Aluminium base alloys
United States Patent 1 Watts et al.  
 I1 Apr. 8, 1975 ALUMINIUM BASE ALLOYS [75] Inventors: Brian Michael Watts, l-laverhill;  
 Edward Frederick Emley, Chalfont Saint Giles; Michael James Stowell, Saffron Walden, all of England [73] Assignee: British Aluminum Company,  
 Limited, London, England and T.I. (Group Services) Limited, Birmingham, England [22] Filed: July 20, 1972 [21] Appl. N0.: 273,639  
 [30] Foreign Application Priority Data July 20, 1971 United Kingdom 33922/71 July 27, 1972 United Kingdom 33922/72 [52] U.S. Cl. 148/32; 75/139; 75/141; 75/142; 75/143; 75/146; 75/147; 148/325 [51] Int. Cl. C22c 21/00 [58] Field of Search 75/138-148; 148/32, 32.5  
 [56] References Cited UNITED STATES PATENTS 1.782.300 11/1930 Hall et al. 75/142 2,245,167 6/1941 3.020.154 2/1962 3,236,632 2/1966 3,666,451 5/1972 Bewlcy 75/147 Primary Examine&#39;rR. Dean Attorney, Agent, or FirmKarl W. Flocks [57] ABSTRACT An aluminum-copper entectic alloy which contains 33% copper exhibits the phenomenon of superplasticity but does not have the low density and good corrosion characteristics of conventional aluminium-base alloys. It has now been found that aluminium-base alloys consisting of the elements normally present in either non-heat treatable aluminium-base alloys containing at least 5%Mg or at least 1%Zn or heat-treatable aluminium-base alloys containing one or more of the elements Cu, Mg, Zn, Si, Li and Mn in known combinations, and at least one of the elements Zr, Nb, Ta and Ni in a total amount of at least 0.30% substantially all of which is present in solid solution, are superplastically deformable. The remainder of the superplastically deformable alloy may be the normal impurities and incidental elements known to be incorporated in heat-treatable and non-heat treatable aluminium-base alloys. Advantageously the alloy contains at least 0.30%Zr and preferably at least 0.40%Zr. The alloys of the invention may in some cases be deformed superplastically under isothermal conditions but it has been found advantageous to heat the alloy quickly to the super-plastic forming temperature and- /or allow the temperature to rise whilst the deformation is in progress.  
 13 Claims, No Drawings ALUMINIUM BASE ALLOYS BACKGROUND OF THE INVENTION It is know that certain alloys under certain conditions can undergo very large amounts &#39;of deformation without failure, the phenomenon being known as superplasiticity and characterised by a high strain rate sensitivity index in the material as a result of which the normal tendency of a stretched specimen to undergo preferential local deformation (necking&#34;) is suppressed. Such large deformation are moreover possible at relatively low stresses so that the forming or shaping of superplastic alloys can be performed more simply and cheaply than is possible with even highly ductile materials which do not exhibit the phenomenon. As a convenient numerical criterion of the presence of superplasticity, it may be taken that a superplastic material will show a strain rate sensitivity (nz-value) of at least 0.3 and a uniaxial tensile elongation at temperature of at least 200%, m-value being defined by the relationship a n 6&#39;&#39;&#34; where represents flow stress, 1 a constant, e strain rate and m strain rate sensitivity index.  
  No known aluminium-base alloy can be superplastically deformed other than the Al-Cu entectic composition which contains 33% copper and has neither the low density nor the good corrosion resistance characteristic of aluminium alloys.  
 SUMMARY OF THE INVENTION According to one aspect of the present invention a superplastically deformable aluminium-base alloy consists of an aluminium-base alloy selected from non&#39;heat treatable aluminium-base alloys containing at least 5%Mg or at least 1%Zn and heat-treatable aluminiumbase alloys containing one or more of the elements Cu, Mg, Zn, Si, Li and Mn in known combinations and quantities, and at least one of the elements Zr, Nb, Ta and Ni in a total amount of at least 0.30% substantially all of which is present in solid solution, the remainder being normal impurities and incidental elements known to be incorporated in the said aluminium-base alloys.  
