U.S. patent application number 12/552510 was filed with the patent office on 2011-03-03 for methods of reducing surface roughness and improving oxide coating thickness uniformity for anodized aluminum-silicon alloys.
This patent application is currently assigned to GM GLOBAL TECHNOLOGY OPERATIONS, INC.. Invention is credited to Yen-Lung Chen, Hong-Hsiang Kuo, Yar-Ming Wang.
Application Number | 20110048958 12/552510 |
Document ID | / |
Family ID | 43623240 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110048958 |
Kind Code |
A1 |
Wang; Yar-Ming ; et
al. |
March 3, 2011 |
METHODS OF REDUCING SURFACE ROUGHNESS AND IMPROVING OXIDE COATING
THICKNESS UNIFORMITY FOR ANODIZED ALUMINUM-SILICON ALLOYS
Abstract
In one exemplary method, an anodized aluminum-silicon alloy work
piece may be formed from an aluminum-silicon alloy substrate
material by applying a friction stir processing treatment to the
aluminum-silicon alloy substrate material to reduce an average
particle size of a plurality of silicon particles contained within
the substrate material while increasing a size uniformity of the
plurality of silicon particles, and subsequently anodizing said
aluminum-silicon alloy substrate material.
Inventors: |
Wang; Yar-Ming; (Rochester
Hills, MI) ; Chen; Yen-Lung; (Troy, MI) ; Kuo;
Hong-Hsiang; (Troy, MI) |
Assignee: |
GM GLOBAL TECHNOLOGY OPERATIONS,
INC.
Detroit
MI
|
Family ID: |
43623240 |
Appl. No.: |
12/552510 |
Filed: |
September 2, 2009 |
Current U.S.
Class: |
205/206 ;
205/205 |
Current CPC
Class: |
C25D 11/04 20130101;
B23K 20/1215 20130101; C25D 5/44 20130101; C25D 11/16 20130101;
B23K 2103/10 20180801 |
Class at
Publication: |
205/206 ;
205/205 |
International
Class: |
C25D 11/16 20060101
C25D011/16; C25D 5/44 20060101 C25D005/44 |
Claims
1. A method for forming an anodized aluminum-silicon alloy work
piece comprising: providing a cast aluminum-silicon alloy substrate
material having a plurality of silicon particles; applying a
friction stir processing treatment to said cast aluminum-silicon
alloy substrate material to reduce an average particle size of said
plurality of silicon particles while increasing a size uniformity
of said plurality of silicon particles; and subsequently anodizing
said cast aluminum-silicon alloy substrate material to form an
oxide coating layer on said aluminum-silicon alloy substrate
material.
2. The method of claim 1, wherein said cast aluminum-silicon alloy
substrate material comprises a cast hypoeutectic aluminum-silicon
alloy substrate material.
3. The method of claim 1, wherein said cast aluminum-silicon alloy
substrate material comprises a cast eutectic aluminum-silicon alloy
substrate material.
4. The method of claim 1, wherein said cast aluminum-silicon alloy
substrate material comprises a cast hypereutectic aluminum-silicon
alloy substrate material.
5. The method of claim 1, wherein applying a friction stir
processing treatment to said cast aluminum-silicon alloy substrate
material comprises: providing a friction stir tool having a
profiled pin; rotating said friction stir tool at a desired
rotational speed; and plunging said rotating profiled pin into an
outer surface of said cast aluminum-silicon alloy substrate
material to alter a microstructure of said substrate material to
reduce an average particle size of said plurality of silicon
particles and to provide a narrower size distribution of said
plurality of silicon particles.
6. The method of claim 5 further comprising: traversing said cast
aluminum-silicon alloy substrate material in a first direction at a
first travel speed such that said rotating profiled pin friction
stirs an additional portion of said outer surface.
