U.S. patent application number 14/830705 was filed with the patent office on 2017-02-23 for processes to avoid anodic oxide delamination of anodized high strength aluminum alloys.
The applicant listed for this patent is Apple Inc.. Invention is credited to William A. Counts, James A. Curran, Brian M. Gable.
Application Number | 20170051425 14/830705 |
Document ID | / |
Family ID | 58157798 |
Filed Date | 2017-02-23 |
United States Patent
Application |
20170051425 |
Kind Code |
A1 |
Curran; James A. ; et
al. |
February 23, 2017 |
PROCESSES TO AVOID ANODIC OXIDE DELAMINATION OF ANODIZED HIGH
STRENGTH ALUMINUM ALLOYS
Abstract
Methods of forming anodic oxide coatings on high strength
aluminum alloys are described. Methods involve preventing or
reducing the formation of interface-weakening species, such as
zinc-sulfur compounds, at an interface between an anodic oxide
coating and underlying aluminum alloy substrate during anodizing.
In some embodiments, a micro-alloying element is added in very
small amounts to an aluminum alloy substrate to prevent enrichment
of zinc at the anodic oxide and substrate interface, thereby
reducing or preventing formation of the zinc-sulfur
interface-weakening species. In some embodiments, a
sulfur-scavenging species is added to an aluminum alloy substrate
to prevent sulfur from a sulfuric acid anodizing bath from binding
with zinc and forming the zinc-sulfur interface-weakening species
at the anodic oxide and substrate interface. In some embodiments, a
micro-alloying element and a sulfur-scavenging species are added to
an aluminum alloy substrate. Resultant anodic oxide coatings have
minimal or no discoloration.
Inventors: |
Curran; James A.; (Morgan
Hill, CA) ; Counts; William A.; (Sunnyvale, CA)
; Gable; Brian M.; (Santa Clara, CA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Apple Inc. |
Cupertino |
CA |
US |
|
|
Family ID: |
58157798 |
Appl. No.: |
14/830705 |
Filed: |
August 19, 2015 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C25D 11/08 20130101;
C25D 11/04 20130101; C22C 21/10 20130101; C25D 11/16 20130101 |
International
Class: |
C25D 7/00 20060101
C25D007/00; C22C 21/10 20060101 C22C021/10; C25D 11/08 20060101
C25D011/08 |
Claims
1. A part, comprising: an aluminum alloy substrate including zinc
as an alloying element; and an anodic oxide coating formed on the
aluminum alloy substrate, wherein the zinc is enriched at an
interface between the anodic oxide coating and the aluminum alloy
substrate, wherein the aluminum alloy substrate includes a
sulfur-scavenging species at a sufficient concentration to bind
with sulfur species within the anodic oxide coating preventing at
least some of the enriched zinc from forming a zinc-sulfur compound
at the interface, the zinc-sulfur compound associated with reducing
an adhesion strength between the anodic oxide coating and the
aluminum alloy substrate.
2. The part of claim 1, wherein the aluminum alloy substrate
includes additional elements other than zinc, magnesium, and the
sulfur-scavenging species, wherein the additional elements
comprise: chromium at no more than 0.01 weight % concentration,
copper at no more than 0.01 weight % concentration, manganese at no
more than 0.01 weight % concentration, zirconium at no more than
0.01 weight % concentration, titanium at no more than 0.02 weight %
concentration, silicon at no more than 0.05 weight % concentration,
iron at no more than 0.08 weight % concentration, and any other
element at no more than 0.01 weight % concentration, to a total
maximum of 0.1 weight % concentration of the additional
elements.
3. The part of claim 1, wherein the anodic oxide coating has a
thickness of at least about 10 micrometers and is characterized as
having b* color space parameter value between -1 and 1, optionally
between -0.5 and 0.5, as defined by CIE Standard Illuminant D65
white spot standard.
4. The part of claim 1, wherein an adhesion strength of the anodic
oxide coating as measured by 5-by-5 array 10 kg Vickers
indentations spaced 350 micrometers apart and as viewed by scanning
electron microscope imaging is less than 10 detached regions of the
anodic oxide coating.
5. The part of claim 1, wherein the aluminum alloy substrate has a
yield strength of at least 330 MPa in a T6 temper.
6. The part of claim 1, wherein the sulfur-scavenging species is
selected from the group consisting of lithium, magnesium, calcium,
strontium, barium, scandium, and yttrium.
7. The part of claim 1, wherein a concentration of the
sulfur-scavenging species ranges from about 0.5 weight % and about
3 weight %.
8. The part of claim 1, wherein magnesium is added in excess over a
balanced ratio for magnesium-zinc precipitate formation so as to
eliminate or reduce a concentration of non-precipitated zinc in the
aluminum alloy substrate in a T6 or T7 temper, thereby reducing a
discrepancy between growth rates of different portions of the
anodic oxide coating on grains of distinct surface orientations,
resulting in the anodic oxide coating having a thickness uniformity
of within 5% among grains of {111} surface orientation and other
surface orientations.
9. The part of claim 8, wherein a concentration of the magnesium is
in excess of a stoichiometric amount required to combine with the
zinc to form .eta.-MgZn.sub.2 precipitates, wherein at least some
of the excess magnesium binds with the sulfur species within the
anodic oxide coating preventing at least some of the zinc from
forming a zinc-sulfur compound at an interface between the anodic
oxide coating and the aluminum alloy substrate.
10. The part of claim 1, wherein the aluminum alloy substrate
comprises magnesium, wherein an atomic concentration of the
magnesium is at least half an atomic concentration of the zinc.
11. The part of claim 1, wherein the part is an enclosure for an
electronic device.
12. The part of claim 1, wherein the aluminum alloy substrate is
comprised of a 7000-series aluminum alloy.
13. A method of anodizing an aluminum alloy substrate comprising
zinc, the method comprising: anodizing the aluminum alloy substrate
in a sulfuric acid-based solution, wherein a sulfur species from
the sulfuric acid-based solution becomes incorporated within a
resultant anodic oxide coating, wherein some of the zinc becomes
enriched at an interface between the anodic oxide coating and
aluminum alloy substrate during the anodizing, wherein the aluminum
alloy substrate includes a sulfur-scavenging species that binds
with the sulfur species preventing at least some of the enriched
zinc from forming a zinc-sulfur compound at the interface, the
zinc-sulfur compound associated with reducing an adhesion strength
between the anodic oxide coating and the aluminum alloy
substrate.
14. The method of claim 13, wherein the sulfur-scavenging species
is selected from the group consisting of lithium, magnesium,
calcium, strontium, barium, scandium, and yttrium.
15. The method of claim 14, wherein a concentration of the
sulfur-scavenging species ranges from about 0.5 weight % and about
3 weight %.
16. The method of claim 13, wherein a concentration of the zinc
within the aluminum alloy substrate is at least 4 weight %.
17. The method of claim 13, wherein the aluminum alloy substrate
includes magnesium as another alloying element, wherein an atomic
concentration of the magnesium is at least half an atomic
concentration of the zinc.
18. An enclosure for an electronic device, the enclosure
comprising: an aluminum alloy substrate including zinc and
magnesium; and an anodic oxide coating formed on the aluminum alloy
substrate, the anodic oxide coating including a sulfur species
incorporated therein, wherein some of the sulfur species is bonded
with a sulfur-scavenging species that prevents the sulfur species
from binding with the zinc.
19. The enclosure of claim 18, wherein the aluminum alloy substrate
with the anodic oxide coating is characterized as having a CIELAB
b* color space parameter value between -1 and 1.
20. The enclosure of claim 18, wherein the sulfur-scavenging
species is selected from the group consisting of lithium,
magnesium, calcium, strontium, barium, scandium, and yttrium.
Description
RELATED APPLICATIONS
[0001] This application is related to U.S. application Ser. No.
14/474,021, entitled "PROCESS TO MITIGATE SPALLATION OF ANODIC
OXIDE COATINGS FROM HIGH STRENGTH SUBSTRATE ALLOYS," filed on Aug.
29, 2014; U.S. application Ser. No. 14/593,845, entitled "PROCESSES
TO REDUCE INTERFACIAL ENRICHMENT OF ALLOYING ELEMENTS UNDER ANODIC
OXIDE FILMS AND IMPROVE ANODIZED APPEARANCE OF HEAT TREATABLE
ALLOYS," filed on Jan. 9, 2015; U.S. application Ser. No.
14/678,881, entitled "PROCESS FOR EVALUATION OF
DELAMINATION-RESISTANCE OF HARD COATINGS ON METAL SUBSTRATES,"
filed on Apr. 3, 2015; and U.S. application Ser. No. 14/678,868,
entitled "PROCESS TO MITIGATE GRAIN TEXTURE DIFFERENTIAL GROWTH
RATES IN MIRROR-FINISH ANODIZED ALUMINIUM," filed on Apr. 3, 2015,
each of which is incorporated herein in its entirety.
FIELD
[0002] This disclosure relates generally to anodic oxide coatings
and methods for forming the same. In particular, methods for
preventing formation of compounds during anodizing of certain
high-strength aluminum alloy substrates that can weaken the
interfacial adhesion of a resultant anodic oxide coating are
described.
[0003] Any publications, patents, and patent applications referred
to in the instant specification are herein incorporated by
reference in their entireties. To the extent that the publications,
patents, or patent applications incorporated by reference
contradict the disclosure contained in the instant specification,
the instant specification is intended to supersede and/or take
precedence over any such contradictory material.
BACKGROUND
[0004] Anodizing of aluminum is most commonly performed in
sulfuric-acid based solutions, for example, using the processes
defined as "Type II" and "Type III" by MIL-A-8625. The resultant
anodic oxide coatings provide good wear and corrosion resistance to
the substrate, and Type II coatings in particular, have a good
cosmetic appearance. On certain alloys, and within certain process
constraints, the resulting oxide layer may be clear and
substantially colorless, giving a bright metallic appearance which
is a highly desirable finish for the aluminum housing of consumer
electronic devices. The anodic oxides are also conducive to taking
on dyes for coloring. Thus, type II and III anodizing processes are
widely used in various industries.
[0005] During type II and III anodizing, sulfur-based anions from
the sulfuric acid solution become incorporated within the resulting
anodic oxide coating. These sulfur-based anions can combine with
certain alloying elements originating from aluminum alloy
substrates and that accumulate at an interface between the anodic
oxide coating and the aluminum alloy substrate. For example, zinc
is a common alloying element found in many high-strength aluminum
alloys, notably the 7000-series, of which it is the defining
alloying element (as per the International Alloy Designation
System). Zinc is less readily oxidized than aluminum, and therefore
accumulates at the interface between the anodic oxide coating and
aluminum alloy substrate. When the sulfur-based anions combine with
zinc enriched at the interface, zinc-sulfur compounds form at the
interface. It has been found that these zinc-sulfur compounds can
weaken adhesion of the anodic oxide coating to the substrate and
cause the anodic oxide coating to be susceptible to delamination
(i.e., chipping or peeling), particularly in alloys designed to
satisfy both a high strength requirement, and anodizing
cosmetics.
