Patent ID: 12252772

DETAILED DESCRIPTION

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

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 inFIG.6.

Example 2—Intermediate Processing Resulting in Bulk Recrystallization

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 inFIG.7.

Example 3—Recovery Anneal Resulting in Bulk Recrystallization

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 inFIG.8.

Example 4—Additional Recovery Anneals Result in Bulk Recrystallization

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 inFIGS.9a-9f.

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

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 inFIG.10.

Example 6—High Temperature Thermal Treatment Results in Unrecrystallized Products

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

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 inFIG.11, indicating the robustness of the process.

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

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 inFIG.12.

Example 9—Grain Size Analysis

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 inFIGS.13a-13e. As shown, the invention alloys all realize much smaller grains. This is confirmed by a computerized analysis. As shown inFIG.13f, the non-invention alloy realized much larger grains than that of the invention alloys. This also can be seen inFIGS.13g-13h, which are micrographs showing particles within a non-invention (13g) and an invention (13h) material. (Note:FIG.13guses a 10 micrometer scale;FIG.13huses a 5 micrometer scale.)

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.

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.

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.

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.

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

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.

The particle size distributions for the various samples are shown inFIG.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 ofFIG.14is provided in Table 1, below.

TABLE 1Particle Size Data for Example 10D10D50D90(micro-(micro-(micro-Practice Typemeters)meters)meters)Ex. 2-type (non-inventive)0.1580.6312.512(TT at 720° F., cool to andTT at 450° F., and then aircool.)Inventive (pink)0.0160.0630.631(Single TT at 945° F., 5.5%stretch)Inventive (green)0.0160.0631.0(TT @ 945° F., 5.5% stretch,TT @ 945° F., 5.5% stretch)Inventive (blue)0.010.0630.631(TT @ 945° F., 5.5% stretch,TT @ 945° F., 6.0% stretch,TT @ 945° F., 7.0% stretch)Inventive (yellow)0.0160.11.0(TT @ 945° F., 5.8% stretch,TT @ 945° F., 6.0% stretch,TT @ 945° F., 5.5% stretch,TT @ 945° F., 6.3% stretch)

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.

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.