7. The method of claim 5 further comprising: rotating said cast
aluminum-silicon alloy substrate material at a first rotational
speed such that said rotating profiled pin friction stirs an
additional portion of said outer surface, wherein said cast
aluminum-silicon alloy substrate material comprises a cylindrical
cast aluminum-silicon alloy substrate material.
8. The method of claim 1, wherein subsequently anodizing said
aluminum-silicon alloy substrate material to form an oxide coating
layer on said cast aluminum-silicon alloy substrate material
comprises anodizing said aluminum-silicon alloy substrate material
using a Type III anodizing process to form an oxide coating
layer.
9. The method of claim 8, wherein said average particle size of
said plurality of silicon particles is about 2 micrometers.
10. The method of claim 1, wherein subsequently anodizing said cast
aluminum-silicon alloy substrate material to form an oxide coating
layer on said aluminum-silicon alloy substrate material comprises
anodizing said aluminum-silicon alloy substrate material using a
Type II anodizing process to form an oxide coating layer.
11. A method for forming an anodized aluminum-silicon alloy piston
comprising: casting a piston having at least one ring groove area
from an aluminum-silicon alloy material, said aluminum-silicon
alloy material including a plurality of silicon particles; applying
a friction stir processing treatment to said ring groove area to
reduce an average particle size of said plurality silicon particles
and to increase the size uniformity of said plurality of silicon
particles; machining ring grooves in the ring groove area; and
anodizing said ring groove area to form an oxide coating layer.
12. The method of claim 11, wherein applying a friction stir
processing treatment to said ring groove area comprises: providing
a friction stir tool having a profiled pin; rotating said friction
stir tool at a desired rotational speed; coupling said piston to a
rotating device; rotating said piston at a desired rotational rate;
and plunging said rotating profiled pin into an outer surface layer
of said ring groove area to alter a microstructure of said ring
groove area to reduce an average particle size of said plurality of
silicon particles and to provide a narrower size distribution of
said plurality of silicon particles.
13. The method of claim 11, wherein anodizing said ring groove
comprises anodizing said ring groove using a Type II anodization
process.
14. The method of claim 13, wherein anodizing said ring groove
using a Type II anodization process comprises: introducing said
piston to a room temperature sulfuric acid bath; and applying a
voltage to said room temperature sulfuric acid bath for a period of
time sufficient to form a barrier oxide layer of a desired
thickness on said ring groove.
15. The method of claim 14, wherein said desired thickness is about
15 micrometers.
16. The method of claim 11, wherein anodizing said ring groove
comprises anodizing said ring groove using a Type II anodization
process.
17. A method for reducing barrier oxide coating layer surface
roughness and increasing barrier oxide coating layer thickness
uniformity in an anodized aluminum-silicon alloy work piece, the
method comprising: providing a cast aluminum-silicon alloy
substrate material having a cast microstructure, said cast
microstructure including a plurality of silicon particles; and
applying a friction stir process to a surface of a cast
aluminum-silicon alloy substrate material prior to anodization to
transform said cast microstructure to a wrought microstructure,
wherein said transformation also reduces an average particle size
of said plurality of silicon particles within said wrought
microstructure and narrows a size distribution of said plurality of
silicon particles.
18. The method of claim 17, wherein applying a friction stir
processing treatment comprises: providing a friction stir tool
having a profiled pin; rotating said friction stir tool at a
desired rotational speed; and plunging said rotating profiled pin
into an outer surface layer of said cast aluminum-silicon alloy
substrate material at sufficient force to alter said cast
microstructure to a wrought microstructure.
Description
TECHNICAL FIELD
[0001] The field to which the disclosure generally relates includes
surface treatment methods and more specifically relates to methods
for reducing surface roughness and improving oxide coating
thickness uniformity for anodized aluminum-silicon alloys.
BACKGROUND
[0002] Many pistons used today are made from hypoeutectic
aluminum-silicon alloys like SAE 332 which contains 81/2 to 101/2
percent silicon, eutectic aluminum-silicon alloy pistons which have
11 to 12 percent silicon, or hypereutectic alloys that have 121/2
to over 16 percent silicon (e.g. B390). Silicon improves high heat
strength and reduces the coefficient of expansion so tighter
tolerances can be held as temperatures change. The piston surface
is often hard anodized to improve wear and scuffing resistance.
