KR20130126644A - Electrolyte solution and electrochemical surface modification methods - Google Patents

Abstract

An aqueous electrolyte solution is disclosed, comprising a citric acid concentration in the range of about 1.6 g / L to about 982 g / L and an effective amount of ammonium bifluoride (ABF), which is substantially free of strong acids. Exposing the surface of the non-ferrous metal workpiece to a bath of an aqueous electrolyte solution comprising citric acid at a concentration of up to about 300 g / L and ammonium bifluoride at a concentration of at least about 10 g / L and having a strong acid of up to about 3.35 g / L Controlling the temperature of the bath above about 54 ° C., connecting the workpiece to an anode of a DC power source, immersing the cathode of the DC power source in the bath, and applying a current across the bath. A method of treating a surface of a non-ferrous metal workpiece is disclosed.

Description

ELECTROLYTE SOLUTION AND ELECTROCHEMICAL SURFACE MODIFICATION METHODS

This application is directed to a commonly owned application with the name “Electrolyte Solution and Electropolishing Methods” filed concurrently with this application.

The solutions and methods relate to the general technical field of electropolishing non-ferrous metal parts and surfaces, and more particularly to surface by electropolishing including crack control and oxide removal for nonferrous and reactive metals, in particular titanium and titanium alloys. It is about processing.

When working with reactive metals from metal ingots to finished mill products and after hot parts of finished parts, or in the case of titanium and titanium alloys, they are usually referred to as alpha cases, which are specific to metal oxides. It is desired to remove the surface layer material. This oxygen-enriched phase occurs when the reactive metal is heated in air or in an oxygen-containing atmosphere. The oxide layer affects the material strength, fatigue strength and corrosion resistance of the metal. Among them, titanium and titanium alloys are reactive metals, which are rigid oxide layers (TiO ₂ for Ti and ZrO for Zr) that react with oxygen and are brittle whenever they are heated in air or an oxidizing atmosphere above about 480 ° C (900 ° F). _2, etc.), depending on the particular alloy and oxidizing atmosphere. The oxide layer is produced by heating the metal to the temperature necessary for typical mill forging or mill rolling as a result of welding, or by heating for finished part forging or hot part forming. Reactive metal oxides and alpha cases are brittle and involve a series of surface microcracks that typically penetrate into the bulk metal during molding, potentially causing immature tensile or fatigue failure and making the surface more susceptible to chemical attack To lose. Thus, the oxide or alpha case layer must be removed before any subsequent hot or cold work, or provision of the final part.

In addition, when working with reactive metals such as titanium and titanium alloys from finished ingots, it is important to remove the cracks formed by thermal and mechanical processes. As mentioned above, these cracks can be deeper than the alpha case and can penetrate the bulk metal. Reactive metals are typically heated, thermally processed (eg, forged, rolling, draw, extrusion), cooled, and reheated during 4-8 additional thermal processes to convert the ingot to a finished mill product. Mill products are completed using techniques including, but not limited to, hot spin forming, ring rolling, superplastic forming, and closed die forming. It is often reheated for the manufacture of finished parts. Each time, the metal cools after the thermal process, cracks form on the surface and expand into the workpiece. In conventional processes, such cracks are removed from the workpiece by a mechanical removal or polishing, including chemical milling, typically in HF-HNO ₃ , to a uniform layer or amount of material from the workpiece, with the bottom of the deepest crack exposed and removed. Is removed until Polishing or chemical milling to this depth ensures that all cracks are removed, but takes a considerable amount of time and labor, and also results in significant expensive material losses. This is because the cracks often extend into the workpiece to a depth of at least 5% of the thickness or diameter of the workpiece or finished part. However, the removal of the cracks is required because if the cracks are not removed prior to the subsequent hot working step or the use of the finished part provided, the cracks can spread and destroy the workpiece or finished part.

In chemistry and manufacturing, electrolysis is a method of using direct current (DC) to induce other non-voluntary chemical reactions. Electropolishing is an application of electrolysis that is well known for deburring metal parts and producing bright, flexible surface finishes. The workpiece to be electropolished is placed in a bath of electrolyte solution and applied to a direct current. The workpiece remains an anode and the cathode connection consists of one or more metal conductors surrounding the workpiece in the bath. Electropolishing relies on two opposing reactions that control the process. The first reaction is a dissolution reaction that occurs during the passage of metal into the solution in the form of ions from the surface of the workpiece. Thus, the metal is removed by ions from the surface of the workpiece. Another reaction is an oxidation reaction that takes place while an oxide layer is formed on the surface of the workpiece. The formation of the oxide film limits the progress of the ion removal reaction. Since these films are thicker than micro depression, thinner than micro projection, and electrical resistivity is proportional to the thickness of the oxide film, the fastest rate of metal dissolution occurs in micro projection, The slowest rate occurs at fine degradation. Thus, electropolishing selectively removes microscopic high points or "peaks" faster than the corresponding rate of attack or attack speed in "valleys."

Another application of electrolysis is in electrochemical machining processes (ECMs). In ECM, high currents (often applied at current densities of more than 40,000 amps and more than 1.5 million amps per square meter) pass between the electrode and the metal workpiece to cause material removal. Electricity is transferred from the negatively charged electrode "tool" (cathode) through the conductive fluid (electrolyte) to the conductive workpiece (anode). The cathode tool is configured to conform to the desired machining operation and moves into the anode workpiece. Pressurized electrolyte is injected into the zone where it is machined to a set temperature. The material of the workpiece is removed and liquefied into the workpiece at a rate essentially determined by the tool feed rate. The distance of the gap between the tool and the workpiece varies within the range of 80 to 800 microns (0.003 to 0.030 inch). As the electrons pass the gap, the material on the workpiece dissolves and the tool is formed into the workpiece in the desired shape. The electrolyte fluid sweeps metal hydroxides formed in the process from the reaction between the electrolyte and the workpiece. Flushing is required because the electrochemical machining process has low resistance to metal complexes that accumulate in the electrolyte solution. In contrast, the process using the electrolyte solution shown herein is stably and effectively maintained even if the electrolyte solution has a high concentration of titanium.

Electrolyte solutions for metal electropolishing are usually mixtures containing concentrated strong acids (completely separated in water), such as mineral acids. Strong acids, as described herein, are generally classified as being stronger than hydronium ions (H ₃ O ⁺ ) in aqueous solutions. Examples of strong acids commonly used in electropolishing are sulfuric acid, hydrochloric acid, perchloric acid and nitric acid, and examples of weak acids include those in carboxylic acid groups such as formic acid, acetic acid, butyric acid and citric acid. Organic compounds such as alcohols, amines or carboxylic acids are sometimes used in admixture with strong acids for the purpose of slowing the dissolution etch reaction to avoid excessive etching of the workpiece surface. See, eg, US Pat. No. 6,610,194, which describes the use of acetic acid as a reaction slower.

It is encouraged to reduce the use of such strong acids in metal finishing baths, mainly due to health risks and the cost of disposable waste of the used solution. Citric acid was previously approved as a passivating agent for stainless steel pieces by both the Department of Defense and ASTM standards. However, previous studies have shown or quantified savings from using a commercial citric acid passivating bath solution to passivate stainless steel, but it was not possible to find a suitable electrolyte solution where significant concentrations of citric acid could reduce the concentration of strong acids. For example, in a publication filed under the name "Citric Acid & Pollution Prevention in Passivation &Electropolishing" (2002), weaker organic acids, particularly citric acid, are low cost, useful and relatively risk free treatments. Substitution of some of these weaker organic acids, in particular citric acid, has been described for several advantages that can reduce the amount of strong mines, but ultimately alternative electrolytes containing mainly mixtures of phosphoric acid and sulfuric acid may be used. Evaluation with organic acids (not citric acid).

Alpha case removal and crack control are typically not cured by the electropolishing process. The strong acid component found in typical electrolyte solutions used in the electropolishing art can cause hydrogen migration to the metal surface and lead to aggressive uncontrolled etching, which can deepen cracks. In the absence of a strong acid component, in the development of new electrolytic polishing bath chemistry using a solution of weak acid and ABF, the inventors have found that both alpha case removal and crack control can be effectively cured through electropolishing. In conclusion, a method of oxide removal and crack control by an electropolishing process using a new bath chemistry suitable for this method is disclosed.

In one embodiment, an aqueous electrolyte comprising about 0.1% to about 59% carboxylic acid and about 0.1% to about 25% fluorinated salt, substantially free of strong acids, is disclosed.

In another embodiment, an aqueous electrolyte comprising ammonium bifluoride of about 1.665 g / L to about 982 g / L citric acid and about 2 g / L to about 360 g / L fluorinated salt is substantially free of strong acids.

In one embodiment, a method of treating a surface of a non-ferrous metal workpiece, the surface of the non-ferrous metal workpiece includes citric acid at a concentration of about 300 g / L or less and ammonium bifluoride at a concentration of about 10 g / L or more, Exposing it to a bath of an aqueous electrolyte solution having a strong acid of about 3.35 g / L or less, adjusting the temperature of the bath to at least about 54 ° C., connecting the workpiece to an anode of a DC power source, and A method of treating a surface of a non-ferrous metal workpiece is disclosed that includes immersing a cathode in the bath, and applying a current across the bath.