SUMMARY
[0006] This paper describes various embodiments that relate to
anodizing processes and anodic oxide coatings using the same. The
methods described are used to form anodic oxide coatings with
strong interfacial adhesion by avoiding formation of
interface-weakening species at an interface between the anodic
oxide coatings and underlying substrates during anodizing of
aluminum alloy substrates.
[0007] According to one embodiment, an enclosure for an electronic
device is described. The enclosure includes an aluminum alloy
substrate including zinc, magnesium, and a micro-alloying element.
A concentration of the micro-alloying element is at most 0.1 weight
%. The enclosure also includes an anodic oxide formed on the
aluminum alloy substrate. The micro-alloying element is enriched at
an interface between the aluminum alloy substrate and the anodic
oxide.
[0008] According to another embodiment, a method of forming an
enclosure for an electronic device is described. The method
includes anodizing an aluminum alloy substrate that includes zinc,
magnesium, and a micro-alloying element. A concentration of the
micro-alloying element within the aluminum alloy substrate is at
most 0.1 weight %. The micro-alloying element reduces enrichment of
the zinc at an interface between the aluminum alloy substrate and a
resultant anodic oxide. Enrichment of the zinc at the interface is
associated with reducing an adhesion of the anodic oxide to the
aluminum alloy substrate.
[0009] According to a further embodiment, a method of forming an
enclosure for an electronic device is described. The method
includes anodizing an aluminum alloy substrate that includes zinc,
magnesium, and a micro-alloying element. A concentration of the
micro-alloying element within the aluminum alloy substrate is at
most 0.1 weight %. The micro-alloying element reduces the
discrepancy between the anodic oxide growth rates on grains having
surface orientations of {111} and those of other orientations.
Grain structures having {111} orientation associated with
preferential anodic oxide growth and defects within the anodized
aluminum alloy substrate.
[0010] According to an additional embodiment, a part is described.
The part includes an aluminum alloy substrate including zinc as an
alloying element. The part also includes an anodic oxide coating
formed on the aluminum alloy substrate, the anodic oxide coating
including a sulfur species incorporated therein, wherein the
anodized part is characterized as having a CIELAB b* color space
parameter value between -1 and 1.
[0011] According to another embodiment, a method of anodizing an
aluminum alloy substrate comprising zinc is described. The method
includes anodizing the aluminum alloy substrate in a sulfuric
acid-based solution. A sulfur species from the sulfuric acid-based
solution becomes incorporated within a resultant anodic oxide
coating. Some of the zinc becomes enriched at an interface between
the anodic oxide coating and aluminum alloy substrate during the
anodizing. The aluminum alloy substrate includes a
sulfur-scavenging species that binds with the sulfur species
preventing at least some of the enriched zinc from forming a
zinc-sulfur compound at the interface. The zinc-sulfur compound is
to be avoided or minimized because it is reduces the interfacial
adhesion between the anodic oxide coating and the aluminum alloy
substrate.
[0012] According to a further embodiment, an enclosure for an
electronic device is described. The enclosure includes an aluminum
alloy substrate including zinc and magnesium. The enclosure also
includes an anodic oxide coating formed on the aluminum alloy
substrate. The anodic oxide coating includes a sulfur species
incorporated therein. Some of the sulfur species is bonded with a
sulfur-scavenging species that prevents the sulfur species from
binding with the zinc. The magnesium may itself act as the
sulfur-scavenging species if it is present in a substantial excess
over the balanced level required for the formation of
zinc-magnesium precipitates to give a certain target strength or
hardness in the alloy.
[0013] These and other embodiments will be described in detail
below.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The disclosure will be readily understood by the following
detailed description in conjunction with the accompanying drawings,
wherein like reference numerals designate like structural
elements.
[0015] FIGS. 1A and 1B show schematic cross-sections of a surface
portion of a part, showing how sulfur-based anodic bath anodizing
of zinc-containing aluminum alloys can form interface-weakening
species.
[0016] FIG. 1C shows an EELS graph and a high-resolution
microscopic image indicating evidence of interfacial zinc
enrichment for an anodized zinc-containing aluminum alloy
substrate.
[0017] FIG. 2 shows a schematic cross-section of a surface portion
of a part formed using a substrate having a micro-alloying element
that prevents or reduces formation of interface-weakening
species.
[0018] FIG. 3 shows a graph indicating interfacial enrichment for a
number of elements as a function of Gibbs free energy.
[0019] FIG. 4 shows an EELS graph and a high-resolution microscopic
image indicating evidence of prevention of interfacial zinc
enrichment when using copper as a micro-alloying element.
[0020] FIG. 5A shows a graph indicating yellowing effects on anodic
films of aluminum alloy substrates with different amounts of
copper.
[0021] FIG. 5B shows a graph indicating anodic oxide grown
uniformity and defect reduction by using copper as a micro-alloying
element.
[0022] FIG. 6A shows a flowchart illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate using a micro-alloying element.
[0023] FIG. 6B shows a flowchart illustrating a process of reducing
grain-related defects in an anodized high-strength substrate using
a micro-alloying element.
[0024] FIG. 7 shows a schematic cross-section of a surface portion
of a part formed using a substrate having a sulfur-scavenging
species that prevents or reduces formation of interface-weakening
species.
[0025] FIG. 8 shows an annotated periodic table summarizing some
criterion for choosing a suitable sulfur-scavenging species in
accordance with some embodiments.
[0026] FIG. 9 shows a graph indicating magnesium and zinc
concentrations of different commercially available 7000 series
aluminum alloys and custom alloy compositions.
[0027] FIG. 10 shows a flowchart illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate using a sulfur-scavenging species.
[0028] FIG. 11 shows a flowchart illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate using a combination of sulfur-scavenging
species and micro-alloying element.
[0029] FIG. 12 shows a graph indicating anodic oxide adhesion
improvement by using copper as a micro-alloying element and lithium
as a sulfur-scavenging species.
DETAILED DESCRIPTION
[0030] Reference will now be made in detail to representative
embodiments illustrated in the accompanying drawings. It should be
understood that the following descriptions are not intended to
limit the embodiments to one preferred embodiment. To the contrary,
they are intended to cover alternatives, modifications, and
equivalents as can be included within the spirit and scope of the
described embodiments.
[0031] Described herein are processes for increasing the adhesion
strength of anodic oxide coatings on certain high-strength aluminum
alloy substrates. Methods involve preventing the formation of
interface-weakening species from forming at an interface between an
anodic oxide coating and underlying aluminum alloy metal base. An
interface-weakening species is an element or compound that resides
at this interface and weakens the bond strength between the anodic
oxide coating and metal base, thereby rendering the anodic oxide
coating susceptible to chipping, peeling, or spalling. A particular
type of interface-weakening species is zinc-sulfur species, such as
a zinc sulfate or a zinc sulfite. The zinc originates from the
aluminum alloy as an alloying element, and the sulfur originates
from a sulfur-containing anodizing solution (e.g., sulfuric
acid-based solution). A number of other aluminum alloying elements
other than zinc-sulfur species have also been shown to form
interface-weakening species at the substrate and anodic oxide
coating interface.
[0032] Methods described herein involve adding one or more elements
to the aluminum alloy substrate prior to anodizing so as to prevent
or reduce the formation of interface-weakening species at the
substrate and anodic oxide coating interface. In some embodiments,
the one or more elements enrich at the interface more favorably
than the interface-weakening species, which prevents or reduces the
enrichment of interface-weakening species at the interface. In some
embodiments, the one or more elements bind with sulfur originating
from an anodizing solution during anodizing. This prevents or
reduces the occurrence of zinc and/or other elements associated
with delamination from combining with the sulfur to form
interface-weakening species at the interface.
[0033] The present paper makes specific reference to aluminum
alloys and aluminum oxide coatings, and particularly to 7000-series
alloys of aluminum, which comprise zinc-based strengthening
precipitates. It should be understood, however, that the methods
described herein may be applicable to other types of aluminum
alloys--such as 8000-series, which contain lithium and zinc
alloying elements--and possibly also to any of a number of other
suitable anodizable metal alloys, such as suitable alloys of
titanium, zinc, magnesium, niobium, zirconium, hathium, and
tantalum, or suitable combinations thereof. As used herein, the
terms anodic oxide, anodic oxide coating, anodic film, anodic
layer, anodic coating, oxide film, oxide layer, oxide coating, etc.
can be used interchangeably and can refer to suitable metal oxide
materials, unless otherwise specified.
[0034] Methods described herein are well suited for providing
cosmetically appealing surface finishes to consumer products. For
example, the methods described herein can be used to form durable
and cosmetically appealing anodized finishes for housing for
computers, portable electronic devices, wearable electronic
devices, and electronic device accessories, such as those
manufactured by Apple Inc., based in Cupertino, Calif.
[0035] These and other embodiments are discussed below with
reference to FIGS. 1A-12. However, those skilled in the art will
readily appreciate that the detailed description given herein with
respect to these Figures is for explanatory purposes only and
should not be construed as limiting.
[0036] FIGS. 1A and 1B show schematic cross-section views of a
surface portion of part 100, showing how sulfur-based anodizing
(e.g., Type II anodizing) of zinc-containing aluminum alloys can
form zinc-sulfur interface-weakening species. Part 100 includes
aluminum alloy substrate 102, a portion of which has been converted
to anodic oxide 104, which includes anodic pores 110 that are
formed during the anodizing process. The region between anodic
oxide 104 and substrate 102 can be referred to as interface 108.
Substrate 102 includes aluminum matrix 112 and zinc 106, which
serves as an alloying element in many aluminum alloy compositions
to increase the strength and hardness of the aluminum alloy, with
7000-series aluminum alloys (per The International Alloy
Designation System) generally having relatively high levels of zinc
106. In some applications, it is necessary for substrate 102 in a
T6 temper to have a yield strength of at least 330 MPa. In some
embodiments, this corresponds to a zinc concentration of at least 4
weight %, in some cases as little as 2 weight %. Note that zinc is
schematically shown as points 106 in FIGS. 1A and 1B, though it may
be either uniformly distributed through aluminum matrix 112, or
concentrated within discrete precipitates, or both. Zinc 106 can
combine with magnesium (not shown) as another alloying element to
form precipitates such as MgZn.sub.2 (the .eta.' or "eta-prime"
phase), which gives substrate 102 its high strength. The atomic %
ratio of magnesium to zinc is thus optimally about 1:2. For
simplicity, magnesium is not shown in FIGS. 1A and 1B. In addition,
most commercially available aluminum alloys contain other alloying
elements that are not shown in FIGS. 1A and 1B for simplicity.
