Method of manufacturing heat treated sheet and plate with reduced levels of residual stress and improved flatness

Abstract

This invention describes a method of producing a flat aluminum sheet product by providing an aluminum alloy stock, hot working the aluminum alloy stock through preheating and hot rolling, side sawing the aluminum alloy stock, subjecting the aluminum alloy stock to solution heat treatment, cold rolling the aluminum alloy stock to reduce its thickness by about 0.25% to about 5%, and finally stretching the aluminum alloy stock.

Description

FIELD OF THE INVENTION

The present invention relates generally to methods for making aluminum alloy sheet. Specifically, the present invention relates to a method of making an aluminum alloy sheet for use in aerospace applications wherein the sheet exhibits lower compressive and tensile stresses and improved flatness.

BACKGROUND OF THE INVENTION

Aircraft wings have traditionally been machined either mechanically or chemically from an aluminum sheet selected from the Aluminum Association's designated 2XXX, 6XXX, or 7XXX series of aluminum alloys. The aluminum alloy sheets used in aerospace applications have to exhibit high fracture toughness, fatigue crack growth resistance, high cycle fatigue resistance, and corrosion resistance. In order to meet these needs, conventional manufacturing techniques have typically required that the aluminum sheet be subjected to solution heat treatment followed by quench in order to enhance the mechanical properties of the sheet. However, a side effect of this process is that distortion of the aluminum sheet can occur during machining to its final product form due to the interaction of inherent residual stresses, such as compressive and tensile stresses, and the severe thermal action of the solution heat treatment and quench. The distortion, which manifests itself as product warping, is considerable when the final product is as large as an aircraft wing.

To minimize these distortions and to improve product flatness, traditional manufacturing techniques have often used shot peening, mechanical bump forming, or stretching after solution heat treatment. Of these techniques, stretching after solution heat treatment has the added advantage of enhancing the mechanical properties of some aluminum alloys. However, products that are formed using conventional processes that utilize solution heat treating and quenching followed by stretching still exhibit compressive stresses near the surface and tensile stresses near the mid-plane. These residual stresses ultimately leads to product warping during the machining of the aluminum sheet into an aircraft wing.

Therefore, there exists a need for producing a large flat aluminum sheet product that is suitable for aerospace applications that does not distort during machining and that exhibits mechanical and physical properties that are equivalent if not improved over aluminum sheet products currently produced through conventional methods of manufacturing.

The present invention is a response to this need for producing a large flat aluminum sheet that would be suitable for aerospace applications by providing a method that reduces compressive and tensile stresses through the aluminum sheet's thickness while improving the mechanical properties of the sheet as compared to aluminum sheets produced through conventional methods of manufacture.

SUMMARY OF THE INVENTION

This invention describes a method of producing a flat aluminum sheet product by providing an aluminum alloy stock, hot working the aluminum alloy stock through preheating and hot rolling, side sawing the aluminum alloy stock, subjecting the aluminum alloy stock to solution heat treatment, cold rolling the aluminum alloy stock to reduce its thickness by about 0.25% to about 5%, including all fractional values and points within this range, and finally stretching the aluminum alloy stock.

The preheating operation can have a temperature range from about 870 degrees Fahrenheit to about 1050 degrees Fahrenheit, including all fractional values and points within this range. It is noted that one skilled in the art would know which temperature to select for a given aluminum alloy and the amount of time the given aluminum alloy should be preheated.

The solution heat treatment operation can have a temperature range from about 870 degrees Fahrenheit to about 1050 degrees Fahrenheit, including all fractional values and points within this range. For the Aluminum Association's designated 7XXX series alloys, the preferred temperature range is from about 880 degrees Fahrenheit to about 900 degrees Fahrenheit, including all fractional values and points within this range. For the Aluminum Association's designated 2XXX series alloys, the preferred temperature range is from about 900 degrees Fahrenheit to about 930 degrees Fahrenheit, including all fractional values and points within this range. For the Aluminum Association's designated 6XXX series alloys, the preferred temperature range is from about 950 degrees Fahrenheit to about 1050 degrees Fahrenheit, including all fractional values and points within this range. Alternatively, the solution heat treatment could conform with the AMS 2772 standard.

