Methods for removal of oxygen-enriched surface layer from metallic workpieces

Abstract

Methods for removing an oxygen-enriched surface layer from a metallic workpiece utilizing electrochemistry generally include immersing in an aqueous solution at least a portion of the metal workpiece having an oxygen-enriched surface layer formed thereon, immersing in the aqueous solution at least a portion of an electrode, and applying an alternating current across the aqueous solution between the metal workpiece and the electrode to thereby remove the oxygen-enriched surface layer from the metal workpiece. At the time of immersing the metal workpiece in the aqueous solution, the oxygen-enriched surface layer formed thereon is free of fractures or cracks formed by mechanical pretreatment of the workpiece. The aqueous solution may include a solvent, and the solvent may be a compound including fluoride, such as ammonium bifluoride.

Description

METHODS FOR REMOVAL OF OXYGEN-ENRICHED SURFACE LAYER FROM METALLIC WORKPIECESCROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 596,180, filed November 3, 2023, the entirety of which is hereby incorporated by reference.BACKGROUND

[0002] In the field of hot working (forging, rolling, extruding) reactive metals (e.g., titanium, niobium, zirconium, vanadium, and their alloys) or exposing reactive metals to heated air (such as during component forging, casting, welding, or heat treatment of reactive metals), a tenacious, hard, brittle oxygen-enriched surface layer develops on the reactive metal. When the reactive metal is titanium or its alloys, this oxygen-enriched surface layer is a microstructural phase called alpha case. For the sake of this disclosure, the term “oxygen-enriched surface layer” and its variants should be interpreted as including alpha case unless otherwise specified. This oxygen-enriched surface layer fully covers the hot worked or heat exposed reactive metal, including cooling cracks, surrounding forging laps, and / or hot working defects that may be present in / on the reactive metal.

[0003] The hard brittle nature of this oxygen-enriched layer is such that it must be fully removed prior to additional metal processing, such as further forging or rolling, or finished part manufacturing. If not removed, the metal tends to crack or defects of greater magnitude are created during the subsequent processing.

[0004] The ability to remove an oxygen-enriched surface layer is currently accomplished using subtractive methods. Generally speaking, the removal method is either all-mechanical or a combined mechanical plus chemical method. All-mechanical methods include, but are not limited to, machining or grinding all over the oxygen-enriched surface layer to a specific depth or final dimension. Machine time, loss of prime metal, and high tooling costs all make this method undesirable. Because machining and grinding operations often smear and re-deposit surface metal, leaving the true surface condition un-clear, it iscommon to follow machining and grinding operations used to remove the oxygen-enriched layer with submerging the metal workpiece in corrosive chemical agents to reveal near- surface remaining defects for elimination by further machining or grinding.

[0005] Combined mechanical plus chemical methods are based on first “fracturing” the hard, brittle oxygen-enriched surface layer, which then allows for chemical dissolution of the oxygen-enriched surface layer and some of the underlying “prime” metal via intrusion of corrosive chemical agents into the fractures. Methods of fracturing the oxygen-enriched surface layer can include abrading via high velocity and / or high-pressure abrasive media. Alternatively, fracturing of the oxygen enriched surface layer may be accomplished by thermal means via molten salt baths followed by a water quench. The resulting 400-500°F rapid change in temperature results in the formation of multiple cracks on the surface of the material. Once the oxygen-enriched surface layer is fractured, the metal workpiece may be submerged in corrosive chemical agents which, due to the fractured surface, have pathways to chemically etch and remove the remaining underlying oxygen-enriched surface layer. Exemplary corrosive chemical agents that are used post-fracturing include a mixture of nitric acid and hydrofluoric acid.

[0006] The above-described methods effectively remove the oxygen-enriched surface layer. The specific method used can be selected based on, e.g., the thickness of the oxygen- enriched surface layer, the material’s form, depth of cracks which form on cooling from hot working, and / or the cost to benefit ratio. However, each processing step carries with it operating and capital cost and associated inefficiencies via their unique operation. For example, fracturing equipment size constraints limit production rates and throughput, but more importantly, total process time is slowed by these operational constraints as well as by the inefficiency of multiple handling sequences. Lean manufacturing operations result in overall higher production rates, lower costs, and high product quality.