[0037] During the anodizing process, alloying elements from
substrate 102 behave in various ways according to their relative
Gibbs free energies for oxide formation. For example, elements that
are more readily oxidized than aluminum matrix 112, such as
lithium, magnesium, calcium, scandium, yttrium, and lanthanum, are
generally anodized along with aluminum matrix 112. They become
incorporated into anodic oxide 104 and can then migrate through
interface 108 between substrate 102 anodic oxide 104 at rates that
depend primarily on the relative mobilities of ions of these
elements through anodic oxide 104, which is, in part, determined by
the relative strengths of bonding between these ions and oxygen
ions within anodic oxide 104. Some ions, including those of
lithium, magnesium, and yttrium, migrate through anodic oxide 104
at a faster rate than aluminum ions.
[0038] Conversely, alloying elements that are less readily oxidized
than aluminum--that is, elements with more positive Gibbs free
energies for oxide formation than that of aluminum--tend to become
enriched at interface 108. Examples of elements that can become
enriched at interface 108 include copper, zinc, nickel, tin,
silver, and gold. More discussion with regard to elements and their
Gibbs free energies for oxide formation will be provided below.
[0039] FIG. 1A shows zinc 106 enriched at interface 108 since zinc
106 has a positive Gibbs free energy for oxide formation compared
to that of aluminum. This enrichment of zinc 106 is an event that
occurs during the anodizing of substrate 102. The presence of zinc
106 has been found to weaken the adhesion between anodic oxide 104
and substrate 102. That is, the presence of zinc 106 at interface
108 can cause anodic oxide 104 to be susceptible to spalling,
chipping, or peeling away from substrate 102. It should be noted
that other elements such as nickel and tin (not shown) have also
been shown to have similar interfacial weakening effects when
positioned at interface 108. In contrast, elements such as copper
and gold (not shown) at interface 108 can strengthen the adhesion
of anodic oxide 104 to substrate 102.
[0040] The enrichment of zinc 106 at interface 108 during the
anodizing process cannot generally be avoided by changing
parameters of the anodizing process, or by using a typical
pre-treatment operation. The enrichment of zinc 106 is a
consequence, primarily of the higher Gibbs free energy of oxide
formation for zinc 106 compared to the aluminum metal of aluminum
matrix 112, and because zinc 106 is less readily oxidized than the
aluminum metal of aluminum matrix 112. That is, the aluminum will
oxidize in preference over zinc 106 during the anodizing process,
resulting in interfacial enrichment of zinc 106 until equilibrium
is achieved. It should be noted, that magnesium (not shown) that is
a common alloying element used in combination with zinc 106 in high
strength 7000-series alloys, has a low Gibbs free energy of
oxidation and is preferentially oxidized over aluminum, resulting
in no accumulation of magnesium at interface 108.
[0041] FIG. 1B shows additional events that occur during the
anodizing process. As described above, anodizing processes such as
Type II and Type III anodizing processes (as defined by Military
Specification Anodizing (MIL-A-8625)) involve the use of a sulfuric
acid solution as an electrolyte. It has been shown that sulfur
species 114 originating from the sulfuric acid solution become
significantly incorporated within anodic oxide 104. Sulfur species
114 can be any sulfur-containing species formed during an anodizing
process using a sulfur-containing anodizing solution, such as
sulfate and/or sulfide ions. During anodizing, sulfur species 114
move toward substrate 102 and interface 108, as indicated by arrows
115, driven by the applied electric current for anodizing. When
sulfur species 114 reaches interface 108, they can combine with
enriched zinc 106 to form zinc-sulfur species 116, which can be any
zinc and sulfur-containing species such as zinc sulfate and/or zinc
sulfide.
[0042] Zinc-sulfur species 116 have been found to further weaken
interfacial adhesion between anodic oxide 104 and substrate 102,
making anodic oxide 104 even more susceptible to spalling,
chipping, or peeling. This can be particularly problematic in
zinc-rich aluminum alloys, such as some 7000-series alloys, due to
the relatively high levels of zinc 106. In this way, both zinc 106
and zinc-sulfur species 116 can be referred to as
interface-weakening species. It should be noted that other elements
could combine with sulfur species 114 to form sulfates and/or
sulfites that can weaken adhesion of anodic oxide 104 to substrate
102. For example, nickel can also form sulfates that detract from
adhesion of anodic oxide 104, whereas copper, silver, gold, and
various other elements can form oxides in preference over sulfates
and generally do not form egregious interfacial sulfates. Thus, the
term "interface-weakening species" is not limited to zinc 106 and
zinc-sulfur species 116, but can refer generally to species that
weakens the adhesion of anodic oxide 104 to substrate 102.
[0043] FIG. 1C shows graph 120 and image 122 indicating evidence of
interfacial zinc enrichment for an anodized zinc-containing
aluminum alloy substrate. Graph 120 represents data collected by
electron energy loss spectroscopy (EELS) and image 122 is collected
using high-resolution microscopy. In graph 120 and image 122, upper
portions correspond to an anodic oxide (Ano) and lower portions
correspond to an aluminum alloy substrate (Al), with the dashed
line labeled "ano-Al interface" corresponding to an interface
region between anodic oxide (Ano) and aluminum alloy substrate
(Al). The EELS graph 120 corresponds to a 20 nanometer scan
(indicated in image 122) taken across the ano-Al interface. Graph
120 shows lines for oxygen (O) and zinc (Zn), corresponding to
relative amounts of oxygen (O) and zinc (Zn) across the ano-Al
interface. As shown, oxygen (O) dramatically increases at the
ano-Al interface since this corresponds to the transition from
metal material (Al) to metal oxide (Ano). Graph 120 also clearly
shows that zinc (Zn) accumulates at the ano-Al interface.
[0044] As described above, the presence of copper within an
aluminum alloy can strengthen the adhesion of an anodic oxide. In
fact, many commercially available aluminum-zinc alloys include
significant amounts of copper as an alloying element. However,
anodizing such commercially available copper-containing alloys
results in entraining the copper into and severely discoloring the
resultant anodic oxide such that the anodic oxide takes on a
distinctly yellow hue. Since the anodic oxide is partially
transparent, this can impart a yellow hue to the silvery-colored
base metal substrate (e.g., as viewed from surface 101 of part 100
in FIGS. 1A and 1B), which can detract from the cosmetic appeal of
the part.
[0045] This yellowing can be measured using conventional techniques
such as colorimetry using a spectrophotometer, and described
according to a color space such as CIE 1976 L*a*b* with a
corresponding standard illuminant and white spot such as CIE
Standard Illuminant D65. The anodized substrate can be measured
while in a non-dyed state--that is, without any color additives
such as anodic dyes (e.g., organic or metallic dyes). Note that the
D65 (daylight) white spot is used as the reference throughout this
document, but F2 (cool white fluorescent) and A (tungsten) will
yield similar results, with the colors all falling within
approximately 0.1 b* in the region of interest, regardless of which
illuminant standard is used.
[0046] In general, L*a*b* color space is a model used to
characterize colors of an object according to color opponents L*
corresponding to an amount of lightness, a* corresponding to
amounts of green and magenta, and b* corresponding to amounts of
blue and yellow. By convention, higher L* values correspond to
greater amounts of lightness and lower L* values correspond to
lesser amounts of lightness. Negative b* values indicate a blue
color, with more negative b* values indicating a bluer color, and
positive b* values indicate a yellow color, with more positive b*
values indicating a yellower color. Anodic oxide 104 having b*
values greater than 1 will generally have a perceptibly yellow hue.
The presence of too much copper or other certain types of alloying
elements within substrate 102 and cause part 100 to have b* values
greater than 1 when anodic oxide 104 is more than five micrometers
in thickness.
[0047] Methods described herein can be used to strengthen the bond
between anodic oxide and underlying aluminum alloy substrate
without substantially yellowing the anodic oxide. A first strategy
involves using a class of elements that become enriched at the
interface more favorably than zinc and/or other alloying elements
associated with delamination. Preventing or reducing the enrichment
of these elements at the interface during the anodizing process
eliminates or reduces the formation of interface-weakening species
at the interface. This first strategy is referred to below as an
interface-weakening species enrichment prevention strategy.
[0048] A second strategy involves using a class of elements,
referred to as sulfur-scavenging elements, which can bind with the
sulfur species originating from an anodizing solution during
anodizing. This prevents or reduces the occurrence of zinc and/or
other elements associated with delamination from combining with the
sulfur species to form the interface-weakening species (e.g.,
zinc-sulfur species 116) at the interface. This second strategy is
referred to below as a sulfur-scavenging strategy. In some
embodiments, a combination of enrichment prevention and
sulfur-scavenging strategies are used. These and other embodiments
are described below.
Interface-Weakening Species Enrichment Prevention
[0049] One way of increasing the bond strength between an anodic
oxide and high-strength aluminum alloy substrate is by preventing
or reducing the enrichment of zinc at the interface between the
anodic oxide and substrate that would otherwise occur during
anodizing. Since zinc can act as an interface-weakening agent,
preventing or reducing accumulation of zinc at the interface can
increase the adhesion of the anodic oxide to the substrate.
Furthermore, if interfacial zinc accumulation is avoided or
reduced, this also prevents or reduces the formation of zinc-sulfur
compounds at the interface, which is also an interface-weakening
agent.
[0050] Preventing or reducing zinc enrichment can be accomplished
by adding one or more additional elements to the substrate that
will enrich at the interface in preference to zinc. To illustrate,
FIG. 2 shows a cross-section view of part 200 formed using such an
enrichment prevention strategy. Part 200 includes aluminum alloy
substrate 202 with anodic oxide 204 formed from a sulfur-containing
bath (e.g., sulfuric acid-based bath) anodizing process. Anodic
oxide 204 includes pores 210 formed during the anodizing process.
Substrate 202 includes alloying element zinc 206, which is
incorporated within aluminum matrix 212. Substrate 202 can also
include magnesium (not shown for simplicity) that can combine with
zinc 206 form MgZn.sub.2 precipitates within substrate 202 to give
substrate 202 high tensile strength, as described above. Sulfur
species 214 originating from the sulfur-containing anodizing
solution becomes incorporated within anodic oxide 204 during the
anodizing process.
[0051] In addition to zinc 206 (and optionally magnesium), aluminum
alloy substrate 202 includes micro-alloying element 216, which is
an element that enriches at interface 208 more favorably than zinc
206, and consequently reduces or eliminates enrichment of zinc 206
at interface 208. Micro-alloying element 216 is added in very small
concentrations, i.e., less than 0.1 weight %, and in some
embodiments preferably in a concentration of 0.02 weight % to 0.05
weight %. These low concentrations have been found to be sufficient
to inhibit zinc 206 enrichment at interface 208, without
significantly yellowing anodic oxide 204 or negatively altering
other alloy properties of substrate 202 such as strength,
elongation, electrical or thermal conductivity, and/or corrosion
resistance.
