Abstract:
A molten metal or molten metal alloy is filtered to remove contaminants. Filters may include imaging agents, especially those useful for N-ray radiography, X-ray analysis, and neutron activation. Currently preferred imaging agents include gadolinia and tungsten. Currently preferred refractory materials include yttria, zirconia, tantalum, tungsten and rhenium. A homogeneous distribution of refractory and imaging materials may be accomplished by vacuum impregnation, chemical vapor deposition, physical vapor deposition, chemical vapor infiltration, fusing, cocalcining, alloying, and physical mixing. Filters substantially resistant to chemical and mechanical degradation may be used. For example, filters may be constructed with at least an outer layer of the filter comprising yttria. Filters may define a porosity and mesh size for efficient filtration and to exclude the maximum allowed size of defects for the particular application. Castings may be analyzed for inclusions by X-ray analysis, neutron activation, and N-ray radiography. Preferred methods for filtering melts include providing a filter including an imaging agent and filtering the melt into a mold to cast a metal article. Filtration may include heating the filter during filtration. The melt may include any molten metal or metal alloy, particularly aluminum, steel, alloys of titanium, chromium, nickel, cobalt and alloys thereof.

Description:
FIELD  
         [0001]    This application concerns filters useful for filtering molten metals and metal alloys and methods for filtering metals and metal alloys using such filters.  
         BACKGROUND  
         [0002]    Investment casting is a process for forming metal or metal alloy articles (also referred to as castings) by solidifying molten metal or alloys in a mold having the shape of a desired article. Investment casting is an important method for producing quality metal articles for a variety of industries that rely upon high integrity precision castings for critical applications. Castings often include undesirable material, referred to as “inclusions”. Inclusions can result from the incorporation of slag or dross material from the melt or from foreign material derived from the casting mold or from contamination from the foundry environment, such as dust and dirt. Inclusions in cast metal articles are detrimental to the mechanical properties of the metal and can lead to catastrophic failure under stress. Therefore, many industries, particularly the aerospace industry, have developed stringent specifications governing the presence and acceptable size of inclusions in cast parts, such as turbine blades and airframe components.  
           [0003]    Filtration is one method for removing impurities, such as slag and dross, from molten metals. References that disclose filters for filtering molten metals include U.S. Pat. Nos. 3,981,352; 4,671,498; 4,719,013; 5,045,511; 5,177,035 and 5,369,063. For example, Nurminen et al.&#39;s U.S. Pat. No. 3,983,352 discloses a filter constructed “of substantially spherical refractory particles. A ceramic binder is utilized for securing the particles in a bonded assembly, the binder composition substantially completely coating the particles.” Column 2, lines 13-17. Nurminen states that, “[t]he binder is characterized by an affinity for dross and slag constituents in the molten metal so that all or substantially all of such constituents can be removed.” Column 2, lines 20-23. Gee et al.&#39;s U.S. Pat. Nos. 5,177,035 and 5,369,063 teach “a porous ceramic body” made from various ceramic powders for filtering molten metals. The &#39;035 patent states that “it is desirable to separate exogenous intermetallic inclusions from the molten metal. Such inclusions result in molten metals from impurities included in the raw materials used to form the melt, from slag, dross and oxides which form on the surface of the melt and from small fragments of the refractory materials . . . ” Column 1, lines 18-24. Sane et al.&#39;s U.S. Pat. No. 5,045,111 describes “a porous ceramic filter preferably comprising particles of alumina and partially stabilized zirconia . . . for separating non-metallic inclusions and contaminants from molten ferrous metal as it is flowed.” Column 1, lines 17-19.  
           [0004]    Despite the teachings of these prior patents, certain problems associated with filtering molten metals have not been adequately addressed. For example, one problem is the premature solidification of the molten metal being filtered. Solidification occurs when the filter acts as a heat sink to cool the metal or unduly restricts the flow of the melt, allowing the melt to solidify on the filter. Efforts to address this premature solidification have met with little success. The Sane patent states, for example that “[p]reheating a filter while it is in the casting mold is impractical.” Column 2, lines 30, 31.  
           [0005]    Another problem is that there are few materials suitable for filtering reactive, high melting point metals and superalloys. Highly reactive metals and metal alloys, such as titanium, titanium alloys, and superalloys, are incompatible with most conventional filter media. For example, typical refractory ceramic filter materials, such as metal oxides, are reduced by molten titanium and titanium alloys, resulting in the incorporation of brittle oxygen-enriched metal in the castings. In addition, filters used for filtering molten metals are subjected to considerable mechanical stress. Therefore, it is possible for filter material to flake, fragment or otherwise become incorporated into the melt. The result is contamination of the casting by the material used to form the filter.  
