Anodic aluminum material and articles and composite articles comprising the material

BACKGROUND OF THE INVENTION

There is an increasing need for aluminum materials having a relatively high anodic solution potential together with substantial strength. Such materials are useful to provide cathodic protection to structural members fashioned from other aluminous materials and are often _3gmore useful if they can be imparted with substantial strength so as to be suitable for bearing loads. One such application is where the anodic material serves as a protective cladding in a composite having a heat treatable core. Accordingly, aluminum alloy materials have been 40 proposed which have relatively anodic electrolytic potentials and which are precipitation hardenable, by which is meant artificially ageable, which materials are useful in providing anodes which can bear loads. However, these materials are marked by variations in solution potential 45 upon thermal exposures such as encountered in precipitation hardening treatments and even such as encountered in actual service. For example, an aluminum alloy containing zinc and magnesium and generally having a suitable anodic electrolytic potential will be observed to ex- 50 hibit widely varying solution potential values varying over a range of up to 100 mv. and even more in response to varying aging or other thermal exposure conditions. In the case of a composite, the desired extent of the difference in solution potential between cladding and core can 55 vary from one application to another although a difference of about 100 mv. is often desired. However, in such a system, it is quite apparent that where the solution potential of the cladding can vary up to 100 mv. because of differing thermal exposures, it becomes quite difficult to 60 assure any desired degree of cathodic protection. In addition, some core compositions require that the cladding solution potential be much greater than the core potential, there being some applications where a difference of 175 mv. is not sufficient. When it is noted that the total span 65 in solution potential bracketing aluminum and all its commercial alloys amounts to about only 300 mv., it is quite apparent that fluctuations approaching 100 mv. can be extremely troublesome. As an example of thermal conditions encountered in service, in an automobile radiator an 70 aluminum member could easily encounter a temperature considerably exceeding 170° F. and the precipitation hardenable aluminum anode materials proposed to date exhibit differences in electrolytic potential upon such exposures which are often considered unacceptably wide. Thus, the extent of the variations in potential renders it difficult to properly balance a dual alloy system from the standpoint of cathodic protection and an anodic material having a thermally stable solution potential would be highly desirable.

STATEMENT OF THE INVENTION

In accordance with the invention, there is provided a strong solution heat treatable and precipitation hardenable aluminum alloy article which exhibits a very stable solution potential which will not vary by more than 50 mv. regardless of any variations in thermal exposure which might occur from situation to situation during thermal treatments such as precipitation hardening treatments or during service.

The foregoing advantages are achieved by providing an article composed of an aluminum base alloy consisting essentially of 2 to 8% zinc, 0.5 to 2% magnesium, 0.3 to 2% silicon, the silicon content amounting to at least 0.45 times the magnesium content, the balance being aluminum and incidental elements and a maximum of 0.25% copper and a maximum of 0.25% chromium, as impurities. The percentages are all by weight. Articles fashioned from this alloy will exhibit upon exposure to varying thermal conditions a very stable electrolytic potential which does not vary by more than 50 mv. even though the temperature, time at temperature and other factors may be varied from one instance to another. Because the alloy is precipitation hardenable, it exhibits substantial strength typified by strength levels of 35 to over 50 k.s.i. tensile and 25 to over 40 k.s.i. yield strength. In practicing the invention, it is essential that, as indicated above, the silicon content must amount to at least 0.45 times the magnesium content. This assures that the desired stable solution potential will be achieved in each case. In order to further assure such and to further improve the strength of the material, it is preferred that the silicon content amount to 1 to 2 times the magnesium content. From the standpoint of improved strength, it is further preferred that manganese be included in an amount of 0.1 to 0.75%. Where higher levels of electrolytic potential are desired, for instance, levels consistently over 980 mv., the zinc content should be 3.5 to 8%.