[0007] In view of the above, a need continues to exist for improved methods for removing oxygen-enriched surface layers from metal workpieces.SUMMARY

[0008] Described herein are various embodiments of a method for removing an oxygen-enriched surface layer from a metallic workpiece, wherein such methods utilize electrochemistry for removing such oxygen-enriched surface layers and expressly eliminate the need for any type of mechanical treatment resulting in the formation of fractures or cracks in the oxygen-enriched surface layer as part of the removal process.

[0009] In some embodiments, the method generally includes immersing in an aqueous solution at least a portion of a metal workpiece having an oxygen-enriched surface layer formed thereon; immersing in the aqueous solution at least a portion of an electrode; and applying an alternating current across the aqueous solution between the metal workpiece and the electrode to thereby remove the oxygen-enriched surface layer from the metal workpiece. In some embodiments, at the time of immersing the metal workpiece in the aqueous solution, the oxygen-enriched surface layer formed thereon is free of fractures or cracks formed by mechanical pretreatment of the workpiece. In some embodiments, the aqueous solution comprises a solvent, the solvent being a compound including fluoride.DETAILED DESCRIPTION

[0010] Embodiments are described more fully below. These embodiments are disclosed in sufficient detail to enable those skilled in the art to practice the invention. However, embodiments may be implemented in many different forms and should not be construed as being limited to the embodiments set forth herein. The following detailed description is, therefore, not to be taken in a limiting sense.

[0011] The technology described herein pertains to safer and more economical removal of an oxygen-enriched surface layer from a metal workpiece. The technology described herein eliminates the need for mechanical fracturing methods as discussed previously by employing an electrochemical method that allows for removal of the oxygen- enriched surface layer. In so doing, product throughput is increased, required capital equipment is minimized, and multiple handling steps are eliminated.

[0012] More specifically, the technology described herein relates to the controlled introduction of electrons via an external power supply to thereby enable, in some cases,simultaneous oxidation and reduction of metal species and their oxides, resulting in the removal of oxygen-enriched surface layers without the need for, e.g., machining, grinding, or fracturing. Control of the reactions involved in the technology described herein, specifically the oxide removal, surface finish, and substrate removal rates, is provided by the external supply of electrons. The ability to customize and configure the compositions and concentrations of the electrochemical bath and the external supply of electrons of the technology described herein allows for the successful removal of an oxygen-enriched surface layer.

[0013] In some embodiments, the process described herein involves submerging the metallic workpiece and an opposing electrode in an electrochemical bath, and then delivering electrical signal between the workpiece and an opposing electrode for a period of time. This electrical signal is designed to target the oxygen-enriched surface layer. Immediately following removal of oxygen-enriched surface layer, additional processing (e.g., chemical etching, gauge removal, crack tip modulation, surface finish improvement, etc.) of the metallic workpiece can be carried out, including while the metallic workpiece remains submerged in the electrochemical bath. The efficiencies of material handling, coupled with eliminating capital and operating costs of previously mentioned fracturing methods, are then realized.

[0014] The specific composition of the metallic workpiece to be treated by the method described herein is generally not limited, provided that the composition of the metallic workpiece is such that oxygen-enriched surface layers form on the metallic workpiece when subjected to various types of processing (e.g., hot working, fabrication and heat treatment as described previously). In some embodiments, the methods described herein are suitable for use with materials whose metallurgical oxides and scales traditionally require mechanical intervention prior to subsequent operations aimed at removing the oxygen-enriched surface layers formed thereon. In one non-limiting example, the metallic workpiece is comprised of titanium or its alloys. As discussed previously, the oxygen-enriched surface layer that forms on titanium and titanium alloys is an alpha case layer. The composition of the metallic workpiece may also include, e.g., zirconium, niobium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, hafnium,tantalum, and / or tungsten and their respective alloys. Other tool steels and stainless steels, super alloys can also be used.

[0015] The material of the opposing electrode submerged in the electrochemical bath along with the metallic workpiece is generally not limited, provided that the counter electrode provides the material necessary for carrying out the desired electrochemical reactions and, in this regard, is complimentary to the metallic workpiece being treated for removal of oxygen-enriched surface layer. In some embodiments, the material of the opposing electrode is any suitable material for the electrochemical reaction that has an electrical conductivity greater than or equal to metallic workpiece. In one non-limiting example, the counter electrode may be a graphite electrode.