           [0006]    Nondestructive evaluation (NDE) of castings can reveal inclusions in cast articles. Some inclusions, if detected, can be removed from the metal article, and the article repaired, without compromising its structural integrity. However, it is difficult, and often impossible to detect, locate and repair flaws using conventional techniques.  
           [0007]    Various techniques for NDE have been used by the investment casting industry. For example, some tungsten and thorium oxide inclusions can be detected by X-ray analysis of titanium castings because there is a sufficient difference in the density of tungsten and thorium oxide and that of titanium. ASTM (American Society for Testing and Materials) publication No. E 1320-90 describes X-ray reference radiographs for analyzing titanium cast articles of less than about two inches thick. Generally, X-ray analysis has proved useful for detecting inclusions in titanium and titanium alloy articles having a maximum thickness of only about two inches. To date, titanium has been used by the investment casting industry primarily for casting articles having relatively small cross sections. However, investment casting is now being considered for producing structural components of aircraft having significantly larger cross sections than articles cast previously. Therefore, N-ray analysis may provide a more general solution to detection and imaging of inclusions in investment castings.  
           [0008]    N-ray imaging agents have previously been used in the investment casting industry. For example, ASTM publication No. E 748-95 states that “[c]ontrast agents can help show materials such as ceramic residues in investment-cast turbine blades.” ASTM E 748-95, page 5, beginning at about line 46. This quote refers to detecting ceramic residues by N-ray on metal articles having an internal cavity produced by initially casting metal about a ceramic core. The ceramic core is removed to form the cavity, and thereafter a solution of gadolinium nitrate is placed in the cavity. The gadolinium nitrate solution is left in the cavity long enough to infiltrate porous ceramic core residue that is on the surface of the article. The residue can then be imaged by N-ray.  
           [0009]    Methods for incorporating imaging materials into refractories are known in the art. For example, Sturgis et al.&#39;s U.S. Pat. No. 6,102,099 teaches “the incorporation of an imaging agent into the investment casting mold, particularly in the facecoat of the mold, prior to casting so that inclusions can be imaged in the cast article.” However, this method does not provide means for detecting filter-derived inclusions.  
         SUMMARY  
         [0010]    The present method concerns preventing inclusions and detecting inclusions in metal articles if they are present. An article or a casting may be an ingot or a processed ingot, processed by forging to form an item such as a billet or a sheet. One disclosed feature includes filters for filtering a molten metal or molten metal alloy. The filters may include one or more N-ray imaging agents. N-ray imaging agents may be selected such that the difference between the linear attenuation coefficient of the imaging agent and the cast metal article sufficient to allow detection of a filter-derived inclusion in the article. Gadolinia is an example of a useful N-ray imaging agent.  
           [0011]    Other types of imaging agents, especially those useful for X-ray spectroscopy, X-ray radiography, and neutron activation may be included in the filters. Imaging agents of particular utility as filter materials may be selected from the group consisting of dysprosium, erbium, europium, gadolinium, holmium, iridium, lutetium, neodymium, osmium, praseodymium, rhenium, samarium, tantalum, tungsten, ytterbium, isotopes thereof, physical mixtures thereof and chemical mixtures thereof. Examples of suitable imaging agent materials used to construct the filters may include metals, metal oxides, intermetallics, borides, carbides, nitrides, metal halides, physical mixtures thereof, and chemical mixtures thereof. Tungsten metal is an example of an X-ray imaging agent used to construct filters.  
           [0012]    Filters may include refractory materials in addition to an imaging agent, and the refractory materials used in the filters may be or include a ceramic, a refractory metal, or combinations of those materials. Some useful refractory materials include those selected from the group consisting of yttria, zirconia, tantalum, tungsten, rhenium, isotopes thereof, physical mixtures thereof and chemical mixtures thereof. The refractory material may be or include a metal oxide, and the refractory materials may be alloyed with particular elements in order to increase their performance characteristics, such as resistance to the chemical and mechanical stress of filtering molten metal. For example, tungsten may be alloyed with rhenium to give an alloy with a high melting point and a high tensile strength.  
           [0013]    The refractory material and imaging material used in these filters may be homogeneously distributed throughout the entire filter or solely or primarily only in a layer of the filter that is directly contacted by molten metal or metal alloy. A homogeneous distribution may be accomplished by vacuum impregnation of one material in another. Other methods for distributing the materials include chemical vapor deposition, physical vapor deposition, chemical vapor infiltration, fusing, cocalcining, alloying, and physical mixing.  
           [0014]    Another feature of the disclosed embodiments involves filtering with filters substantially resistant to chemical and mechanical degradation. For example, filters may be constructed such that at least an outer layer of the filter comprises a chemically robust material, such as yttria. The filters may be constructed by a method such as vacuum impregnation, such that the filters are resistant to mechanical degradation.  