  According to another aspect of the present invention a method of making a superplastically deformable aluminium-base alloy semi-fabricated product comprises casting a liquid alloy having a composition according to the immediately preceding paragraph at a temperature of at least 775C to produce a cell size in the cast alloy not exceeding /.LM and subjecting the cast alloy to plastic working at a temperature not substantially in excess of 550C.  
 By cell size is meant secondary dendrite arm spacing.  
  The invention also extends to an aricle shaped by the plastic forming of an alloy according to the said one aspect of the invention.  
 DESCRIPTION OF THE PREFERRED EMBODIMENTS Throughout this specification all percentages of elements are given as percentages by weight.  
  By heat-treatable alloys is meant those classes of alloys in which the mechanical properties can be improved by precipitation hardening treatments, for example alloys of the Al-Cu, Al-Cu-Mg, Al-Mg-Si and A]- Zn-Mg systems.  
  By non-heat-treatable alloys is meant those classes of alloys in which the mechanical properties cannot be significantly improved by precipitation hardening treatments, for example alloys of the Al-Mn, Al-Mg and Al-Zn systems.  
  Of the elements Zr, Nb, Ta and Ni it is preferred to use zirconium (Zr) in the alloy according to the invention as niobium (Nb), tantalum (Ta) and nickel (Ni) have been found to be less effective than zirconium in inducing superplastic behaviour in the alloy. These four elements have low solubility, high temperature coefficient of solubility and diffuse only very slowly in aluminium even at temperatures as high as 500C. When zirconium only is used in the alloy it is used in a quantity of at least 0.30% and preferably of at least 0.40%.  
  It is believed that the alloys according to the invention owe their superplastic properties to the presence of a supersaturated solid solution of one or more of the elements Zr, Nb, Ta and Ni in a sufficient quatity physically to restrict aluminium grain growth by giving rise at the temperatures employed for hot forming to a fine sub-optical precipitate capable of restricting grain boundary movements. The formation of such a fine sub-optical precipitate has been verified in alloys containing each ofthe elements Zr, Nb, Ta or Ni, but it was not found with Cr. or Mn.  
  Zirconium is already known to confer on certain aluminium-base alloys both grain refinement of the cast alloys and to restrict grain coarsening of the worked alloys. However, the maximum liquid solubility of zirconium in aluminium at the peritectic temperature is approximately 0.11% and additions of zirconium to aluminium alloys do not normally exceed 0.20%.  
  Tests carried out on alloys formed from pure grades of aluminium with 0.2% and 0.571 zirconium additions did not result in superplastic behaviour at any temperature of testing in the range 350C to 500C. Tests have shown that an aluminum-manganese alloy also does not deform superplastically after the addition of zirconium. These tests indicated that for an aluminium-base alloy to be superplastically deformable it is necessary not only to provide a slow diffusing element such as zirconium which would precipitate in the form of finely dispersed and relatively stable second phase particles from a supersaturated solution during hot forming, but also to provide one or more additional elements which inhibit recovery processes and allow the alloy to crystallise to an ultra fine grain structure, for example by lowering the high stacking fault energy of aluminium, thereby making possible the occurrence of dynamic recrystallisation during or prior to the hot forming.  
  These additional elements include Cu, Mg, Zn, Li and Si in such combinations and in such quantitites as are commonly used in heat treatable aluminium alloys and Mg and Cu in such combinations and quantities as may be used to produce non-heat treatable alloys of Al-Mg or Al-Zn systems containing at least 5% Mg or at least 1% Zn respectively.  
