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
  • C22C21/10—Alloys based on aluminium with zinc as the next major constituent
• • 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
  • C22F1/053—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 with zinc as the next major constituent

Definitions

• TITLE ALUMINIUM ALLOY SUITABLE FOR CAN MAKING
• This invention relates to the use of aluminium based alloys for the manufacture of liquid containers and in particular to a high zinc content aluminium base alloy suitable for liquid container construction.
• Conventional two-piece aluminium beverage containers are generally fabricated from two distinct alloys such as the Aluminium Association Specification 3004 and 5182 (See Table 1).
• 3004 alloy is generally used for body stock by deep drawing and wall ironing forming methods but lacks the necessary rigidity and strength properties to be a useful lid stock whereas 5182 alloy which is unsuitable for body stock has the properties desirable for can lid fabrication.
• Increased demand for this type of container and the need for stronger cans of thinner gauge material has spurred development of new alloys and in particular alloys which can be used for both body and lid stock.
• the AA3004 type alloy composition alloys are traditionally produced by casting the alloy using the direct chill casting method (DC-cast) into an ingot block of cross-section around 500 mm thick x 700 mm wide. The ingots are then homogenised at temperatures between 500 and 620°C for 4-24 hours and hot rolled.
• DC-cast direct chill casting method
• the hot rolling procedure reduces the ingot thickness to a gauge of about 2-3 mm by a series of breakdown passes.
• the material is then usually annealed at temperatures of 300 - 400°C for periods between 0.5 and 4 hrs to allow the metal to recrystallise.
• the annealed metal is then subjected to a cold rolling schedule to develop strength and other properties. This normally consists of 2 - 5 passes using 20 - 50% reductions to achieve a final gauge of about 0.3 - 0.33 mm.
• the final gauge sheet is then fabricated into aluminium cans by use of a cupping machine and can body maker. Circular discs approximately 135 mm diameter are cut or punched from the cold worked sheet on the cupping machine and drawn into shallow cups. The cup then enters the bodymaker and is first redrawn into a cup close to that of its final diameter. The sidewalls are then reduced in thickness in one or more wall ironing operations to produce a can body around 65 mm diameter and 140 mm high with a wall thickness of between 0.10 - 0.18 mm.
• the material from which the body is drawn has anisotropic properties (i.e. the properties differ with direction) and so the top of the drawn body has a scalloped top, with approximately 4 peaks oriented 90° apart the peaks of which are called ears.
• the degree of "earing" is determined simplistically by the following equation:
• h a is the distance between the bottom of the cup and peak of the ears and h t is the distance between the bottom of the cup and valley of the ear.
• the formed body must have earing levels of no more than 3-5% and preferably less than 3%.
• the tops of the can bodies are trimmed off (fixed for a given process), during a slitting operation and the "eared" area is scrapped.
• Another major consideration of the quality of the finished body is its ability to form in the body maker without tearing and to have a smooth surface finish, free of drawing streaks and lines. Deep grooves caused by "galling" may appear on the finished can walls if the material is not of the correct microstructure.
• a container can be formed from a fabricated downgauged high strength 3004 type alloy of non-galling, low earing characteristics, unless the strength is great enough to offset the strength lost from reduced wall thickness the buckle resistance of the can will be reduced.
• Buckle strength of resistance is determined by applying pressure within a drawn and wall ironed can and then gradually increasing the pressure until the bottom end of the can deforms and bulges out, i.e. it buckles or the inverted dome reverses. The pressure at which the bottom buckles is then designated as the buckle strength or dome reversal pressure.
• a can formed from the alloy sheet must exhibit a buckle strength of at least 85 p.s.i.
• Cans drawn from high strength 3004 alloy produced using conventional direct chill (D.C.) cast methods exhibit a buckle strength of about 90 p.s.i.
• an aluminium alloy suitable for can stock which has non-galling, low earing and high strength characteristics with good formability at least comparable with existing AA3004 type alloys which will allow can bodies to be made from thinner sheet feed stock. It is also an object of the present invention to provide can stock which is also suitable for the manufacture of can lids thus, allowing manufacturing of a complete two-piece beverage can from one alloy composition.
• the alloy will be amenable to fabrication into can stock via both tbe direct chill cast and the continuous strip cast methods. Accordingly, the invention relates to a can stock and process for producing a can stock from an aluminium alloy comprising
• the incidental impurity level in the alloy is less than a total of 0.15 wt%.