[0052] The types of micro-alloying element 216 that preferably
enrich at interface 208 compared to zinc 206 can be identified
based on how easily micro-alloying element 216 oxidizes compared to
zinc 206. That is, since the interfacial enrichment of
micro-alloying element 216 during anodizing is primarily a
consequence of higher Gibbs free energies of oxide formation
compared to that of aluminum (of aluminum matrix 212), it may be
assumed, to first approximation, that elements with higher Gibbs
free energies for oxide formation than zinc 206 will, in turn be
preferentially enriched at interface 208 over zinc 206. This
approximately limits elements of interest as possible candidates
for micro-alloying element 216 to vanadium, phosphorus, tin,
tungsten, iron, germanium, cadmium, molybdenum, nickel, cobalt,
phosphorus, antimony, bismuth, arsenic, indium, tellurium, copper,
thallium, osmium, selenium, iridium, mercury, platinum, silver, and
gold (ranked in approximate order of increasing Gibbs free energy
for oxide formation, and consequent enrichment relative to zinc
206).
[0053] FIG. 3 shows graph 300 indicating interfacial enrichment of
a number of elements as a function of Gibbs free energy
(.DELTA.G.sup.0). Graph 300 is a modified version of data provided
in Corrosion Science, Vol. 39, No. 4, pp. 731-737 (1997). The
x-axis of graph 300 indicates Gibbs free energy (.DELTA.G.sup.0)
for oxide formation of each element. The y-axis of graph 300
indicates an amount of enrichment of each element at the interface
between an anodic film and aluminum alloy substrate, expressed in
atoms (.times.10.sup.15) per cm.sup.2. Graph 300 indicates that
vanadium (V), tin (Sn), nickel (Ni), molybdenum (Mo), bismuth (Bi),
antimony (Sb), indium (In), copper (Cu), mercury (Hg), silver (Hg),
and gold (Au) have higher .DELTA.G.sup.0 for oxide formation than
zinc (Zn), and that these elements also enrich at the interface.
This indicates that using higher .DELTA.G.sup.0 for oxide formation
compared to that of zinc is a good first approximation for
determining types of micro-alloying elements that can accumulate at
the anodic oxide-substrate interface.
[0054] FIG. 4 shows graph 400 and image 402 indicating evidence of
prevention of interfacial zinc enrichment when using copper (Cu) as
a micro-alloying element. Graph 400 represents data collected by
electron energy loss spectroscopy (EELS) and image 402 is collected
using high-resolution microscopy. In graph 400 and image 402, upper
portions correspond to an anodic oxide (Ano) and lower portions
correspond to an aluminum alloy substrate (Al), with the dashed
line labeled "ano-Al interface" corresponding to an interface
region between anodic oxide (Ano) and aluminum alloy substrate
(Al). The EELS graph 400 corresponds to a 20 nanometer scan
(indicated in image 402) taken across the ano-Al interface. Graph
400 shows lines for oxygen (O), zinc (Zn), and copper (Cu)
corresponding to relative amounts of oxygen (O), zinc (Zn), and
copper (Cu) across the ano-Al interface. As shown, copper (Cu)
accumulates at the ano-Al interface while zinc (Zn) does not. This
EELS scan confirms that copper (Cu) micro-alloying element
preferentially enriches at the ano-Al interface over zinc (Zn), and
can result no zinc (Zn) enrichment at the ano-Al interface.
[0055] Some elements may be eliminated as candidates for a
micro-alloying element for various reasons. For example, phosphorus
is known to weaken the interface between an anodic oxide and
substrate, and can therefore be avoided. Lead, mercury, cadmium,
thallium, and arsenic may be avoided due to their toxicity, whilst,
nickel may be undesirable for applications where skin contact is
anticipated. Mercury, bismuth, lead, tin, cadmium, and indium may
not constitute practical alloying elements for aluminum due to
their low melting points or phase changes occurring within a
typical aluminum alloy's thermal processing window.
[0056] Assuming that the micro-alloying element is uniformly
distributed within aluminum alloy substrate (and preferably within
the aluminum matrix) rather than in discrete second phase particles
(which can themselves be a cause of cosmetic defect in anodizing),
solubility in the aluminum matrix may also be a selection
criterion. Iron, for example, can form Al.sub.13Fe.sub.4
precipitates, which act as a grain refiner, limiting grain growth
during thermo-mechanical processing of the alloy. This may in turn
be perceived as a cosmetic defect in the anodic oxide. Assuming
that a solubility of about 0.05 weight % or more in the aluminum
matrix is a further condition can eliminate platinum, palladium,
selenium, tellurium, arsenic, antimony, nickel, cobalt, molybdenum,
and tungsten--although some of these elements will be considered
and explored in some cases.
[0057] Elements such as tungsten, germanium, tellurium, osmium,
selenium, iridium, rhodium, platinum, palladium, silver, and gold
may be less desirable candidates due to their scarcity or cost.
However, since they may only be needed in low concentrations, they
may be considered in some cases. Of the remaining elements having
higher Gibbs free energies for oxide formation than zinc identified
above, including the rare or expensive metals, some, such as copper
and gold, enhance precipitation strengthening, and can therefore
possibly be used in combination with lower amounts of zinc.
[0058] A major consideration for applications that are to be used
for cosmetic surface finishes of consumer products is the intrinsic
color of the surface of the part, including the anodic oxide, after
anodizing. Elements such as iron, copper, and silver can discolor
the anodic oxide. As described above, copper, in particular,
results in adding a yellow or bronze color to the anodic oxide.
This yellow discoloration is noticeable even when copper is added
in quantities as low as about 0.1 weight % to about 0.2 weight %,
with b* values of greater than 3 when the anodic oxide has a
thickness of 10 micrometers or more using processing conditions of
a typical Type II anodizing process. Typical zinc-magnesium-copper
aluminum alloys such as commercially available aluminum alloy 7010
(with 1.5-2.0 weight % copper) and 7075 (with 1.2-2.0 weight %
copper) have severely discolored anodic oxide (b*>>1). This
makes anodized 7010 and 7075 aluminum alloys unsuitable for use in
certain products, where a silvery-colored aluminum appearance is
desired.
[0059] Other commercially available zinc and magnesium alloying
element-based 7000-series alloys specify maximum levels of copper:
notably 0.2 weight % and 0.1 weight % for 7003 and 7005
respectively. But such permitted levels would still be too high for
a desired degree of color control (i.e., b*<1), especially as
other elements such as manganese are similarly tolerated or
specified as 0.3 weight % max and 0.2-0.7 weight % in 7003 and
7005, respectively. The anodic oxide film thickness on such alloys
could be restricted to just a couple of micrometers to minimize
discoloration, but that approach severely limits the process window
for anodizing parts, and consequently limits the wear and corrosion
protection offered by the anodic oxide.
[0060] To achieve a desirable level of high clarity (e.g.,
L*>80, and preferably L*>85) and substantially colorlessness
(e.g., b*<1, and preferably b*<0.5) of an anodic oxide,
formed under typical Type II anodizing conditions to thicknesses of
10 micrometers or more, the aluminum alloy composition
specification for a high-strength 7000-series alloy must, for
example, specify that with the exception of zinc and certain
corresponding precipitate-forming strengthening element or elements
(e.g., magnesium or lithium). For example, aluminum alloy can have
strict limits on all elements that would result in discoloration of
the anodic oxide or cause other cosmetic defects. For example,
limits might be set at 0.01 weight % maximum for chromium, copper,
manganese and zirconium, 0.02 weight % maximum for titanium, 0.05
weight % maximum for silicon, 0.08 weight % maximum for iron and
0.01 weight % maximum for any other non-specified element, to a
total maximum concentration of 0.1 weight % of other non-specified
elements. Note that this range of elemental composition is provided
by way of example for yielding substantially colorless anodic
oxides, and are not intended to limit the possibility of other
variations that would fall within the scope of inventive
embodiments presented herein. That is, the concentrations of
chromium, copper, manganese, zirconium, titanium, silicon, iron,
and/or other non-specified elements can be slightly varied from
those listed above and still achieve anodic oxides with acceptable
levels of clarity.
[0061] Aluminum alloy substrate compositions without copper can
offer maximum clarity and colorlessness of the anodic oxide.
However, the absence of copper in these alloys can result in more
egregious accumulation of zinc at the interface, and necessitates
the development of the alternative strategies for delamination
mitigation. In the present work, the micro-alloying element is
added in controlled "micro-alloying" amounts (<0.1 weight %) to
substrates made of high strength aluminum alloys, such as
7000-series alloys, for the specific purpose of eliminating
interfacial enrichment of zinc and/or other delamination species.
The micro-alloying element is added in specified levels just
sufficient to inhibit enrichment of zinc and/or other delamination
species at the interface without significant discoloration of the
anodic oxide and the resulting surface finish of the part (i.e.,
unlike the commercially available 7003, 7005, and 7010 alloys), and
without significantly altering other alloy properties of the
substrate such as strength, elongation, electrical or thermal
conductivity, or corrosion resistance.
[0062] The amount of micro-alloying element can depend on the type
of micro-alloying element and on cosmetic requirements. For
example, as described above, even relatively small amounts of
copper within aluminum alloy substrates have been found to result
in discolored anodized part. To illustrate, FIG. 5A shows graph 500
indicating yellowing effects of different amounts of copper as a
micro-alloying element to an aluminum alloy substrate. Graph 500
shows b* color space values for zinc-magnesium aluminum alloy
substrates having different amounts of copper anodized using
different anodizing bath temperatures. Data for three type of
zinc-magnesium aluminum alloy substrates having different
concentrations of copper: 1A=0.05 weight % copper; 1B=0.1 weight %
copper; and 1C=0.15 weight % copper. In addition to the copper,
each type of aluminum alloy substrate 1A, 1B, and 1C include only
magnesium and zinc as alloying elements.
[0063] Each type of substrate 1A, 1B, and 1C was anodized using 1.5
A/dm.sup.2 current density in 200 g/L sulfuric acid solution, to a
target thickness of 15 micrometers. Three different anodizing bath
temperatures were explored: 17 degrees Celsius, 20 degrees Celsius,
23 degrees Celsius, and 26 degrees Celsius. Graph 500 indicates a
linear relationship between levels of copper and b* value, with
higher concentrations of copper correlating with higher b* values.
As described above, higher b* values correspond to yellower
appearance--thus the more copper added, the yellower the appearance
of the anodized part. Graph 500 also indicates that increasing
anodizing bath temperatures can reduce values b*. However, higher
anodizing temperatures can result in a softer the anodic oxide.
Thus, higher anodizing temperatures may not be suitable for certain
applications. For good wear protection, a temperature of 20 degrees
Celsius is generally preferred, together with a thickness of 10
micrometers or more. Graph 500 indicates that in embodiments where
zinc-magnesium aluminum alloy substrates are required to have b*
values no more than 1, the copper concentration should not exceed
about 0.1 weight %, depending, in part, on the temperature of the
anodizing bath. Similarly, lower current density may be used to
reduce the yellowing, but the effect is not as strong as that of
raising temperature, and reduced current density again softens the
resulting anodic oxide, with 1.5 A/dm.sup.2 being preferred for
sufficient hardness at room temperature.