[0003] During anodizing, since silicon particles are non-reactive,
the aluminum oxide coating grows around the silicon sites. Due to
the large silicon particle size and non-uniform size distribution,
the typical 15 to 20 micron thick coating has high surface
roughness and non-uniform coating thickness, which may not be
overcome by changing anodizing parameters. The combination of high
surface roughness and non-uniform coating thickness are thought to
contribute to decreased wear durability of the piston surface
during use, and may contribute to premature piston failure.
SUMMARY OF EXEMPLARY EMBODIMENTS OF THE INVENTION
[0004] In one exemplary method, an anodized aluminum-silicon alloy
work piece may be formed from a cast aluminum-silicon alloy
substrate material by applying a friction stir processing
(hereinafter referred to as FSP) treatment to the cast
aluminum-silicon alloy substrate material to reduce an average
particle size of a plurality of silicon particles contained within
the substrate material while increasing a size uniformity of the
plurality of silicon particles, and subsequently anodizing the FSP
treated cast aluminum-silicon alloy substrate material.
[0005] Other exemplary embodiments of the invention will become
apparent from the detailed description provided hereinafter. It
should be understood that the detailed description and specific
examples, while disclosing exemplary embodiments of the invention,
are intended for purposes of illustration only and are not intended
to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] Exemplary embodiments of the invention will become more
fully understood from the detailed description and the accompanying
drawings, wherein:
[0007] FIG. 1 is a schematic representation of a cross-section of a
relatively flat B390 aluminum alloy substrate material magnified to
20 micrometers after conventional anodization;
[0008] FIG. 2 is a perspective view of a friction stir process for
modifying the surface of a relatively flat aluminum alloy work
piece prior to anodization according to an exemplary
embodiment;
[0009] FIG. 3 is a schematic representation of a cross-section of
an B390 aluminum alloy substrate material magnified to 20
micrometers after friction stir processing according to FIG. 2 and
anodization according to an exemplary embodiment;
[0010] FIG. 4 is a perspective view of a friction stir process for
modifying the surface of a cylindrical aluminum alloy work piece
prior to anodization according to another exemplary embodiment;
[0011] FIG. 5 is a perspective view of a piston having ring-grooves
that are FSP treated and anodized in accordance with an exemplary
method;
[0012] FIG. 6A is a graphical illustration plotting size range of
silicon particles versus frequency for an anodized aluminum alloy
piston without FSP treatment; and
[0013] FIG. 6B is a graphical illustration plotting size range of
silicon particles versus frequency for an anodized aluminum alloy
piston of similar composition having FSP treatment.
DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS
[0014] The following description of the embodiment(s) is merely
exemplary (illustrative) in nature and is in no way intended to
limit the invention, its application, or uses.
[0015] The exemplary embodiments describe a method for reducing
surface roughness in aluminum-silicon alloy work pieces in which a
hard-anodized coating, preferably using either a Type II or Type
III anodizing process, has been formed on the underlying substrate
material to achieve hardness, high thermal insulation, corrosion
resistance, decreased friction, and increased wear resistance.
[0016] Exemplary embodiments of aluminum-silicon alloys may
include, but are not limited to, material such as a hypoeutectic
aluminum-silicon alloy like SAE 332 which includes between about
8.5 and 10.5 percent silicon, a eutectic aluminum-silicon alloy
which includes between about 11 to 12 percent silicon, or a
hypereutectic aluminum-silicon alloy that includes between about
12.5 to over 16 percent silicon. One exemplary hypereutectic
aluminum-silicon alloy that may be used as the substrate material,
as shown below in FIGS. 1 and 3, is aluminum alloy B390, which
includes silicon particles having a particle size of up to about 40
microns in diameter. However, additional alloying elements may be
included in the aluminum-silicon alloys.
