METHODS OF COLD FORMING ALUMINUM LITHIUM ALLOYS

Abstract

New methods of making cold formed, extruded aluminum lithium alloys, and unrecrystallized products made therefrom are disclosed. A method may include one or more of heating an unrecrystallized extruded aluminum-lithium product to a treatment temperature, cooling the unrecrystallized extruded aluminum-lithium product from the treatment temperature to a post-treatment temperature, and cold forming the unrecrystallized extruded aluminum-lithium product into a second product form. Due to the unique processing conditions of the method, the second product form may wholly or partially retain the unrecrystallized microstructure.

Claims

1. A method comprising: cold forming an unrecrystallized extruded aluminum-lithium product into a second product form; (i) wherein the unrecrystallized extruded aluminum-lithium alloy comprises from 0.2-5.0 wt. % Li; (ii) wherein the unrecrystallized extruded aluminum-lithium product is predominately unrecrystallized; (iii) wherein the cold forming comprising initiating the cold forming when the unrecrystallized extruded aluminum-lithium product has a temperature of not greater than 400° F.; (iv) wherein the cold forming comprises plastically deforming the unrecrystallized extruded aluminum-lithium product, wherein the plastically deforming comprises at least one of: (A) non-uniformly deforming the unrecrystallized extruded aluminum-lithium product, wherein, due to the non-uniform deforming, a first portion of the second product form realizes a first strain amount and a second portion of the second product form realizes a second strain amount, wherein the first strain amount is at least 1% different than the second strain amount; and (B) applying curvature to the unrecrystallized extruded aluminum-lithium product, wherein the second product form comprises at least one arcuate surface. (v) wherein the second product form is predominately unrecrystallized.

2. The method of claim 1, wherein, prior to the cold forming step, the method comprises: heating the unrecrystallized extruded aluminum-lithium product to a treatment temperature, wherein the treatment temperature is at least 750° F.; and then cooling the unrecrystallized extruded aluminum-lithium product from the treatment temperature to a post-treatment temperature and at a cooling rate of not greater than 500° F./minute.

3. The method of claim 2, comprising: second heating the second product form to a second treatment temperature, wherein the second treatment temperature is at least 750° F.; second cooling the second product form from the second treatment temperature to a second post-treatment temperature; second cold forming the second product form into another product form, wherein the another product form is predominately unrecrystallized.

4. The method of claim 3, wherein the second product form is an intermediate product form, wherein the another product form is a final product form, and wherein the second cooling comprises cooling the second product form from the second treatment temperature to the second post-treatment temperature at a rate of at least 1000° F./minute.

5. The method of claim 2, wherein the treatment temperature is below a solidus temperature of the unrecrystallized extruded aluminum-lithium product.

6. The method of claim 2, wherein the treatment temperature is from 85° F. below a solidus temperature of the unrecrystallized extruded aluminum-lithium product to 15° F. below a solidus temperature of the unrecrystallized extruded aluminum-lithium product.

7. The method of claim 2, wherein the heating comprises heating the unrecrystallized extruded aluminum-lithium product from the pretreatment temperature to the treatment temperature at a heating rate of at least 1° F. per minute.

8. The method of claim 2, wherein the heating comprises heating the unrecrystallized extruded aluminum-lithium product from the pretreatment temperature to the treatment temperature at a heating rate of not greater than 100° F. per minute.

9. The method of claim 2, wherein the heating comprises holding the unrecrystallized extruded aluminum-lithium product at the treatment temperature for a period of time sufficient to dissolve a predominate amount of precipitate phase particles but without recrystallizing the unrecrystallized extruded aluminum-lithium product.

10. The method of claim 2, wherein the cooling comprises cooling the unrecrystallized extruded aluminum-lithium product from the treatment temperature to the post-treatment temperature at a cooling rate of not greater than 400° F./minute.