           [0015]    Filters may be constructed from metals or metal alloys suitable for imaging and having melting points substantially the same as or higher than that of the molten metal or metal alloy to be filtered. Particular useful metals for filter construction include those selected from the groups described above. Certain of these materials may be alloyed with other elements to improve their chemical or mechanical properties, such as high temperature strength. A particularly useful metal for making filters is tungsten, and hafnium and platinum are two elements that, when alloyed with tungsten, confer desirable chemical and mechanical properties. For example, one application for the disclosed filters is filtration of titanium and titanium alloys, which have a typical melting point of about 3,000° F. (1,600° C.). Thus, the filter material should be chosen such that it has a melting point sufficiently close to or in excess of that of the metal or metal alloy being cast.  
           [0016]    A further embodiment of the disclosure includes a method for filtering impurities from molten metals or molten metal alloys using filters comprising the imaging agents described above. Moreover, filter-derived inclusions may be detected and imaged in a product casting. The filter may be heated to a temperature suitable to substantially avoid solidification of the molten metal or metal alloy during filtration. Filtering a molten metal may be followed by casting the metal to form a metal article. The molten metal or metal alloy may include any molten metal or metal alloy, including reactive metals, aluminum, steel, titanium, alloys and superalloys, particularly alloys of titanium, chromium, nickel, cobalt, and zirconium. In further embodiments of the present method, cast metal articles may be analyzed for inclusions by X-ray analysis (including X-ray radiography and X-ray spectroscopy), neutron activation, and/or N-ray spectroscopy, and/or N-ray radiography.  
           [0017]    Further embodiments of the present method include procedures for making the filters described above. One embodiment includes providing a filter comprising a refractory material, such as those described above, and providing an imaging agent by a surface coating or precipitation method. For example, the filter may comprise partially-stabilized zirconia and the imaging agent may comprise gadolinia. The imaging agent may be provided such that at least an outer surface region of the filter comprises the imaging agent, and such methods for providing layers of different refractory materials are known to those of ordinary skill in the art. The imaging agent may be applied by one of the methods described above, and may be applied with additional refractory material. For example, yttria and gadolinia can be applied to a filter or filter pattern.  
           [0018]    The imaging agent should be supplied in a sufficient amount to allow detection of filter-derived inclusions. For working embodiments, the imaging agent used to make the filters typically comprised from about 0.5 to about 100 weight percent imaging agent, more typically from about 1 to about 100 weight percent, even more typically from about 1 to about 65 weight percent, and preferably from about 2 to about 25 weight percent, imaging agent. Systems for filtering molten metals may include a crucible for pouring a molten metal or metal alloy through a filter. The filter may be one of the filters described above. The filter may be positioned such that the filtered metal or metal alloy is directed into a mold. Systems also may include a heater for heating the filter. Filters may be heated by an electrical arc or by a method selected from the group consisting of resistive, conductive, inductive, convective and radiative heating and combinations thereof. Systems may include a thermocouple for monitoring the filter temperature, and means for positioning the filter in the melt pour path. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0019]    [0019]FIG. 1 is diagram of a system for filtering molten metal into a mold.  
         [0020]    [0020]FIG. 2 is a plan view of one embodiment of a wire mesh filter.  
         [0021]    [0021]FIG. 3 is a cross-sectional view of filter  30  taken along line  3 - 3 .  
         [0022]    [0022]FIG. 4 is a perspective view of one embodiment of a wire mesh filter for filtering molten metals including connections for heating the filter.  
         [0023]    [0023]FIG. 5 is an X-ray radiograph of a wire mesh filter after filtration of a titanium melt containing inclusions showing inclusions retained on the filter.  
         [0024]    [0024]FIG. 6 is an N-ray radiograph of a ceramic filter after filtration of a titanium melt; inclusions comprising gadolinium are visible on the filter as light flakes.  
         [0025]    [0025]FIG. 7 is an N-ray radiograph of a GdNO 3  infiltrated filter after filtration of a titanium melt. 
     
    
     DETAILED DESCRIPTION  
       [0026]    Embodiments of the present disclosure concern removing inclusions from molten metals, particularly titanium and titanium alloys, via filtration prior to casting. Since filtration can result in the incorporation of filter-derived material into the molten metal, and thus the casting, it is desirable to detect this material in the product casting. To ensure high quality castings, it may be desirable to use filters containing an imaging agent, such that filter-derived inclusions can be detected by NDE. If there are inclusions larger than the allowed size for the particular application, the article may be repaired in some cases. To aid accurate repairs, it may be desirable to image inclusions three-dimensionally in the cast article. Filters have been constructed to include imaging agents, particularly X-ray and N-ray detectable materials, such that filter-derived inclusions can be detected and imaged. The filters and methods of filtering are applicable to filtering virtually all metals or metal alloys, with particular examples being titanium and titanium alloys.  