  Particular suitable combinations of additional elements include.  
 a. Cu 1.75  
 MgO  
 b. Cu 2.5  
 MgO  
  Mn0.25 d. Zn2  
  tional incidental elements, excluding Pb and Bi, not ex- -Cont1nued ceedmg 0.75%. To improve the machinabllitv of the al- Mg 0.75 to 4 &#39;71 Cu O m 3 loys small additions of Pb and/or B1 may be made in Zn 3 w 2 quantities up to 0.6% of each and up to 1% in total. 4: 5 When Pb and/or Bi are present in the alloy, the total t. Zn 4 w 7.5 /1 quantity of incidental elements, including Pb and/or Bi,  
  3: will not exceed l.257(. g. Si 0.4 m 0.9 (/1 The alloys according to the invention may in some h yet f H W cases be deformed superplastically under isothermal 0 8 pie l0 conditions following prolonged soaking at superplastic C to .5 forming temperaure but it has been found advantao 8 (Z prefcmbii geous to heat the alloy quickly to the superplastic forming temperature and/or allow the temperature to rise whilst the deformation is in progress. Under the latter it wiii be appreciated from what has previously been conditions elongation values of 800% to 1200% were stated that the additional elements of either h or i when b i d on A] 6%C 0 5%Z anoys hi h h d iaiioyed with aiumihium give hoihheai &#34;eaiiibie alloy ously shown elongation values of 500% to 700% after while the fldditiOllili elements Of any one Of the remain- Soaking at the plastic forming temperature and isothering combinations when liiioyed with aluminium g 11 mal deformation. The following table illustrates the difheat-treatable alloy. Alloys containing the additional ferences i h lt bt i ed by the two forming elements lz may need a higher forming temperature techniques on four other alloy compositions together range for best results e.g. up to 550C. with isothermal data on two further compositions.  
 TABLE A Elongation &#39;7! at forming temperature Alloy Type Approx. Composition* Isothermal test Rapid heating after soaking and/or rising at temperature temperature during test BA 733 Al; 4.5%Zn; 0.8Mg l50 330 BS L88 Al: ti /(Zn; .WMg:  
  l.5/(Cu 540 BS 2L70 Al: 571C111 0.9%Si; 170 300 (Liv/(Mu; (Mi /(Mg AA 2219 Al; 6.5&#39;/1Cu: 0.3 /rMn: I40 540 (LP/1v BS M20 A]; 0.7% Mg; (m /(Si; 200 288 0.2571Cu BS M20 Al; 7 /(M 250 Al: l0/(Zn 600 AI; 3&#39;/(Zn 360 Exclusive of Zirconium at approximately 05&#39; level except for the Al: 792 Mg alloy where 0.87: zirconium was present.  
  It is to be understood that the alloy according to the invention may contain the impurities normally to be found in heat treatable and non-heat-treatable aluminum-base alloys and one or more of the incidental elements known to be added to such aluminium-base alloys. These incidental elements include in percentages by weight:  
 B 0 to 0.05  
 O to 0.4 when not present as a specified constituent.  
 Rare earth metals and Mn All the alloys were rapidly cast from temperatures in the excess of 850C.  
 Attempts to determine the dissolved zirconium content in alloys according to the invention by wet chemi-,  
 cal processes have not yet proved entirely satisfactory,  
 but a suitable content can be assured by casting from,  
 much higher temperatures than are usual in the production of aluminium semi-fabricated wrought products together with the use of more rapid solidification .of the liquid alloy. Thus, whilst casting temperatures for known aluminium wrought alloys are in the range 665C to 725C, the alloy of the present invention is cast by temperatures in the range 775C to 925C and preferaably above 800C. For best results a casting temperature in the range 825C to 900C is preferred. Similarly, whilst the normal solidification rates obtaining in semi-continuous direct chill casting result in an average cell size or secondary dendrite arm spacing of 40 to uM, the solidification rates of the alloys according to the invention are designed to be such that the average cell size does not exceed 30 uM, and preferably does not exceed 25 uM. In this way the miniumum dissolved zirconium content required, believed to be 0.25% represents 0.2% in excess of the equilib rium solubility of zirconium at 500C.  
