Patent Application
20070262120
An aqueous lubricating composition according to the invention is made by mixing together a first mass of water and at least the following components:
(A) a second mass of at least one nitrate salt of a divalent metal;
(B) a third mass of at least one alkali metal hydroxide;
(C) a fourth mass of at least one ionic surfactant; and
(D) a fifth mass of at least one lubricant/release agent.
The aqueous lubricating composition is applied to surfaces of aluminum, titanium and/or their alloys to be formed. Upon drying the composition forms a dry film lubricant that is stable to heating to a temperature of at least with increasing preference in the order given about 200, 250, 300, 350, 375, 400, 450, 500, 550, 600, 650, 700, 750, 800, 850, 900, 950, 100, 1050, 1100° C. Those of skill in the art will understand “stable to heating” to mean that the substance heated does not decompose, smoke excessively, flash, burn, flake off, fracture or lose its lubricity or plasticity during or after heating.
The aqueous lubricating composition comprises (A) at least one nitrate salt of a divalent metal. The nitrate salt is water soluble and present in an amount, in increasing order of preference of at least 0.4, 0.45, 0.5, 0.6, 0.7, 0.8, 0.9, 1.0, 1.4, 1.45, 1.5, 1.6, 1.7, 1.8, 1.9, 2.0 wt % and not more than, in increasing order of preference 5.0, 4.75, 4.5, 4.0, 3.5, 3.0, 2.5, 2.25 wt % of the nitrate salt. Suitable nitrate salts are those of water soluble salts of divalent metals of Groups 2-6 of the periodic table of elements, preferably calcium. In a preferred embodiment, (A) comprises calcium nitrate in an amount of 1.41 wt %.
Suitable caustics for component (B) include at least one alkali metal hydroxide, such as lithium, sodium and potassium hydroxides and mixtures thereof. The alkali metal hydroxide is present in an amount, in increasing order of preference of at least 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20 wt % and not more than, in increasing order of preference 0.35, 0.33, 0.30, 0.28, 0.26, 0.25, 0.24, 0.23, 0.22, 0.21 wt %. In one embodiment the at least one alkali metal hydroxide comprises sodium hydroxide in an amount of 0.05 wt %.
The lubricating composition of the invention further comprises (C) at least one surfactant. Any heat stable surfactant known to those of skill in the art can be used provided it does not interfere with the performance of the lubricating composition or dry film lubricant. Ionic surfactants are preferred. Suitable surfactants include those falling within the classification of substances known as acetylenic diols and ethoxylated acetylenic diols. By way of non-limiting example, surfactants such as ethoxylated 2,4,7,9-tetramethyl-5-decyne-4,7-diol and 2,4,7,9-tetramethyl-5-decyne-4,7-diol are suitable. The surfactant is present in an amount, in increasing order of preference of at least 0.025, 0.03, 0.035, 0.04, 0.045, 0.05, 0.06, 0.07, 0.08, 0.09, 0.10, 0.14, 0.15, 0.16, 0.17, 0.18, 0.19, 0.20 wt % and not more than, in increasing order of preference 0.35, 0.33, 0.30, 0.28, 0.26, 0.25, 0.24, 0.23, 0.22, 0.21 wt %. In one embodiment, the at least one surfactant comprises an ionic surfactant present in an amount of 0.10%.
The aqueous lubricating composition also comprises (D) at least one lubricant/release agent. Suitable lubricant/release agents are those heat stable compositions that provide sufficient lubricity to the metal sheet during quick plastic forming operations, even at temperatures of 200 to 550° C., preferably to 1100° C., such that the flow of the metal over the die is improved as compared to the flow of metal in the presence of a comparable lubricating composition in the absence of the lubricant/release agent. The lubricant/release agent is present in an amount, in increasing order of preference of at least 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10 wt %, and not more than, in increasing order of preference 20, 19, 18, 17, 16, 15, 14, 13, 12, 11 wt %. In one embodiment, the lubricant/release agent is present in an amount of to 1.5 to 10 wt %.
