CA1113282A - Superplastic aluminium alloy - Google Patents

Abstract

Abstract of the Disclosure Aluminum alloy is disclosed which consists essentially of (a) Cad and Zn within the coordinates 2.0% Ca, 8.0% Zn; 6.0% Ca, 8.0% Zn; 7% Cad 3.0% Zn;
and 3.0% Ca, 3.0% Zn; (b) not more than 1.0% each of Si and Mn, not more than 0.2% each of Cu and Mg, not more than 0.5% each (not more than 1.0% total) of Fe, Ti, V, Cr, Zr and Sr, not more than 0.25% each (not more than l.0% total) of other elements; (c) balance Al, the percentages mentioned being by weight.
This alloy is useful for making products having superplastic properties.

Description

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This application is a divisional application based on Canadian Application Serial No. 299~997 filed March 30, 1978.
This invention relates to aluminium alloy which is useful for making products having superplastic properties. The invention further relates to novel aluminium alloys for use in the production of metal sheet and other pro-ducts having superplastic properties.
Superplastic alloys are able to undergo extensive deformation under small forces at temperatures within a range determined by alloy composition.
Swperplastic alloy sheet at appropriate temperature can be formed into complex shapes by blow moulding with compressed air at relatively low pressure in a manner similar to plastic or glass.
The most satisfactory cr~terion applied to define superplastlcity is a tensile elongation of at least lQO% and more preferably at least 200%. It is -also considered desirable that a superplastic alloy should exhibit a strain rate sensitivity index value m of at least about 0.3. The alloy should exhibit these properties at a selected forming te~perature within the range 300-600C
(more usually 400-50QC~ and need not exhibit these values throughout this range. In general it may be said that both tensile elongation and strain rate sensitivity index values increase with increase in temperature.
Known superplastic alloys have been found to have utility in making metal parts of configurations difficult to produce from sheet metal by conven-tional techniques. One known superplastic alloy is a zinc~base alloy containing 22% aluminium. A known superplastic aluminium-based alloy containing 6% copper and 0.5% zirconium, is advantageous for various applications because lt is lighter in weight, and has better creep resistance and surface finish than the zinc-based alloy, but it is relatively difficult to produce and so~ewhat suscep-tible to corrosion. The binary eutectic alloy of aluminium with 7.6% calcium : . - , , ~
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Z~2 is also superplastic, but calmot readily be cold-worked owing to its brittleness.
According to one aspect of the present invention an aluminium alloy containing calcium and zinc, in proportions relatively close to a ternary eutectic composition, can be treated to develop useful superplastic properties when cast and worked in a particular manner as hereinafter described. For the present purpose the term "worked" implies that the alloy has been subjected to one or more of the operations of rolling, drawing, extruding or forging. The superplastic products of these alloys, in addition to having the attributes of light weight and superior creep resistance and surface finish characteristic of other superplastic aluminium alloys(as compared with zinc-based alloys), are easy to produce and afford an improved combination of corrosion resistance and cold-working properties ~as compared with known superplastic Al alloys).
The invention may be generally defined as an aluminium alloy consist-ing essentially of ~a) Ca and Zn within the coordinates 2.0%.Ca, 8.0% Zn; 6.0% Ca, 8.0% Zn; 7% Ca, 3.0% Zn; and 3.0% Ca, 3.0% Zn;
~b) not more than 1.0% each of ~i and Mn, not more than 0.2% each of Cu and Mg, not more than 0.5% each ~not more than 1.0% total~ of Fe, Ti, V, Cr, Zr and Sr, not more than 0.25% each (not more than 1.0% total) of other elements;
(c) balance Al, the percentages mentioned being by weight.
Th.e accompanying drawing is a graph illustrating broad and preferred Al-Ca-Zn composition ranges and showing the relationship of these ranges to the eutectic trough of th.e ternary Al-Ca~Zn system.
The method of making products which exhibit superplastic properties from the already mentioned Al Ca-Zn alloys inYolves the performance of certain steps on alloys having those compositions.
The pertinent features of composition may be explained with reference ~ 2 ~
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thc accompanying drawing. It has been discovered that, for the ternary system Al-Ca-Zn, i.e. the system of alloys constituted of a major proportion of aluminium ~ith calciuln and ZillC as principal alloying elements, there exists aeutectic trough which is represented in the drawing by line 10. Al-Ca-Zn alloys having a composition close to this eutectic trough can be cast to produce a cellular eutectic structure including, in an aluminium matrix, a substantial volume fraction (10 to 30 volume percent, usually 1~ to 23 volume percent) of fine eutectic rods of one or more Ca-Zn-Al intermetallic compounds, formed from the melt in the casting operation and having an a~erage diameter of 0.05-1.5 microns. These rods can be fractured into particles having an average particle diameter ~as later defined) in the range of 0.05-2 microns. It is believed that this intermetallic phase is (CaZn)A12 as distinct from the brittle CaA14 phase found in a binary Al-Ca alloy.
In the broadest sense, superplastic wrought products can be produced from alloys having proportionS of Ca and Zn ~ithin the limits defined by the broken line rectangle 12, viz. 2-8% Ca and 1.5-15% Zn. Although the best super-plastic properties are exhibited by alloy products haYing compositions close to the eutectic trough, decreasing but still useful superplastic properties ~ -are attainable with compositions lying to the left or right of the trough, within the broad limits of rectangle 12.
The degree of superplasticity attainable decreases progressively with decreasing Ca content, until at less than 2% Ca the Yolume fraction of the Al-Ca-Zn intermetallic particles becomes too small to pro~ide useful superplastic behaviour. Increase in Ca content to the righ* of the eutectic trough tends to result in undesirable formation of coarse primary intermetallic crystals.
Coarse primary crystals can be somewhat suppressed by increasing the casting temperature, but this expedient becomes very difficult with compositions ~ 3 .