In one embodiment, a method of controlling cracking at a surface of a non-ferrous metal workpiece, wherein the surface of the non-ferrous metal workpiece includes citric acid at a concentration of about 300 g / L or less and ammonium bifluoride at a concentration of about 10 g / L or more Exposing it to a bath of an aqueous electrolyte solution having a strong acid of about 3.35 g / L or less, adjusting the temperature of the bath to about 54 ° C. or more, connecting the workpiece to an anode of a DC power source and a cathode of a DC power source. A method of controlling cracks in the surface of a non-ferrous metal workpiece is disclosed, comprising immersing the bath in the bath, and applying a current of less than about 53.8 amps per square meter across the bath.

In one embodiment, a method of removing metal oxides from a surface of a non-ferrous metal workpiece, wherein the surface of the non-ferrous metal workpiece comprises citric acid at a concentration of about 60 g / L or less and ammonium bifluoride at a concentration of about 60 g / L or more; Exposing it to a bath of an aqueous electrolyte solution having a strong acid of about 3.35 g / L or less, adjusting the temperature of the bath to about 54 ° C. or more, connecting the workpiece to an anode of a DC power source and Disclosed is a method of removing metal oxides on a surface of a non-ferrous metal workpiece comprising immersing in the bath, and applying a current of less than about 53.8 amps per square meter across the bath.

In one embodiment, a method of removing an alpha case from a surface of a titanium or titanium alloy workpiece, the surface of the titanium or titanium alloy workpiece comprising citric acid at a concentration of about 60 g / L or less and ammonium bifluoride at a concentration of about 60 g / L or more; Exposing it to a bath of an aqueous electrolyte solution having a strong acid of about 3.35 g / L or less, adjusting the temperature of the bath to about 54 ° C. or more, connecting the workpiece to an anode of a DC power source and A method of removing an alpha case from a surface of a titanium or titanium alloy workpiece is disclosed that includes immersing in the bath and applying a current of less than about 53.8 amps per square meter across the bath.

1A-1B show data showing changes in material removal rate and surface finish with citric acid concentration at a high current density of 1076 A / m 2 over various temperatures in an aqueous electrolyte solution having a low to medium concentration of ammonium bifluoride of 20 g / L. It is a graph.
2A-2B are graphs of data showing material removal rates according to ammonium bifluoride concentration in an aqueous electrolyte solution comprising 120 g / L citric acid at representative low and high temperatures, respectively, over various current densities.
2C-2D are graphs of data showing changes in surface finish with ammonium bifluoride, respectively, under conditions corresponding to FIGS. 2A-2B.
2E-2F are graphs of data showing changes in material removal rate and surface finish, respectively, with current density in an aqueous electrolyte solution substantially free of citric acid at a temperature of 85 ° C.
3A-3D show material removal rates according to citric acid concentration in aqueous electrolyte solutions for various concentrations of ammonium bifluoride at current densities of 53.8 A / m 2 and temperatures of 21 ° C., 54 ° C., 71 ° C. and 85 ° C., respectively. A graph of data representing
4A-4D show the concentrations of citric acid in aqueous electrolyte solutions for various concentrations of ammonium bifluoride at temperatures of 54 ° C. and current densities of 10.8 A / m 2, 215 A / m 2, 538 A / m 2 and 1076 A / m 2, respectively. A graph of data showing the rate of material removal.
4E-4G show data of material removal rates with current density at 85 ° C. in aqueous solutions with 120 g / L, 600 g / L and 780 g / L citric acid, respectively, for various concentrations of ammonium bifluoride Is a graph of.
4H-4J are graphs of data showing changes in surface finish with current density under conditions corresponding to FIGS. 4E-4G, respectively.
5A-5B are graphs of data showing changes in surface finish and the amount of material removed, respectively, at low temperatures (21 ° C.) and high current densities (538 A / m 2) at various combinations of citric acid and ammonium bifluoride concentrations.
6A-6B are graphs of data showing the amount of material removed and the change in surface finish at low temperature (21 ° C.) and high current density (1076 A / m 2), respectively, at various combinations of citric acid and ammonium bifluoride concentrations.
7A-7B are graphs of data showing the amount of material removed and the change in surface finish at high temperature (85 ° C.) and high current density (1076 A / m 2), respectively, at various combinations of citric acid and ammonium bifluoride concentrations.
8A-8B are graphs of data showing changes in surface finish and the amount of material removed, respectively, at high temperature (85 ° C.) and low current density (10.8 A / m 2) at various combinations of citric acid and ammonium bifluoride concentrations. to be.
9A-9B are graphs of data showing the amount of material removed and changes in surface finish, respectively, at representative high temperatures (85 ° C.) and high current densities (538 A / m 2) at various combinations of citric acid and ammonium bifluoride concentrations. .
10A-10B are graphs of data showing changes in surface finish and amount of material removed, respectively, at representative high and medium temperatures (71 ° C.) and medium current density (215 A / m 2) at various combinations of citric acid and ammonium bifluoride concentrations. to be.
FIG. 11 is a schematic diagram of a sequence occurring in the prior art method of removing cracks extending from the surface of a material to a workpiece.
12 is a schematic diagram of a sequence that takes place in a method using an electrolyte disclosed herein for controlling cracks extending from the surface of a material to a workpiece.

Although not limited thereto, an aqueous electrolyte solution is particularly useful for surface treatment of reactive metals including titanium and titanium alloys. Relatively small amounts of fluoride salts and carboxylic acids are dissolved in water substantially in the absence of strong acids such as mineral acids so that the solution is substantially free of strong acids. Such electrolyte solutions include, but are not limited to, previous attempts in the electrolytic bath for surface treatment of reactive metals including titanium and titanium alloys, typically using strong acids and requiring that the amount of water in the electrolyte solution be kept to an absolute minimum. Notable departure from.

The fluoride salt provides a source of fluoride ions to the solution, which may be, but is not limited to, ammonium bifluoride, NH ₄ HF ₂ (hereinafter also abbreviated as “ABF”). Without wishing to be bound by theory, carboxylic acids are believed to mitigate fluoride ion attack on the reactive metal surface to be treated, but may not be limited to citric acid. No amount of strong acid or mineral acid is intentionally added to the solution, even if trace amounts of strong acid are present. As used herein, the terms “substantially in the absence of” and “substantially free of” refer to about 3.35 g / L or less, preferably about 1 g / L or less, and More preferably used to refer to a strong acid at a concentration of less than about 0.35 g / L.

A test piece of commercially pure (CP) titanium was immersed at 54 ° C. in a bath of an aqueous solution containing 60 g / L citric acid and 10 g / L citric acid, and a current was applied at 583 A / m 2. The test piece cut from the mill-surface titanium strip (0.52 μm surface roughness) exposed to this solution for 15 minutes was uniformly smooth (0.45 μm surface roughness) and was apparently reflective. A small amount of 42 ° Be HNO ₃ (nitric acid) was then added gradually and the prepared test article was treated repeatedly until a surface change was detected. The test article was not affected by the process after every nitric acid addition until the nitric acid concentration reached 3.35 g / L, at which point the test panel was in the periphery of the test specimen having a surface roughness in the range of 0.65 to 2.9 μm and higher. Uneven cosmetic appearance including pitting and spalling with irregular attack on the body. Nitric acid is considered to be a borderline strong acid with a dissociation constant that is not much higher than hydronium ions. Thus, for other stronger acids with dissociation constants equal to or higher than nitric acid, a similar electrolyte solution is expected to be similarly effective at controlled material removal and micropolishing at concentrations of strong acids less than about 3.35 g / L. However, other electrolyte solutions disclosed herein having different concentrations of citric acid and ABF, and having different ratios of citric acid and ABF concentrations, have lower resistance to the presence of strong acids, as well as certain strong acids, depending on operating parameters such as temperature and current density. It is expected to have Thus, an aqueous electrolyte solution of less than about 1 g / L, preferably about 0.35 g / L, can be used effectively to improve material removal and surface finish over a wide range of citric acid and ABF concentrations within a wide range of temperature and current densities. The following strong acids should be present.

Extensive electropolishing tests were performed on titanium and titanium alloy samples using various chemical concentrations, current densities and temperatures. Specifically, the tests are performed to remove bulk metals, improve or refine the surface finish on sheet metal products at low material removal rates, and / or micropolish the metal surfaces to achieve very fine surface finishes at very low material removal rates. When delivered at the beginning of a typical mill manufacturer conforming to "clean" mill products (American Society for Testing and Materials (ASTM) or Aerospace Material Specification (AMS) to measure the ability of solutions and methods In the same condition as the "as delivered" condition. In addition, although most of the tests focused on titanium and titanium alloys, the tests also showed that the same solutions and methods are more generally applicable to treating multiple nonferrous metals. For example, in addition to, but not limited to, titanium and titanium alloys, good results are obtained for metals including gold, silver, chromium, zirconium, aluminum, vanadium, niobium, copper, molybdenum, zinc, and nickel. Got it. In addition, alloys such as titanium-molybdenum, titanium-aluminum-vanadium, titanium-aluminum-niobium, titanium-nickel (Nitinol ^® ), titanium-chromium (Ti 17 ^® ), Waspaloy and Inconel ^® (nickel-based alloys) It was also treated positively.