The lubricant/release agent can be soluble, insoluble or sparingly soluble in the lubricating composition, provided that it does not interfere with the formation of the dry film lubricant upon evaporation of water from the as-coated metal sheet. Desirably the lubricant/release agent is selected from the group consisting of at least one of polytetrafluoroethylene, silicon dioxide, sodium thiosulfate, calcium oxide, sodium nitrite and hectorite clay. Embodiments of the invention having coefficients of friction of less than 0.35 are preferred and had lubricant/release agents comprising PTFE alone or a combination of PTFE, sodium nitrite and calcium oxide; or a combination of sodium thiosulfate and calcium oxide.
It is further desirable that the lubricant/release agent does not interfere with removal of the dry film lubricant after forming or build-up on the die.
Lubricating compositions as described herein are generally used in quick plastic forming where high temperature stability and easy removal of lubricants is necessary. The invention also includes a method of forming a metal sheet of a superplastic aluminum or titanium alloy by forcing a side of the metal sheet into conformance with the surface of a shaping tool or die, the method comprising applying a lubricant to at least one of (a) the surface of the shaping tool or die and (b) the side of the metal sheet to be contacted with and conformed to the shape of the tool or die, drying the applied lubricating composition to form a dry film lubricant, heating the metal sheet to a superplastic forming temperature, applying fluid pressure to the opposite side of the metal sheet so as to deform the metal sheet at a superplastic strain rate into conformance with the tool or die surface, and thereafter removing the deformed metal sheet from the tool or die surface; wherein the lubricating composition comprises at least one nitrate salt of a divalent metal, at least one alkali metal hydroxide, at least one surfactant, and at least one lubricant/release agent. The method may further comprise the optional step of cleaning the dry film lubricant from the deformed metal sheet and optionally etching the surface of the sheet, during cleaning or in a separate etching step. Suitable compositions for cleaning and etching in one step are known to those of skill in the art and include hydrofluorosilicate compositions.
The manner of depositing the lubricating composition can be those typically used for applying waterborne lubricants, including but not limited to roll coating, dipping, spraying or using a drawdown bar. The time of contact with the lubricating composition is in increasing order of preference at least about 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15 minutes, and not more than, in increasing order of preference, at least for economy 60, 50, 40, 35, 30, 25, 20 minutes. Working temperatures for the coating bath range from ambient temperature to below the boiling point of the working bath. Generally, the bath is heated to accelerate the deposition of the lubricating composition onto the metal sheets, but not to greater than 200° F. to limit evaporation of water from the bath. Typical working bath temperatures are in increasing order of preference at least 100, 110, 120, 130, 140 or 150° F. and not more than, in increasing order of preference, 220, 210, 200, 190, 180, 170, 160° F.
The metal sheet is then dried at ambient or at elevated temperature to form a dry film lubricant on the surfaces of the metal sheet. Forming may take place as soon as the film has dried. The metal sheet is then subjected to quick plastic forming processes which are known to those of skill in the art.
Suitable substrates for use in the method include superplastic metal alloys such as aluminum alloys and titanium alloys. Desirably, the metal sheet is an aluminum alloy and the lubricating composition applied to the metal sheet forms a calcium aluminate salt on at least one surface of the metal sheet.
The practice of this invention may be further appreciated by consideration of the following, non-limiting, working examples.
Calcium aluminate is known to form a powdery film that could be rubbed off of an aluminum panel surface. Applicants recognized that calcium aluminate was adherent enough to bind on a surface, would not decompose under high heat (decomposes at 1535° C., and yet could be easily cleaned from a surface. Distilled water (hereinafter referred to herein as “DI water”) was selected as a carrier to prevent water hardness interference from tap water in the formation of the lubricious dry film by generation of undesirable salts.