[0064] An additional consideration in determining how much
micro-alloying element should be used is the commercial
recyclability of the part. By restricting the level of the
micro-alloying element to 0.05 weight % or less, there is no
implication for recycling of the part. This is because most
commercial 7000-series alloys specify a maximum of 0.05 weight %
for "other" alloying elements and could therefore accommodate the
part having micro-alloying element at levels of 0.05 weight % or
less. Thus, in some embodiments, the micro-alloying element is
preferably added in amounts less than about 0.05 weight %. This is
particularly true of less common alloying element candidates (such
as silver, antimony), and is generally not a limiting factor for
copper, which is used in most 7000-series alloys at levels of at
least 0.5 weight %.
[0065] In one preferred embodiment, a specified micro-alloying
addition of between 0.02 weight % and 0.05 weight % copper is made
to an aluminum alloy that includes about 5.5 weight % zinc and
about 1 weight % magnesium, with substantially no other alloying
elements. This composition corresponds to a relatively pure and
balanced aluminum-zinc-magnesium 7000-series alloy optimized for a
high yield strength (about 340-350 MPa), hardness (about 125 HV),
and heat-treatability, with a very small amount of copper. It has
been found that anodizing this substrate composition using Type II
anodizing (200 g/L sulfuric acid bath, at 20 degrees Celsius, with
1 A/dm.sup.2 current density) to form an anodic oxide having a
thickness of about 15 micrometers completely eliminates
delamination of the anodic oxide, as assessed by rock tumble
testing or by indentation testing such that described in U.S.
application Ser. No. 14/678,881, which is incorporated by reference
herein in its entirety. The color of the anodized surface of this
substrate composition remains silvery, with a b* value of about
0.4, corresponding to very little yellowing.
[0066] Similar results were obtained with silver as the
micro-alloying element, although for a 0.05 weight % addition, the
corresponding atomic % of silver is lower than copper, and the
delamination resistance was not as improved as found with copper.
In some embodiments, approximately 0.1 weight % silver
micro-alloying element is required to eliminate delamination in a
15 micrometer thick anodic oxide, and at that level, the
discoloration is found to be higher than that using copper, albeit
b* is still less than 1.
[0067] In some embodiments, the micro-alloying element includes a
combination of copper and silver so as to give compounded benefits
in terms of delamination resistance, but also compounded color
shifts. One or more of germanium, osmium, iridium, rhodium, and
gold may be used similarly to silver and copper, in some cases even
without any corresponding yellow discoloration. However the cost
premium over silver may make these elements less desirable.
[0068] Vanadium was also evaluated, even though its Gibbs free
energy for oxide formation is very close to that of zinc. A very
slight improvement in delamination resistance was observed, when
using 0.05 weight % vanadium concentration, and there was no yellow
significant discoloration. Titanium, and zirconium were also
evaluated. However, in one embodiment, titanium and zirconium
showed no ability to reduce interfacial enrichment of zinc and no
significant improvement in delamination resistance, even at 0.3
weight %. This is consistent with the Gibbs free energies for oxide
formation for each of titanium and zirconium being lower than that
of zinc.
[0069] Of the elements as candidates for the micro-alloying element
that might be dismissed on the basis of limited solubility in the
aluminum matrix, nickel, molybdenum, and antimony were explored at
0.05 weight %. Nickel was detrimental--perhaps forming even worse
interfacial compounds than zinc. Molybdenum was of no significant
benefit--possibly because its Gibbs free energy for oxide formation
is not far enough from that of zinc, compounded by the fact that
its relatively high atomic weight makes its corresponding atomic
concentration low. Antimony was of significant benefit--comparable
to silver, but without the undesirable yellow discoloration of
silver. Instead, antimony gave a slight blue discoloration, with a
b* value of -0.2. This could possibly be used in combination with a
yellowing element to neutralize color. However, antimony can
introduce small spherical inclusions to the anodic oxide--probably
corresponding to aluminum-antimony precipitates in the alloy, which
inhibit growth of a completely uniform anodic oxide film. However,
it is possible for more than one type of micro-alloying element to
be used to achieve a cumulative effect in offsetting interfacial
enrichment of zinc and/or discoloration of the anodic oxide. For
example, an element that results in adding a yellow hue (b*>0)
to the anodic oxide, such as copper, can be added to increase
interfacial enrichment of zinc 206, while an element that results
in adding a blue hue (b*<0) to the anodic oxide, such as
antimony, can be added to offset the yellowing of the copper. In
some embodiments, a target b* for the part is between -1 and 1. In
some embodiments, a target b* for the part is between -0.5 and
0.5.
[0070] Given the above-described considerations and limitation, in
some embodiments, preferred candidates for the micro-alloying
element include one or more of vanadium, germanium, cobalt,
antimony, copper, tellurium, osmium, selenium, iridium, rhodium,
palladium, silver, and gold. In some embodiments, preferred
candidates for the micro-alloying element include one or more of
copper, silver, and antimony. In some embodiments, more than one
element is used in combination. In some embodiments, the
micro-alloying element is added at levels of no more than about 0.1
weight %. In some embodiments, the micro-alloying element is
preferably added at levels of between 0.02 weight % and 0.05 weight
%. In some embodiments, the anodized part, as viewed from an
exposed surface of the anodic oxide (surface 201), has a b* value
of less than 2, in some embodiments a b* value less than 1, even
when anodized to thicknesses of 10 micrometers or more, at current
densities of 1 A/dm.sup.2 or more and an anodizing bath temperature
of 25 degrees Celsius or less. In some embodiments, the anodized
part has an L* value (corresponding to a level of brightness)
greater than 75, and in some preferred embodiments, the anodized
part has an L* value greater than 85.
[0071] In one preferred embodiment, the specified addition of
copper as a micro-alloying element at a level of between 0.02 and
0.05 weight % is notably within the typical tolerances for copper
as an impurity in certain commercially available 7000-series
alloys, such as 7003 and 7005, which respectively specify 0.1 and
0.2 weight % maximum for copper. A crucial distinction between such
tolerated impurity levels in commercially available 7000-series
alloys and the specified micro-alloying additions made in
embodiments presented herein, however, is the specified
micro-alloying addition. In particular, in embodiments presented
herein, copper has both a specified minimum (e.g., 0.02 weight %)
to ensure sufficient interfacial adhesion of the anodic oxide, and
a specified maximum (e.g., 0.05 weight %) that is carefully
selected to limit the maximum discoloration of the anodic oxide,
and as such is also generally lower than any commercial high
strength 7000-series alloys' tolerated impurity level for
copper.
[0072] There can be other advantages of using micro-alloying
element within the aluminum alloy substrates. For example, one
problem encountered in the anodizing of aluminum alloys only having
zinc and magnesium as alloying elements is the differential growth
of an anodic oxide on grains of different surface orientation
within the substrate, resulting in grain texture-related thickness
variation. In the presence of zinc, grains of {111} surface
orientation are relatively anodic, as compared to grains of {110}
and {100} orientation, and are thus anodized anomalously fast. This
can detract from the aesthetics of the anodized finish of a
part--notably as apparent pits at the anodic oxide and substrate
interface.
[0073] One solution to the problem of differential growth rates for
grains of different surface orientation is described in U.S.
application Ser. No. 14/678,868, which is incorporated by reference
herein in its entirety. In the U.S. application Ser. No.
14/678,868, an electrolyte to enable anodizing at low current
density and/or increased temperature whilst maintaining adequate
anodic oxide hardness is described. In the present application, the
micro-alloying element can induce faster growth of the anodic oxide
in other orientations (e.g., {110} and/or {100}) to mitigate the
discrepancy. For example, copper micro-alloying element has shown
to induce faster anodic oxide grown on grains of {110} orientation.
In one embodiment, 0.05 weight % copper proved sufficient to dilute
the cosmetic defect observed when Type II anodizing (200 g/L
sulfuric acid with 1.5 A/dm.sup.2 current density) beyond the limit
of perception--even on a substrate with a mirror-lapped surface
anodized to a thickness of 10 micrometers or more. This corresponds
to a thickness discrepancy of less than 5% between grains of
distinct orientation--far less than the typical 10% thickness
discrepancy that would result in the absence of the micro-alloying
addition. In this way, the presence of a micro-alloying element can
also reduce non-uniform growth of the anodic oxide and related
cosmetic defects.
[0074] FIG. 5B shows graph 510 indicating the effect of adding 0.05
weight % copper on a thickness uniformity of a resultant anodic
oxide. Graph 510 shows that the absence of the copper addition (5.5
Zn, 1.0 Mg sample), the presence of zinc in the alloy results in
very non-uniform growth rates for different crystallographic
orientations, with surfaces of orientations close to {111}
orientation in particular growing at an accelerated rate. This
results in grains of {111} orientation having an oxide film of
about 3-9% thicker than the average film thickness--appearing as
distinct "pit"-like features in the anodic oxide. These are
particularly evident on a substrate that has been lapped to a
mirror finish. The addition of 0.05 weight % copper (5.5 Zn, 1.0
Mg, 0.05 Cu sample) is sufficient to overcome preferential growth
of oxide on {111} surfaces and to ensure film thickness uniformity
within 5%, without having to resort to such methods as that
disclosed in U.S. patent application Ser. No. 14/678,868, which is
incorporated herein in its entirety.
[0075] FIG. 6A shows flowchart 600, illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate. At 602, a micro-alloying element is
incorporated into an aluminum alloy substrate. The aluminum alloy
substrate can include zinc and magnesium alloys that give the
substrate high tensile strength. In some embodiments, the substrate
is an enclosure, or part of enclosure, for an electronic device.
The micro-alloying element is added in an amount that is less than
conventionally used for alloying purposes. In some embodiments, a
concentration of the micro-alloying element within the aluminum
alloy substrate is at most 0.1 weight %, and in some embodiments
about 0.05 weight % or less. The micro-alloying element can be
characterized as having a higher Gibbs free energy of oxide
formation than the zinc. In some embodiments, the micro-alloying
element includes one or more of vanadium, tin, nickel, molybdenum,
germanium, bismuth, cobalt, antimony, tellurium, osmium, selenium,
indium, iridium, rhodium, palladium, copper, mercury, silver, and
gold. In some embodiments, the micro-alloying element includes one
or more of copper, silver, and antimony. In particular embodiments,
two or more types of micro-alloying elements are used, such as
copper and silver.
[0076] At 604, the aluminum alloy substrate is anodized. In some
embodiments, the anodizing takes place in an anodizing solution
comprising sulfuric acid. In some embodiments, a type II anodizing
process is used. During anodizing, the micro-alloying element
enriches at an interface between the substrate and a resultant
anodic oxide, thereby preventing or reducing enrichment of zinc at
the interface. Since zinc can be an interface-weakening species,
prevention or reduction of zinc accumulation at the interface can
increase an adhesion strength of the anodic oxide to the substrate.