Electrolyte solutions containing citric acid and ammonium bifluoride have surprisingly proved effective in etching non-ferrous metals and metal alloys at dilute concentrations of both components. In such a situation, etching is understood to include substantially uniform surface removal. In addition, improvements in surface finish have been seen over a wide range of citric acid and ammonium bifluoride concentrations. Any citric acid concentration (59% by weight or about 982 g / L aqueous solution at standard temperature and pressure) up to the saturation point containing water can be used, but the rate of material removal is dramatically reduced and the micropolishing of the material surface is increased, It appears that there is a correlation between the concentration of citric acid and the concentration of ammonium bifluoride, in which citric acid moderates the etching effect of fluoride ions produced by dissociation of ammonium bifluoride. For both etching and micropolishing, several mixtures having an amount of citric acid concentration as low as 3.6% by weight or about 60 g / L of solution, far exceed the amount comprising up to about 36% by weight or about 600 g / L of solution Etch rates and surface micropolishing results in titanium are comparable to the concentration of. Thus, in such a solution, the etch rate is clearly more directly affected by the concentration of ABF than the concentration of citric acid. Effective etching and micropolishing have been shown at significantly lower citric acid concentrations of solutions of less than about 1% by weight or less than 15 g / L. However, even the presence of the smallest amount of fluoride appears to be sufficient for some metal removal to occur.

Etch rates are substantially lowered at citric acid concentrations above about 600 g / L. However, at such high concentrations of citric acid, at least in the case of mitigating high current densities, the surface finish result is improved, while the etching rate is lowered. Thus, when direct current is applied, a more diluted citric acid mixture can speed up surface material removal, while higher concentrations of citric acid mixture, which are as high as about 42% by weight or about 780 g / L of solution, are lower. Compared to pieces finished with a concentration of citric acid mixture, it provides a smoother, more glossy finish with uniform, superior particles and no corona effect.

Highly controlled metal removal can be achieved using the bath solutions and methods described herein. Specifically, the level of control is so good that bulk metals can be removed in thicknesses as small as 0.0001 inches to thicknesses as large as 0.5000 inches. Such good control can be achieved by adjusting the combination of citric acid and ABF concentration, temperature, and current density, as well as by varying the application period and periodic application of direct current. Removal is generally performed uniformly on all surfaces of the workpiece or may be selectively applied only on a particular selected surface of the mill product or manufactured part. Control of removal is achieved by fine tuning various parameters including, but not limited to, temperature, power density, power cycle, ABF concentration and citric acid concentration. Nevertheless, by keeping the concentrations of citric acid and ABF within certain desired ranges, high levels of micropolishing can also be achieved at high temperatures, as would be expected.

The removal rate depends on how the DC power is applied. Unexpectedly, the removal rate appears to be inversely proportional to the DC power applied continuously, and when applied continuously, increasing the DC output density decreases the removal speed. However, by cycling the DC power supply, the removal rate can be advanced. In conclusion, where significant material removal rates are desired, the DC power source is cycled from off to on throughout the processing operation. Conversely, if fine control of the removal rate is desired, the DC power source is applied continuously.

While not wishing to be bound by theory, removal is slowed in proportion to the thickness of the oxide layer formed on the metal surface, and higher applied DC power sources cause higher oxidation at the metal surface, which can act as a barrier to fluoride ion attack of the metal. It is thought to be. Thus, cycling the DC power source on and off at a predetermined rate may overcome this oxygen barrier or create a mechanism in which thick oxides are encouraged to break the surface periodically. As described herein, varying bath temperatures, applied voltages, citric acid concentrations and ammonium bifluoride concentrations, operating parameters of the electrolyte, are beneficial results, ie the ability to tailor highly controlled bulk metal removal and micropolishing to specific applications. To provide. In addition, changing the operating conditions within the operating parameters of a given process set can change and increase the ability for fine-tuning control of metal removal and surface finish.

For example, FIGS. 8A and 9A show that at 85 ° C., 300 g / L citric acid, 10 g / L ammonium bifluoride, the material removal rate increases as the current density increases from 10.8 A / m 2 to 538 A / m 2. 8b and 9b show that, under the same conditions, the surface finish deteriorates when the current density increases from 10.8 A / m 2 to 538 A / m 2. By cycling the DC power supply between these two current densities, better final results can be achieved compared to the case of performing either of the current densities alone for the entire process. In particular, the process time for removing a certain amount of material can be reduced compared to the case performed alone at 10.8 A / m 2. In addition, because of the lower current density smoothing, the overall surface finish of the final product is better than that obtained by treatment alone at 538 A / m 2. Thus, a circulation of two or more power settings (as shown in current density) enables favorable results of both improved surface and precise bulk metal removal, the process being a separate process for either surface enhancement or bulk metal removal alone. It requires less total time.

In addition to varying the duty cycle, power can be applied across the electrolyte solution and the workpiece, including but not limited to creating additional beneficial results and / or enhancements to processing speed without sacrificing final surface finish. However, there may be various waveforms available from the DC power supply, including half wavelength, rectified propagation, square wave and other intermediate rectification. DC switching speeds as fast as 50 kHz to 1 MHz or as slow as 15 to 90 minutes cycles may be beneficial depending on the surface to be treated, the mass of the workpiece and the specific surface conditions of the workpiece. In addition, the DC switching cycle itself may optimally require its own cycle. For example, large mass workpieces with very rough initial surface finishes may benefit most by the slow switching cycle initially, followed by increased frequency of switching cycles as material is removed and surface finish is improved. .

Testing of electrolyte baths of the type described herein has shown that electropolishing occurs in certain embodiments without increasing the hydrogen concentration in the metal surface, and in some cases the hydrogen concentration decreases. Oxygen barriers at the material surface can play an important role in preventing hydrogen from moving into the metal matrix. The data suggest that such an oxygen barrier may also be present by removing hydrogen from the metal surface. Higher fluoride ion concentrations result in faster removal rates but have an unknown effect on hydrogen adsorption to the metal matrix. Higher citric acid concentrations slow the removal rate and tend to require higher power densities during electropolishing, as well as adding 'smoothing' or 'luster' to the surface.

Compared to the prior art solutions for finishing and / or pickling metal products, several advantages are obtained using an aqueous electrolyte solution of ABF and citric acid. The disclosed electrolyte solution allows precise control of the finish gauge to be achieved. Finishing of conventional production alloy flat products (sheets and plates) utilizes fine grinding media in the finish gauge, typically followed by an acid bath comprising hydrofluoric acid (HF) and nitric acid (HNO ₃ ). and "rinse pickling" in an acid bath to remove residual grinding material, ground-in smeared metal and multi-step grinding. HF-HNO ₃ acid washes are exothermic and thus difficult to control, often causing the metal to be below the dimension, resulting in a higher scrap rate or lower value of refurbishing of the metal. By using the electrolyte solution of the present invention, typical secondary and tertiary grinds that may be required for rinse pickling can be omitted. A precisely set finish gauge can be achieved which cannot be achieved with conventional grinding and pickling. In addition, the electrolyte solution of the present invention does not introduce stress into the portion to be treated. In contrast, any mechanical grinding process imparts significant surface stress, which causes some percentage of the material to fail to meet typical or customer contract planar specifications.

Typical processes using HF-HNO ₃ acid washes charge hydrogen into the target material, which must often be removed by expensive vacuum degassing to inhibit embrittlement of the material. Tests performed using an aqueous electrolyte bath containing citric acid and ABF on typical mill production actual size sheets of Ti-6Al-4V and on specimens of CP titanium, 6Al-4V titanium and nickel-based alloy 718, Reduced hydrogen infiltration results were observed compared to samples exposed to strong acid wash solutions. In particular, an aqueous electrolyte solution composition comprising ammonium bifluoride and citric acid may be prepared by treating Ti-6Al-4V and CP titanium to achieve the same alpha-case free, clean surface finish results as are typically achieved by strong acid washing, Hydrogen was not charged into the material of the workpiece when using a wide range of temperature and current density conditions, and in many of these operating conditions, hydrogen was found to actually escape from the material. For all metals and alloys, the test proceeded to improve the desired operating range, and the results so far were filled with less hydrogen into the material than under the same operating conditions with a strong acid wash bath, even under conditions that were not optimal. Consistently. In general, lower concentrations of ammonium bifluoride form more hydrogen removal from the material exposed to the electrolyte solution or allow less hydrogen penetration into the material.

Highly regulated metal removal, surface finish and Micropolishing

Micropolishing or microsmoothing of parts, in particular microsmoothing of already relatively smooth surfaces, can be achieved using the solutions and methods of the invention with better precision compared to manual or mechanical polishing. Micropolishing occurs without generating harmful residual stress in the target workpiece or material, which is a problem inherent in current mechanical methods, and without smearing of metal in the workpiece. In addition, by eliminating human variability, the resulting level of polishing is specific and reproducible. In addition, cost reduction can be achieved using the electrolyte solution of the present invention compared to the existing method.