• the alloy is processed into feed for a can line by a process comprising, forming a melt of the alloy metal suitable for casting, casting the melt into a form suitable for rolling, performing an intermediate rolling to an intermediate thickness, treating the alloy with heat, performing finish rolling by cold rolling within the range of 2 to 85%, temper heat treating the material to the desired ductility and strength.
• the alloy preferably comprises constituents in following weight percent ranges, zinc 4.0 - 6.5, magnesium 1.0 - 2.5, manganese 0.3 - 0.8, silicon 0.15 -
• zirconium up to a maximum level of 0.25 wt% may be suitable for can stock, to produce a can body with low earing, it is preferable that the zirconium level is less than 0.08 wt% and most preferable that it is not added to the melt and its level is below tbh standard level of impurity for that element of 0.01 wt%.
• alloy which falls within the above composition range achieves high tensile and yield strength properties as well as good formability and "non-galling" wall ironing quality.
• the alloy is preferably used to produce both the body and the lid of the beverage container.
• the alloy is capable of producing a can body which has a dome reversal strength in excess of 90 p.s.i. and a wall ironed thickness of less than 0.16 mm.
• a 3004 alloy is generally only capable of achieving such a dome reversal strength with a wall thickness equal to or greater than 0.16 mm.
• the above alloy may suitably be cast by the direct chill casting method, continuous roll casting or continuous strip casting. However, if the alloy is to be produced using roll or strip casting the compositional range of the alloy can be broadened. The amount of Zn, Mg, Mn, Fe and Si may be increased which results in higher volume fraction of alpha phase particles after casting and a greater amount of precipitation during final processing.
• the alloy sheet may be produced by a combination of rolling reduction and heat treatmant in the final stages of manufacture. Consequently, fabrication steps vary with end use requirements. However, during the performance of the heat treatment processes, it has been found that the alloy thus formed was especially adaptable to solution heat treatment of very short duration, while more lengthy heat treatments can be used to simultaneously anneal the sheet using recrystallisation anneal or recovery anneal techniques.
• the ingot or strip cast material may be heat treated to homogenise the cast material and preferably hot rolled and then cold rolled. The material is then solution heat treated and given a cold rolling reduction. Lastly to improve strength and ductility, the alloy is aged to a temper dependent on the end use requirements.
• the cast material if ingot cast, is subjected to an additional heat treatment step to homogenise the alloy and preferably followed by a hot rolling step.
• the intermediate rolling preferably is a cold rolling stage and may employ a full anneal or recovery intermediate anneal during the cold rolling practice.
• the step of treating the alloy with heat is preferably a solution heat treatment which may be followed by a natural ageing stage.
• the finish rolling step is essential to the invention to provide the necessary strength properties to the final product but still must maintain sufficient ductility and formability properties to be a suitable can stock.
• the finish rolling is thus a cold rolling reduction in the range 2-85% and preferably within the range of 10-80% and most preferably within the range of 30-80%.
• the final temper heat treatment is preferably artificial ageing.
• a can line feed produced from the alloy of the invention which preferably has yield and tensile strength of in excess of 400 MPa and a total elongation of 4% or more.
• the alloy of the invention may also be used to produce can endstock which in a tempered state must have a miniumum yield stress of 310 MPa, a minimum ultimate tensile stress of 5 MPa and MIN 6% elongation.
• the strengths and elongations specified above for can end stock relate to the post bake strength of the material.
• endstock is manufactured in coil form and is sent to tbe end makers who then coat tbe material and bake the coated sheet at modest temperatures of between
• a further aspect of the invention is a two piece beverage container produced from the alloy of the invention.
• the container preferably has a wall thickness of less than 0.12 mm and a dome reverasal strength of greater than 90 p.s.i.
• Table 1 contains details of the invention alloy range and preferred range.
• Figure 1 is a flow diagram showing the overall processing details for the alloys;
• Figure 2 is a Differential Scanning Calorimetry Thermal Analysis curve for the invention alloy;
• Figure 3(a) is a flow diagram representing the final stage fabrication of the alloy;
• Figure 3(b) illustrates the effect of the final cold rolling reduction, and
• Figure 3(c) represents the effects of cold working and artificial ageing on the alloy of Example 1;
• Figure 4 shows micrographs of alloy 2 of Example 2 at various stages of processing in which
• 4(b) is the microstructure after the material has been annealed
• 4(c) is the microstructure after the final cold rolling reduction
• Figure 5 shows the effect of natural ageing on Alloys 1, 2 and 3 (Example 2).