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containing more than 8% Ca. As indicated by broken-line rectangle 14, a prefer:red upper limit of Ca content is 7%.
Alloys containing less than 1.5% Zn may be superplastic but they are very brittle and tend to crack badly during bending and/or cold rolling; alloys containing more than 10 to 15% Zn may also be superplastic but have very poor corrosion resistance. The variation of superplasticity (in terms of percent tensile elongation at forming temperature~ with zinc content is such that the best superplastic properties are attainable by compositions containing less than about 8.5% corrosion resistance of the higher zinc alloys, a zinc content in the lower port-on of the broad range affords: an advantageous com~ination of superplasticity and corrosion resistance. As rectangle 14 further indicates, 10% is a preferred upper limit of Zn content.
The most preferred range of Ca and Zn proportions, affording the best combination of superplastic hehaviour, corrosion res~istance, and resistance to cracking under cold working or bending, is that defined by the figure ABCD in the drawing, viz. alloys having proportions of Ca and Zn ly~ng within the coordinates 2.0% Ca, 8.0% Zn; 6.0% Ca, 8.0% Zn; 3.0% Ca, 3.0% Zn; and 7.0% Ca, 3.0% Zn. For a specif~c zinc content with~n the range of 1.5 - 15% Zn and particularly within the range of 3 - 8% Zn lt is pre~erred that the Ca content is within 0.5% of the Ca value at the eutectic trough.
With the exception o S~, ~n, Cr, Cu, Zr and Sr, impurities and minor additions of other elements tend to coarsen the as-cast eutectic structure and are thus undesirable. ~gain stated broadly, the upper limit.s of additions and impurities in alloys suitable for the pract~ce of the invention are 2.0% each of Mg, Si, Mn and Cu; other elements, 1.0% each, 2% total. Preferably, how-ever, the following maxima are obser~ed ; - . . ....................... .- ~ . :;

