EP0981653A1

EP0981653A1 - Method of improving fracture toughness in aluminum-lithium alloys

Info

Publication number: EP0981653A1
Application number: EP98903777A
Authority: EP
Inventors: Alex Cho; Michael H. Skillingberg; Richard E. Greene; Michael M. Niedzinski
Original assignee: Reynolds Metals Co
Current assignee: McCook Metals LLC
Priority date: 1997-01-31
Filing date: 1998-01-30
Publication date: 2000-03-01
Anticipated expiration: 2018-01-30
Also published as: EP1359232B2; DE69836569T3; EP1359232B1; DE69836569T2; EP0981653B1; ES2278093T5; ATE250675T1; DE69818448D1; ATE346963T1; ES2278093T3; EP0981653A4; DE69836569D1; EP1359232B9; DE69818448T2; EP1359232A2; WO1998033947A1; EP1359232A3

Abstract

An aluminum-lithium alloy is processed with controlled amounts of copper, lithium, manganese and zirconium to produce a product having improved fracture toughness in the short longitudinal (S-L) direction and acceptable strength in the short transverse (ST) direction.

Description

METHOD OF IMPROVING FRACTURE TOUGHNESS IN ALUMINUM-LITHIUM ALLOYS

This application claims the benefit of U.S. Provisional Application No. 60/036,329, filed January 31, 1997.

Field of the Invention Tne present invention is directed to a method of improving tne fracture tougnness in the short longitudinal direction m aluminum-lithium alloys and a product therefrom and, in particular, to a method which controls the levels of copper, manganese, lithium, and zirconium in the alloys to obtain the improved fracture toughness.

Background Art It is well known that adding lithium as an alloying element to aluminum alloys results in beneficial mechanical properties . Aluminum-lithium alloys exhibit improvements in stiffness and strength while reducing density. Consequently, these types of alloys have utility as structural materials in airplane and aerospace applications. Examples of known alummum-lithium alloys include AA2097 and AA2197. The chemical compositions of these alloys are shown in Table 1 below. Problems exist with alummum-lithium alloys, particularly in thick plate of about 3 inches (76.2 mm) or greater, m terms of fracture toughness in the short longitudinal (S-L) direction. Tougnness values in this direction tend to be significantly lower than toughness values in other directions such as the longitudinal (L-T) direction or the long transverse (T-L) direction . 1 In view of the drawbacks in alummum-lithium alloys with

2 respect to fracture toughness, a need has developed to provide a

3 methoα of improving the short longitudinal (S-L) direction

4 fracture toughness for these types of alloys. In response to this

5 need, the present invention provides both a method and a product

6 therefrom which significantly increases the fracture toughness of

7 alummum-lithium alloys in the short longitudinal (S-L)

8 direction, thereby improving their suitability for more

9 commercial applications.

I 0 Summary of the Invention

I I A first ob ect of the invention is to improve the fracture

12 toughness in the short longitudinal (S-L) direction of alummum-

13 lithium alloys.

14 Another object of the invention is to provide a method of

15 making an alummum-lithium alloy having improved short

1 6 longitudinal direction fracture toughness.

1 1 A still further object of the present invention is to

1 8 utilize an alummum-lithium alloy having controlled amounts of

19 copper, lithium, manganese, zinc and zirconium to acnieve

20 fracture toughness improvements.

21 Yet another object of the present invention is to provide an

22 alummum-lithium alloy product having both improved fracture

23 toughness the short longitudinal (S-L) direction and

24 acceptable strength in the short transverse direction.

25 Other objects and aαvantages of the present invention will

26 become apparent as a description thereof proceeds.