Furthermore, this prevents or reduces formation of zinc-sulfur
species, another interface-weakening species, at the interface.
[0077] FIG. 6B shows flowchart 610 illustrating a process of
reducing grain-related defects in an anodized high-strength
substrate. The aluminum alloy substrate can include zinc and
magnesium alloys that give the substrate high tensile strength. At
612, a micro-alloying element having a higher Gibbs free energy of
oxide formation than the zinc is incorporated into an aluminum
alloy substrate. A concentration of the micro-alloying element
within the aluminum alloy substrate can be at most 0.1 weight %,
and in some embodiments about 0.05 weight % or less. In some
embodiments, the micro-alloying element includes one or more of
vanadium, tin, nickel, molybdenum, germanium, bismuth, cobalt,
antimony, tellurium, osmium, selenium, indium, iridium, rhodium,
palladium, copper, mercury, silver, and gold. In some embodiments,
the micro-alloying element includes one or more of copper, silver,
and antimony. In particular embodiments, two or more types of
micro-alloying elements are used, such as copper and silver.
[0078] At 614, the aluminum alloy substrate is anodized, using for
example, a sulfuric acid anodizing solution. In some embodiments, a
type II anodizing process is used. During anodizing--even at 1.5
A/dm.sup.2 in a sulfuric acid solution, the presence of the
micro-alloying element reduces the discrepancy between the growth
rates of anodic oxide on grains of {111} orientation and other
orientations. This may be because the micro-alloying element
increases the growth rate of the anodic oxide on surface
orientations other than {111}--such as {110} and {100} grain
orientations. It may also depress the anomalously high growth rate
of {111} oriented grains. The result is an oxide with thickness
uniformity to within 2-3% between grains of distinct
orientations--far less than the 10% discrepancy that would result
in the absence of the micro-alloying addition. The resultant
anodized substrate is free from pitting defects related to
accelerated anodic oxide grown at {111} grains. This can be
especially important in anodized substrates that have an underlying
polished and highly uniform surface (e.g., having a mirror
shine).
Sulfur-Scavenging
[0079] Another way of increasing the bond strength between an
anodic oxide and high-strength aluminum alloy substrate is by
preventing or reducing the occurrence of sulfur bonding with zinc
during anodizing, thereby preventing or reducing formation of
zinc-sulfur species at the interface. As described above,
zinc-sulfur species can act as an interface-weakening
species--thus, eliminating or minimizing the formation of such
zinc-sulfur can increase the adhesion strength of the anodic
oxide.
[0080] This can be accomplished by adding different class of
alloying elements to aluminum alloy substrate that will
preferentially bond with sulfur, thereby preventing the sulfur from
bonding with zinc. To illustrate, FIG. 7 shows a cross-section view
of part 700 formed using such a sulfur-scavenging strategy. Part
700 includes aluminum alloy substrate 702 with anodic oxide 704
formed from a sulfur-containing bath (e.g., sulfuric acid-based
bath) anodizing process. Pores 710 of anodic oxide 704 are formed
during the anodizing process. Substrate 702 includes alloying
element zinc 706 and optionally magnesium (not shown for
simplicity) that can combine with zinc 706 form precipitates such
as MgZn.sub.2 and give substrate 702 high tensile strength, as
described above. Sulfur species 714 originating from the
sulfur-containing anodizing solution becomes incorporated within
anodic oxide 704 during the anodizing process. Sulfur species 714
is likely in ionic form, such as a sulfide, and possibly compounded
with oxygen ions as a sulfate and/or sulfide ion.
[0081] Substrate 702 includes sulfur-scavenging species 705 that
has a strong affinity for bonding with sulfur species 714, and
therefore readily bonds with sulfur species 714 forming bound
sulfur species 707. In this way, sulfur-scavenging species 705
"scavenges" inward-diffusing sulfur species 714 and prevents sulfur
species 714 from reaching the zinc 706 at interface 708. Bound
sulfur species 707 becomes locked within anodic oxide 704 and away
from interface 708, and thus does not interfere with the adhesion
capability of anodic oxide 704 to substrate 702.
[0082] Criteria for choosing sulfur-scavenging species 705 include
how readily oxidized it is, its affinity for sulfur species 714,
and its ionic mobility. Sulfur-scavenging species 705 should be
more readily oxidized than aluminum such that, sulfur-scavenging
species 705 is oxidized together with aluminum matrix 712 during
anodizing. Table 1, below, lists a number of elements based on
their calculated Gibbs free energy for oxide formation
(-.DELTA.G.sup.0 (kCal/mol O.sub.2)).
TABLE-US-00001 TABLE 1 Oxide Formation Energies Element Oxide
-.DELTA.G.sup.0 (kCal/mol O.sub.2) Yttrium (Y) Y.sub.2O.sub.3 320
Calcium (Ca) CaO 320 Scandium (Sc) Sc.sub.2O.sub.3 310 Europium
(Eu) EuO 290 Gadolinium (Gd) Gd.sub.2O.sub.3 287 Magnesium (Mg) MgO
286 Lanthanum (La) La.sub.2O.sub.3 285 Lithium (Li) LiO.sub.2 285
Cerium (Ce) Ce.sub.2O.sub.3 285 Strontium (Sr) SrO 280 Aluminum
(Al) Al.sub.2O.sub.3 277 Hafnium (Hf) HfO.sub.2 265 Zirconium (Zr)
ZrO.sub.2 262 Erbium (Er) Er.sub.2O.sub.3 260 Titanium (Ti)
Ti.sub.2O.sub.3 242 Silicon (Si) SiO.sub.2 217 Tantalum (Ta)
Ti.sub.2O.sub.5 198 Vanadium (V) VO 198 Manganese (Mn) MnO 184
Chromium (Cr) Cr.sub.2O.sub.3 179 Niobium (Nb) NbO.sub.2, NbO 176,
187 Zinc (Zn) ZnO 166 Rubidium (Rb) Rb.sub.2O 158 Indium (In)
In.sub.2O.sub.3 157 Tin (Sn) SnO.sub.2 139 Tungsten (W) WO.sub.3
133 Iron (Fe) Fe.sub.3O.sub.4 130 Germanium (Ge) GeO.sub.2 129
Molybdenum (Mo) MoO.sub.2, MoO.sub.3 127, 106 Cobalt (Co) CoO 115
Nickel (Ni) NiO 114 Antimony (Sb) Sb.sub.2O.sub.3 111 Bismuth (Bi)
Bi.sub.3O.sub.4 104 Copper (Cu) Cu.sub.2O 80 Tellurium (Te)
TeO.sub.2 77 Thallium (Tl) Tl.sub.2O 69 Osmium (Os) OsO.sub.2 62
Selenium (Se) SeO.sub.2 54 Iridium (Ir) IrO.sub.3 43 Rhodium (Rh)
Rh.sub.2O.sub.3 42 Platinum (Pt) Pt.sub.3O.sub.4 22 Silver (Ag)
Ag.sub.2O 14
[0083] Table 1 indicated that yttrium (Y), calcium (Ca), scandium
(Sc), europium (Eu), gadolinium (Gd), magnesium (Mg), lanthanum
(La), lithium (Li), cerium (Ce), and strontium (Sr) have more
negative .DELTA.G.sup.0 for oxide formation than aluminum (Al), and
thus are more readily oxidized than aluminum (Al).
[0084] Another consideration for choosing viable candidates for the
sulfur-scavenging species is an element's ability to form a stable
compound with sulfur species 714. As described above,
sulfur-scavenging species 705 can bind with sulfur species 714 to
form a sulfate, a sulfide or other suitable stable bound sulfur
species 707. Thus, one approximation as to an element's ability to
bond with sulfur species 714 can be its enthalpy of sulfate
formation.
[0085] Table 2, below, lists a number of elements based on enthalpy
(-.DELTA.H) of sulfate formation.
TABLE-US-00002 TABLE 2 Sulfate Formation Energies Element Sulfate
-.DELTA.H (kJ/mol SO.sub.4) Barium (Ba) BaSO.sub.4 1473 Radium (Ra)
RaSO.sub.4 1471 Strontium (Sr) SrSO.sub.4 1453 Lithium (Li)
LiSO.sub.4 1437 Rubidium (Rb) RBSO.sub.4 1436 Calcium (Ca)
CaSO.sub.4 1435 Sodium (Na) Na.sub.2SO.sub.4 1387 Cerium (Ce)
Ce.sub.2(SO.sub.4).sub.3 1318 Magnesium (Mg) MgSO.sub.4 1285
Beryllium (Be) BeSO.sub.4 1250 Aluminum (Al)
Al.sub.2(SO.sub.4).sub.3 1147 Hafnium (Hf) Hf(SO4).sub.2 1115
Zirconium (Zr) Zr(SO.sub.4).sub.2 1109 Manganese (Mn) MnSO.sub.4
1065 Zinc (Zn) ZnSO.sub.4 982 Chromium (Cr)
Cr.sub.2(SO.sub.4).sub.3 970 Iron (Fe) FeSO.sub.4 929 Palladium
(Pd) PdSO.sub.4 920 Cadmium (Cd) CdSO.sub.4 935 Cobalt (Co)
CoSO.sub.4 888 Iron (Fe) Fe.sub.2(SO.sub.4).sub.3 861 Bismuth (Bi)
Bi.sub.2(SO.sub.4).sub.3 848 Nickel (Ni) NiSO.sub.4 872 Copper (Cu)
Cu.sub.2O 771 Gold (Ag) Ag.sub.2SO.sub.4 716
[0086] Table 2 indicates that barium (Ba), radium (Ra), strontium
(St), lithium (Li), rubidium (Rb), calcium (Ca), sodium (Na),
cerium (Ce), magnesium (Mg), beryllium (Be), aluminum (Al), hafnium
(Hf), zirconium (Zr), and manganese (Mn) each have very large
negative (i.e., exothermic) values for .DELTA.H for sulfate
formation, and are therefore strong sulfate formers. Barium (Ba),
radium (Ra), strontium (St), lithium (Li), rubidium (Rb), calcium
(Ca), sodium (Na), cerium (Ce), magnesium (Mg), beryllium (Be) have
more negative .DELTA.H for sulfate formation than aluminum (Al),
and therefore may be preferable in some embodiments.
[0087] In some preferred embodiments, the criterion for choosing
the sulfur-scavenging species is based on both oxide formation and
sulfate formation. Based on Tables 1 and 2, these elements include
one or more of lithium (Li), magnesium (Mg), strontium (Sr), and
calcium (Ca). It should be noted, however, that this does not
necessarily include all possible suitable sulfur-scavenging
species.
