US3187428A - Method of treating aluminum and aluminum alloys preparatory to bright finishing - Google Patents

Description

United States Patent METHOD OF TREATING ALUMINUM AND ALUMI- NUM ALLOYS PREPARATORY TO BRIGHT FINISHING This invention relates to chemical and electrolytic brightening of aluminum and aluminum alloys, and particularly concerns an improved metallurgical technique for obtaining a brightly reflective and yet relatively heavily anodized article. 7 V

This application is a division of application Serial No. 63,503 (now US. Patent 3,164,494).

The production of decorative, anodized aluminum articles has been known in the art for many years. The lustrous surface has an attractive appearance, and the anodized coating overlying the reflective metallic surface has the advantage of furnishing a'hard-surfaced protective coating to preserve the bright aluminum substrate. Such an anodized coating is not itself reflective, but is generally transparent so that light can be reflected through it 1 It has been found, however, when using conventional alloys and practices, that the reflectance of an anodized article progressively diminishes to a noticeable extent as the thickness ofthe anodized coating is increased. In an effort to preserve most of the bright reflective qualities of the anodized article, while still retaining some anodized coating for protective purposes, it has usually been accepted in the trade that the anodized coating is allowed to fall'on the sample, and the reflected'light is v automatically integrated inside a magnesium oxide coated The average light density, as measured by the two circumstances: the brightness of an anodized article depends not only upon the transparency of the coating, but also upon the surface regularity of the metallic substrate. Impurities or alloying constituentscreate the possibility of both inclusions in the anodized coating and pitting or other surface. irregularities on the substrate. Of course, ultra high-purity aluminum is extremelyexpensive, and it lacks various physical properties which maybe required for a given application. The use of magnesium as a suitable alloying element is known toyield satisfactory strength characteristics, but the quantity of that element I has necessarily been restricted due to its deleterious effect the types of alloys which are presently preferred in the trade forbrightly reflective anodized articles, viz. the 5X57 series (Aluminum Association designation).

" sphere.

The published compositions of several 5X57 alloys are given below. 1

Constituents Manganese Magnesium Others, each Others, total Aluminum B THE METHOD USED TO EVALUATE REFLECTIVE CHARACTERISTICS The light reflecting characteristics of metallic surfaces When an incident light beam strikes a surface, the

intensity, distribution and color of the reflected light are a measure of the nature of thejsurface. Instruments for the photometric measurement of opaque surfaces typically consist of a suitable light source, an integrating light sphere, a photoelectric cell, signal multiplier, and recording or indicating equipment. The incident light beam energyxdistribution to the sensitivity of the eye. Such a A light source is employed in the modified Gardner Pivotable Sphere Hazemeter which was used to determine the characteristics reported herein. The conversion filter employed consists of two bonded glass plates (blue and amber) designated to give atransmittance curve closely approximating the visual response ofequal energy light at various wave lengths. With .this filter, surfaces having different lusters can be measured and the result compared with visual impressions.

The pivotable integrating sphere allows the angle of incident light to be varied from normal (or perpendicular) to the sample surface to a second value which is usually less than 10 from the normal. In the, second position, virtually all reflected light is retained inside the sphere. When the angle of incidence is 0 (perpendicular to the surface), only the diffuse or scattered light is retained inside the sphere and the specular reflection is emitted outside the sphere.

The intensity of the reflected light from any particular Patented June 8, 1965 light source, as measured on an instrument as described, may be evaluated under the following definitions:

1) Total reflectance (TR percent).The percent intensity of the incident light reflected from a flat opaque surface at all angles of observation.

('2) Diflase reflectance (DR pcrcent).-The percent intensity of the incident light reflected from a flat opaque surface at all angles and planes, other than the portion of the light reflected at an angle equal to the incident beam and in the same plane to the normal.

(3) Specular reflectance (SR percent).The percent intensity of the incident light reflected at an angle equal to the incident beam and in the same plane.