21 In satisfaction of the foregoing objects and advantages, the

28 present invention provides a method for improving the fracture

29 toughness in the short longitudinal (S-L) direction an

30 alummum-lithium alloy article comprising the steps of providing

31 an aluminum alloy consisting essentially of, in weight percent

32 all subsequent alloying levels are weight percent unless otherwise indicated): 2.5 to 4.0% copper, 0.8% to less than 1.3% lithium, 0.05 to 0.8% manganese, 0.04 to 0.18% zirconium, with the oalance aluminum and inevitable impurities . The aluminum alloy can also include gram refining elements such as at least one of boron, titanium, vanadium, manganese, hafnium, scandium and chromium. Preferably, the aluminum alloy has only impurity levels of zinc so that it is essentially zinc-free, e.g., less than 0.05 weight percent zinc, more preferably less than or equal to 0.02%. Preferably, the copper content is controlled between 2.7 and 3.0 weight percent. The lithium content is preferably controlled between about 1.2 to 1.28 weight percent to provide a low density product with good fracture toughness in the short longitudinal direction. Manganese is preferably between 0.30 and 0.32 weight percent, with zirconium being about 0.10 weight percent. It should be appreciated that the amounts of alloying elements, other than the amounts of lithium and copper, can be within the ranges described m the preceding paragraph. In a modification of the composition described in the preceding paragraph, the lithium content ranges from about 0.8% to less than 1.2% and the copper content is between about 2.8 and 4%. This composition should provide even higher combined properties of fracture toughness and strength, with slightly higher density. In this composition range, additional theta' precipitate particles (Al₂Cu) would precipitate in addition to Tj. precipitate particles (Al₂CuLι) at the gra boundaries. This would increase the combined properties of strength and fracture toughness in the short longitudinal direction. Magnesium can be added if desired, m an amount up to 0.35% weight percent, preferably, up to 0.25 weight percent. Small amounts of magnesium may be beneficial in terms of strength and lowering of density. However, excessive amounts may create susceptibility to stress corrosion cracking and do not provide furtner oenefits terms of strength and density reduction. The aluminum alloy is cast into an ingot and homogenized for a select period of time. The homogenized ingot is then hot worked into a shape such as a plate and solution heat treated for a select period of time. The solution heat treated shape is then quencneα, preferably in water, cold worked, preferably by stretching, and aged for a select period of time. With this processing, the cold worked (stretched) and aged shape exhibits equivalent strengths but higher fracture toughness in the short longitudinal (S-L) direction than similar aluminum allovs having lithium contents greater than 1.3%. The homogenization and solution heat treating temperatures can range between about 900° and 1030°F (482 to 554°C), preferably between about 930° F (499°C) and 1000°F (538°C) . More preferably, homogenization temperatures will range between about 940°F (505°C) to 975°F (524°C), and solution heat treating temperatures will range between about 975°F (524°C) to 1000°F (538°C) . The preferred temperature often depends on the particular alloy comoos^tion as will be understood by one skilled in the art. homogenization times can be about 8 to 48 hours, preferably aoout 24 to aoout 36 hours. Solution heat treating times can range from about 1 to 10 hours, preferably about 1 hour to 6 hours, more preferably about 2 hours, once the metal reaches a desired temperature. The plate may be artificially aged without any cold work. However, it is preferred to provide between about 4% and 8% cold work, preferably by stretching. The plate is preferably artificially aged between about 300 and 350°F (149 to 177°C) for between about 4 and about 48 hours, preferably between about 12 and about 36 hours, with the aging time being a function of the aging temperature Using the inventive processing, an alummum-lithium alloy article is made having vastly improved fracture toughness in the short longitudinal (S-L) direction. The fracture toughness value tne short longitudinal (S-L) direction is at least about 68% of the fracture toughness m the long transverse (T-L) direction. While exhibiting improved fracture toughness in the short longitudinal (S-L) direction, the inventive alummum-lithium alloy articles have tensile yield strengths exceeding about 54 KSI.

Brief Description of the Drawings Reference is now made to the drawings of tne invention wnerem: Figure 1 compares the invention to the prior art m terms of tensile yield strength in the short transverse direction and fracture toughness in the short longitudinal (S-L) direction; Figure 2 compares prior art alloy products and the invention alloy products with respect to lithium content and fracture tougnness in the short longitudinal (S-L) direction; and Figure 3 compares the prior art alloy products and tne invention alloy products with respect to copper content and fracture toughness in the short longitudinal (S-L) direction.