[0016] The specific composition of the electrochemical bath is generally not limited, provided the electrochemical bath provides for the transport of electrons and / or ions between the metallic workpiece and the opposing electrode. In some embodiments, the electrochemical bath includes a solvent component and optionally a solubilizer component. In embodiments including both solvent and solubilizer, the solubilizer component of the bath can be from 1 to 99 wt% of the bath, and the solvent component of the bath can be from 99 to 1 wt% of the bath.

[0017] The specific solvent component used is generally not limited, provided that the solvent component can serve to provide ions for facilitating the electrochemical reaction. It may also be desirable to select a solvent that has relatively low conductivity. In some embodiments, the solvent component comprises fluoride so that the solvent can provide fluoride ions for the electrochemical reaction. In one non-limiting example, the solvent is ammonium bifluoride. Another non-limiting example of a suitable solvent is ammonium hydrogen difluoride. In some embodiments, the concentration of the solvent component in the electrochemical bath is in the range of from 0.01 g / L to 100 g / L. Other operating conditions of the electrochemical reaction, including voltage, amplitude, frequency, and bath temperature may need to be adjusted based on the specific concentration of the solvent used in the electrochemical bath. The impact of solvent concentration on these other operating conditions is discussed in greater detail below.

[0018] The specific solubilizer component used, if the solubilizer is included in the electrochemical bath, is generally not limited, and may be selected based on, for example, processing steps to be carried out on the workpiece following the removal of alpha case layers per the methods described herein. In one non-limiting example, the solubilizer component is citric acid. In another non-limiting example, the solubilizer component is 2- hydroxypropane-1 ,2,3-tricarboxylic acid. In some embodiments, the concentration of the solubilizer component in the electrochemical bath is in the range of from 0 g / L to 1 ,000 g / L.

[0019] In some embodiments, the chemical bath is expressly free of material-specific corrosive compounds and or free of nitrous oxide (NOx) compounds. For example, the chemical bath is preferably free of any compounds that corrosive to material of the workpiece. Corrosive compounds are unique to each metal or metal alloy of workpiece. An example of a corrosive compound that may be excluded from the chemical bath treating certain metallic species includes nitric acid (HNO3), among others.

[0020] In some embodiments, the electrochemical bath has a conductivity that is greater than 0.011649 micro-siemens per cm at 0°C.

[0021] In application, the method described herein generally begins by providing the metallic workpiece having untreated oxygen-enriched surface layer deposited thereon, and submerging some or all of the workpiece in an electrochemical bath. As used herein, the phrase “untreated oxygen-enriched surface layer” and variants thereof means that no processing steps having the intent or goal of removing some or all of an oxygen-enriched surface layer formed on the metallic workpiece, including steps which do not directly remove some or all of the oxygen-enriches surface layer but which are considered to put the oxygen- enriched surface layer in better condition for removal (e.g., fracturing), have been performed. The methods described herein are specifically directed at processing metallic workpieces having untreated oxygen-enriched surface layers deposited thereon so as to avoid the need for any such processing steps.

[0022] In some embodiments, the method includes submerging in the electrochemical bath a metallic workpiece directly following whatever processing steps were carried out on the metallic workpiece that led to the formation of the oxygen-enriched surface layer thereon. In this regard, the term “directly” does not necessarily mean that no or little time has passedbetween the processing that led to formation of the oxygen-enriched surface layer and submerging the metallic workpiece in the electrochemical bath. Instead, the term “directly” refers to the absence of intermediate processing steps directed at removal of oxygen- enriched surface layer between the formation of the oxygen-enriched surface layer and submerging the metallic workpiece in the electrochemical bath (including the exclusion of fracturing steps). For example, an extended period of time (e.g., hours or days) can have passed between the processing that led to formation of the oxygen-enriched surface layer and submerging the metallic workpiece in the electrochemical bath, provided that no additional processing aimed at affecting the oxygen-enriched surface layer is carried out during this extended time period.

