In the prior art, it is known to produce aluminum in coil form from a single or unitary roll continuous casting apparatus wherein molten aluminum is delivered from a tundish and cast in the form of a metal sheet or strip and wound onto a coiler. Generally, in this process, molten aluminum is deposited on a moving chill surface from a tundish having an open outlet. An inlet is provided for the flow of molten metal into the tundish from a source of molten metal. The direct casting of the molten aluminum metal onto a chill wheel, preferably a grooved chill wheel, produces a cast aluminum product at a rapid rate. The aluminum cast strip is wound on a coiler in heated form, generally at a temperature in the range of about 400.degree.-1000.degree. F.
Drag casting apparatus and methods of this type are described, for example, in U.S. Pat. Nos. 4,828,012, 4,896,715, 4,934,443, 4,945,974, 4,940,077 and 4,955,429. The disclosures of these patents are hereby specifically incorporated by reference with respect to the method and apparatus for the production of aluminum strip and coil from molten aluminum or aluminum alloys.
In the manufacture of aluminum foils, typically, an AA8111 aluminum alloy is twin roll cast, cold rolled, homogenized, cold rolled and final annealed to produce a aluminum foil product. These alloys contain about 0.56 wt. % silicon and 0.67 wt. % iron and are rolled to final gauges ranging between 0.00066 inches and 0.0009 inches. As is well known in the art, these types of alloys are usually formed by adding silicon and iron to high impurity aluminum. For purposes of aluminum foil manufacture, the starting aluminum material has an impurity content of copper which is essentially zero, for example, 0.004 weight percent copper or less.
With regard to impurity levels in aluminum alloys, it is also well known that the Aluminum Association designations for chemical composition limits for wrought aluminum and wrought aluminum alloys identifies impurity limits in terms of a maximum tolerable amount (i.e. limits) for a specific element. These limits reflect a judgement of what can be tolerated for a particular alloy while still providing an acceptable alloy. However, these Aluminum Association designations do not set forth the quantities of impurities actually present or expected to be present in an alloy.
Generally, aluminum foils are designated as rolled products less than 0.006 inches in thickness and having a rectangular cross section. However, attempts at single roll or drag casting of AA8111 alloys have,been unsuccessful. When the AA8111 alloy is drag cast and rolled to foil gauge, an unacceptable level of pinholes are present and the foil is stiff. Even using higher homogenization temperatures and obtaining recrystallization during the final anneal, unacceptable levels of pinholes are still present in the AA8111 drag cast foil. In view of the inherent advantages of drag casting over twin roll casting, e.g., increased casting speed, increased productivity and a decrease in required cast strip thickness cold reduction, a need has developed to provide a method and alloy composition capable of being single roll or drag cast and further reduced into an acceptable aluminum foil product.
In response to this need, the present invention provides a method of making an aluminum foil product and a cast strip stock for foil making which utilizes an aluminum-based alloy composition which, when single roll cast, provides an acceptable product for aluminum foil making. The inventive method and product utilize an aluminum-based alloy composition containing critical amounts of iron, silicon and copper which provides a narrow freezing band range during single roll casting and produces a cast strip having a minimum of microshrinkage porosity and a foil having minimal pinholes resulting in an acceptable quality single roll cast strip and aluminum foil product produced therefrom.