[0088] FIG. 8 shows an annotated periodic table illustrating some
criteria for choosing a suitable sulfur-scavenging species, in
accordance with some embodiments. Elements that have Gibbs free
energy (.DELTA.G.sup.0) for oxide formation greater than or equal
to .DELTA.G.sup.0 for aluminum oxide (Al.sub.2O.sub.3) formation
can be eliminated. This leaves lithium (Li), beryllium (Be),
magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), radium
(Ra), scandium (Sc), yttrium (Y), lutetium (Lu), lawrencium (Lr),
lanthanum (La), cerium (Ce), samarium (Sm), europium (Eu),
gadolinium (Gd), actinium (Ac), and thorium (Th).
[0089] Some elements indicated in FIG. 8 can be further eliminated
due to cost or other factors, such as toxicity or radioactivity (as
indicated). For example, the cost of beryllium (Be), radium (Rd),
scandium (Sc), yttrium (Y), lutetium (Lu), lawrencium (Lr),
actinium (Ac), thorium (Th), samarium (Sm), europium (Eu), and
gadolinium (Gd) may be too high for many practical purposes.
Beryllium (Be), radium (Rd), yttrium (Y), lanthanum (La), actinium
(Ac), cerium (Ce), and thorium (Th) may be toxic and/or
radioactive. This leaves one or more of lithium (Li), magnesium
(Mg), calcium (Ca), strontium (Sr), and barium (Ba) as preferred
sulfur-scavenging candidates, according to some embodiments.
[0090] An additional consideration for choosing a sulfur-scavenging
species is its ionic mobility within the anodic oxide during
anodizing. Those species that are lighter in weight, such as
lithium, sodium and magnesium, have high mobility within an anodic
oxide, thereby increasing the likelihood of reaching and binding
with the counter-flowing sulfur species. Moreover, their high ionic
mobility relative to aluminum ensures that under the applied
electric field, they diffuse away from the metal oxide interface
more rapidly than aluminum. Upon encountering counter-flowing
sulfur species, they will form compounds away from the anodic
oxide-substrate interface. They effectively present a
counter-flowing barrier to the flow of such anions towards the
interface.
[0091] It should be noted, however, that ion mobility might not be
the only factor to consider. For example, Table 2 above shows that
barium (Ba) forms a very strong bond with sulfate. However, the
barium atom is much heavier than elements such as sodium and
magnesium, generally making barium less effective for a given
weight percent of lighter elements such as lithium, sodium and
beryllium. Never the less, barium's higher sulfate energy formation
may counterbalance its higher atomic weight.
[0092] Other considerations can include the solubility of the
element within aluminum and the abundance of the element. Ideally,
the elements in question should also present solubility within the
aluminum matrix to the desired level, possibly also eliminating
barium, calcium, cerium, or gadolinium in certain embodiments, For
example, scandium is more soluble in face-centered cubic aluminum
(0.04 atomic % at 500 Celsius) compared to yttrium (0.008 atomic %
at 500 Celsius). However, this solubility condition may not be
essential as being a suitable candidate, provided that the element
does not have an adverse effect on the aluminum alloy's
microstructure or on the anodic oxide cosmetics.
[0093] Low stability or low melting point may also be considered,
which may rule out sodium, potassium, and rubidium. Elements such
as scandium, europium, and hafnium are extreme scarce and may be
further ruled out for this reason. In some embodiments where the
above factors are considered, preferable sulfur-scavenging species
include one or more of lithium, magnesium, calcium, strontium, and
barium. In some embodiments, preferable sulfur-scavenging species
include one or more of lithium, magnesium, calcium, strontium,
barium, scandium, and yttrium. In some embodiments, a combination
of two or more of lithium, magnesium, calcium, strontium, barium,
scandium, and yttrium are used.
[0094] Other considerations include the cosmetic effects of adding
the sulfur-scavenging species. As described above, some elements
can noticeably discolor a resultant anodic oxide, with yellow
discoloring particularly undesirable in certain applications.
Lithium, magnesium, calcium, strontium, and barium have no
significant yellowing effect on a resultant anodic oxide in amounts
sufficient to increase delamination resistance, and can therefore
be used without negatively affecting the cosmetic qualities of the
anodic oxide.
[0095] The amount of sulfur-scavenging species can vary depending
on the type of sulfur-scavenging species. For example, it may be
desirable to add the sulfur-scavenging species to an amount
sufficient to result in an anodic oxide having a predetermined
determined delamination resistance, such as determined by
techniques described in U.S. application Ser. No. 14/678,881, which
is incorporated herein in its entirety. It should be noted,
however, that the amount of sulfur-scavenging species added to the
aluminum alloy to provide sufficient sulfur-scavenging capability
is significantly greater than the micro-alloying amounts described
above with respect to preventing enrichment of interface-weakening
species. For example, it has been found that 2 weight % or more of
lithium should be used in order to provide sufficient
sulfur-scavenging and prevent delamination. However, high
concentrations of alloying elements reduce the thermal conductivity
of the alloy, which may be undesirable in some applications. In
some embodiments, the sulfur-scavenging species is added to the
substrate at a concentration ranging from about 0.5 weight % and
about 3 weight %.
[0096] One alloying element that has proven to be a good
sulfur-scavenging species is lithium. As described above, lithium
added in a concentration of about 2 weight % proved to eliminate
delamination. Whilst interfacial enrichment of zinc is shown to
occur, the counter-migration of lithium through the anodic oxide is
sufficient to limit the diffusion of sulfur species towards the
interface, and to eliminate the formation of the
interface-weakening species of zinc and sulfur--even when the
anodizing is conducted under conditions which would result in a
very weak interface in the absence of lithium (such as a
conventional Type II anodizing process conducted in 200 g/L
sulfuric acid at 1.5 A/dm.sup.2 current density and 20 degrees
Celsius). Furthermore, if the addition of lithium is used in
combination with other approaches to minimized sulfate
incorporation from the electrolyte, such as the mixed acid
electrolyte compositions described in U.S. application Ser. No.
14/678,868 (which is incorporated herein in its entirety), a more
robust solution can be achieved, and one or more of the necessary
conditions may be relaxed (e.g., use less lithium, or higher
current density, or lower anodizing bath temperature).
[0097] Magnesium, in particular, as a sulfur-scavenging species can
be of interest since most commercially available 7000-series
aluminum alloys already include magnesium and zinc as alloying
elements. As described above, magnesium can be a key to the
strengthening mechanism of many 7000-series alloys by virtue of its
propensity to form precipitates such as MgZn.sub.2, and
specifically an .eta.' phase, within the aluminum matrix, within
the aluminum matrix. FIG. 9 shows graph 900 indicating magnesium
and zinc concentrations of different commercially available 7000
series aluminum alloys: 7005, 7108, 7003, 7029, 7075, 7050, 7030,
7046A, 7046, as well as custom aluminum alloy composition 904,
which is based on optimal .eta.' precipitation strengthening, and
custom aluminum alloy composition 906. The x-axis of graph 900
indicates weight % of zinc content and the y-axis indicates weight
% of magnesium content within the aluminum alloys. Most of the
commercial alloys include significant concentrations of other
alloying elements which are not shown: 7029, 7030, 7046, 7050, and
7075, for example, all include copper at levels that would
significantly discolor an anodic oxide.
[0098] Line 902 represents balanced zinc and magnesium compositions
for providing MgZn.sub.2 precipitates for enhancing the strength of
the aluminum substrate. That is, line 902 represents stoichiometric
amounts of zinc and magnesium to form MgZn.sub.2 .eta.'
precipitates. Alloy compositions below line 902 can be
characterized as being zinc-rich, and alloy compositions above line
902 can be characterized as being magnesium-rich. Excess zinc or
magnesium in zinc-rich or magnesium-rich alloy compositions will
reside in the aluminum matrix of the aluminum alloy, reducing the
thermal or electrical conductivity of the alloy. Thus, for some
applications, and notably for electronics enclosures, which play a
role in dissipating heat, it is preferable to avoid this by
choosing aluminum alloys having a generally balanced magnesium and
zinc composition, such as custom aluminum alloy composition 904. In
addition to the Mg:Zn ratio defined by the precipitate
stoichiometry, an optimal level (volume fraction) of precipitate
strengthening has informed the selection of exact composition
target 904, allowing for a given homogenization, quenching,
extrusion and artificial ageing process to achieve a target
strength, such as 340 MPa.
[0099] Since magnesium can also act as an effective
sulfur-scavenging species, an excess of magnesium over the balanced
composition of custom alloy 904 can be made, such as indicated
magnesium-rich custom alloy 906. That is, a magnesium-rich custom
alloy 906 may be beneficial in providing the added benefit of
sulfur-scavenging and thereby improving adhesion of an anodic
oxide. The amount of excess magnesium relative to zinc to provide
such sulfur-scavenging benefit can be significant. Namely the
atomic concentration of magnesium should be at least equal to half
the atomic concentration of zinc, and preferably equal to or
greater than the atomic concentration of zinc, placing it in excess
by a factor of two. The excess of magnesium in this illustrative
example is not intended to change the strengthening precipitate
from MgZn.sub.2 (though it may do so, as detailed in the next
paragraph), and as such, it does not significantly contribute
strength to the alloy. Nor does the excess of magnesium have a
significant effect on the level of zinc accumulation at the
interface, even though it can reduce or eliminate the level of free
zinc in the matrix. This is because the interfacial accumulation of
zinc during anodizing occurs whether the zinc is in the matrix or
bound in a precipitate phase. In sufficient excess, however, the
excess magnesium does prevent delamination of the anodic oxide
through this sulfur-scavenging mechanism. It may reduce the thermal
and electrical conductivity of the alloy somewhat, but may be
beneficial in terms of corrosion resistance.
[0100] Although, the excess of magnesium in the previous
illustrative example is not intended to significantly contribute
strength to an alloy where the strengthening phase was assumed to
be MgZn.sub.2, alternative strengthening phases may be formed in
some cases, and their roles must then be considered in determining
precise alloying concentrations for optimal strengthening under any
given thermo-mechanical processing route, and in turn for
determining an appropriate excess of magnesium. This may allow
equivalent strength to alloy 904 with lower zinc concentrations.
Whatever the precise alloy composition selected for optimal
strengthening, an excess of magnesium can be preferred. A wide
region of interest for such candidate alloys exists, in the region
outlined 908. Note that, in addition to custom compositions
904/906, region 908 encompasses lower zinc compositions, including
those above and to the left of 904/906 on graph 900. A further
benefit of the excess of magnesium will be discussed later.