After the solution heat treatment operation, the aluminum alloy stock is cold rolled to reduce the thickness of the stock by about 0.25% to about 5%. One or more cold rolling passes may be used to reduce the thickness of the stock to the desired thickness.

After the cold rolling process, the aluminum alloy stock is stretched or elongated by about 0.5% to about 4%, including all fractional values and points within this range. Preferably, the aluminum alloy stock is stretched by about 1% to about 3%.

The total amount of cold working (cold rolling+stretching) is about 2% to about 6%, including all fractional values and points within this range. Preferably, the total amount of cold working is about 3% to about 5%.

The aluminum alloys that may be used in this invention would include several of the Aluminum Association's designated 2XXX, 6XXX, and 7XXX series of aluminum alloys. For example, some aluminum alloys that may be used in connection with this invention would include, but shall not be limited to, aluminum alloy 2024 and its variants, 6061, 6013, 7075, and 7085.

An aspect of this invention is to provide an aluminum sheet product that does not exhibit or has minimal through-thickness stress gradients such as compressive and tensile stresses.

Another aspect of this invention is to provide an aluminum sheet product that does not distort during machining to its final product form.

Another aspect of this invention is provide an aluminum sheet product that has similar if not greater mechanical properties than aluminum products utilizing conventional methods of making aluminum sheet.

Another aspect of this invention is to provide an aluminum sheet that would be suitable for aerospace applications.

Another aspect of this invention is to provide an aluminum sheet that would be suitable for machining into an aircraft wing or stabilizer.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph plotting the flatness of the sheet or plate after stretch along the length of the plate.

FIG. 2A is a schematic that depicts an aluminum alloy sheet or plate in a dish condition.

FIG. 2B is a schematic that depicts an aluminum alloy sheet or plate in a crown condition.

FIG. 2C is a schematic that depicts an aluminum alloy sheet or plate in a dish and crown condition.

FIG. 3 is a graph plotting stored elastic strain energy as a function of percent cold rolling.

FIG. 4 depicts the predicted lateral bow reduction as a function of percent cold rolling.

FIG. 5 is a graph plotting the strength/toughness of the aluminum sheet or plate that was cold rolled prior to being stretched.

FIG. 6 is a graph plotting the yield strength of the aluminum sheet or plate as a function of percent cold worked.

FIG. 7 is a flowchart depicting one preferred embodiment of the method that is disclosed in this invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The accompanying figures and the description that follows set forth this invention in its preferred embodiment. However, it is contemplated that persons generally familiar with the manufacture of aluminum alloy sheet or plate will be able to apply the novel characteristics of the structures and methods illustrated and described herein in other contexts by modification of certain details. Accordingly, the figures and description are not to be taken as restrictive on the scope of this invention, but are to be understood as broad and general teachings. It is noted that any ranges disclosed in the figures and description shall include all fractional values and points within those ranges.

The method disclosed includes providing an aluminum alloy stock selected from the 2XXX, 6XXX, or 7XXX series of aluminum alloys, hot working the aluminum alloy stock through preheating and hot rolling, side sawing the aluminum alloy stock to remove side cracks that would cause the aluminum sheet or plate to break during the stretching process, subjecting the aluminum alloy stock to solution heat treatment, cold rolling the aluminum alloy stock to reduce its thickness by about 0.25% to about 5%, and finally stretching the aluminum alloy stock by about 0.5% to about 4%, preferably stretching the aluminum alloy stock by about 1% to about 3%. The total amount of cold working (cold rolling+stretching) is about 2% to about 6%. Preferably, the total amount of cold work done on the aluminum sheet is about 3% to about 5%.

