EP3500689B1

EP3500689B1 - Anodized aluminum with dark gray color

Info

Publication number: EP3500689B1
Application number: EP17758674.0A
Authority: EP
Inventors: DaeHoon KANG; Martin Frank; Simon Barker; Devesh Mathur
Original assignee: Novelis Inc Canada
Current assignee: Novelis Inc Canada
Priority date: 2016-08-17
Filing date: 2017-08-11
Publication date: 2023-10-04
Anticipated expiration: 2037-08-11
Also published as: AU2017312853B2; JP7149262B2; MX2019001837A; US20180051387A1; BR112019002606B1; KR20190040007A; SA519401070B1; CA3033962C; WO2018034960A1; PL3500689T3; CN109642300A; BR112019002606A2; EP3500689A1; CN109642300B; CA3033962A1; JP2019531404A; AU2017312853A1; HUE063474T2; ES2960834T3; RU2717622C1

Description

This application claims the benefit of U.S. Provisional Patent Application No. 62/375,932, filed August 17, 2016 .

FIELD

Described herein are anodized aluminum alloy sheets and, in particular, dark gray colored anodized aluminum alloy sheets.

BACKGROUND

A dark gray color is a desirable property in certain anodized aluminum products, such as anodized quality ("AQ") architectural sheets. An anodization process is an electrochemical process that converts the aluminum alloy surface to aluminum oxide. Because the aluminum oxide forms in place on the surface, it is fully integrated with the underlying aluminum substrate. The surface oxide layer produced by an anodization process is a highly ordered structure that, when pure, can be clear and colorless so that the anodized sheet has a shiny, light gray color. The surface oxide layer is also porous and susceptible to additional colorization by treatment subsequent to and/or separate from the anodization process. Conventional colored anodized alloys are colored by additional absorptive or electrolytic coloration processes, which increase production costs for colored alloys relative to alloys that are not colored. JP2009209426 A discloses an aluminum alloy for a housing comprising: 5000 type aluminum alloy having less than 0.05 wt% Cu and an anodized film and its production method.

SUMMARY

Covered embodiments of the invention are defined by the claims, not this summary. This summary is a high-level overview of various aspects of the invention and introduces some of the concepts that are further described in the Detailed Description section below. This summary is not intended to identify key or essential features of the claimed subject matter, nor is it intended to be used in isolation to determine the scope of the claimed subject matter. The subject matter should be understood by reference to appropriate portions of the entire specification, any or all drawings, and each claim.
Provided herein are aluminum alloys that have a dark gray color when anodized. The alloys do not require any absorptive or electrolytic coloration process separate from the anodization process to achieve the dark gray coloration. The alloys have economic and environmental advantages over conventional anodized aluminum alloys that require a separate coloration process in order to achieve a desired color.
In one aspect, this invention provides an aluminum alloy as defined in claim 1 with preferred composition defined in depend claim 2. In another aspect, this invention provides an aluminum sheet as defined in claim 3 with preferred embodiments defined in dependent claims 4-7. In the third aspect, this invention provides a method of preparing an aluminum sheet as defined in claim 8 with preferred embodiments defined in dependent claims 9 and 10.
Other objects and advantages will be apparent from the following detailed description of non-limiting examples.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1A is a scanning transmission electron microscopy (STEM) image of dispersoids in a comparative aluminum alloy.
FIG. 1B is a STEM image of dispersoids in a comparative aluminum alloy.
FIG. 1C is a STEM image of dispersoids in an aluminum alloy with a dark anodized color, as described herein.
FIG. 2A is a high-resolution scanning electron microscopy (SEM) image of dispersoids in a comparative anodized aluminum alloy.
FIG. 2B is a high-resolution SEM image of dispersoids in a comparative anodized aluminum alloy.
FIG. 2C is a high-resolution SEM image of dispersoids in an anodized aluminum alloy with natural dark anodized color, as described herein.
FIG. 3A is a phase diagram of phases in a comparative alloy.
FIG. 3B is a phase diagram of phases in a comparative alloy.
FIG. 3C is a phase diagram of phases in an anodized aluminum alloy with natural dark anodized color.

