Recrystallized 6XXX Aluminum Alloy with Improved Strength and Formability

Abstract

A substantially recrystallized extrusion may include from about 0.7 to about 1.3 wt % silicon, up to about 0.50 wt % iron, from about 0.03 to about 0.2 wt % copper, up to about 0.5 wt % manganese, from about 0.6 to about 1.2 wt % magnesium, up to about 0.05 wt % chromium, up to about 0.2 wt % zinc, up to about 0.10 wt % titanium and the balance consisting essentially of aluminum and incidental elements and impurities. The disclosed alloy may be homogenized at a temperature above 1030° F. (554.4 ° C.) and have a recrystallized grain structure. Extrusions made from the disclosed alloys and having a wall thickness ranging from about 0.050 to about 0.500 inches (1.5-˜12.5 mm) provides a tensile yield strength of greater than 42 ksi (290 MPa).

Description

BACKGROUND

1. Technical Field

This disclosure relates to 6XXX aluminum alloys. More specifically, this disclosure relates to 6XXX aluminum alloys with improved strength and formability properties despite having a recrystallized grain structure.

2. Description of the Related Art

A significant economic factor in operating a vehicle is the cost of fuel. As a consequence, vehicle designers and manufacturers are constantly striving to improve overall fuel efficiency. One way to increase fuel efficiency is to reduce the structural weight of the vehicle. Because steel alloys are used for many structural components of most vehicles, significant efforts have been expended to develop aluminum alloys with sufficient strength to replace steel alloys in current use, while maintaining the same or higher tensile yield strengths (TYS) and ultimate tensile strengths (UTS).

6XXX series are high strength aluminum alloys that can be strengthened by heat treatment (precipitation hardening), through the presence of their main alloying elements silicon and magnesium. Typically, silicon (Si) and magnesium (Mg) are present in the range of from about 0.3 to about 1.5 wt % as the major alloying elements. Copper (Cu), manganese (Mn), chromium (Cr), zinc (Zn), boron (B), lead (Pb) and bismuth (Bi) may be added to the alloys of 6xxx series as minor alloying elements. 6XXX alloys are generally less strong than the 2xxx and 7xxx series aluminum alloys, but 6XXX alloys have good formability and are weldable, which makes them attractive for use in various types of vehicles. 6XXX alloys also have excellent corrosion resistance.

Extruded 6XXX series alloys are also often used for machined products. By adding low melting phase elements such as lead, bismuth and/or tin, 6XXX series alloys have been shown to exhibit good machinability. 6XXX alloys can be easily anodized when a hard surface and improved corrosion resistance are required.

Chromium is included in 6XXX alloys in amounts generally less than 0.35% to control grain structure; chromium prevents recrystallization during hot working or heat treatment. The fibrous structures that develop increase strength and reduce stress corrosion susceptibility and/or improve toughness in certain orientations. Chromium in solid solution or as fine dispersoids increases strength slightly. The disadvantage of chromium in heat-treatable 6XXX alloys is an increase in quench sensitivity as the Mg₂Si tends to precipitate on pre-existing chromium-phase precipitates.

The microstructure of an alloy is important to obtaining the desired strength and hardness properties. Further, it is also desirable to extrude the alloy as quickly as possible for higher productivity. However, extruding an alloy too quickly can compromise the strength and hardness properties. Specifically, extruding an aluminum alloy too quickly can result in recrystallization. Recrystallization is a process by which deformed grains are replaced by a new set of undeformed grains that nucleate and grow until the original grains have been entirely consumed. Recrystallization is usually accompanied by a reduction in the strength and hardness of a material and a simultaneous increase in the ductility. Thus, if ductility is a primary concern, recrystallization may be introduced as a deliberate step in metals processing, but recrystallization may also be an undesirable byproduct of a processing step, such as extruding. In contrast, unrecrystallized grain structures provide texture strengthening where the formed alloy is stronger in the extrusion direction and weaker transverse to the extrusion direction.

6082 alloys are medium strength aluminum alloys with excellent corrosion resistance. 6082 alloys have some of the highest strengths of the 6000 series alloys. 6082 alloys are also known as structural alloys. The higher strength of 6082 alloys have resulted in 6082 alloys replacing 6061 alloys in many applications. The addition of a large amount of manganese in excess of 0.5% controls the grain structure which in turn results in a stronger alloy.

Compositions for 6082 alloys typically fall within the ranges of Table 1.

TABLE 1
Element % Present
Si      0.7-1.3
Fe      0-0.5
Cu      0-0.10
Mn      0.4-1.0
Mg      0.6-1.2
Cr      0-0.25
Zn      0-0.2
Ti      0-0.1
Al      Balance
Melting point temperature ranges from about 1070 to about 1200° F. (577-649° C.); density ~0.098 lb/in3 (27.1 g/cm3).