' Thus, the sum of SR+DR=TR or specular and diffuse components equal the total reflectance. In measuring these components, the angle of incidence should be less than 10 (as measured from the normal).

A perfectly diffuse surface would scatter or diffuse the incident light at all positions (angles) of the sphere giving identical readings for both TR and DR. In contrast, a specular surface would reflect substantially all of the incident light, if the incident light is normalto the surface, and give extremely low DR. If the angle of incidence has a value other than 0, all the reflected light (TR) is retained in the sphere. The two readings TR and DR, therefore, distinguish all surfaces if the illuminant and angle of incidence (for TR readings) are kept constant. The difference TRDR, referred to as the specular component of the reflected light, is then readily calculated.

The generally adopted standard of reflectance consists of a specially prepared magnesium oxide surface. The preparation of this, standard is outlined in ASTM Specification 11986-50. This surface (a perfect diffuser) is considered to give 100% reflectance (TR percent and DR percent). In order to balance the measuring circuit of the instrument, a black standard is used to define 0% reflectance. This standard consists of a black dyed velvet surface roviding, a nearly perfect light trap and has an absolute reflectance of 0.4%. Under some circumstances, it has been found tobe advantageous to use an etched aluminum surface for an, 80% reflectance standard, together with a black polished glass for 0% standard. This latter system of secondary standards is the basis upon which the data herein Was determined.

Certain additional definitions are essential to an understanding of the significance of the accompanying data. Specular reflectance is defined as the ability of a plain surface to reflect an image without distortion.

Specular reflectance factor (SRF percent) is a measure of the ratio of specular reflectance to total reflectance.

By reporting the TR and DR readings, the average reader will only with diificulty comprehend the real specularity of a surface whereas the specular reflectance factor is a. direct measure of the mirror qualities of the surface. As an example, a sample of highly specular aluminum showed these readings as compared to polished chrome plate:

TR D R SRF percent percent percent Aluminum 87. 5 12. 5 90. 8 Chrome 67. 5 9. 5 90. 9

a. the standard silver mirror and the previously mentioned aluminum and chrome surfaces:

From the above discussion, it follows that specular surfaces should be reported in terms of specular reflectance (SRFEimage quality) and specular brightness (SBFEimage. brightness) As an additional example, two commercial mirrors (silver and lead sulfide) gave identical SRF readings of 96% but had SBF values of 92% and 45% respectively.

The following equations are employed to compute the two factors used herein to differentiate the various surfaces discussed:

SRF percent=100- TR or if the instrumental correction factor C has been determined using the specular standard,

.W SRFpement-IOO [TR] (b) Also, 4

100 SBF percent (TR DR) Where TR and DR are the previously defined reflectance readingsg TR and; DR are the corresponding readings for the standard; and

The following examples furnish specific illustrations of present preferred embodiments of the invention.

Example I An alloy was made consisting of .03% copper, .08% iron, .05% silicon, .03% manganese, and 0.80% magnesium, the balance being substantially aluminum (other constituents each being no more than .02% and totaling less than .05%). A standard sheet ingot of this, alloy was cast by conventional D.C. casting methods, and homogenized in the temperature range 1l00-ll75 F. for a period of several hours in order to obtain satisfactory grain size control'and uniformity of structure. The ingot was then scalped to remove surface irregularities and then hot-rolled (750900 F.) in a slabbing mill. The slabbed ingot was then cooled to about 525-575 F. and given an additional reduction in thickness while the metal temperature was above about 350 F. The ingot thickness was reduced from approximately 15 inches to about 1 inch in the hot-rolling step, and to about 0.15 inch in the subsequent rolling. Thereafter, the sheet was brought to a finished gage of about .040 inch by means of a conventional cold-rolling operation. Portions of the sheet were tempered to approximately H25 condition, while others were annealed. The sheet portions were finished by conventional mechanical butting, then solvent degreased, bright dipped in a nitric acid solution, and anodized with 15 %-18% sulfuric acid in deionized water at 65-75 F., using 10-15 amperes per square foot for different periods of time (ranging from about 8 minutes to minutes) for different specimens in order to produce anodized coatings of various thicknesses on the respective specimens. The coatings were all sealed in deionized water at 210-212 F.

