US3196528A - Metal sheet article and process for making - Google Patents

Description

United States Patent The present invention relates to novel alloy compositions having improved strength combined with ductility, to the methods of forming the alloys and of forming articles therefrom. More particularly, this invention relates to the formation of metal sheet articles having I hollow portions developed therein by inflation, wherein 5 one side of the sheet has higher strength than the other and remains essentially smooth and flat following the I inflation.

The process of forming metal articles in sheet form having hollow portions developed therein by inflation is disclosed in the patent of Grenell, US. 2,690,002. A substantial amount of art has been built up based on the concepts and procedures disclosed in the original patent.

1 Many of the articles formed by the Grenell and related processes are employed in refrigeration applications. This use of the panels is a natural outgrowth of the properties possessed by the composite sheets and the intricate patterns of coolant flow passages which can be developed therein. In forming such a sheet the pattern itself is produced by disposing a foreshortened pattern of weld inhibiting material onto sheets to be joined together at their confronting surfaces prior to the pressure bonding and pressure bonding the sheets to extend the patterns to the desired dimensions.

One of the problems which has persisted in the use of refrigeration sheets of this type is the puncture of the tube passages by a person using some sharp instrument to assist in removal of ice deposits formed on the sheets during normal refrigeration uses. This puncture of the coolant flow tubes is commonly referred to as ice pick damage in the art. One way of reducing this damage is by forming sheets having tubes which are flat on one side and lie in the plane of one surface of the sheet. Damage of this type may also be reduced if one of the sheets, in addition to being essentially flat, is the stronger of the two sheets of the composite.

In accordance with the Grenell patent, 2,690,002, sheets which are flat on one side may be formed by the use of formed dies. However, it will be appreciated that the cost of formed dies and the additional labor and material needed results in a higher cost product over the products obtained from processes which can be used in forming coolant flow passages bulged out of one side of a sheet, but which does not rely. on the preparation and use of formed dies. It is particularly evident that there is an extreme latitude in the selection of the patterns of flow channels which can be developed in a sheet where no formed die must be employed, and that all that is needed for a change from one pattern to another is a change of the foreshortened pattern of weld inhibiting material which is printed on one of the component sheets before the sheets are pressure welded together.

Problems are encountered in the large scale production of composite sheets having coolant passages expressed from one side only principally because'of the difliculty of working with pairs of component metal sheets, each sheet of which has a distinct combination of physical and/ or metallurgical properties.

For example, products having a substantial difference in the radius of curvature of the tube walls formed by free inflation have been prepared using bi-alloy blanks, i.e., blanks formed with component sheets of two distinct alloys; and also using bi-gauge blanks, i.e., blanks formed with component sheets of different gauge. No product has been formed commercially, however, which is flat on one side as a result of free inflation, nor has any product been formed heretofore which remains flat, after or inflation into a one-side-flat configuration when subjccted to high fluid pressures as in pressure testing at pressures of pounds per square inch or in use with fluids at the higher pressures.

To some extent this has been overcome byuse of a cavity die inflation employing two hydraulic pressures, one for holding the flat side of the panel against a flat platen and the other for inflating the expressed side at a higher pressure and against the hold down pressure. However, unless the flat component layer is substantially stronger than the, expressed layer, there is a tendency for the flat sheet to bulge out when the tube passages are pressurized for test purposes, or in use, with a sufliciently high pressure. Accordingly, a very desirable product is one which will maintain the flatness of the flat side when pressure tested and in actual use.

A very desirable condition for forming one-side-flat refrigeration panels, and particularly for forming the blanks from which such panels are subsequently formed by inflation of the coolant passageways, is an essential identity of properties of the components employed in making the blank up to the point where the joining of the panel-s is completed. However, from this point on it is desirable that there be a substantial difference between certain properties of one sheet and the same properties of the other sheet of the composite.

In fact, the desirable combinations of properties for formation of a product of this type are essentially selfcontradictory. This is so because, although an essentially identical set of metal working characteristics is desired in the component starting sheets for formation of the blank, a very different set of properties is desired for formation of the expanded panel from the blank by inflation.

One of the problems, which is encountered in forming a blank capable of ready conversion into a bi-alloy oneside-flat refrigeration panel, is that of controlling the dimensions and alignment of the component sheets during the pressure bonding operation. Where substantial differences of physical or metalurgical properties, or other metal working characteristics, exist between the starting sheets, such problems as dimensional control, curling, and misalignment are aggravated during the operations leading up to the formation of the completed composite blank.