Many vehicle manufacturers are specifying 6XXX alloys having strength properties above the requirements of the Aluminum Association. The 6082 alloy has significant amounts of manganese and chromium, which are used to produce an unrecrystallized grain structure that provides strength properties that are acceptable for vehicles and other commercial transportation parts/components. However, the ability to achieve an unrecrystallized grain structure with thinner extrusions (i.e., <12.7 mm (<0.50 in.)) requires a very slow extrusion process and therefore results in significant losses in extrusion productivity.

Thus, many of the vehicle applications require good formability for producing the components with higher strengths. An unrecrystallized grain structure does not provide the formability required by many vehicle applications. In fact, it is not unusual for an unrecrystallized grain structure to be accompanied with coarse grains at the surface of the wrought product. In these cases the formability is further compromised by the grain structure.

As a result, there is a need for 6XXX alloys, and specifically 6082 alloys, that meet the strength requirements imposed by various vehicle manufacturers but which also can be extruded quickly enough to keep manufacturing costs low.

SUMMARY OF THE DISCLOSURE

In one aspect, an aluminum alloy is disclosed which may include from about 0.7 to about 1.3 wt % silicon, up to about 0.50 wt % iron, from about 0.03 to about 0.2 wt % copper, up to about 0.5 wt % manganese, from about 0.6 to about 1.2 wt % magnesium, up to about 0.05 wt % chromium, up to about 0.2 wt % zinc, up to about 0.10 wt % titanium, and the balance consisting essentially of aluminum and incidental elements and impurities.

In another aspect, an extrusion has been disclosed that has a recrystallized grain structure. The extrusion may include from about 0.7 to about 1.3 wt % silicon, up to about 0.50 wt % iron, from about 0.03 to about 0.02 wt % copper, up to about 0.5 wt % manganese, from about 0.6 to about 1.2 wt % magnesium, up to about 0.05 wt % chromium, up to about 0.2 wt % zinc, up to about 0.10 wt % titanium, and the balance consisting essentially of aluminum and incidental elements and impurities.

In another aspect, a method of forming an extrusion having a recrystallized grain structure is disclosed. The extrusion may have a thickness ranging from about 0.125 to about 0.50 inches (0.3175 to about 1.27 cm). The method may include providing a billet from an alloy consisting essentially of: from about 0.7 to about 1.3 wt % silicon, up to about 0.50 wt % iron, from about 0.03 to about 0.2 wt % copper, up to about 0.5 wt % manganese, from about 0.6 to about 1.2 wt % magnesium, up to about 0.05 wt % chromium, up to about 0.2 wt % zinc, up to about 0.10 wt % titanium, and the balance consisting essentially of aluminum and incidental elements and impurities. The method may further include extruding the billet at a temperature exceeding 800° F. (426.7° C.) at an extrusion speed exceeding 40 fpm (12.1 m/min).

In any one or more of the embodiments described above, chromium may be present in an amount less than 0.02 wt %.

In any one or more of the embodiments described above, manganese may be present in an amount less than 0.5 wt %.

In any one or more of the embodiments described above, the alloy may be a 6082 alloy.

In any one or more of the embodiments described above, the tensile yield strength (TYS) may be at least 42 ksi (290 MPa).

In any one or more of the embodiments described above, the ultimate tensile strength (UTS) may be at least 45 ksi (310 MPa).

In any one or more of the embodiments described above, the TYS and UTS may be at least 42 ksi (290 MPa) and at least 45 ksi (310 MPa) respectively.

In any one or more of the embodiments described above, the extrusion speed may be at least 40 fpm (12.1 m/min) at a temperature of at least 800° F. (426.7° C.), more preferably equal to or greater than 70 fpm (21.3 m/min), more preferably greater than 75 fpm (22.9 m/min).

In any one or more of the embodiments described above, the extrusion may have a thickness ranging from about 0.05 inches (1.27 mm) to about 0.500 inches (12.7 mm)

In any one or more of the embodiments described above, the extrusion may be a 6082 aluminum alloy. Alternatively, an increased copper content and/or a decreased manganese content will result in an alloy that does not fall under the 6082 classification.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a photograph illustrating the unrecrystallized grain structure in a tube having an outer diameter of 2.00 inches (5.08 cm) and a wall thickness of 0.150 inches (0.381 cm) that were extruded from a billet having a length of about 21.5 inches (54.61 cm), an extrusion length of about 145 feet (44.2 m), a front trim of about 12 feet (3.658 m), a rear trim of about 18 feet (5.486 m), a billet temperature of about 920° F. (493.3° C.) an extrusion speed of about 30 fpm (9.14 m/min)

FIG. 2 is a photograph of a tube having the same dimensions of the tube shown in FIG. 1 but which was extruded from a billet formed from a disclosed alloy, with a recrystallized grain structure and that was extruded at a speed of about 75 fpm (22.86 m/min).