The reflectance characteristics of the specimens are tabulated below. The values for O-temper material are given in Table 1, while Table 2 presents similar information for H25 temper material.

For comparison, specimens of 5457 alloy produced by conventional procedures typically exhibit much poorer characteristics as the coating thickness is increased to one mil, viz. down to about 79 (SRF percent) and about 60 (SEE percent). However, by utilizing the procedures as specified in Example I, especially the intermediate Warmcool rolling, the results for even 5457 alloy can be improved to maximums of about 87 (SRF percent) and 73 (SEE percent) and'minimums, respectively; of about 81 and 64. Thus, the remarkable characteristics tabulated inTable l are attributable both to the improved fabrication practice and to the improved alloy composition.

Certain comparisons again indicate the superiority of the alloy and practice described in Example I. For example, tempered 5457' alloy (approximately half hard) exhibits characteristics ranging down to about 80 (SRF percent) and 64 (SEE percent) as the coating thickness is increased to about 1 mil. The corresponding values for 5357 alloy are about 6 6 (SRF percent)and 50 (SBFpercent); and the char-acteristics for that alloy seldo'rn exceed 73 and 61, respectively, even for a 0.1 mil surface coating.

Another comparative reference of interest is high purity aluminum (generally referred to as tour ninef metal) which has an aluminum content of at least 99.99% and is generally produced by expensive refining processes. When this material is fabricated by conventional techniques, its image characteristics are typically about 90 (SRF percent) and 78 (SEE percent) for H25 temper. It is apparent therefore, thatthe alloy and fabricating practice of the present invention are capable of producing a surface finish substantially equivalent to those prev-iously attainable only with more expensive high purity aluminum.

6 7 Example II An alloy similar to that in Example I was prepared in the same manner, but having the following spectrographic analysis:

Cu Fe Si Mn Mg "211 Ni Cr Ti Al .75 |.00 .00 .00 .01 Bill.

Table presents the image characteristics which were observed in the case of the Example II alloy.

i The specimens of Example II were produced in H25 temper, and therefore the results shown in Table 3 correspond to those of Table 2. It is readily apparent from a comparison of Tables 2 and 3 thatthe results achieved with the alloys of Examples I and II are equivalent. It should be noted that the increasein the copper content of the Example 11 alloy apparently had "no deleterious effect on the brightness of the finished sheet. 7

'- Analyses of variant forms of theExample II alloy, which showed comparable characteristics, are the follow ing:

Cu Fe Si Mn Mg Zn Ni Cr Ti Al The sheet of alloy (1) above was produced in .070inch thickness, while that of alloy (2) was- .125 inch.

In order to evaluate the critical upper limit of copper, additional tests were made using alloys within the general designation: aluminum and '0.'60 1.0% 'ma'gn'esiurn; up to 0.12% iron, up to 118% silicon, manganese, .03% max. and varying amounts of copper. It foiind that 08% copper did not detract appreciably from the brightness of the finished alloy. In fact, as much as 0.15% copper was indicated to be satisfactory provided the metal was annealed; however, tempered stocles (such as H25) exhibited an undesired hazing or clouding when the copper content reached this level.

In addition, the presence of small amounts of zinc, 'niclrel, chromium, and titaniumfwithin the broad limits of 03% each and 0.10% total) did not significantly affect the brightness characteristics Beyond such limits, however, additions of chromium have progres sivelydetrimental effects upon surfacebrightness. For example, additions of 05% and 0.10% chromium toan alloy su'oli as that of Example I result in characteristics-not differing substantially from those attainable with 'conventional'al- 10y 5457.

