SURFACE MODIFICATION OF ALUMINUM ALLOY PRODUCTS FOR MICRO-ARC OXIDATION PROCESSES

METHODE DE MODIFICATION DE LA SURFACE DE PRODUITS EN ALLIAGE D'ALUMINIUM POUR PROCEDES D'OXYDATION MICRO-ARC

English Abstract

This invention relates to an effective method of accelerating micro-arc oxidation processes to form a thick, smooth and modified oxide layer on aluminum alloy products. Said method involves the modification of aluminum alloy products by forming a thin, dense and nonporous alumina barrier layer (or dielectric layer) and a modification substance layer. These two layers coexist and together serve as promoting layers which accelerate the subsequent micro-arc oxidation process.

French Abstract

La présente invention a pour objet une méthode efficace pour accélérer des processus d'oxydation sous micro-arc pour former une épaisse couche lisse et modifiée d'oxyde sur des produits en alliage d'aluminium. Ladite méthode fait intervenir la modification des produits en alliage d'aluminium en formant un fine couche barrière dense et non poreuse d'alumine (ou couche diélectrique) et une couche de substance de modification. Ces deux couches coexistent et ensemble servent de couches qui accélèrent le processus ultérieur d'oxydation sous miro-arc.

Claims

6
Claims
The embodiments of the invention in which an exclusive property or privilege
is claimed are defined
as follows:
1. A method for producing a coating on aluminum and aluminum alloy products
comprising the
following order of operations:
(a) forming a thin, dense and nonporous alumina barrier layer having thickness
of 50 - 250
.ANG. on the surface of aluminum and aluminum alloy products;
(b) forming an additional modification substance layer of metal oxides,
carbides, borides,
nitrides and/or silicides, and/or graphite, molybdenum disulfide, tungsten
disulfide, boron
nitride and tin metal, having thickness of 5 - 30 µm on top of the said
alumina barrier layer;
(c) processing the modified aluminum and aluminum alloy products using micro-
arc
oxidation in the micro-arcing region of 450 - 600 V.
2. The method as recited in claim 1 wherein the alumina barrier layer is
formed by using
anodizing, electrolytic oxidation, chemical oxidation, physical vapour
deposition and/or chemical
vapour deposition.
3. The method as recited in claim 1 wherein the modification substance layer
is formed by
using anodizing, electrolytic and/or chemical process, physical vapour
deposition, chemical vapour
deposition and/or powder spray techniques.
4. The method as recited in claim 1 wherein the alumina barrier layer and
modification
substance layer are formed by using a single or multiple treatments.
5. The method as recited in claim 4 wherein conventional anodizing is used to
simultaneously
form an alumina barrier layer and modification substance layer in one
treatment.

Description

CA 02540340 2008-01-16
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Description of the Invention
This invention relates to a surface modification process of aluminum alloy
products using surface
treatment techniques to produce a thin, dense and nonporous alumina barrier
layer and a
modification substance layer which together accelerate the micro-arc oxidation
process.
Background of the Invention
Aluminum and its alloys are extremely desirable metals for cookwares,
automotive components and
machinery parts because they are relatively inexpensive, lightweight (low
density) and have good
conductivity. However, aluminum is very soft, and consequently has poor wear
resistance. As well,
it is chemically active, and thus corrodes easily.
Using various surface treatments, the disadvantageous properties of aluminum
and its alloys can be
improved. For example, a conventional anodizing process to form an oxide
layer, followed by a
sealing treatment, can be used to improve the corrosion resistance of
aluminum. A hard anodizing
process can be used to form an oxide film with hardness values up to a maximum
of HV 600,
drastically improving the wear resistance of aluminum. For this reason, hard
anodizing processes
are being applied to the production of machinery parts, auto brakes, auto
pistons and textile
machinery parts. For applications that require higher wear resistance, such as
aluminum cylinders
and bearings, the hard anodizing process is not sufficient. Therefore, other
surface treatments, such
as ceramic powder coating, hard chromium electroplating and chemical nickel
plating, must be used.
However, because these films are different from the aluminum substrate, many
difficulties are
encountered in the different stages of the manufacturing process, including
ineffective bonding
between the aluminum substrate and its film.
The recent development of the micro-arc oxidation process has provided a
potential solution to these
problems. Using micro-arc oxidation processes, a thick oxide film with HV 2000
can be obtained.
Until now, an effective, technical and economical micro-arc oxidation process
has not been
developed and commercially used. This is due to the high consumption of energy
through long
treatments which use high voltage and high current density. In addition, low
energy efficiency and a
low coating growth rate have made this process less commercially viable.
Furthermore, the oxide
layer formed through this conventional method produces a high percentage of a
soft, rough and