FIG. 3 is a photograph of a tube having the same dimensions of the tube shown in FIG. 1 but which was extruded from a billet formed from a disclosed alloy, with a recrystallized grain structure and that was extruded at a speed of about 100 fpm (30.48 m/min).

FIG. 4 is a photograph of a tube having the same dimensions of the tube shown in FIG. 1 but which was extruded from a billet formed from a disclosed alloy, with a recrystallized grain structure and that was extruded at a speed of about 90 fpm (27.43 m/min).

DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EMBODIMENTS

For the description of preferred alloy compositions that follows, all percentage references are to weight percent (wt %) unless otherwise indicated. All temper and alloy designations used herein are generally described in the Aluminum Association Standards and Data book, the pertinent disclosures of which are incorporated by reference herein.

The term “ksi” means kilopounds per square inch.

By “substantially recrystallized”, it is meant that the disclosed extruded products are preferably 85 to 100% recrystallized, or at least 60% of the entire thickness of the extruded products are recrystallized.

The terms “6XXX”, “6000 series” and 6082, when referring to aluminum alloys, means those structural aluminum alloys with silicon and magnesium as the major alloying elements present in the greatest weight percent as defined by the Aluminum Association.

When referring to any numerical range of values disclosed herein, such ranges are understood to include each and every number, decimal and/or fraction between and including the stated range minimums and maximums. A range of about 0.03 to 0.08 wt. % copper, for example, would expressly include all intermediate values of about 0.030, 0.031, 0.032, 0.033 . . . wt. %, and so on, all the way up to and including 0.071, 0.072 . . . 0.080 wt. % Cu. The same applies to all other elemental ranges, property values (including strength levels) and/or processing conditions (including aging temperatures) set forth herein.

The term “substantially-free” means having no significant amount of that component purposefully added to the composition to import a certain characteristic to that alloy, it being understood that trace amounts of incidental elements and/or impurities may sometimes find their way into a desired end product.

The expression “consisting essentially of” is meant to allow for adding further elements that may even enhance the performance of the invention so long as such additions do not cause the resultant alloy to materially depart from the invention and its minimum properties as described herein and so long as such additions do not embrace prior art.

The high ultimate tensile strength (UTS) and high tensile yield strength (TYS), weldability and adequate corrosion resistance properties of the disclosed alloys are dependent upon a chemical composition as set forth below, upon a controlled heat treatment, and for extrusion products, upon a microstructure that is substantially recrystallized.

The disclosed aluminum alloys may include from about 0.7 to about 1.3 wt % Si, up to about 0.5 Mn, from about 0.6 to about 1.2 Mg, up to about 0.05 Cr, the balance substantially aluminum, minor alloying elements, incidental elements and impurities. The trace and impurity elements may be included as illustrated in Table 2 below.

TABLE 2
Element % Present
Si      0.7-1.3
Fe      0-0.5
Cu      0.03-0.2
Mn      0.0-0.5
Mg      0.6-1.2
Cr      0.0-0.05
Zn      0-0.2
Ti      0-0.1
Al      Balance

The example extrusions presented in the figures and tables below are tubes. However, the reader will note that this disclosure applies to other extrudable structures having a thickness or a wall thickness of about 0.5 inches (12.7 mm) or less.

FIG. 1 is a photograph of a tube having an outer diameter of 2.00 inches (5.08 cm) and a wall thickness of 0.150 inches (0.381 cm) that was extruded from a billet having a length of about 21.5 inches (54.61 cm), an extrusion length of about 145 feet (44.2 m), a front trim of about 12 feet (3.658 m), a rear trim of about 18 feet (5.486 m) and which was extruded at a billet temperature of about 920° F. (493.3° C.), but at an extrusion speed of only about 30 fpm (9.14 m/min) After quenching, the extrusion was stretched to a level of about 1 to about 1.5%. The tube shown in FIG. 1 demonstrates no “orange peel” appearance after stretching, which indicates that the billet underwent substantially no recrystallization during the extrusion process and, accordingly, the tube of FIG. 1 represents an unrecrystallized grain structure. The problem with the alloy of FIG. 1 is that, to maintain the unrecrystallized grain structure, the extrusion speed must be limited to a speed of about 30 fpm or less (<9.14 m/min). An extrusion speed of only about 30 fpm (9.14 m/min) is unsatisfactory for many manufacturers because it is too slow and therefore too costly.