DETAILED DESCRIPTION

Described herein are alloys and processes providing colorized anodized substrates designed based on in-depth microstructure and metallurgical analysis. Generally, an anodized layer on a conventional aluminum alloy substrate is almost transparent and the anodized substrate shows a deep and shiny light gray metallic color due to light reflectance from both the surface of the anodized layer and the surface of the base metal. In the alloy products prepared according to the present methods, fine intermetallic particle dispersoids (alternately called precipitates) inside the normally-transparent anodized oxide layers of the anodized alloys described herein affect the color of the anodized material by interrupting light as it passes through the anodized layer before it can reach the surface of the base metal. By controlling alloy composition and process parameters, the number density of certain dispersoids inside the anodized layer is maximized. Those dispersoids give the anodized substrate a dark gray color without an additional coloring process.
The alloys and methods disclosed herein provide dark anodized sheets that can be prepared with significantly reduced processing and cost as compared to known dark anodized sheets. The methods described herein eliminate conventional adsorptive or electrolytic coloration steps which are required in current production of dark colored anodized materials. The methods described herein result in fewer byproducts and are more environmentally friendly than conventional methods of producing similarly colored products.
In some examples, an anodized aluminum sheet as described herein has a dark gray color. The color of the anodized aluminum sheet can be quantified by colorimetry measurement by CIE lab 1931 standard and/or ASTM E313-15 (2015). In some examples, the anodized aluminum sheet has an L^∗ value lower than 60, lower than 55, or lower than 50, as measured by CIE lab 1931 standard. In some examples, the anodized sheet has a white balance of lower than 35, lower than 30, or lower than 25, as measured by ASTM E313-15 (2015).

Definitions and Descriptions

Aluminum alloys are described herein in terms of their elemental composition in weight percentage (wt. %) based on the total weight of the alloy.
As used herein, the meaning of "room temperature" can include a temperature of from about 15 °C to about 30 °C, for example about 15 °C, about 16 °C, about 17 °C, about 18 °C, about 19 °C, about 20 °C, about 21 °C, about 22 °C, about 23 °C, about 24 °C, about 25 °C, about 26 °C, about 27 °C, about 28 °C, about 29 °C, or about 30 °C.