1968 grams of DI water was heated to 150° F. Thirty grams of calcium nitrate salt was dissolved in the hot DI water to form a clear solution. In order to drive the reaction, five grams of sodium hydroxide (50%) was introduced to the formula. A white flocculent formed, which was determined to be CaOH. When an aluminum panel was introduced to the bath, a grey powdery substance was developed on the panel surface. This powdery film was identified as calcium aluminate. Without being bound by a single theory, it is believed that the calcium aluminate was generated by reactions as shown in the following equation:
CaO.Al2O3: calcium aluminate is the powdery grey material deposited on the aluminum panel surface;
Ca(OH)2: calcium hydroxide is an insoluble flocculent reaction product; and
NaAlO2: sodium aluminate is a water soluble salt that is a reaction by-product.
Applicants initially investigated three materials to improve lubricity and help as anti-stick or lubricant/release agents. Polytetrafluoroethylene (PTFE), silicon dioxide, and sodium thiosulfate, as well as surfactant, were introduced to an aqueous solution of nitrate salt and caustic as recited in Table 1, according to the procedure of Example 1. The amounts of components are in grams.
For each formulation a clean 6×4 inch panel of 6111 aluminum was weighed and immersed in the solution for 20 minutes at 150° F. The panels were removed, inspected for coating and reweighed, see Table 1.
Polytetrafluoroethylene (PTFE), silicon dioxide, sodium thiosulfate, calcium oxide, sodium nitrite and hectorite clay (a montmorillonite mineral having an empirical formula of Na0.67(Mg, Li)6Si8O20(OH,F)4) were selected for further study as lubricant/release agents based on their relative heat resistance. The silicon dioxide used had to disperse readily in water so nanoparticle size silicon dioxide, brand name Ludox CL-P, commercially available from W. R. Grace & Co., was chosen.
Compositions were made according to the procedure of Example 1, with the following ingredients making up CA-1, in weight percent:
To improve wetting and minimize foam, addition of various surfactants was investigated. Commercially available acetylenic diols are known to wet out surfaces, are low foam and generally are compatible with dry film coatings such as paints. The compositions were built according to Example 3, with the addition of 5 grams of surfactant. Panels were coated according to Example 3. Each surfactant tested produced similar results for lubricating compositions M1-M3 upon observing the solution and resulting dry film, see Table 3.
Performance parameters for use in manufacturing were assessed including uniformity of the dry film and lubricity of the dry film lubricant, as expressed in coefficient of friction measurements. Panels were coated using M1-M6 formulations according to the procedure of Example 3, and dried in an oven set at 150° F. The films were then observed and recorded in Table 4.
Procedure: Panels were coated using M1-M6 formulations according to the procedure of Example 3, and dried in an oven set at 150° F. Used Cetr UMT-2 Lubrication tester on the thus coated 6111 aluminum coupons. The coefficient of sliding friction is reported in Table 5, below. In general, when comparing “like chemistries”, the lower the coefficient of friction, the better the lubricant.
Measurements of adhesion to the substrate metal were made for all film coated panels of Table 2, at room temperature and after heating treatments at 450° C. during five minutes under an air atmosphere. Adhesion can be measured using a Crockmeter; the Crockmeter rubs a given area using a felt protected tool called the finger to consistently rub a known area with a predetermined amount of strokes. Aluminum panels prepared with the dry film lubricant candidates of Table 2 were stroked (the stroke rubs an area one-half inch by 4 inches) 20 times on an area. Both unheated and heated coated panels (460° C.) were tested with the Crockmeter. The weight loss in milligrams per square foot is reported in Table 6, below. The initial coating weights on the tested panels were between 700-1000 milligrams per square foot per side for this experiment.
Analysis of the results showed that the lubricant formulation comprising CA-1+sodium thiosulfate shows the best adhesion before heating and after heating for the QPF process.
This invention provides a lubricant combination that can be used at the high temperatures of superplastic forming of aluminum alloy and titanium alloy sheets. It can be used in many variations of the processes that are employed in the superplastic forming of metal sheet materials. While the invention has been described in terms of specific embodiments thereof, it will be appreciated that other forms could readily be adapted by one skilled in the art. Accordingly, the scope of the invention is to be considered limited only by the following claims.
This invention claims priority from the U.S. Provisional Application Ser. No. 60/799,155, filed May 10, 2006.
| Number | Date | Country | |
|---|---|---|---|
| 60799115 | May 2006 | US |
