FR2782668A1 - Aluminum alloy laminate of at least two plated sheets or strips finished by cold bright rolling has high reflectivity on one external face and low reflectivity on other external face - Google Patents

Abstract

The two sheets or strips making up the external faces are chosen such that, after the same surface treatment, i.e. chemical or electrochemical etching or brightening, one has a bright face with reflectivity at 85 deg measured in the rolling sense (R85SL) greater than 60%, and the other matt face has reflectivity (R85SL) less than 20 deg . The bright face is, for example, made of aluminum of purity greater than 99.5% or an aluminum-magnesium alloy produced from aluminum of such purity. The matt face is made of an aluminum alloy with a composition such that the surface fraction of the intermetallic compounds is greater than 2.5%, and preferably greater than 7.5%.

Description

High-reflectivity aluminum alloy sheet or strip on one side and

  weak on the other face Field of the invention The invention relates to a sheet or strip of aluminum alloy such that it exhibits, after surface treatment, asymmetrical optical properties on its two faces,

  with a high reflectivity on one and very low on the other side.

  STATE OF THE ART Aluminum alloy sheets and strips having a specular reflecting surface have been used for many years, in particular for the manufacture of mirrors and reflectors. To obtain a high reflectivity, high purity aluminum, more than 99.5% aluminum, is generally used, or, if higher mechanical properties are desired, aluminum alloys, in particular of the AI-Mg type, made from high purity aluminum. The manufacturing process for these sheets and strips generally comprises one or more final cold rolling passes on polished rectified cylinders, and a chemical or electrochemical brightening treatment, possibly followed by anodization. It is thus possible to obtain, on the best face, a reflectivity at 85 of incidence, measured in the long direction (R85SL)

  according to ISO 7668, greater than 70%.

  Although the lamination introduces a slight difference between the reflectivity of the two faces, similar optical properties are generally obtained on these two faces. However, there are applications for which it is desired to have a single glossy face, the other being matt, such as for example certain types of reflectors in which it is desirable to avoid parasitic reflections. We are then led to cover one side with a black plastic film to avoid the reflection of the

  light on this face. This operation is however expensive.

  OBJECT OF THE INVENTION The object of the invention is to obtain directly, without subsequent coating, sheets and strips of aluminum alloy having asymmetrical optical properties on

  both sides, in particular a high reflectivity on one side and low on the other.

  The subject of the invention is a sheet or strip of aluminum alloy, finished by brilliant cold rolling, consisting of at least two sheets or strips pressed one on the other, the two sheets or strips constituting the external faces. being chosen in such a way that after the same surface treatment, in particular pickling, chemical or electrochemical brightening and / or anodization, one becomes a glossy face with a reflectivity at 85 measured in the direction of rolling (R85SL ) greater than%, and preferably 70%, and the other a matt face with R85SL reflectivity

  less than 20%, and preferably 10%.

  The reflectivity at 85 measured in the direction perpendicular to that of the rolling (R85ST) of the glossy face is greater than 70%, and preferably 80%. The reflectivity at 20 in the rolling direction (R20SL) of the matt face is less than 5

  and preferably less than 2%.

  Another object of the invention is a sheet or strip of aluminum alloy plated with high reflectivity on one side and low on the other, comprising an aluminum side of purity greater than 99.5% and preferably greater than 99, 7%, or an aluminum-magnesium alloy made from aluminum of the same purity, and an aluminum alloy face of composition such as the surface fraction of the intermetallic compounds, measured by image analysis on a micrograph, obtained by scanning electron microscopy, of a sample of this alloy, ie greater than 2.5%, and preferably 7.5%, the sheet or strip having undergone, after brilliant cold rolling, pickling or chemical brightening or electrochemical. After polishing, the sheets or strips can be anodized to give the surface finish obtained sufficient durability; for practical reasons, this anodizing treatment is indeed inevitable for a certain number of applications. The aluminum alloys leading to a high surface fraction of the intermetallic compounds are in particular alloys containing at least one of the elements Fe, Mn and Si, with a total weight content greater than 1%, for example AISi alloys containing 6 at 14% by weight of Si and preferably less than 1.7% of Mn and Fe. The sheet or strip may also include, between the two external faces, an aluminum alloy core of any composition, or in another metal alloy such

than steel.