  If desired the approximate proportion of dissolved zirconium in an alloy of known total zirconium content can be determined by microprobe analysis; alernatively optical microscopy can be used to provide a rapid check as to whether or not there is a substantial proportion of the zirconium not in solution, the phase ZrAl being easily recoognisable.  
  When the alloy conains Nb or Ta in place of Zr, a high casting temperature and fine cell size are required; with Ni in place of Zr a high casting temperature is not essential.  
  To assist in the maintenance of a high level of supersaturated zirconium, the alloys of the present invention may be prepared by splat cooling or spray casting in known manner or by compacting blown powder.  
  To illustrate the invention aluminium-base alloys containing copper as an essential alloying element, but containing other optional alloying elements as will be mentioned, are now described by way of example.  
  Ordinary commercial aluminium of minimum purity 99.5% may be used for preparing the alloy but better results are obtained by limiting the iron and silicon content, e.g. by preparing the alloy from high purity aluminium of about 99.85% purity. Metal with purity lower than 99.5% (e.g. 99.3%) has nevertheless given acceptable results.  
  At a given purity level the adverse effects of iron and silicon are minimised if these elements are present in approximately equal atomic proportions. Thus as good results are obtained from 99.8% aluminium with atomically balanced iron and silicon as from 99.9% aluminium with an FezSi atomic ratio of 1:2 or b 2:1. A 1:1 atomic ratio corresponds almost exactly to an FezSi ratio of 2:1 by weight, the FezSi will therefore desirably between 1.521 and 2.521 by weight.  
  Preferably the copper content is in the range 2.5% to 7% and particularly in the range 3.5% to 6.5%. For high tensile properties in the formed or shaped object after subsequent full heat treatment, combined with good rolling properties, a copper content of 5.75% to 6.25% may be used. A substantially higher copper content than 7% can be tolerated where the alloy is to be extruded rather than rolled or can be pre-extruded prior to rolling, for example up to 10%.  
  Small amounts of some elements may be tolerated or added with a view of conferring certain properties on the resulting alloy. Magnesium may be added in amounts up to about 0.5%; manganese and cadmium may each be added in amounts preferably not exceeding 0.25%, whilst small amounts ranging from 0 to 0.2% of one or more grain refining elements Ti, Ta and Sc may be added to assist in obtaining a fine grained cast structure. Germanium may also be added in quantities up to 0.5% to control ageing behaviour.  
  To achieve superplasticity it appears to be necessary for the alloy when cast to contain a minimum level of zirconium in supersaturated solid solution so that the zirconium is then available to precipitate in such a manner during the hot forming operation as will assist in the production or maintenance of a very fine grained structure of average grain size below uM similar to that observed in other superplastic materials. This minimum content of dissolved zirconium will not be achieved unless the total zirconium content of the metal is at least 0.30%, and preferably at least 0.40%.  
  To obtain superplastic behaviour the copper content should desirably exceed the solid solubility level at the hot forming temperature. Thus for forming at a temperature of 400-425C the miniumum copper content is desirably about 2%.  
  Hot forming will generally be carried out in the temperature range 300500C and preferentially in the range 350-475C.  
  Although the slow diffusion rate of zirconium in aluminium allows the cast alloy to be hot worked by rolling or extrusion to a considerable degree without excessive precipitation from the alloy of the zirconium in excess of saturation (it being on the presence of excess zirconium that the capability for subsequent superplastic forming depends) it is clearly desirable to avoid excesive pre-heating of the alloy prior to hot working and to carry out the working operations at tempera tures below those at which the precipitation of zirconium is rapid, e.g. in the range 300C to 500C. If desired the cast metal may be held for some time at temperatures in the range 300C to 400C prior to hot working without detriment and sometimes with benefit to the final superplastic forming properties.  
  The hot formed objects may be heat treated to develop maximum tensile properties, e.g. the components may be solution heat treated for 40 min at 535C, rapidly cooled and then artificially aged (precipitation heat treated) for 6 hr at C. Alternatively, though at some sacrifice in their final properties, the objects may be rapidly cooled after hot forming and then artificially aged.  