FIG. 1 shows a graph plotting the flatness of the plate after stretch as measured along the length of the plate. The aluminum alloy sheet is perfectly flat when the y-axis value is 0. A negative value on the y-axis indicates that the plate is in a dished condition. As seen in FIG. 2A, if the aluminum sheet or plate 2 is in a dish condition, then both ends 4 and 6 of the plate are elevated above a flat table surface 8 and the middle section of the sheet or plate 10 is in direct contact with the flat table surface 8. In contrast, a positive value on the y-axis of FIG. 1 indicates that the aluminum sheet or plate is in a crown condition. As seen in FIG. 2B, if the aluminum sheet or plate 2 is in a crown condition, then both ends of the plate 4 and 6 are in contact with a flat table surface 8 and the middle section of the plate 10 is elevated above the flat table surface 8.

In FIG. 1, “CR” represents the percent the aluminum plate was cold rolled and “Str” represents the percent the aluminum plate was stretched. All of the plates in FIG. 1 were made from Aluminum Association designated 7055 aluminum alloy and were subjected to AMS 2772 solution heat treatment. As can be seen in FIG. 1, six tests were conducted using different combinations of cold rolling and stretching. FIG. 1 shows that the aluminum plate was significantly flatter when the cold rolling operation was applied to the aluminum alloy plate prior to the stretching operation

Plates #1 and #2 were stretched by 2.75% but the plates were not cold rolled. Plate #1 exhibited a dished condition at the first and second ends (0 and 200 on the x-axis) having approximate values of −7.62 mm (−0.3 inches) and −4.57 mm (−0.18 inches), respectively, while the middle section of the plate, having a range between −1.52 mm (−0.06 inches) to −2.54 mm (−0.10 inches), had a flatter orientation when compared to the two ends. In contrast, plate #2 was dished and crowned. As can be seen in FIG. 1, the first and second ends (0 and 200 on the x-axis) having values of −6.35 mm (−0.25 inches) and −3.56 (−0.14 inches), respectively, exhibited a dished condition. In contrast, the middle section of the plate (100 on the x-axis) having a value of 0.51 mm (0.02 inches) was crowned. FIG. 2C is a schematic depicting the dish and crown configuration of aluminum plate #2. As stated above, aluminum plates #1 and #2 were not cold rolled. The only cold working process these plates were subject to was a 2.75% stretch. Despite being stretched, both plates still exhibited distortion in the form of dishing as in plate #1 or dishing and crowning as in plate #2.

In contrast to plates #1 and #2, plates #3-6 were subjected to a combination of cold rolling and stretching. Plate #3 was cold rolled 3.3% and stretched by 1%. The total amount of cold work done on Plate #3 was 4.3%. As can be seen in FIG. 1, the dishing effect was significantly reduced when cold rolling was applied prior to stretching. The first end of plate #3 had an approximate value of −2.79 mm (−0.11 inches). When compared to plate #1, the first end of plate #3 was approximately 60% flatter than the first end of plate #1. When compared to plate #2, the first end of plate #3 was 53% flatter than the first end of plate #2. The middle section of plates #3 and #1 both had values of −2.03 mm (−0.08 inches). The second end of plate #3 had an approximate value of −2.03 mm (−0.08 inches). This is 56% flatter than the second end of plate #1 and 45% flatter than the second end of plate #2.

Plate #4 was cold rolled 2.8% and stretched by 1%. The total amount of cold work done to plate #4 was 3.8%. Again, as seen in FIG. 1 the dishing effect of the plate was significantly reduced when the plate was cold rolled prior to stretching. The first end of plate #4, having a value of −3.30 mm (−0.13 inches), was 56% flatter than the first end of plate #1 and 49% flatter than the first end of plate #2. The middle portion of plate #4 had a value of −2.01 mm (−0.079 inches), which was 9% flatter than the middle portion of plate #1. The second of plate #4 (180 on the x-axis because plate #4 was shorter) had a value of −2.03 mm (−0.08 inches). When compared to the same point in plates #1 and #2, plate #4 was 59% flatter than plate #1 and 37% flatter than plate #2.

Plate #5 was cold rolled 1.7% and stretched by 2%. The total amount of cold work done to plate #5 was 3.7%. As seen in FIG. 1, the dishing effect of the plate was significantly reduced when cold rolling was applied prior to stretching. The first end of plate #5, having a value of −3.30 mm (−0.13 inches), was 56% flatter than the first end of plate #1 and 50% flatter than the first end of plate #2. The middle portion of plate #5, having a value of −1.52 mm (−0.06 inches), was 28% flatter than the middle portion of plate #1. The second end of plate #5, having a value of −1.78 mm (−0.07 inches), was 64% flatter than the second end of plate #1 and 55% flatter than the second end of plate #2.