Description of the invention

  The invention is based on the new combination of two phenomena, the principle of which is known. It is well known that, on high purity aluminum, polishing or pickling reduce the roughness -Ra, which can drop below 1 gm, and increase the reflectivity. It is also known that on certain alloys, the effect is strictly opposite, in particular alloys containing iron, manganese or silicon. The applicant has carried out numerous tests on the difference in behavior, during the chemical attack treatment of the metal during pickling or polishing operations, between the various aluminum alloys. These tests enabled it to relate this behavior to the presence of particles of intermetallic compounds of these elements with aluminum. It can be assumed that the presence of these compounds on the surface of the sheet causes preferential local attacks which cause an increase in roughness. The surface density of these intermetallic compounds can be measured by image analysis on micrographs carried out on samples of the alloy, for example by scanning electron microscopy. This gives a surface fraction of intermetals, which is measured in% of the total area of the observed area. The applicant has observed that when this surface fraction exceeds 2.5%, and preferably 7.5%, the reflectivity of the sheet after brilliant and glossy rolling can reach very low levels, less than 10%, while the same treatment leads to a reflectivity of more than 60% or even more than 70% for pure aluminum (for example AA 1085 according to the nomenclature of

the Aluminum Association).

  When an anodization is carried out leading to a transparent anodic oxide layer with a thickness of the order of 1 to 2 μm, it is observed, for alloys with a large surface fraction of intermetallics, a decrease in reflectivity. It can be assumed that the high concentration of intermetallic particles leads to coloration of the anode layer which further reduces the reflectivity. The two effects, namely the decrease in reflectivity due to preferential attacks during the polishing, and the decrease in reflectivity due to the opacity of the anode layer linked to the incorporation of intermetallic particles, add up. We can thus obtain a very matt surface with a reflectivity of less than 10% and even less than 5%,

  with a very high reflectivity (> 70%) on the other side.

  The applicant has further observed that in order to obtain a surface fraction of intermetals greater than 2.5%, the alloy must contain, for example, iron, silicon and / or manganese at a total weight content of more than 1%. . High rates, greater than 7.5%, are obtained with the AISi alloys of the 4xxx series according to the

  designation of the Aluminum Association.

  The surface fractions are measured as follows. For alloys with a high surface fraction of intermetallics, in particular alloys of the 4xxx series, a sample is taken, which is coated with resin and polished under decane, to avoid oxidation of the Mg2Si phases leading to the surface, up to 0.25 µm diamond paste, according to metallographic techniques known to those skilled in the art. A very thin conductive layer can optionally be deposited by evaporation. The shots are taken by scanning electron microscopy, at a magnification of 1000, in contrast mode of atomic number (backscattered electrons), with an electron acceleration voltage of 15 kV In the alloys of the 4xxx series, there are several families of intermetallic phases: most often Mg2Si and AIMgSi (in black), primary silicon (in gray) and AlFe (in clear). After filtering and reconstructing the edges, gray level thresholds make it possible to extract the image of the phases and to measure the size and the surface fraction of these phases. The measurements are cumulated on 15 fields per sample, distributed at random on the polished section. The area fraction is expressed as the sum of the areas of the different types of phases, related to the total area of the field analyzed. By way of example, for an alloy 4004, well suited for obtaining the mat face of the sheet or strip according to the invention, the following results are found for the different phases (the experimental standard deviation is indicated in parentheses for a cumulative of 15 fields and a magnification of 1000) Phases Phases with iron Si primary Mg2Si Fraction surf. (%) 2.97 (0.40) 7.95 (1.36) 3.58 (0.63) Size (gim) 2.02 (0.95) 2.77 (2.01) 1.26 (0.89) For alloys with a small surface fraction of intermetallics, such as for example alloys 1085, 1200, 8011 and 8079, a magnification of 2000 is adopted and

  the observation is made on a polished section covered with a thin layer of carbon.

  The electron acceleration voltage is 15 kV. To ensure sufficient stability of the measurement results, they are accumulated on 30 fields distributed randomly over the polished section. The measurements are generally stable from 5 to 10 fields, but sometimes the surface fractions are more dispersed. The surface fraction thus measured takes into account all the intermetallic phases, the most important

being AIFe and AlFeSi.

  Table 1 gives the surface fractions of intermetallic for some alloys.

  When the surface fraction of pure aluminum containing less than 500 ppm of iron and less than 400 ppm of silicon is measured, a rate of less than 0.2% is obtained, which is perfectly suitable for the shiny side. Instead of 99.5% pure aluminum, an AlMg or AICu alloy can also be used for the shiny side, provided that an aluminum base of 99.5% purity is used, in particular respecting the same

  limit values than previously for iron and silicon.

