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Method of removing an alpha-case titanium layer from a beta-phase titanium alloy
EP1947217B1
European Patent Office
- Other languages
German French - Inventor
Joseph Parkos Curtis H. Riewe James O. Hansen - Current Assignee
- RTX Corp
Worldwide applications
Application EP08250140A events
2008-07-23
2012-05-23
Application granted
2012-05-23
Status
Ceased
2028-01-11
Anticipated expiration
Description
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[0001] This invention relates to chemically treating metallic articles and, more particularly, to chemically removing a surface layer to expose a core of the metallic article. -
[0002] A metallic article is often manufactured from a raw work piece using a variety of steps. Typically, one or more of the steps includes heating the raw work piece to form it into a desired shape or to obtain desirable mechanical properties. One problem of using heat is that the heat may cause an undesirable surface layer to form on the raw work piece that diminishes the appearance or inhibits subsequent manufacturing steps. For example, heating a work piece made of titanium alloy may cause preferential alpha-phase precipitation at the surface of the work piece, i.e. an alpha-phase surface layer, also known as a casing. -
[0003] One proposed solution is to remove the surface layer in a removal step. However, conventional removal steps using a chemical treatment have been unsuccessful. For example, the treatment results in smut, such as oxides, intermetallics, or other impurities, on the surface that must subsequently be removed mechanically. Mechanical removal of the smut is labor intensive and often results in incomplete removal, which may subsequently be cause for rejection of the final article. Additionally, the treatment produces intergranular attack and thereby diminishes the mechanical integrity of the final article. An example of a prior art chemical removal process having the features of the preambles of claims 1 and 8 is disclosed inEP 1533391 A1 . -
[0004] Therefore, what is a needed is a treatment system and method for uniformly chemically removing the surface layer without need for mechanical removal. This invention addresses these needs while avoiding the shortcomings and drawbacks of the prior art. -
[0005] A method of surface treating a metallic article is provided, as set forth in claim 1. The surface layer includes a higher volume fraction of titanium alloy alpha-phase and the core may include either single phase beta titanium or alpha precipitates in a beta titanium matrix. -
[0006] A solution system for chemically removing the surface layer is also provided, as set forth inclaim 10. The first solution is to chemically remove a thin layer of titanium. The second solution is to prevent smut formation. -
[0007] The first solution includes about 45 vol% - 50 vol% nitric acid that is at least 42° Baume (e.g., 70wt% or 1.42 g/cc reagent grade nitric acid), hydrofluoric acid in the range of from about 5.6-8.4 vol% at 70% grade to about 8-12% at 49% grade, and a remainder of water. This solution non-selectively removes the surface layer with no or minimal smut formation. The second solution may include about 50 vol% - 60 vol% nitric acid that is at least 42° Baume and a remainder of water. This solution reacts with the surface to remove any created smut and prevent further smut formation in the final rinse. -
[0008] The various features and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the currently preferred embodiment. The drawing that accompanies the detailed description can be briefly described as follows. -
[0009] Figure 1 illustrates selected portions of a work piece having a casing and a core, where the casing is chemically removed to expose the core. -
[0010] Figure 1 illustrates selected portions of an examplemetallic work piece 10, such as a sheet, stamping, forging, casting, or other type of pre-manufactured article. For example, thework piece 10 is used to manufacture an aircraft engine nozzle bracket, and aircraft nozzle honeycomb structure, or other type of article. Thework piece 10 was previously heated in a known process, such as a vacuum heat treatment process, hot forming process, or other process that utilizes heat. The heating resulted in formation of a casing 12 (i.e., a surface layer) on acore 14 of thework piece 10. Thecasing 12 is uniformly chemically removed in a treatment process to expose thecore 14 of treated work piece 10', as will be described below. -