In the case of a composite, a core material most highly suited is one of the alloy system usually designated the Al-Zn-Mg-Cu system. This type of alloy carries the Aluminum Association Registration designation 7XXX and these alloys contain up to about 3 to 9% Zn, 1 to 3% Mg, copper in an amount of up to 3 % together with additions of Cr, Mn and other elements. Specific examples of these alloys include the following nominal compositions: 7075: 5.6 Zn, 2.5 Mg, 1.6 Cu, 0.3 Cr, bal. Al 7079: 4.3 Zn, 3.3 Mg, 0.6 Cu, 0.2 Mn, 0.2 Cr, bal. Al

The 7XXX type of alloy is known to exhibit high strength after solution heat treatment and precipitation hardening although it is also known that in some cases it is advisable to provide cathodic protection by means of an anodic cladding to which purpose the invention is especially suited. However, while the invention contemplates use of the improved anodic material as a cladding on a 7XXX aluminum alloy, in a broader sense, the improved anodic material should be useful with other heat treatable aluminimum cores and it is not necessarily intended to limit the invention to the use of a 7XXX core. In any event, the solution potential of the core will most often vary from about 780 to about 850 mv. after precipitation hardening and the improved material provides suitable cathodic protection for such.

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The articles can be manufactured by more or less standard techniques. Accordingly, an alloy of proper composition is cast into an ingot as by continuous casting and the ingot broken down by hot or cold working or both. Suitable working operations would include rolling, extruding, drawing and the like.

In the case of a composite which may be sheet, the core and cladding materials are separately processed as sheet or plate and then hot rolled together to provide a metallurgically bonded composite laminate. In such rolled composites, the cladding may suitably constitute about 3 to 10% of the total composite thickness. In some cases, especially where the core contains magnesium, it may be desirable to provide an interliner, intermediate the core and cladding and containing little or no magnesium, to assure a quality bond. Such has proven useful in producing composites utilizing the improved anode material with 7XXX type cores.

Articles produced from the improved material may have their strength improved by solution heat treatment and precipitation hardening. Solution heat treatment may be effected by heating to a temperature of 825° F., or preferably a somewhat higher temperature of about 870° to 970° F. or more. Where the highest strength is desired, the solution heat treatment temperature should be at least 950° F. The article is maintained at solution heat treating temperature for a period of time sufficient to take into solution substantially all the soluble alloy constituents. This time varies from case to case depending on the relative size of the article and the heating equipment employed and will generally vary from a matter of several minutes to several hours. Where the article is a composite article, the solution heat treatment time and temperature may be fixed by the particular core material utilized and the improved material’s solution potential stability is especially desirable in such a case.

After the solution heat treatment, the article is quenched as by cold water immersion and then subjected to a precipitation hardening treatment wherein it is heated to a temperature of up to 400° F., but which generally ranges from 200° to 375° F., for varying durations depending on the exact physical and corrosion properties needed. In the case of a composite, the precipitation hardening conditions may be fixed by the particular core material employed and again the stable solution potential of the improved anodic material is especially useful. As indicated earlier, prior heat treatable anode compositions normally exhibit widely varying electrolytic potential levels in response to varying solution heat treatment and precipitation hardening treatments whereas the improved material exhibits highly stable solution potential levels. This permits the selection of thermal treatment conditions without regard for any sensitivity on the part of the cladding.

In order to demonstrate the improvement derived from practicing the invention, the following examples proceed:

Example 1

Alloys of three compositions were compared as to their response to different solution heat treating and precipitation hardening conditions. The compositions could be considered as Al-4.5 Zn-1.2 Mg plus varying amounts of Si. The articles here were 2 inch thick plates and the compositions were as follows:

table I [Anodie alloy compositions]

<td></td><td> Zn</td><td> Mg</td><td> Si</td><td> Mn</td><td> Or</td>

<td> Composition: A...............</td><td> 4.52</td><td> 1.23</td><td> 0.08</td><td> 0.50</td><td> 0.12</td>

<td> B...............</td><td> 4.53</td><td> 1.21</td><td> 0.46</td><td> 0.58</td><td> 0.11</td>