In testing, good results for micropolishing have been obtained at high concentrations of citric acid, low to medium concentrations of ABF, high temperature and high DC current densities, which can be applied continuously or periodically. However, the DC current density must be adjusted based on the alloy to be treated. Aluminum-containing titanium alloys (typically alpha-beta metallurgical alloys, including common Ti-6Al-4V alloys) tend to lose gloss at applied DC voltages in excess of 40 volts. However, for such metals, capping the voltage at about 40 volts and applying a higher current (ie, to achieve higher current density) can cause the material to be polished again. While not wishing to be bound by theory, this may be the result of an alpha stabilizing element, and for most alpha-beta alloys (including Ti-6Al-4V), this is aluminum anodization with Al ₂ O ₃ rather than polished. In addition, however, titanium-molybdenum (both beta phase metallurgy) and commercially pure (CP) titanium (both alpha phase) are not brightly limited to similar upper limit voltages but become brighter with increasing DC current density. In particular, for other metals, higher voltages, for example up to at least 150 volts, for nickel based alloy 718 may be used to produce beneficial results in electropolishing, micropolishing and surface treatment using the electrolyte solutions of the present invention. Was found.

The solutions and methods disclosed herein provide a machine by preferentially processing burrs on machined metal parts, in particular when the machined parts are made difficult to machine metals such as titanium and nickel base alloys. It can be used to deburr machined parts. In the state of the art, the deburring of machined parts is typically performed by manual work and thus suffers from a number of problems associated with human error and human mismatch. Testing with the solution of the present invention showed that deburring was most effective at low citric acid concentrations due to the resistance of citric acid in electrochemical cells, and best at high fluoride ions in ABF. In addition, similar solutions can be used to remove surface impurities, or can be used to clean workpieces after machining, such as can be done differently using strong acid washing with an HF-HNO ₃ bath.

Non-ferrous and especially reactive metals demonstrate the effective rate of chemical etching in a wide range of diluted citric acid mixtures as described above. This allows customization of the finishing process for certain nonferrous metal workpieces, which selectively selects the residence in the bath before applying a current to remove and react to a portion of the surface metal before starting electropolishing to reduce the peak area. May include time.

Citric acid-based electrolytes have significantly lower viscosity than conventional electropolishing mixtures due in part to the significantly lower dissociation constants of citric acid compared to the strong acids commonly used in electrolytic polishing electrolytes. Lower voltages can be used compared to conventional electropolishing with lower viscosity acids and lower electrical resistance in material transport. The final electropolishing finish obtained is substantially influenced by the viscosity and resistance of the electrolyte used. The best (high micropolished) surface finish can be achieved using a high resistance electrolyte solution with a high electrolytic polishing voltage (and therefore medium to high current density). In addition, when a slightly higher conductive (less highly resistant) electrolyte solution is used, good micropolishing can still be achieved at high voltages and high current densities.

The corresponding gain is subsequently followed by application to electrochemical machining. Specifically, electrolyte baths having the compositions described herein can be effectively used in place of conventional electrochemical machining and / or pickling solutions with substantial environmental and cost benefits. Since the electrolyte solutions disclosed herein are essentially free of strong acids, the problem of hazardous waste handling and handling is minimized. In addition, the required current density is significantly less than that required for conventional electrochemical machining.

In general, increasing the concentration of ammonium bifluoride reduces the electrical resistance of the electrolyte solution (ie, ammonium bifluoride increases the electrical conductivity of the electrolyte solution), while on the other hand, the presence of citric acid or the concentration of ammonium bifluoride Increasing the concentration of citric acid to the alleviates the effect of ammonium bifluoride on the electrical resistance. That is, in order to maintain a high level of electrical resistance of the electrolyte solution to promote micropolishing, it is preferable to keep the ammonium bifluoride concentration low or to use a higher concentration of ammonium bifluoride with higher concentrations of citric acid. . Thus, by varying the concentration of ammonium bifluoride and the relative concentrations of ammonium bifluoride and citric acid, the electrical resistance of the electrolyte solution can be advantageously adjusted to achieve the desired level of micropolishing of the workpiece surface.

In the methods disclosed herein, the proximity of the workpiece (anode) to the cathode need not be precise, unlike conventional electropolishing or electrochemical machining. Successful processing takes place at the cathode in the range of about 0.1-15 cm from the workpiece. The practical limits on the maximum distance between the cathode and the anode workpiece are mostly commercially derived, including bath size, workpiece size and electrical resistance of the electrolyte solution. Since the total current density is lower and often considerably lower than that required by electrochemical machining, it is possible to use larger workpiece-to-cathode distances, thus simply increasing the power supply capacity. Moreover, because the lower viscosity electrolyte solutions disclosed herein allow for highly controlled bulk metal removal, surface finish and micropolishing, the same solution is also expected to be effective in electrochemical machining.

Electropolishing of the metal workpiece is performed by exposing the workpiece and the at least one cathode to a bath of electrolyte solution and connecting the workpiece to the anode. The electrolyte solution comprises an amount of carboxylic acid in the range of about 0.1% to about 59% by weight. The electrolyte solution may also comprise from about 0.1% to about 25% by weight of a fluoride salt selected from alkali metal fluorides, alkaline earth metal fluorides, silicate etching compounds, and / or mixtures thereof. Current is applied from a power source between at least one anode connected to the workpiece and a cathode immersed in the bath to remove metal from the surface of the workpiece. The current is applied at a voltage ranging from about 0.6 millivolts direct current (mVDC) to about 100 volt direct current (VDC). Citric acid is a preferred carboxylic acid, but is not limited to formic acid, acetic acid, propionic acid, butyric acid, valeric acid, caproic acid, enanthic acid, caprylic acid, pelagonic acid, capric acid, lauric acid, palmitic acid and stearic acid. Other carboxylic acids including can be used. ABF is the preferred fluoride salt.

In another aspect of the electropolishing method, the current is applied at a voltage of about 0.6 VDC to about 150 VDC. The current can be applied at a current density of less than about 255,000 A / m 2 (amperes per square meter) (approximately 24,000 amps per square foot), where the denominator represents the total effective surface area of the workpiece. For some nonferrous metals, such as nickel-based alloys, current densities up to and including about 5,000 A / m 2 (about 450 A / ft ² ) can be used, and for titanium and titanium alloys, about 1 to about 1100 A / m 2 Current densities of (approximately 0.1 to 100 A / ft ² ) are preferred. The electropolishing process using an electrolyte solution may be performed, for example, at a temperature of about 2 ° C to about 98 ° C, preferably at a temperature in the range of about 21 ° C to about 85 ° C.

In practice, the material can be removed from the metal substrate at a rate of about 0.0001 inches (0.00254 mm) to about 0.01 inches (0.254 mm) per minute. The following examples show the effect of the electrolyte on changing concentrations and operating conditions.

Example  1: etching of commercially pure titanium

In an electrolyte that consists essentially of 56% water, 43% citric acid (716 g / L) and 1% (15.1 g / L) ammonium bifluoride, in weight percent, and is carried out at 185 ° F. (85 ° C.), commercially Pure titanium plate samples were treated to improve the surface finish of the material (ie, to make the mill-standard finish more smooth). The material started at a surface finish of about 160 microinches, and after treatment, the surface finish was reduced by 90 microinches with a final reading of 50 microinches, or showed an improvement of about 69%. The treatment carried out for 30 minutes led to a material thickness reduction of 0.0178 inches.

Cold formability, an important feature of titanium plate products in many end-use applications, is highly dependent on the surface finish of the product. Using the implementation of the electrochemical processes disclosed herein, material surface finish enhancement can be achieved at lower cost compared to conventional grinding and pickling methods. Finishes obtained using implementations of the disclosed solutions and methods have been demonstrated to improve the cold formability of plate products to a higher degree than conventional methods.

Example  2: Etching 6A1-4V Specimen

The following examples were processed on 6Al-4V titanium alloy sheet stock specimens measuring 52mm x 76mm. The electrolyte consisted of water (H ₂ O), citric acid (CA) and ammonium bifluoride (ABF) at various concentrations and temperatures. Results observations and readings are shown in Table 1 below.

Table 1

Example  3: 6 Al Electropolishing of -4V Specimen

The following examples were processed on 6Al-4V titanium alloy sheet stock specimens measuring 52mm x 76mm. The electrolyte consisted of water (H ₂ O), citric acid (CA) and ammonium bifluoride (ABF) at various concentrations and temperatures. Results observations and readings are shown in Table 2 below.

Table 2

More extensive tests range from about 0 g / L to about 780 g / L (about 0% to about 47% by weight) citric acid and about 0 g / L to about 120 g / L (about 0% to about 8% by weight) Baths in the range of about 21 ° C. to about 85 ° C., using an aqueous electrolyte solution that contains ammonium bifluoride and is substantially free of strong acids (ie, less than about 1 g / L or less than 0.1 wt%). Temperature and applied current density in the range of about 0 A / m 2 to about 1076 A / m 2 of the workpiece surface area. (780 g / L citric acid in water is saturated at 21 ° C) Current densities as high as at least 225,000 A / m 2 can be used for applied voltages above 150 volts. Metals tested included some spot tests on 6Al-4V titanium and nickel-based alloy 718 as well as commercially pure titanium. Based on these results, it is expected that similar electropolishing, micropolishing and surface treatment results can be obtained in nonferrous metals and alloys. The results are summarized with reference to the drawings in the following table and description. Unless otherwise specified, tests conducted at temperatures of about 21 ° C., about 54 ° C., about 71 ° C. and about 85 ° C., and about 0 A / m 2, about 10.8 A / m 2, about 52.8 A / m 2, about 215 A / m 2, The current density was about 538 A / m 2 and about 1076 A / m 2. Although traces of strong acid did not appear to significantly affect the results, no amount of strong acid was intentionally added to any test solution.