Alloys

The aluminum alloys useful for providing dark anodized aluminum alloy sheets as described herein comprise up to 0.40 wt. % Fe, up to 0.25 wt. % Si, up to 0.2 wt. % Cr, 2.0 wt. % to 3.2 wt. % Mg, 0.8 wt. % to 1.5 wt. % Mn, up to 0.1 wt. % Cu, up to 0.05 wt.% Zn, up to 0.05 wt.% Ti, and up to 0.15 wt. % impurities, with the remainder as Al. Preferably, the aluminum alloy includes 0.05 wt. % to 0.20 wt. % Fe, 0.03 wt. % to 0.1 wt. % Si, up to 0.05 wt. % Cr, 2.5 wt. % to 3.2 wt. % Mg, 0.8 wt.% to 1.3 wt. % Mn, up to 0.05 wt. % Cu, up to 0.05 wt.% Zn, up to 0.05 wt.% Ti, and up to 0.15 wt. % total impurities, with the remainder as Al. In some examples, the aluminum alloy includes up to 0.30 wt. % Fe, up to 0.13 wt. % Si, up to 0.07 wt. % Cr, from 2.0 wt. % to 2.75 wt. % Mg, from 0.80 wt. % to 1.5 wt. % Mn, up to 0.05 wt. % Cu, up to aout 0.05 wt.% Zn, up to 0.05 wt.% Ti, and up to 0.15 wt. % impurities, with the remainder as Al. Optionally, the aluminum alloy includes 0.1 wt. % Fe, 0.06 wt. % Si, 0.005 wt. % Cr, 2.74 wt. % Mg, 1.13 wt. % Mn, 0.024 wt. % Cu, aout 0.005 wt.% Zn, 0.005 wt.% Ti, and up to 0.15 wt. % total impurities, with the remainder as Al.
The aluminum alloy includes iron (Fe) in an amount of from 0 % to 0.4 % (e.g., from to 0.05 wt. % to 0.20 wt. %) based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, 0.05 %, 0.06 %, 0.07 %, 0.08 %, 0.09 %, 0.1 %, 0.11 %, 0.12 %, 0.13 %, 0.14 %, 0.15 %, 0.16 %, 0.17 %, 0.18 %, 0.19 %, 0.2 %, 0.21 %, 0.22 %, 0.23 %, 0.24 %, 0.25 %, 0.26 %, 0.27 %, 0.28 %, 0.29 %, 0.3 %, 0.31 %, 0.32 %, 0.33 %, 0.34 %, 0.35 %, 0.36 %, 0.37 %, 0.38 %, 0.39 %, or 0.4 % Fe. In some cases, Fe is not present in the alloy (i.e., 0 %). All expressed in wt. %.
The aluminum alloy includes silicon (Si) in an amount of from 0 % to 0.25% (e.g., from 0.03 % to 0.1 %) based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, 0.05 %, 0.06 %, 0.07 %, 0.08 %, 0.09 %, 0.1 %, 0.11 %, 0.12 %, 0.13 %, 0.14 %, 0.15 %, 0.16 %, 0.17 %, 0.18 %, 0.19 %, 0.2 %, 0.21 %, 0.22 %, 0.23 %, 0.24 %, or 0.25 % Si. In some cases, Si is not present in the alloy (i.e., 0 %). All expressed in wt. %.
The aluminum alloy includes chromium (Cr) in an amount of from 0 % to 0.2% (e.g., from 0.001 % to 0.15 %) based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, 0.05 %, 0.06 %, 0.07 %, 0.08 %, 0.09 %, 0.1 %, 0.11 %, 0.12 %, 0.13 %, 0.14 %, 0.15 %, 0.16 %, 0.17 %, 0.18 %, 0.19 %, or 0.2 % Cr. In some cases, Cr is not present in the alloy (i.e., 0 %). All expressed in wt. %.
The aluminum alloy includes magnesium (Mg) in an amount of from 2.0 % to 3.2 % (e.g., from 2.5 % to 3.2 %) based on the total weight of the alloy. In some examples, the alloy can include 2.0 %, 2.1 %, 2.2 %, 2.3 %, 2.4 %, 2.5 %, 2.6 %, 2.7 %, 2.75 %, 2.8 %, 2.9 %, 3.0 %, 3.1 %, or 3.2 % Mg. All expressed in wt. %.
The aluminum alloy includes manganese (Mn) in an amount of from 0.8 % to 1.5 % (e.g., from 0.8 % to 1.3 %) based on the total weight of the alloy. In some examples, the alloy can include 0.8 %, 0.9 %, 1.0 %, 1.1 %, 1.2 %, or 1.3 % Mn. All expressed in wt. %.
The aluminum alloy includes copper (Cu) in an amount of from 0 % to 0.1 % (e.g., from 0 % to 0.05 %) based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, 0.05 %, 0.06 %, 0.07 %, 0.08 %, 0.09 %, or 0.1 % Cu. In some cases, Cu is not present in the alloy (i.e., 0 %). All expressed in wt. %.
The aluminum alloy includes zinc (Zn) in an amount of from 0 % to 0.05 % based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, or 0.05 % Zn. In some cases, Zn is not present in the alloy (i.e., 0 %). All expressed in wt. %.
The aluminum alloy includes titanium (Ti) in an amount of 0 % to 0.05 % based on the total weight of the alloy. For example, the alloy can include 0.001 %, 0.002 %, 0.003 %, 0.004 %, 0.005 %, 0.006 %, 0.007 %, 0.008 %, 0.009 %, 0.01 %, 0.02 %, 0.03 %, 0.04 %, or 0.05 % Ti. In some cases, Ti is not present in the alloy (i.e., 0 %). All expressed in wt. %.
