Patent Application
20070029207
This invention relates generally to metal forming. More specifically, this invention discloses a method in which an enhanced oxide layer is generated on an aluminum alloy to reduce or eliminate the total amount of lubricant that is applied to the aluminum alloy prior to the metal forming process.
Metal forming, in most instances, is performed with the aid of lubricants in order to control the friction that is generated between the forming tool and the surface of the metal workpiece during the forming process, and to limit or eliminate the transfer of metal from the metal workpiece to the tooling surface. Most metals, including aluminum and its alloys, which are used in a forming process have a natural oxide layer that is present on the surface of the metal. The natural oxide layer's formation is due to a reaction between the metal's surface and the oxygen, water, and other oxidizing species in the environment. Under dry conditions at room temperature, the oxide layer that naturally forms on the surface of an aluminum object has thickness ranging from about 2.5 nm to about 3.0 nm. Metal transfer tends to occur when the contact conditions during a forming process are severe enough to substantially disrupt the naturally formed oxide layer that is present on the surface of the metal workpiece. Some lubricants, however, can minimize the occurrence of metal transfer even under these severe forming conditions. Additionally, some lubricants can also facilitate the release of the formed part from the forming tool. As metal deformation and temperature increase, however, the ability of the lubricant to perform these functions is significantly inhibited. This is due in part to fresh reactive metal being exposed and the thickness of the lubricant film being reduced during the metal deformation process and to the lubricant's organic components rapidly degrading due to the elevated temperature. Even solid lubricants, such as graphite or boron nitride, also suffer from a number of problems despite their high thermal stability. For instance, solid lubricants are difficult to apply uniformly onto the metal product, may adhere poorly once applied, and may not spread readily to provide lubrication to newly formed surfaces. The utilization of solid lubricants in a forming process can also lead to frequent manufacturing line interruptions because of lubricant accumulation on the forming tool. Additionally, solid lubricant residue can interfere with downstream processing and finishing. Furthermore, some solid lubricants can promote corrosion of the surface of the metal product onto which they are applied. Finally, in an industrial environment, solid lubricants not only present inhalation risks but they can also present challenges to safety and housekeeping by being difficult to clean from the work environment.
Therefore, there exists a need for a method that reduces or eliminates the amount of lubricants used during a forming process. There also exists a need for a method that reduces or eliminates the amount of lubricants used during a forming process and that is compatible with the downstream processes to which the metal is subjected.
This invention responds to these needs by disclosing a method of using oxides that are generated electrochemically on the surface of an aluminum product thereby reducing or eliminating the need for using lubricants during the forming process.
This invention discloses a method for preparing a metal product for forming that includes providing a metal product and anodically generating an oxide layer on at least one surface of the product.
This invention also discloses a method for reducing or eliminating the amount of lubricants used during a forming process that includes providing a metal product having at least one surface, anodically generating an oxide layer on the surface of the product, and forming the metal product into a shape using a forming process.
This invention also discloses a metal product suitable for forming that includes an oxide layer on at least one surface of the metal product, the oxide layer being anodically generated on the surface of the metal product prior to forming the metal product into a desired shape using a forming process.
One aspect of this invention is to reduce the costs, e.g. labor costs, lubricant costs, and process inconsistencies, associated with metal forming by reducing or eliminating the need of having to purchase and apply a lubricant to the surface of the metal prior to a forming process.
Another aspect of this invention is to eliminate the step of having to remove and dispose of excess lubricant that has remained on the surface of the metal workpiece prior to subsequent, i.e. post-forming, processing steps.
Another aspect of this invention is the generation of a surface film on the surface of the metal product, which can provide protection against corrosion and scratching.
The accompanying figures and the description that follow set forth this invention in its preferred embodiments. However, it is contemplated that persons generally familiar with metal forming and anodizing techniques will be able to apply the novel characteristics of the structures and methods illustrated and described herein in other contexts by modification of certain details. Accordingly, the figures and description are not to be taken as restrictive on the scope of this invention, but are to be understood as broad and general teachings. When referring to any numerical range of values, such ranges are understood to include each and every number and/or fraction between the stated range minimum and maximum.
This invention discloses using an oxide layer that is generated electrochemically on one or more surfaces of an aluminum product to supplement the natural oxide layer and to minimize the need for lubricating the aluminum product prior to a forming process. Based on anodizing conditions, the electrochemically generated oxide layer can be either porous or non-porous. For clarity, the oxide layer that is generated electrochemically on the surface of the aluminum product shall be referred to as the enhanced oxide layer hereafter. The enhanced oxide layer is formed on the aluminum product using anodizing techniques that are commonly known in the art. For instance, batch processing whereby individual panels or pieces are anodized as well as coil anodizing may be used with the disclosed invention. Additionally, the anodizing process that is used in conjunction with this invention may also use an electrical current that is either a direct current (DC) or an alternating current (AC). The anodizing technique can be used to manipulate the oxide layer's physical properties by controlling variables such as bath composition, concentration, temperature, current density, use of direct or alternating electrical current, electrode geometry, and the total amount of time the metal product is exposed to the anodizing process. Additionally, subsequent processes, e.g. a sealing process, can alter the properties of the enhanced oxide layer in order to provide other benefits such as increased corrosion resistance.
