US3312535A - Aluminum reflectors - Google Patents

Description

United States Patent 3,312,535 ALUMINUM REFLEQTORS William A. Anderson, Verona, and Ronald Andrew, Pittsburgh, Pa., assignors to Aluminum Company of America, Pittsburgh, Pa, a corporation of Pennsylvania No Drawing. Filed July 16, 1963, Ser. No. 295,517 8 Claims. (Cl. 29-183.5)

This invention relates to the production of high quality aluminum light reflectors, that is, reflectors which have a high reflective efliciency. The surface of such reflectors is generally chemically or electrolytically treated to provide a protective artificial transparent oxide film.

To obtain a maximum light reflectance from an aluminum surface it has been found that it is necessary to employ high purity metal, usually not less than 99.8%. While such metal responds well to brightening and oxide coating treatments, it suffers from diffi-c-ulties associated with grain size. High purity metal under usual casting, working and annealing conditions develops relatively large or coarse grains. If such metal in the form of annealed or lightly worked sheet, for example, is subsequently bent, formed or stretched, the surface acquires a roughened appearance, especially where the deformation is most severe. This appearance is commonly referred to as orange peel and it detracts from the appearance of the reflective surface if not the reflective efiiciency. The defect is, of course, especially conspicuous on an otherwise smooth specularly reflective surface.

Conventional means of controlling the size of grains in such a high purity metal have been ineffective or introduced other problems or added to the cost of production to such an extent as to be considered impractical. The use of small amounts of the well-known grain refining agents boron and titanium, for example, develop hard spots which give rise to streaks on the polished aluminum surface. The problem of controlling grain size without adversely affecting the reflective quality of the aluminum has received much attention over the years. We have found, after careful investigation, that the desired result can be achieved simply by the controlled addition of certain elements to the high purity aluminum.

It is an object of our invention to provide a highly reflective aluminum surface free from surface defects.

Another object is to provide an aluminum light refleet-or which has undergone deformation without the occurrence of the orange peel defect.

A further object is to provide a specularly reflective aluminum surface having a fine grained structure.

These and other objects and advantages can be attained by our invention which is predicated upon the discovery that the addition to aluminum of a purity of not less than 99.8% of small amounts of manganese, copper and magnesium provides grain size control without any adverse effects upon the light reflective quality of the aluminum. Although it has been considered necessary in the past to limit the amount of unavoidable impurities, including manganese, copper and magnesium, and other elements in aluminum to achieve a high reflective quality, we have found that the addition of the three named elements within prescribed limits does not detract from the reflective quality. More specifically it has been discovered that by adding 0.l to 0.3% manganese, 0.05 to 0.2% copper and 0.1 to 0.25% magnesium to aluminum containing a maximum of 0.2% impurities a small uniform grain size is assured in reflectors made of the alloy. This combination of elements is essential to the attainment of the desired results, no one of them, or two of them, being capable of producing the desired reduction in grain size.

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Referring to the individual elements, we have found that manganese, which is substantially insoluble in aluminum and forms a dispersoid, must be present in amounts of at least 0.1% to minimize grain growth during hot Working and annealing operations. On the other hand, more than 0.3% tends to decrease reflectance. Copper, an element soluble in solid aluminum, also has a grain refining effect, however, more than 0.2% produces a yellowish tinge in the oxide coatings which is objectionable. At least 0.05% copper is necessary in order to produce a marked reduction in grain size. The third element, magnesium, is also soluble in solid aluminum, and has a grain refining effect, the minimum amount needed to obtain this result being 0.1% While the upper limit is 0.25%. Larger amounts diminish workability to the point of making forming diflicult.

In respect to the impurities iron and silicon, not more than 0.1% of each should be permitted and preferably the maximum should be 0.05% each. Other impurities are also apt to be present but the amounts are generally too small to affect the working of the metal and its reflecting quality. By using aluminum having a minimum purity of 99.8%, and preferably 99.9%, the impurity limits are not exceeded.

Reflectors are generally formed from sheet but other wrought forms than sheet can be employed depending upon the desired shape of the reflector. The sheet or other wrought form may be in the fully annealed condi tion or in a worked condition such that the fragments of grains are large enough to cause orange peel or other defects when the product is formed or otherwise shaped and for this reason such fragments are referred to herein as grains. As a generality, the size of grain fragments is not a problem where the reduction in cross section from that at which the product was anealed, or from the initial thickness of the stock, is more than 50%.

It will be appreciated that While reflectors can be formed from sheet or other wrought products composed entirely of the alloy described above, that alloy may also be used as a cladding on another alloy having different properties. The same considerations relating to grain or grain fragment size apply to the cladding and hence in referring to reflectors both the clad and non-clad wrought products are contemplated.