Plate #6 was cold rolled 1.2% and stretched by 2.5%. The total amount of cold work done to plate #6 was 3.7%. Again, as seen in FIG. 1 the dishing effect of the plate was significantly reduced when the plate was cold rolled prior to stretching. The first end of plate #6 having a value of −2.79 mm (−0.11 inches) was 64% flatter than the first end of plate #1. Additionally, the first end of plate #6 was 58% flatter than the first end of plate #2. The middle portion of the plate, having a value of −1.52 mm (−0.06 inches), was 36% flatter than the middle portion of plate #1. The second end of plate #4, having a value of −1.27 mm (−0.05 inches), was 72% flatter than the second end of plate #1 and 64% flatter than the second end of plate #2.

FIG. 3 shows a graph plotting strain energy as a function of percent cold rolled. All of the plates in FIG. 1 were made from Aluminum Association designated 7055 aluminum alloy and were subjected to AMS 2072 solution heat treatment. “eL” is the measure of the longitudinal component of stored elastic strain energy and “eLT” is the measure of the long-transverse component of stored elastic strain energy as determined using equations (1) and (2) below. “eTotal” is the measure of the combined strain energies of “eL” and “eLT” as determined using equation (3) below. $\mathrm{eL}=\frac{1}{2\mathrm{Eb}}{\int }_0^b{\sigma }_L\left({\sigma }_L-v{\sigma }_{\mathrm{LT}}\right)dz$ $\mathrm{eLT}=\frac{1}{2\mathrm{Eb}}{\int }_0^b{\sigma }_{\mathrm{LT}}\left({\sigma }_{\mathrm{LT}}-v{\sigma }_L\right)dz\left(1,2,3\right)$ $\mathrm{eTotal}=\mathrm{eL}+\mathrm{eLT}$

E is the elastic modulus, v is Poisson's ratio, b is the sheet or plate thickness and σ_L and σ_LT are the longitudinal and long-transverse residual stresses measured as a function of depth through the thickness of the sheet or plate using any of several methods common to the industry.

