosed herein. For example, it is 35 contemplated that the subject low temperature underaging trentment mny be applicable to other alloying combinations not now under development, and specifically to Elluminum-lithium alloys with substantial amo-lnts of zinc, silicon, iron, nickel, beryllium, bismuth, germanium, al)d/or zirconium. It is therefore .

~IL2B03~

intended th~t the scope of Letters Pntent grllnted hereon will be limited only by the definition contuined in the nppended cl~ims nnd equivnlents thereof.

Claims (7)

1. A process for improving the relative strength and fracture toughness of an aluminum alloy containing lithium as an alloying clement, said alloy consisting essentially of:
Element Amount (wt%) Li 1.0 to 3.2 Mg 0 to 5.5 Cu 0 to 4.5 Zr 0.08 to 0.15 Mn 0 to 1.2 Fe 0.3 max Si 0.5 max Zn 0.25 max Ti 0.15 max Other trace elements each 0. 05 max total 0.25 max Al Balance, said alloy first being formed into an article, solution heat treated and quenched, said process comprising the step of aging said alloy at temperatures ranging from about 200°F to less than 300°F for a period of time ranging from about one up to eighty or more hours.

2. The process of Claim 1 wherein said aging temperature is less than about 275° F.

3. The process of Claim 1 wherein said aging temperature is between about 250°F and about 275°F.

4. The process of Claim 1 wherein said aging temperature is less than about 250° F.

5. The process of Claim 1 wherein said alloy consists essentially of:
Element Amount (wt%) Li 1.5 to 3.0 Mg 0 to 4.2 Cll 0 to 3.7 Zr 0.08 to 0.15 Mn 0 to 0.8 Fe 0.15 max Si 0.12 max Zn 0.10 max Ti 0.10 max Other trnce elements each 0.05 max total 0.25 max Al Balance.

6. The process of claim 5 wherein said alloy consists essentially of:
Element Amount (wt%) Li 1.8 to 2.7 Mg 0 to 3.2 Cu 0.5 to 3.0 Zr 0.08 to 0.15 Mn 0 to 0.6 Fe 0.10 max Si 0.10 max Zn 0.10 max Ti 0.10 max Other trace elements each 0.05 max total 0.25 max Al Balance.

7. The product produced by the process of Claim 1.