Description of the Preferred Embodiment The present invention solves a significant problem in the field of alummum-lithium materials for structural applications such as those found in the aerospace and airplane industry. That is, by controlling the compositional amounts of copper, lithium, manganese and zirconium in these types of alloys, acceptable fracture toughness in the short longitudinal (S-L) direction with acceptable strength the short transverse (ST) direction is obtained. This unexpected improvement m fracture toughness in the S-L direction permits the use of these types of alloys in a wide variety of structural applications requiring low weight, high strength and stiffness, and good fracture tougnness. Accordmg to the present invention, the alloy elements of copper, lithium, manganese and zirconium are controlled in the following ranges to achieve the improvements m fracture toughness: about 2.5 to 4.0 weight percent copper, about 0.8 to less than about 1.2 or 1.3 weight percent lithium, about 0.05 to 0.8 weight percent manganese, about 0.04 to 0.16 weight percent zirconium, with the balance aluminum and inevitable impurities. One or more gram refining elements can also be added to the alummum-lithium composition described above. The gram refining elements can be selected from the group consisting of titanium in an amount up to 0.2 weight percent, boron in an amount of up to 0.2 weight percent, vanadium in an amount of up to 0.2 weight percent, manganese an amount of up to 0.8 weight percent, hafnium in an amount up to 0.2 weight percent, scandium in an amount up to 0.5 weight percent, and chromium m an amount up to 0.3 weight percent. Preferably the aluminum is free of zinc. In other words, zinc is present only as an impurity and at levels less than 0.05 weight percent. It is believed that zinc in levels greater than such impurity level adversely affects tne mechanical properties of these types of alummum-lithium alloys. The copper content should be kept higher than 2.5 weight percent to achieve high strength but less than 4.0 weight percent to avoid undissolved particles during solution heat treatment. Higher levels of copper are preferred due to the lower levels of lithium in the alloy. The lithium content should be kept higher than 0.8 weight percent to achieve good strength and low density but less than 1.3 weight percent to avoid a loss of fracture toughness in the short longitudinal (S-L) direction. The manganese content should be kept below 0.8 weight percent to avoid large non-dissolvable particles which would be detrimental to fracture toughness. The manganese should be higher than 0.05 weight percent to control gram size and homogenous slip behavior during plastic deformation processing. More preferably, the lithium content should be controlled between 1.2 and to less than 1.3 weight percent. Still more preferably, in one embodiment, the lithium content is controlled to less than 1.2 weight percent. The manganese is more preferably between 0.3 and 0.32 weight percent with the copper level ranging between about 2.7 and 3.0 weight percent. Magnesium can be added if desired, in an amount preferably up to 0.35% weight percent, preferably, up to 0.25 weight percent. Small amount of magnesium may be beneficial m terms of strength and lowering of density. However, excessive amounts may create susceptibility to stress corrosion cracking and do not provide further benefits in terms of strength and density reduction. In conjunction with specifying the alloy composition in the alummum-lithium alloy composition above, the alloy is processed by the steps of casting, homogenizing, hot working (for instance, by rolling, forging, extruding and combinations thereof), solution heat treating, quenching, cold working (for instance by stretching) and agmg to form an alummum-lithium article having the improvements in fracture toughness in the S-L direction. As part of this processing, the alummum-lithium alloy described above is cast into an mgot, billet or other shape to provide suitable stock for the subsequent processing operations. Once the shape is cast, it can be stress-relieved as is known in the art prior to homogenization. The cast shape is then homogenized at temperatures m the range of 930°F to 1,030°F, 499°C to 5544°C, for a sufficient period of time to dissolve the soluble elements and homogenize the internal structure of the metal. A preferred homogenization residence time is in the range of 1 to 36 hours, while longer times do not normally adversely affect the article. The homogenization can be conducted at one temperature or m multiple steps utilizing several temperatures. After homogenization, the cast shape is then hot worked to produce stock such as sheet, plate, extrusions, or other stock material depending on the desired end use of the alummum-lithium alloy article. For example, an mgot having a rectangularly shaped cross section could be hot worked into a plate form. Since this hot working step is conventional in the art, a further description thereof is not deemed necessary for understanding of the invention. Following the hot working step, the hot worked shape is then solution heat treated and quenched. Preferably, the hot worked shape is solution heat treated between 930° to 1030°F (499° to 554°C) at a time from less than an hour to up to several hours. This solution heat treated shape is preferably rapidly quenched, e.g. quenched in ambient temperature water, to prevent or minimize uncontrolled precipitation of strengthening phases m the alloy. The rapid quenching can also include a subsequent air cooling step, if desired. The quenched shape is then preferably stretched up to 8% and artificially aged m the temperature range of 150° to 400°F (66° to 204°C) for sufficient time to further increase the yield strength, e.g., up to 100 hours, depending on the temperature, for instance, 24 hours at 300°F (149°C) . The stretched and aged shape is then ready for use in any application, particularly an aerospace or airplane application. Alternatively, prior to ag g, the shape may be formed into an article and then aged. In order to demonstrate the unexpected improvements associated with the present invention, a comparison was made between properties of articles made from alummum-lithium alloys of tne prior art and articles made from alummum-lithium alloys accordmg to the invention. In this comparison, four prior art cnemistries were selected along with four chemistries accordmg to the invention. An aluminum alloy melt was made from each of the eight chemistries and processed by casting, homogenizing, solution heat treating, quenching, stretching and agmg to produce an alummum-lithium alloy article or product. The alummum-lithium alloy articles were then subjected to tensile and fracture toughness testing to compare the mechanical properties of the prior art chemistries to those corresponding to the instant invention. The following details tne processing used and test methods to compare the mechanical properties of the prior art and inventive alummum-lithium alloy articles. In the comparison, the prior art articles are designated as Examples 1-4, and the articles of the invention are designated as Examples 5-8. It should be understood that the processing variables and chemistries disclosed in Examples 5-8 are more preferred embodiments of the invention.

