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.
Many pistons used today are made from hypoeutectic aluminum-silicon alloys like SAE 332 which contains 8½ to 10½ percent silicon, eutectic aluminum-silicon alloy pistons which have 11 to 12 percent silicon, or hypereutectic alloys that have 12½ 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.
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.
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.
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.
Exemplary embodiments of the invention will become more fully understood from the detailed description and the accompanying drawings, wherein:
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.
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.
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
Referring first to
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 μm 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.
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.
In a friction stir processing process (FSP), in accordance with one exemplary embodiment as shown in
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.
In the work piece 20 of
In one exemplary embodiment for transforming the cast microstructure 27, as shown in
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
To confirm the results, both surface roughness and uniformity of the oxide layer 35 of
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.
While the exemplary embodiments of
Referring now to
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.
One exemplary product that may utilize the exemplary process described above in
More specifically, in one exemplary embodiment, the piston 60, being a cylindrical work piece similar in shape to the cylindrical work piece 120 of
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.
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.
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.
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
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.
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.