       I. Introduction  
       [0027]    With reference to FIGS.  1 - 4 , the following paragraphs describe several working embodiments of the present method and apparatus. An embodiment of a system  10  for filtering molten metals is depicted in FIG. 1. Crucible  12  is provided to pour the molten metal or metal alloy to be cast. The melt is poured into pour cup  16 . Path  14  shows the flow of the molten metal through filter  18  and into mold or die  20 . A casting is produced upon solidification of the melt.  
         [0028]    A working embodiment of a filter  30 , suitable for use as part  18  in FIG. 1, is shown in FIG. 2. Wire mesh screen  32  is placed over a refractory ceramic frame  34 . The wire of the mesh screen may be selected with a wire diameter sufficient to withstand the mechanical stress of filtration for the particular pour size. Working embodiments of the present method use wire mesh filters with a wire diameter of about 0.004 inch (0.1 mm). The screen is held in place by rods  36 , which also serve as electrodes for connecting the filter to a power source (not shown) for resistive heating or as heat conductors so that the filter may be heated conductively. FIG. 3 shows a cross-sectional view  40  of the filter  32 , held in place by rod  36 . The screen may be welded to the electrode or may be fastened by clamps (not shown).  
         [0029]    [0029]FIG. 4 illustrates a further embodiment of a filter apparatus  50 . Wire mesh  52  is clamped by clamps  54  to supports 56. Wire mesh 52 is wrapped around rods 60 and insulative half-tubes  58  are clamped (clamps not shown) around the wire mesh-wrapped rods. The filter of FIG. 4 also can be heated, as with the embodiment illustrated by FIG. 3.  
         [0030]    The following paragraphs concern pertinent details of filter construction, the use of filters for filtering molten metal, apparatus for filtering molten metals and the incorporation of filtering in the casting process, as well as working examples of such methods of construction and of imaging filter-derived inclusions in cast metal articles.  
       II. Filters for Filtering Molten Metals  
       [0031]    Filters must be chemically and mechanically robust to withstand the considerable stress of filtering molten metals. To maintain their integrity and capture inclusions, materials used to make filters must be stable at high temperatures and have only “low reactivity” with the molten metal or metal alloy being filtered. “Low reactivity” may as used herein is defined with reference to Aerospace Material Specification Number 4985B (AMS 4985B), published by SAE International, January 1997. Pertinent data from AMS 4985B are provided by Table 1. With reference to Table 1, a filter&#39;s reactivity with molten titanium 6-4 alloy should be such that the composition of a casting made from filtered metal comprises no more than approximately the amounts of a contaminant listed in Table 1. Similarly, the solubility of the filter materials in the molten metal or metal alloy to be filtered should be low. Low solubility may be defined, with reference to AMS 4985B, as solubility that yields no more than about the percentage of a material listed in Table 1 in the product casting. Reference to the AMS 4985B requirements is made solely to illustrate the industry standard for certification of investment castings and not to limit the scope of the present method.  
                                           TABLE 1                           Composition Specification of Titanium 6-4 Alloy Investment Castings a              Element   Minimum (%)   Maximum (%)                    Al   5.50   6.75       V   3.50   4.50       Fe   —   0.30       O   —   .20       C   —   .10       N   —    0.05 (500 ppm)       H   —   0.015 (150 ppm)       Y   —   0.005 (50 ppm)        Residual Elements, each   —   0.10       Residual Elements, total   —   0.40       Ti   remainder                          
 
         [0032]    High melting point metals that are useful for constructing filters include, without limitation, those selected from the group consisting of osmium, rhenium, tantalum, tungsten and their alloys. The melting points of these metals range from about 5,250° F. (2,900° C.) to about 6,150° C. (3,400° C.).  
         [0033]    Alternatively, filters may be constructed from ceramic materials suitable for filtering molten metals. As discussed in the Background, molten metals such as titanium and titanium alloys reduce many refractory ceramic materials, resulting in brittle, oxygen-enriched metal castings. Therefore, it may be preferable to use a refractory material on at least an outer surface of a filter that is substantially resistant to reduction by the molten metal or metal alloy being filtered. For example, a ceramic foam filter may be coated or impregnated with additional ceramic refractory material, which may include an imaging agent. The filters typically are coated by applying a slurry, by a method such as dip coating, comprising the desired refractory or imaging material. The material then can be deposited on the filter or impregnated in the filter via vacuum impregnation. Other deposition methods include methods selected from the group consisting of chemical vapor deposition, physical vapor deposition, chemical vapor infiltration and combinations thereof.  
