CHEMICAL POLISHING OF ALUMINUM
A highly polished surface on an aluminum substrate is formed using any number of machining processes. During the machining process, intermetallic compounds are typically generated at a top surface area of the aluminum substrate caused by spot heat generated between the tool edge and the cut tip of the aluminum substrate during the cutting process. The intermetallic compounds can leave surface imperfections after conventional mechanical polishing operations that render the surface of the aluminum substrate difficult to obtain a desired high glossiness due to exfoliation of the intermetallic compounds from the top surface. In order to remove the effect of the intermetallic compounds, an acid etching solution is applied to the surface resulting in removal of intermetallic compounds across a surface portion of the aluminum substrate.
Latest Apple Patents:
- MEASUREMENT BEFORE RADIO LINK FAILURE
- TECHNOLOGIES FOR DISCARDING MECHANISM
- DETERMINATION AND PRESENTATION OF CUSTOMIZED NOTIFICATIONS
- Mesh Compression with Base Mesh Information Signaled in a First Sub-Bitstream and Sub-Mesh Information Signaled with Displacement Information in an Additional Sub-Bitstream
- Systems and methods for performing binary translation
1. Technical Field
The described embodiments relate generally to surface treatment of metals. In particular, acid etching of an aluminum substrate is described.
2. Related Art
In some cases extruded aluminum blocks require a machining process to be applied to achieve a shape more closely resembling a desired geometry. During that machining process, intermetallic compounds are typically generated at a top surface area of the aluminum block caused by spot heat generated between the tool edge and the cut tip of the aluminum block during the machining process. The intermetallic compounds can cause surface imperfections (referred to as “orange peels”) to be left behind after conventional polishing. These orange peels render the surface of the aluminum block difficult to polish to a desired high glossiness and miller surface due to exfoliation of the intermetallic compounds from the top surface during mechanical polishing operations.
Therefore, what is desired is a technique for polishing aluminum parts in a manufacturing efficient manner.
SUMMARYThis paper describes various embodiments that relate to a method, system, and computer readable medium for non-mechanical polishing of aluminum.
In a first embodiment a method of polishing a surface of an aluminum part is disclosed. The method includes acid etching the aluminum part, which includes at least the following steps: (1) supporting the aluminum part with a metal plate; (2) immersing the metal plate into a mixed acid bath of nitric and phosphoric acid at a temperature of about 60° C. to about 75° C.; and (3) vibrating the metal plate in the acid bath for between about 5 minutes and about 15 minutes. During the acid etching the immersed metal plate acts as an anode creating a galvanic potential gap through the mixed acid bath to the aluminum part which acts as a cathode. This results in electron concentration on a number of convex protrusions occurring across a surface portion of the aluminum part that are subsequently dissolved in the mixed acid bath, thereby improving surface quality of the aluminum part.
In another embodiment an acid etching assembly is disclosed. The acid etching assembly includes at least the following: (1) a number of metal plates, each metal plate configured to support a plurality of aluminum parts; (2) a plate holder configured to support a number of metal plates; (3) an acid etching tank containing a mixed acid bath; (4) a heat exchanger configured to heat the mixed acid bath to a temperature of about 60° C. to about 75° C.; and (5) a vibration apparatus configured to vibrate the plate holder when the plate holder is positioned within the acid etching tank. The mixed acid bath inside the acid etching tank includes the following acids by weight percentage: 66-71 percent phosphoric acid (H3PO4); and 5-9 percent nitric acid (HNO3). A galvanic potential gap develops between the plurality of metal plates and the plurality of aluminum parts when immersed in the mixed acid bath causing an electro-polishing operation to be applied to a surface portion of the plurality of aluminum parts.
In yet another embodiment a non-transitory computer readable medium for storing computer instructions executed by a processor is disclosed. The non-transitory computer readable medium includes at least the following: (1) computer code for preparing a mixed acid bath within an acid etching tank, where the mixed acid bath includes 66-71 percent by weight phosphoric acid (H3PO4), and 5-9 percent by weight nitric acid (HNO3); (2) computer code for setting a temperature of the mixed acid bath to a temperature of about 60° C. to about 75° C.; (3) computer code for immersing an aluminum part supported by a titanium plate within the mixed acid bath for a time of about 5 to 15 minutes; and (4) computer code for vibrating the titanium plate while it is immersed within the mixed acid bath.
The embodiments will be readily understood by the following detailed description in conjunction with the accompanying drawings, wherein like reference numerals designate like structural elements, and in which:
Representative applications of methods and apparatus according to the present application are described in this section. These examples are being provided solely to add context and aid in the understanding of the described embodiments. It will thus be apparent to one skilled in the art that the described embodiments may be practiced without some or all of these specific details. In other instances, well known process steps have not been described in detail in order to avoid unnecessarily obscuring the described embodiments. Other applications are possible, such that the following examples should not be taken as limiting.