Optionally, the alloy compositions described herein can further include other minor elements, sometimes referred to as impurities, in amounts of 0.05 % or below, 0.04 % or below, 0.03 % or below, 0.02 % or below, or 0.01 % or below each. These impurities may include, but are not limited to, V, Zr, Ni, Sn, Ga, Ca, or combinations thereof. Accordingly, V, Zr, Ni, Sn, Ga, or Ca may be present in alloys in amounts of 0.05 % or below, 0.04 % or below, 0.03 % or below, 0.02 % or below, or 0.01 % or below. In some cases, the sum of all impurities does not exceed 0.15% (e.g., 0.10%). All expressed in wt. %. The remaining percentage of the alloy is aluminum.
As further described below, the alloys described herein can be prepared as sheets and can be anodized. The surface oxide layer produced by an anodization process of a conventional alloy is a highly ordered structure that, when pure, can be clear and colorless. The alloys described herein, in contrast, are designed to form fine intermetallic particles (e.g., dispersoids or precipitates) in the substrate that are maintained inside the oxide layer formed during the anodization process.
The alloys comprise at least 1.5 weight percent intermetallic particles Al₁₂(Mn,Fe)₃Si, and/or Al₆Mn having an average dimension of greater than 50 nm in any direction. While many intermetallic particles contain aluminum, there also exist intermetallic particles that do not contain aluminum, such as Mg₂Si. The composition and properties of intermetallic particles are described further below.
In some examples, the alloys described herein include various weight percent of phases Al_x(Fe,Mn), Al₁₂(Fe,Mn)₃Si, and Al₆Mn, Mg₂Si. When an element in an intermetallic particle designation is italicized, that element is the dominantly present element in the particle. The notation (Fe,Mn) indicates that the element can be Fe or Mn, or a mixture of the two. The notation (Fe,Mn) indicates that the particle contains more of the element Fe than the element Mn, while the notation (Fe,Mn) indicates that the particle contains more of the element Mn than the element Fe.
The weight percent of each phase differs at different annealing temperatures used in the methods for preparing the aluminum alloy sheets, as detailed below. An alloy having a higher weight percent of Al_x(Fe,Mn) (such as Al₆Mn) and/or Al₁₂(Fe,Mn)₃Si particles will have a darker natural anodized color. In some examples, the aluminum alloy includes at least 1.5 weight % Al₆Mn and/or Al₁₂(Fe,Mn)₃Si at 400 °C (e.g., at least 1.5 %, or at least 1.75 %, all weight %). In some examples, the aluminum alloy includes at least 2.0 weight % Al₆Mn and/or Al₁₂(Fe,Mn)₃Si at 500 °C (e.g., at least 2.0 %, at least 2.2 %, or at least 2.4 %, all weight %).
In some examples, the aluminum sheet having a dark gray color includes dispersoids at a density of at least 1 dispersoid per 25 square micrometers (e.g., at least 1 dispersoid per 25 square micrometers, at least 2 dispersoids per 25 square micrometers, at least 4 dispersoids per 25 square micrometers, at least 10 dispersoids per 25 square micrometers, or at least 20 dispersoids per 25 square micrometers).
The dispersoids have an average dimension of greater than 50 nanometers in any direction. For purposes herein, "any direction" means height, width, or depth. For example, the dispersoids can have an average particle dimension of greater than 50 nanometers, greater than 100 nanometers, greater than 200 nanometers, or greater than 300 nanometers. In some examples, the dispersoids additionally include one or more of Al₃Fe, Al₂₀Cu₂Mn₃, Al(Fe,Mn)₂Si₃, Al₃Zr, Al₇Cr, Mg₂Si, and AhCuMg.
In some examples, the aluminum sheet has a grain size of from 10 microns to 50 microns. For example, the aluminum sheet can have a grain size of from 15 microns to 45 microns, from 15 microns to 40 microns, or from 20 microns to 40 microns.