[0023] With respect to submerging the workpiece in the electrochemical bath, the method may include submerging in the electrochemical bath only the surface or surfaces of the metallic workpiece on which the oxygen-enriched surface layer resides. In embodiments where the oxygen-enriched surface layer is incorporated into the metallic workpiece and possibly in zones of the metallic workpiece, the method may include partial submersion or localized treatment. In some embodiments, the method may also include fully submerging the metallic workpiece in the electrochemical bath.

[0024] Once the workpiece is submerged in the electrochemical bath in the desired manner, a voltage is applied across the electrochemical bath between the workpiece and the opposing electrode. Alternating current is used exclusively when supplying the voltage. That is to say, the method described herein does not employ direct current at any point during the process of removing oxygen-enriched surface layer from the workpiece. Alternating current as used herein refers to providing a voltage whose slope is continually changing. In some embodiments, the signal is comprised of a voltage ranging from -4800V to +4800 volts, varying in frequency from 0.001 Hz to 30MHz. In some embodiments, the voltage is in the range of from -480V to -480V and the frequency is in the range of from 24 Hz to 666 Hz. A preferred frequency may be in the range of about 60 Hz.

[0025] As mentioned previously, the amplitude is one operation variable that may be adjusted based on various aspects of the method described herein. In some embodiments, the amplitude is adjusted based primarily on the concentration of the solvent (and morespecifically, the concentration of the fluoride) in the electrochemical bath. The amplitude used generally increases as solvent concentration is decreased, as the conductivity of the solution creates excessive amperage.

[0026] Similarly, frequency may be adjusted based on various aspects of the method described herein. In some embodiments, the frequency is adjusted based on the concentration of the solvent in the electrochemical bath, although frequency only marginally affects the results of the process regardless of solvent concentration.

[0027] The duration of supplying the voltage is still another operation variable that may be adjusted based on various aspects of the method described herein. The duration of supplying the voltage is generally not limited. In some embodiments, the duration of supplying voltage may be dependent on aspects of the workpiece being treated and / or the composition of the electrochemical bath. For example, the thickness of an oxygen-enriched surface layer formed on the workpiece can vary, and is influenced by, e.g., the length of time the workpiece was exposed to elevated temperature and the environment (oxidizing or reducing) of the furnace in which the metal workpiece was heated and which resulted in the formation of the oxygen-enriched layer. Due to factors such as the thickness of the oxygen- enriched surface layer, the concentration of the solvent component of the electrochemical bath, and the frequency and amplitude used with the voltage source, there is potentially a wide range of possible electrochemical exposure times required to accomplish the removal of oxygen-enriched surface layer. In some embodiments, the time period during which voltage is applied is in the range of from as little as one second to potentially as much as 24 hours.

[0028] While the workpiece is submerged in the bath and voltage is being applied, other process parameters may be adjusted or controlled in order to ensure the oxygen-enriched surface layer is removed and, in some cases, components of the oxygen-enriched surface layer are dissolved in the bath. For example, the temperature of the electrochemical bath can be controlled. Generally speaking, the temperature of the electrochemical bath is about room temperature prior to supplying the voltage to the electrochemical bath. During the application of voltage, the temperature of the bath generally increases, and therefore steps may be taken to ensure the bath temperature does not get too high, such as preventing thetemperature of the bath from exceeding the boiling point temperature of the bath. In some embodiments, external refrigeration means are used in order to limit the temperature of the bath. However, other means for controlling bath temperature may also be used, such as using a larger processing tank for carrying out the method described herein.

[0029] Agitation of the chemical bath can also be used to promote reaction between the bath and the oxygen-enriched surface layer and / or, in some cases, to encourage dissolution into the chemical process solution. In yet another example, the sequencing of voltage, amperage, wave form, dwell, and periodicity may all be controlled to promote specific reactions. Each unique application of the method will require specific methods applied for successful operation of the proposed invention.

[0030] In some embodiments, the amplitude and / or frequency is altered during the application of alternating current. For example, the alternating current may first be applied at a first amplitude and frequency for a period of time, after which the amplitude and / or frequency is changed and alternating current is applied at the new amplitude and / or frequency for a second period of time. In one non-limiting example, alternating current is applied at +65V and -65V at 60Hz for 5 minutes, followed by applying alternating current between +24V and -24V at 666Hz for 5 minutes. When the parameters of the alternating current are altered during application in this manner, the sequence can be repeated one or more times.