11. The method of claim 2, wherein the treatment temperature is at least 800° F.

12. The method of claim 1, wherein the unrecrystallized extruded aluminum-lithium product is a 2xxx-Li product, and wherein the 2xxx-Li product comprises from 2.0-5.0 wt. % Cu, 0.2-2.0 wt. % Li, up to 1.5 wt. % Mg, up to 1.0 wt. % Ag, up to 1.0 wt. % Mn, up to 1.5 wt. % Zn, up to 0.25 wt. % each of Zr, Ti, Sc, and Hf, the balance being aluminum, optional incidental elements and impurities.

13. The method of claim 1, wherein the unrecrystallized extruded aluminum-lithium product is at least 60% unrecrystallized.

14. The method of claim 1, wherein the cold forming comprising including from 3% to 20% strain in the second product form.

15. The method of claim 1, wherein the cold forming comprises initiating the cold forming when the unrecrystallized extruded aluminum-lithium product has a temperature of not greater than 300° F.

16. The method of claim 1, wherein the cold forming comprises initiating the cold forming when the unrecrystallized extruded aluminum-lithium product is at ambient temperature.

17. The method of claim 1, wherein the second product form is predominately unrecrystallized, or at least 60% unrecrystallized.

18. The method of claim 1, wherein the cold forming is stretch forming.

19. A method comprising: (a) heating an unrecrystallized extruded aluminum-lithium product to a treatment temperature, wherein the treatment temperature is at least 750° F. but below a solidus temperature of the unrecrystallized extruded aluminum-lithium product; (i) wherein the heating comprises heating the unrecrystallized extruded aluminum-lithium product from the pretreatment temperature to the treatment temperature at a heating rate of from 10° F. to not greater than 100° F. per minute; and (ii) wherein the heating comprises holding the unrecrystallized extruded aluminum-lithium product at the treatment temperature for a period of time sufficient to dissolve a predominate amount of precipitate phase particles but without recrystallizing the unrecrystallized extruded aluminum-lithium product; (b) cooling the unrecrystallized extruded aluminum-lithium product from the treatment temperature to a post-treatment temperature and at a cooling rate of not greater than 500° F./minute.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0101] FIG. 1 is a flow chart illustrating one embodiment of an inventive method for producing unrecrystallized extruded products.

[0102] FIG. 2a illustrates additional embodiments of step (100) of FIG. 1 relating to aluminum alloy compositions and extruded product types.

[0103] FIG. 2b illustrates additional embodiments of step (100) of FIG. 1 relating to thermal treatment practices.

[0104] FIG. 2c is a flow chart illustrating cycles of the heating (100), cooling (200) and cold forming (300) steps, including the initial (100i-300i) and final (100f-300f) cycles.

[0105] FIG. 3 illustrates additional embodiments of step (200) of FIG. 1 relating to cooling rates.

[0106] FIG. 4a illustrates additional embodiments of step (300) of FIG. 1 relating to deformation and related items.

[0107] FIG. 4b illustrates additional embodiments of FIG. 1 relating to particle size distributions due to the methodology.

[0108] FIG. 5a illustrates optional additional processing (400) relating to the methodology.

[0109] FIG. 5b illustrates additional embodiments of FIG. 5a.

[0110] FIGS. 6a-6b are micrographs showing the microstructure of the typical products of Example 1.

[0111] FIG. 7 is a micrograph showing the microstructure of the product of Example 2.

[0112] FIG. 8 is a micrograph showing the microstructure of the product of Example 3.

[0113] FIGS. 9a-9f are micrographs showing the microstructure of the products of Example 4.

[0114] FIG. 10 is a micrograph showing the microstructure of the product of Example 5.

[0115] FIG. 11 is a micrograph showing the microstructure of the product of Example 7.

[0116] FIG. 12 is a micrograph showing the microstructure of the product of Example 8.

[0117] FIGS. 13a-13e are SEM images from Example 9.

[0118] FIG. 13f is a graph showing grain size distributions based on the SEM images of Example 9.

[0119] FIGS. 13g-13h are micrographs showing particles within a non-invention (13g) and an invention (13h) material.

[0120] FIG. 14 is a graph showing particle size distribution results for a non-inventive practice and various inventive practices.

[0121] FIGS. 15a-15b are optical micrographs showing an inventive microstructure (FIG. 15a) versus a non-inventive microstructure (FIG. 15b).