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Si, Mnup to 1. 0% each Cu, Mgup to 0.2% each Fe, Ti, V, Cr, Sr up to 0.5% each, up to 1.0% total Othersup to 0~25% each, up to 1.0% total The above preferred limits are set for Cu and Mg because Mg levels over 0.25%
lead to cracking during cold-rolling while Cu levels over 0.2% reduce corrosion resistance. -An especially preferred alloy composition is that consisting essential-ly of Ca and Zn within the ranges of proportions defined by the figure ABCD, with all additions and impurities held below the a~ove-specified preferred maxima, balance aluminium. ~:
As stated, Al-Ca-Zn alloys having compositions within the broad or 10 preferred limits set forth above are capable of developing a cast structure : :
of fine eutectic Ca-Zn-Al intermetallic rods which, upon working, break up ~ .
into particles that impart superplasticity to the alloy product. The method of the invention includes the steps of casting the Al-Ca-Zn alloy in such manner as to produce the requisite cast structure, and then working the cast mass to fragment the rods into the desired particles by procedures generally described ~ .;`.
in Patent Application No. 2QQ,289. ~.
As set forth.in that patent, the most convenient method for producing .~. : .
rod-like lintermetallic phases in an aluminium mass is to cast a eutectic or near-eutectic allo~, incorporating allo~ing el.ements which form intermetallic .
~20 phases with aluminium on sol~diication, under selected casting conditions to produce a fine coupled growth stTUCture. That phenomenon is well known and is ~: explained in an article ~ 3.D. Livlngston in ~aterial Science Engineering, Vol. 7 (1971~, pp. 61-70.
The Al-Ca-Zn eutectic, ~hen cast in ingot form by the direct chill semi-continuous casting process or cast by other continuous or semi-continuous casting process involving a high solidification rate, produces a rod-like eutectic structure. For the purpose of the present invention it i5 preferred ~O*
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that the rod-likc phases should not be aligned with the axis of the cast mass.
In consequence, ingots may be produced by conventional direct chlll semi-continuous casting under conditions selected to ensure coupled growth of the intermetallic phase in fine rods in the matrix composed of the more ductile aluminium. Very satisfactory superplastic products can be achieved provided that the cast ~ass is produced in such a manner that the intermetallic phase grows in the form of fine closely spaced rods that can be broken up by subsequent working to produce a uniform dispersion of fine intermetallic particles which are on an average less than 2 microns in diameter. These particles tend to coarsen somewhat during superplastic forming, i.e. up to an average particle size of 3 microns or higher.
In contrast to these particles formed by fracturing the rod-like Al-Ca-Zn eutectic phase, coarse primary intermetallic particles are generally in the form of faceted polyhedra, resulting from nucleation ahead of the solidi-fication front during casting, and range upwardly in size from about 3 microns, and typically upwards o 10 microns. In the practice of the present invention, the cast alloy is considered to be essentially free of such coarse primary particles when their total volume is not more than 2%.
The average particle diameter of the particles formed by fracturing the rods is determined by counting the number of particles present in unit area in a micrograph of a cross section, ignoring coarse primary intermetallic particles and fine particles that are precipitated from solid solution. Such coarse and fine particles are easily recognizable byand then given by the formula:
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d ~ 1.13 / Np where: d = particle diameter.

Np = number of particles per unit area measured from photomicrographs Y = volume fraction of intermetallics measured by point analysis of a ~ .,, :
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metallographic section using visual observation through a micro-scope eyepiece fitted with a fine-meshed square grid - see pages 165, 168 and 169 of Modin and Modin reference given below.
The above formula, taken from H. Modin and S. Modin~ Metallurgical , trans. G. G. Kinnane (London: Butterworths, 1973), p. 164, ex-presses the si~e of the particles in terms of the diameter of a sphere of equal volume. The diameter of an elongated particle formed by segmenting a cylin-drical rod is, when expressed in these terms, usually larger than the diameter of the rod from which it formed. -Since there is no requirement for the coupled phases (intermetallic -rods) to be aligned in a single direction, it is unnecessary to supress the formation of eutectic cellular growth (caused by the segregation of impurities), ~-~
and therefore commercial purity aluminium metal can be used for the production of the cast alloy. This cellular or "colony" mode of solidifications prodaces unaligned intermetallic rods. In producing the cast alloy, the metal should be cast under such conditions that substantially no nucleation of intermetallicsoccurs in the molten metal in ad~ance of the $ront between the liquid metal and solid metal, i.e. so that the cast alloy ~ill be essentially free of coarse primary particles. T~e solidification rate Crate of deposition of solid metal in a direction substantially perpendicular to the solidification front) should be at least 1 cm/minute to achieve the growth of the rod-like intermetallic phase.
Thus ingots having the desired characteristics may be produced by the conven-tional direct-chill ~"D.C."~ continuous casting process in which coolant is applied direct to the surface of the ingot as it emerges from an open-ended mould or by twin-roll casting processes such as the "Hunter-Engineering" processin which molten metal is drawn $rom a nozzle and solidified by a pair of heavilychilled rolls. Unsatisfactory structures are produced by sand casting and per-. . - : . . .:

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manent mould casting and other processes that produce a non-uniform micro-structure. T~e D.C. casting process, particularly when employing a hot-top mould in conjullction with a glass-cloth distributor, maintains relatively stable con-ditions in the vicinity of the solidification front, while applying a heavy chill to the solidified metal by the application of coolant to the surface of the ingot emerging from the mould. This enables the desired high solidification rate to be achieved as requlred for coupled growth of metal matrix and inter-metallic phase in conjunction with provis-ion of a steep thermal gradient in the immediate vicinity of the solidification front, or avoidance of growth of coarse primary intermetallic particles.
~hen the cast alloy is deformed by ~orking, the intermetallic rods tend to fracture evenly along their length, creating somewhat elongated particles of relatively uniform size. These particles tend to disperse themselves evenly throughout the ductile metal matrix during the subsequent deformation of the ingot. The aspect ratio (ratio of length to diameter) of the majority of particles formed by the disintegration of the intermetallic rods falls in the range of 1:1 to 5:1. By contrast, the average length of the rod-like inter-metallics in the cast alloy is usually substantially more than 100 times their diameter.
Having produced a cast alloy of the necessary structure, the breakdown of the brittle intermetallic phase into dispersed particles less than 2 microns in average diameter (as calculated by the formula given above) may be achieved by either hot and/or cold working the cast alloy in a variety of ways. A reduc-tion of at least 60% is required for the necessary dispersion of the particles formed by fracturing the intermetallic rods. ~n the production of rolled sheet suitable for subsequent superplastic deformation, it is preferred to perform the major part of the reduction of the initial ingot by~hot rolling, but it is also ~ 8 ~

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preferable to apply a subsequent cold-rolling operation. Indeed, stated generally, it is preferable that the working step include final cold working in an amount equal to at least about 6Q% cold reduction. By the term "cold working", it should be understood that the alloy has been sub~ected to working at a temperature below about 25Q~C.
Preheating before hot rolling s*ould be kept to a minimum. Hot roll-ing temperatures of 400 to 500QC have been found satisfactory; use of lower hot rolling temperatures (~ith~n this range) tends- to reduce particle coarsen-ing. Subsequent cold rolling can be performed without inter-annealing, and no treatment is needed after cold rolling, since the as-rolled sheet has the required superplastic microstructure.
Typical conditions for superplastic forming of shapes from a sheet alloy product of the present invention are as follows: sheet thickness 1 mm, temperature 450C, pressure 5.25 kg/cm2, time 2 minutes. The blanks tsheets to be formed) are usually preheated Ce.g. to 45QC) to ensure an even tempera-ture distribution, but successful forming has been achieved starting with cold blanks, which are heated in position in the forming apparatus.
The alloy products of the invention, e.g. sheet, can be super-plastically formed by blow-moulding using equipment and techniques heretofore known and used for forming other superplastic alloys, at appropriate temperatures within the above-specified forming range. The mechan~cal properties at room temperature of the articles thus produced vary~to some extent, depending on the time and temperature of the forming operation (increase ~n forming time and temperature decreases yield strength and ultimate tensile strength and increases elongation~, but typical properties are as follows; Q.2% y~ield strength, 1480-19aO kg/cm2; ultimate tensile strength 176Q-1970 kglcm2; elongation ~5 cms) 13-19%. ~hese properties allo~ convent~onal cold-forming after super-~7 ~ :, . ~ ' ' . , -:

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2~'2 plastic forming.
The creep resistance o~ the alloy products of the present invention is found to be similar to that of other aluminium alloys, i.e. very much better than zinc-based alloys. In addition, these products exhibit good corrosion resistance, as determined by neutral salt spray and tap-water pitting tests.
By way of further illustration of the invention, reference may be made to the following examples.
EXA~PLE 1 An alloy containing 5.0% Ca, 4.8% Zn was prepared from super-purity Al and commercial purity Ca and Zn and cast in the form of a 95 mm x 229 mm D.C.
ingot, using a glass cloth screen in the mould. Casting speed was 102 mm per minute and casting temperature 700C. The ingot was scalped 6 mm on each face, hot rolled at 490C to 6 mm thickness, and then cold rolled to l mm or 0.6 mm final thickness. The resultant slleet was superpla~tic in the temperature range 450C to 500C as judged by the following measurements:
(1) Strain rate sensitivity index "m"; values of 0.3 were obtained at both 450C and 500C measured in hot tensile tests- on 51 mm gauge length sheet specimens at an initial strain rate of 2 ~ 10 3 sec. 1 ~2) Tensile elongat~on, values of 232% and 267% were measured at 450C and 500C respectively, using sheet tensile specimens of 50 mm-gauge length tested at a strain rate of 3 x 10 2 sec. 1 ~3) Shapes, such as hemispherical domes, were formed at 450C by low pressure compressed air orming: e.g. a sheet of 0.6 mm thickness was formed at a pressure of 1.4 kg/cm2 at 450C to a dome in a time of 50 seconds.

An alloy containing 4.94% Ca, 5.25% Zn was prepared from commercial purity Al containing 0.16% Fe and 0.07% S~ and from commercial grade calcium - 10 ~
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and zinc. The alloy was cast in the form of a 127 mm x 508 mm x 1016 mm D.C.
ingot, using similar casting conditions to those described in Example 1. The -ingot was scalped 9 mm on each face, hot rolled to 6 mm gauge, and cold rolled to various final gauges in the range 1.5 mm to 0.38 mm. This sheet exhibited superplastic behaviour. The strain rate sensitivity index, m, was measured by means of a blow moulding technique as described by Belk, Ing. J Mech. Sci., Vol. 17, p. 505 (1975). Values of m ranged between 0.26 and 0.37 over the range of testing temperatures from 375C to 525C.
10After superplastic forming at 450C, this alloy exhibited room tem-perature mechanical properties as follows:
0.2% yield strength 1620 kg/cm2 Ultimate tensile strength 1830 kg/cm2 Elongation 19%
EXA~PLE 3 Alloys COntaiTIing approximately 5% Ca, 5% Zn, and various third element additives (Pemainder cGmmercial pur~ty All ~ere cast ~n the form of 89 mm x 229 mm D.C. ingots and fa~ricated to s~eet in the manner described in Example 1.
The compositions and values of percentage elongation and m at 450C of these alloys are listed in Table r..

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TABLE I

Superplasticity Parameters, Percentage Elongation and m at at 450C for the Alloys of Example 3 Example Composition (wt %) _ ;
Ca Zn Other Remainder % Elongation A 4.73 4.81 0.5 ~n Al 338 0.29 B 4.78 5.0 0.26 ~n " 408 0.33 C 5.23 5.00 0.10 Zr " 300 0.28 D 5.13 4.88 0.45 Cr " 323 0.22 ~ -E 5.33 4.97 0.073 ~g " 478 0.32 5.0 5.0 0.2 ~g " 345 0.51 G 5.00 4.98 0.21 Cu " 395 0.34 An alloy containing 5.0% Ca and 5.0% Zn (6alance commercial purity Al) was cast in the form o~ a 178 mm diameter D.C. cylindrical extrusion ingot using similar casting conditions to those given in Example 1. The ingot was preheated to approximately 5aOQC and extruded to a tu~ular section with an exter-nal diameter of 33 mm and an internal diameter of 25 mm. This section was then cold drawn down to a tube of external diameter of 25 mm and an lnternal diameter of 21 mm. This cold-drawn tube exh~bited superplastic behaYiour at 450C as evidenced by the ability to expand t~e tube ~nto a mould by compressed air pres-sure of only 5.6 kg/cm2 in a time of 15 minutes.