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porous layer which must be removed before using the aluminum alloy product.
This further
increases the cost of micro-arc oxidation processes.
In this invention, the surface of the aluminum product will be modified prior
to the micro-arc
oxidation process. As a result, a hard and thick film can be obtained using
the micro-arc oxidation
process in a very short time. This film is also smoother than that produced
through conventional
micro-arc oxidation processes, potentially removing the need for any
additional machining prior to
commercial usage. This will effectively reduce the costs of the process,
creating a feasible and
economical process. In addition, using this invention, the aluminum oxide film
can be modified to
contain solid lubricants including graphite, molybdenum disulfide and tungsten
disulfide and/or
other hardening materials including metal oxides, carbides, borides, nitrides
and silicides.
Currently, the only preparation step completed prior to using micro-arc
oxidation processing is
surface cleaning of the aluminum product. The diluted alkaline electrolyte
used contains 1-10 g/L
metal hydroxides, such as sodium, potassium, calcium or magnesium, in addition
to 1-10 g/L
metal silicates and aluminates. The pH of the bath is maintained in the range
of 8 to 13. Various
types of power sources, including DC, pulsed DC, symmetrical AC and unbalanced
AC sources are
used, resulting in growth rates in the range of 0.5 - 3.0 m/min.
Summary of the Invention
The first step of this invention involves the formation of a thin (50 - 250
A), dense and nonporous
alumina barrier layer (or dielectric layer) through anodizing, electrolytic
oxidation, chemical
oxidation, physical vapour deposition and/or chemical vapour deposition.
In addition, a modification substance layer, consisting of 5 - 30 m of metal
oxide, carbide, boride,
nitride, silicide and/or solid lubricant or composites, is formed on top of
the alumina barrier layer
using one or more of the following techniques: anodizing, electrolytic and/or
chemical process,
physical vapour deposition, chemical vapour deposition, and powder spray
techniques. The
modified product then undergoes micro-arc oxidation, reaching an appropriate
voltage (450 - 600
V) in the micro-arcing region within the preset short time (usually the first
minute) and maintaining
this voltage range for the duration of the process. Using this process, a
thick, smooth and modified
oxide layer is formed within a short period of time.

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The alumina barrier layer and modification substance layer coexist and both
serve as promoting
layers to accelerate the micro-arc oxidation process. Therefore, the micro-
arcing region can be
reached and maintained within a short time without damaging the aluminum
substrate through high
energy arcing due to high voltage and high current density. This process is
different from
conventional micro-arc oxidation processes which use low current density; high
voltage and low
duty cycle; or apply a high current density within the first minute then drop
it to reasonable levels.
Applying a low current density requires a longer period of time to reach the
effective micro-arcing
region during the micro-arc oxidation process. Applying a high voltage with a
low duty cycle will
prevent plasma damage; however, the low duty cycle will reduce the oxide film
growth. Applying a
high current density within the first minute followed by a drop to the low
preset current density will
also prevent plasma damage but the growth rate is still low because a high
current density is not
maintained.
In addition to more effective micro-arc oxidation, the existence of the
modification substance layer
mentioned in the invention provides many useful functions. First, it provides
a micro-arc fusing
substance which will absorb the localized energy during the process, thus
protecting the aluminum
substrate from plasma damage. Second, in the local micro-arcing area, this
modification substance
layer will locally fuse and solidify and thus incorporate constituents of the
electrolyte and promote
oxide growth. Third, any modifying substances within the modification
substance layer can be
permanently incorporated within the aluminum oxide layer during micro-arc
oxidation processing.
This can be used to provide permanent lubricating or hardening effects, for
example. Fourth, using
this method, the surface of the oxide layer after micro-arc oxidation
processing will be smoother,
denser and harder, potentially removing the need for any additional machining.
In practical applications of this invention, the barrier layer and
modification layer formation can be
done in a single treatment or multiple treatments. For example, in one
treatment of conventional
anodizing, an alumina barrier layer of approximately 100 - 150 A and a porous
modification
aluminum oxide layer of 5- 30 m can be formed for the succeeding micro-arc
oxidation process.
The following three examples illustrate the versatility of the modification
process. The first
example describes a method of forming an alumina barrier layer and porous
modification layer in a
single conventional anodizing process for A1319 cast alloys. Example 2
describes a two-step
process: first, conventional anodizing is used to form a barrier layer and
porous A1203 layer

