This invention relates to aluminum alloy sheet products and methods for making them. Specifically this invention relates to a new aluminum alloy which can be substituted for conventional homogenized DC cast 3003 alloy in any temper; as rolled, partially annealed or fully annealed and method of making it. An important aspect of the present invention is a new aluminum alloy suitable for use in household foil and semi rigid foil containers having a combination of strength and formability and an economical method for its manufacture using a continuous caster.
Semi rigid foil containers are manufactured from aluminum sheet rolled to a thickness of 0.002-0.010 inches. The sheet is then cut to a desired shape and formed into a self supporting container commonly used for food items such as cakes, pastries, entrees, cooked vegetables, etc. Conventional DC cast 3003 alloy is commonly used for this application. Generally the term sheet will be used herein to refer to as cast or rolled alloy having a thickness that is relatively thin compared to its width and includes the products commonly referred to as sheet, plate and foil.
The conventional method for manufacturing 3003 alloy is to direct chill (DC) cast an ingot, homogenize the ingot by heating to a temperature sufficient to cause most of the manganese to go into solid solution, cool and hold at a temperature where a significant portion of the manganese precipitates out of solution, hot roll the ingot to a predetermined intermediate gauge, cold roll to final gauge optionally with interannealing between at least some of the cold rolling passes and then annealing the cold rolled alloy sheet to the desired temper. Typical mechanical properties of 3003 alloy produced in this manner is shown in Table 1:
TABLE 1 ______________________________________ Typical Mechanical Properties of 3003 Alloy Elong. Temper UTS (Ksi) YS (Ksi) % Olsen ______________________________________ As Rolled 34.8 30.8 2 -- H26 24.6 23.3 11 0.208 H25 23.1 20.5 15 0.248 H23 22.2 18.5 18 0.251 O 15.1 7.0 20 0.268 ______________________________________
Furthermore, DC cast 3003 alloy is relatively insensitive to variations in the final annealing process allowing for reproducible properties that are consistent from coil to coil. For example, variations in the properties of DC cast 3003 annealed at various temperatures are shown in Table 2;
TABLE 2 ______________________________________ Properties of DC Cast 3003 Annealing Temp .degree.C. UTS (Ksi) YS (Ksi) Elongation % ______________________________________ As rolled 42.2 37.5 2.0 250 27.2 24.5 2.2 260 24.7 21.5 10.4 270 23.8 20.2 13.8 280 22.6 17.8 16.4 290 21.6 14.0 -- 350 16.4 7.5 22.4 ______________________________________
Because of these useful properties DC cast 3003 has found numerous uses and DC cast 3003 is a commonly used alloy. A typical composition for 3003 including maximum and minimum limits is :
Cu: 0.14 (0.05-0.20) % PA1 Fe: 0.61 ((0.7 max.) % PA1 Mn: 1.08 (1.0-1.5) % PA1 Si: 0.22 (0.6 max.) % Zn: 0.00 (0.10 max.) % Ti: 0.00 (0.10 max,) % PA1 Balance Al and incidental impurities.
This alloy belongs to the category of dispersion hardened alloys. With aluminum alloys dispersion hardening may be achieved by the addition of alloying elements that combine chemically with the aluminum or each other to form fine particles that precipitate from the matrix. These fine particles are uniformly distributed through the crystal lattice in such a way to impede the movement of dislocations causing the hardening effect. Manganese is such an alloying element. Manganese is soluble in liquid aluminum but has a very low solubility in solid aluminum. Therefore as 3003 cools down after casting dispersoids form at the expense of Mn in solution. The dispersoids are fine particles of MnAl.sub.6 and alpha manganese (Al.sub.12 Mn.sub.3 Si.sub.2). The formation of these dispersoids is a slow process and in practice more than 60% of the Mn remains in solution after DC cast 3003 ingots have solidified. During homogenization the dispersoids tend to go into solid solution until equilibrium is reached. The ingot is then cooled to a lower temperature and maintained for a prolonged period of time to form dispersoids from about 80% of the available Mn.
Continuous casting, on the other hand, can produce substantially different properties from dispersion hardening alloys because cooling rates are generally much faster than with DC casting. Continuous casting can also be more productive than DC casting because it permits the casting of a shape that is closer to common sheet dimensions which then requires less rolling to obtain the final gauge. Several continuous casting processes and machines have been developed or are in commercial use today for casting aluminum alloys specifically for rolling into sheet. These include the twin belt caster, twin roll caster, block caster, single roll caster and others. These casters are generally capable of casting a continuous sheet of aluminum alloy less than 2 inches thick and as wide as the design width of the caster. Optionally, the continuously cast alloy can be rolled to a thinner gauge immediately after casting in a continuous hot rolling process. The sheet may then coiled for easy storage and transportation. Subsequently the sheet may be hot or cold rolled to the final gauge, optionally with one or more interannealing or other heat treatment steps.