[0017] Referring first to FIG. 1, a magnified schematic
representation of a prior art cast aluminum-silicon alloy work
piece 20, anodized with a Type II anodization process, may be
illustrated as including a substrate layer 21 having a cast
microstructure 27 that may include aluminum-containing portion 22
and silicon particles 23 of a predetermined size range having
non-uniform size distribution. The silicon particles 23 may be
classified as having larger particles, or primary particles, that
may approach about 40 micrometers in diameter, and as having
smaller particles, or secondary particles, that may include silicon
flakes.
[0018] As one of ordinary skill recognizes, during a conventional
anodizing process such as a Type II process, elemental aluminum and
possibly other alloying elements of the aluminum-containing portion
22 close to the surface 26 of the cast substrate material 21 may
react with oxygen to form a barrier oxide layer, or oxide coating
layer 24, of varying thickness, depending upon the anodization
conditions, but typically ranges between about 5-15 .mu.m thick.
Given the fact that the silicon particles 23 do not react in the
anodization process, the resultant oxide coating layer 24 may
therefore have non-uniform film thickness as a result of the random
distribution of large (primary) and small (secondary) silicon
particles 23, and may also have high surface roughness. This high
surface roughness and non-uniform film thickness is thought to
contribute to reduced durability characteristics.
[0019] The exemplary embodiments utilize a friction stir processing
(FSP) treatment prior to anodizing to reduce the average particle
size of the silicon particles and provide a more uniform and narrow
distribution of silicon particles, thereby reducing surface
roughness and increasing the uniformity of film thickness for the
oxide coating layer 24. This is thought to improve the wear
resistant properties of the resultant anodized work piece.
[0020] In a friction stir processing process (FSP), in accordance
with one exemplary embodiment as shown in FIG. 2, a rotating
friction stir tool 30 having a profiled pin 32 may be rotated at a
predetermined rotational speed (signified by arrow 41) and pressed
into substrate material 21 of the cast work piece 20 at a
predetermined force (shown by arrow 43). In this exemplary process,
the tool 30 may be held in place while the non-anodized substrate
material 21 traverses (i.e. the non-anodized substrate material 21
is moved in a desired direction at a predetermined travel rate,
shown by arrow 45, to allow the profiled pin 32 to contact and
penetrate the surface 26 of the substrate material 21 at any
desired surface location for a desired amount of time).
Alternatively, the rotating tool 30 may traverse across a
stationary substrate 21. The predetermined rotational speed and
force of the friction stir action of the profiled pin 32, coupled
with the predetermined travel rate of the substrate material 21,
create heat, which in turn soften the surface 26 of the substrate
material 21 and force it to flow, generally without melting the
substrate 21. The force may also break down the silicon particles
23 to smaller, and more uniformly distributed, silicon particles.
The original microstructure 27 of the substrate material 21 may
thereby be modified to a wrought microstructure (shown as 32 in
FIG. 3) prior to anodization. The process that occurs in FIG. 2 to
modify the microstructure may be done in a single pass or in
multiple passes. In another exemplary process, multiple overlapping
passes may be utilized.
[0021] The shape of the profiled pin 32 may vary to achieve a
desired result, but generally may include a stepped spiral feature
(not shown) including a shoulder at a particular pin height that
may be preferred for silicon particle 23 breakdown. The pin height
and shoulder diameter of the profiled pin 32 may be adjusted to
achieve a desired depth with the surface 26 of the substrate
material 21 to achieve the desired silicon particle breakdown at
the given pin 32 rotational speed and substrate material traverse
speed.
[0022] In the work piece 20 of FIG. 1, for example, the cast
microstructure 27 of the substrate material 21 may be broken up and
refined during FSP prior to anodization, resulting in a modified
substrate material 31 having a wrought microstructure 39, as shown
in FIG. 3, which includes smaller and more uniformly distributed
silicon particles 33 Furthermore, the porosity is greatly reduced
or eliminated in the resultant wrought microstructure 39 as
compared to the microstructure 27 of the substrate material 21 of
FIG. 1.