The pattern of stopweld is placed on the harder of the two component alloy sheets because there is appreciable difference in the degree of extension of the softer sheet. The result is the production of products with poorer specifications and an increase in the amount of the waste produced.

However, where the metal working properties of the component sheets of a blank are very similar or essentially identical, the cost of producing bi-alloy one-sideflat panels is higher partly because of the necessity and the use of dies or die substitutes, as referred to above. Also, where identical properties exist in each of the components of the composite blank, a less desirable product is formed, because both component layers will be equally soft, and the softness of both layers will thus be the degree which is needed for expansion of one side only. It is undesirable to have the flat side of the finished panel as soft as the expanded side because of the lowered resistance to ice pick damage which is ofiered by softer metal as noted above.

One object of the present invention is to provide novel aluminum alloy compositions having desirable combinations of properties such as combined strength and ductility.

duce the precipitation of the dissolved zirconium into the a hollow sheet article having one side'essentially flat and a' preheat practice consisting of a soak of the ingot at a having substantial differences in properties between the component layers of the article.

A further object is to provide a method for fabricating a one-sidedlathollow panel. 7 7

Still another object is to form a blank subject to being expanded into a -oneside-flat pressure bonded metal sheet where the layers of said blank have essentially the same plasticity during a pressure bonding operation.

' panel-s of the general type disclosed in US. Patent No.

2,690,002 in which one of-the outer surfaces of the formed panel hasna high degree of smoothness andv a period of between 8 hours and 15 hours.

desired extremely fine form, the ingot was subjected to preferred temperature between 850 F. and 900 F. for The time interval specified'is the time during which the alloyis within the stated temperature range.

. 10 Another objeet is to provide a method for fabricating Where zirconium concentrations are higherfor exam ple approaching 0.3 percent,'longer soak periods, up to .about hours, are used to make possible production of the best mechanical properties after cold work. For

' lower zirconium concentrations, in the preferred range of ti-on may be achieved 'by'preparing a sheet from an ingot of 1100 aluminum alloy containing from about 0.1 to

about 0.3 percent zirconium uniformly dispersed; therein in an extremely fine form, aligning said sheet with a sheet of an equal size and thickness of 1100 aluminum alloy, either one 'of the confronting surfaces of said sheets having applied thereon a design of weld' inhibiting material, pressure welding said sheets together in-the areas free of inhibiting material byrolling the assembly at high temperatures, cold rolling the composite product and par tially annealing said composite sheet to develop diiferential yield strength in the component layers thereof, and inflating thec-omposite in the unwelded areas by fluid pres-sure. r

Among the articles which may be produced in accordance with the foregoing method is an article comprising a composite sheet of aluminum :1'100'alloy having hollow portions between component layers thereof, one layer of said sheet being essentially fiat and containing between Examples A sample of aluminum alloy 1100 was melted :and alloyed with zirconium to give a composition containing a 0.12 percent zirconium inthe alloy. The molten com position was then cast by the DC casting process, entering the DC casting mold at a temperature between 1325 F. and 1375 'F. Care was exercised to avoid having the vmolten composition enter the mold at a temperature below 1325 F.

as used herein, is meant particles of a size the. larger of which are clearly resolvable, by metallographic microscope techniques only at higher magnifications of 500X andabove;

Regarding the lower limit of the soak period, although 1 some precipitation occurs'from use of soak periods below I 8 hours, the use of a; soak period atleast eight hours in 5 length is necessary for attainment of optimum anodizing 77 properties in the sheet surface. Where less than an eighthour soak is employed with an aluminum 1100 alloy con taining at least 0.10 percent zirconium, some of the unprecipitated zirconium will come out 'of solution during Lower temperatures of the order of 1325" F. or below ed out below. The use of melt temperatures in the range of 1325 to 1375" F. permi-ts'casting of ingots containing 0.12 percent Zr with substantially all of the precipitated ZrAl in the desirable finely divided form.

The cast ingot contained zirconiumiin solid solutionand ZrAl dispersed'in extremely fine form of microscopic and submicroscopic particles. In order to in hot rolling, and will produce a surface which shows undesirable streaking and/or discoloration from the anodizing treatment.

It has been found that this preheat treatment is essential in ensuring the presence of ,a' maximum amount of the zirconium in the form of a very fine or microscopic dispersion of ZrAl uniformly distributed through the process ingot. Although this dispersion has essentially no effect on the hot rolling properties of the alloy, an opti-v posite product. Alternatively, the ingot can be hot rolled to some intermediate gage and then cold rolled to final gage wherethis cold rolling is followed by an anneal.