In the following detailed description, references are made to the accompanying drawings, which form a part of the description and in which are shown, by way of illustration, specific embodiments in accordance with the described embodiments. Although these embodiments are described in sufficient detail to enable one skilled in the art to practice the described embodiments, it is understood that these examples are not limiting; such that other embodiments may be used, and changes may be made without departing from the spirit and scope of the described embodiments.
In some cases extruded aluminum parts can be formed in a shape closely matching the geometry of a finished part. Unfortunately the extrusion process generally results in surface cuts and nicks exceeding a maximum depth where polishing and surface finishing alone does not fully remove such flaws. A standard practice in industry is to extrude the blocks at a size somewhat larger than otherwise desired so that a machining process can be applied to effectively remove large defects in the aluminum part. The surface machining process can include machining features into the surface of the aluminum part such as rounded corners. Alternatively corners can also be rounded at later points in a finishing operation by for example a sanding operation. Even though the machining process tends to reduce the occurrence of large defects or pits in the surface of the aluminum part, the machining process can still leave significant ridges and tooling lines that can make polishing problematic. In this case an acid bath process can be introduced in which machining artifacts such as ridges or pits are removed or substantially reduced. An acid bath can also be effective at removing any oxide layer that has formed over the aluminum. While the acid bath does smooth the overall surface of the part a collection of intermetallic compounds can still remain embedded in the surface of the aluminum part as a result of the surface machining process. During the surface machining process, intermetallic compounds are typically generated at a top surface area of the aluminum substrate caused by spot heat generated between a tool edge and a cut tip of the aluminum part during the cutting process. The intermetallic compounds formed are generally along the lines of an Aluminum Iron alloy such as Al3Fex. In some cases trace amounts of Silicone can also be found in the intermetallic compounds. Intermetallic compounds tend to have small grains and also tend to include numerous different alloys, thereby resulting in a surface portion which can include widely different material properties along an outer surface portion of the aluminum part. Consequently, surface processing the aluminum parts can be quite difficult due to these differing material properties. Conventional machining operations can result in the formation of surface imperfections (sometimes referred to as orange peels) that tend to flake off or exfoliate during a mechanical polishing operation. A relatively wide beam electron beam polishing process can be used to dissolve or evaporate intermetallic compounds within 10-20 microns of the surface of the aluminum part. In this way a substantially homogenous surface can be created along the surface of the aluminum part making subsequent polishing or buffing much easier to achieve.
In one embodiment a chemical etching process can be utilized. The chemical etching process involves inserting the aluminum part into an acid bath. The acid bath can have the effect of removing surface artifacts (such as burrs) and surface oxides. In one particular arrangement, the acid bath process can include supporting a number of aluminum parts in a plate formed of an alloy of titanium, graphite or mild steel. The plate (and part therein) can then be immersed into an acid bath that includes a solution of phosphoric acid (H3PO4:66-71 wt %), and nitric acid (HNO3 :5-9 wt %) at a temperature of about 60° C. to about 75° C. A galvanic potential gap voltage between the aluminum part and the metal alloy cage generates an electron concentration on a portion of the aluminum part that is then dissolved in the mixed acid by vibrating the metal alloy cage holder in the acid bath for between about 5 and 15 minutes. Because the chemical etching process is not mechanical in nature it does not tend to cause the orange peeling process generally associated with intermetallic compounds and mechanical polishing. In some applications the chemical etching process can provide a sufficiently smooth surface finish to forgo additional surfacing operations. In this case a protective layer such as an anodization layer can be applied to the polished aluminum surface subsequent to the chemical etching process.
In another embodiment an electronic beam polishing step can be performed subsequent to the chemical etching step. While electron beams have been used to polish steel and titanium alloys they have not been previously used to polish aluminum. Electron beams configured to polish titanium and steel generally have a diameter of about 0.5 mm while it has been discovered in the case of aluminum a diameter of between about 20 mm and 30 mm is more appropriate due in part to the softness of aluminum. In many cases additional cooling is also required to keep the aluminum from being excessively heated. One such case is when an aluminum enclosure requires machining In such a case a water cooled heat exchanger can be required to prevent the enclosure from deforming due to heat buildup caused by the electron beam. The electron beam accomplishes its polishing step by scanning across the surface of an aluminum part. During its scan the electron beam heats the surface of the aluminum part to a temperature sufficient to cause evaporation or dissipation of intermetallic compounds embedded within the surface of the aluminum part. In this way a substantially homogenous surface can be created free of intermetallic compounds down to a depth of about 10-20 microns. The electron beam is also capable of affecting off-axis surfaces; however, at about 30 degrees performance drops off quickly. By attaching the aluminum parts to a jig during the electron beam processing step various surfaces of the aluminum part can be re-oriented towards the electron beam during the polishing process, thereby allowing multiple faces of an aluminum part to be electron beam polished at any given time. Furthermore, the off-axis beam performance also allows polishing of surfaces with significant curvatures. In one set of trials only about 1 minute of exposure was required to properly polish a batch of aluminum parts. After the electron beam polishing operation is complete a mechanical buffing and polishing operation can be initiated since the intermetallic compounds have been removed from the surface of the aluminum part. This allows the surface of the aluminum part to attain a high glossiness and miller surface.
