Lithium bearing alloys free of Luder lines

Aluminum-lithium alloy sheets are stretched under predetermined temperature and stretch rate conditions to provide contoured metal sheets. The temperature and stretch rate conditions provide a stretched sheet which is substantially free of Luder lines that are conventionally associated with stretch-formed aluminum-lithium alloy sheets.

BACKGROUND OF THE INVENTION 
The invention relates to aluminum alloys containing lithium as an alloying 
element, and particularly to a process for stretching the aluminum-lithium 
alloys without producing strain-induced imperfections known as Luder 
lines. 
It has been estimated that some 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 for every pound of weight saved. At current fuel costs, a significant 
investment to reduce the structural weight of the aircraft can be made to 
improve overall economic efficiency of the aircraft. 
The need for improved performance in aircraft of various types can be 
satisfied by the use of improved engines, improved airframe design, or by 
the use of new or improved structural materials. Improvements in engines 
and aircraft design have been vigorously pursued, but only recently has 
the development of new and improved structural materials received 
commensurate attention, and their implementation in new aircraft designs 
is expected to yield significant gains in performance. 
Materials have always played an important role in dictating aircraft 
structural concepts. Since the early 1930's, structural materials for 
large aircraft have remained remarkably consistent, with aluminum being 
the primary material of construction in the wing, body and empennage, and 
with steel being utilized for landing gears and certain other specialty 
applications requiring very high strength. Over the past several years, 
however, several important new material concepts have been under 
development for incorporation into aircraft structures. These include new 
metallic materials, metal matrix composites and resin matrix composites. 
It is believed by many that improved aluminum alloys and carbon fiber 
resin matrices 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 materials of this type. 
Heretofore, aluminum-lithium alloy products of the types described 
hereinafter have not been used in aircraft structure. Aircraft 
applications for alloys of the type have heretofore been restricted to 
uses wherein the mill product has been adapted by machining or otherwise 
contouring the product form without the need for stretching. The 
state-of-the-art in producing suitably strong, yet damage-tolerant 
aluminum lithium alloy sheets, has progressed to a point that its inherent 
properties are attractive for air transport body skins. Body-skin 
applications, however, have been restricted because of the alloys' 
propensity to form Luder-lines at low relative amounts of contour 
stretching. These Luder lines are aesthetically objectionable, and may 
compromise engineering properties. 
It is generally understood that Luder line phenomena are associated with 
non-homogeneous deformation of the metal alloy. Although other 
aluminum-based alloy materials exist that only occasionally suffer from 
the formation of Luder lines, lithium additions to aluminum provide a 
substantial density reduction which has been determined to be very 
important in decreasing the overall structural weight of the aircraft. 
While substantial strides have been made in improving the aluminum-lithium 
processing technology, a major challenge remains to obtain a 
stretch-formed sheet of these aluminum-lithium alloys whose surfaces are 
substantially free of Luder lines. 
SUMMARY OF THE INVENTION 
The present invention provides sheets of aluminum-lithium alloys which are 
substantially free of Luder lines, that also have suitably high tensile 
strengths yet retain high damage tolerance. The sheets of aluminum-lithium 
alloy are formed by stretching the sheets under specific combinations of 
temperature and stretch rate conditions that prevent the formation of 
Luder lines. Generally, the sheets can be stretched at least 3% of their 
original dimensions without forming Luder lines by choosing a temperature 
ranging from about -50.degree. to about 350.degree. F. and a stretch rate 
ranging from about 0.1%/minute to about 50%/minute. 
The stretching process provides sheets of aluminum-lithium alloy which are 
substantially free of Luder lines, a condition that is not achieved when 
aluminum-lithium alloy sheets are stretched by conventional means. These 
sheets will have engineering properties, including tensile strength and 
damage tolerance, that will allow them to be used as contoured body skin 
structures for aircraft. Success of the process depends on controlling the 
stretching parameters (i.e., temperature and stretch-rate) both of which 
can be simply and accurately monitored, thus resulting in a Luder 
line-free product with consistent properties.

DETAILED DESCRIPTION OF THE INVENTION 
An aluminum-lithium alloy formulated in accordance with the present 
invention can contain from about 1.7 to about 2.3 percent lithium. The 
current data indicates that the benefits of the stretching process in 
accordance herewith are most apparent at lithium levels of between 1.7 to 
about 2.3 percent, however other alloys containing more or less lithium 
may benefit equally as much from the present invention. All percentages 
herein are by weight percent (wt %) based on the total weight of the alloy 
unless otherwise indicated. Additional alloying agents such as magnesium 
and copper can also be included in the alloy. Alloying additions function 
to improve the general engineering properties but also affect density 
somewhat. Zirconium is also present in these alloys for grain size control 
at levels between 0.04 to 0.16 percent. Zirconium is essential to the 
development of the desired combination of engineering properties in 
aluminum-lithium alloys, including those subjected to our stretching 
process. 