The'upper temperature limit of 950 F. for soaking or subsequent primary heat treatment of the alloy is necessary because above this temperature the zirconium begins to redissolve, and this can lead to subsequent reprecipitation of the ZrAl in the less desirable larger particle form.

Apiece taken from this starting stock containing 0.12

' percent zirconium, is prepared for assemblywith a dimensionally corresponding sample of aluminum alloy 1100. The faces of the components to be confronted in theassembly may be cleaned by conventional steps such as brushing, organic. solvent degreasing, etching in acid solutions, or. similar conventional steps. 7

After such cleaning a pattern of stop weld is applied a to one of thesurfaces-to be confronted. The assembly may then .be tack welded atits corners topreserve align ment during subsequent processing. a

Thefassembly is then heated as part of the secondary metal processing operation to a temperature between 900 F. and 975 F. and is pressure bonded by hot rolling with a reduction of about 65 percent. This hot reduction is followed by a cold reduction by rolling of about 30 percent.. Excellent metallurgical pressure bonding is pro-. duced by this combination of steps.

Following the hot and cold rolling, a large difference in yield strength is developed between the two alloys by subjecting the composite blank to a critical partial annealing practice. To obtain the larger differences in yield strength the partial annealing is carried out between a temperature of 550 F. and 600 F. for a period of between 30 and 60 minutes.

The yield strength of aluminum alloy 1100 containing zirconium after annealing at 700 F. for to 20 minutes is 5,900 p.s.i. and 5,700 p.s.i. respectively, while that of commercially available annealed aluminum alloy 1100 is 5,900. For a composite of aluminum alloy 1100 a partial annealing resulted in the alloy having a yield strength of ranging from 11,000 p.s.i. to 4,000 p.s.i. after 30 to 60 minutes annealing at 550 F., respectively, and was almost constant at 4,000 p.s.i. after 30 minutes annealing at 600 F. By contrast an aluminum alloy 1100 containing 0.20 percent Zr as a component of a composite, has yield strength values of about 20,000 p.s.i. and 17,500 p.s.i. after annealing at temperatures of 550 F. and 600 F. respectively, almost independent of annealing times between 30 and 60 minutes. The preferred partial annealing cycle is a heating at 550 F. for about 60 minutes to develop the higher yield strength differential without any significant sacrifice of the tensile properties.

This partial annealing is followed by inflation of the partially annealed blank using the differential pressure inflation procedure employing a cavity dye as referred to above to expand the softer aluminum alloy 1100 component layer and to maintain the alloy 1100 containing 0.12 percent Zr in a smooth flat configuration.

Alternatively, the inflation can be carried out by inflation of the partially annealed blank between platens as taught in the patent art pertinent to inflation methods although the same degree of smoothness and flatness of one side is not achieved by this inflation.

The product formed in accordance with the foregoing described procedure is a hollow sheet article having internal tubes formed between the component sheets thereof. The tubes, when formed by free inflation, have a larger radius of tube wall curvature on the harder component containing the finely dispersed zirconium, and a smaller radius of tube wall curvature in the component of aluminum sheet free of zirconium.

It will be appreciated that in carrying out the subject process substantial variation may be made in certain of the process variables whereas others must be retained within closely fixed limits.

For example, although larger differences of yield strength are produced by partial annealing within the above indicate-d temperature and time ranges, it will be appreciated that substantial differences in yield strength result from partial annealing treatments outside the stated ranges due to the inhibition of the recovery of the zirconium aluminide bearing alloy. However, the presence of the zirconium aluminide in the indicated concentration, form and distribution is useful in inhibiting the recovery only and has no appreciable effect on the recrystallization of the alloy. Accordingly, any heat treatment must be carried out at temperatures below the recrystallization temperature.

Regarding next the composition of the aluminum base alloy, the use of the aluminum alloy 1100 as a base is preferred where the desirable combination of properties possessed by this alloy are needed for the application to the use described. However, it will be appreciated that the inhibition of recovery and the development of increased yield properties as described above can be achieved in accordance with the present invention in any aluminum base alloy in which zirconium solubility is appreciable and from which the ZrAl can be precipitated in the extremely finely divided form. It should be understood that this distribution does not affect the yield strength of the fully annealed alloy but is useful in developing increased yield strength coupled with high ductility by partial annealing following cold working,'when this combination of yield strength and ductility are compared to those of the same base metal which is free of the finely divided ZrAl particles. Accordingly, it is apparent that a composite product may be prepared pursuant to this invention to contain the finely divided zirconium aluminum in only one of the layers of the composite so that essentially identical physical properties are present in the fully annealed state, but so that substantial differences in yield strength result from partial annealing after cold working. Use of composite sheet articles containing the novel alloy component in metal forming operations such as rolling, cupping, and the like permit special operations to be carried out, and new and distinct products to be formed.