Table 1

Table 1 Notes 1. Chemical compositions are expressed m a weight percent maximum unless shown as a range. 2. In addition to the listed elements, each alloy may contain other elements, with the maximum amount of each other element not exceeding 0.05 wt. % and the total of other elements not exceeding 0.15 wt. %. Exampl e 1 An aluminum alloy consisting of, in weight percent, 2.84 Cu- 1.36 Lι-,32 Mn . - .1 Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The ingot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial ag g, the stretched samples were aged m an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane in accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests in the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and T-L directions. The results of these tests are listed in Table 2.

TABLE 2 Chemistry (weight %) Cu L Mn Zr 2.84 1.36 .32 .1

1 KSI = 6.894757 MPa KSI Vin = 1.0988434 MPa vm

Example 2 An aluminum alloy consisting of, weight percent, 2.71 Cu- 1.37 Lι-.32 Mn . - .1 Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched in water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane m accordance with ASTM B-557. Tensile tests in the longitudinal direction and long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests m the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (S-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and T-L directions. The results of these tests are listed in Table 3.

TABLE 3 Chemistry (weight % ) Cu Li Mn Zr 2.71 1.37 .32 .1

1 KSI = 6.894757 MPa

KSI Vin = 1.0988434 MPa im

Example 3 An aluminum alloy consisting of, m weight percent, 2.77 Cu- 1.33 Lι-.32 Mn.-.ll Zr, the balance aluminum and impurities, was cast into an got with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched in water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane m accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and T-L directions. The results of these tests are listed in Table 4.

TABLE 4 Chemistry (weight %) Cu Li Mn Zr 2.77 1.33 .32 .11

* KSI = 6.894757 MPa ** KSI Vin = 1.0988434 MPa Vm

Example 4 An aluminum alloy consisting of, in weight percent, 2.89 Cu- 1.36 Lι-,32 Mn.-O.l Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched n water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane in accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests in the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture tougnness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (S-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and T-L directions. The results of these tests are listed in Table 5.

TABLE 5 Chemistry (weight % ) Cu Li Mn Zr 2.89 1.36 .32 .10

KSI = 6.894757 MPa ** KSI Vin = 1.0988434 MPa Vm

Example 5 An aluminum alloy consisting of, m weight percent, 2.78 Cu- 1.21 Lι-.31 Mn.-O.l Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched in water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane n accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests in the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12. ~ mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (S-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and T-L directions. The results of these tests are listed m Table 6.

