Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C21/00—Alloys based on aluminium
  • C22C21/12—Alloys based on aluminium with copper as the next major constituent
  • C22C21/16—Alloys based on aluminium with copper as the next major constituent with magnesium
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22F—CHANGING THE PHYSICAL STRUCTURE OF NON-FERROUS METALS AND NON-FERROUS ALLOYS
  • C22F1/00—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working
  • C22F1/04—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon
  • C22F1/05—Changing the physical structure of non-ferrous metals or alloys by heat treatment or by hot or cold working of aluminium or alloys based thereon of alloys of the Al-Si-Mg type, i.e. containing silicon and magnesium in approximately equal proportions

Definitions

• This invention relates to ends or lids for aluminum alloy beverage cans and the like, and to methods of making these can ends, as well as to the production of can end stock from which such ends are made.
• a typical present-day commercial beverage can has a thin-walled, one-piece, drawn and ironed aluminum alloy can body, and a can end or lid secured around its rim to the free upper edge of the body.
• the lid is formed from aluminum alloy sheet of suitable composition, microstruc­ture, properties and gauge (such sheet being hereinafter sometimes termed "can end stock") by drawing and stretching.
• AA 5182 Aligninum Association designation
• H28 temper H28 temper at a gauge of, e.g. 0.29 mm.
• This stock is lacquered, and the lacquer is cured with heat before being formed into can ends.
• the lacquered stock has acceptable strength and formability for such use.
• the present invention in a first aspect, broadly con­templates the provision of a method of making aluminum alloy can ends, comprising the successive steps of pro­viding a progressively cast, hot-rollable ingot of an alloy consisting essentially of 0.25-0.60% Cu, 1.5-2.4% Mg, 0.15-0.50% Mn, 0.15-0.40% Si, up to 0.29% Fe, up to 0.20% Cr, other elements up to 0.05% each, up to 0.15% total, balance Al; hot rolling the ingot to produce hot-rolled strip; cold rolling the strip to an intermediate gauge from which a reduction of at least about 20% is required to achieve a predetermined final gauge; solution-heat-treating the strip by successively heating and quenching it; there­after, without any intervening heat treatment, cold rolling the strip to final gauge; artificially aging the strip by heating it for increasing its yield strength and formabi­lity (as represnted by % elongation); and forming portions of the strip into can ends, e.g.
• strip refers to a slab or sheet article or workpiece of indeterminate length advanced longitudinally in rolling operations, and typically coiled for storage and/or batch-type thermal treatments. All composition proportions and percentages herein are given by weight.
• the ingot-­providing step may comprise direct-chill-casting an ingot of the alloy and homogenizing the ingot to remove as-cast Mg2Si constituents.
• this step may be performed by casting a strip ingot of the alloy between chilled moving surfaces (e.g. twin belt casting); in such case, hot rolling is performed directly on the as-cast ingot without intervening homogenization.
• the strip is naturally aged after quenching and before the final cold rolling step.
• the artificial aging step may be performed simply as the heating operation for curing lacquer, i.e. after lacquer is applied to the final-gauge cold rolled strip; preferably, however, it includes a heating operation prior to lacquering. Also preferably, in that operation the strip is heated to a predetermined temperature for a period of time shorter than that required to achieve maximum yield strength attainable by artificial aging of the strip at such temperature, and the heating is terminated at the end of that time period.
• composition features include an upper limit of 2.1% Mg (most preferably an Mg content of 1.9-2.0%), a lower limit of 0.20% Si (especially in the case of direct­chill-cast ingot), a ratio of % Fe to % Si greater than one, and, in many though not all instances, an alloy essentially free of Cr.
• One particularly preferred compo­sition is an alloy consisting essentially of about 0.40% Cu, about 1.9% Mg, about 0.40% Mn, about 0.22% Si, about 0.23% Fe, balance Al.
• the invention contemplates the provision of aluminum alloy can ends formed of an alloy consisting essentially of 0.25-0.60% Cu, 1.5-2.4% Mg, 0.15-0.50% Mn, 0.15-0.40% Si, up to 0.29% Fe, up to 0.20% Cr, other elements up to 0.05% each, up to 0.15% total, balance Al, and preferably or advantageously produced by the foregoing method.
• the invention contemplates methods of making can end stock, and can end stock produced by such methods.