         [0034]    Investment castings for different applications have various requirements for the size of inclusions that may be tolerated. Thus, the aperture size of filters for filtering molten metals will depend upon the intended application of the product casting. The aperture shape and size may be chosen such that the molten metal flows efficiently while all matter above a certain predetermined size is retained on the filter. Various different aperture shapes also may be used, including circular, polyhedral and irregular shapes. Working embodiments of filters had aperture sizes ranging from about 0.005 inch (0.13 mm) to about 0.05 inch (1.3 mm). Additional working embodiments of filters had porosity of from about 10 pores per inch to about 30 pores per inch.  
       III. Imaging Agents Useful for Imaging Inclusions  
       [0035]    The choice of imaging agent to be used in a filter depends upon the type of NDE to be employed. Table 2 provides data concerning some materials that may be used for N-ray and X-ray imaging of inclusions in investment castings. Data for titanium is provided for purposes of comparison.  
         [0036]    If X-ray radiographic analysis is to be performed, the primary consideration is the density of the imaging agent relative to the density of the metal article to be analyzed. Other considerations include the size and orientation of the inclusion and the thickness of the casting cross-section being analyzed. If the difference between the density of the cast material and the inclusion is small, X-ray radiography may yield insufficient image contrast for thick articles. For example, Sturgis teaches that if the difference in density between the article and the inclusion is less than about 0.5 g/cc for titanium or titanium alloy castings, insufficient image contrast may be obtained for inclusion detection in articles of about a 1 inch (2.5 cm) thickness or less. Sturgis, column 5, lines 58-65.  
                                                                                                                           TABLE 2                           Densities and Thermal Neutron Linear Attenuation Coefficients       Using Average Scattering and Thermal Absorption Cross       Sections for the Naturally Occurring Elements a                      Density of                       Metals or   Linear       Element       Metal   Attenuation            Atomic       Cross Section (barns) b     Oxides   Coefficient   Technique            No.   Symbol   Scattering   Absorption   (g/cc)   (cm −1 ) c     Used                    3   Li   0.95   70.6    2.01 (Li 2 O)   3.31   N-ray       5   B   4.27   767    2.46 (B 2 O 3 )   101.79   N-ray       22   Ti   4.09   6.09    4.5 (Ti)   0.58   Reference       41   Nb   6.37   1.15    7.03 (NbO)   0.42   X-ray       49   In   2.45   193.8    6.99 (In 2 O)   7.52   Both                        7.18 (I 2 O 3)         60   Nd   16   60.6    7.24 (Nd 2 O 3 )   1.89   X-ray       62   Sm   38   5670    8.3 (Sm 2 O 3 )   171.86   Both           Sm 149         41000           Sm 151         15000       63   Eu   . . .   4565    7.42 (Eu 2 O 3 )   94.82   Both           Eu 155         14000       64   Gd   172   48890    7.4 (Gd 2 O 3 )   1483.88   Both           Gd 155         61000           Gd 157         254000       66   Dy   105.9   940    7.81 (Dy 2 O 3 )   33.13   Both       67   Ho   8.65   64.7    8.79 (Ho)   2.35   Both       68   Er   9   159.2    8.64 (Er 2 O 3 )   5.49   Both           Er 167         700       70   Yb   23.4   35.5    9.2 (Yb 2 O 3 )   1.43   X-ray       71   Lu   6.8   76.4    9.4 (Lu 2 O 3 )   2.82   Both       73   Ta   6.12   20.5    16.6 (Ta)   1.47   X-ray           Ta 82         8200       74   W   4.77   18.4    4.2 (Na 2 WO 4 )   1.46   X-ray                       19.35 (W)       77   Ir   14.2   425.3    22.4 (Ir)   30.86   Both       90   Th   12.97   7.97    11.7 (Th)   0.62   Both                       9.86 (ThO 2 )                                          
 
         [0037]    Imaging agents that may be used to construct filters and detect filter-derived inclusions in investment castings using X-ray radiography include materials comprising metals selected from the group consisting of dysprosium, erbium, europium, gadolinium, hafnium, holmium, iridium, lutetium, neodymium, osmium, praseodymium, rhenium, samarium, tantalum, tungsten, ytterbium, isotopes thereof, physical mixtures thereof and chemical mixtures thereof. The oxides of these metals (e.g., Dy 2 O 3 , Er 2 O 3 , Gd 2 O 3 ) and the salts of such metals (e.g., GdNO 3 , YNO 3 ) are examples of types of compounds that may be employed. Materials that form these compounds upon further treatment, such as heating, also can be used for filter construction.  