As can be seen in FIG. 3, the total amount of strain energy is significantly reduced when the cold rolling operation was applied to the aluminum alloy plate prior to the stretching operation. Plates #1, 2, and 7 were stretched by 2.75% but the plates were not cold rolled. The total amount of cold work that plates #1, 2, and 7 were subjected to was 2.75%. The total strain energy (eTotal) for plate #1 was 5.28 KJ/m³. The total strain energy for plates #2 and #7 were 6.03 KJ/m³ and 4.264 KJ/m³ respectively. In contrast to plates #1, 2, and 7, plates #3-6 and #9-11 were subjected to a combination of cold rolling and stretching. Plate #9 was cold rolled 2% prior to being stretched by 1.5%. The total amount of cold work plate #9 was subjected to was 3.5%. As can be seen in FIG. 3, Plate #9 had a total strain energy of 1.27 KJ/m³, which was a 76% reduction in total strain energy when compared to plate #1, a 79% reduction in total strain energy when compared to plate #2, and a 70% reduction in total strain energy when compared to plate #7. Plate #10 was cold rolled 2.7% prior to being stretched by 1%. The total amount of cold work plate #10 was subjected to was 3.7%. Plate #10 had a total strain energy of 1.43 KJ/m³, which was a 73% reduction in total strain energy when compared to plate #1, a 76% reduction in total strain energy when compared to plate #2, and a 66% reduction in total strain energy when compared to plate #7. Plate #3 was cold rolled 3.3% prior to being stretched by 1%. The total amount of cold work plate #3 was subjected to was 4.3%. As can be seen in FIG. 3, Plate #3 had a total strain energy of 1.66 KJ/m³, which was a 69% reduction in total strain energy when compared to plate #1, a 73% reduction in total strain energy when compared to plate #2, and a 51% reduction in total strain energy when compared to plate #7. Plate #4 was cold rolled 2.8% prior to being stretched by 1%. The total amount of cold work plate #4 was subjected to was 3.8%. Plate #4 had a total strain energy of 2.00 KJ/m³, which was a 62% reduction in total strain energy when compared to plate #1, a 67% reduction in total strain energy when compared to plate #2, and a 53% reduction in total strain energy when compared to plate #7. Plate #5 was cold rolled 1.7% and stretched 2%. The total amount of cold work plate #5 was subjected to was 3.7%. FIG. 3 shows that plate #5 had a total strain energy of 2.83 KJ/m³, which was 46% reduction in total strain energy when compared to plate #1, a 53% reduction in total strain energy when compared to plate #2, and a 34% reduction in total strain energy when compared to plate #7. Plate #6 was cold rolled 1.2% prior to being stretched by 2.5%. The total amount of cold work plate #6 was subjected to was 3.7%. As can be seen in FIG. 3, plate #6 had a total strain energy of 2.498 KJ/m³, which was 53% lower than the total strain energy of plate #1, 59% lower than the total strain energy of plate #2, and 41% lower than the total strain energy of plate #7. Plate #8 was cold rolled 1.3% prior to being stretched by 2.5%. The total amount of cold work plate #8 was subjected to was 3.8%. The total strain energy of plate #8 was 3.36 KJ/m³, which was a 36% reduction in total strain energy when compared to plate #1, a 44% reduction in total strain energy when compared to plate #2, and a 21% reduction in total strain energy when compared to plate #7. Plate #11 was cold rolled 3.4% prior to being stretched by 1%. The total amount of cold work plate #11 was subjected to was 4.4%. As can be seen in FIG. 3, plate #11 had total strain energy of 2.16 KJ/m³, which was a 59% reduction in total strain energy when compared to plate #1, a 64% reduction in total strain energy when compared to plate #2, and a 36% reduction in total strain energy when compared to plate #7.

FIG. 4 depicts the predicted reduction in lateral bow as the percentage of cold rolling is increased. Lateral bow is best described as the lateral inward movement of a side of the aluminum plate toward the center of the plate. This is synonymous to “in-plane” distortion. The end result of lateral bow is that the aluminum plate, from an overhead perspective, does not have a rectangular shape but rather the plate exhibits a concave shape. All of the plates in FIG. 4 were made from Aluminum Association designated 7055 aluminum alloy and were subjected to AMS 2772 solution heat treatment. As can be seen in FIG. 4, the predicted lateral bow should be significantly reduced when the cold rolling operation is applied to the aluminum alloy plate prior to the stretching operation