One such product, of course, is the product resulting from the above process, namely, a composite sheet having an aluminum alloy in one component layer and an aluminum alloy containing zirconium in extremely finely divided and uniformly dispersed form in the other sheet. Maximum difference in yield strength can be obtained only where substantially all of the zirconium is present in an extremely finely divided and uniformly dispersed form as a fine aluminide of zirconium.

It is possible to combine a first component alloy layer containing the finely divided zirconium in uniformly dispersed form, with a second component alloy layer having a different and distinct aluminum base. However, it will be realized that when such a composite sheet article is made, advantageous properties otherwise attainable in forming a bi-alloy one-side-flat inflated refrigeration panel or like product are sacrificed. For example, the advantage of a substantial identity of rolling, or similar initial metal working properties of the two components, is lost where base alloys having different metal working properties are included in the composite.

While substantial advantages are obtained by use of a composite article, particularly a composite sheet, made up from a component formed of a first alloy, and a second component made of the same alloy containing the very finely divided uniformly distributed zirconiumaluminum particles, it is to be understood that the novel aluminum alloy compositions of this invention also have substantial uses, because of the novel properties thereof, although not provided in combination with distinct metals as composite structures.

The novel alloy compositions have distinctly improved properties only when that portion of the zirconium additive, present in the mircoscopically subdivided and uniformly distributed form, is at a concentration effective to inhibit the recovery of the alloy after cold work when subjected to a partial anneal. Maximum effectiveness of the zirconium additive is achieved when substantially all of the additive is present in the microscopically subdivided and uniformly distributed form.

A number of substantial uses exist for this novel composition when in the partially annealed state. One such use is as a building sheet material. Such use of the novel composition in the cold worked, partially annealed sheet form is based on the sheet having a lower cost than sheet such as aluminum alloy 3004 of equivalent strength.

Another use is in forming articles of small thickness such as finstock of heat exchangers. In a material such as 1100 aluminum alloy, the stock must be cold rolled to achieve the desired combination of both thickness and hardness. Achieving such a combination of properties by rolling leaves much to be desired in the way of accuracy, and results in the production of much Waste material. The problem of controlling both tihckness and hardness of small dimension fabricated material is greatly simplified through use of the novel composition of this invention because the final dimension can be achived by rolling directly with substantially no concern for hardness, and the hardness can be independently controlled by a subsequent partial heat treating operation.

With particular reference now to the cleaning opera-i tions it will be appreciatedthat the several conventional cleaning steps mentioned'in the above example or alternative cleaning operations may be' employed. Thernanner of cleaningis not an'f'essential part of the invention.

However, thebrushing or a similar step is necessary '-to provide clean metal surfaces 'for the pressure bonding operation in order to obtain satisfactorily strong bonds between the component layers.

In the same way theparticular pattern of stop weld is not critical in practice of this invention forknown alternatives. may be substituted in place thereof. For example, the procedures and compositions as described in the basic Grenell patent referred to above, or in other patents which have resulted from inventive improvements over, and extensions of the basic teaching in this may be employed as well.

patent tics.

form does not contribute at all to development of an increase in yield strength on partial annealing after cold Work, but rather that the contribution is, of a low efiiciency only for the percentage of the zirconium contained in the alloy. f Moreover, it is undesirable to include more In other words, for commercial scale-processing of alloy to form a product having the desired properties there The temperature at which the composite blank, i,e., a