         [0038]    Tungsten is an excellent imaging agent for X-ray radiography, and is sufficiently chemically robust to withstand filtration of reactive molten metals such as titanium and titanium alloys. Furthermore, tungsten has the physical properties to withstand the mechanical stress of filtration. For example, tungsten has the highest melting point and lowest vapor pressure of all metals, and at temperatures over 3,000° F. (1,650° C.) the highest tensile strength. Handbook of Chemistry and Physics, p. B-35, CRC Press, Boca Raton, Fla., 1973.  
         [0039]    However, it is possible that other materials with the desired density may be used to practice the present invention. Indeed, additional X-ray imaging agents can be selected by choosing materials with a density sufficiently different from the density of the metal or alloy being cast. Generally, the imaging agents described above have sufficiently different densities from the metals and/or alloys used to produce investment castings, such as stainless steel, titanium and titanium alloys, and the chromium, cobalt and nickel based superalloys.  
         [0040]    N-ray imaging is discussed in ASTM E 748-95, entitled Standard Practices for Thermal Neutron Radiography of Materials, which is incorporated herein by reference. N-ray imaging is a process whereby radiation beam intensity modulation by an object is used to image certain macroscopic details of the object. N-ray uses neutrons as penetrating radiation for imaging inclusions. The basic components required for N-ray imaging include a source of fast neutrons, a moderator, a gamma filter, a collimator, a conversion screen, a film image recorder or other imaging system, a cassette, and adequate biological shielding and interlock systems. See, ASTM E 748-95.  
         [0041]    Suitable imaging agents for N-ray imaging of inclusions may be selected by the linear attenuation coefficient or the thermal neutron absorption cross section of the material relative to that of the metal or metal alloy being cast. N-ray imaging agents currently considered most useful for filtering and detecting filter-derived inclusions in investment castings include those materials comprising metals selected from the group consisting of boron, dysprosium, erbium, europium, gadolinium, holmium, iridium, lutetium, neodymium, osmium, praseodymium, rhenium, samarium, tantalum, tungsten, ytterbium, isotopes thereof, physical mixtures thereof, and chemical mixtures thereof. The oxides of the above metals (e.g., Dy 2 O 3 , Er 2 O 3 , Gd 2 O 3 ) and the salts of such metals (e.g., GdNO 3 , YNO 3 ) are particularly useful. Gadolinium, for example, has one of the highest linear attenuation coefficients of any element (about 1483.88 cm −1 ), whereas the linear attenuation coefficient of titanium is about 0.68 cm −1 . The relatively large difference between the linear attenuation coefficient of titanium or titanium alloys and the linear attenuation coefficient of gadolinium makes gadolinia particularly suitable for N-ray imaging. However, other materials with the desired linear attenuation coefficient or thermal neutron absorption cross section can be used to practice the present method. Finally, certain isotopes of the elements above have superior neutron attenuation properties. For example, gadolinium 157 has a thermal neutron absorption coefficient of 254,000 barns. Additional isotopes that may be useful for N-ray imaging are included in Table 2. Other materials suitable for N-ray imaging may be identified by comparison of the linear attenuation coefficient or the thermal neutron absorption cross section of the material to the metal or metal alloy being cast.  
       IV. Construction of Filters  
       [0042]    It is possible to construct filters comprising substantially solely an imaging agent or imaging agents. Preferred imaging agents for constructing filters often are expensive, and hence it may be preferable to use a mixture of imaging agent or agents with another material suitable for filter construction. It is possible to distribute the imaging agent substantially uniformly throughout the filter material. Alternatively to use the imaging agent most efficiently, the imaging agent can be distributed substantially uniformly on the outer surface of the filter, and also perhaps in one or more of the outer layers of the filter material. If the imaging agent is not distributed substantially uniformly within the desired layer, it is possible that an inclusion could comprise solely undetectable material.  
         [0043]    The outer regions or layers of filters for filtering molten metals contact the metal or metal alloy in its molten state during filtration. Most metals and metal alloys used for investment casting are highly reactive, particularly at elevated temperature, so that at least the outer layers of the filter preferably should be substantially non-reactive with the molten metal or alloy being filtered.  
         [0044]    A list of materials useful for filtering molten metals includes those selected from the group consisting of zirconia, zircon, yttria, tantalum, titania, tungsten, hafnium, iridium, osmium, rhenium, isotopes thereof, physical mixtures thereof and chemical mixtures thereof. The choice of the filter material depends on the metal being filtered. For example, yttria is a useful component of filters, especially outer regions of filters, for molten titanium and titanium alloys, primarily because it is less reactive with molten titanium and titanium alloys than many other refractory materials.  