The impurity elements iron and silicon can be present in amounts up to 0.30 
and 0.20 percent, respectively. It is preferred, however, that these 
elements be present only in trace amounts of less than 0.12 and 0.10 
percent, respectively. Certain trace elements such as zinc and titanium 
may be present in amounts up to but not to exceed 0.25 percent and 0.10 
percent, respectively. Certain other trace elements such as manganese and 
chromium must each be held to levels of 0.10 percent or less. If these 
maximums are exceeded, the desired properties of the aluminum-lithium 
alloy will tend to deteriorate. The trace elements potassium and sodium 
are also thought to be harmful to the properties of aluminum-lithium 
alloys and should be held to the lowest levels practically attainable, for 
example, on the order of 0.003 percent maximum for potassium and 0.0015 
percent maximum for sodium. The balance of the alloy, of course, comprises 
aluminum. 
The following table represents the preferred proportions in which the 
alloying and trace elements may be present to provide the best set of 
overall properties for use in aircraft structures. The broadest ranges are 
acceptable under some circumstances. The present invention will be equally 
applicable to other aluminum-lithium alloys that suffer from the formation 
of Luder lines, though not within the preferred ranges disclosed below. 
TABLE 
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Amount (wt %) 
Element Acceptable 
Preferred 
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Li 1.7-2.8 1.7 to 2.3 
Mg 2.0 max 1.1 to 1.9 
Cu 1.0-3.0 1.8 to 2.5 
Zr 0.04-0.16 0.06 to 0.16 
Mn 0.10 max 0.10 max 
Fe 0.30 max 0.12 max 
Si 0.20 max 0.10 max 
Zn 0.25 max 0.25 max 
Ti 0.15 max 0.10 max 
Cr 0.10 max 0.10 max 
K 0.05 max 0.0030 max 
Na 0.05 max 0.0015 max 
Other trace 
elements 
each 0.05 max 0.05 max 
total 0.15 max 0.015 max 
Al Balance Balance 
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An aluminum-lithium alloy formulated in the proportions set forth in the 
foregoing paragraphs and table 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 
980.degree. F. to approximately 1010.degree. F. Thereafter, the alloy is 
converted into a usable article by conventional mechanical 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 980.degree. to 1010.degree. F., followed by 
quenching into a medium such as water that is maintained at a temperature 
on the order of 40.degree. F. to 90.degree. F. Alloys of this type are 
commercially available from Pechiney Aluminum or the Aluminum Company of 
America (Alcoa) under the designation 2091. Each alloy is produced in 
various tempers by varying the particular conditions such as solution 
treatment, quench, stretch and aging under which the alloy is produced. 
Examples of suitable tempers include T4, T6, and T8 that are in 
accordance with the guidelines and definitions of ANSI H35.1 as published 
by the Aluminum Association. 
Thereafter, in accordance with the present invention, a sheet of the 
aluminum-lithium alloy is stretched at least about 3% up to about 9% of 
its original dimensions to contour it into various shapes, such as 
aircraft structures, without the formation of Luder lines. The percent of 
the original dimensions that the sheets are stretched is measured in the 
direction of the applied stretching force. In order to provide these 
stretched sheets in a condition substantially free of Luder lines, the 
sheet is stretched under a combination of temperature and stretch rate 
conditions that range from about -50.degree. F. to about 350.degree. F. 
and 0.1%/minute to about 50%/minute, respectively, depending on the total 
amount of stretch desired. In the aircraft industry, where 6 to 7 percent 
stretching of the aluminum-lithium alloy sheet is often desired, the 
options for stretching at low temperatures (-30.degree. F. to +40.degree. 
F.) and high strain rates (1% per min. to 10% per min.), or, at higher 
tempertures (140.degree. F. o 200.degree. F.) and low strain rates (0.1% 
per min. to 5% per min.) need to be balanced economically based on 
available facilities and the production rates required. For example, when 
body-skin contouring requires 6% longitudinal stretch in the T6 temper 
without Luder line formation, the sheet may be stretched at about 
30.degree. F. using a strain rate of about 10% per minute. Alternatively, 
the same degree of longitudinal stretch could be accomplished by forming 
at about 180.degree. F. using a strain rate of about 1% per minute. Other 
stretch conditions will provide substantially the same result but will not 
be as economical. 
