Extrusion of aluminum-lithium alloys

Disclosed is a method of making lithium-containing aluminum base alloy extrusion having at least a section thereof having a low aspect ratio or which is generally axisymmetrical, the extrusions having improved properties in sections thereof having the low aspect ratio or which are axisymmetrical. The method comprises providing a body of a lithium-containing aluminum alloy, pressing a portion of the body which is to form the axisymmetrical or low aspect ratio section through a tortuous path and extruding an axisymmetrical or a low aspect ratio extrusion section. The axisymmetrical or low aspect ratio section of the extrusion has a tensile strength of at least 60 ksi and an ultimate yield strength at least 4.5 ksi greater than the tensile yield strength.

This invention relates to a process for extruding aluminum alloys and in 
particular to a process for extruding aluminum alloys containing lithium 
to improve the properties of the extrusions so produced. 
BACKGROUND OF THE INVENTION 
It has been generally recognized that one of the most effective ways to 
reduce the weight of an aircraft is to reduce the density of aluminum 
alloys used in the aircraft construction. For purposes of reducing the 
alloy density, lithium additions have been made. However, the addition of 
lithium to aluminum alloys is not without problems. For example, extrusion 
of aluminum--lithium alloys using conventional flat faced extrusion dies 
to produce thick and/or low aspect ratio aluminum--lithium extrusions 
results in materials which sometimes display low elongations, poor 
fracture toughness and high anisotropy of mechanical properties. These 
characteristics have been associated with cracking problems in the 
extruded product during certain fabrication procedures, especially during 
machining operations which are performed on some extrusions. There is also 
concern regarding the load transfer capability of thick and/or low aspect 
ratio aluminum--lithium extrusions. This is because such extrusions have a 
small spread between ultimate tensile strength (UTS) and tensile yield 
strength (TYS). This problem is also sometimes present in portions of the 
extruded shape which are formed by generally axisymmetrical metal flow, 
even if the portion has a high aspect ratio. 
As used in the art, the term aspect ratio means the ratio of the width to 
thickness in cross section of an extrusion or a portion of an extrusion. A 
low aspect ratio is in the range of approximately 1-4:1. A 1:1 ratio means 
that the extrusion or the portion of the extrusion has approximately a 
round or square cross section. A ratio of 4:1 means that the width of the 
extrusion would be approximately 4 times the thickness of the extrusion in 
cross section. 
The influence of the extrusion process parameters on mechanical properties 
of different aluminum--lithium extrusions was investigated by Tempus et al 
as published in Aluminum Lithium, Vol. 4, (1987). The strength of the 
extrusion was shown to be influenced more by the extrusion aspect ratio, 
(width/thickness) which determines texture, than by extrusion temperature 
and extrusion ratio. With increasing extrusion aspect ratio, the strength 
declines considerably. Tempus et al revealed that a small difference 
between UTS and TYS in aluminum--lithium extrusions is associated with a 
fiber texture. If an extrusion does not undergo recrystallization during 
solution heat treatment, the region of the extrusion where the flow 
resembles axisymmetric deformation exhibits a fiber texture. In contrast, 
when deformation conditions are more like plane strain, the resulting 
extrusion or section of an extrusion contains a more equiaxed type 
texture. The later is typical of extrusions with a high aspect ratio which 
have an acceptable difference between UTS and TYS and which will machine 
well with little or no cracking. 
A paper entitled Texture and Properties of 2090, 8090 and 7050 Extruded 
Products, by D. K. Denzer, P. A. Hollingshead, J. Liu, K. P. Armanie and 
R. J. Rioja, which was presented at the Sixth Annual International 
Aluminum--Lithium Conference, Garmisch-Partenkirchen, Germany, Oct. 7-11, 
1991, contains a further discussion of the correlation between extrusion 
aspect ratio and tensile strength in aluminum--lithium extrusions. 
U.S. Pat. No. 5,151,136 discloses a method for producing low aspect ratio 
aluminum--lithium extrusions with improved properties in which the low 
aspect ratio section of the extrusion is reduced by at least 4:1 
(extrusion ratio) during the extrusion reduction. 
A method is desired for producing aluminum--lithium alloy extrusions having 
portions thereof with a low aspect ratio with improved elongation, 
improved fracture toughness and other mechanical properties. 