<td> C...............</td><td> 4.51</td><td> 1.21</td><td> 0.76</td><td> 0.58</td><td> 0.08</td>

In the above list, Composition A is clearly outside the practice of the invention and exemplifies an aluminumzinc-magnesium alloy which, in turn, typifies prior heat treatable aluminum alloys having an anodic electrolytic potential. Even though the alloy constituents, Zn, Mg and Si, broadly speaking, can be viewed as present within the ranges claimed herein, Composition B is nonetheless outside the invention as the ratio of its Si to Mg content amounts to only 0.38 which is less than the 0.45 level set out as essential in the practice of the invention. Composition C is within the practice of the invention with respect to the Zn, Mg and Si contents along with the silicon to magnesium ratio which is slightly over 0.62. Several plates of each composition were solution heat treated for two hours at two different temperatures. The plates were quenched by immersion in cold water and then precipitation hardened under varying conditions according to Table II. One-eighth inch diameter short transverse, that is, across the 2 inch plate thickness, specimens were removed and tested for tensile properties. In addition, electrolytic potential measurements were made by measuring the specimens against a 0.1 N calomel electrode in 1.0 N NaCl plus 0.3% H₂O₂ electrolyte, a procedure commonly used in the aluminum alloy art for such measurements. The results of these measurements are set forth in Tables III to V which set out the tensile and yield strength (T.S. and Y.S.) together with the elongation in 2 inches and the solution potential (Pot.) for each of the compositions in Table I under the seventeen different conditions of thermal treatment described in Table Π.

Referring to Table III and Composition A, it can be seen that the different thermal treatments result in widely varying strength properties. It is highly significant that for the solution heat treatment at 870°, the potential measurement spread was over a range of 93 mv. For the other solution heat treatment temperature, 970° F., the spread was a little less at 74 mv. but still is considered excessively high. Similar results were encountered with the Composition B specimens as is seen in Table IV. Turning now to Table V and the Composition C results, it can again be seen that some variation in strength is caused by differing thermal treatments. However, it is readily apparent that the potential measurements did not vary appreciably. On the contrary, they were very stable. At a solution heat treatment of 870° F., the total potential spread amounted to only 10 mv. regardless of the precipitation hardening treatment. Equally significant, the results for solution heat treatment temperature of 970° F. indicate a total variation of only 13 mv. The total variation of all the results for Composition C amounts to only 29 mv. whereas for Compositions A and B, the total variation amounts to over 3 times that much, i.e., respectively, 93 and 96 mv. Where it is deemed essential for cathodic protection that

TABLE II [Solution heat treatment and precipitation hardening conditions) Sol. H.T., hr. at ° F. Prec. hdn., hr. at ° F.

Condition No.:

I—............2 hr. at 870°.............24 at 200°.

....................do............... 24 at 250°.

....................do............... 24 at 300°.

....................do...................24 at 325°.

....................do................... 24 at 350°.

....................do...................3 at 250° plus 3 at 325°.

7- ...................do...................3 at 250° plus 16 at 325°.

8- ...................do...................6 at 225° plus 30 at 325°.

9- ...................do...................8 at 225° plus 16 at 325°.

...................do...................8 at 225° plus 16 at 350°.

........... 2 hr. at 970°.............24 at 200°.

12—.................do...................24 at 260°.

...................do...................24 at 300°.

..................-do...................24 at 320°.

16...................do...................24 at 340°.

............. do...................3 at 250° plus 3 at 326°.

.......... ..do...................3 at 250° plus 15 at 325°.