The inventors of this invention hypothesize that a suitable oxide layer can provide enhanced forming performance when compared to the naturally occurring oxide layer in aluminum workpieces. Additionally, the inventors believe that such an oxide layer can reduce or completely remove the need for having to add a lubricant prior to a forming process. It is noted that this invention is not limited to hot or cold forming processes but that it may be applied to any forming process where control or reduction of the coefficient of friction is needed and the tendency for metal transfer to the tooling is present. For instance, processes that involve sheet or plate forming, drawing or stamping as well as forging and extruding would most likely benefit from the disclosed invention.
In each of the following trials, the coefficient of friction of the aluminum product was measured using the following method. First, the aluminum product was heated in a furnace that was set to a temperature of about 454° C. (849.2° F.). The aluminum product was held at this temperature for about 15 to 20 minutes in order to stabilize the temperature of the product. While in the furnace, the coefficient of friction of the aluminum product was obtained using a sliding contact test in which an array of tooling specimens traveled across the surface of the product in a direction substantially perpendicular to the rolling direction. The frictional force was measured by a transducer that was attached to the tooling piece and recorded into a computer program. The computer program was used to generate the coefficient of friction, which is shown in
In each of the trials involving an anodized oxide layer (i.e. an enhanced oxide layer) on the surface of the aluminum product, the integrity of the enhanced oxide layer was evaluated with a microscope to determine to what extent the tooling piece stripped the enhanced oxide layer from the surface of the aluminum product during the sliding contact test described in the preceding paragraph. If the tooling piece had no visual effect on the enhanced oxide layer, i.e. the enhanced oxide layer remained intact and attached to the surface of the specimen, then a rating of “excellent” was given to that trial. Conversely, if the tooling piece stripped or scraped the enhanced oxide layer completely from the surface of the product and/or disrupted the surface of the product at the areas of contact, then a rating of “poor” was given to that particular trial. Finally, if the tooling piece partially stripped the enhanced oxide layer from the surface of the aluminum product, then a rating of “fair” or “good” was given to the trial.
The thickness of the oxide layer in trials 2-13 was measured using a Permascope instrument.
Prior to anodizing the surface of the 5083 alloy, the surfaces of the alloy were cleaned for about 2 minutes at about 60° C. (140° F.) with Henkel Surface Technologies' (32100 Stephenson Hwy, Madison Heights, Mich. 48071) A31K cleaner, which is a non-etching alkaline medium containing borate, phosphate, and sulfate. Excess A31K cleaner was removed by submerging the aluminum alloy in a tank of deionized water and later rinsing the surfaces of the alloy with additional deionized water.
In trial #1, a bare 5083 aluminum alloy with an as-rolled surface texture was used to create a base line against which the coefficient of friction of the other trials may be compared. In other words, the 5083 aluminum alloy in trial #1 does not have the enhanced oxide layer on any surface of the alloy. The surface of the bare 5083 alloy was cleaned using the method that is described in the previous paragraph. The cleaned alloy was then placed in a heating furnace and the coefficient of friction of the bare 5083 alloy was obtained using the sliding contact test as described above. As can be seen from
The next two trials involved creating a barrier coating, which served as an enhanced oxide layer, on the surface of the 5083 alloy. In trial #2, an enhanced non-porous oxide layer was generated anodically on the aluminum product by submerging the product in a neutral borate solution medium that was at a temperature of about 22.2° C. (72° F.). The solution medium was prepared by adding about 20 grams per liter of boric acid to deionized water and adding sodium borate until the solution had a pH of about 6.8. After submerging the aluminum product in the solution medium, a direct electrical current having a potential of about 50 volts was applied to the aluminum product for about 2 minutes until the current diminished thereby generating on the surface of the aluminum product an enhanced non-porous oxide layer having a thickness of about 0.07 μm. Once the enhanced oxide layer was generated on the surface of the 5083 aluminum product, the coefficient of friction was measured using the method that is disclosed above. As can be seen in
In trial #3, the solution medium in which the aluminum product was submerged was prepared in the same manner as described in the preceding paragraph. However, in trial #3 the electrical potential that was applied to the aluminum product was doubled to about 100 volts, which was applied to the aluminum product for about 4 minutes until the current diminished. The resulting enhanced oxide layer had a thickness of about 0.14 μm. As can be understood from
These three trials involved submerging the 5083 aluminum product in a tank containing a 20% by weight sulfuric acid (H2SO4) solution, which was held at a temperature of about 32.2° C. (90° F.). After the product was submerged in the sulfuric acid solution an electrical current was applied to the aluminum product to form an enhanced porous oxide layer on the surface of the product.