SUMMARY OF THE INVENTION 
This invention provides a method of extruding a lithium-containing aluminum 
alloy to form an extrusion having at least a portion thereof with a low 
aspect ratio and/or which is formed by axisymmetric deformation. In this 
method, the ingot is forced to flow through a tortuous path to work the 
alloy prior to extruding the alloy through a die aperture. One embodiment 
of this invention utilizes a spreader plate in front of the extrusion die 
to improve ductility and increase the spread between UTS and TYS in the 
extrusion that is produced. Improvements in ductility and spread between 
UTS and TYS in accordance with this invention facilitates the use of 
aluminum--lithium extrusions in many otherwise unacceptable applications. 
This invention results in overall improvement in mechanical behavior of 
aluminum--lithium alloys which are produced in accordance with the 
invention. 
An object of this invention is to provide a method for producing an 
improved lithium-containing aluminum base alloy extrusion. 
Another object of this invention is to provide lithium-containing aluminum 
base alloy extrusion having low aspect ratio sections thereof having 
improved properties. 
A further object of this invention is to provide a lithium-containing 
aluminum base alloy extrusion having low aspect ratio sections (4:1 or 
less) having TYS greater than 70 ksi and having a UTS of at least 4.0 ksi 
greater than the TYS. 
Another object of this invention is to provide a method for producing 
aluminum lithium extrusions having portions thereof which are generally 
axisymmetric and which have improved properties. 
These and other objects will become apparent from the specification, 
drawings and claims appended hereto.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
By low aspect ratio is meant a ratio in the range of 1-4 to 1. By high 
aspect ratio is meant a ratio greater than 4 (4 to 1). By aspect ratio is 
meant the ratio of width to thickness, as shown in FIG. 1, for example. In 
a simple extrusion, e.g., an extrusion having a rectangular, square or 
circular cross section, the aspect ratio is the ratio of the width to the 
thickness of the extrusion. For extrusions having square or circular cross 
sections, the aspect ratio is 1 (1 to 1). 
FIG. 1 shows an aircraft floor beam 10 extrusion having both a high aspect 
ratio section A and low aspect ratio sections B, C, D and E. Section A has 
a high aspect ratio and generally has satisfactory properties when formed 
by using conventional flat faced dies. Sections B, C, D and E have low 
aspect ratios and have typically exhibited "chunkiness" which means they 
have a low UTS to TYS spread, low ductility, poor fracture toughness, high 
anisotropy of mechanical properties, and a fibrous texture. The sections 
of the extrusions having low aspect ratios have inferior properties 
compared to the section having high aspect ratios. The low aspect ratio 
section may have: (1) very high longitudinal TYS, e.g., 90 ksi; (2) small 
difference between longitudinal UTS and TYS, e.g., 1.6 ksi or less; and 
(3) poor fracture toughness, e.g., less than 15 ksi .sqroot.in. Such 
properties can exist even when the low aspect ratio section has undergone 
considerable work, e.g., even after an extrusion ratio of 7:1. 
In accordance with this invention, the properties in the sections having 
low aspect ratios or which are formed by axisymmetric metal flow are 
improved by pressing the portion of the ingot to be formed into such 
section through a tortuous path before it is extruded through the die 
aperture. As used herein, a tortuous path that includes at least one bend 
or turn in the metal flow in addition to that required for the metal to 
flow through the extrusion die. In a preferred method of this invention, 
the tortuous path results from the use of a spreader die as shown in FIG. 
2. The ingot or billet 12 in container 14 is pressed or pushed by an 
extrusion stem (not shown) through an aperture 16 in a spreader die 18, 
and then through an aperture 20 in a flat faced die 22 to form extrusion 
24 having a cross-sectional profile as shown in FIG. 3. The extrusion 24 
has a section A having a low aspect ratio and a sections B, C, D and E 
having high aspect ratios (less than 4). The spreader die 18 has a lip 
portion 26 across the bottom of the aperture 16 in the die, as best been 
in FIG. 3. The lip 26 interferes with the flow of metal and forces the 
aluminum alloy in ingot 12 to change directions and flow in a tortuous 
path over the lip before the alloy is expressed through the die aperture 
20. Surprisingly, the use of the spreader plate permits the extrusion of 
aluminum lithium alloys that do not exhibit chunkiness in the low aspect 
ratio areas of the extruded product. It is believed that this movement of 
the alloy in a tortuous path results in working of the alloy to a greater 
extent than would normally be the case in forming low aspect ratio 
portions of an extrusion and that this increased working improves the 
properties of such low aspect portions of the extrusion. The tortuous path 
also results in increased localized strain rates of the metal. 