DETAILED DESCRIPTION

Example 1—Conventional Extrusion, Cold Forming and Solution Heat Treatment Results in Recrystallized Products

[0122] A 2055-style aluminum alloy was extruded into a Z-shaped extrusion, resulting in an unrecrystallized aluminum-lithium extrusion. The material was cold formed into a final product shape by stretch forming. The material was then solution heat treated and cold water quenched. The final material was recrystallized as shown in FIG. 6.

Example 2—Intermediate Processing Resulting in Bulk Recrystallization

[0123] Given conventional processing (Example 1) yielded a recrystallized product, an intermediate thermal treatment practice was developed to stop/restrict transformation of the unrecrystallized extruded product into a recrystallized product. Specifically, a 2055-style aluminum alloy was extruded into a rectangular bar, resulting in a unrecrystallized aluminum-lithium extrusion. The rectangular bar was then thermally treated by rapidly heating to a 720° F. treatment temperature in a furnace. The material was held at the 720° F. treatment temperature (+/−10° F.) for 1 hour (the soak time started when the material reached a temperature of 690° F.). The material was then slowly cooled by changing the temperature of the furnace to 450° F. The material cooled from the 720° F. treatment temperature to the 450° F. treatment temperature at a rate of 50° F./hour. The material was held at the 450° F. treatment temperature (+/−10° F.) for 4 hours (the soak time started when the material reached a temperature of 465° F.). The material was then removed from the furnace and allowed to air cool. The material was then cold formed by uniaxially stretching the material to yield 8% permanent strain. The material was then solution heat treated and then quenched in cold water. Despite the intermediate thermal practice, the final material was still recrystallized, as shown in FIG. 7.

Example 3—Recovery Anneal Resulting in Bulk Recrystallization

[0124] Additional efforts to produce final unrecrystallized products surrounded the use of a post cold-forming recovery anneal. Specifically, a 2055-style aluminum alloy was extruded into a rectangular bar, resulting in a unrecrystallized aluminum-lithium extrusion. The rectangular bar was then thermally treated as per Example 2, i.e., treated at both 720° F. and 450° F., and then allowed to air cool. The material was then cold formed by uniaxially stretching the material to yield 6% permanent strain. The material was then thermally treated by heating to 215° F. (3 hours), and then 400° F. (2 hours), and then 500° F. (3 hours), and then 600° F. (4 hours). The material was then solution heat treated and then quenched in cold water, as per Example 2. The final material was also recrystallized as shown in FIG. 8.

Example 4—Additional Recovery Anneals Result in Bulk Recrystallization

[0125] Building on the efforts of Example 3, additional recovery anneal tests were completed. Specifically, a 2055-style aluminum alloy was prepared and thermally treated prior to cold forming, as per Example 3. The material was then cold formed by uniaxially straining to yield 7% stretch. Various samples of this material were then rapidly heated to various anneal temperatures (525° F., 575° F., 675° F., 725° F., 775° F., and 875° F.). The materials were then solution heat treated and then quenched in cold water, as per Example 2. All final materials were recrystallized as shown in FIGS. 9a-9f.

Example 5—Recovery Anneals Without Cold Forming Result in Bulk Recrystallization

[0126] In Example 5, a 2055-style aluminum alloy was prepared and thermally treated, as per Example 2, except the material was extruded into a Z-shape. This time, the thermal treatment cycle was repeated three times (i.e., 3× at 720° F. and 450° F. as per Example 2). No cold forming operation was employed in this Example 5. Instead, after the three thermal cycle operations, the material was solution heat treated and then quenched in cold water, as per Example 2. Despite receiving no cold forming, the final material was still recrystallized as shown in FIG. 10.