TABLE 6 Chemistry (weight %) Cu Li Mn Zr 2.78 1.21 .31 .1

KSI = 6.894757 MPa

KSI Vin = 1.0988434 MPa Vm

example 6 An aluminum alloy consisting of, in weight percent, 2.86 Cu- 1.28 Lι-.3 Mn.-O.l Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated m a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched in water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at tne T/4 plane in accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests m the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at THE T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (S-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and the T-L directions. The results of these tests are listed in Table 7.

TABLE 7

Chemistry (weight % )

Cu Li Mn Zr

2.86 1.28 .30 .10

KSI = 6.894757 MPa

KSI Vin = 1.0988434 MPa Vm

Example 7 An aluminum alloy consisting of, in weight percent, 2.73 Cu- 1.28 Lι-.3 Mn.-O.l Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The mgot was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quenched m water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane in accordance with ASTM B-557. Tensile tests in the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests m the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using W=1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (s-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and the T-L directions. The results of tnese tests are listed in Table 8.

TABLE 8 Chemistry (weight %) Cu Li Mn Zr 2.73 1.28 .30 .1

KSI = 6.894757 MPa KSI Vin = 1.0988434 MPa Vim

Example

An aluminum alloy consisting of, in weight percent, 2.83 Cu- 1.26 Lι-.32 Mn.-O.ll Zr, the balance aluminum and impurities, was cast into an mgot with a cross section of 16" (406.4 mm) and 45" (1143 mm) wide. The got was homogenized at 950°F (510°C) for 36 hours, then hot worked to 4" (101.6 mm) thick plate. The plate was then solution heat treated in a heat treating furnace at a temperature of 990°F (532°C) for 2 hours and then quencned m water. The plate was then stretched by 6% in the longitudinal direction at room temperature. For artificial agmg, the stretched samples were aged in an oven at 320°F (160°C)for 24 hours. Tensile properties were determined at the T/4 plane in accordance with ASTM B-557. Tensile tests n the longitudinal direction and the long transverse direction used round tensile specimens with .5" (12.7 mm) diameter and 1" (25.4 mm) gauge length. Tensile tests m the short transverse direction were conducted with round tensile specimens with .160" (4.1 mm) diameter and .5" (12.7 mm) gauge length. Fracture toughness was determined at the T/4 plane by ASTM standard practice E266 using =1.5" (38.1 mm) Compact Tension specimens for the short longitudinal (S-L) direction and W=2" (50.8 mm) Compact Tension specimens for the L-T and the T-L directions. The results of these tests are listed m Table 9.

TABLE 9

Chemistrv weiαht %

Cu Li Mn Zr

2.83 1.26 .32 .11

KSI = 6.894757 MPa

KSI in = 1.0988434 MPa Vm

The advantage of the present invention is shown in Figure 1 which correlates the fracture toughness values in Tables 2-9 in the S-L direction with tensile yield strengths in the S-T direction. As is evident from Figure 1, no compromise is made in the tensile yield strengths between the prior art examples and the examples of the invention. More specifically, the prior art tensile yield strength values range from just above 54 KSI to almost 60 KSI. In comparison, the tensile yield strengths of the examples accordmg to the invention range from ust below 55 KSI to ust above 57 KSI. Figure 1 demonstrates that the articles made of the present invention provide significantly improved fracture toughness n the S-L direction while maintaining acceptable strength levels in the S-T direction. Figure 2 illustrates the unexpected improvements n fracture toughness in the S-L direction over the prior art. The values depicted in Figure 2 demonstrate that the fracture toughness in the S-L direction for Examples 5-8 is vastly superior to that shown for Examples 1-4. This improvement, which relates to lithium content, is quite unexpected in view of the prior art. Figure 3 emphasizes the fact that the improvements m fracture toughness are related to the lithium content of the alloys. Figure 3 demonstrates that the fracture toughness does not vary widely with respect to copper content. For Examples 5-8, the fracture toughness appears to remain relatively the same with increasing or decreasing amounts of copper. Similarly, the fracture toughness of Examples 1-4 does not vary widely with increasing or decreasing copper content. Referring again to Figure 2, it is believed that the lithium content can be as low as 0.8 weight percent while still giving improvements in fracture toughness and maintaining the acceptable strength in the short transverse direction. It is further believed that the same results are obtainable when practicing the inventive processing m accordance with the broad processing variable ranges disclosed above. Accordingly, an invention has been disclosed in terms of preferred embodiments thereof which fulfill each and every one of the objects of the present invention as set forth above and provides a new and improved method for improving the short longitudinal direction fracture toughness of alummum-lithium alloys. Various changes, modifications and alterations from the teachings of the present invention may be contemplated by those skilled m the art without departing from the intended spirit and scope thereof. Accordingly, it is intended that the present invention only be limited by the terms of the appended claims.