In trial #4, the enhanced oxide layer was generated on the surface of the 5083 aluminum product by applying a direct electrical current of about 24 amps per square foot for about 2.5 to about 3 minutes to form an enhanced oxide layer having a thickness of about 2.0 μm. The aluminum product containing the enhanced oxide layer was then placed in a heating furnace and the coefficient of friction of the 5083 alloy was obtained using the sliding contact test as described above. Referring to
Trial #5 involved applying a direct electrical current of about 24 amps per square foot to the 5083 aluminum product for about 15 minutes to generate an enhanced oxide layer having a thickness of about 16 μm on the surface of the aluminum product. As can be seen from
Similar to trials #4 and #5, the oxide layer in trial #6 was generated on the surface of the 5083 aluminum product by applying a direct electrical current of about 24 amps per square foot. However, unlike trials #4 and #5 the electric current in trial #6 was applied for about 20 minutes. The resulting enhanced oxide layer had a thickness of about 58 μm. As can be understood from
In trials 7 and 8, the 5083 aluminum products were treated electrochemically to generate an enhanced porous oxide layer containing tin. The enhanced oxide layer was initially generated on the aluminum product by submerging the aluminum product in a tank containing a 20% by weight sulfuric acid solution held at a temperature of about 21.1° C. (70° F.). After submerging the 5083 product into the tank, a direct electrical current of about 12 amps per square foot was applied to the aluminum product. The aluminum product was then transferred to a second tank containing a stannous sulfate solution that was held at a temperature of about 22.2° C. (72° F.) and had a pH of about 1.0. The stannous sulfate solution contained about 20 grams per liter of sulfuric acid, 20 grams per liter of stannous sulfate, and 20 grams per liter of a stabilizing agent. In trials 7 and 8, the stabilizing agent that was used in the stannous sulfate solution was catechol. Once the aluminum product was submerged in the second tank, the 5083 aluminum products were subjected to an alternating electrical current ranging from about 5 amps per square foot to about 7 amps per square foot.
In trial #7, an enhanced tin-containing oxide layer having a thickness of about 3.3 μm was generated on the surface of the 5083 aluminum product by applying the alternating electrical current for about 3 minutes to the product generated using direct current in the preceding paragraph. After the enhanced tin-containing oxide layer had been generated on the surface of the aluminum product, the coefficient of friction was determined using the sliding contact test described above. Referring to
Trial #8 involved applying an alternating electrical current for about 20 minutes to the direct current anodized 5083 aluminum product in order to generate an enhanced oxide layer containing tin having a thickness of about 30 μm on the surface of the product. As can be understood from
The 5083 aluminum products in trials #9 and #10 were submerged in a tank containing a 5% oxalic acid solution, which was held at a temperature of about 20° C. (68° F.). A direct current at a potential of about 40 volts was then applied to the 5083 aluminum product to form an enhanced porous oxide layer on the surface of the product.
In trial #9, the 40 volt potential was applied to the aluminum product for about 3 minutes thereby producing an enhanced oxide layer having a thickness of about 3.5 μm. Once the enhanced oxide layer was generated on the surface of the 5083 aluminum product, the coefficient of friction was measured using the method that is disclosed above. As can be understood from
Trial #10 involved applying the 40 volt potential to the aluminum product for about 60 minutes. The resulting enhanced oxide layer that was produced had a thickness of about 58 μm. As can be seen in
In these three trials, the 5083 aluminum alloy product was submerged in a 20% by weight phosphoric acid solution that was held at a temperature of about 29.4° C. (85° F.). Air agitation was used to draw the heat that was generated from the electrochemical reaction during the anodizing process away from the surface of the aluminum product in order to provide uniform temperature conditions at the metal surface. The resulting enhanced porous oxide layers in each of these trials had a thickness of less than about 1.2 μm.
Trial #11 involved applying a direct current at a potential of about 15 volts to the aluminum product for about 1 minute. After the enhanced oxide layer had been generated on the surface of the 5083 product, the coefficient of friction was determined using the method that is described above. As can be seen in
In trial #12, a direct current having a potential of about 15 volts was applied to the aluminum product for about 10 minutes. As can be seen in
Trial #13 involved applying a direct current of about 15 volts to the aluminum product for about 20 minutes. As
Trials 2-13 all show that an enhanced oxide layer that is generated on the surface of the 5083 aluminum product can decrease the coefficient of friction of the product by about 23% to about 65% when compared to bare 5083 aluminum alloy. Because the amount of friction is substantially reduced when the oxide layer is present on the surface of the aluminum product, the amount of lubricant needed to protect the aluminum product during a forming process can be reduced or eliminated.