In contrast, the tubes shown in FIGS. 2-4 were extruded at speeds exceeding 70 fpm (21.3 m/min), more specifically, 75 fpm (22.86 m/min; FIG. 2), 74 fpm (22.56 m/min; FIGS. 3) and 76 fpm (22.86 m/min; FIG. 4). Table 3 shows the billet temperatures as well as the front and rear extrusion speeds for the tubes shown in FIGS. 2-4.

TABLE 3
Sample ID	Billet Temperature	Front Speed (fpm)	Rear Speed (fpm)
FIG. 2	892	75	78-86
FIG. 3	872	74	90-105
FIG. 4	890	76	86-91

After quenching, the extrusions were minimally stretched to a straight form. Measurements showed the stretch was approximately 0.5%. In FIGS. 2-4, each sample shows a resulting grain structure that is fine grain recrystallized. Further, the tube of FIG. 3, which was extruded at a speed of about 100 fpm (30.48 m/min) shows the smallest grain size. Further, the tube of FIG. 4 shows a grain structure that is closer to that shown in FIG. 3 than the tube of FIG. 2. The tube of FIG. 4 was extruded at a speed of about 90 fpm (27.43 m/min). The tube of FIG. 2 was extruded at a slower speed of about 75 fpm (22/86 m/min)

The tensile strength results are shown below. All tensile testing is performed on a full cross section of the tube sample that were approximately 19 inches (approximately 48.26 cm) in length. The T4 tensile properties are listed in Table 4 and the T6 tensile properties are listed in Table 5. While the T4 properties of Table 4 were measured on tubes that were held at 100° F. (37.78° C.) for about 5 days, one skilled in the art will realize that aging practices can vary widely while still providing similar to identical results. As shown in Table 4, the results for all tubes were relatively consistent. The T6 properties in Table 5 were measured on tubes that were aged approximately 18 hours at a temperature of about 320° F. (160° C.), or “artificially aged”. Again, the aging practice can vary widely and the durations and temperatures listed here are but mere examples.

TABLE 4
T4 Tensile Reuslts
Sample ID	Location	UTS (ksi)	TYS (ksi)	% elong.
FIG. 2	Middle	37.6	21.1	31
	Rear	37.8	22.5	32
FIG. 3	Middle	38.2	21.5	30
	Rear	38.0	22.3	30
FIG. 4	Front	36.0	21.0	37
	Middle	38.0	22.3	30
	Rear	37.9	22.2	32
	AVG.*		21.8

TABLE 5
T6 Tensile Results
Sample ID	Location	UTS (ksi)	TYS (ksi)	% elong.
FIG. 2	Middle	47.5	46.3	20
	Rear	47.7	46.5	20
FIG. 3	Middle	47.9	46.7	20
	Rear	48.4	47.2	19
FIG. 4	Front	47.3	45.9	19
	Middle	47.7	45.6	21
	Rear	47.7	46.8	20
	AVG.*	47.7	46.4
*Includes samples not shown in FIGS. 2-4.

The grain structure illustrated in FIGS. 2-4 is clearly fine grain recrystallized and is anticipated to achieve superior forming performance. Further, the tubes shown in FIG. 2-4 are superior in strength to counterpart tubes made to be unrecrystallized.

Accordingly, a fine grain recrystallized structure as shown in FIGS. 2-4 for tubes extruded from a billet formed from an alloy as described in Table 2 and tempered at T6 conditions produces a high yield strength even though the extrusion process results in a fine grain recrystallized structure. As a result, high productivity is possible using the 6XXX alloy described above. Accordingly, applicants surprisingly found that a 6XXX alloy extruded at speeds in excess of 40 fpm (12.1 m/min) and more preferably extruded that speeds in excess of 70 fpm (21.3 m/min) can produce tubes having a wall thickness of 0.150 inches (0.381 cm) or smaller but with a UTS and a TYS in excess of 42 ksi (290 MPa) or, more specifically, a UTS of greater than 45 ksi (310 MPa) and a TYS of greater than 42 ksi (290 MPa).