Table 1

  Alloy Mn (%) Fe (%) Si (%) surface fraction (%)

1085 0.036 0.04 0.1

1200 0.5 0.1 0.8

3003 1.0 0.4 0.2 4.1

4045 0.5 10 12.0

8011 0.8 0.7 2.7

8079 1.3 0.15 3.6

  The pickling or polishing treatments are carried out in a manner known per se. For example, pickling can be carried out for five minutes in an alkaline bath (50 g / l) at a temperature of 50 ° C., followed by neutralization with nitric acid. The polishing can be carried out for one minute in an acid bath (H3PO4 1200 g / l and H2SO4 300 g / l) at a temperature of 75 C under a voltage of V. Anodization can be carried out for five minutes in a H2SO4 (200 g / l) at 20 C through which a current of 1.5 A / dm2 passes. Anodizing is normally followed by plugging which can be done for four minutes

  in an aqueous solution of nickel salts at 100 C.

  The reflectivity measurements are carried out according to ISO 7668, with a lamp with

  tungsten filament and at fixed angles of 20 and 85.

  The invention can be achieved in different ways. One can use a basic alloy which leads after pickling or chemical brightening or electrochemical brightening to a very low reflectivity, such as an alloy of the 4xxx series, for example alloy 4045, plated on one side with an alloy which develops after surface treatment a

  high reflectivity, such as alloy 1085.

  We can also proceed in the opposite way and use as base metal an alloy which leads to a high reflectivity, plated with an alloy which leads to a low reflectivity. And then, in a preferred embodiment of the invention, it is possible to interpose, between the two external faces of alloys chosen for their influence on the optical properties (for example alloys 4045 and 1085), a sheet or strip of any alloy, chosen for economic reasons or to obtain certain mechanical characteristics. For example, this central sheet or strip can be in

aluminum alloy, or steel.

  The sheets or strips in the various alloys can be plated, in a manner known per se, by colaminating, the colaminated strip then being subjected to a glossy rolling with rectified cylinders. The thickness of the cladding (which can be determined according to standard EN 2004-5) is advantageously between 1 and 20% of the total thickness of the sheet or strip, for each face, and preferably between and 12%. However, regardless of the total thickness of the sheet, the thickness of the

  plating must be greater than 10 lm per side.

Examples

  The applicant has determined the reflection characteristics of different sheets before and after surface treatment (see Tables 2 and 3). In Table 2, for each type of sheet, four sheets of different industrial origin have been characterized

  as is or subjected to surface treatments and then characterized.

Table 2

  Sample Ref. R20SL R20ST R85SL R85ST

[%] [%] [%] [%]

  1050 alloy, "High Gloss" quality, Ala 40.7 38.3 55.2 52.8 electrochemical polishing 60 sec Alb 39.5 39.2 54.9 54.1 anodic oxide 1, im Alc 40.4 39, 3 55.4 51.5 Ald 40.2 38.9 55.2 52.8 1050 alloy, "High Gloss" quality A2a 67.5 68.5 83.2 84.4 A2b 67.7 68.4 85, 6 84.5 A2c 67, 8 68.2 85.3 83.5 A2d 67.7 68.4 84.7 84.1 alloy 3003 A3a 12.3 7.5 48.9 43.1 electrochemical polishing 60 s A3b 12.5 7.6 49.1 41.6 anodic oxide 1 gm A3c 11.9 7.6 51.0 38.2 A3d 12, 2 7.6 49.7 41.0 alloy 8079 A4a 7.6 4, 9 42.3 35.9 electrochemical brightening 60 s A4b 8.3 5.6 49.6 33.9 anodic oxide 1 gm A4c 5.6 4.4 39.6 33.3 A4d 7.2 5.0 43, 8 34.4

Table 3

  ref alloy sheet treatment R20SL R20ST R85SL R85ST (see codes below) 00 [%] [%] [%] []

B 1 1085 (0) 75.7 76.2 77.5 85.0

  B2 1085 (1) I Hm 74.7 74.8 72.5 85.4 B3 1085 (2) 2.5 tm 67.6 67.4 73.0 76.3

B4 1085 (3) 83.1 82.6 73.6 80.9

  B5a 1085 (3) + (2) 1 plm 79.8 79.1 72.8 78.1 B5b 1085 (3) + (2) 2.5 Him 77.0 76.9 73, 1 75.3

B6 4045 (0) 54.0 54.1 81.3 76.4

  B7 4045 (1) 1 pm 3.4 2.3 35.4 19.4 B8 4045 (2) 2.5 lpm 0.94 0.79 54.7 32.4

B9 4045 (3) 2.4 2.0 4.5 4.5

  B10Oa 4045 (3) + (2) 1 lm 0.60 0.56 4.1 4.2 BlOb 4045 (3) + (2) 2.5 Hm 0.20 0.20 0, 3.3 3.6 code sheet metal treatments (0) uncoated, without surface treatment (1) pickling (with indication of the etched thickness) (2) anodization (with indication of the thickness of the anodic oxide layer) (3) electrochemical brightening It is noted that to obtain a high reflectivity after brightening, it is preferable to use an aluminum with a purity greater than 99.5% or an AI-Mg alloy produced from an aluminum with a purity greater than 99.5%, in order to avoid the harmful effect of intermetallics. An excellent choice, in terms of technical performance and cost, is represented by the alloy 1085 for the shiny face, and an alloy of type 4045

  for the mat face, preferably in the form of a sheet plated on both sides.