[0101] It should be noted that the commercially available aluminum
alloys shown in FIG. 9 include significant amounts of elements
other than zinc and magnesium, such as copper, iron, silicon and
manganese, which can have undesirable cosmetic effects. As
described above, the presence of copper, manganese, and certain
other elements in more than micro-alloying amounts can yellow a
resultant anodic oxide to unacceptably high b* value levels. As
described above, in some applications the b* value of the anodized
part, as viewed from an exposed surface of the anodic oxide (e.g.,
surface 701), should be less than 2, in some cases less than 1, or
even less than 0.5. Thus, in some embodiments, custom alloy 904 and
magnesium-rich custom alloy 906, have very strictly controlled
maxima for all other elements: e.g., 0.01 weight % for magnesium,
0.01 weight % for chromium, 0.01 weight % for zirconium, 0.02
weight % for copper, 0.02 weight % for titanium, 0.05 weight % for
silicon, and a maximum of 0.01 weight % for any other non-specified
element, to a total maximum of 0.1 weight % for non-specified
others. Other elements may, however, be specifically added as
micro-alloying elements in accordance with the approach described
earlier in this paper.
[0102] It has been found that zinc and magnesium do not yellow a
resultant anodic oxide. In fact, magnesium and/or zinc may tend to
provide a bluish high to the anodic oxide, which in certain
applications is more desirable than a yellow hue. Thus,
magnesium-rich custom alloy 906 can provide sulfur-scavenging
capability as well as desired cosmetic (color) quality to an
anodized part.
[0103] FIG. 10 shows flowchart 1000, illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate using a sulfur-scavenging strategy. The
aluminum alloy substrate can include zinc and magnesium alloys that
give the substrate high tensile strength, with a balanced
proportion of magnesium and zinc (e.g., atomic % zinc=2 times
atomic % magnesium to yield MgZn.sub.2 .eta.' precipitates). At
1002, a sulfur-scavenging species is incorporated into an aluminum
alloy substrate. In some embodiments, the sulfur-scavenging species
is added at a concentration ranging from about 0.5 weight % and
about 3 weight %. In a particular embodiment, the sulfur-scavenging
species is an excess of magnesium (i.e., a significant addition of
magnesium, over and above the balanced level of half that of the
atomic % zinc).
[0104] In some embodiments, the sulfur-scavenging species has a
Gibbs free energy of oxide formation lower than that of aluminum.
In some embodiments, the sulfur-scavenging species is additionally
a strong sulfate former, i.e., has a large negative enthalpy for
sulfate formation--in some embodiments, more negative than that for
aluminum sulfate formation. In some embodiments, the
sulfur-scavenging species includes one or more of lithium,
magnesium, calcium, strontium, and barium. In particular
embodiments, two or more types of sulfur-scavenging species are
used.
[0105] At 1004, the aluminum alloy substrate is anodized, using,
for example, an anodic solution comprising sulfuric acid (e.g.,
type II anodizing process). During anodizing, the sulfur-scavenging
species binds with sulfur species originating from the anodic
solution. For example, lithium and/or magnesium and bind with
sulfate ions to form lithium sulfate and/or magnesium sulfate. In
this way, the sulfur-scavenging species "scavenges" the sulfur
species and prevents the sulfur species from binding with zinc to
form a zinc-sulfur species, which is an interface-weakening
species, at an interface between the substrate and a resultant
anodic oxide.
[0106] A further possible benefit of engineering an excess of
magnesium, and the correspondingly reduced level of free zinc in
the matrix is that the differential growth rates of grains of
different orientations (by virtue of zinc making the matrix more
anodic in {111} orientations) may be eliminated. Thus,
aluminum-zinc-magnesium alloys with an excess of magnesium are
preferred for avoiding the afore-mentioned grain-orientation
related cosmetic defects. In a zinc-rich alloy, or an alloy with a
balanced ratio (such as Zn:2.times.Mg where MgZn.sub.2 precipitates
are expected), anodic oxide forms on grains of {111} surface
orientation approximately 10% faster than on grains on other
surface orientations (under typical Type II anodizing conditions at
1.5 A/dm.sup.2). When a substantial excess of magnesium is employed
to eliminate free zinc in the matrix, this discrepancy is reduced
to about 1-3% and the visual defect is eliminated. That is, the
magnesium can be added in excess over a balanced ratio for
magnesium-zinc precipitate formation so as to eliminate or reduce a
concentration of non-precipitated zinc in the aluminum alloy
substrate in a T6 or T7 temper. This reduces a discrepancy between
growth rates of different portions of the anodic oxide on grains of
distinct surface orientations, resulting in the anodic oxide having
a thickness uniformity of within 5% among grains of {111} surface
orientation and other surface orientations.
[0107] FIG. 12 shows how the addition of copper in micro-alloying
amounts and lithium as a sulfur-scavenging species can improve
adhesive of an anodic film to a substrate. In particular, FIG. 12
shows scanning electron microscope (SEM) images of three anodized
substrate samples (1200, 1202, 1204) after performing a 5-by-5
array of 10 kg Vickers indentations spaced 350 micrometers apart,
using an interfacial adhesion testing method as disclosed in U.S.
patent application Ser. No. 14/678,881, which is disclosed herein
in its entirety. In all samples 1202, 1204, and 1204, the anodic
oxides are of 14 micrometer thickness, and were formed using 1.5
A/dm.sup.2 current density in 200 g/L sulfuric acid solution.
[0108] Sample 1200 is an aluminum alloy substrate having 5.5 weight
% of zinc and 1.0 weight % of magnesium (corresponding to a
balanced zinc and magnesium aluminum alloy) without added
micro-alloying element or sulfur-scavenging species. The SEM image
of sample 1200 shows evidence of significant anodic oxide
detachment due to interface weakening by interfacial enrichment of
zinc, and its interaction with sulfur from the anodizing
electrolyte. In particular, the back-scattered compositional
scanning electron microscope image of sample 1200 shows a number of
light areas corresponding to the bare aluminum substrate--where the
applied load has detached the anodic oxide. Some manufacturing
requirements require samples having less than 10 detachment regions
to be acceptable.
[0109] Sample 1202 is an aluminum alloy substrate having 5.5 weight
% of zinc, 1.0 weight % of magnesium, and 0.05 weight % copper as a
micro-alloying element. As shown, the indentation test shows there
are only four very much smaller light areas corresponding to
interfacial adhesion failure. Thus, the addition of just a small
amount of copper is sufficient to overcome the weak interfacial
adhesion caused by interfacial enrichment of zinc and zinc-sulfur
species. Sample 1204 is an aluminum alloy substrate having 5.5
weight % of zinc, 1.0 weight % of magnesium, and 1.75 weight %
lithium as a sulfur-scavenging element. The SEM image of sample
1204 shows substantially no bright spots, thereby indicating the
addition of a sulfur-scavenging element (e.g., lithium) also
overcomes the delamination problem.
Combinations and Other Embodiments
[0110] In some cases, using a combination of a micro-alloying
element and a sulfur-scavenging species has been found to provide a
combined benefit. For example, adding one or more micro-alloying
elements and adding one or more sulfur-scavenging species to an
aluminum alloy composition can result in an anodic oxide having an
even higher resistance to delamination than the micro-alloying
element or sulfur-scavenging species individually. For example,
copper micro-alloying (which minimize zinc enrichment at the
interface) may be used in combination with lithium and/or magnesium
for their sulfur-scavenging ability. Alternatively, one or more
sulfur-scavenging species can be added in lower amounts than would
be used to prevent delamination alone, and one or more
micro-alloying elements can be added to make up for the deficiency
in sulfur-scavenging species, resulting in an anodic film that is
resistant to delamination. This strategy can be used to add lesser
amounts of elements that can cause yellow discoloration, such as
copper, iron, and/or silver.
[0111] FIG. 11 shows flowchart 1100, illustrating a process of
increasing an adhesion strength of an anodic oxide to a
high-strength substrate using a combination of sulfur-scavenging
species and micro-alloying element. At 1102, relative amounts of
one or more sulfur-scavenging species and one or more
micro-alloying elements required to achieve a pre-determined
delamination resistance of an anodic oxide on a high aluminum
strength substrate is determined. The pre-determined delamination
resistance can be a threshold value determined using, for example,
the delamination-resistance methods described in U.S. application
Ser. No. 14/678,881, which is incorporated herein in its entirety.
Additionally or alternatively, the relative amounts of a
sulfur-scavenging species and a micro-alloying element can be
determined by a pre-determined discoloration of the anodic oxide.
For example, the pre-determined discoloration may be not be allowed
to exceed a particular b* value (e.g., b*>1 or b*>0.5).
[0112] At 1104, the one or more sulfur-scavenging species and the
one or more micro-alloying element are incorporated into an
aluminum alloy substrate. At 1106, the aluminum alloy substrate is
anodized such that during anodizing, the sulfur-scavenging species
binds with sulfur species, and the micro-alloying element prevents
at least some of the zinc from enriching at the interface between
the substrate and anodic oxides. The combination of the
sulfur-scavenging species and the micro-alloying element increases
the adhesion strength of the anodic oxide and/or reduces the
discoloration of the anodic oxide compared to using a
sulfur-scavenging species or a micro-alloying element
individually.
[0113] In some cases, the addition of one or more sulfur-scavenging
species and/or one or more micro-alloying elements can reduce the
amount of other alloying elements required to provide the
high-tensile strength to the aluminum alloy. For example, adding
0.05 weight % of copper micro-alloying element has been found to
reduce the amount of zinc required for optimum strengthening by a
corresponding 0.05 weight %, with the resultant aluminum alloy
substrate having substantially the same mechanical properties
(e.g., yield strength of 340-350 MPa, and hardness of 125 HV), as
the alloy having the full concentration of zinc. In a particular
embodiment, this reduces the zinc composition to 5.45 weight %
compared to a nominal 5.5 weight % for the custom alloy 904 of FIG.
9. This can be some benefit since aluminum alloys having higher
zinc composition can present corrosion problems while lower zinc
compositions (without the micro-alloying element) can detrimentally
affect the strength of the alloy substrate. Once the amount of zinc
is fixed, a corresponding amount of magnesium to provide a balanced
alloy can be determined. In some embodiments, the magnesium content
is substantially increased over this balanced amount to provide the
sulfur-scavenging benefits described above or to minimize free zinc
in the matrix, so as to reduce grain-orientation related defects.
In this way, customized anodized alloys having prescribed tensile
strength, anodic oxide delamination resistance, and/or color can be
designed.
[0114] It should be noted that embodiments presented herein can be
used in combination with one or more embodiments described in
related U.S. application Ser. Nos. 14/474,021, 14/593,845,
14/678,881, and 14/678,868, which are incorporated herein in their
entireties. For example, the embodiments described herein may be
used in combination with a post-anodizing heat treatment to diffuse
zinc away from the interface, either for greater robustness, or to
allow for shorter or lower temperature heat treatments to achieve
the same effect.
[0115] The foregoing description, for purposes of explanation, used
specific nomenclature to provide a thorough understanding of the
described embodiments. However, it will be apparent to one skilled
in the art that the specific details are not required in order to
practice the described embodiments. Thus, the foregoing
descriptions of the specific embodiments described herein are
presented for purposes of illustration and description. They are
not targeted to be exhaustive or to limit the embodiments to the
precise forms disclosed. It will be apparent to one of ordinary
skill in the art that many modifications and variations are
possible in view of the above teachings.
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