• Figures 6 - 8 are graphs representing the response of the alloys of Example 2 to time at a given temperature in the final ageing treatment.
• Figure 9 represents the effects of natural ageing and coldwork on the alloy
• Figure 10 represents the effect of using a double heat treatment in the artificial ageing step and the effect of coldwork
• Figure 11 represents tensile results for the alloys of the invention.
• Figure 12 represents strength and ductility characteristics for the alloys of the invention compared to those of conventional 3004 body stock.
• FIG 13 represents an overview of the benefits of the alloys of the invention.
• An alloy in accordance with the invention comprises the following basic alloying elements (wt%): copper (0.05 - 0.9), manganese (0.1 - 1.1), magnesium (0.5 - 3.0), and a relatively high concentration of zinc (3.0 - 8.0). Furthermore, the following additives are included in the melt: chromium (0.0 -0.30), silicon (0.1 - 2.0) and iron (0.0 - 0.5).
• the silicon, manganese and iron additives be within the ranges specified above as these elements are required to form the desired dispersed second phase distribution (alpha phase) in the alloy which is critical for the production of a non-galling wall ironed can.
• Zirconium and chromium are typically found in the melt at impurity levels of less than 0.01 wt%, but if already present or added to the melt for additional strength properties, it is preferable that they are present at levels less than 0.08 wt% and less than 0.05 wt% respectively.
• the effect of the very low levels of grain refining elements chromium and preferably zirconium, is that full recrystallisation of the product after, for example, hot rolling or low temperature annealing (345°C for 0.5 - 2 hours) can take place.
• the zirconium and chromium levels in the alloy exceed the specified levels in the present invention, then the wrought structure created by hot and cold rolling will be retained during solution heat treatment or anneal and the properties of the final gauge sheet will be more anisotropic. This is particularly disadvantageous if the sheet material is to go through deep drawing operations as the anisotropic properties result in material of high earing which is undesirable for the can body making process. High strengths can be obtained by increasing the concentration of these elements but formability and earing suffer as a result. The yield strength and tensile strength of the 10 alloy in accordance with the invention is far greater than that of a 3xxx series can body material.
• the 3004 alloy has yield and tensile strengths of around 285 and 330 MPa respectively with 4% elongation whereas the alloy of the invention exhibits yield and tensile strengths around 420 and 480 MPa respectively with a total elongation measured on a 50 mm gauge length of 4% or more.
• a sheet is drawn and wall ironed into a body for a two-piece beverage container it possesses can buckle strengths in excess of 100 p.s.i. with a wall thickness of 0.12mm.
• the very high strength and improved ductility of this alloy over any type of 3004 or modified 3xxx series alloy allows the gauge of sheet used for the initial forming operation (can line feed) to be reduced by at least 10% while still retaining buckle strength and formability in the finished body.
• alloy AA5182 which is currently used for beverage can lids has a tensile strength of 395 MPa and an elongation of 4%.
• the 3004 alloy is generally not used to produce lid stock as it does not have sufficient strength for the desired lid thickness.
• the invention alloy can match the properties of AA5182, and thus a two-piece can may be made from one alloy composition instead of the conventional two.
• the alloy is cast by the DC-casting method to produce can bodies it is first homogenised.
• the cast is preferably homogenised as a block between 480° and 500°C for a period of 5 to 10 hours.
• the material is then hot rolled preferably from a temperature of up to 500°C down to a thickness suitable for coiling (preferably less than 5 mm). It is preferable that the hot rolling coil finishing temperature is above about 300°C as the alloy will automatically anneal during the coiling operation.
• the material is then cold rolled and preferably employs a full anneal or recovery intermediate anneal during the cold rolling practice. After this rolling stage the strip preferably has a thickness of between 0.8 and 0.4 mm. These thicknesses are required as a rolling reduction must be effected during the final processing to can sheet gauge to provide the necessary sheet flatness and final gauge strength.
• a solution heat treatment at temperatures preferably between 480°C and 595°C for duration times between 5 seconds and 1 hr is followed by a water quench to ambient temperature.