pressure-bonded butuninflated composite sheet product, one layer of which is composed of the novel alloy of the invention is rolled, or to which it is heated in prepara tion for rolling, may vary and may range for short periods up to about 9755 1 withoutrapparent deleterious effects in the finished sheet. 'The temperature of the ina- I terial containing the aluminum should not normally be allowed to exceed 950 F. during primary metal processing, i.e., up to the formation of the primary metal stock, however, as this may lead to' the partial redissolution or ZrAl As indicated above, redissolution of ZrA l from the uniformly distributed microscopically subdivided form detracts from the effective contribution of the zirconium bearing phase to the desired combination of properties following cold working and partialannealing, particularly where the redissolutionis followed by hot rolling because the zirconium 'alurninide; is caused tobe precipitated by such rolling in an anisotropic form; Such precipitation causes non-uniform dispersions of the ZrAl and results in accentuated textural streaking and/or dis coloration on anodizing. V In rolling'a composite aluminum alloy 1100 with the aluminum alloy 1100 containing the uniformly and finely dispersed zirconium, a reduction of 65 percent during hot rolling, may be employed; However, it isknown somewhat different results to be obtained without loss of the advantages of the invention described herein. For ex that percentages in reductions, may be changed to permit ample, the assembly may be prepared for hot rolling by heating to temperatures between 700 F. and 975 F.

' ponent alloysof the composite sheet of this invention in the cold rolled state after the prior hot rolling reduction by an amount such as 65 percent. It is only when the material has been partially annealed that the substantial difference in the yield strengths is'developed. The zir-' conium content of the alloy of thezirconium-bearmg component may be varied from between 0.1 percent and 0.3 percent; The optimum concentration range for ,zirconium content in1100 aluminum alloy to produce the maximum difference in yield strength for the inflation is between 0.12 percent and 0.18 percent;

' Where an 1100- aluminum alloy containingfrom 0.12

saw.

is little gain for zirconium" added above 0.3' percent in view of the higher' melt temperatures needed to solubili'ze thezirconium preparatory to precipitating it in finely divided form. Where the ZrAl is not in'the finely divided form, it does not lend the improved yield strength property to the alloy and can even produce deleterious effects.

-Another factor which contributes to the uniformity of the distribution of zirconiurnin its extremely finely divided form is the soak treatment described in the example. J It is important to appreciate that this soak treatment is useful in converting only the zirconium in solid solution to the desired finely dividedand uniformly dispersed form of precipitate. Accordingly, for optimum results a combination of the preferred casting practice as described above with the preferred soak treatment is neces Since many examples of the foregoing procedures and compositions maybe carried out and made and since many modifications can be made therein without depart-v ing from the scope of the subject invention, the foregoing is to be interpreted as illustrative only and not as defining or limiting the scope of the invention.

What is claimed is the following: i v 1. A composite article'of aluminum sheet metal comprising V V ,7 w

(A) a first component sheet of an aluminum base alloy havingfrom 0.1% to 0.3% zirconium distributed therein as a finely divided uniform dispersion of zirconium aluminide precipitate, (B) and a second component sheet of 'the'same aluminumbase alloy integrally'unifiedwith said first cornponent sheet, said second component sheet being p. free of said finely divided and uniformly dispersed zirconium aluminide.

' 2. The article of claim 1 wherein the aluminum base 1 a smooth hard component layer and'distensions raised rom the opposite softer component layer to form hollow portions in said panel, said panel comprising, as the hard to 0.18 percentzirconium is cast at a temperature below 1325 F., a smaller portion ofthe zirconium is foundto occur in the alloy in the extremely finely divided state. Moreover, itcannot be converted to the finely divided and uniformly dispersed state by use of special heat treatment described in theexample above; Thisdoes not mean, of course, that the zirconiumpresent in a form other than the extremely finely divided and uniformly distributed component, a sheet of aluminumcontaining from 0.1% to 0.3 zirconium'uniformly dispersed therein in a finely divided form, and a sheet of said aluminum as the softer component, comprising the steps of (A) forming ablank by positioning said sheets'adja- I cent one another, one of. said sheets having applied to a confronting surface thereof a'pattern of weld inhibiting material, p

(B) heating said blank to a temperature within the range of 700 to 975 F., v I

(C) pressure welding said sheets together in the areas 7 not covered by said weldinhibiting material,

1 (D) cooling and cold working saidv blank to develop a high strength levelin saidsheets, 1 .(E) partially, annealing said .blank in a' critical temperature range in which said softer aluminum component undergoes considerable loss of strength by recovery-..and .recrystallization' but in which said aluminum with zirconium component undergoes 8,1 9 only slight loss of strength by recovery relative to said softer aluminum component thereby developing a differential in yield strength between the component layers thereof while preserving the finely divided zirconium dispersion,

(F) and inflating the blank in the unwelded areas by the application of fluid pressure therein.

5. The process as set forth in claim 4 wherein said partial annealing is carried out within the temperature range of 550 to 600 F. for a period of 30 to 60 minutes.

