CA2185216A1 - Aluminium foil - Google Patents

Abstract

Aluminium foil is composed of an alloy of composition Fe 1.2 - 2.0 %; Mn 0.2 1.0 %; Mg and/or Cu 0.1 - 0.5 %; Si up to 0.4 %; Zn up to 0.1 %; balance A? of at least commercial purity. The foil has an average grain size below 5 .mu.m and is continuously recrystallised with a substantially retained rolling texture. The solute elements Mg and/or Cu increase strength without inhibiting continuous recrystallisation.

Description

2t85216 .
Wo 95/25825 ~ r~"~i~ sl~
.
AL~MINIU~ FOIL
This invention is c~n~-orn~d with aluminium foil having improved strength. In the current Al-Fe-Mn based foil alloys, such as AP. 8006 and AA 8014, the good balance of strength and formability of thin gauge 10 foil is obtained by achieving a combination of fine grain size after final Anne~l ;n5 and dispersion strengthening. This invention describes the use of an additional strengthening ~ An; om to achieve increased strength; namely solid solution strengthening, and 5 specif ies the range within which the solute level must be controlled in order to avoid loss of other beneficial properties associated with the solute-free versions of these alloys.
British Patent Specification 1 479 429 20 described dispersion-strengthened aluminium alloys based on the Al-Fe-Mn system, such as AA ~006 and AA 8014. (from Registration record of ;nt~rnAtional alloy designations and chemical composition limits for wrought Al and wrought Al alloys, A~ Inc. May 1987) .
25 The as-cast ingot comprised unaligned intermetallic rods. These were broken up during working to provide a wrought aluminium alloy product cr~n~;n;n~ dispersed ;nt~ -tAlliC particles. The invention was applicable to the production of rolled sheet, which was to some 3 extent anisotropic. It was possible to reduce the relative proportions of the anisotropy by introducing small proportions of Cu and/or Mg which 1~ ; n~d in solid solution in the Al phase and had known strength providing properties. The 1055 of anisotropy implies 35 discontinuous recrystallisation and 1055 of grain size control, which changes would have been acceptable in

- 2 -the sheet products mainly envisaged and ~X~ r 3~
The successful production of aluminium foil having useful properties depends on several critical pd~d~ r~. The metal to be rolled must not be too hard, otherwise rolling down to the very low ~l ,ickl,e:.a~s below 100 ,um required is not co"""e",i~"y viable. After rolling, the foil has to be heated, to a temperature sufficient to remove rolling lubricant but not so high that adjacent sheets of foil stick together. This temperature window is quite narrow, generally 220 - 300C, and results in a final annealing treatment of the foil. During this annealing treatment, recry~ takes place, and it is necessary that this be continuous recry: ' " " n, which retains a desired small grain size, rather than discontinuous recry~ , which results in grain growth. If large grains are present, the foil has reduced Ill~ulldlli~dl properties. While these critical pdldlllt:L~ have long been achieved using Al-Fe-Mn alloys, it was not apparent that they could be achieved in CUI I Ibi~ IdliUI I with solid solution hardening. And indeed, as the inventors have discovered, the nature and amount of solute that can be added is critically circumscribed.
In one aspect this invention provides aluminium foil composed of an alloy of co",l,o~i~ion by weight %:
Fe 1.2 - 2.0%
Mn 0.2 - 1.0%
Mg and/or Cu 0.1 - 0.5%
Si up to 0.4%
Zn uptoO.1%
Ti up to 0.1%
balance Al of at least c~" ", It31 Uidl purity which foil has an average grain size beiow 5 ~um after annealing.
In another aspect, the invention provides ~MENDED SHEET