• a solution heat treatment for duration times toward the upper end of the range will also anneal the material if it has been cold rolled prior to the solution heat treatment. Nevertheless, the composition of the alloy allows the material to be heat treated for much shorter times than would normally be performed in processing 3004 alloys.
• the alloy strip may then be allowed to naturally age at room temperature preferably for a period of zero to 48 hours prior to cold rolling of the strip.
• the finish rolling step has a cold rolling reduction within the range 2 to 85 percent, the reduction preferably being 10 and 80% and most preferably 30 and 70%. It has been found that even with a cold rolling reduction towards the upper limits of this range, the necessary ductility and formability properties for a viable can stock are present.
• the sheet material is aged to a temper between the underaged and over-aged states.
• the ageing will depend on the equipment used and the can manufacturer's strength and ductility specifications and is preferably within the range 120 to 26oC for a time of between 1 minute and 4 hours.
• Solution heat treatment of the material prior to the final processing recrystallises the material and reduces the anisotropic properties created by the cold rolling schedule. This means that when given a final rolling reduction and fully aged to the desired temper, a deep drawing material is created which has very low anisotropic properties and very low earing levels.
• a cup formed from final gauge 3004 or modified alloy typically has an earing level of 3%, whereas a cup formed from an alloy of the present invention has earing levels of less than 2%.
• the alloy can be strip cast in a conventional strip caster and solidified into a web approximately an inch or less in thickness.
• the molten alloy feedstock for this may be richer in composition than that used in the DC cast alloy but would be of the preferred invention alloy range (see Table 1). It is then preferred that the strip is given a hot or cold rolling reduction in sheet thickness of at least 25% and more preferable 50 to 85%.
• the alloy composition of the present invention and processing technique allows liquid container bodies to be made from thinner gauge sheet stock and achieve cost reductions. Furthermore, one alloy can be used for body end stock and easy-open tab stock by varying the processing steps of the alloy. The use of a single alloy type for all of the component parts of liquid container results in production costs benefits and improvements in scrap recycling efficiency.
• the ingot was subjected to the following schedule to produce a coil of the sheet. - homogenise 5 hrs 4800° - 595°C - hot roll to 3.175 mm coil exit gauge temperature 295 - 315°C - standard edge trim applied - anneal 370°C - 373 for 3 hours - cold roll 60% reduction to 1.22 solution heat treatment at 1.22
• the coil, once heat treated, was rolled in back-to-back passes to final gauge with a maximum delay of 48 hours before commencing to roll.
• Figure 3(b) illustrates the effect of the final cold rolling reduction on the strength and ductility of the alloy
• Figure 3(c) demonstrates the effect of artificial ageing time on strength and ductility of the above alloy after solution heat treatment (S.H.T.) and 70% cold rolling reduction.
• the ageing temperature is 250°F (121°C).
• Heomogenisation is shown in Figure 2.
• the following letters represent the positions in Figure 2, where changes in the various phases occur.
• B is the precipitation of (MgZn 2 ) phase
• C the dissolution of phase
• D the precipitation of T phase
• E the dissolution of T phase
• F the localized GB liquidation.
• the scalped blocks were cooled to a rolling temperature of between 500 and 485°C in the furnace.
• the blocks were then removed from the furnace and individually rolled from a gauge of 100 mm to a finished gauge of approximately 2 - 3 mm.
• the microstructure of the hot rolled material is shown in Figure 4a.
• the finishing temperature of hot rolling was often above 200°C which was sufficient to allow some recovery of the rolled structure prior to cold rolling to the solution heat treatment gauge.
• the material was given a recrystallisation anneal at a temperature of 345°C for 3 hours. This allowed the material to fully recover and recrystallise.
• the fully recrystallised grain size of the alloy had an average diameter of 19m (ASTM 8.5)
• Figure 4(c) shows the microstructure of alloy 2 in Example 2 after the final cold rolling reduction and as can be seen from the micrograph the material has an alpha phase ( ⁇ - Al(Fe,Mn)Si) totally dispersed within the matrix.
• This microstructure results in a wall ironed can body with excellent non-galling properties.
• Solution heat treatment of the plate was conducted at a temperature of 500°C to put elements into solution in preparation for the final ageing procedures.
• a study of the natural ageing behaviour of the three alloys after solution heat treatment of 2 hours at 500°C is shown in Figure 5.
• the material was cold water quenched and then given a number of heat treatments.
• Ageing studies and hardness measurements were used to charactise the response of the alloys to various treatments.