1A-1B show an aqueous electrolyte solution comprising medium and low concentrations of ammonium bifluoride at 20 g / L and citric acid at a concentration of about 0 g / L to about 780 g / L, at four different temperatures, and at 1076 A / m 2 At the current density, respectively, the change in material removal rate and surface finish. 1a shows that the rate of material removal changes directly at temperature, especially at lower concentrations of citric acid. As the bath temperature increases, the rate of removal also increases. At low temperatures of 21 ° C., 54 ° C. and 71 ° C., 180 g / L of citric acid is sufficient to begin to mitigate the material removal effect of ammonium bifluoride, while relatively fast material removal at high temperatures of 85 ° C. is up to about 300 g / l. Continues to L citric acid. In contrast, FIG. 1B shows that at lower citric acid concentrations, especially at citric acid concentrations below 120 g / L to 180 g / L, the surface finish decays at all temperatures except the lowest. That is, fluoride ions that act on significant material removal at lower citric acid concentrations produce surface damage, but the presence of insufficient concentrations of citric acid appears to act as a beneficial barrier to fluoride ion attack. However, as the citric acid concentration increases above 180 g / L, the surface finish actually improves, especially at citric acid levels above 600 g / L, where the rate of material removal is significantly reduced. Moreover, even at citric acid levels of about 120-600 g / L where material removal still occurs, an improvement in surface finish can be achieved simultaneously.

Testing has shown that a source of fluoride ions such as ammonium bifluoride is needed to achieve the desired material removal and surface finish enhancement. In an electrolyte solution consisting essentially of citric acid alone in water, substantially no material removal was obtained, regardless of the temperature of the bath or current density, substantially in the absence of ammonium bifluoride, and the change in surface finish was minimal. When titanium or other reactive metals are treated in an aqueous electrolyte containing only citric acid, the surface of the material is anodized and rapidly formed into an oxide layer that is essentially very thin (ie, about 200 nm to about 600 nm thick). After the anode oxide layer is formed, it hydrolyzes water because the applied DC power can no longer attack the material surface. The resulting generated oxygen quickly finds other monoatomic oxygen and is given to the anode as O ₂ gas.

2A-2B and 2C-2D illustrate the removal of materials and surface finish, respectively, when aqueous electrolyte solutions comprising 120 g / L citric acid and ammonium bifluoride at a concentration of about 0 g / L to about 120 g / L are used. Indicates a change. 2A and 2C show data at a representative low temperature of 21 ° C. and FIGS. 2B and 2C show data at a representative high temperature of 71 ° C. FIG. 2A-2B show that material removal is highly related to ammonium bifluoride concentration and temperature, but is minimally affected by current density. Higher material removal rates are generally obtained by increasing one or both of ammonium bifluoride concentration and temperature. 2C-2D show that material removal occurs with some surface decay. Unexpectedly, however, as the temperature increases and the material removal rate increases, the amount of surface finish decay decreases. As in FIG. 2C, at low temperatures of 21 ° C., the increase in current density mitigates the surface decay effect, and at the highest current density, some surface finish improvement is demonstrated. As in FIG. 2D, at higher temperatures of 71 ° C., the change in surface finish does not change significantly with a change in current density.

2E-2F show, respectively, when performed at a high temperature of 85 ° C. when using an aqueous electrolyte solution consisting essentially of ammonium bifluoride in water and without intentionally adding citric acid, Changes in material removal rate and surface finish are shown. High rates of material removal can be achieved with ABF-only electrolytes, but such material removal results in damage to the surface finish, which is often mitigated to significantly decay by the electrolyte solution. Nevertheless, at certain operating conditions (not shown), minimal decay or moderate improvement of the surface finish was achieved. For example, the improvement of surface finish from the ABF-only electrolyte solution is at 21 ° C. and 215-538 A / m 2 and at 10 ° C./L ABF solution at 54-71 ° C. and 1076 A / m 2, and at 21 ° C. and 215-1076 A / m 2. 20 g / L ABF solution and 60 g / L ABF solution at 21 ° C. and 538-1076 A / m 2.

While not wishing to be bound by theory, a description of the possibility of increased current density for improved surface finish and, on the other hand, minimally affecting material removal rates, suggests that a function of the current is to provide a natural oxide layer on the material surface. It is to grow. Such excess oxygen, along with citric acid, is believed to act as a beneficial barrier to attack of the material surface. Thus, as the current density increases, higher concentrations of oxygen are produced at the anode, which can then act as a mass transfer barrier. Alternatively, if the surface morphology of the material is extremely simplified as a series of "peaks" and "valleys," citric acid and oxygen lie in the valleys, exposing only the peaks of the surface morphology to fluoride ions. It is assumed to be. As the citric acid and oxygen barriers increase in strength (ie, higher citric acid concentration and higher current density), only the highest peak on the surface is available for chemical attack. Under this theory, low current density and low citric acid concentration provide the least possible process for surface smoothing, and high current density and high citric acid concentration are expected to provide the maximum possible process for surface smoothing. Regardless of whether the theory is correct or not, the data appear to prove a result consistent with the analysis.

Oxygen and citric acid (generated by current) appearing to act as micro-barriers to the removal process, which is the variable that ABF concentration and temperature seem to best accept for use to control material removal and micropolishing results. It is understood that. Thus, in the processes described herein, current density appears to primarily play a role in producing oxygen mostly and is not an important agent for increasing total material removal. Rather, material removal appears to be induced almost exclusively by fluoride ions, the activity of which is controlled to some degree by thermodynamic impact of temperature. In sum, the current density as a control variable unexpectedly appears to have a relatively small importance, where the presence of fluoride ions overwhelms the influence of the current density.

3A-3D show that at a typical current density of 53.8 A / m 2, the rate of material removal can vary directly with temperature, resulting in better material removal for the same mixture of citric acid, ammonium bifluoride and water. It occurs at high temperatures. Similar trends were observed at all current densities from 0 to 1076 A / m 2.

4A-4D show that, at a representative temperature of 54 ° C., the rate of material removal is relatively consistent with the current density such that the rate of material removal is equal to the current density for the same mixture of citric acid and ammonium bifluoride at any given bath temperature. Relatively insensitive to change. Similar trends were observed at all temperatures of 21-85 ° C., and this trend remained below 21 ° C. (but above the freezing point of the solution) and above 81 ° C. (but below the boiling point of the solution). As it occurs at almost all temperature and current conditions irrespective of the ABF concentration, as the citric acid concentration rises above a certain level, typically from 600 g / L to 780 g / L, the rate of material removal is significantly reduced. If workpiece molding is desired, the citric acid concentration should generally be kept below 600 g / L to maintain the ability to achieve some level of material removal.

4E-4G show the effect of current density on material removal rate at representative high temperatures of 85 ° C. and three different concentrations of citric acid, and FIGS. 4H-4J show the current density for surface finish under the same set of conditions. Influence. FIG. 4E shows that, on a smaller scale, as shown in FIGS. 4F and 4G, the material removal capability of the electrolyte solution is maximum at the highest concentration of ammonium bifluoride and is significant at high temperatures. 4E only shows data at 120 g / L citric acid, but it should be noted that essentially the same material removal rates are seen at citric acid concentrations of 60 g / L, 120 g / L and 300 g / L. However, as shown in FIG. 4F, at 600 g / L citric acid, the concentration of citric acid provides some amount of protection to the surface from large-scale attack, and the rate of material removal appears to be lower compared to lower citric acid concentrations. At 780g / L, as shown in FIG. 4G, the removal rate is further reduced. Regardless of the concentration of ammonium bifluoride and citric acid, material removal appears to be hardly affected by the current density.

4H shows that moderate amounts of surface finish decay at high temperatures and moderate citric acid concentrations occur at almost all ammonium bifluoride concentrations and current densities. 4E and 4H, however, one process condition stands out. At a citric acid concentration of 120 g / L, at low levels of 10 g / L ammonium bifluoride and a high current density of 1076 A / m 2, material removal is suppressed and a significant improvement in surface finish occurs. This is due to the fact that elevated current densities can create sufficiently excessive oxygen at the material surface to fill the "valley" at the surface such that the "peak" is preferentially attacked by the fluoride ions produced by dissociation of ammonium bifluoride. Further evidence of the above-mentioned ions may be provided. This effect, together with the possible micro-barrier effects of citric acid, can be seen more strongly in FIGS. 4I (600 g / L citric acid) and 4J (780 g / L citric acid), which surface at higher citric acid concentrations and higher current densities alone. It shows a reduced decay of the finish, in some cases an improvement of the surface finish, and also a combination of higher citric acid concentrations and higher current densities. For example, there is a marked improvement in surface finish at 10 g / L and 20 g / L ammonium bifluoride in 600 g / L to 780 g / L citric acid.

However, the surface finish is such an effect as it can be seen that the highest concentration of ammonium bifluoride of 120 g / L and higher current densities of 120 g / L to 600 g / L and further worsen at 780 g / L citric acid. There seems to be a limit. Similar results were obtained at 60 g / L ammonium bifluoride in citric acid concentrations of at least 600 g / L to 780 g / L.