• the broad method of making end stock in accord­ance with the invention corresponds to the above-described broad method of making can ends, up to and including the artificial aging step (but not necessarily including the lacquering operation).
• composition and processing features cooperatively provide sheet stock characterized by a superior combination of properties of strength and form­ability for the production of can ends, and, upon forming, achieve highly satisfactory can end products, with benefi­cially low processing costs.
• the alloy composition shares the advantages of alloys such as AA 6061 with respect to high strength (afforded by precipitation of Mg2Si) and corrosion resistance after aging, but offers significantly improved post-aging formability as compared to AA 6061, principally (as at present believed) because its excess Mg and Cu additions, together with processing features, improve post-aging work-hardening ability by providing for solute hardening after the aging treatment.
• the low (0.29%, and preferably 0.25%) upper limit of Fe in the alloy is the low (0.29%, and preferably 0.25%) upper limit of Fe in the alloy.
• coarse intermetallic particles which can cause "voiding" in the sheet (resulting in tearing) during the can-end-forming operation.
• the invention will be described as embodied in a method of making aluminum alloy can ends for beverage cans having one-piece drawn-and-ironed bodies, and in can ends produced by this method.
• the method in the des­cribed embodiment includes the production of can end stock (sheet in final gauge for forming can ends) by successively casting, hot rolling, and cold rolling an alloy of speci­fied composition, with intervening and final thermal treat­ments, and forming the final-gauge stock into can ends.
• the final gauge of the can end stock is typically about 0.33 mm or less, e.g. (in accordance with current preference) about 0.28 mm.
• the hot rolling, cold rolling and thermal treatment steps of the present method conform generally to the corresponding steps of the method des­cribed in Jeffery et al., U.S. Patent 4,637,842, issued January 20, 1987.
• the present invention utilizes an alloy of composition different from that specified in U.S. Patent 4,637,842.
• the end-forming step or steps of the present method are conventional.
• the method of the invention employs an aluminum alloy consisting essentially of the following elements, within the specified ranges or maxima:
• the present alloy provides for solute hardening after the aging step (which follows cold rolling to final gauge, as explained below) by including excess Mg (i.e. above the stoichiometric amount for formation of Mg2Si) and additions of Cu. This feature aids in imparting to the can end stock the post-­aging formability required for the manufacture of beverage can ends.
• alloys containing excess Mg have not attracted much attention, because the solubility of Mg2Si decreases as a function of excess Mg.
• there exists an approximate "window" of compositions bordered by about 1.5-2.4% Mg, about 0.15-0.4% Si, and about 0.25-0.60% Cu which will allow for the dissolution of the Mg2Si phase during homogenization and subsequent solution heat treatment and which provides both sufficient age-hardening response to achieve requisite strength at final gauge, and excess solutes affording increased work-­hardening ability after thermal aging.
• the effective solubility of Mg2Si is not controlled solely by alloy chemistry but is also affected by process variables such as solidification rate during casting and cooling rate after solution heat treatment. Rapidly cooled continuously cast alloy strip, such as strip produced from twin-belt-­cast ingots (further discussed below), owing to the nature of the solidification process, offers in effect a larger "window" of alloy compositions.
• an "excess Mg" type of magnesium silicide alloy in the present invention provides an improved work-hardening coefficient n , which contributes very significantly to the added strength of the final can end by virtue of the work hardening that develops during the actual forming (drawing and stretching) of the can end.
• a preferred maximum for Mg in the alloy compositions used in the practice of the invention is 2.1% and the most preferred range for Mg is 1.9-2.0%.
• the preferred range for Si is 0.2-0.4%, but in the case of strip produced from twin-belt-cast ingot, good results are obtained with alloys containing as little as 0.15% Si.
• a currently most preferred value for Si is about 0.22%.
• Cu in the alloys employed in the practice of the invention, has the dual function of providing some solid solution hardening and speeding up the kinetics of the precipitation of Mg2Si.
• Mn helps to avoid the formation of large Fe intermetallics and provides grain size control in the wrought product.
• the starting ingot for the present method is direct chill cast
• the required long-time, high-temperature homogenizing treatment coupled with the hot rolling condi­tions sometimes causes an unfavorable Mn morphology, and in such cases a Cr addition is helpful in controlling the grain size during solution heat treatment.
• Cr is not needed if the starting ingot is tin-belt cast, because there is no homogenization step and the Mn, being held in solution, can be used alone for grain size control.