As stated above, plates #1, 2, and 7 were not cold rolled prior to being stretched by 2.75%. As can be seen in FIG. 4, it is predicted that the lateral bow in plates #1, 2, and 7 would be reduced by 1%, −5%, and 4% respectively. In contrast, plates #3-6, #8-11 were subjected to a combination of cold rolling and stretching. On average, the cold rolling operation reduced the thickness of plate #3 by approximately 3% prior to plate #3 being stretched by 1%. It is predicted that plate #3 would exhibit an 83% reduction in lateral bow when compared to plate #1, an 89% reduction when compared to plate #2, and an 80% reduction when compared to plate #7. The cold rolling operation reduced the thickness of plate #4, on average, by approximately 2.32% prior to plate #4 being stretched by 1%. As shown in FIG. 4, it is predicted that plate #4 would exhibit a 74% reduction in lateral bow when compared to plate #1, a 79% reduction when compared to plate #2, and a 71% reduction when compared to plate #7. The cold rolling operation reduced the thickness of plate #5, on average, by approximately 1.24% prior to plate #5 being stretched by 2%. It is predicted that plate #5 would exhibit a 31% reduction in lateral bow when compared to plate #1, a 38% reduction when compared to plate #2, and a 29% reduction when compared to plate #7. On average, the cold rolling operation reduced the thickness of plate #6 by approximately 1.11% prior to plate #6 being stretched by 2.5%. As can be seen in FIG. 4, it is predicted that plate #6 would exhibit a 42% reduction in lateral bow when compared to plate #1, a 48% reduction when compared to plate #2, and a 39% reduction when compared to plate #7. The cold rolling operation reduced the thickness of plate #8, on average, by approximately 1.03% prior to plate #8 being stretched by 2.5%. It is predicted that plate #8 would exhibit a 26% reduction in lateral bow when compared to plate #1, a 26% reduction when compared to plate #2, and a 23% reduction when compared to plate #7. On average, the cold rolling operation reduced the thickness of plate #9 by approximately 1.87% prior to plate #9 being stretched by 1.5%. As FIG. 4 shows, it is predicted that plate #9 would exhibit a 72% reduction in lateral bow when compared to plate #1, a 78% reduction when compared to plate #2, and a 69% reduction when compared to plate #7. The cold rolling operation reduced the thickness of plate #10, on average, by approximately 2.39% prior to plate #10 being stretched by 1%. It is predicted that plate #10 would exhibit a 74% reduction in lateral bow when compared to plate #1, an 80% reduction when compared to plate #2, and a 71% reduction when compared to plate #7. The cold rolling operation reduced the thickness of plate #11, on average, by approximately 2.95% prior plate #11 to being stretched by 1%. As can be seen in FIG. 4, it is predicted that plate #11 would exhibit a 59% reduction in lateral bow when compared to plate #1, a 65% reduction when compared to plate #2, and a 56% reduction when compared to plate #7.

	TABLE 1
	Column 1:
	Approximation	Column 2:
	of a	6 Month Avg.	Column 3:
	Commercially	(stretching	Trial Results
	Acceptable	operation	(Cold Roll +
	Standard	only)	Stretch)
LT Ult (MPa)	580-610	625.4	639.5
LT Yield (MPa)	550-580	596.4	610.0
LT Elg (%)	5-9	9.4	9.5
L Ult (MPa)	580-610	628.8	640.0
L Yield (MPa)	570-595	606.8	617.2
L Elg (%)	5-9	9.8	10.0
EXCO	EC	—	EA-EB
L Comp Yield	570-595	617.8	628.4
(MPa)
LT Comp Yield	—	—	657.3
(MPa)
T-L Klc (MPa {square root over (m)})	21-25	26.5	28.9
L-T Klc (MPa {square root over (m)})	21-25	29.9	28.4

Table 1 depicts the mechanical property summaries of aluminum plates that were cold rolled prior to stretch (column 3) and aluminum plates that were not cold rolled prior to stretch (column 2). Additionally, an approximation of a commercially acceptable standard is also listed in Table 1. “LT” stands for long transverse direction or across the width of the aluminum plate. “L” stands for longitudinal direction or down the length of the plate. “Ult” represents ultimate tensile strength, “Yield” represents tensile yield strength, and “Elg” represents percent elongation. “T-L Klc” refers to the fracture toughness of the aluminum plate when the plate is pulled in the transverse direction and the crack propagates in the longitudinal direction. “L-T Klc” refers to the fracture toughness of the plate when the plate is pulled in the longitudinal direction and the crack propagates in the transverse direction. “L Comp Yield” represents longitudinal compressive yield strength, which represents the yield strength of the plate when it is subjected to compressive forces. All of the plates in Table 1 were made from Aluminum Association designated 7055 aluminum alloy and were subjected to AMS 2772 solution heat treatment.

As can be seen in Table 1, column 2, the mechanical properties of the aluminum plates were higher than the approximated commercial standard when the plates were stretched. In the long-transverse direction, the stretched aluminum plates exhibited a 2% to 7% increase in ultimate tensile strength (LT Ult), a 3% to 8% increase in yield strength (LT Yield), and a 4% to 36% increase in elongation (LT Elg) over the approximated commercial standard found in column 1. In the longitudinal direction, the stretched aluminum plates exhibited a 3% to 8% increase in ultimate yield strength (LT Ult), a 2% to 6% increase in yield strength (LT Yield), and an 8% to 39% increase in elongation (L Elg) over the approximated commercial standard. Additionally, the stretched plates in column 2 exhibited a 4% to 8% increase in longitudinal compressive yield strength (L Comp Yield) over the approximated commercial standard. As can be seen in Table 1, the stretched plates in column 2 exhibited a 6% to 21% increase in fracture toughness over the approximated commercial standard in the “T-L” direction and a 17% to 30% increase in fracture toughness in the “L-T” direction.