As shown in Tables 3a-3c and 4a-4c, sheet product finishes that require minimal material removal and want to improve medium to high surface finish, and micropolishs that virtually eliminate material removal and require very high surfaces Process conditions for can be achieved over a wide range of electrolyte mixtures, temperatures and current densities. Although these solutions can achieve essentially zero material removal and medium to high surface enhancement over a wide range of temperature and current densities, since these conditions are discussed separately with reference to FIGS. 1A-1C, Table 3A -3c and 4a-4c consist essentially of water and citric acid and contain no electrolyte substantially free of ammonium bifluoride. Likewise, since these conditions were discussed separately with reference to FIGS. 2A-2D, Tables 3A-3C and 4A-4C consist essentially of water and ammonium bifluoride and contain substantially no citric acid-containing electrolyte. Do not. 3A-3C are divided by the level of surface finish improvement and then arranged in order of increased ABF concentration. Tables 4a-4c break down the levels of citric acid concentrations and then sort them in order of increased ABF concentrations.

Several trends emerge from the data in Tables 3a-3c. First, low or near zero material removal and improved surface finish include full range of citric acid concentrations (60 g / L to 780 g / L), aluminum non-fluoride concentrations (10 g / L to 120 g / L), temperature (21-85). ° C) and current density (10.8-1076 A / m 2). Thus, aqueous solutions of citric acid and ABF in the presence of substantial strong acid are minimal at concentrations as low as 60 g / L citric acid and 10 g / L ABF and at concentrations as high as 780 g / L citric acid and 120 g / L ABF, and in various combinations between them. Material loss can produce excellent surface finish.

Table 3A: Improved Top Surface Finish

In general, as shown in Table 3a, the minimum level of surface finish improvement (ie, a reduction of more than 30% in surface roughness) is high current density of 538-1076 A / m 2, high citric acid concentration of 120-780 g / L, and general As obtained at low ABF concentrations of 10-20 g / L. If the ABF concentration is lower in the range of 10-20 g / L, higher temperatures of 71-85 ° C. produce better surface finish at higher citric acid concentrations of 600-780 g / L, while medium temperatures of 54 ° C. A degree of temperature tends to produce good surface finish at medium citric acid concentrations of 120-300 g / L. Nevertheless, also significant improvements in surface finish are low ABF, medium citric acid and high temperature conditions (10 g / L ABF, 120 g / L citric acid, 85 ° C.) and low ABF, medium citric acid and low temperature conditions (20 g / L ABF, 180 g / L citric acid, 54 ° C.). If the ABF concentration is higher in the range of 60-120 g / L, lower temperatures of 21-54 ° C. tend to produce better surface finish at higher citric acid concentrations of 600-780 g / L and higher current densities. have. In addition, significant surface finish improvements were achieved at 10.8-53.8 at high citric acid concentrations of 780 g / L and high temperatures of 71-85 ° C. for both low ABF concentrations of 10 g / L and high ABF concentrations of 120 g / L, as shown in FIG. 4H. Achieved at lower current densities of A / m 2.

Table 3B: Top Surface Finish Improvements

In general, as shown in Table 3B, the high but not high level of surface finish improvement (ie, a reduction of about 15% to about 30% of surface roughness) is achieved at lower ABF concentrations of 10-20 g / L and at 54-85 ° C. Acquired at medium temperature, a generally but not exclusive surface finish improvement was obtained at higher currents of 538-1076 A / m 2. Typically, these results have been achieved at high citric acid concentrations of 600-780 g / L. For example, 10-20 g / L concentrations of ABF generally produce good results at higher current densities and higher citric acid concentrations, but good results are also at low current densities of 10.8 A / m 2 and at high temperatures of 85 ° C., and It was obtained using a lower current density of 53.8 A / m 2 and lower citric acid concentration of 60-300 g / L at an intermediate temperature of 54 ° C. The high improvement in surface finish is also at high temperatures and low current densities (71-85 ° C. and 10.8-53.8 A / m 2) and low temperature and high current densities (21 ° C. and 1076 A / m), in all cases with a high citric acid concentration of 780 g / L. M 2) at high levels of ABF of 120 g / L. In this regard, temperature and current in that similar surface finish results can be achieved for solutions with high concentrations of citric acid by using higher current densities at lower temperatures or lower current densities at higher temperatures. There appears to be some complementary activity between the densities. Referring also to FIGS. 4H-4J, high temperature conditions in combination with high current densities show a tendency to produce the best surface finish enhancement.

Table 3C: Improved intermediate surface finish

In general, as shown in Table 3C, moderate surface finish enhancement (ie, less than about 15% reduction in surface roughness) results in lower ABF concentrations of 10-20 g / L and higher temperatures of 71-85 ° C. and Typically obtained over a full range of current densities of 10.8-1076 A / m 2. Typically, these results have been achieved at high citric acid concentrations of 600-780 g / L. One notable exception to this trend is that used surface finish enhancements are also obtained at all ABF concentrations of 10-120 g / L and low weight citric acid concentrations of 60-300 g / L at low temperatures of 21 ° C. and high current densities of 1076 A / m 2. It is.

Table 4A: Lowest Citrate Concentration

As shown in Table 4A, the improvement of surface finish at low citric acid concentrations of 60-180 g / L was consistently shown to require high current density. Typically, the best surface finish improvements were obtained at low ABF concentrations of 10-20 g / L and high temperatures of 54-85 ° C. Low surface finish improvements were achieved at ABF concentrations of 10-60 g / L and temperatures of 21 ° C.

Table 4B: Medium Citric Acid Concentration

As shown in Table 4B, at intermediate citric acid concentrations of 300-600 g / L, a marked improvement in surface finish generally requires higher current densities of 538-1076 A / m 2, mainly low ABF of 10-20 g / L ABF. Occurs in concentration. At the lowest ABF concentration of 10 g / L, higher temperatures of 54-85 ° C. achieve the best results, while good results at the ABF concentration of 20 g / L are achieved in the 21-85 ° C. range. At higher ABF concentrations of 60-120 g / L, surface finish enhancement occurs more typically at lower temperatures of 21 ° C.

Table 4C: Maximum Citric Acid Concentration

Comparing Tables 4C and 4A and 4B, the highest process conditions to achieve surface enhancement with virtually no material loss or minimal material loss occur at high citric acid concentrations of 780 g / L. As shown in Table 4C, a significant improvement in surface finish at high citric acid concentrations of 780 g / L is achieved with almost all current densities of 10.8-1076 A / m 2 and low to high temperatures of 21-85 ° C. and 10-20 g / L ABF. Both low ABF concentrations and high ABF concentrations of 120 g / L ABF can be obtained.

5A and 5B show material removal rates and surface finish changes at representative low temperatures of 21 ° C. and representative high current densities of 538 A / m 2. From FIG. 5B, surface finish decay is common at all citric acid concentrations below 600 g / L for ABF concentrations below 60 g / L, and surface finish is actually 10-120 g at high citric acid concentrations above 600 g / L, in particular 780 g / L It can be seen that for every ABF concentration of / L it is improved. 5A also shows that the rate of material removal under these process conditions is relatively low. Thus, it may be desirable to perform at this range of compositions, temperatures and current densities to achieve moderate controlled material removal with minimal surface decay, or perhaps particularly effective for large-scale material removal, although moderate surface finish enhancement occurs. It may not be.

Likewise, FIGS. 6A and 6B show material removal rates and surface finish changes at representative low temperatures of 21 ° C. and high current densities of 1076 A / m 2. From FIG. 6B, small or moderate surface finish enhancement is achieved at all citric acid concentrations below 600 g / L for ABF concentrations above 10 g / L and below 120 g / L, with surface finish most significantly at citric acid concentrations above 600 g / L. It can be seen that the improvement. 6A also shows that the rate of material removal at these process conditions is relatively low except for compositions near 300 g / L citric acid and 120 g / L ABF where the material removal rate is higher without causing any significant surface decay. . Thus, it may be desirable to perform at this range of compositions, temperatures and current densities to achieve moderate controlled material removal with minimal surface decay, or perhaps particularly effective for large-scale material removal, although moderate surface finish enhancement occurs. It may not be.

7A and 7B show that under certain conditions controlled material removal and surface finish enhancement can be achieved simultaneously. In particular, at an ABF concentration of about 10 g / L, FIG. 7A shows a constant normal material removal rate across all citric acid concentrations when the workpiece is exposed to electrolyte solution at high temperatures of 85 ° C. and high current densities of 1076 A / m 2. 7B shows a substantial improvement in surface finish at all citric acid concentrations above 60 g / L. Even at higher ABF concentrations of 20 g / L to 120 g / L ABF, material removal can be obtained in direct correlation with the ABF concentration without substantial loss of surface finish. However, at the highest citric acid concentrations above 600 g / L citric acid, the rate of material removal is significantly shortened.

A range of performance conditions have been identified in which controlled material removal can be achieved while not having much surface finish decay to increase roughness, typically less than about 50%. 8A-8B, 9A-9B and 10A-10B show exemplary performance conditions that fall within this category.