         [0045]    Simple physical mixtures of refractory materials for filter construction and imaging materials generally work to practice the present invention. Alternatively, “intimate mixtures” formed between the imaging agent and other components of the filter may be used. Intimate mixtures are discussed in detail in U.S. Pat. No. 6,102,099, which is incorporated herein by reference. “Intimately mixed” or “intimate mixture” is used to differentiate binary mixtures that result simply from the physical combination of two components. Typically, an “intimate mixture” means that the first material is atomically dispersed in a second, such as with a solid solution or as small precipitates in a crystal matrix. Alternatively, an intimate mixture may refer to compounds that are fused, such as, fused yttria-alumina or fused yttria-titania. By way of example and without limitation, intimate mixtures may be formed in the following ways: (1) finely dispersed in a matrix; (2) provided as a coating on the surface of particles; or (3) provided as a diffused surface layer of on the outer surface of particles. The intimate mixture may be a solid solution, or it may be in the form of small precipitates in a crystal matrix, or it may be a coating on the surface of a particle or portions thereof.  
         [0046]    The filter material may be physically mixed with the imaging material. The filter material also may be fused with the imaging material. Fused materials may be generated by first forming the desired weight mixture of filter-forming material and imaging material. The mixture is then heated until molten and then cooled to yield the fused material. Fused material can then be used to construct a filter or a filter layer in the same way as the physical mixture would be used. For example, the filter can be constructed without the use of an imaging agent, and can be coated or impregnated subsequently with one or more imaging agents. The imaging agent may be supplied as a physical mixture, a solution or an intimate mixture with other components. For example, a filter may be impregnated with a slurry comprising an imaging agent or mixture of imaging agents with or without additional refractory materials. Similarly a filter may be infiltrated with a solution comprising one or more imaging agents. The filter may then be fired at a temperature sufficient to sinter the filter components. Typical firing temperatures are from about 2,000° F. (1,090° C.) to about 3,200° F. (1,800° C.).  
       V. Methods of Filtering Molten Metals  
       [0047]    One embodiment of the present method includes providing a filter comprising an imaging agent, providing a molten metal or metal alloy and filtering the molten metal or metal alloy. The filter can include any filter comprising an imaging agent or an imaging agent and additional refractory material. The imaging agent may be selected such that filter-derived inclusions may be imaged in a product casting.  
         [0048]    Filters may be heated to a temperature sufficient to avoid substantial solidification of the melt during filtration. For example, working embodiments of the present invention involve heating a tungsten wire mesh filter to above about the melting point of titanium prior to filtration of a titanium melt.  
         [0049]    Filters may be placed in the molten metal flow at any point in the flow path. For example the filters may be attached to a crucible used to pour the molten metal, to the mold gating, or to any other point in the flow path of the molten metal.  
         [0050]    Filtration of reactive metals and metal alloys including aluminum, steel, titanium, including superalloys, particularly alloys of titanium, chromium, nickel, zirconium and cobalt may be performed under an inert atmosphere or at reduced pressure or both. Any gas of low reactivity may be used to provide an inert atmosphere. Typical examples of inert gases include helium, nitrogen, neon, argon, krypton and xenon.  
       VI. Alloys Comprising Imaging Agents  
       [0051]    Embodiments of the present method include using alloys comprising an imaging agent. Alloys comprising any imaging agent may be used to practice the present method. Typically, embodiments of the present method include titanium alloys comprising any imaging agent listed in Table 2. More typically, embodiments of the present method include titanium alloys comprising an imaging agent selected from the group consisting of gadolinium, samarium, europium, and mixtures thereof.  
         [0052]    Generally, the method for making alloys of the present invention comprise providing desired, predetermined material amounts, such as by stating weight percents. These predetermined material amounts are then combined and then heated to a temperature effective to form the alloy. Working embodiments have heated the constituent materials to a temperature above the melting point of the constituent metals. Additional information concerning working embodiments of making and using such alloys in casting processes. Additional techniques for processing materials are known to those skilled in the art, for example, hot isostatic pressing (HIP) and can be used to make desired alloys.  
       EXAMPLE 1  
       [0053]    This example describes a tungsten wire mesh filter useful for filtering molten metals. Tungsten wire cloth was purchased from Twin City Wire, Inc., Eagan Minn. The wire cloth is available in various appropriate gauges and aperture sizes to withstand the physical stresses of melt filtration while capturing any undesired foreign materials. The wire cloth may be fashioned into a filter apparatus, such as that illustrated by FIG. 4. A 12-inch×12-inch (30 cm×30 cm) section of tungsten wire cloth  52  (aperture size 0.029 inch (0.74 cm), having wire of diameter 0.004 inch (0.1 mm) was secured to ceramic supports  56  by clamps  54 . Tungsten rod  60  was coupled to the tungsten wire mesh by clamping (clamps not shown) insulative, alumina half-tube  58  around the mesh-wrapped rod. Tungsten rods  60  were connected to a power source, and the filter was heated by application of electrical current to approximately 3,300° F. (1,800° C.) as measured by a thermocouple (not shown). Molten titanium 6-4 alloy (33 lbs., 15 kg) was then poured through the filter.  