[0011] Thework piece 10 is made of a titanium alloy. For example, the titanium alloy includes a nominal composition having about 14 wt% - 16 wt% molybdenum, about 2.5 wt% - 3.5 wt% aluminum, about 2.4 wt% - 3.2 wt% niobium, about 0.15 wt% - 0.25 wt% silicon, and a remainder titanium. The nominal composition also includes trace amounts of other elements. For example, the nominal composition also includes about 0.4 wt% iron, 0.05 wt% carbon, 0.1 wt% copper, 0.11 wt% - 0.17wt% oxygen, 0.05 wt% nitrogen, 150 ppm hydrogen, and 50ppm yttrium. In some examples, the trace amounts of the other elements are essential to obtaining desirable properties of the titanium alloy, such as processing properties and mechanical properties. -
[0012] For the above example titanium alloy composition thecasing 12 includes alpha-phase precipitation (i.e. an alpha-phase surface layer) caused by the prior heating process, and thecore 14 includes beta-phase titanium. For example, thecasing 12 includes a higher volume fraction of titanium alloy alpha-phase and thecore 14 includes either single phase beta titanium or alpha precipitates in a beta titanium matrix. If not removed, the alpha-phase precipitation in thecasing 12 may compromise the mechanical integrity of an article produced from thework piece 10 or inhibit subsequent manufacturing steps, such as forming or welding. Other titanium alloy compositions may also form alpha-phase casings. -
[0013] Thecasing 12 is removed in a treatment process that utilizes a first solution that includes nitric acid and hydrofluoric acid, and a second solution that includes nitric acid. The first solution chemically removes thecasing 12 to thereby expose thecore 14 with minimal or no smut formation. The first solution reacts with thecasing 12 to remove any pre-existing smut, and the second solution prevents further smut formation before a final rinse in water. Using the two solutions provides the benefit of removing thecasing 12 with little or no hydrogen diffusion into the titanium alloy, without intergranular attack (i.e., selective oxidation between metallic microstructural grains of the work piece 10), and without smut remaining on thecore 14. -
[0014] In a first step, the treatment process includes wetting thecasing 12 with the first solution, such as by immersing thework piece 10 in a container having the first solution. The first step is conducted at a temperature of about 75°F (24°C) for about one minute and thirty seconds, depending on the thickness of thecasing 12 to be removed, for example. Given this description, one of ordinary skill in the art will, recognize that a higher temperature or a lower temperature than disclosed may be used. -
[0015] In a second step, thework piece 10 is wetted with the second solution, such as by immersing thework piece 10 in a container having the second solution. The second step is conducted at a temperature of about 55°F - 120°F (13-49°C) for about thirty seconds, for example. Given this description, one of ordinary skill in the art will recognize that a higher temperature or a lower temperature than the disclosed range may be used. -
[0016] Thework piece 10 is maintained in a "wet" state between the steps of wetting with the first solution and wetting with the second solution. For example, thework piece 10 is moved from the first solution directly into the second solution within a short time, without rinsing. This prevents residual smut from setting and permanently bonding to thecore 14, which would require mechanical removal. A short dwell time is permitted between the first solution and the second solution to allow drainage of the first solution off of thework piece 10. After the second solution, thework piece 10 is then rinsed in water. -
[0017] In one example, the first solution and the second solution contain predetermined compositions to uniformly remove thecasing 12. The following example solutions are described with reference to acid concentrations that are expressed in terms of density, specific gravity, or weight percent. However, it is to be understood that these conventions of expression may be converted into other conventions of expression. -
[0018] The first solution includes about 45 vol% - 50 vol% nitric acid that is at least 42° Baume (e.g., 70wt% or 1.42 g/cc reagent grade nitric acid), about 5.6 vol% - 8.4 vol% hydrofluoric acid that is about 70%grade, and a remainder of water. The term "grade" as used in this description refers to the concentration of the acid wherein the percentage is the weight percent of the acid in an "as received" technical or reagent acid solution. The second solution includes about 50 vol% - 60 vol% nitric acid that is at least 42° Baume and a remainder of water. Alternatively, the first solution includes about 45 vol% - 50 vol% of the nitric acid, about 8 vol% - 12 vol% hydrofluoric acid that is 49% grade, and a remainder of water. The term "about" as used in this description relative to percentages or compositions refers to possible variation in the compositional percentages, such as normally accepted variations or tolerances in the art. It is to be understood that other equivalent solution compositions than disclosed may be determined based on the purities of the acids. -