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TABLE III [Strength and potential for Comp. A (0.08 Si)]

<td></td><td> T.S. (Ks.i.)</td><td> Y.S. (Ks.i.)</td><td> Elong. (percent)</td><td> Pot. (mv.)</td><td> Spread (mv.)</td>

<td> Condition No.: 1.............</td><td> 51.1</td><td> 32.7</td><td> 15.5</td><td> 1,010 Ϊ</td><td></td>

<td> 2.............</td><td> 52.9</td><td> 42.3</td><td> 12.0</td><td> 987</td><td></td>

<td> 3_____________</td><td> 49.9</td><td> 40.0</td><td> 11.0</td><td> 945</td><td></td>

<td> 4.............</td><td> 44.3</td><td> 32.8</td><td> 12.0</td><td> 939</td><td></td>

<td> 5.............</td><td> 42.3</td><td> 29.8</td><td> 14.0</td><td> 929</td><td></td>

<td> 6.............</td><td> 53.0</td><td> 42.9</td><td> 14.0</td><td> 962</td><td> ,</td>

<td> 7.............</td><td> 51.8</td><td> 41.0</td><td> 13.0</td><td> 935</td><td></td>

<td> 8.............</td><td> 49.3</td><td> 37.8</td><td> 14.0</td><td> 933</td><td></td>

<td> 9.............</td><td> 51.2</td><td> 39.9</td><td> 12.5</td><td> 944</td><td></td>

<td> 10............</td><td> 43.6</td><td> 31.4</td><td> 15.5</td><td> 917 J</td><td></td>

<td> 11</td><td> 51.1</td><td> 33.1</td><td> 17.0</td><td> 995</td><td></td>

<td> 12............</td><td> 53.5</td><td> 42.4</td><td> 13.0</td><td> 982</td><td></td>

<td> 13............</td><td> 50.0</td><td> 39.9</td><td> 13.0</td><td> 923</td><td></td>

<td> 14............</td><td> 45.8</td><td> 34.8</td><td> 12.5</td><td> 928</td><td> 74</td>

<td> 15............</td><td> 43.1</td><td> 31.5</td><td> 12.5</td><td> 921</td><td></td>

<td> 16............</td><td> 52.2</td><td> 42.2</td><td> 12.0</td><td> 944</td><td></td>

<td> 17............</td><td> 52.3</td><td> 41.9</td><td> 13.0</td><td> 929</td><td></td>

TABLE-IV [Strength and potential for Comp. B (0.46 Si)]

<td></td><td> T.S. (Ks.i.)</td><td> Y.S. (Ks.i.)</td><td> Elong, (percent)</td><td> Pot. (mv.)</td><td> Spread (mv.)</td>

<td> Condition No.: 1.............</td><td> 46.6</td><td> 29.4</td><td> 12.0</td><td> 1,025 )</td><td></td>

<td> 2..............</td><td> 43.6</td><td> 31.6</td><td> 8.5</td><td> 1,021</td><td></td>

<td> 3.............</td><td> 41.2</td><td> 31.1</td><td> 10.5</td><td> 983</td><td></td>

<td> 4.............</td><td> 38.0</td><td> 27.8</td><td> 12.0</td><td> 951</td><td></td>

<td> 5.............</td><td> 35.3</td><td> 25.0</td><td> 12.5</td><td> 959</td><td></td>

<td> 6.............</td><td> 41.3</td><td> 30.4</td><td> 9.0</td><td> 1,017J</td><td> 74</td>

<td> 7.............</td><td> 42.9</td><td> 33.1</td><td> 10.0</td><td> 968</td><td></td>

<td> 8.............</td><td> 44.9</td><td> 35.2</td><td> 9.0</td><td> 965</td><td></td>

<td> 9.............</td><td> 46.2</td><td> 36.6</td><td> 8.0</td><td> 967</td><td></td>

<td> 10............</td><td> 41.7</td><td> 31.3</td><td> 10.0</td><td> 959</td><td></td>

<td> 11............</td><td> 49.4</td><td> 32.2</td><td> 11.5</td><td> 1,017 1</td><td></td>

<td> 12............</td><td> 48.2</td><td> 33.7</td><td> 12.5</td><td> 1, 011</td><td></td>

<td> 13............</td><td> 51.0</td><td> 40.5</td><td> 11.5</td><td> 990</td><td></td>