When the stretching of the aluminum-lithium alloy sheet in the T6 temper is 
conducted in accordance with the parameters set forth above as represented 
graphically in FIG. 1, the process will result in a stretched 
aluminum-lithium alloy sheet which is substantially free of Luder lines. 
Similar graphs can be constructed for the T4 and T8 tempers of the 
aluminum-lithium alloy. Analogous "safe" zones exist for the F, O, W, T3, 
or T7 tempers, but require secondary heat treatment and/or greater 
extremes in temperature and strain rate during forming. 
The following Example is presented to illustrate the Luder line free sheet 
achieved by the stretching process of an aluminum-lithium alloy in 
accordance with the present invention and to assist one of ordinary skill 
in making and using 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. 
EXAMPLE 
An aluminum alloy containing 2.0 percent lithium, 1.5 percent magnesium, 
2.2 percent copper, 0.12 percent zirconium with the balance being aluminum 
is formulated. The trace elements present in the formulation constituted 
less than 0.15 percent of the total. The alloy is cast and homogenized at 
1000.degree. F. Thereafter, the alloy is hot rolled to a thickness of 
0.063 inches. The resulting sheet is then solution treated at 990.degree. 
F. for about 0.5 hour. The sheet is then quenched in water and maintained 
at about 75.degree. F. and aged at 275.degree. F. for 12 hours. A similar 
aluminum-lithium alloy is commercially available from Pechiney Aluminum or 
Alcoa under the designation 2091 with a T6 temper. 
The specimens having original dimension of 3 inches by 10 inches are then 
stretched with a tensile machine under a plurality of combined temperature 
(.degree.F) and stretch rate (%/minute) conditions, ranging from 
350.degree. F. to -50.degree. F. and 0.1%/minute to 50%/minute. The 
percent stretch (i.e., % increase in the original dimension of the sheet 
in the direction of the stretch) attained for a given temperature and 
stretch rate at the onset of Luder lines appearing in the sheet anyway 
between the grips of the tensile machine is determined using visual 
observation of the specimen surface and load-deflection recordings. The 
summary of the percent stretch attained is graphically illustrated in FIG. 
1 as a function of the temperature and the stretch rate. This example 
illustrates that Luder-free stretching is not possible with conventional 
methods at room temperature. The T6 temper is the least prone toward Luder 
formation using conventional stretch-processing, and lends itself to the 
least amount of stretch rate and temperature control process modification. 
As illustrated in FIG. 1 by the "LUDER-FREE ZONE" regions, the specimens 
stretched at least 3%, as represented by lines 3, under a combination of 
temperature and stretch rate conditions that fall outside the polygon 
ABCDA do not exhibit Luder line formation. Likewise, those specimens 
stretched at least 6%, as represented by lines 6, under a combination of 
temperature and stretch rate conditions that fall outside the polygon 
EFGHE do not exhibit Luder lines. Finally, specimens that are stretched at 
least 9%, as represented by lines 9, under temperature and stretch rate 
conditions outside the polygon IJKLI do not exhibit Luder line formation. 
Similar polygons are defined for other percent stretch values and are 
summarized in Table I below. 
TABLE I 
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Percent Stretch at 
Onset of Luder Lines Polygon 
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4 MNOPM 
5 QRSTQ 
7 UVWXU 
8 YZA'B'Y 
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Some of the stretched (6%) specimens which were free of Luder lines are 
then tested for total yield strength, ultimate tensile strength, % 
elongation and Young's Modulus, by known methods. The recorded values are 
summarized in Table II. 
TABLE II 
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Total yield strength (psi) 
58,000-63,000 
Ultimate tensile strength (psi) 
65,000-75,000 
% Elongation (%) .gtoreq.13 
Young's Modulus (10.sup.6 psi) 
11-11.4 
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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 able to effect various changes, substitutions of 
equivalents and other alterations without departing from the broad 
concepts disclosed herein. For example, it is contemplated that the 
subject stretching process treatment may be applicable to other alloying 
combinations not now under development, and specifically to 
aluminum-lithium alloys with substantial amounts of zinc, silicon, iron, 
nickel, beryllium, bismuth, germanium, and/or zirconium. It is therefore 
intended that the scope of Letters Patent granted hereon will be limited 
only by the definition contained in the appended claims and equivalents 
thereof.