Claims

What is Claimed Is:

1. A method of improving the fracture toughness in a short longitudinal direction of an alummum-lithium alloy article comprising the steps of: a) providing an aluminum alloy consisting essentially of, in weight percent:

2.5 to 4.0% copper, 0.8 to less than 1.3% lithium, 0.05 to 0.8% manganese, up to 0.35% magnesium, 0.04 to 0.18% zirconium, the balance aluminum and inevitable impurities; b casting the aluminum alloy into an mgot; c homogenizing the mgot; d hot working the homogenized got into a hot worked shape; e solution heat treating the hot worked shape; f quenching the solution heat treated shape; and g cold working and agmg the quenched shape;

whereby the cold worked and aged shape exhibits substantially equivalent strength and higher fracture toughness in the short longitudinal (S-L) direction than similar aluminum alloys having lithium amounts greater than 1.3%.

2. The method of claim 1 wherein homogenization and solution heat treating occurs at between about 930┬░ and 1030┬░F (499 to 554┬░C) .

3. The method of claim 1 wherein the shape is stretched between 4 and 8% and then aged between about 300 and 350┬░F (149 to 177┬░C) .

4. The method of claim 1 wherein the lithium content is less than 1.2% by weight.

5. The method of claim 1 wherein the copper ranges between about 2.7% and 3.0%, the lithium ranges between about 1.20 and 1.28% and the manganese ranges between 0.30% and 0.32.

6. The method of claim 1 wherein the fracture toughness of the plate in the short longitudinal (S-L) direction is at least about 68.5% of the fracture toughness of the plate in the long transverse (T-L) direction.

7. The method of claim 1 wherein the lithium content is at least 0.8% by weight and less than 1.2% by weight and the copper content is more than 2.8% by weight.

8. The method of claim 1 wherein the lithium content ranges between about 1.2 and 1.28%.

9. The method of claim 1 wherein the aluminum alloy includes at least one gram refining alloying element selected from the group of titanium in an amount up to 0.2% weight percent, vanadium m an amount up to 0.2% by weight, hafnium in an amount up to 0.2% by weight, scandium in an amount up to 0.5% by weight, and chromium in an amount of up to 0.3% by weight and combinations thereof.

10. The method of claim 1 wherein the aluminum alloy is essentially zinc free.

11. The method of claim 1 wherein any zinc present m the aluminum alloy is at an impurity level of no more than 0.05% by weight.

12. The method of claim 1 wherein the amount of manganese is between about 0.30 and about 0.32% by weight.

13. The method of claim 1 where the aluminum alloy has an amount of magnesium up to 0.25%.

14. A product made by the method of claim 1.

15. A product made by the method of claim 5.

16. A product made by the method of claim 7.

17. The product of claim 14 having a fracture toughness of at least 21.0 KSI Vm the short longitudinal direction.

18. The product of claim 14 having a fracture toughness of at least 21.0 ksi Vm the short longitudinal direction and a tensile yield strength of at least 54.0 KSI in the short transverse direction.

19. The product of claim 18 wherein the fracture toughness is at least 22.7 KSI Vm and the tensile yield strength is at least 54.7 KSI.