<td> 14............</td><td> 47.2</td><td> 37.4</td><td> 11.0</td><td> 929</td><td> 88</td>

<td> 15............</td><td> 45.6</td><td> 35.0</td><td> 9.0</td><td> 929</td><td></td>

<td> 16............</td><td> 46.5</td><td> 33.6</td><td> 12.0</td><td> 1,011</td><td></td>

<td> 17............</td><td> 48.2</td><td> 38.4</td><td> 11.0</td><td> 955</td><td></td>

<td colspan="6"> TABLE V [Strength and potential for Comp. C (0.76 Si)]</td>

<td></td><td> T.S. (Ks.i.)</td><td> Y.S. (K s.i.)</td><td> Elong, (percent)</td><td> Pot. (mv.)</td><td> Spread (mv.)</td>

<td> Condition No.: 1..........</td><td> ..... 40.2</td><td> 25.2</td><td> 12.5</td><td> 1,035 )</td><td></td>

<td> 2..........</td><td> ..... 37.5</td><td> 25.4</td><td> 12.0</td><td> 1,035</td><td></td>

<td> 3..........</td><td> ..... 38.3</td><td> 29.8</td><td> 8.5</td><td> 1,033</td><td></td>

<td> 4..........</td><td> ..... 35.6</td><td> 27.5</td><td> 8.0</td><td> 1,028</td><td></td>

<td> 5..........</td><td> ..... 33.3</td><td> 24.7</td><td> 9.0</td><td rowspan="2"> 1,027 1,039</td><td>₍ 10</td>

<td> 6..........</td><td> ..... 36.3</td><td> 24.8</td><td> 12.0</td><td></td>

<td> 7..........</td><td> ..... 37.4</td><td> 29.5</td><td> 9.0</td><td> 1,035</td><td></td>

<td> 8..........</td><td> ..... 37.9</td><td> 30.0</td><td> 9.0</td><td rowspan="2"> 1,025 1,031</td><td></td>

<td> 9..........</td><td> ..... 38.1</td><td> 30.2</td><td> 8.0</td><td></td>

<td> 10-........</td><td> ..... 35.3</td><td> 26.4</td><td> 9.0</td><td> 1,027 J</td><td></td>

<td> 11.........</td><td> ..... 48.8</td><td> 29.9</td><td> 15.5</td><td> 1,021</td><td></td>

<td> 12.........</td><td> ..... 47.2</td><td> 32.2</td><td> 15.0</td><td> 1,021</td><td></td>

<td> 13.........</td><td> ..... 50.4</td><td> 42.5</td><td> 8.5</td><td> 1,021</td><td></td>

<td> 14.........</td><td> ..... 47.4</td><td> 40.1</td><td> 8.0</td><td> 1,021</td><td> f ¹³</td>

<td> 15.........</td><td> ..... 46.0</td><td> 37.9</td><td> 9.0</td><td> 1,010</td><td></td>

<td> 16.........</td><td> ..... 47.4</td><td> 35.6</td><td> 12.0</td><td> 1,023</td><td></td>

<td> 17.........</td><td> ..... 48.6</td><td> 42.0</td><td> 9.0</td><td> 1,019</td><td></td>

the anode potential be 100 mv. more anodic than the material being protected, this potential difference being a common value often chosen in the industry, it can be seen that variations of 93 to 96 mv. can render positive assurance of this difference quite difficult. It is apparent that the comparatively minuscule potential variation associated with the practice of the invention minimizes this problem.

Referring again to the tables, particularly Table V, it is apparent that the improved material exhibits substantial strength especially when solution heat treated at a temperature of over 950° F. In such cases, the tensile strength is over 45 K.s.i. regardless of the thermal treatment. Composite laminates, which included the improved anodic material as cladding, exhibited substantially higher strength than similar laminates featuring a non-heat treatable cladding and this strength improvement is realized without substantial variations in the solution potential of the cladding in response to varying thermal treatments thus rendering the improved material very useful as an anodic cladding layer.