8A shows that at high temperature (85 ° C.) and low current density (10.8 A / m 2) conditions, a fairly constant material removal rate can be achieved at all ABF concentrations for citric acid concentrations ranging from about 60 g / L to about 300 g / L. As a result, higher material removal rates are obtained in direct correlation with ABF concentrations. FIG. 8B shows that for these citric acid and ABF concentration ranges, the surface finish decay is consistently medium regardless of the specific citric acid and ABF concentration. Citric acid concentrations above 600 g / L significantly reduce or even stop the material removal capacity of the electrolyte solution, and also, except for ABF concentrations of 60 g / L, tend to mitigate surface finish deterioration and slightly improve surface finish. There may be. 9A and 9B show very similar results at high temperature (85 ° C.) and high current density (538 A / m 2) conditions, and FIGS. 10A and 10B show somewhat lower temperatures of 71 ° C. and an intermediate current density of 215 A / m 2. Similar results can be obtained from.

Based on the test data disclosed herein, by adjusting the temperature and current density, the same aqueous electrolyte solution bath first removes moderately controlled amounts of material at a relatively low current density, and then maintains the temperature or It can be used in a multi-step process including healing the surface by increasing the current density to a high level while slightly lowering it. For example, when using a solution with 300 g / L citric acid and 120 g / L ABF, normal material removal rates are obtained at temperatures of 85 ° C. and current densities of 53.8 A / m 2, with less than 30% deterioration of surface finish. Surface enhancement can then be obtained at the same temperature and current density of 1076 A / m 2 (see FIGS. 7A and 7B) with less material removal.

More combinations of conditions for the multi-step process can be achieved by changing the citric acid concentration in addition to the temperature and current density, due to the strong material removal mitigation effect that occurs when the citric acid concentration increases above 600 g / L. For example, referring to FIGS. 8A and 8B, in the case of using an electrolyte solution having 120 g / L ABF at a temperature of 85 ° C. and a current density of 10.8 A / m 2, aggressive material removal occurs with normal surface decay. This can be achieved by performing at a concentration of 300 g / L citric acid in the second process step and then simply increasing the concentration of citric acid to 780 g / L in the second process step, and material removal can be virtually stopped while the surface finish is significantly improved. . Similar results can be obtained using the high temperature, high current density conditions of FIGS. 9A and 9B, or the high and medium current density conditions of FIGS. 10A and 10B.

Very low concentrations of ammonium bifluoride have been shown to be effective in both material removal and micropolishing. As shown in FIG. 1, the rate of material removal is good at elevated temperatures, so that lower concentrations of ammonium bifluoride are expected to be effective at higher temperatures, such as 85 ° C. or higher. In one exemplary electrolyte solution with both citric acid and ammonium bifluoride concentrations of 2 g / L, material removal and surface finish changes were observed. At 285 A / m 2, a material removal rate of 0.008 mm / hr was recorded with a surface finish change (decay) corresponding to -156%. A material removal rate of 0.0035 mm / hr was recorded with a surface finish change corresponding to -187% at 0 A / m 2.

Similarly, when treated with an applied current of 271 A / m 2 in an aqueous solution containing 2 g / L ABF and citric acid, the material removal rate of 0.004 mm / hr with a change in surface finish (decay) corresponding to -162% The material removal rate of 0.0028 mm / hr was recorded at 0 A / m 2 with a surface finish change corresponding to -168%.

It may be desirable to use the minimum amount of ABF required to have an effect, but at levels as high as 240-360 g / L, concentrations significantly above 120 g / L, including ammonium bifluoride concentrations, even in supersaturated water Can be used. The effect of the electrolyte solution at high concentrations of ABF was tested by the incremental addition of ABF to a solution of 179.9 g / L citric acid at a fixed temperature of 67 ° C. and a current density in the range of 10.8-255.000 A / m 2. Because such solutions have relatively low electrical resistance, it was predicted that higher concentrations of ABF could provide higher conductivity to the solution, especially at higher levels of current density. In addition, the temperature was raised above room temperature in order to reduce the resistance of the electrolyte. Samples of both CP titanium and nickel base alloy 718 were exposed to the electrolyte and bulk material removal and micropolishing continued as ABF was added. ABF was added above its saturation point to the electrolyte. Under these parameters the saturation point of ABF (depending on temperature and pressure) was about 240-360 g / L. The data in Table 5 show that the electrolyte solution was effective for both bulk metal removal and micropolishing at ABF concentrations above the supersaturation concentration in water.

The test was conducted to determine the effect of the electrolyte solution on micropolishing and bulk metal removal at relatively high current densities, including close to 255.000 A / m 2. It is understood from the literature that electrolytes having low resistance values can tolerate high current densities. Particular combinations of citric acid concentration and ABF concentration show particularly low resistance. For example, an electrolyte solution containing about 180 g / L citric acid in the temperature range of about 71-85 ° C. was irradiated at high current density. Samples of commercially pure (CP) titanium and nickel base alloy 718 were exposed to this electrolyte solution with a gradual increase in current density in the range of 10.8-255.000 A / m 2. The data in Table 5 shows that bulk material removal and micropolishing were achieved at all current densities tested in the range including 255.000 A / m 2. Compared to treating titanium and titanium alloys, it may be useful for treating nickel-based alloys at higher current densities, particularly at about 5000 A / m 2.

CP titanium is effectively treated with relatively low voltages of about 40 volts or less, although higher voltages may also be used. In one exemplary test, CP titanium was treated at 85.6 ° C. by applying a potential of 64.7 VDC and a current density of 53.160 A / m 2 in a bath of an aqueous electrolyte solution comprising about 180 g / L citric acid and about 120 g / L ABF. Under these conditions, a 5 mm / hr bulk metal removal rate was achieved with a 37.8% improvement in surface roughness roughness, which formed a surface with a visually uniformly bright and transparent appearance. The same chemical electrolyte remained effective in CP titanium samples for bulk metal removal after increasing the voltage to 150 VDC and reducing the current density to 5,067 A / m2, but under these conditions the metal removal rate was slowed to 0.3 mm / hr and finished Slightly deteriorated with satin appearance.

For some metals and alloys, higher voltages may be equally or more effective at achieving one or both of bulk material removal and surface finish enhancement. In particular, certain metals, including but not limited to nickel-based alloys (such as Waspalloy and Nickel Alloy 718), 18k gold, pure chromium and nitinol alloys, provide faster bulk metal removal and / or better surface finish enhancement. It appears to be beneficial from higher voltage processes. In one exemplary experiment, specimens treated in an aqueous electrolyte comprising about 180 g / L citric acid and about 120 g / L ABF at a relatively high voltage in nickel alloy 718 using a potential of 150 VDC and a current density of 4,934 A / m 2. Formed only a bulk metal removal rate of 0.09 mm / hr, but a 33.8% uniform surface finish improvement based on surface roughness measurements.

Table 5

In order to evaluate the effect of the metal accumulated and dissolved in the electrolyte solution, a batch of 21 Ti-6Al-4V rectangular bars having dimensions of 6.6 cm x 13.2 cm x about 3.3 meters was continuously subjected to a bath of about 1135 liters. Treated. The process was to demonstrate high controlled metal removal in typical mill product forms. For 21 pieces of rectangular bars, a total volume of 70.9 kg of material was removed from the bars and suspended in the electrolyte solution. The first bar started the process with 0 g / L of dissolved metal in solution and the last bar performed the process with an excess of dissolved metal content of 60 g / L. At the beginning of the process and at the end of the process, no deleterious effects on metal surface conditions or metal removal rates were detected and no significant change in any operating parameters was required as a result of the increased dissolved metal content in the electrolyte solution. This is in contrast to the HF / HNO ₃ acid wash results of titanium, where the solution is substantially less effective at titanium concentrations of 12 g / L in solution. Likewise, electrochemical machining interferes with high levels of molten metal in the electrolyte solution because the metal particles can interfere with the gap between the cathode and the anode workpiece and even cause short circuits if the solid is electrically conductive. Receive.

Crack control and oxide layer removal

The aqueous electrolytes disclosed herein control or round the bottom of the surface cracks, and thus are formed at various depths across the metal surface, including but not limited to reactivity including titanium and titanium alloys, nickel-based alloys, zirconium, and the like. These metals in the metal family can be used to remove the sharpest tip crack tips that are most harmful when cooled from elevated temperatures in an oxygen-containing atmosphere. For example, such surface cracks may occur upon cooling from processes including, but not limited to, hot working (eg, forging, rolling, superplastic forming, etc.), welding, and heat treatment. Moreover, the aqueous electrolyte solution can control these cracks with relatively little yield loss. Rounded or controlled crack tips are suitable for subsequent thermal metal processes because such cracks may be "heal" during subsequent thermal operations, while typical metal-cooled cracks may not be removed in subsequent processes if not completely removed. "Runs" and the metal is broken or broken.

11 shows a conventional process for removing cracks, which typically involves mechanically removing the entire uniform layer of material by grinding or machining to expose the bottom of the deepest surface crack. As shown schematically, this causes substantial loss of material. In contrast, Figure 12 shows the use of an electrolyte solution with the application of a current to widen the crack and round the tip of the crack at the bottom thereof, so that if the workpiece is subjected to further heat treatment or placed in service, A method of controlling cracks that does not propagate.

Thus, the preferred process conditions for crack tip control or removal are to produce very fast removal rates while minimizing or eliminating hydrogen pickup. As described further below, such conditions are generally obtained when the concentration of carboxylic acid (eg, citric acid) is low, the fluoride ions (eg in the form of ammonium bifluoride) are high, and the temperature is high. Effective large-scale removal can be performed at all power densities, but good results are obtained when the power is cycled to have a low power density and significant off duty that the process will withstand to minimize the hydrogen content imparted to the material. Was achieved.