[0023] In one exemplary embodiment for transforming the cast
microstructure 27, as shown in FIG. 1 without FPS treatment after
Type II anodization, to form the wrought microstructure 39, as
shown in FIG. 3 with FSP treatment and Type II anodization, the
predetermined rotational speed for the friction stir tool 30 of
FIG. 2 may be approximately 2000 rpm with a travel speed of about 6
mm/s. More preferably, a second overlapping pass in the same or
opposite direction may be utilized to further reduce the silicon
particle size and obtain more uniform microstructure. Further, in
this example, the profiled pin 32 had a shoulder diameter of about
12 millimeters and a pin height of 2.5 millimeters.
[0024] The FSP-treated substrate 31 may then be anodized with a
Type II sulfuric acid anodizing process (not shown), similar to
that for the substrate material 21 of FIG. 1, to form an anodized
work piece 39 having the oxide layer 35, as shown in FIG. 3. The
resultant oxide layer 35 may achieve decreased oxide surface
roughness and increased oxide layer thickness consistency as
compared with the oxide layer 24 of FIG. 1, wherein each work piece
20 or 39 was formed from similar starting materials, here a B390
aluminum-silicon alloy, and processed in the same manner with the
exception of the afore-mentioned.
[0025] To confirm the results, both surface roughness and
uniformity of the oxide layer 35 of FIG. 3 were measured and
compared with the oxide layer 24 of FIG. 1 without FSP treatment.
The FSP-treated anodized oxide layer 35 showed substantial
improvement in both oxide layer thickness consistency and
smoothness that is believed to be due to the refinement of the
silicon particles in both size and size distribution. By reducing
the oxide surface roughness and increasing the oxide thickness
uniformity, an increase in wear resistant properties of the oxide
coating layer 35 may be realized, as well as improvements of other
properties such as corrosion resistance and thermal insulating
ability.
[0026] Moreover, by adjusting the working parameters of the
friction stir processing treatment and the ensuing anodization
process, one can tailor the anodized coating property, in terms of
the afore-mentioned material characteristics and material
properties, including coating uniformity, roughness, corrosion
resistance, and thermal insulation characteristics. Adjustments in
working parameters to adjust these properties and characteristics
include adjusting the tool design (such as the pin height, pin
profile and shoulder diameter), adjusting the number of FSP passes
and/or the rotational speed of the friction stir tool 30, and/or
adjusting the applied pressure (force) on the surface 26 of the
substrate material 21 on any given pass or passes.
[0027] While the exemplary embodiments of FIGS. 1-3 may be
illustrated for a relatively flat work piece, other exemplary
embodiments may utilize other shaped work pieces. For example, as
shown in FIG. 4, a cylindrical work piece 120 may be FSP treated
and anodized in substantially the same manner as the work pieces
20, 21 and 39 in FIGS. 1-3 respectively.
[0028] Referring now to FIG. 4, a cylindrical work piece 120 of the
same composition as work piece 20 of FIG. 1 may be mounted on a
rotating structure (not shown) and rotated at a predetermined
rotational speed (as shown by arrows 121). The mounted rotating
friction stir tool 130 having a profiled pin 132 may be rotated at
a predetermined rotational speed (signified by arrow 141) and
pressed into surface 126 of the cylindrical work piece 120 at a
predetermined force (shown by arrow 143). In the same manner
described above with respect to the flat work piece 20, the
microstructure of the cylindrical work piece 120 may be broken up
and refined during FSP, resulting in a modified substrate material
having a wrought microstructure, which includes smaller and more
uniformly distributed silicon particles similar to particles 33 in
FIG. 3. Furthermore, the porosity in the resultant wrought
microstructure of the FSP-treated anodized material may be greatly
reduced or eliminated as compared to the microstructure of the
non-FSP-treated anodized substrate material. Subsequent anodization
of the cylindrical work piece 120 forms an oxide coating layer on
the exterior of the cylindrical work piece, similar to oxide layer
35 in FIG. 3, having reduced surface roughness and improved coating
thickness uniformity as described above with respect to the
anodized work piece 39 of FIG. 3.