• Tensie studies were then made of sheet in certain conditions to establish the yield strengths and elongations of the specific alloy treatments.
• Final ageing treatment studies are show in Figures
• Figures 6(a), 6(b) and 6(c) show the response of the alloys to ageing at 155°C after a first ageing stage at 121oC for 1, 2 and 3 hours respectively.
• Figures 7(a), 7(b) and 7(c) represents the ageing response of the alloys at 111°C with a preceding natural ageing stage of 0, 24 and 48 hours respectively.
• Figures 8(a), 8(b) and 8(c) represent the ageing response of the alloys at 131oC with a preceding natural ageing stage of 0, 24 and 48 hours respectively.
• Figures 10(a) and 10(b) demonstrate the effects of secondary artificial ageing on material of alloy 2 with prior cold word (cw) treatment from 0-60%.
• the alloy of Figure 10(a) has undergone a solution heat treatment of 500°C for 2 hours, 0 hours natural ageing, cold working and united artificial ageing at 1210°C for 3 hours.
• the alloy has undergone solution heat treatment at 500°C for 2 hours, 48 hours of natural ageing, cold working and an initial
• Table 4 demonstrates the response of alloys 1,2 and 3 of Example 2 to varying effects of cold working and artificial ageing.
• the column marked C .W. represents the %
• the columns AA(1) and AA(2) are expressed in the form T/t where T is the temperature °C of that step of heat treatment and t is the time in hours the material is held at that temperature.
• the column marked N. A. represents the natural ageing time.
• SHT/ANN is the solution heat
• a number of sheet alloys were formed into cups for body making and the earing levels measured.
• the cups were then formed into bodies on a can body maker.
• Each of the example results is from a successful can body free of holes or surface marking.
• the buckle resistance of selected cans was measured on a
• the alloys demonstrate a very high tensile strength with ductilities in excess of 4%.
• the earing levels of can bodies made from the alloy of the invention show earing levels between 0 - 2.4%. These levels are on average 1% lower for a given condition than that of can bodies made from 3004 alloys.
• dome reversal pressures for these cans made from alloys of the invention at the same thickness as cans made from 3004 are far in excess of 3004 values.
• Dome reversal pressures for cans of 3004 alloy are typically about 90 p.s.i. maximum whereas the cans of the same thickness made from the invention alloys is between 100 and 115 p.s.i.
• the graph of Figure 11 illustrates the effect ageing has on the tensile properties of alloys 2 and 3 and Figure 12 shows a comparison between tensile properties for the invention alloys and that of 3004.
• Figure 12 shows how the invention alloys between lines
• a and B have tensile strengths about 20 - 40% higher than 3004 type alloys (conventional can stock) with ductilities in excess of 4% and are therefore superior in a number of properties.
• a stronger can body material means that the wall thickness of a can body can be reduced whilst
• An alloy with the composition of alloy 3 in Example 2 are subjected to the following processing steps.
• the material was sectioned into 2 samples with one sample cold rolled to 0.43 - and the other to 0.355 mm. 5. Both samples were subjected to a solution heat treatment at 500°C for 1 hour before being water quenched.
• the samples were subjected to artificial ageing at 121°C for 3 hours or 6 hours.
• Table 6 illustrates the % drop in strength and ductility properties as a result of a bake used by can end producers.
• Figure 13 basically illustrates the main
• Cans with comparable strength mean lower cost of alloy from thinner material allowing less material to be consumed to make the same number of cans.
• can body can be produced with strength, ductility and anisotropic properties which far exceeds properties obtainable from conventional body stock alloys. As it is
• a two piece beverage container can be produced from the same alloy type with the same alloy composition.

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• 1991
  • 1991-08-21 WO PCT/AU1991/000376 patent/WO1992003586A1/en not_active Application Discontinuation
  • 1991-08-21 EP EP91915278A patent/EP0544758A1/en not_active Withdrawn
  • 1991-08-21 BR BR919106787A patent/BR9106787A/pt unknown
  • 1991-08-21 CA CA002091355A patent/CA2091355A1/en not_active Abandoned
  • 1991-08-21 JP JP3513794A patent/JPH06503854A/ja active Pending
  • 1991-08-22 ZA ZA916651A patent/ZA916651B/xx unknown
  • 1991-08-22 CN CN91108893A patent/CN1060115A/zh active Pending

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