When the aluminum plates were subjected to a cold rolling operation prior to stretching, the mechanical properties of the plates were further enhanced. As can be seen in Table 1, in the long-transverse direction the cold rolled and stretched plates in column 3 exhibited a 5% to 9% increase in ultimate tensile strength (LT Ult), a 5% to 10% increase in yield strength (LT Yield), and a 5% to 37% increase in elongation (LT Elg) over the approximated commercial standard found in column 1. The plates in column 3 also exhibited, in the long-transverse direction, a 2% increase in ultimate tensile strength (LT Ult), a 2% increase in yield strength (LT Yield), and a 1% increase in elongation (LT Elg) when compared to the plates in column 2 that underwent only a stretching operation. In the longitudinal direction, the plates in column 3 exhibited a 5% to 9% increase in ultimate tensile strength (L Ult), a 4% to 8% increase in yield strength (L Yield), and a 10% to 40% increase in elongation (L Elg) over the approximated commercial standard. The cold rolled and stretched plates also exhibited, in the longitudinal direction, a 2% increase in ultimate tensile strength (L Ult), a 2% increase in yield strength (L Yield), and a 2% increase in elongation (L Elg) when compared to the plates found in column 2. When compared to the approximated commercial standard found in column 1, the plates in column 3 exhibited a 5% to 9% increase in longitudinal compressive yield strength (L Comp Yield). When compared to the plates in column 2, the plates in column 3 exhibited a 6% increase in longitudinal compressive yield strength (L Comp Yield).

As can be seen in Table 1, the cold rolled and stretched plates in column 3 exhibited a 13% to 27% increase in fracture toughness over the approximated commercial standard in the “T-L” direction, a 12% to 26% increase in fracture toughness over the approximated commercial standard in the “L-T” direction, and an 8% increase in fracture toughness in the “T-L” direction when compared to the plates in column 2. FIG. 5 graphically depicts the fracture toughness of the aluminum plates that were cold rolled prior to being stretched. The fracture toughness was measured in both the “L-T” and the “T-L” orientation. As seen in FIG. 5, the cold rolling operation prior to stretch did not degrade the tensile properties of the aluminum plates. Additionally, as can be seen in FIG. 5, the overall fracture toughness of the aluminum plates that were cold rolled prior to being stretched was well above the approximated commercial standards in both the “L-T” and the “T-L” directions. FIG. 5 also shows that there was no change in toughness as the tensile yield strength increased. This result is atypical because as toughness increases, the tensile yield strength usually decreases.

In addition to tensile properties, Table 1 also shows that the plates in column 3 exhibited an EA-EB rating, which is a higher rating than the commercially accepted EC rating, in the ASTM G34 EXCO 48 hour corrosion test.

FIG. 6 graphically depicts the longitudinal yield strength of the aluminum plates as a function of total percent cold work. As can be seen in FIG. 6, the overall longitudinal yield strength of the aluminum plates that were cold rolled prior to being stretched was well above the approximated commercial standard.

FIG. 7 is a flowchart depicting one preferred embodiment of the method that is disclosed in this invention. As can be seen in FIG. 7, this invention discloses a method of producing a flat aluminum sheet product by providing an aluminum alloy stock, hot working the aluminum alloy stock through preheating and hot rolling, side sawing the aluminum alloy stock, subjecting the aluminum alloy stock to solution heat treatment, cold rolling the aluminum alloy stock to reduce its thickness by about 0.25% to about 5%, and finally stretching the aluminum alloy stock.

Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.