.
Wo 9~2582 aluminium foil of the stated composition, wherein at least 50g~ by volume of the as-rolled texture is retained after final anneal.
In another aspect, the invention provides aluminium foil of the stated composition, wherein the crystallographic texture of the final ~nn~ 1 product is a retained rolling texture.
The aluminium foil preferably ha9 a thickness below 100 ~Lm, particularly in the range 5 - 40 llm e.g.
10 - 20 /~m. The improved strength of foil according to this invention should enable thinner gauge9 to be marketed .
Fe and Mn are present to provide dispersion strengthening properties, as described in the aforesaid GB 1 479 42g. Preferably the Fe content is 1.4 - 1.896;
the Mn content is 0 . 3 - 0 . 6%; and the Fe + Mn content is 1.8 - 2.15~6.
If the Fe + Mn concentration in the melt exceeds this value of 2.15 then coarse primary intermetallic particles (typically up to 100 ~lm length) can form during soli~;fir~t;nn as a consec~uence of nucleation of these phases on the cooler parts of the molten metal distribution system. These coarse particles will break-up somewhat during subse~uent processing but will still persist as relatively coarse non-deformable particles in the final product. For the case of sheet products this will not cause 9i~nif icant problems, but in the case of foil products will give rise to problems with pin-hole formation in the rolled

3 strip and give rise to excessive strip breaks during processing. It is thus preferred to ca9t a composition where primary intermetallic particles cannot form, and this imposes an upper limit on the Fe and Mn levels for use of this invention for foil products.
Mg and/or Cu is added to provide solution strengthening, in a cnnt-~ntration of 0.1 - 0.5%

. - 2185216 Wo 95l25825 _ r~ . s.

preferably 0.15 - 0.359~. At the lower end of these ranges, little strengthening is observed. At the upper end of these ranges, there is a risk that the solute will encourage disrnnt; nllnus recrystallisation and will 5 result in undesired grain growth. This risk iB
particularly apparent at relatively high ~nn~Al in~
temperatures. As shown in the examples, Mg provides a better solution strengthening effect than Cu at equivalent cnnr~ntrations and i8 accordingly preferred.
The inventors have tried other solution strengthening elements, but have f ound that they tend to encourage discnnt;nllnllc recrystallisation during final anneal or are otherwise lln~tic~Actnry. It is therefore believed that Mg and Cu are the only two 15 usable solution strengthening additives.
Si and Zn are included in the AA
specifications of AA 8006 and AA 8014. But they are preferably not deliberately included here. It is an advantage of the invention that recycled scrap metal 20 can be used to make the foil.
The foil is specified as having an average (or mean) grain size below 5 ~m, preferably below 3 ~m.
The grain size is preferably subst~nti~lly uniform, and is achieved as a result of rnnt;nllnus recryst:~ll;R;~t;nn 25 during final anneal. Alternatively a non-uniform grain size may be acceptable provided that gross discnnt;nllmlc recryst~ll;qa~t;nn during final anneal is avoided. For example, the majority of grains may have a size of 2-3 ~Lm with a minor proportion of grains of 3 10-30 l~m. This duplex grain size structure may reduce the ductility of the f oil, but the overall properties may nevertheless be satisfactory.
Grain size may be det~rmi nPd by the mean linear intercept method. On a micrograph of a section 35 of the alloy under test, a line (e.g. a straight line or a circle) of known length is drawn, and a count is made Wo95l25825 1~I/~1. ~ :