Methods of Preparing

Methods of producing an aluminum sheet comprise: casting an aluminum alloy to form an ingot; homogenizing the ingot in a two-step homogenization process to form a homogenized ingot, wherein a first homogenization step is heating to attain a peak metal temperature of 500-550°C for 2-24h and soaking for a period of time, and a second homogenization step is decreasing the temperature to a temperature of from 480-550°C; hot rolling the homogenized ingot to produce a hot rolled intermediate product; cold rolling the hot rolled intermediate product to produce a cold rolled intermediate product; interannealing the cold rolled intermediate product to produce an interannealed product; cold rolling the interannealed product to produce a cold rolled sheet; and annealing the cold rolled sheet to form an annealed aluminum sheet comprising dispersoids having an average dimension of greater than 50 nanometers in any direction. In some examples, the method further includes etching the annealed aluminum sheets (e.g., in an acid or base bath) and anodizing the annealed aluminum sheets.
In some examples, the alloys described herein can be cast into ingots using a direct chill (DC) process. The resulting ingots can optionally be scalped. In some examples, the alloys described herein can be cast in a continuous casting (CC) process. The cast product can then be subjected to further processing steps. In some examples, the processing steps further include a homogenization step, a hot rolling step, a cold rolling step, an optional interannealing step, a cold rolling step, and a final annealing step. The processing steps described below exemplify processing steps used for an ingot as prepared from a DC process.
The first homogenization step dissolves metastable phases into the matrix and minimizes microstructural inhomogeneity. An ingot is heated to attain a peak metal temperature of 500-550 °C for about 2-24 hours. In some examples, the ingot is heated to attain a peak metal temperature ranging from about 510 °C to about 540 °C, from about 515 °C to about 535 °C, or from about 520 °C to about 530 °C. The heating rate to reach the peak metal temperature can be from about 30 °C per hour to about 100 °C per hour. The ingot is then allowed to soak (i.e., maintained at the indicated temperature) for a period of time during the first homogenization stage. In some examples, the ingot is allowed to soak for up to 5 hours (e.g., up to 1 hour, up to 2 hours, up to 3 hours, up to 4 hours, inclusively). For example, the ingot can be soaked at a temperature of about 515 °C, about 525 °C, about 540 °C, or about 550 °C for 1 hour to 5 hours (e.g., 1 hour, 2 hours, 3 hours, 4 hours, or 5 hours).
In the second homogenization step, the ingot temperature is decreased to a temperature of from about 480 °C to 550 °C prior to subsequent processing. In some examples, the ingot temperature is decreased to a temperature of from about 450 °C to 480 °C prior to subsequent processing. For example, in the second stage the ingot can be cooled to a temperature of about 450 °C, about 460 °C, about 470 °C, or about 480 °C and allowed to soak for a period of time. In some examples, the ingot is allowed to soak at the indicated temperature for up to eight hours (e.g., from 30 minutes to eight hours, inclusively). For example, the ingot can be soaked at a temperature of about 450 °C, of about 460 °C, of about 470 °C, or of about 480 °C for 30 minutes to 8 hours.
Following the second homogenization step, a hot rolling step is performed. The hot rolling step can include a hot reversing mill operation and/or a hot tandem mill operation. The hot rolling step can be performed at a temperature ranging from about 250 °C to about 450 °C (e.g., from about 300 °C to about 400 °C or from about 350 °C to about 400 °C). In the hot rolling step, the ingots can be hot rolled to a thickness of 10 mm gauge or less (e.g., from 3 mm to 8 mm gauge). For example, the ingots can be hot rolled to a 8 mm gauge or less, 7 mm gauge or less, 6 mm gauge or less, 5 mm gauge or less, 4 mm gauge or less, or 3 mm gauge or less. Optionally, the hot rolling step can be performed for a period of up to one hour. Optionally, at the end of the hot rolling step (e.g., upon exit from the tandem mill), the aluminum sheet is coiled to produce a hot rolled coil.