  The alloys are fusion weldable provided they have a magnesium content not materially exceeding about 0.25%.  
  If prepared using high purity aluminium the alloys may be chemically brightened and anodised or subjected to other forms of decorative anodising treatment. For bright anodising the copper content may usefully be about 2.5%, and the combined content of iron and silicon should not exceed 0.2%. Alternatively, the alloys may be clad, e.g. with pure aluminium, to improve their corrosion resistance.  
  By virtue of their superplastic behaviour the alloys may be formed into complex shapes with sharp angles by applying air pressure for a few minutes to the alloy heated to a temperature in the range 300C to 500C.  
  Reference is now made to the following more specific Examples and experiments.  
 EXAMPLE 1 TABLE B 1 Total Zr content Maxim-um elongation (wt%) m-value Nil 127 plastic behaviour 1t will be seen from Table B that for superplastic behaviour a minimum total Zirconium content of about 0.3% is required.  
 EXAMPLE ll In a series of bulge test experiments some 0.030in thick sheets having the composition A1-6&#39;/(Cu-0.471.Zr were submitted to bulge tests at 440C and 455C. The sheet was blown by air pressure through an open circular die so as to form an unsupported bulge as shown by the results in Table C.  
 TABLE C Forming temp Pressure applied Height/dia Time taken (C) (p.s.i.) ratio of (mini bulge EXAMPLE III In other experiments an alloy of the composition Al- 6%Cu 0. 5%Zr was rolled and subjected to 200% isothermal deformation at 400C at a velocity of 0.05 in/- min. Tensile tests were carried out on specimens taken from the deformed alloy and also after full heat treatment on the deformed alloy with the results shown in Table D.  
 TABLE D Tensile properties at room temperature Condition 0.1% proof U.T.S. /r elong Hardstress MNmZ (on 50 ness MNm-Z (tsl) (tsl) mm g.l.) HV  
 As deformed 99 (6.4) 190 16 62 12.3) Fully heat treated 40 min at 535C 304 437 12 140 water quench (19.7) (28.3)  
 6 hrs at 170C It will be seen therefore that an alloy of the present invention is capable of being superplastically deformed and subsequently heat treated to give very attractive tensile properties. By modification of the ageing cycle LII even higher tensile properties can be obtained at some sacrifice of elongation. The alloy moreover has high resistance to both creep and fatigue.  
  A further advantage of the Al-Cu alloys at present being discussed is that the superplastic behaviour is not limited to a narrow range of temperature. Typical resalts from two casts of alloy are shown in Table E.  
 TABLE E Cast Forming Maximum No. Composition temp C m-value 71 elong Al-fi /lCu-052 /lZr 400 0.45 210 425 0.45 300 450 0.42 320 2 Al-o /(Cu0.50&#39;/1 Zr 400 0.41 410 425 0.41 300 450 0.40 250 The effect of additions of titanium or chomium in place of zirconium to an Al-6%Cu alloy have been investigated, but even with many tenths per cent of Cr and/or Ti present it was only possible to induce at most a marginal degree of superplasticity in the rolled metal. It appears therefore that an additive which will grain refine the cast structure or which will hinder grain growth after hot working is not sufficient and that performance of both functions by two additives is not sufficient for superplasticity to be developed in the absence of the fine sub-optical precipitate of the kind produced with Zr. Nb, Ta and Ni but not by Cr and Mn.  