6. A composite article of aluminum sheet metal comprising (A) a first component sheet of an aluminum base alloy having from 0.1% to 0.3% zirconium distributed therein as a finely divided uniform dispersion of zirconium aluminide precipitate,

(B) and a second component sheet of essentially the same aluminum base alloy integrally unified with said first component sheet, said second component sheet being free of said finely divided and uniformly dispersed zirconium aluminide.

'7. A process of fabricating a sheet metal panel having a smooth hard component layer and distensions raised from the opposite softer component layer to' form hollow portions in said panel, said panel comprising, as the hard component, a sheet of aluminum containing from 0.1% to 0.3% zirconium uniformly dispersed therein in a finely divided form, and a sheet of essentially said aluminum as the softer component, comprising the steps of (A) forming a blank by positioning said sheets adjacent one another, one of said sheets having applied to a confronting surface thereof a pattern of weld inhibiting material,

(B) heating said blank to a temperature within the range of 700 to 975 F.,

(C) pressure welding said sheets together in the areas not covered by said weld inhibiting material,

(D) cooling and cold working said blank to develop a high strength level in said sheets,

oases References Cited by the Examiner UNITED STATES PATENTS 1,716,943 6/29 Archer et al 148-159 2,252,421 8/41 Stroup 75--138 2,941,282 6/60 Fromson. 3,067,491 12/62 Neel et al. 29-157.3 X

FOREIGN PATENTS 843,824 8/60 Great Britain.

OTHER REFERENCES De Pierre and Bernstein: Grain Refinement in Cast Aluminum, The Iron Age, Jan. 20, 1949, pages 66-70, vol. 163, No. 3.

Metal Handbook, 1948 ed, page 1168.

Harrington: Abstract of application Serial No. 18,483, published Jan. 23, 1951, 642 O.G. 1225.

Harrington: The Effect of Single Addition Metals on the Recrystallization, Electrical Conductivity and Rupture Strength of Pure Aluminum, ASM 1 948 preprint, No. 15.

WHITMORE A. WILTZ, Primary Examiner.

NEDWIN BERGER, Examiner.

Claims (1)

1. 7. A PROCESS OF FABRICATING A SHEET METAL PANEL HAVING A SMOOTH HARD COMPONENT LAYER AND DISTENSIONS RAISED FROM THE OPPOSITE SOFTER COMPONENT LAYER TO FORM HOLLOW PORTIONS IN SAID PANEL, SAID PANEL COMPRISING, AS THE HARD COMPONENT, A SHEET OF ALUMINUM CONTAINING FROM 0.1% TO 0.3% ZIRCONIUM UNIFORMLY DISPERSED THEREIN IN A FINELY DIVIDED FORM, AND A SHEET OF ESSENTIALLY SAID ALUMINUM AS THE SOFTER COMPONENT, COMPRISING THE STEPS OF (A) FORMING A BLANK BY POSITIONONG SAID SHEETS ADJACENT ONE ANOTHER, ONE OF SAID SHEETS HAVING APPLIED TO A CONFRONTING SURFACE THEREOF A PATTERN OF WELD INHIBITING MATERIAL, (B) HEATING SAID BLANK TO A TEMPERATURE WITHIN THE RANGE OF 700* TO 975*F., (C) PRESSURE WELDING SAID SHEETS TOGETHER IN THE AREAS NOT COVERED BY SAID WELD INHIBITING MATERIAL, (D) COOLING AND COLD WORKING SAID BLANK TO DEVELOP A HIGH STRENGTH LEVEL IN SAID SHEETS, (E) PARTIALLY ANNEALING SAID BLANK IN A CRITICAL TEMPERATURE RANGE IN WHICH SIAD SOFTER ALUMINUM COMPONENT UNDERGOES CONSIDERABLE JLOSS OF STRENGTH BY RECOVERY AND RECRYSTALLIZATION BUT IN WHICH SAID ALUMINUM WITH ZIRCONIUM COMPONENT UNDERGOES ONLY SLIGHT LOSS OF STRENGTH BY RECOVERY RELATIVE TO SAID SOFTER ALUMINUM COMPONENT THEREBY DEVELOPING A DIFFERENTIAL IN YIELD STRENGTH BETWEEN THE COMPONENT LAYERS THEROF WHILE PRESERVING THE FINELY DIVIDED ZIRCONIUM DISPERSION, (F) AND INFLATING THE BLANK IN THE UNWELDED AREAS BY THE APPLICATION OF FLUID PRESSURE THEREIN.