It is important to note that the lip 26 in the spreader die 18 extends only 
across the bottom of the die 18 that the spreader die results in tortuous 
flow path only for the metal that is extruded into the low aspect ratio 
portion of the extruded product. The flow path for the portion of the 
billet that becomes the high aspect ratio portion of the extruded product 
is not tortuous. Instead, the flow path for the high ratio portion of the 
billet is a more typical laminar flow path. If all portions of an extruded 
product have a low aspect ratio(s), as with a square or round extrusion, 
then in accordance with this invention substantially all of the metal in 
the billet should flow along a tortuous path. 
The lip 26 is not disposed across portions of the spreader die which forms 
the high aspect ratio portions of the extruded product so the metal that 
forms such high aspect ratio portions of the product do not flow along a 
tortuous path. This is because pressing the alloy that forms the high 
aspect ratio portions of the product along a tortuous path would require 
an unnecessary increase in the working and little or no improvement in 
properties for that portion of the product. In fact, some test data 
suggests that the properties may even be reduced if the metal that forms 
the high aspect ratio sections also flows along a tortuous path. Thus it 
is important to this invention that the tortuous flow path be applied only 
for that portion of the billet that forms the low aspect ratio portion of 
the extruded product. 
FIG. 4 shows the profiles of lips 30 and 32 on a spreader plate 34 for 
forming the floor beam 10 extrusion of FIG. 1 in apparatus similar to that 
shown in FIG. 2. The lips 30 and 32 on the spreader plate 34 force the 
aluminum lithium alloy that will form the low aspect portions of the 
extrude body to flow along a tortuous path before it is extruded through 
the die aperture 36. The profile of the exit aperture 38 in the spreader 
plate 34 is shaped like a dog bone with the lips 30 and 32 partially 
obstructing the flow of metal through the ends of the aperture. 
Aluminum--lithium alloys which may be formed into extrusions with this 
invention can contain 0.2 to 5.0 wt. % Li, 0 to 5.0 wt. % Mg, up to 6.5 
wt. % Cu, 0 to 1.0 wt. % Zr, 0 to 2.0 wt. % Mn, 0.05 to 12.0 wt. % Zn, up 
to 2 wt. % Ag, 0.5 wt. % max. Fe, 0.5 wt. % max. Si, the balance aluminum 
and incidental elements and impurities. The impurities are preferably 
limited to about 0.05 wt. % each, and the combination of impurities 
preferably should not exceed 0.35 wt. %. 
A preferred alloy in accordance with the present invention can contain 0.2 
to 5.0 wt. % Li, at least 0.5 wt. % Cu, 0 to 1.0 wt. % Ag, 0.05 to 5.0 wt. 
% Mg, 0 to 1.0 wt. % Mn, 0.05 to 0.16 wt. % Zr, 0.05 to 12.0 wt. % Zn, the 
balance aluminum and incidental elements and impurities as specified 
above. A typical alloy composition would contain 1.0 to 3.0 wt. % Li, 2.0 
to 2.9 wt. % Cu, 0.05 to 2.5 wt. % Mg, 0.05 to 11.0 wt. % Zn, 0 to 0.09 
wt. % Zr, 0 to 1.0 wt. % Mn and max. 0.1 wt. % of each of Fe and Si. In a 
preferred typical alloy, Zn may be in the range of 0.05 to 2.0 and Mg 0.05 
to 2.0 wt. %. 
In the present invention, lithium is important not only because it permits 
a significant decrease in density but also because it improves tensile and 
yield strengths markedly as well as improving elastic modulus. 
Additionally, the presence of lithium improves fatigue resistance. Most 
significantly though, the presence of lithium in combination with other 
controlled amounts of alloying elements permits aluminum alloy products 
which can be worked to provide unique combinations of strength and 
fracture toughness while maintaining meaningful reductions in density. 
Toughness or fracture toughness as used herein refers to the resistance of 
a body, e.g., extrusion, sheet or plate, to the unstable growth of cracks 
or other flaws. 