The hot rolled coil can be uncoiled into a hot rolled sheet which undergoes a cold rolling step. The hot rolled sheet temperature can be reduced to a temperature ranging from about 20 °C to about 200 °C (e.g., from about 120 °C to about 200 °C). The cold rolling step can be performed for a period of time to result in a final gauge thickness of from about 1.0 mm to about 3 mm, or about 2.3 mm. Optionally, the cold rolling step can be performed for a period of up to about 1 hour (e.g., from about 10 minutes to about 30 minutes) and the sheet can be coiled to produce a cold rolled coil.
The cold rolled coil undergoes an interannealing step. The interannealing step can include heating the coil to a peak metal temperature of from about 300 °C to about 400 °C (e.g., about 300 °C, 305 °C, 310 °C, 315 °C, 320 °C, 325 °C, 330 °C, 335 °C, 340 °C, 345 °C, 350 °C, 355 °C, 360 °C, 365 °C, 370 °C, 375 °C, 380 °C, 385 °C, 390 °C, 395 °C, or 400 °C). The heating rate for the interannealing step can be from about 20 °C per minute to about 100 °C per minute (e.g., about 40 °C per minute, about 50 °C per minute, about 60 °C per minute, or about 80 °C per minute). The interannealing step can be performed for a period of about 2 hours or less (e.g., about 1 hour or less). For example, the interannealing step can be performed for a period of from about 30 minutes to about 50 minutes.
The interannealing step is followed by another cold rolling step. The cold rolling step can be performed for a period of time to result in a final gauge thickness between about 0.5 mm and about 2 mm, between about 0.75 and about 1.75 mm, between about 1 and about 1.5 mm, or about 1.27 mm. Optionally, the cold rolling step can be performed for a period of up to about 1 hour (e.g., from about 10 minutes to about 30 minutes).
The cold rolled coil then undergoes an annealing step. The annealing step can include heating the cold rolled coil to a peak metal temperature of from about 180 °C to about 350 °C. The heating rate for the annealing step can be from about 10 °C per hour to about 100 °C per hour. The annealing step can be performed for a period of up to 4.8 hours or less (e.g., 1 hour or less). For example, the annealing step can be performed for a period of from 30 minutes to 50 minutes.
Following the annealing step and before the anodizing step, the aluminum sheets can be etched. Any known etching process may be used, including alkaline etching or acidic etching. As an example, an alkaline etching process can be performed with sodium hydroxide (e.g., a 10% aqueous sodium hydroxide solution) followed by a desmutting process. As another example, an acidic etching process can be performed with phosphoric acid, sulfuric acid, or a combination of these. For example, the acidic etching process can be performed using 75% phosphoric acid and 25% sulfuric acid at an elevated temperature. As used herein, an elevated temperature refers to a temperature higher than room temperature (e.g., greater than 40 °C, greater than 50 °C, greater than 60 °C, greater than 70 °C, greater than 80 °C, or greater than 90 °C, such as 99 °C). During the etching process, the bulk aluminum matrix and intermetallic particles/dispersoids are dissolved. Depending on the etching process, the degree and uniformity of etched surface can be varied.
After the etching step, the aluminum sheets described herein are anodized. In some examples, the aluminum sheets described herein are anodized by placing the aluminum in an electrolytic solution and passing a direct current through the solution. In some examples, the electrolytic solution is an acidic solution, such as, but not limited to, a solution including hydrochloric acid, sulfuric acid, chromic acid, phosphoric acid, and/or an organic acid. Anodization creates an oxide surface layer on the aluminum alloy. In some examples, the aluminum sheet includes an oxide surface layer.