• Cr is not a desired addition because of a potential detrimental effect in the remelt/casting operations.
• the Fe maximum for the present invention is set at 0.29%, with a preferred maximum of 0.25%.
• can body stock sheet from which is drawn and ironed can bodies are produced
• a rela­tively high volume percent of fairly coarse (e.g. about 2 micron size) intermetallic particles in order to prevent scoring during the ironing operation employed in forming the can body.
• coarse intermetallics may tend to cause voiding as the metal is formed, the body stock is restricted (in the ironing operation) between die walls so that the metal is effectively pushed back together whenever any voiding starts.
• Can end stock is not thus restricted during the end-forming operation; hence voiding caused by coarse intermetallics will result in tearing. For this reason, presence of such coarse intermetallics (as would be produced by Fe levels conventional for can body stock) would render can end stock unacceptable in formability, and it is therefore critical to maintain a low Fe content in end stock.
• a currently most preferred composition for the practice of the invention consists essentially of about 0.40% Cu, about 1.9% Mg, about 0.40% Mn, about 0.22% Si, about 0.23% Fe, balance essentially Al.
• a progressively cast ingot of the above-described alloy composition is pro­vided in hot-rollable condition.
• the alloy may be cast by a conventional direct chill casting tech­nique to produce a sheet ingot (shaped for convenient production of strip of sheet gauge by successive hot and cold rolling operations), and homogenized (typically at a temperature of 540-580°C) for a sufficient period of time to substantially remove as-cast Mg2Si constituents.
• the ingot may be a strip ingot produced by continuous strip casting.
• Continuous strip casting is performed by supplying molten metal to a cavity defined between chilled, moving casting surfaces such as substan­tially parallel, extended runs of a pair of chilled endless metal belts, thereby to produce a thin (typically less than 25 mm thick) continuous cast strip.
• Belt-casting apparatus for such casting of strip is described, for example, in U.S. Patents Nos. 4,061,177 and 4,061,178.
• Each of the belt runs defining the casting space or cavity, in this apparatus has a surface facing away from the casting space; chilling is effected by direct impingement of coolant on the last-mentioned surfaces of the belt runs.
• the as-cast ingot has a very fine distribution of Mg2Si, which is suitable for hot rolling, and may subsequently be readily taken into solution during the solution heat treatment step.
• the strip is naturally aged by being maintained at ambient (room) temperature for at least about a day.
• the cold rolled sheet in final gauge is lacquered, and then subjected to a generally conventional lacquer-curing (heating) operation, before being formed into can ends.
• the heating of the strip for lacquer curing may itself constitute the artificial aging step.
• the artificial aging step includes a separate heat treatment prior to application of the lacquer coat. This treatment is performed by heating the strip to a relatively low predetermined temperature (e.g. 160°C) for a time period (e.g. 3 hours) shorter than that required to achieve the maximum yield strength attainable by artificially aging the same final-gauge strip at that temperature; and the heating is terminated at the end of such time period.
• a relatively low predetermined temperature e.g. 160°C
• a time period e.g. 3 hours
• the final-gauge cold rolled strip when the final-gauge cold rolled strip is sub­jected to heating for artificial aging, its % elongation value initially increases to a maximum and then, with continued heating, progressively declines, so that a pro­longed aging treatment may result in a value of % elonga­tion lower than that at the start of heating.
• the yield strength also undergoes an initial increase during aging, and typically reaches a peak value somewhat later than the time at which peak % elongation is attained.
• the heating of the strip for arti­ficial aging in the method of the present invention is terminated at a time at which both the yield strength and the % elongation are above their respective values at the start of heating. Suitable illustrative procedures for such an artificial aging step (though, as stated, with different alloy compositions) are described in United States Patent 4,637,842.
• a series of alloy compositions (respectively designated BRB, BRC, BRD, BRE and BRF) were prepared and cast into ingots under laboratory conditions. Strip of can end stock gauge was produced from these ingots in accordance with the above-described fabrication procedure. After lacquer curing, the five alloy strips, as well as a sample of commercially produced AA5182-H28 can end stock, were tested for mechanical properties (0.2% yield strength, ultimate tensile strength, percent elongation, and work hardening coefficient n ).
• Yield strength was measured at 45° to the rolling direction, as well as in the longi­tudinal (L) and transverse (T) directions, since the buckle pressure (an important property of can ends) is controlled by the yield strength in the weakest direction, which is usually the 45° direction.