Trials 15-17 involved measuring the coefficient of friction of three automotive sheet samples corresponding to a single alloy in the Aluminum Association's 6XXX series that were imprinted with electrical discharge textured (EDT) rolls having a Ra roughness of about 0.51 μm, 0.84 μm, and 1.12 μm. The goal of trials 15-17 was to determine the effect the texture imparted by the electro-discharge texturing rolls had in lowering the coefficient of friction of the aluminum sheet without an enhanced oxide layer generated on the surface of the sheet. In each of these trials, the coefficient of friction was obtained using the sliding contact test that was used in the previous trials.
In trial #14, the coefficient of friction of bare 6XXX aluminum sheet was measured using the sliding contact test that is described above. Unlike trials 15-17, the aluminum sheet in trial #14 was rolled with rolls having a typical ground surface used in normal rolling mill operations. As can be understood from
The surface of the 6XXX aluminum sheet in trial #15 had a Ra roughness of about 0.51 μm (20 μin.). As can be seen in
In trial #16, the surface of the 6XXX aluminum sheet had a Ra roughness of about 0.84 μm (33 μin.). Referring to
The surface of the 6XXX aluminum sheet in trial #17 had a Ra roughness of about 1.12 μm (44 μin.). Similar to trials 14 and 15, the aluminum sheet in trial #17 had a coefficient of friction of about 2.61 at a temperature of about 454° C. (849.2° F.).
Unlike the trials that had an enhanced oxide layer generated by anodizing the surface of the aluminum product, the modified surface texture on 6XXX sheet imparted by EDT rolls in these trials afforded little protection against the friction that was generated by the sliding contact test. Even though trials 14-17 involved the use of 6XXX aluminum as opposed to 5083 aluminum (trials 2-13), it is hypothesized that the EDT texture would provide an equivalent amount of protection to a 5083 aluminum alloy. In other words, without the use of lubrication or some other type of coating the EDT texture would provide minimal to no protection to a 5083 aluminum alloy that is undergoing a forming process. From the test results obtained from trials 14-17, one skilled in the art would appreciate the protective benefits of having an enhanced oxide layer generated on a surface of the aluminum alloy by anodizing as opposed to having the pattern from EDT rolls imparted into the surface of the aluminum alloy.
The goal of these trials was to determine what affect a selected commercially available conversion coating treatment would have in lowering the coefficient of friction of an aluminum workpiece. In trials 19-21, the conversion coating was applied onto a single 6XXX aluminum product at a maximum thickness of about 0.1 μm. As in the previous trials, the coefficient of friction of the aluminum sheet in trials 18-21 was obtained using the sliding contact test that is described above.
In trial #18, the coefficient of friction of bare 6XXX aluminum sheet was measured in order to provide a baseline to which trials 19-21 may be compared. As can be understood from
The 6XXX aluminum sheet in trial #19 was coated with a Henkel B1453 conversion coating. Henkel B1453 is a titanium and zirconium based coating containing silicates and some organic particles. The Henkel B1453 conversion coating may be obtained from Henkel Surface Technologies, 32100 Stephenson Highway, Madison Heights, Mich. 48071. In this trial, the coating was applied onto the aluminum product at 7.7 msf (mg per square foot). As depicted in
Trial #20 involved coating the aluminum sheet with a Chemetall Oakite X4591 conversion coating. Similar to the Henkel B1453 coating, the Chemetall coating is also titanium and zirconium based. In addition, the coating weight of X4591 is substantially similar to the coating weight of the Henkel conversion coating. The Chemetall Oakite X4591 conversion coating may be obtained from Chemetall Oakite, 50 Valley Rd., Berkeley Heights, N.J. 07922. As can be seen in
In trial #21, the 6XXX aluminum sheet was coated with a Permatreat 1021B conversion coating. The Permatreat 1021B conversion coating may be obtained from Betz-Dearborn Inc., 4636 Somerton Road, Trevose, Pa. 19053. Unlike the Henkel or the Chemetall Oakite conversion coatings used in trials 19 and 20, respectively, Permatreat 1021B is a fluorotitanate based coating having a coating weight of about 10 msf. As depicted in
Unlike the trials that involved anodically generating an oxide layer over the surface of the aluminum product, the conversion coatings in trials 19-21 afforded little protection against the conditions that were generated during the sliding contact test. Additionally, the conversion coatings failed to prevent the tooling piece from severely damaging the surface of the underlying aluminum product.
In one embodiment, the enhanced oxide layer can have a thickness up to about 100 μm.
In another embodiment, the thickness of the oxide layer ranges from about 2 μm to about 60 μm.
Having described the presently preferred embodiments, it is to be understood that the invention may be otherwise embodied within the scope of the appended claims.