[0029] Exemplary products that may benefit from such a reduction in
surface roughness and an increase in oxide layer thickness
uniformity, that utilize cast aluminum-silicon alloy work pieces,
include but are not limited to automotive parts such as
pistons.
[0030] One exemplary product that may utilize the exemplary process
described above in FIG. 3, as shown in FIG. 5, is a piston 60
having a ring groove area 62. Here, the ring groove area 62 has
been FSP-treated, ring grooves machined out and subsequently
anodized using a Type II anodization technique as will be described
in further detail below.
[0031] More specifically, in one exemplary embodiment, the piston
60, being a cylindrical work piece similar in shape to the
cylindrical work piece 120 of FIG. 4 and similar in composition to
the substrate material 21 of FIGS. 1 and 3, may be mounted on a
rotating structure (not shown) and rotated at a desired rotational
speed. A mounted rotating friction stir tool, similar to the
friction stir tool 130 of FIG. 4 and having a profiled pin similar
to the profiled pin 132 of FIG. 4, may be rotated at a
predetermined rotational speed and pressed into piston groove area
62. The microstructure of the piston groove area 62 may be broken
up and refined during the FSP treatment, resulting in a modified
piston groove area 62 having a wrought microstructure with smaller
and more narrowly uniformly distributed silicon particles, and is
thus similar to the microstructure 39 of FIG. 3.
[0032] In an alternative embodiment, the ring groove area 62 may be
machined prior to the FSP treatment, and then finish-machined to
remove the flash formed during the FSP treatment and obtain the
desired ring groove geometry.
[0033] The surface of the piston groove area 62 may then be
anodized through a Type III hard-anodizing process, resulting in a
piston ring groove area 62 having reduced coating roughness and
improved coating uniformity.
[0034] In a Type III anodizing process, the piston 60 is introduced
to a room temperature bath containing 100 g/L sulfuric acid at a 20
volt applied voltage. An aluminum oxide coating of about 15
micrometers in thickness is achieved, having silicon particles with
an average particle size of 2 micrometers or less with a relatively
narrow size distribution. As one of ordinary skill recognizes, a
Type II anodizing process utilizes lower bath temperatures, higher
voltages, and lower concentrations of sulfuric acid than a
conventional Type II anodizing process, and therein forms a barrier
oxide layer 35 that is thicker (0.001-0.003 inches) than Type II
anodized barrier layers 24. In addition, Type II oxide barrier
layers may penetrate approximately 0.001 inches into the underlying
substrate material, therein forming a integral, harder barrier
layer.
[0035] A comparison of an anodized piston 60, with and without the
FSP treatment and with a subsequent Type III anodization, showed
silicon particle size reduction from about 6 micrometers to about
1.4 micrometers with the FSP treatment. In addition, as shown in
FIGS. 6A and 6B, a comparison of the silicon particle size
distribution without (FIG. 6A) and with (FIG. 6B) the FSP treatment
followed by the Type III anodization confirmed a more uniform
silicon particle distribution within the aluminum oxide layer when
FSP treatment was utilized.
[0036] In either related embodiment, the piston 60 having the
FSP-treated anodized piston groove area 62 may avoid or reduce
microwelding that may occur between a piston ring and the piston
60. In addition, the FSP anodized ring-groove 62 offered
significantly improved durability as compared with a conventional
anodized ring-groove, therein reducing piston failure and reducing
costs associated with replacement or repair.
[0037] The above description of embodiments of the invention is
merely exemplary in nature and, thus, variations thereof are not to
be regarded as a departure from the spirit and scope of the
invention.
* * * * *