. Example III In order to evaluate the influence of iron and silicon,

as well as the general impurity level, an alloy was prepared in the manner described in Example I, having the 71. It will be apparent that this alloy may be formulated even from commercial reduction cell aluminum, since the aluminum content is only about 99.75% exclusive of the magnesium. The image characteristics of H25 material having the above analysis are summarized in Table 4.

Example IV In order to evaluate the influence of magnesium upon the image characteristics of the finished metal, two alloys were constructed, having compositionsas follows:

Cu Fe Si Mn Mg Zn Ni Cl Ti A1 (a)... .06 .07 .05 01 1.95 01 .01 01 Bal. (b)... 02 08 01 3. 22 03 00 01 02 Dal.

The image characteristics for H25 temper material having compositions (a) and (b) as noted above are shown respectively in Tables 5 and 6.

TABLE 5 Image reflecting characteristics Approximate thickness of anodie coating (mils) Quality, Brightness, SRF percent SBF percent TABLE 6 Image reflecting characteristics Approximate thickness of anodie coating (mils) Quality, Brightness, SRF percent SBF percent While these readings are somewhat lower than the cor- Cu Fe Si Mn Mg Zn Cr 8 produced in H25 temper by conventional fabricating procedures exhibited substantially lower image characteristics, viz. from about 74 down to about 53 (SRF percent) and from about down'to about 35 (SBF percent), as the coating thickness ranged from 0.1 mil to 1 mil.

It is apparent, therefore, that novel alloys such as those of Example IV are particularly suitable for end uses which demand both high strength and good image characteristics. Previously, the image characteristics of an alloy of comparable purity were sacrificed whenever improvement of strength was essential. Similarly, previous alloys constructed to exhibit optimum image characteristics were necessarily of lower strength. The opportunity to increase the magnesium content is, therefore, a significant advantage of the present invention.

By way of summation, the present invention encompasses an improved fabricating method typified by a warm-cool rolling step which has been found to contribute materially to the superior image characteristics of the novel alloys disclosed herein, as well as aluminum and other aluminum alloys generally. With respect to that aspect of the invention which relates to fabricating procedures, it has been found that optimum development of image characteristics may be achieved by use of a novel step between conventional hot and cold-rolling operations in the preparation of the metal. The intermediate rolling step may be conducted while the metal is in the temperature range from about 300600 F. Preferably, the metal is cooled following hot-rolling to a temperature in the range of about 525-575 F., and a substantial reduction in thickness is accomplished before the metal temperature falls below about 350 F.

While various present preferred embodiments of the invention have been described, it will be recognized that the invention may be otherwise variously embodied and practiced within the scope of the following claims.

I claim:

1. A method of forming a surface of high reflectivity on a body of a substance belonging to the group consisting of aluminum and aluminum alloys containing aluminum up to about 99.95% and up to about 3% magnesium, the balance consisting of copper up to about 0.15%, iron up to about 0.12%, silicon up to about .08%, others up to about .03% each and 0.10% total, comprising the steps of: homogenizing a cast ingot of said metal to achieve grain size control and uniformity of structure; hot-rolling said ingot; cooling the resulting slab to about 525-575 F. and warm-cool rolling the slab to a substantial reduction in thickness, said warm-cool rolling being completed before the metal temperature drops below about 300 F.; cold-rolling to final gage; and bright finishing.

2. A method of forming a surface of high reflectivity on a body of a substance belonging to the group consisting of aluminum and aluminum alloys consisting essentially of magnesium up to about 1.20%, copper .0l.08%, manganese .03% maX., iron .01-.l2%, silicon .01.08%, balance aluminum, comprising the steps of: homogenizing a cast ingot of said metal to achieve grain size control and uniformity of structure; hot-rolling said ingot; cooling the resulting slab to about 525-575 F.; and warm-cool rolling the slab to a substantial reduction in thickness, said warm-cool rolling being completed before the metal temperature drops below about 350 F.; cold-rolling to finished gage; and anodizing.