Other lithium-containing aluminum alloys which may be extruded to provide a 
product in accordance with the invention include Aluminum Association 
Alloy (AA) 2090, 2091, 2094, 2095, 2096, 2097, 8090, 8091, 8190, 2020, 
2195 and Russian alloys, 1420, 1421, 1430, 1440, 1441, 1450 and 1460. 
The aluminum--lithium alloy for use with this invention is prepared 
according to specific method steps in order to provide the most desirable 
characteristics of the extrusion. Thus, the alloy as described herein can 
be provided as an ingot or billet for fabrication into a suitable extruded 
product by casting techniques currently employed in the art for cast 
products. The ingot or billet may be preliminarily worked or shaped to 
provide suitable stock for subsequent working operations. Prior to the 
principal working operation, the alloy stock is preferably subjected to 
homogenization, and preferably at metal temperatures in the range of 800 
to 1050.degree. F. for a period of time of at least one hour to dissolve 
soluble elements such as Li and Cu, and to homogenize the internal 
structure of the metal. A preferred time period is about 20 hours or more 
in the homogenization temperature range. Normally, the heat-up and 
homogenizing treatment does not have to extend for more than 40 hours; 
however, longer times are not normally detrimental. A time of 20 to 40 
hours at the homogenization temperature has been found quite suitable. In 
addition to dissolving constituent phases to promote workability, this 
homogenization treatment is important in that it aids precipitation of Mn 
and/or Zr-bearing dispersoids which help to control final grain structure. 
After the homogenizing treatment, the ingot is usually scalped and then 
extruded to produce extrusions. 
When the ingot is comprised of the preferred alloy noted above, and Zn is 
maintained at less than 1 wt. %, typically 0-1 wt. % and Zr in the range 
of 0 to 0.12 wt. %, then preferably the ingot is heated in the temperature 
range of 500 to 1050.degree. F., typically 500 to 950.degree. F., and 
maintained in this range during the extruding process. 
After extruding the ingot to the desired shape, the extrusion is preferably 
subjected to a solution heat treatment to dissolve soluble elements. The 
solution heat treatment is preferably accomplished at a temperature in the 
range of 900 to 1050.degree. F. and preferably produces a recovered or 
recrystallized grain structure. 
Solution heat treatment can be performed in batches. Solution effects can 
occur fairly rapidly, for instance in as little as 30 to 60 seconds, once 
the metal has reached a solution temperature of about 900 to 1050.degree. 
F. However, heating the metal to that temperature can involve substantial 
amounts of time depending on the type of operation involved. In batch 
treating in a production plant, the extrusions are treated in a furnace 
load and an amount of time is required to bring the entire load to 
solution temperature, and accordingly, batch solution heat treating can 
consume one or more hours. 
To further provide the desired strengths necessary in the final product, 
the extrusions should be rapidly quenched to prevent or minimize 
uncontrolled precipitation. 
An extrusion produced by the present invention may be artificially aged to 
provide the combination of fracture toughness and strength which is so 
highly desired in extrusion members of this type. This can be accomplished 
by subjecting the extrusion product to a temperature in the range of 150 
to 400.degree. F. for a sufficient period of time to further increase the 
yield strength. Preferably, artificial aging is accomplished by subjecting 
the alloy product to a temperature in the range of 275 to 375.degree. F. 
for a period of at least 30 minutes. A suitable aging practice 
contemplates a treatment of about 8 to 24 hours at a temperature of about 
325.degree. F. Further, it will be noted that the alloy product in 
accordance with the present invention may be subjected to any of the 
typical underaging treatments, including natural aging. Also, while 
reference has been made herein to single aging steps, multiple aging 
steps, such as two or three aging steps, are contemplated and may be used. 
While the ingot may be extruded in a one-step extrusion, two or even 
multiple steps may be employed. Thus, in the first step, the ingot may be 
extruded to preliminarily work the ingot without extruding to shape. That 
is, a 16" diameter ingot may be first extruded to 9" diameter rod before 
extruding to a final shape. Or, the ingot may be preliminarily shaped by a 
shaping step such as forging or rolling and thereafter extruded to a final 
shape. Between the extruding steps, the preliminarily worked or shaped 
ingot may be subjected to a thermal treatment, prior to extruding to the 
final shape. The thermal treatment is designed to minimize undesirable 
crystallographic texture. The thermal treatment can be in the temperature 
range of 400 to 1020.degree. F., preferably 500 to 900.degree. F., for a 
time period in the range of 8 to 24 hours. Usually, time in the 
temperature range is not needed to exceed 20 hours. In the first or 
preliminary working or extruding step, the amount of work should be at 
least 30% and preferably at least 40%. 