Methods of Using,

The materials described herein are particularly useful in architectural quality applications as well as other decorative applications, such as decorative panels, street signs, appliances, furniture, jewelry, artwork, boating and automotive components, and even consumer electronics where high quality dark gray color in anodized sheets are required by customers.
The following examples will serve to further illustrate the present invention without, at the same time, however, constituting any limitation thereof. On the contrary, it is to be clearly understood that resort may be had to various embodiments, modifications, and equivalents thereof which, after reading the description herein, may suggest themselves to those skilled in the art without departing from the spirit of the invention. During the studies described in the following examples, conventional procedures were followed, unless otherwise stated. Some of the procedures are described below for illustrative purposes.

EXAMPLE 1

An inventive alloy sheet and three comparative alloy sheets having the compositions detailed in Table 1 were prepared. The sheets were prepared by casting an ingot at approximately 650 °C, homogenizing the ingot at 525 °C for less than 1 hour soaking time, hot rolling the homogenized ingot for 10 minutes at 250-450 °C to produce a hot rolled intermediate product, and cold rolling the hot rolled intermediate product for 10 minutes at 150-180 °C to produce a cold rolled intermediate product.

Table 1. Alloy elemental compositions, with up to 0.15 weight % total impurities, the balance Aluminum.

	Si	Fe	Cu	Mn	Mg	Cr	Zn	Ti
Comparative Alloy
1	0.14	0.32	0.050	0.77	2.88	0.071	0.013	0.013
Comparative Alloy 2	0.20	0.37	0.050	0.30	2.76	0.092	0.048	0.025
Comparative Alloy 3	0.18	0.31	0.019	0.23	2.87	0.008	0.01	0.01
Alloy 4	0.06	0.10	0.024	1.13	2.74	0.005	0.005	0.005

EXAMPLE 2

The aluminum sheets of Alloy 4 and Comparative Alloys 1 and 2 described in Example 1 were imaged with scanning transmission electron microscopy (STEM). FIG. 1A and FIG. 1B are STEM images of Comparative Alloy 1 and Comparative Alloy 2, respectively. FIG. 1C is a STEM image of Alloy 4. Alloy 4 showed a much higher density of dispersoids than the comparative alloys. Alloy 3 had a lower density of dispersoids than Alloys 1 and 2, and thus is not pictured.

EXAMPLE 3

Sheets of Comparative Alloys 1 and 2 and Alloy 4 prepared as described in Example 1 were alkaline etched with 10% sodium hydroxide solution and anodized to a 10 micrometer (µm) anodized layer thickness. The resulting anodized layer cross section was imaged with high-resolution scanning electron microscopy (SEM). The SEM images of Comparative Alloys 1 and 2 and Alloy 4 are shown in FIGs. 2A-2C, respectively. As identified in Figure 2A, fine particles were Al₆Fe and Mg₂Si in these example alloys. The anodized aluminum sheet from Alloy 4 has a significantly darker gray color, with many dispersoids visible (see FIG. 2C), whereas the two comparative anodized aluminum alloy sheets have a light gray color and fewer dispersoids (see FIGs. 2A-2B).

EXAMPLE 4

Thermodynamic modelling by Thermo-Calc software (Thermo-Calc Software, Inc., McMurray, PA) was used to calculate the equilibrium phase transformation behavior of Comparative Alloys 1-2 (see FIGs. 3A and 3B, respectively) and Alloy 4 (see FIG. 3C). Equilibrium phases at each temperature of given alloy composition was calculated by CALPHAD (Computer Coupling of Phase Diagrams and Thermochemistry) technique. Each line represents specific phase. Line 1: liquid; line 2: Al matrix; line 3: Al₆Mn; line 4: Al(Fe,Mn)₂Si₃; line 5: Mg₂Si; line 6: AlCuMn; line 7: AlCuMg; line 8: Al₈Mg₅; line 9: Al₁₂Mn. Modeling results indicate that the amount of Al₆Mn dispersoids (line 3) is the most in alloy 4. (Figure 3C). Not intending to be bound by theory, the inventive alloy's higher Mn content relative to the comparative alloys results in a greater concentration of Al₆Mn dispersoids in the inventive alloy oxide layer, which provides scattering of incoming light.