of the number of intercepts of: that line with grain boundaries. The mean linear intercept grain size (mean grain size) is the length of the line divided by the number of intercepts.
The foil is generally anisotropic. Cold-rolling develops an as-rolled texture typical of dilute Al alloys. Texture i8 conv-nt;nn~lly measured from an orientation distribution function in terms of six parameters (cube, goss, copper, S, brass and random). C~onventionally, these are measured a8 a volume fraction of crystals orientated over a ~15 spread about the c.~r.~ iate Miller indices which are {001}~100~, lllO}<001~, {112}<111~, {123}<634~, {011}<211~ respectively, the 5 random t being the r~ ;n;n~ volume fraction.
The copper, S and brass c ^ntq are generated by cold rolling . DiSrnnt; nllnus recrystallisation would tend to destroy the as-rolled texture and favour the formation of cube and/or goss and/or random. In the 20 foil of this invention, at least 509,i by volume, preferably 75~ of this as-rolled texture as represented by the copper, S and brass components is retained after f inal anneal . Preferably the crystallographic texture of the final ~nn~ product is substantially the same 25 as the as-rolled product with no significant levels of recryst~ll;c~t;nn texture r_ ^ntC.
It has surprisingly been found that the foil of this invention may have a surface roughness greater than that of its solute-free counterpart. This 3 increase in ro~hn~cq was confirmed by optical profilometry (Perthometer) measurements, giving an Ra of 0.38 for foil of this invention ~Example 2) compared with an Ra of 0.24 for a commercial foil of corr~qp~ nr1; ng composition without Mg . The rougher 35 surface improves the matt appearance of the foil.
In making the aluminium foil of this : 2185216 ~
WO95l2~825 1~I,... ~c invention, a molten aluminium alloy of desired composition iE cast, e.g. by direct chill (D.C. ~
casting, or alternatively by roll casting or belt casting or other known casting techniques. The cast 5 metal is rolled by successive rolli~g steps in conventional manner down to the re~uired foil th; nkn~qq . These steps typically involve hot rolling followed by cold rolling, possibly with one or more inter~nn-~l in~ steps. Finally, the fQil is heated to a 10 temperature su-ficient to remove the rolling lubricant.
The heating rate is preferably 1C - 100C per hour.
As noted above, this temperature is typically in the range 220 - 3D0C, preferably 230 - 280C, more preferably 230 - 250C, and also effects continuous 5 recrystallisation of the foil. The aluminium foil of this invention is preferably subst;~nt;Ally free of surface ~nnt~m;n;ltion by rolling lubricar,t.
The technical basis of the invention, as presently understood by the i~ventors, is ~ ; n~d in 20 the following paragraphs.
A range of aluminium alloys are known to achieve a fine grain size after final ~nn~l inS by a gradual coarsening of the cold-rolled substructure, sometimes called ~nnt;nllml~ recryst~ll;q~t;nn, which 25 allows a good ~ ' ;n~tion of strength and formability to be achieved . During the f inal Ann~A l; n~ of ~ lmin;llm foil products it is important to avoid the oc~uLLt:~lce of large recrystallised grains which severely ~;m;n;sh formability, often as a result of 3 strain loc~ t; on leading to premature f ailure during loading. These grains are formed in the ~1Aqq;~
di8-nnt;nllnus manner whereby individual grainE nucleate and grow to a large size. It is known that in these type of alloyE this transition from dis~-nnt;nllnus to 35 cnnt~ nllnus recrystallisation occurs when the level of :`

WO ~snss2~ J/~ ' cold work is increased above a critical level typical of foil rolling.
If there is a sufficient high concentration of non-deformable intermetallic particles, such as the 5 FeA16 and/or (FeMn)A16 eutectic rods formed during solidification of Al-Fe-Mn alloys such as AA 8006 and AA 8014, then after deformation to high strains these particles must have increased dislocation activity associated with them in order to Tn-int~;n r ^nt;nll;ty 10 across the aluminium/particle interface. Under conYentional solute-free conditions, the8e dislocations are capable of rearranging themselves into dislocation walls, or sub-grain boundaries. As deformation proceeds the geometrically necessary dislocations 5 generated during the rolling process crntinll~ to migrate to, and recover into, the sub-grain boundaries, increasing their misoriont~ti~n. Eventually these boundaries will attain high misorientations with their neighbours, i . e . high angle grain boundaries . When 20 these boundaries are then annealed, they can all migrate at similar rates, thus encouraging r~ntinll~uS
recryst~l 1 i q~tion In addition this is helped by the ability of the now broken up rod eutectic to pin grain boundaries and prevent excessive rates of grain growth.
25 Dispersoids formed during hot processing of the ingot will also assist this pinning process.
Thus, the conventional (solute-free) AA 8006 achieves a fine grain size after anneal, which imparts the good balance of strength and ductility associated 3 with these alloys. The strength is inversely proportional to the grain (or sub-grain) size, and follows a d~l r.ol~t;r~nch;r This invention still m~;nt~;nq this strengthening m~h~n; r- whilst using the additional 35 strengthening ~ h~n; ~-- of solid solution strengthening. If the amount of solute added is too ; ` 21 852~ 6 w09s/2s82s r~"~