[0031] The method described herein results in the removal of the oxygen-enriched surface layer from the underlying workpiece. In some embodiments, a portion of the thickness of the workpiece is removed from the workpiece as part of the removal of the oxygen-enriched surface layer. The portion of the thickness of the workpiece removed by the methods described herein may include the oxygen-enriched surface layer and some of the material of the underlying workpiece. However, the portion of the thickness of the workpiece removed by the methods described herein is generally minimal. For example, in some embodiments, the thickness of the workpiece is reduced by 0.001” to 0.090”.

[0032] EXAMPLES

[0033] Experiment 1

[0034] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 30 g / L of citric acid (solubilizer) and 10 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +24V and -24V at 66Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 150°F and cycling continuously for 420 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.030" of material.

[0035] Experiment 2

[0036] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 10 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +48V and -48V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 149°F and cycling continuously for 30 minutes, process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a uniform material finish, and having removed 0.006" of material.

[0037] Experiment 3

[0038] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 13 g / L of ammonium bifluoride (solvent). The Ti-6A4v workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +48V and-48V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 129°F and cycling continuously for 10 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a uniform material finish, and having removed 0.005" of material.

[0039] Experiment 4

[0040] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 13 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +48V and -48V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 150°F and cycling continuously for 15 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a uniform material finish, and having removed 0.005" of material.

[0041] Experiment 5

[0042] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 50 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 196°F and cycling continuously for 4 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a uniform material finish, and having removed 0.002" of material.

[0043] Experiment 6

[0044] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 12 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 86°F and cycling continuously for 20 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform reflective material finish, and having removed 0.006" of material.

[0045] Experiment 7

[0046] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 12 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 88°F and cycling continuously for 40 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.010" of material.

[0047] Experiment 8

[0048] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 10 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior tobeing placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 105°F and cycling continuously for 15 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.058" of material.

[0049] Experiment 9

[0050] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 30 g / L of citric acid (solubilizer) and 13 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 132°F and cycling continuously for 15 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.052" of material.

[0051] Experiment 10

[0052] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 45 g / L of citric acid (solubilizer) and 8 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 142°F and cycling continuously for 15 minutes, the process resulted in theTi-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with a light brown anodized color, and having removed 0.013" of material.

[0053] Experiment 11

[0054] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 30 g / L of citric acid (solubilizer) and 3 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 115°F and cycling continuously for 60 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with a light blue anodized color, and having removed 0.004" of material.

[0055] Experiment 12

[0056] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 20 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece for 5 minutes. The voltage source was then changed to a continually changing slope alternating between +24V and -24V at 666Hz for 5 minutes. This sequence was repeated once for a total processing time of 20 minutes. At an average solution temperature of 136°F and cycling continuously for 20 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with base grey color, and having removed 0.001" of material.

[0057] Experiment 13

[0058] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 20 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece for 5 minutes. The voltage source was then changed to a continually changing slope alternating between +24V and -24V at 666Hz for 5 minutes. This sequence was repeated once for a total processing time of 20 minutes. At an average solution temperature of 118°F and cycling continuously for 20 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with base grey color, and having removed 0.001" of material.

[0059] Experiment 14

[0060] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 20 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +65V and -65V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece for 5 minutes. The voltage source was then changed to a continually changing slope alternating between +24V and -24V at 666Hz for 5 minutes. This sequence was repeated once for a total processing time of 20 minutes. At an average solution temperature of 120°F and cycling continuously for 20 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with base grey color, and having removed 0.001" of material.

[0061] Experiment 15

[0062] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 6 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +240V and -240V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 107°F and cycling continuously for 8 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with base metal grey color, and having removed 0.075" of material.

[0063] Experiment 16

[0064] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 6 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +240V and -240V at 666Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 110°F and cycling continuously for 8 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.073" of material.

[0065] Experiment 17

[0066] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 6 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +240V and-240V at 666Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 118°F and cycling continuously for 8 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth, bright, uniform material finish, and having removed 0.067" of material.

[0067] Experiment 18

[0068] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 3 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +480V and -480V at 60Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 126°F and cycling continuously for 6 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish with base metal grey color, and having removed 0.083" of material.