By performing within these process parameters, the crack control capability of the aqueous electrolyte solution is significantly greater than the current treatment where the metal surface is uniformly or locally mechanically removed until the deepest crack bottom is rounded to the same height as the surrounding metal. To improve metal yield. In addition, the treatment and consumption costs when using the electrolyte solutions and methods disclosed herein are significantly lower than current mechanical removal.

Another significant process advantage that can be obtained with the aqueous electrolyte solution is the removal of the oxide layer of the reactive metal or the alpha case in the case of titanium and titanium alloys. Similar to oxide layers of other reactive metals, the alpha case is an oxygen-enriched phase that occurs when titanium and its alloys are exposed to heated air or oxygen. Alpha cases are brittle and tend to produce a series of surface microcracks, which reduce performance including the strength, fatigue properties and corrosion resistance of metal parts. Titanium and titanium alloys are particularly reactive metals, which are tough oxide layers (TiO ₂ , Ti for Ti) that react with oxygen whenever they are heated in air or an oxidizing atmosphere at or above the temperature at which the natural oxide layer is formed. ZrO _2, etc.) to Zr, which depends on the particular alloy and oxidizing atmosphere. As noted above, the oxide layer or alpha case oxide layer may be produced by heating the metal to the temperature required for mill forging or mill rolling as a result of welding, or by heating for forging or heating of the finished portion. Alpha cases are brittle and filled with micro-cracks penetrating into the bulk metal, potentially causing immature tensile or fatigue failure, making the surface more susceptible to chemical attack.

Thus, the alpha case layer must be removed before any subsequent hot or cold work or final part service. The aqueous electrolyte solutions described herein, and methods using such solutions, remove the alpha case to reveal unaffected base metals. Alpha case removal is a challenging problem for titanium and titanium alloy processes. The reason is that alpha cases are quite resistant to attack, and there is a common belief that some mechanical intervention is required before the electrochemical process.

This difficulty was emphasized in the initial test, where the titanium alpha case was lightly ground as a pre-treatment for primary grit-blasted and / or electrochemical treatment. Once abraded, the residual oxide layer is most easily removed by a DC power cycle on, followed by a simple scrub brush wear, followed by an off cycle of the DC power source, which is then repeated several times. Note that an alternative to scrub brush wear may be a high pressure, high capacity pump system. When the alpha case layer is removed, the material removal rate of the electrochemical etch increases. However, this multi-step process cycle is very costly to carry out and produces low profits.

To overcome this difficulty, workpieces with oxide or alpha case layers have low carboxylic acid (eg, citric acid) concentrations, high fluoride ion (eg, ammonium nonfluoride) concentrations, high temperatures, and preferably low power densities. It is treated in a bath of aqueous electrolyte solution. Low citric acid concentrations, high ammonium bifluoride concentrations, and high temperatures maximize the rate of material removal, which allows less hydrogen to penetrate the material surface because it is relatively insensitive to current density when power is applied periodically. Lower power densities are used.

In summary, aqueous electrolyte solutions and methods using such solutions disclosed herein enable the removal of oxide layers and titanium alpha cases from reactive metal surfaces in controlled repeatable forms. This is a strongly exothermic reaction and in the case of titanium uses evolved titanium as a catalyst in the reaction, thus causing a continuous change in acid concentration and reaction rate, making it difficult to repeat and eliminating accumulated surface metals. This is in contrast to the current HF-HNO ₃ acid wash, which makes it impossible. Moreover, the aqueous electrolyte solution and method do not charge harmful hydrogen into the bulk metal. As a result, the process of the invention can be carried out in a manner that removes hydrogen from the bulk metal. In contrast, current processes inherently introduce harmful hydrogen into the bulk metal, requiring an additional costly degassing step to remove the hydrogen.

The aqueous electrolyte solution does not produce environmentally friendly and hazardous waste, but the current process uses environmentally challenging and hazardous hydrofluoric acid (HF) and nitric acid (HNO ₃ ), which are quite difficult to handle and can only be used under strict acceptance programs. will be. As a result, the method using the aqueous electrolyte solution can be carried out without significant air handling equipment, and no dangerous chemical protection equipment required for using the current HF-HNO ₃ ) process is required of the operator.

For crack control and alpha case removal, surface finish is not particularly important and if surface finish improvement is unnecessary due to further work steps to be followed, it will not severely deteriorate the material surface or will cause pitting or other deep defects in the material. It is still desirable not to cause it.

In Figures 3d, 4e-4j, 7a, 8a, 9a and 10a, the highest mass removal rate is achieved by maximizing the ABF concentration and temperature while maintaining the citric acid concentration below 300 g / L (i.e. 120 g / L as shown in the graph). At an ABF concentration and a temperature of 85 ° C.). In addition, as cited in various figures including FIGS. 4E-4G, citric acid tends to mitigate the attack of fluoride ions on the material surface, so that the rate of material removal increases as the citric acid concentration approaches 0 g / L. It can be seen that there is a tendency towards. However, at zero citric acid concentrations, Figures 4H, 7B, 8B, 9B and 10B show that surface fit and severe surface finish deterioration can occur.

Thus, when using the aqueous electrolyte solution for crack control where severe surface deterioration is preferably avoided, at least a small amount of citric acid, such as, for example, 1 g / L or 10 g / L, has the most severe effect of fluoride ion attack. It should be used to mitigate. However, when using an aqueous electrolyte solution for alpha case removal where severe surface deterioration may not be particularly detrimental, fluoride ion attack can be tolerated as aggressive as possible without causing severe fitting, thus allowing citric acid concentration to be Can be reduced to nearly zero.

In both crack control and alpha case removal environments, it is desirable to avoid introducing hydrogen into the material so that embrittlement is suppressed. In particular, as shown in Figures 1A-1C, 2A-2B and 4E-4G, because the material removal rate is relatively insensitive to the current density, and because the higher current density tends to induce hydrogen into the material, It is desirable to perform at the lowest effective current density.

Although described in connection with an exemplary embodiment, one of ordinary skill in the art may make additions, deletions, modifications and substitutions that are not specifically described without departing from the spirit and scope of the invention as defined in the appended claims. It will be appreciated that the invention is not limited to the specific implementations disclosed herein.

Claims (14)

About 0.1% to about 59% by weight of carboxylic acid; And
About 0.1% to about 25% by weight of fluorinated salt
An aqueous electrolyte solution, substantially free of strong acids.
The method of claim 1,
Wherein said fluorinated salt is selected from the group consisting of alkali metal fluorides, alkaline earth metal fluorides, silicate etching compounds and combinations thereof.
3. The method of claim 2,
Wherein said fluorinated salt is ammonium bifluoride.
The method of claim 1,
Wherein the carboxylic acid is citric acid.
From about 1.665 g / L to about 982 g / L citric acid; And
Ammonium bifluoride of about 2 g / L to about 360 g / L fluorinated salt
An aqueous electrolyte solution, comprising substantially no acid.
Exposing the surface of the non-ferrous metal workpiece to a bath of an aqueous electrolyte solution comprising citric acid at a concentration of up to about 300 g / L and ammonium bifluoride at a concentration of at least about 10 g / L and having a strong acid of up to about 3.35 g / L step;
Adjusting the temperature of the bath to about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the bath; And
Applying a current across the bath
A method of treating a surface of a non-ferrous metal workpiece comprising a.
The method according to claim 6,
Wherein the temperature of the bath is adjusted to about 71 ° C. or greater.
The method according to claim 6,
Wherein the current applied across the bath is about 538 A / m 2 or less.
9. The method of claim 8,
Wherein the current applied across the bath is less than or equal to about 53.8 A / m 2.
The method according to claim 6,
And wherein said concentration of citric acid is less than about 60 g / L.
The method according to claim 6,
And wherein said concentration of ammonium bifluoride is at least about 60 g / L.
Exposing the surface of the non-ferrous metal workpiece to a bath of an aqueous electrolyte solution comprising citric acid at a concentration of about 600 g / L and less than ammonium bifluoride at a concentration of about 60 g / L and having a strong acid of about 3.35 g / L or less. step;
Adjusting the temperature of the bath to about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the bath; And
Applying a current of less than about 53.8 A / m 2 over the bath;
Comprising a crack at the surface of the non-ferrous metal workpiece.
Exposing the surface of the non-ferrous metal workpiece to a bath of an aqueous electrolyte solution comprising citric acid at a concentration of about 60 g / L or less and ammonium bifluoride at a concentration of about 60 g / L or more and having a strong acid of about 3.35 g / L or less step;
Adjusting the temperature of the bath to about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the bath; And
Applying a current of less than about 53.8 A / m 2 over the bath;
A method of removing metal oxides from the surface of a non-ferrous metal workpiece comprising a.
The surface of the titanium or titanium alloy workpiece is exposed to a bath of an aqueous electrolyte solution comprising citric acid at a concentration of about 60 g / L and ammonium bifluoride at a concentration of at least about 60 g / L and having a strong acid of about 3.35 g / L or less. step;
Adjusting the temperature of the bath to about 54 ° C. or higher;
Connecting the workpiece to an anode of a DC power source and immersing the cathode of the DC power source in the bath; And
Applying a current of less than about 53.8 A / m 2 over the bath;
A method of removing an alpha case from a surface of a titanium or titanium alloy workpiece comprising a.

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