high then the ability to control the grain size during the final anneal is lost, giving rise to a decrease in grain size strengthening and formability. This presumably is because dynamic recovery is prevented 5 during rolling, and 80 the driving force for di8cnnt;nllnus recrystallisation is increased. This al80 makes it increasingly difficult to roll the foil to the reguired thin gauge because of the increased rolled strength, giving a loss of the roll softening 10 normally found in solute-free alloys of this type.
Another aspect of the reaLLa.,ly t of dislocations into high angle grain boundaries during the rolling process is that the strength of the foil decreases as the rolling s~rain is increased (roll 5 sof tening), instead of the usual roll hardening associated with most aluminium alloys. Adding solute to the alloy will hinder the ability of the dislocations to rearrange themselves into low energy configuration in the sub-grain boundaries, and will 20 prevent roll softening from occurring. Thus, if too much solute is added the cold rolled strength of the foil will be significantly increased, losing the ability to roll the material to the thin gauges needed for household foil and packaging applications (within 25 the range 40 ~Lm to 5 /~m).
Reference is directed to the ~~ ~nying drawings in w~lich :-Figure 1 shows the effect of ::nnP~l ;n~temperature on tensile strength of laboratory processed 3 alloys rolled to 140 l~m;
Figure 2 shows the effect of ~nn~ l in~
temperature oll tensile yield stress of laboratory proceæsed alloys rolled to 140 ~m;
Figure 3 shows the effect of ~nn~l ;n~
35 temperature o~ tensile elongation of laboratory processed alloys rolled to 140 llm; and Wo95l25825 5 ~