These and other embodiments are discussed below with reference to
Equation 1
2 Al+6 HNO3→Al2O3+3 H2O+6NO2
After an aluminum oxide layer forms the phosphoric acid dissolves the oxidized aluminum (Al2O3). The cycle of oxidation and dissolving continues while aluminum blocks 102 are immersed in the mixed acid bath; however, since a natural galvanic potential gap voltage of 0.8V exists between aluminum blocks 102 and titanium alloy plates 104 removal of electrons from the surface of aluminum blocks 102 are as previously stated concentrated along convex protrusion resulting in substantial polishing of the affected surface. Such a natural electro-polishing process can produce a more homogenous surface finish of the aluminum when compared to a more conventional electro-polishing process. In more conventional electro-polishing processes an externally applied current can be inadvertently concentrated non-uniformly resulting in undesirable surface variation. Acid etching step 220 can be carried out for a duration of between about 5 and 25 minutes. During that time electron concentrations can be removed from a surface portion of aluminum parts 102. Profile normalization results are further detailed in
While the ionization based chemical etching process can substantially improve a surface finish across a surface portion of aluminum parts 102 it does not solve the problem of intermetallic particles arranged along the surface, since the acid etch comes in contact only with a surface portion of aluminum parts 102. Subsequent mechanical polishing across the surface of an acid etched aluminum part 102 can still result in exfoliation of intermetallic particles embedded just below a surface portion of aluminum parts 102, thereby preventing or at least substantially hindering an effective mechanical polishing operation. In cases where a finer polish is required than can be obtained by the ionization based chemical etching process a subsequent electron beam polishing process can be applied.
In
Electronic device 1300 can also include user input device 1308 that allows a user of the electronic device 1300 to interact with the electronic device 1300. For example, user input device 1308 can take a variety of forms, such as a button, keypad, dial, touch screen, audio input interface, visual/image capture input interface, input in the form of sensor data, etc. Still further, electronic device 1300 can include a display 1310 (screen display) that can be controlled by processor 1302 to display information to the user. Data bus 1316 can facilitate data transfer between at least file system 1304, cache 1306, processor 1302, and controller 1313. Controller 1313 can be used to interface with and control different manufacturing equipment through equipment control bus 1314. For example, control bus 1314 can be used to control a computer numerical control (CNC) mill, a press, an injection molding machine or other such equipment. For example, processor 1302, upon a certain manufacturing event occurring, can supply instructions to control manufacturing equipment through controller 1313 and control bus 1314. Such instructions can be stored in file system 1304, RAM 1320, ROM 1322 or cache 1306.
Electronic device 1300 can also include a network/bus interface 1311 that couples to data link 1312. Data link 1312 can allow electronic device 1300 to couple to a host computer or to accessory devices. The data link 1312 can be provided over a wired connection or a wireless connection. In the case of a wireless connection, network/bus interface 1311 can include a wireless transceiver. Sensor 1326 can take the form of circuitry for detecting any number of stimuli. For example, sensor 1326 can include any number of sensors for monitoring a manufacturing operation such as for example a Hall Effect sensor responsive to external magnetic field, an audio sensor, a light sensor such as a photometer, computer vision sensor, a temperature sensor to monitor a chemical reaction and so on.
The foregoing description, for purposes of explanation, used specific nomenclature to provide a thorough understanding of the invention. However, it will be apparent to one skilled in the art that the specific details are not required in order to practice the invention. Thus, the foregoing descriptions of specific embodiments of the present invention are presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention 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.
The embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.
Claims
1. A method of polishing a surface of an aluminum part, comprising:
- acid etching the aluminum part, comprising: supporting the aluminum part with a metal plate, immersing the metal plate into a mixed acid bath of nitric and phosphoric acid at a temperature of about 60° C. to about 75° C., and vibrating the metal plate in the acid bath for between about 5 minutes and about 15 minutes, wherein the immersed metal plate acts as an anode creating a galvanic potential gap through the mixed acid bath to the aluminum part which acts as a cathode, resulting in electron concentration on a plurality of convex protrusions occurring across a surface portion of the aluminum part that are subsequently dissolved in the mixed acid bath, thereby improving surface quality of the aluminum part.