To obtain the desired reflecting surface, the article shaped from sheet or other wrought form must be subjected to a sequence of surface treatments. Once the surface has been cleaned, mechanically polished or buffed as required, it is subjected to conventional etching or brightening treatments depending upon whether a diffuse or specularly reflective surface is desired. An alkaline fluoride solution is exemplary of those Which produce a diffuse reflecting surface and a nitric-phosphoric acid mixture as described in United States Patents 2,650,157 and 2,729,551 illustrate a type which produces a brightened surface. Following such treatments the surface is subjected to a conventional oxide coating treatment that produces a transparent film. Electrolytic treatment in a sulfuric acid bath is an example of those that yield the desired result. Ordinarily the coating should not be more than about 0.5 mil in thickness.

The sheet or other products are fabricated according to customary practices of melting, casting, rolling or other working, and annealing.

The following examples illustrate the improvement gained by our invention over a well-known clad commercial product, identified as No. 12 Reflector Sheet, the surface of which nominally consists of aluminum, 0.04% copper and a maximum of 0.15% total of iron, silicon and other impurities. The alloy, representative of our invention, nominally consisted of 0.2% manganese, 0.1%

copper, 0.15% magnesium, a maximum of 0.15% total.

3 iron and silicon and balance aluminum. When cold rolled to 0.050 inch thick sheet and fully annealed, the commercial product had an average grain count of 56 grains per square millimeter while the sheet made from our alloy had a grain count of 195 grains per square millimeter.

Samples of both annealed sheets were treated in a conventional nitric-phosphoric acid solution to brighten the surface and then anodically treated in a 15% sulfuric acid solution at a current density of 12 amps per square foot and a voltage of 15. One group of samples was removed when the oxide coating acquired a thickness of 0.2 mil, a second group was left in the solution until the coating became 0.3 mil in thickness while a third group was exposed long enough to develop a coating 0.4 mil in thickness. These samples were tested in a Taylor-Baumgartner reflectometer to determine total reflectance values with the following results.

It is evident from the foregoing that the combined presence of manganese, copper and magnesium in reflector grade aluminum has produced a finer grain size in annealed sheet than found in the commercial product when annealed under the same conditions. Also, that the addition of the three elements to the aluminum did not reduce the reflectance values as compared to those of the commercial material.

Having thus described our invention and certain embodiments thereof, we claim:

1. An aluminum base alloy wrought article consisting of 0.1 to 0.3% manganese, 0.05 to 0.2% copper, 0.1 to 0.25% magnesium, a total of not over 0.2% impurities, and balance aluminum, said article being characterized by a smaller grain size than the same article made from aluminum containing no copper, magnesium and manganese except as unavoidable impurities and capable of developing as high a reflectance value by surface treatment as said aluminum containing no manganese, copper and magnesium.

2. A wrought article according to claim 1 wherein the alloy has a maximum of 0.1% impurities.

3. A light reflector having a surface composed of an aluminum base alloy consisting of 0.1 to 0.3% manganese, 0.05 to 0.2% copper, 0.1 to 0.25% magnesium, a maximum of 0.2% all impurities and balance aluminum, the surface of said reflector having a high uniform reflective quality surface and coated with a transparent artificial oxide film.

4. A light reflector according to claim 3 formed from a sheet of the defined alloy.

5. A light reflector according to claim 3 formed from a sheet consisting of a base and cladding thereon composed of the defined alloy.

6. A light reflector according to claim 3 having a specularly reflective surface.

7. A light reflector according to claim 3 having a diffusely reflective surface.

8. A light reflector according to claim 3 wherein the oxide film is less than 0.5 mil in thickness.

References Cited by the Examiner UNITED STATES PATENTS 2,383,511 8/1945 Reynolds 29l97.5 3,164,494 1/1965 English 75-139 X HYLAND BIZOT, Primary Examiner.

Claims (1)

1. 3. A LIGHT REFLECTOR HAVING A SURFACE COMPOUND OF AN ALUMINUM BASE ALLOY CONSISTING OF 0.1 TO 0.3% MANGANESE, 0.05 TO 0.2% COPPER, 0.1 TO 0.25% MANGNESIUM, A MAXIMUM OF 0.2% ALL IMPURITIES AND BALANCE ALUMINUM, THE SURFACE OF SAID REFLECOR HAVING A HIGH UNIFORM REFLECTIVE QUALITY SURFACE AND COATED WITH A TRANSPARENT ARTIFICIAL OXIDE FILM.