[0019] Using the first solution and the second solution in two steps provides the benefits of removing thecasing 12 with little or no hydrogen diffusion into the titanium alloy, little or no intergranular attack, and little or no smut remaining on thecore 14. For example, using a more aggressive solution than disclosed (e.g., using stronger acids, or using greater volume percentages of the acids) to remove thecasing 12 in a single step may result in non-uniform removal, intergranular attack, and promote hydrogen diffusion. Conversely, using a less aggressive solution (e.g., using weaker acids, or using lower volume percentages of the acids) to remove thecasing 12 may result in incomplete removal of thecasing 12. The disclosed treatment utilizing two steps and two different solutions avoids these drawbacks by removing thecasing 12 in a controlled manner in the first step, and subsequently preventing further smut formation in the second step. -
[0020] Although a preferred embodiment of this invention has been disclosed, a worker of ordinary skill in this art would recognize that certain modifications would come within the scope of this invention. For that reason, the following claims should be studied to determine the true scope and content of this invention.
Claims (9)
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Claims (9)
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- A method of surface treating a metallic article, comprising the steps:chemically removing a titanium alloy alpha-phase surface layer to expose a core having titanium alloy beta-phase, the method including wetting the surface layer with a first solution in a first step to remove at least a portion of the surface layer and expose the core, then wetting any remaining surface layer and the core with a second solution in a second step, and maintaining the surface layer and the core in a wet state between the first step and the second step;characterised in that the first solution includes about 45 vol% - 50 vol% nitric acid that is at least 42° Baume, hydrofluoric acid in the range of from 5.6-8.4 vol% at 70% grade to 8-12 vol% at 49% grade, and a remainder of water, and wherein the second solution includes nitric acid.
- The method as recited in claim 1, including performing the first step at a temperature of about 75° F (24°C) and performing the second step at a temperature of about 55° F - 120° F (13-49°C).
- The method as recited in claim 1 or 2, wherein the second solution includes about 50 vol% - 60 vol% nitric acid that is at least 42° Baume, and a remainder of water.
- The method as recited in any preceding claim, wherein the first solution has about 45 vol% - 50 vol% nitric acid that is at least 42° Baume, about 8 vol% - 12 vol% hydrofluoric acid that is 49% grade, and a remainder of water.
- The method as recited in any preceding claim, wherein the first solution has about 45 vol% - 50 vol% nitric acid that is at least 42° Baume, about 5.6 vol% - 8,4 vol% hydrofluoric acid that is about 70% grade, and a remainder of water.
- The method as recited in any preceding claim, including forming the surface layer and the core from a titanium alloy having a nominal composition that includes about 14 wt% - 16 wt% molybdenum, about 2.5 wt% - 3.5 wt% aluminum, about 2.4 wt% - 3.2 wt% niobium, about 0.15 wt% - 0.25 wt% silicon, and a remainder titanium.
- The method as recited in any preceding claim, including uniformly removing the surface layer without hydrogen embrittlement and intergranular attack of the titanium alloy beta-phase.
- A solution system for chemically removing a titanium alloy alpha-phase surface layer to expose a core having titanium alloy beta-phase of a metallic article, characterised by the solution system comprising:a first solution that includes about 45 vol% - 50 vol% nitric acid that is at least 42° Baume, hydrofluoric acid in the range of from 5.6 vol% - 8.4 vol% at about 70% grade to 8 vol% - 12 vol% at about 49% grade, and a remainder of water; anda second solution that includes nitric acid, where the first solution and the second solution cooperate to remove the surface layer without smut formation to expose the core.
- The solution system as recited in claim 8, wherein the second solution includes about 50 vol% - 60 vol% nitric acid that is at least 42° Baume and a remainder of water.