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
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/057—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys with copper as the next major constituent

Definitions

• 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.
• 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
• L-T longitudinal
• T-L long transverse
• the present invention provides both a method and a product
• a first ob ect of the invention is to improve the fracture
• Another object of the invention is to provide a method of
• a still further object of the present invention is to
• Yet another object of the present invention is to provide an
• the aluminum alloy can also include gram refining elements such as at least one of boron, titanium, vanadium, manganese, hafnium, scandium and chromium.
• 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%.
• 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.
• the amounts of alloying elements can be within the ranges described m 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 (Al 2 Cu) would precipitate in addition to Tj.
• 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 quencne ⁇ , preferably in water, cold worked, preferably by stretching, and aged for a select period of time.
• 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) .
• 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
• 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.
• 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
• 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.
• 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.
• 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.
• the aluminum is free of zinc.
• 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.
• 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.
• 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.
• 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.
• 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.
• the hot worked shape is then solution heat treated and quenched.
• 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.
• the shape may be formed into an article and then aged.
• 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.
• 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 L ⁇ -,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.
• 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.
• Example 2 An aluminum alloy consisting of, weight percent, 2.71 Cu- 1.37 L ⁇ -.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.
• 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.
• Example 3 An aluminum alloy consisting of, m weight percent, 2.77 Cu- 1.33 L ⁇ -.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.
• Example 4 An aluminum alloy consisting of, in weight percent, 2.89 Cu- 1.36 L ⁇ -,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.
• 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.
• Example 5 An aluminum alloy consisting of, m weight percent, 2.78 Cu- 1.21 L ⁇ -.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.
• 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.
• example 6 An aluminum alloy consisting of, in weight percent, 2.86 Cu- 1.28 L ⁇ -.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.
• 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.
• Example 7 An aluminum alloy consisting of, in weight percent, 2.73 Cu- 1.28 L ⁇ -.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.
• 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.
• 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.
• Figure 1 correlates the fracture toughness values in Tables 2-9 in the S-L direction with tensile yield strengths in the S-T direction.
• 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.
• 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.

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• 1998
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