Figures 4a and 4b are pole diagrams of a foil sample before and after ;InnP~l ;n~, Exa~Ple 5 Laboratorv Processinq The effect of different levels of copper and magnesium additions have been investigated using laboratory processing of 200 mm x 75 mm cross-section D.C. ingots of 1.6% Fe, 0.40% Mn, 0.15~ Si (denoted by 0 8006 in the figures) and modified alloys r~nt~;n;nr~
0 . 2% and 0 . 4~ of Cu or Mg . At the cooling rates associated with this ingot cross-section, the addition of the solutes does not prevent the formation of the preferred rod eutectic, with only slight coarsening 15 being observed.
The above ingots have been heated to 525C, hot rolled to 20 mm, and annealed at 330C for 3 hours to simulate commercial hot processing. The materials have then been cold rolled to 4.5 mm, inter~nnP~1Pd at 360C, and cold rolled to 145 ~m. This reproduces the strain levels achieved during rolling of 14 ~m household f oil . Table 1 shows the ef f ect of the rolling rP~lllrt;r~n on the tensile strength of the materials. Adding solid solution strengtheners 25 prevents the usual roll softening associated with AA 8006 from occurring, thus; ~o'::;n~ an upper limit on how much solute could be added and still enable thin gauge products to be rolled commercially.
The 140 ~m foil has been annealed for 2 hours 3 at a range of temperatures using a simulation of batch ~nn~l; nrj, involving heating to temperature at 25C/hour and longitudinal tensile properties measured.
The variation of UTS, O . 2~6 proof stress, and elongation-to-failure are shown in Figures 1, 2 and 3, 35 respectively, for the five alloys. All alloys r- nt~;n;nrj the solute additions show an i, L~V. ~ in ` 2185216 W0 9~2s82s F~~ a UTS over the solute-free AA 8006, with the ; , uv - t being of the order of 20 to 40 MPa after the commercially u~able anneal at temperatures in the region of 220 - 260C ~owever, after ~nnP~l ;n~ at the highest temperature investigated (300C)_ the more c~n~-Pntr~tPd allûys have lower strengths than the 0 . 2~
additions. This is more prnn~unrP~ in the proof stress data, and indicates that in the more r~n-Pntrated alloys there is a loss in strength as a consequence of loss of grain size control caused by rl;~cnntiml~lus recrystallisation. This is confirmed by the optical metallography of the grain structures after ;3nnP:~l in~
where coarse grained regions are apparent in the solute ~;n;n~ alloyg annealed at the highest temperatures.
The loss of grain size control is often associated with loss of formability and ductility, although the ductilities do not shûw any reduction here, pûssibly as a consequence of the much thicker gauges P~m; nP~l here (140 ~um vs 14 ~m) preventing strain localisation.
This 1088 of grain size control at the higher temperatures in the 0.49~ rr~nt~;n;n~ alloys shows that there will be ~n upper limit on the amount of solute which can be added for solid solution strengthening without running intû problems with loss of strength (in particular yield strength) caused by coarser recrystallised grains.
r 1~- 2 p~ ;~nt Trials 3 Based on the wish to achieve a significant strength increase over the standard AA 8006 composition, a full scale processing trial has been performed with A~ 8006 plus 0.2 wt.~ Mg. This was DC cast as an ingot of 1600 mm x 600 mm cross section. The ingot was then processed, the processing route consisting of hot rolling to 3 mm, cold rolling to 450 ~m, and Wo 95/25825 ~ .~ 5,~c ~
interannealing at 360C. It was then cold rolled to the f inal gauge of 14 ~lm .
Tensile testing of the as-rolled foil showed the yield stress to be significantly higher than the standard Mg-free version (Table 2).
Commercial batch anneal is carried out at a temperature of 220 - 260C during a heating cycle of at least 8 hours f rom room temperature to the ~nn~ 1; ng temperature. Preferably the metal is held in the temperature range f or at least 3 0 minutes . The total cycle time depends on the coil width. Tensile properties of the plant annealed foil are shown in Table 2, showing that a significant strength imp, ,-v~ t is achieved over AA 8006 . The results of the plant trial clearly demonstrate that there will be an upper limit on the amount of magnesium which can be added to large cross-section D.C. cast ingot and still give the re~uired microstructure for rnnt;nllmlR
recrySt;,11; R~t inn .
The process of rolling is highly anisotropic as a result of crystal plasticity and inevitably leads to a product with preferred orientations or crystallographic texture. In order to describe crystallographic texture a system has been devised that 25 enables reference directions on the sample to be related to the crystallographic directions of a large number of grains on a simple diagram called a pole figure. The t~rhn;-r~ s for measuring crystallographic texture in metals are well established and an excellent 3 reference is Hatherley and ~ trh;n~on "An Introduction to Textures in Metals ~ The Institute of Metallurgists, r/lnnnrr~rh No 5, 1979.
The crystallographic texture of the foil samples before and after ~nnl~l ;nr, have bee~ rlf~t~rl~;nf~d 35 using x-ray diffraction from a laminate made from the 14 llm f oil . Figures 4a and 4b show the pole f igures `~ '. ` 21 8521 6 Wo 95/25825 1 generated from the ~111} aluminium planes orientated relative to the rolling direction (vertical), transverse direction (horizontal) and the foil plane normal ~into the page). Figure 4a ls the as-rolled 5 f oil . Fi~ure 4b is the annealed f oil . The contour levels are 1.00 1.60 2.20 2.80 3.40 4.00 4.60. This shows that the crystallographic texture i8 PC5Pnt i ~ y unaltered by the anneal, i . e . the texture is a retained rolling texture. Pole figures corresponding to other 0 aluminium reflections have also been obtained from which the Or;Pnt~t;nn Dlstribution Function (ODF) in the rolled and annealed conditions have been generated.
The volume fractions of specific texture rn~monpnts have been extracted from the ODF ' s and these are shown 5 in Table 3.
The grain size of the 14 llm foil has been determined after commercial i~nnPAl ;n~ using the mean linear intercept technique . This has been perf ormed on micrographs ~ht~; nPr~ in the Transmission Electron 0 Microscope (TEM). A total line length of lmm has been m;nPcl and the mean linear intercept grain size ~lPtPrm;nP-l to be 3.1 ~m.