Claims

1. A method of producing an aluminum sheet or plate product comprising:

providing an aluminum alloy stock;

hot working said aluminum alloy stock;

side sawing said aluminum alloy stock;

solution heat treating and quenching said aluminum alloy stock;

cold rolling said aluminum alloy stock to reduce the thickness of said aluminum alloy stock by about 0.25% to about 5%; and

stretching said aluminum alloy stock.

2. A method according to claim 1 wherein:

providing an aluminum alloy stock selected from the 2XXX, 6XXX, or 7XXX series of aluminum alloys.

3. A method according to claim 1 wherein:

hot working comprises preheating and hot rolling.

4. A method according to claim 1 wherein:

solution heat treating said aluminum alloy stock from about 870° F. to about 1050° F.

5. A method according to claim 2 wherein:

solution heat truncating said 7XXX aluminum alloy stock from about 880° F. to about 900° F.

6. A method according to claim 2 wherein:

solution heat treating said 2XXX aluminum alloy stock from about 900° F. to about 930° F.

7. A method according to claim 2 wherein:

solution heat treating said 6XXX aluminum alloy stock from about 950° F. to about 1050° F.

8. A method according to claim 1 wherein:

stretching said aluminum alloy stock by about 0.5% to about 4%.

9. A method according to claim 1 wherein:

cold rolling and stretching said aluminum alloy stock by about 2% to about 6%.

10. An aluminum sheet or plate exhibiting improved flatness and mechanical strength made by a process comprising:

providing an aluminum alloy stock;

hot working said aluminum alloy stock;

side sawing said aluminum alloy stock;

solution heat treating and quenching said aluminum alloy stock;

cold rolling said aluminum alloy stock to reduce the thickness of said aluminum alloy stock by about 0.25% to about 5%; and

stretching said aluminum alloy stock.

11. An aluminum sheet or plate according to claim 10 wherein:

providing an aluminum alloy stock selected from the 2XXX, 6XXX, or 7XXX series of aluminum alloys.

12. An aluminum sheet or plate according to claim 10 wherein:

hot working comprises preheating and hot rolling.

13. An aluminum sheet or plate according to claim 10 wherein:

solution heat treating said aluminum alloy stock from about 870° F. to about 1050° F.

14. An aluminum sheet or plate according to claim 11 wherein:

solution heat treating said 7XXX aluminum alloy stock from about 880° F. to about 900° F.

15. An aluminum sheet or plate according to claim 11 wherein:

solution heat treating said 2XXX aluminum alloy stock from about 900° F. to about 930° F.

16. An aluminum sheet or plate according to claim 11 wherein:

solution heat treating said 6XXX aluminum alloy stock from about 950° F. to about 1050° F.

17. An aluminum sheet or plate according to claim 10 wherein:

stretching said aluminum alloy stock by about 0.5% to about 4%.

18. An aluminum sheet or plate according to claim 10 wherein:

cold rolling and stretching said aluminum alloy stock by about 2% to about 6%.

19. A method of producing an aluminum sheet or plate product comprising:

providing an aluminum alloy stock;

hot working said aluminum alloy stock;

side sawing said aluminum alloy stock;

solution beat treating and quenching said aluminum alloy stock;

cold rolling said aluminum alloy stock to reduce the thickness of said aluminum alloy stock by about 0.25% to about 5%; and

stretching said aluminum alloy stock by at about 0.5% to about 4%.

20. A method according to claim 19 wherein:

providing an aluminum alloy selected from the 2XXX, 6XXX, or 7XXX scrics of aluminum alloys.

21. A method according to claim 19 wherein:

hot working comprises preheating and hot rolling.

22. A method according to claim 19 wherein:

solution heat treating said aluminum alloy stock from about 870° F. to about 1050° F.

23. A method according to claim 20 wherein:

solution heat treating said 7XXX aluminum alloy stock from about 880° F. to about 900° F.

24. A method according to claim 20 wherein:

solution heat treating said 2XXX aluminum alloy stock from about 900° F. to about 930° F.

25. A method according to claim 20 wherein:

solution heat treating said 6XXX aluminum alloy stock from about 950° F. to about 1050° F.

26. A method according to claim 19 wherein:

cold rolling and stretching said aluminum alloy stock by about 2% to about 6%.