Claims

An aluminum alloy comprising up to 0.40 wt. % Fe, up to 0.25 wt. % Si, up to 0.2 wt. % Cr, 2.0 wt. % to 3.2 wt. % Mg, 0.8 wt. % to 1.5 wt. % Mn, up to 0.1 wt. % Cu, up to 0.05 wt.% Zn, up to 0.05 wt.% Ti, and up to 0.15 wt. % impurities, with the remainder as Al, wherein the aluminum alloy comprises at least 1.5 weight percent Al₆Mn and/or Al₁₂(Fe,Mn)₃Si dispersoids having an average dimension of greater than 50 nanometers in any direction.
The aluminum alloy of claim 1, wherein the aluminum alloy comprises 0.05 wt. % to 0.2 wt. % Fe, 0.03 wt.% to 0.1 wt. % Si, up to 0.05 wt. % Cr, 2.5 wt.% to 3.2 wt. % Mg, 0.8 wt.% to 1.3 wt.% Mn, up to 0.05 wt. % Cu, up to 0.05 wt.% Zn, up to 0.05 wt.% Ti, and up to 0.15 wt. % impurities, with the remainder as Al.
An aluminum sheet comprising the aluminum alloy of any of claims 1-2.
The aluminum sheet of claim 3, wherein the aluminum alloy sheet comprises an oxide surface layer.
The aluminum sheet of claim 3, wherein the aluminum alloy sheet has a white balance of lower than 35 as measured by ASTM E313-15 (2015).
The aluminum sheet of claim 3, further comprising dispersoids at a density of at least 2 disperoids per 25 square micrometer.
The aluminum sheet of claim 6, wherein the dispersoids comprise Al₆Mn and/or Al₁₂(Fe,Mn)₃Si and one or more of Al₃Fe, Al_x(Fe,Mn), Al₃Fe, Al₇Cu₂Fe, Al₂₀Cu₂Mn₃, Al₃Ti, Al₂Cu, Al(Fe,Mn)₂Si₃, Al₃Zr, Al₇Cr, Al_x(Mn,Fe), Al₃,Ni, Mg₂Si, MgZn₃, Mg₂Al₃, Al₃₂Zn₄₉, and AhCuMg.
The aluminum sheet of claim 6, wherein the dispersoids comprise Al₆Mn and/or Al₁₂(Fe,Mn)₃Si and one or more of Al₃Fe, Al₂₀Cu₂Mn₃, Al(Fe,Mn)₂Si₃, Al₃Zr, Al₇Cr, Mg₂Si, and AhCuMg.
The aluminum sheet of any of claims 4-8, comprising a grain size of from 10 microns to 50 microns.
A method of preparing an aluminum sheet comprising dispersoids, the method comprising:
casting an aluminum alloy to form an ingot;

homogenizing the ingot in a two-step homogenization process to form a homogenized ingot, wherein a first homogenization step is heating to attain a peak metal temperature of 500-550°C for 2-24h and soaking for a period of time, and a second homogenization step is decreasing the temperature to a temperature of from 480-550°C;

hot rolling the homogenized ingot to produce a hot rolled intermediate product;

cold rolling the hot rolled intermediate product to produce a cold rolled intermediate product;

interannealing the cold rolled intermediate product to produce an interannealed product;

cold rolling the interannealed product to produce a cold rolled sheet; and

annealing the cold rolled sheet to form an annealed aluminum sheet comprising dispersoids having an average dimension of greater than 50 nanometers in any direction,

wherein the aluminum alloy comprises up to 0.40 wt. % Fe, up to 0.25 wt. % Si, up to 0.2 wt. % Cr, 2.0 wt. % to 3.2 wt. % Mg, 0.8 wt. % to 1.5 wt. % Mn, up to 0.1 wt. % Cu, up to 0.05 wt. % Zn, up to 0.05 wt. % Ti, and up to 0.15 wt. % impurities, with the remainder as Al, and wherein the aluminum alloy comprises at least 1.5 weight percent Al₆Mn and/or Al₁₂(Fe,Mn)₃Si.
The method of claim 10, further comprising anodizing the aluminum sheet.
The method of claim 10 or 11, wherein the dispersoids comprise Al₆Mn and/or Al₁₂(Fe,Mn)₃Si and one or more of Al₃Fe, Al _x (Fe,Mn), Al₃Fe, Al₇Cu₂Fe, Al₂₀Cu₂Mn₃, Al₃Ti, Al₂Cu, Al(Fe,Mn)₂Si₃, Al₃Zr, Al₇Cr, Al_x(Mn,Fe), Al₃,Ni, Mg₂Si, MgZn₃, Mg₂Al₃, Al₃₂Zn₄₉, AhCuMg.
The method of claims 10 or11, wherein the aluminum sheet has a white balance of lower than 35 as measured by ASTM E313-15 (2015) or,
wherein the dispersoids are present in the aluminum sheet at a density of at least 2 disperoids per 25 square micrometers.