3. A method of forming a surface of high reflectivity on a body of a substance belonging to the group consisting of aluminum and aluminum alloys consisting essentially of magnesium 0.60-1.0%, manganese .03% max, copper .0l.06%, iron .01.10%, silicon .01.08%, balance aluminum, comprising the steps of: homogenizing a cast ingot of said metal to achieve grain size control and uniformity of structure; hot-rolling said ingot; cooling the resulting slab to about 525575 F.; and warm cool rolling the slab to a substantial reduction in thickness,

said warm-cool rolling being completed before the metal temperature drops below about 350 F.; cold-rolling to finished gage; and anodizing.

4. In a method of bright finishing aluminous metal from the group consisting of aluminum and aluminum alloys of the X57 series, the step of rolling said metal in the temperature range of 300-600" F. following a hotrolling operation at higher temperature and prior to coldrolling, whereby the luster of the finished surface is enhanced.

5. In the production of aluminum and aluminum alloy sheet preparatory to bright finishing, the method which comprises hot-rolling an ingot of said metal above 600 F. to provide a slab suitable for rolling into sheet; warmcool rolling the slab in an intermediate operation, following said hot-rolling and prior to cold-rolling, during which warm-cool rolling operation the metal temperature is below the hot-rolling temperature and above 350 F., the thickness of the workpiece at the end of said operation being reduced to about 85% of its thickness at the beginning :of said operation; and cold-rolling the metal to finished gage.

6. A fabricating method for improving the surface finish of aluminum and aluminum alloys which are susceptible to chemical and electrolytic brightening, comprising the steps of: hot-rolling an ingot therefore at about 750-900 F.; cooling the slabbed ingot to below about 600 F.; rolling the slab to a substantial reduction of thickness while the temperature of the slab is above about 300 F.; and cold-rolling to final gage.

7. The method of claim 6 wherein the intermediate rolling step is completed while the metal is above about 350 F.

8. A method of fabricating aluminum and aluminum alloys which are susceptible to chemical and electrolytic brightening, comprising the steps of: homogenizing a cast ingot thereof to achieve grain size control and uniformity of structure; hot-rolling the ingot; warm-cool rolling the resulting slab in the temperature range of 300-600" F.; and cold-rolling to final gage.

9. The method of claim 8 wherein said hot-rolling is accomplished above about 600 F.; the resulting slab is then cooled to about 525-575 F.; and said warm-cool rolling is completed above about 350 F.

References Cited by the Examiner UNITED STATES PATENTS WHITMORE A. WILTZ, Primary Examiner.

JOHN F. CAMPBELL, Examiner.

Claims (1)

1. 2. A METHOD OF FORMING A SURFACE OF HIGH REFLECTIVELY ON A BODY OF A SUBSTANCE BELONGING TO THE GROUP CONSISTING A ALUMINUM AND ALUMINUM ALLOYS CONSISTING ESSENTIALLY OF A MAGNESIUM UP TO ABOUT 1.20%, COPPER .01-.08%, MANGANESE .03% MAX., IRON .01-12%, SILICON .01-.08%, BALANCE ALUMINUM, COMPRISING THE STEPS OF; HOMOGENIZING A CAST INGOT OF SAID METAL TO ACHIEVE GRAIN SIZE CONTROL AND UNIFORMITY OF STRUCTURE; HOT-ROLLING SAID INGOT; COOLING THE RESULTING SLAB TO ABOUT 525-575*F.; AND WARM-COOL ROLLING THE SLAB TO A SUBSTANTIAL REDUCTION IN THICKNESS, SAID WARM-COOL ROLLING BEING COMPLETED BEFORE THE METAL TEMPERATURE DROPS BELOW ABOUT 350*F.; COLD-ROLLING TO FINISHED GAGE; AND ANODIZING.