.` 21~5216 Wo 95l25825 1 ~l, ,.,~.~C ~ -~

Table 1 - Effect o~ solute additions on the as-rolled strength of laboratory procesaed alloys rolled to give the eguivalent strain as commercially rolled housefoil.
Alloy 0 . 29~ Proof Stressl UTS ~longation (MPa ) (MPa ) ( % ) 8006 159 210 6 . 1 8006 + 0.2~ Cu 217 268 5.9 8006 + 0.4~ Cu 249 309 3.3 8006 + 0.2~6 Mg 259 311 2.2 8006 + 0.4~6 Cu 274 331 2.4 ~ ~ 2185216 WO 95/2~825 - 14 - r~
Table 2 - Te~8i.le properties O~ comnLercially pr.,.luc~d 14 ~n foil.
Alloy Cond:Ltion 0.2% Proo~ Stress ~TS 17lnr1~t;nn (MPa) (MPa) (~) AA8006 P.~-rolled 165 190 l.0 AA8006 Plant annealed 98 115 2.7 8006 + 0.2 Mg ~s-rolled 231 255 0.6 8006 + 0.2 Mg Plant annealed 102 123 l.9 WO 9S/2S825 - 15 - r ~ l l~. ,,, _ - '~ E
Table 3 - 14 ~m Foil r~ _ .1 Volume ~6 i 15~
As-rolled Annealed I
Cube {001)<100> 2.2 2.8 Goss { 110 } ~001~ 3 . 2 2 . 4 Copper {112}<111~ 20.2) 25.3) S {123}c634~ 35.2) 72.5 39.5) 79.2 Brass { 011 } ~211~ 17 .1) 14 . 4 ) Random 22 .1 15 . 6

Claims (6)

- 16 -

1. Aluminium foil composed of an alloy of composition in wt %:
Fe 1.2 - 2.0%
Mn 0.2 - 1.0%
Mg and/or Cu 0.1 - 0.5%
Si up to 0.4%
Zn up to 0.1%
Ti up to 0.1%
balance A? of at least commercial purity which foil has an average grain size below 5 µm after annealing.

2. Aluminium foil composed of an alloy of composition in wt %:
Fe 1.2 - 2.0%
Mn 0.2 - 1.0%
Mg and/or Cu 0.1 - 0.5%
Si up to 0.4%
Zn up to 0.1%
Ti up to 0.1%
balance AI of at least commercial purity produced by rolling followed by final anneal wherein at least 50% by volume of the as-rolled texture is retained after the final anneal.

3. Aluminium foil composed of an alloy of composition in wt %:
Fe 1.2 - 2.0%
Mn 0.2- 1.0%
Mg and/or Cu 0.1 - 0.5%
Si up to 0.4%
Zn up to 0.1%
Ti up to 0.1%
balance A? of at least commercial purity wherein the crystallographic texture of the final annealed product is a retained rolling structure.

4. Aluminium foil as claimed in any one of claims 1 to 3, wherein the foil thickness is 40 µm or less.

5. Aluminium foil as claimed in any one of claims 1 to 4, wherein the alloy composition in is Fe 1.4 - 1.8%
Mn 0.3 - 0.6%
Fe + Mn 1.8 - 2.15%
Mg 0.15 - 0.35%
Si up to 0.4%
balance A? of at least commercial purity.

6. A method of making the aluminium foil of any one of claims 1 to 5, which method comprises providing a billet of required composition, converting the billet to foil, and heating the foil to an annealing temperature of 220°C - 300°C.