• the strips of alloys BRE and BRF did not attain the minimum desired yield strength of 3165 Kg/cm2 in all test directions and were therefore excluded from further testing.
• Examination of the microstructure of the final-­gauge BRE and BRF samples revealed the presence of some coarse Mg2Si particles, indicating that these composi­tions exceed the aforementioned alloy "window.”
• the micro­structure of the BRD alloy sample revealed some small Mg2Si particles indicating that this composition is probably close to the upper permissible limit of Mg and Si.
• can ends were formed from the fully processed (final gauge, aged, lacquered and cured) strips of BRB, BRC and BRD alloys and from the commercially produced AA 5182-H28 can end stock. About 40 to 50 can ends were formed from each alloy sample, and the produced can ends were subjected to various performance tests.
• composition notes following Table I additionally set forth estimates, for each alloy, of the maximum possible amount (%) of Mg2Si and the resulting amount (%) of excess Mg (above that stoichiometrically required to form Mg2Si). (control is AA 5182, meeting the Aluminum Association composition limits for that alloy) (balance aluminum in each composition) *stoichiometric composition Mg:Si ratio 1.73 assuming sufficient solubility at elevated temperatures and all Si as Mg2Si. **Excess Mg over that to form Mg2Si.
• All alloys BRB, BRC, BRD, BRE and BRF were direct chill cast in the laboratory as 9.5 cm ⁇ 22.9 cm ingots.
• the ingots of BRB, BRC and BRD were homogenized 6 hrs. at 580°C; the ingots of BRE and BRF were homogenized 6 hours at 590°C.
• the ingots of all five alloys were processed as follows: cooled to 520°C., hot rolled to 2.29 mm gauge, cold rolled to 0.63 mm gauge, solution heat treated and water quenched, final cold rolled to 0.335 mm gauge, aged for three hours at 160°C, and then lacquer coated and subjected to a lacquer curing treatment.
• control material was AA 5182 alloy sheet, commercially produced, in H28 temper; it was not subjected to an aging heat treatment but simply lacquer-cured.
• compositions and processing conditions are identified by the code letters in the table; the meanings of the code letters are set forth in the notes following the table.
• the alloys of the invention in conjunction with the specified fabricating schedule, provide can end material having very favorable properties in respect of strength, formability, and low anisotropy.
• a - alloy composition 0.37% Cu, 0.22% Fe, 1.76% Mg, 0.25% Mn, 0.24% Si, 0.024% Ti, 0.10% Cr, balance Al B - alloy composition: 0.43% Cu, 0.20% Fe, 2.09% Mg, 0.24% Mn, 0.25% Si, 0.036% Ti, 0.10% Cr, balance Al C - alloy composition: 0.38% Cu, 0.21% Fe, 1.91% Mg, 0.28% Mn, 0.23% Si, balance Al D - fabrication schedule: direct chill case, homogenized, hot rolled to 3.2 mm. gauge, initial cold rolled to code F or code G gauge (see below), solution heat treated and water quenched, final cold rolled to 0.287 mm. gauge, aged 2 hrs.
• E - fabrication schedule twin belt cast, directly hot rolled (from caster exit) to 3 mm. gauge, initial cold rolled to code F or code H gauge (see below), solution heat treated and water quenched, final cold rolled to 0.287 mm. gauge, aged 2 hrs. at 170°C. and lacquered-­cured at a peak metal temperature of 230°C. max.
• G - initial cold rolled to 0.64 mm. gauge (55% cold reduction in final cold roll) H - initial cold rolled to 1.3 mm.
• Can end stock produced in accordance with the invention though being of a gauge and having properties and microstructure (including absence of coarse inter­metallics) especially suitable for formation of can ends, also has utility for formation of other articles such as food can bodies that are produced by drawing and redrawing with at most a small amount of ironing restricted to the upper portion of the can wall.
• reference herein to can end stock is descriptive of the attributes but not restrictive as to the actual end use of the alloy strip so designated.

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• 1988
  • 1988-02-09 EP EP88301074A patent/EP0282162A1/fr not_active Withdrawn
  • 1988-02-23 JP JP4053888A patent/JPS63277744A/ja active Pending
  • 1988-02-23 NO NO880795A patent/NO880795L/no unknown

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