  It is surprising to note that, in the case of the 4045 alloy, the anodization alone greatly reduces the reflectivity measured at 20 o, but leaves the reflectivity measured at 85 at a fairly high level. On the other hand, the brightening alone lowers the reflectivity measured at 85 o, but leaves a reflectivity measured at 20 at an appreciable level. Successive execution of the two processes therefore makes it possible to obtain

  both a low reflectivity measured at 20 o, and a low reflectivity measured at 85.

  This is why the shine followed by anodization is the embodiment of

the most effective invention.

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Claims (19)

  1. Sheet or strip of aluminum alloy, finished by bright cold rolling, consisting of at least two sheets or strips plated one on the other, characterized in that the two sheets or strips constituting the external faces are chosen in such a way that after the same surface treatment, one becomes a shiny face with a reflectivity at 85 measured in the rolling direction   (R85SL)> 60% and the other a matt face with a reflectivity R85SL <20%.   2. Sheet or strip according to claim 1, characterized in that the treatment of   surface is a chemical or electrochemical pickling or polishing.   3. Sheet or strip according to one of claims 1 or 2, characterized in that the   R85SL reflectivity of the shiny side is more than 70%.   4. Sheet or strip according to one of claims 1 to 3, characterized in that the   R85SL reflectivity of the mat face is less than 10%.   5. Sheet or strip according to one of claims 1 to 4, characterized in that the   reflectivity at 85 o measured perpendicular to the rolling direction   (R85ST) of the shiny side is more than 70% and preferably 80%.   6. Sheet or strip according to one of claims 1 to 5, characterized in that   reflectivity at 20 measured parallel to the rolling direction (R20SL) of   the matt face is less than 5% and preferably less than 2%.   7. Sheet or strip according to one of claims 1 to 6, characterized in that the   shiny side is made of aluminum with purity greater than 99.5% and preferably greater than 99.7%, or aluminum-magnesium alloy   made from aluminum of such purity.   8. Sheet or strip according to claim 7, characterized in that the high-alloy   purity contains less than 500 ppm of iron and less than 400 ppm of silicon.   9. Sheet or strip according to one of claims 1 to 8, characterized in that the face   matt consists of an aluminum alloy of composition such that the surface fraction of the intermetallic compounds is greater than 2.5%, and preferably 7.5%.   10. Sheet or strip according to claim 9, characterized in that the aluminum alloys leading to a high surface fraction of the intermetallic compounds are alloys containing at least one of the elements Fe, Mn and   If, at a total weight content greater than 1%.   11. Sheet or strip according to claim 10, characterized in that the aluminum alloy leading to a high surface fraction of the compounds   intermetallic is an AISi alloy containing 6 to 14% (by weight) of Si.   12. Sheet or strip according to claim 11, characterized in that Si is included   between 6 and 14% and Mn <1.7% and Fe <1.7%.   13. Sheet or strip according to one of claims 1 to 12, characterized in that it   comprises, between the two external faces, a core of aluminum alloy or steel.   14. Sheet or strip according to one of claims I to 13, characterized in that   the thickness of the cladding is between, for each face, between I and 20% and preferably between 5 and 12% of the total thickness of the sheet, and always greater than 10 gtm.   15. Clad sheet or strip, cold-rolled bright, then pickled or brightened chemically or electrochemically, with a shiny face of aluminum with a purity greater than 99.5%, or an aluminum-magnesium alloy made from aluminum of same purity, and a matte face of aluminum alloy of composition such that the surface fraction of the intermetallic compounds is   greater than 2.5%, and preferably 7.5%.   16. Sheet or strip according to claim 15, characterized in that the aluminum alloy leading to a high surface fraction of the intermetallic compounds is an alloy containing at least one of the elements Fe, Mn and Si,   at a total weight content greater than 1%.   17. Sheet or strip according to claim 16, characterized in that the aluminum alloy leading to a high surface fraction of the compounds   intermetallic is an AlSi alloy containing 6 to 14% (by weight) of Si.   18. Sheet or strip according to one of claims 15 to 17, characterized in that it   comprises, between the two external faces, a core of aluminum alloy or steel.   19. Sheet or strip according to one of claims 15 to 18, characterized in that after   pickling or polishing, it is anodized.