Following these steps can result in an extrusion with section thereof 
having low aspect ratios, yet exhibiting improved properties. That is, 
differences of at least 4.5 ksi can be achieved between TYS and UTS in 
both the low aspect ratio portions of the extrusion as well as the high 
aspect ratio portions of the extrusion. 
The following example is further illustrative of the invention: 
EXAMPLE 
Ingots 15".times.60".times.100" long were cast having the composition, in 
wt. %, 2.07 Li, 2.79 Cu, 0.10 Zr, and remainder Al (known as Alloy 2090). 
The ingots were homogenized for 8 hours at 950.degree. F. and 24 hours at 
1000.degree. F. and then machined to an extrusion billet 9" in diameter. 
For extruding, the billets were heated to about 850.degree. F., and the 
extrusion cylinder was maintained at about the same temperature during 
extrusion. The billets were extruded to the shape shown in FIG. 1 at 2 
inches per minute ram speed or product speed with an extrusion ratio of 
approximately 7 to 1. The billets were extruded using three different die 
arrangements. The first arrangement was a conventional flat die with no 
feeder or spreader die. The second was a conventional feeder die which 
provided a restriction for the billet to flow through before extrusion 
through the die aperture. The third included a spreader plate for 
extrusion in accordance with this invention. The extrusions were solution 
heat treated for about 0.5 to 1 hours at about 1000.degree. F., then cold 
water quenched and stretched about 6% of their original length. 
Thereafter, the extrusions were aged at 300.degree. F. for 30 hours. Table 
I below gives the results for floor beam extrusions (FIGS. 1 and 4), and 
Table II gives the results for hinge extrusions (FIGS. 2 and 3). The 
letters R and F in table stand for "rear" and "front", meaning that the 
test results are for two sections of each extrusion with one (R) being 
approximately 21/2 ft. from the "rear" end of the extrusion and the other 
(F) being approximately 21/2 ft. from the "front" end of the extrusion. 
From the Tables, it is seen that extrusions produced in accordance with 
this invention have improved properties, as shown by the difference 
between UTS minus TYS and the elongation percents compared against an 
extrusion made by prior art practices (flat and feeder dies). 
TABLE I 
__________________________________________________________________________ 
LOW ASPECT RATIO HIGH ASPECT RATIO 
REGION B REGIONS A 
(FIG. 1) (FIG. 1) 
UTS TYS UTS - TYS 
% UTS TYS UTS - TYS 
% 
DIE (ksi) 
(ksi) 
(ksi) el (ksi) 
(ksi) 
(ksi) el 
__________________________________________________________________________ 
FLAT 
R 90.5 
88.3 
2.2 8.0 82.1 
78.3 
3.8 7.0 
F 89.8 
88.4 
1.4 10.0 
82.9 
78.0 
4.9 9.0 
FEEDER 
R 92.8 
90.0 
2.8 8.0 86.0 
81.7 
4.3 8.0 
F 92.7 
91.2 
1.5 9.0 84.0 
80.9 
3.1 7.0 
SPREADER 
R 77.9 
71.3 
6.6 16.0 
80.0 
74.8 
5.2 9.0 
F 78.0 
72.5 
5.5 8.0 83.6 
79.0 
4.6 9.0 
__________________________________________________________________________ 
TABLE II 
______________________________________ 
Test Location in Center of Section A (FIG. 2) 
TYS (ksi) UTS (ksi) % elongation 
UTS-TYS 
______________________________________ 
Flat - F 
97.6 97.9 2.0 0.3 
R 98.8 99.2 2.0 .04 
Spreader - F 
77.6 87.4 10.0 9.8 
R 82.7 87.3 10.0 4.6 
______________________________________ 
While the invention has been described in terms of direct extrusion in 
which the metal is forced by a ram through a stationary die, it is not so 
limited. Those skilled in the art will recognize that the invention can 
also be practiced in an indirect extrusion process. 
It is also contemplated the required tortuous flow path for the metal 
extruded in accordance with this invention can be provided by means other 
than a spreader die. Such means might be in the form of bridges or other 
obstruction positioned in the extrusion chamber which force the billet to 
follow a tortuous path or change of direction before being extruded 
through the die aperture.