An allor containing 4.0% Ca and 4.0% Zn Cbalance commercial purity Al) was cast in the form of a 89 mm x 229 mm D.C. ingot and rolled down to metal sheet in the manner described in ~xample 1. Tensile tes*s were carried out at 450C using 25.4 mm gauge-length test pieces. At a strain rate of 1.67 x 10 3 sec. 1, an elongation of 226~ was recorded, thus indicating the fully super-plastic nature of the allo~.

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An alloy containing 4.94% Ca, 5.25% Zn was prepared from commercial purity Al containing 0.16% Fe and 0.07% Si and from commercial grade calcium and zinc. The alloy was cast in the form of a 127 mm x 508 mm x 1016 mm D.C. ingot uslng similar casting conditions to those described in Example 1. The ingot was scalped 9 mm on each face and was hot-rolled to 6 mm gauge. Tensile specimens, cut from this plate and tested at 450~C at a strain rate of

3 x 10 2 sec. 1, exhibited an elongation of 4Q8% without failure, thus confirm-ing the superplastic nature of the hot-rolled product.

Samples of the 6 mm thlck hot-rolled plate, described in Example 6, were stamped into 31~8 mm diameter blanks (or "slugs"). These were impact-extruded at room temperature to cylindrical cups 31.8 mm in diameter and approxi-mately 100 mm long. These cups exhibited superplastic behaviour, demonstrated by the fact that they could be expanded into complex shapes at 450C using compressed air at 4.2 kgs/cm pressure.
EXA~PLE 8 The alloys listed in Table ~I ~ere cast as 89 mm x 229 mm D.C. ingots.
These were hot rolled to 6 mm thicknes~s and then cold rolled to 1 mm thickness.
Tensile tests were carried out at 450~C at a strain ~ate of 5 x 10 3 sec. and '~!, the elongations shown in Table II measured.
TABLE IT
Alloy % Ca % Zn % Elongation 1 1.0 5.0 65 2 3.5 5.0 198 3 5~0 5~Q 300 These res-ults s~ow tnat ~hereas 1% Ca is insufficlent to confer super-plastic properties, additions o 3.5% and 5.0% Ca in conjunction with 5% Zn both ~ ~ , . . .

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confer superplastic behaviour, the latter composition being superior and having a composition closer to the eutectic trough 10 in the drawing.

Alloys having the composition indicated below ~remainder commercial purity Al) were cast as in Example 1 and were rolled to 1 mm. sheet. The sheet was subjected to bend tests-at room temperature and tensile tests at 450C.
From the bend tests the minimum radius mandrel over which samples could be bent without cracking are listed below. These show that higher zinc levels are associated with low minimum bending radii, i.e. are less brittle. The high temperature tensile tests gave ~alues of elongation that show the alloys to be superplastic.

~inimum bend radius ~% Elongation % Ca _ (in.) {at Room Temperature) at 450C
A 6.2 2.0 0.146 470 B 5.0 5.0 0.040 408 C 3.9 8.5 0.018 155 D 3.6 10.0 O.Q18 133 E 3.2 15 o.a26 230 :;: , : :
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Claims (2)

1. THE EMBODIMENTS OF TIE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   l. An aluminium alloy consisting essentially of (a) Ca and Zn within the coordinates 2.0% Ca, 8.0% Zn; 6.0% Ca, 8.0% Zn; 7% Ca, 3.0% Zn; and 3.0% Ca, 3.0% Zn; (b) not more than 1.0% each of Ci and Mn, not more than 0.2% each of Cu and Mg, not more than 0.5% each (not more than 1.0% total) of Fe, Ti, V, Cr, Zr and Sr, not more than Q.25% each (not more than 1.0% total) of other elements;
   (c) balance Al, the percentages mentioned being by weight.
2. 2. An aluminium alloy according to claim 1, in which, in relation to the Zn content, the Ca content is within 0.5% of the value of the Ca content at the eutectic of the system Al-Ca-Zn.
   
   FETHERSTONHAUGH &CO
   
   OTTAWA, CANADA