2. The method as recited in claim 1, wherein the mixed acid bath comprises:
- 66-71 percent by weight phosphoric acid (H3PO4); and
- 5-9 percent by weight nitric acid (HNO3).
3. The method as recited in claim 2, wherein the metal plate is a titanium alloy plate.
4. The method as recited in claim 3, wherein the galvanic potential gap is about a 0.8 Volts.
5. The method as recited in claim 3, wherein an iron mesh in contact with both the surface of the aluminum part and the titanium alloy plate facilitates an increase in polishing performance.
6. The method as recited in claim 3, wherein the acid etching removes intermetallic compounds along the surface of the aluminum part.
7. The method as recited in claim 1, further comprising:
- rinsing residual acid from the mixed acid from off of the aluminum part by circulating water across the surface of the aluminum part in a neutralization tank.
8. The method as recited in claim 1, further comprising:
- anodizing the surface of the aluminum part.
9. The method as recited in claim 1, wherein the acid etching of the aluminum part results in a reduction in surface variation of the aluminum part of at least 50 percent.
10. An acid etching assembly comprising:
- a plurality of metal plates, each metal plate configured to support a plurality of aluminum parts;
- a plate holder configured to support a plurality of metal plates;
- an acid etching tank containing a mixed acid bath, the mixed acid bath comprising: 66-71 percent by weight phosphoric acid (H3PO4), and 5-9 percent by weight nitric acid (HNO3);
- a heat exchanger configured to heat the mixed acid bath to a temperature of about 60° C. to about 75° C.; and
- a vibration apparatus configured to vibrate the plate holder when the plate holder is positioned within the acid etching tank,
- wherein a galvanic potential gap develops between the plurality of metal plates and the plurality of aluminum parts when immersed in the mixed acid bath causing in an electro-polishing operation to be applied to a surface portion of the plurality of aluminum parts.
11. The acid etching assembly as recited in claim 10, wherein the plurality of metal plates are comprised of a metal selected from the group consisting of mild steel and titanium alloy.
12. The acid etching assembly as recited in claim 11, further comprising:
- an acid neutralization tank configured to rinse residual acid from the mixed acid bath off of the plate holder subsequent to an acid etching operation.
13. The acid etching assembly as recited in claim 10, further comprising:
- an iron mesh disposed between each of the plurality of aluminum parts and one of the plurality of titanium alloy plates.
14. The acid etching assembly as recited in claim 11, wherein the plate holder has a number of fluid access openings configured to allow easy circulation of the mixed acid bath through the plate holder.
15. The acid etching assembly as recited in claim 10, wherein the plurality of metal plates each include a plurality of perforations that ease circulation of the mixed acid bath across the plurality of aluminum parts.
16. A non-transitory computer readable medium for storing computer instructions executed by a processor, the non-transitory computer readable medium comprising:
- computer code for preparing a mixed acid bath within an acid etching tank, the mixed acid bath comprising: 66-71 percent by weight phosphoric acid (H3PO4), and 5-9 percent by weight nitric acid (HNO3);
- computer code for setting a temperature of the mixed acid bath to a temperature of about 60° C. to about 75° C.;
- computer code for immersing an aluminum part supported by a titanium plate within the mixed acid bath for a time of about 5 to 15 minutes; and
- computer code for vibrating the titanium plate while it is immersed within the mixed acid bath.
17. The non-transitory computer readable medium as recited in claim 16, wherein the immersing the aluminum part comprises immersing a plurality of aluminum parts in the mixed acid bath, the aluminum parts supported by a titanium plated and in direct contact with an iron mesh.
18. The non-transitory computer readable medium as recited in claim 17, wherein the titanium plate acts as an anode during the acid immersion step causing oxidation to form on a surface portion of the plurality of aluminum parts.
19. The non-transitory computer readable medium as recited in claim 18, further comprising:
- computer code for rinsing residual acid from the mixed acid bath from the plurality of aluminum parts in a neutralizing tank.
20. The non-transitory computer readable medium as recited in claim 17, wherein the computer code for immersing the aluminum part comprises supporting the titanium plate by a plate holder while immersing the titanium plate and aluminum part.
Type: Application
Filed: Sep 26, 2012
Publication Date: Sep 26, 2013
Applicant: Apple Inc. (Cupertino, CA)
Inventors: Simon R. Lancaster- Larocque (Gloucester), Purwadi Raharjo (Niigata), Kensuke Uemura (Kanagawa)
Application Number: 13/626,882
International Classification: C25F 3/20 (20060101); C25D 11/08 (20060101); C25F 7/00 (20060101);