Without being bound to theory, it is believed that controlling the chromium levels to blow about 0.05 wt % and controlling the manganese levels to blow about 0.5 wt %, and preferably between about 0.3 and about 0.5 wt %, in combination with purposely adding copper in a range of from about 0.03 to about 0.2 wt % provides the superior tensile results shown above in Table 5. Further, homogenizing the ingot at higher temperatures, in excess of 1030° F. (554.4° C.) helps to coarsen the manganese dispersoids, which helps promote fine grain recrystallization. As noted above, it was previously thought that fine grain recrystallization was disadvantageous and provided an extruded structure of inferior strength. Thus, applicants have surprisingly found that extruding thin wall structures at high speeds, using the chemistry described above in Table 2, produces tubes of improved formability and higher strengths for demanding applications such as structural components in vehicles. The extrusion speeds disclosed herein are approximately twice that of typical or standard 6082 alloys.

While only certain embodiments have been set forth, alternatives and modifications will be apparent from the above description to those skilled in the art. These and other alternatives are considered equivalents and within the spirit and scope of this disclosure and the appended claims.

Claims

1. An aluminum alloy comprising:

from about 0.7 to about 1.3 wt % silicon,

up to about 0.50 wt % iron,

from about 0.03 to about 0.2 wt % copper,

up to about 0.5 wt % manganese,

from about 0.6 to about 1.2 wt % magnesium,

up to about 0.05 wt % chromium,

up to about 0.2 wt % zinc,

up to about 0.10 wt % titanium, and

the balance consisting essentially of aluminum and incidental elements and impurities.

2. The alloy of claim 1 wherein chromium is present in an amount less than 0.02 wt %.

3. The alloy of claim 1 wherein manganese is present in an amount less than 0.5 wt %.

4. The alloy of claim 1 wherein the alloy is a 6082 alloy.

5. An extrusion that has a recrystallized grain structure, the extrusion comprising:

from about 0.7 to about 1.3 wt % silicon,

up to about 0.50 wt % iron,

from about 0.03 to about 0.2 wt % copper,

up to about 0.5 wt % manganese,

from about 0.6 to about 1.2 wt % magnesium,

up to about 0.05 wt % chromium,

up to about 0.2 wt % zinc,

up to about 0.10 wt % titanium, and

the balance consisting essentially of aluminum and incidental elements and impurities.

6. The extrusion of claim 5 wherein chromium is present in an amount less than 0.02 wt %.

7. The extrusion of claim 5 wherein manganese is present in an amount less than 0.5 wt %.

8. The extrusion of claim 5 wherein the extrusion has a tensile yield strength (TYS) of at least at least 42 ksi (290 MPa).

9. The extrusion of claim 5 wherein the extrusion has an ultimate tensile strength (UTS) of at least at least 45 ksi (310 MPa).

10. The extrusion of claim 5 wherein the extrusion has a tensile yield strength (TYS) of at least at least 42 ksi (290 MPa) and an ultimate tensile strength (UTS) of at least at least 45 ksi (310 MPa).

11. The extrusion of claim 5 wherein the extrusion was extruded at an extrusion speed of at least 40 fpm (12.1 m/min) and with a billet temperature of at least 800° F. (426.7° C.).

12. The extrusion of claim 5 wherein the extrusion was extruded at an extrusion speed of at least 70 fpm (21.3 m/min) and with a billet temperature of at least 800° F. (426.7° C.).

13. The extrusion of claim 5 wherein the extrusion has a thickness ranging from about 0.050 inch to about 0.500 inch (˜1.5-˜12.7 mm)

14. The extrusion of claim 5 wherein the extrusion is a 6XXX aluminum alloy.

15. A method of forming an extrusion having a recrystallized grain structure and a wall thickness ranging from about0.05 to about 0.500 inch (˜1.5-˜12.7 mm), the method comprising:

providing a billet from an alloy consisting essentially of: from about 0.7 to about 1.3 wt % silicon, up to about 0.50 wt % iron, from about 0.03 to about 0.2 wt % copper, up to about 0.5 wt % manganese, from about 0.6 to about 1.2 wt % magnesium, up to about 0.05 wt % chromium, up to about 0.2 wt % zinc, up to about 0.10 wt % titanium, and the balance consisting essentially of aluminum and incidental elements and impurities;

extruding the billet at a temperature exceeding 800° F. (426.7° C.) and at an extrusion speed exceeding 40 fpm (12.1 m/min)

16. The method of claim 15 wherein chromium is present in an amount less than 0.02 wt %.

17. The method of claim 15 wherein manganese is present in an amount less than 0.5 wt %.

18. The method of claim 15 wherein the tube was extruded at an extrusion speed of at least 70 fpm (21.3 m/min) and at a temperature of at least 800° F. (426.7° C.).

19. The method of claim 15 wherein the tube was extruded at an extrusion speed of at least 75 fpm (22.86 m/min) and with a billet temperature of at least 875° F. (468.3° C.).

20. The method of claim 15 wherein the billet is a 6XXX aluminum alloy.