 We claim:  
  1. A superplastically deformable wrought aluminumbase alloy consisting of 1. an aluminium-base alloy selected from the group consisting of (l-a) non-heat-treatable aluminium-base alloys of aluminium and one of the elements selected from the group consisting of Mg and Zn, the quantity of Mg being from 5% to 10% with zero to 0.5% Cu, and the quantity of Zn being from 1% to 15% with zero to 0.5% Mg and zero to 0.5% Cu, and  
 (l-b) heat-treatable aluminium-base alloys of aluminium and one of the elements selected from the group consisting of Cu, Mg, Zn, Si, Li, Mn and mixtures thereof in known combinations and quantities in a total quantity not exceeding 10%, and  
 2. Zr in an amount of 0.3% to 0.8% in total content of which at least 0.25% is present in solid solution,  
 3. the remainder of said superplastically deformable alloy being normal impurities and incidental elements known to be incorporated in said aluminium-base alloys.  
  2. A superplastic aluminum-base alloy according to claim 1, wherein said (l-b) heat-treatable aluminiumbase alloy consists essentially of aluminium and Cu 1.75 to 10 Mg 0 to 2 Si 0 to 1.5 in percentages by weight based on the total composition.  
  3. A superplastic aluminium-base alloy according to claim 1, wherein said (142) heat-treatable aluminiumbase alloy consists essentially of aluminium and Cu 2.5 to 7 Mg 0 to 0.5  
 in percentages by weight based on the total composition.  
  4. A superplastic aluminium-base alloy according to claim 3, in which the normal impurities include iron and silicon, the iron content ranging from 1.5 to 2.5 times the weight of the silicon content.  
  5. A superplastic aluminum-base alloy according to claim 1, wherein said (l-b) heat-treatable aluminiumbase alloy consists essentially of aluminium and Cu 3.5 to 5.5  
 Mg 0.25 m 1.25  
 Si 0.25 to 1 Mn 0.25 to l in percentahes by weight based on the total composition.  
  6. A superplastic aluminium-base alloy according to claim 1, wherein said (l-b) heat-treatable aluminiumbase alloy consists essentially of aluminum and Zn 2 to 8 Mg 0.75 to 4 Cu to 2 in percentages by weight based on the total composition.  
  7. A superplastic aluminium-base alloy according to claim 1, wherein said (l-b) heat-treatable aluminiumbase alloy consists essentially of aluminium and Zn 3 to 5.5  
 Mg 1 to 2 Cu 0 to 0.3 in percentages by weight based on the total composition.  
  8. A superplastic aluminium-base alloy according to claim 1, wherein said (l-b) heat-treatable aluminiumbase alloy consists essentially of aluminium and Zn 4 to 7.5  
 Mg 2 to 3 Cu 1 to 2 in percentages by weight based on the total composition.  
 - 9. A superplastic aluminium-base alloy according to claim 1, wherein said (lb) heat-treatable aluminiumbase alloy consists esseptially of aluminium and Si 0.4 to 0.9  
  Mg 0.5 to 1 in percentages by weight based on the total composition.  
  10. A superplastic aluminium-base alloy according to claim 1, wherein said l-a) non-heat-treatable aluminium-base alloy consists essentially of aluminium and Zn 1 to 15 Mg 0 to 0.5  
  Cu O to 0.5 in percentages by weight based on the total composition.  
  11. A superplastic aluminium-base alloy according to claim 1, wherein said l-a) non-heat treatable aluminium-base alloy consists essentially of aluminium and Mg 5 to 10 Cu 0 to 0.5 in percentages by weight based on the total composition.  
  12. A superplastic aluminium-base alloy according to claim 1, containing at lest 0.40% Zr.  
  13. A superplastic aluminium-base alloy according to claim 1, containing at least one of the following incidental elements, in a total amount not exceeding 1.25  
 percent by weight.  
  UNITED STATES. PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,876,474 Dated April 8 l975 Inventor(s) Brian Michael WATTS. Edward Frederick EMLEY and Michael James STOWELL It is certified that error-appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:  
 In column 1, cover page, the first Assignee&#39;s name should read: THE BRITISH ALUMINIUM COMPANY LIMITED Signed and Sealdthis Arrest.-  
 RUTH C. MASON C. MA Arrestin Offi RSHALL DANN ummissiuner nj&#39;latems and Trademarks