[0069] Experiment 19

[0070] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 3 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +480V and -480V at 666Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 130°F and cycling continuously for 6 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth uniform material finish, and having removed 0.083" of material.

[0071] Experiment 20

[0072] An aerospace titanium alloy, Ti-6AI-4V, was processed in accordance with the methods described herein. More specifically, the Ti-6AI-4V workpiece having an alpha case layer disposed thereon was placed in an aqueous solution of 3 g / L of ammonium bifluoride (solvent). The Ti-6AI-4V workpiece was not subjected to surface layer fracturing prior to being placed in the aqueous solution. A counter electrode was also placed in the aqueous solution, and a voltage source of continually changing slope alternating between +480V and -480V at 666Hz was used to apply a current across the aqueous solution between the counter electrode and the Ti-6AI-4V workpiece. At an average solution temperature of 136°F and cycling continuously for 6 minutes, the process resulted in the Ti-6AI-4V workpiece having a surface free of the alpha case layer, exhibiting a smooth, bright, uniform material finish, and having removed 0.078" of material.

[0073] From the foregoing, it will be appreciated that specific embodiments of the invention have been described herein for purposes of illustration, but that various modifications may be made without deviating from the scope of the invention. Accordingly, the invention is not limited except as by the appended claims.

[0074] Although the technology has been described in language that is specific to certain structures and materials, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific structures and materials described. Rather, the specific aspects are described as forms of implementing the claimed invention. Because many embodiments of the invention can be practiced without departing from the spirit and scope of the invention, the invention resides in the claims hereinafter appended.

[0075] Unless otherwise indicated, all number or expressions, such as those expressing dimensions, physical characteristics, etc., used in the specification (other than the claims) are understood as modified in all instances by the term "approximately". At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the claims, each numerical parameter recited in the specification or claims which is modified by the term "approximately" should at least be construed in light of the number of recited significant digits and by applying rounding techniques. Moreover, all ranges disclosed herein are to be understood to encompass and provide support for claims that recite anyand all sub-ranges or any and all individual values subsumed therein. For example, a stated range of 1 to 10 should be considered to include and provide support for claims that recite any and all sub-ranges or individual values that are between and / or inclusive of the minimum value of 1 and the maximum value of 10; that is, all sub-ranges beginning with a minimum value of 1 or more and ending with a maximum value of 10 or less (e.g., 5.5 to 10, 2.34 to 3.56, and so forth) or any values from 1 to 10 (e.g., 3, 5.8, 9.9994, and so forth).

Claims

CLAIMSI / We claim:1 . A method of treating the surface of a metal workpiece, comprising: immersing in an aqueous solution at least a portion of a metal workpiece having an oxygen-enriched surface layer formed thereon; immersing in the aqueous solution at least a portion of an electrode; and applying an alternating current across the aqueous solution between the metal workpiece and the electrode to thereby remove the oxygen-enriched surface layer from the metal workpiece.

2. The method of claim 1 , wherein at the time of immersing the metal workpiece in the aqueous solution, the oxygen-enriched surface layer formed thereon is free of fractures or cracks formed by mechanical pretreatment of the workpiece.

3. The method of claim 1 , wherein the aqueous solution comprises a solvent, and the solvent is a compound containing fluoride.

4. The method of claim 3, wherein the solvent is ammonium bifluoride.

5. The method of claim 3, wherein the concentration of the solvent in the aqueous solution is from 0.01 g / L to 100 g / L.

6. The method of claim 3, wherein the aqueous solution further comprises an organic acid.

7. The method of claim 5, wherein the organic acid is citric acid.

8. The method of claim 1 , wherein the alternating current continually changes slope and has a voltage in the range of +480V and -480V.

9. The method of claim 1 , wherein the method is free of the application of direct current.

10. The method of claim 1 , wherein the frequency of the alternating current is in the range of from 0.001 Hz to 30MHz.11 . The method of claim 1 , further comprising: controlling the temperature of the aqueous solution to below a boiling point of the aqueous solution after commencing the application of the alternating current.

12. The method of claim 1 , wherein the workpiece is a titanium-based workpiece or a titanium alloy-based workpiece.

13. The method of claim 1 , wherein the workpiece comprises zirconium, niobium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, yttrium, zirconium, niobium, molybdenum, hafnium, tantalum, tungsten or any combination thereof.