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followed by electrolytic tin deposition. This porous A1z031ayer, together with
the tin deposits,
forms a composite as the modification substance layer. Example 3 describes a
three-step process:
first, electropolishing is used to form an alumina barrier layer. Next, a
M.B.V. conversion coating
process is used to form a boehmite + solution anion (eg. Cr04, P04). In the
third step, a graphite
powder is applied to the top of the boehmite + solution anion surface to form
the composite as the
modification substance layer. These three examples describe modification
processes which are
followed by micro-arc oxidation processes using a pulsed DC power source, 100%
duty cycle and 1
- 2 kHz frequency.
Example 1:
An A1319 cast cylindrical specimen, 5 cm in diameter and 8 cm long, was
cleaned. The specimen
went through a conventional anodizing process for 25 minutes using an 18 wt%
sulfuric acid
solution at 15V. The specimen was rinsed with water before undergoing a micro-
arc oxidation
process in a solution containing 2- 3 g/L KOH and 2 - 5 g/L Na2SiO3. The power
was switched on
and raised to 550V within the first minute and maintained this constant
voltage for 5 minutes. At
the end of the process, the electric power was switched off; the specimen was
disconnected from the
anode, removed from the electrolyte tank, cleaned with warm water and dried
with warm air. A
smooth oxide coating with average thickness of 110 m was measured along the
curved edge of the
specimen.
Example 2:
An A16061 cylindrical bar sample, 5 cm in diameter and 8 cm long, was cleaned
and put through a
conventional anodizing process at room temperature using an 18 wt% sulfuric
acid electrolyte for 30
minutes, resulting in a total oxide film (barrier and porous) of 20 m. The
specimen was rinsed
thoroughly with water before undergoing a tin deposition process for 1-3
minutes in a room
temperature solution containing 2 - 15 g/L stannous sulfate, 10 - 20 g/L
sulfuric acid and 8 - 12 g/L
tartaric acid. The specimen was rinsed again before undergoing the micro-arc
oxidation process.
The voltage was raised to 560 - 570 V within the first minute and maintained
for 5 minutes in the
electrolyte described in Example 1. A smooth oxide coating with average
thickness of 120 m was
measured along the curved edge of the specimen.

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Example 3:
An Al 6061 cylindrical bar sample, 5 cm in diameter and 8 cm long, was cleaned
and put in a
solution of 80% phosphoric acid and 10% n-butyl alcohol at 50 - 60 C for
electropolishing.
Depending on the applied voltage, a barrier layer of 50 - 100 A can be formed.
The specimen was
rinsed with water before undergoing a 1- 5 minute chemical conversion coating
process using a
M.B.V. solution of 3% sodium carbonate and 1% sodium chromate. Afterwards,
graphite powder, a
solid lubricant, was applied to the top of the surface before undergoing the
micro-arc oxidation
process. The voltage was raised to the micro-arc region at 500V within the
first minute and
maintained for 5 minutes. A smooth oxide layer with average thickness of 125
m was found.
Under microscope examination, graphite was found to be imbedded within the
oxide layer,
providing a permanent self-lubricating effect.