Example 6—High Temperature Thermal Treatment Results in Unrecrystallized Products

[0127] In Example 6, a 2055-style aluminum alloy was extruded into a rectangular bar, resulting in a unrecrystallized aluminum-lithium extrusion. The rectangular bar was then thermally treated by rapidly heating to a 945° F. treatment temperature in a furnace. The material was held at the 945° F. treatment temperature (+/−10° F.) for 1 hour (the soak time started when the material reached a temperature of 935° F.). Upon conclusion of the soak, the material was removed from the furnace and allowed to air cool to ambient temperature. The cooling rate for this cooling step was about 25° F. per minute. The material was then cold formed by uniaxially straining to yield 6% permanent strain. The material was then solution heat treated and quenched in cold water, as per Example 2. This time, the final material remained unrecrystallized.

Example 7—Multiple High Temperature and Multiple Straining Operations Results in Unrecrystallized Products

[0128] To test the robustness of this process, the same process as Example 6 was performed on an unrecrystallized 2055 extruded product, but with 4 thermal treatment cycles at 945° F. and with 4 corresponding strain operations following each thermal treatment cycles, each strain operation applying 6% permanent strain to the prior product. After the 4th strain operation, the material was solution heat treated and quenched in cold water, as per Example 2. Even after four strain operations, the final material remained unrecrystallized, as shown in FIG. 11, indicating the robustness of the process.

Example 8—Multiple High Temperature and Multiple Straining Operations Results in Unrecrystallized Products

[0129] In Example 8, an unrecrystallized 2055 extruded product was thermally treated as per Example 2, i.e., treated at both 720° F. and 450° F., and then allowed to air cool. The material was not cold formed after this thermal treatment. Instead, an additional thermal treatment cycle was employed as per Example 6, i.e., treated by rapidly heating to a 945° F. treatment temperature in a furnace, holding at the 945° F. treatment temperature (+/−10° F.) for 1 hour (the soak time started when the material reached a temperature of 935° F.), and then removing the material from the furnace and allowing to air cool to ambient temperature. The material was then cold formed by uniaxially straining to yield 8% permanent strain. The material was then solution heat treated and quenched in cold water, as per Example 2. Again, the final material remained unrecrystallized as shown in FIG. 12.

Example 9—Grain Size Analysis

[0130] SEMs of several alloys made by the invention process and one alloy made by a non-invention process were obtained as per the Microstructure Determination Procedure. The grain sizes of these SEMs were calculated as per the Grain Size Computer Analysis Procedure. The SEMs are provided in FIGS. 13a-13e. As shown, the invention alloys all realize much smaller grains. This is confirmed by a computerized analysis. As shown in FIG. 13f, the non-invention alloy realized much larger grains than that of the invention alloys. This also can be seen in FIGS. 13g-13h, which are micrographs showing particles within a non-invention (13g) and an invention (13h) material. (Note: FIG. 13g uses a 10 micrometer scale; FIG. 13h uses a 5 micrometer scale.)

[0131] Given the foregoing examples, and without being bound to any particular theory, it is believed that the high temperature thermal treatment practice in combination with reasonable amounts of strain allows for the production of cold formed aluminum-lithium extruded products that retain unrecrystallized grains. Indeed, the final products generally contain a significant amount of unrecrystallized grains and relative to the starting products in the as-extruded condition.

[0132] Thus, in some embodiments, a “recrystallized” cold formed product is one who, based on the EBSD data and SEMs gathered above, realizes a microstructure (as per the SEMs) having an area fraction of at least 0.20% of large grains (≥67.5 micrometers (i.e., greater than or equal to 67.5 micrometers)) and in any one of the obtained samples. That is, if even one of the samples realizes these criteria, the material is categorized as recrystallized. In one embodiment, a recrystallized cold formed product realizes a microstructure having an area fraction of at least 25% of large grains. In another embodiment, a recrystallized cold formed product realizes a microstructure having an area fraction of at least 30% of large grains. In yet another embodiment, a recrystallized cold formed product realizes a microstructure having an area fraction of at least 35% of large grains. In another embodiment, a recrystallized cold formed product realizes a microstructure having an area fraction of at least 40% of large grains. In yet another embodiment, a recrystallized cold formed product realizes a microstructure having an area fraction of at least 45% of large grains, or higher.

[0133] In some embodiments, an unrecrystallized cold formed product is any product that is outside the above definition of a “recrystallized” cold formed product. In one embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure (as per the SEM and EBSD data) having an area fraction of not greater than 0.2% of grains of a size of from ≥57.5 to 67.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.15% of grains of a size of from ≥57.5 to 67.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.10% of grains of a size of from ≥57.5 to 67.4 micrometers.

[0134] In one embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.2% of grains of a size of from ≥47.5 to 57.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.15% of grains of a size of from ≥47.5 to 57.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.10% of grains of a size of from ≥47.5 to 57.4 micrometers.

[0135] In one embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.22% of grains of a size of from ≥37.5 to 47.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.17% of grains of a size of from ≥37.5 to 47.4 micrometers. In another embodiment, an unrecrystallized cold formed product also realizes or alternatively realizes a microstructure having an area fraction of not greater than 0.12% of grains of a size of from ≥37.5 to 47.4 micrometers.

Example 10—Particle Size Analysis

[0136] Various samples were obtained from materials processed consistent with the practices of Examples 2 (non-inventive) and Examples 6-8 (inventive). All samples were thermally treated and then air quenched in accordance with, or similar to, these examples. Backscattered SEM images of the sample were obtained and the images were then computer analyzed to determine the particle distributions/sizes for the various materials as per the

Particle Size Computer Analysis Procedure.

[0137] The particle size distributions for the various samples are shown in FIG. 14. As shown, the inventive practices (shown by the solid bars) generally have a much higher volume of small particles and the distribution is more even. The non-inventive practice (shown by the bars with hatching) of Ex. 2 realizes much larger particles and the distribution is more condensed. The specific D10, D50, and D90 values for the data of FIG. 14 is provided in Table 1, below.

TABLE-US-00001 TABLE 1 Particle Size Data for Example 10 D10 D50 D90 (micro- (micro- (micro- Practice Type meters) meters) meters) Ex. 2-type (non-inventive) 0.158 0.631 2.512 (TT at 720° F., cool to and TT at 450° F., and then air cool.) Inventive (pink) 0.016 0.063 0.631 (Single TT at 945° F., 5.5% stretch) Inventive (green) 0.016 0.063 1.0 (TT @ 945° F., 5.5% stretch, TT @ 945° F., 5.5% stretch) Inventive (blue) 0.01 0.063 0.631 (TT @ 945° F., 5.5% stretch, TT @ 945° F., 6.0% stretch, TT @ 945° F., 7.0% stretch) Inventive (yellow) 0.016 0.1 1.0 (TT @ 945° F., 5.8% stretch, TT @ 945° F., 6.0% stretch, TT @ 945° F., 5.5% stretch, TT @ 945° F., 6.3% stretch)

[0138] Given the foregoing examples, and without being bound to any particular theory, it is believed that the high temperature thermal treatment practice in combination with the post-thermal treatment cooling rates and appropriate amounts of post-cooling strain produces unique unrecrystallized products having a distribution of small precipitate phase particles. As explained above in Section iii, higher concentrations of smaller precipitate phase particles (e.g., within the D10, D50, and D90 amounts described in Section iv) may facilitate grain boundary pinning while also reducing the amount of solute present during cold forming operations. The grain boundary pinning may restrict/prevent recrystallization. Further having a relatively low amount of nano-scale precipitate phases (e.g., <20 nanometers) may facilitate working of the material. Larger particles may also act as nucleation sites for recrystallization. Accordingly, the methods described herein seek to restrict/avoid the production of large scale and nano-scale particles, while having an appropriate amount of small precipitate phase particles. Thus, shape forming may be completed in a low number of cycles to achieve the final part geometry and in the unrecrystallized condition, followed by appropriate post-cold forming operations (e.g., solution heat treatment, post-SHT stretch to facilitate nucleation of aging precipitates, aging (natural and/or artificial), and machining, to name a few). Significant costs reductions may accordingly be realized.

[0139] While various embodiments of the present disclosure have been described in detail, it is apparent that modifications and adaptations of those embodiments will occur to those skilled in the art. However, it is to be expressly understood that such modifications and adaptations are within the spirit and scope of the present disclosure.