       EXAMPLE 2  
       [0054]    This example describes the preparation of a ceramic filter useful for filtering molten metals. The preparation includes a method for making a slurry useful for impregnating a filter comprising a refractory material with an imaging agent. Amounts stated in this example are percentages based upon the total weight of the slurry (weight percents), unless noted otherwise. Moreover, unless otherwise stated, the materials were combined in the order given.  
         [0055]    In this particular example, the filter was made from partially stabilized zirconia impregnated with yttria. Yttria is a robust material suitable for filtering reactive metals and metal alloys.  
         [0056]    Deionized water (15.6 weight %) was mixed with 1.00 weight % tetraethyl ammonium hydroxide (TEAH). To this solution was added 1.2% latex (Dow 460 NA), 0.7 weight % surfactant (NOPCOWET C-50) and 0.2 weight % of a defoamer (Dow 1410 antifoam). Finally, 81.3 weight % yttria was added to the mixture under continuous stirring to form a slurry.  
         [0057]    A partially stabilized zirconia filter having 20 pores per inch, purchased from HiTech Ceramics, Alfred, N.Y. was then positioned in the yttria slurry and placed under vacuum (28 inches Hg, 711 mm Hg) to infiltrate the slurry into the pores of the filter. The filter was removed from the slurry and excess solid material was removed from the filter. After drying at 160° F. (71° C.), the filter was fired at 2900° F. (1,600° C.) to sinter and densify the filter.  
       EXAMPLE 3  
       [0058]    In this particular example, a partially stabilized zirconia filter having 10 pores per inch was vacuum infiltrated with a gadolinium nitrate solution. Gadolinium is particularly useful for imaging inclusions by N-ray because it has a relatively high linear attenuation coefficient of 1483.88 cm −1 .  
         [0059]    A partially stabilized zirconia filter having 10 pores per inch, purchased from HiTech Ceramics, Alfred, N.Y. was placed in a 1.66 M solution of GdNO 3  and placed under vacuum (28 inches Hg, 711 mm Hg) to infiltrate the solution into the pores of the filter. The filter was removed from the solution and dried at 160° F. (71° C.). The filter was fired at 2,900° F. (1,600° C.) to sinter and densify the filter.  
       EXAMPLE 4  
       [0060]    In this particular example, a partially stabilized zirconia filter having 10 pores per inch was vacuum infiltrated with a yttrium nitrate solution to produce a filter suitable for filtering molten metals including titanium and titanium alloys.  
         [0061]    A partially stabilized zirconia filter having 10 pores per inch, purchased from HiTech Ceramics, Alfred, N.Y., was placed in a 2.66 M solution of YNO 3  and placed under vacuum (28 inches Hg, 711 mm Hg) to infiltrate the solution into the pores of the filter. The filter was removed from the solution and dried at 160° F. (71° C.). The filter was fired at 2,900° F. (1,600° C.) to sinter and densify the filter.  
       EXAMPLE 5  
       [0062]    This example concerns the attempted filtration of a titanium alloy melt through a non-heated tungsten filter. The filter was constructed by drilling apertures of three different sizes (0.025, 0.050 and 0.1 inch) in a tungsten plate. Ti 6-4 ally was melted over the tungsten plate. The molten metal solidified on the filter, blocking the apertures before any molten metal could flow through the apertures.  
       EXAMPLE 6  
       [0063]    This example concerns the filtration of a titanium alloy melt through a tungsten wire mesh filter. In this example, the filter apparatus of Example 1, comprising a tungsten wire mesh approximately 12 inches×12 inches (30 cm×30 cm), having wire diameters of about 0.004 inch (0.1 mm) and apertures of about 0.029 inch (0.74 cm), was heated to approximately 2,000° F. A melt comprising approximately 33 pounds (15 kg) of molten Ti 6-4 alloy was prepared, and the melt poured through the filter into a mold to form a casting. The molten metal solidified on the filter, and the filter failed during filtration, fracturing in several places. This resulted in tungsten inclusions in the casting.  
       EXAMPLE 7  
       [0064]    This example concerns the filtration of a titanium alloy melt through a tungsten wire mesh filter, and metal articles cast using the filtration process.  
         [0065]    In this particular example, the filter apparatus of Example 1, comprising a tungsten wire mesh approximately 12 inches×12 inches (30 cm×30 cm) having wire of diameter 0.004 inches (0.1 mm) and apertures of 0.029 inches (0.74 mm), was heated to approximately 3200° F. (1760° C.). A melt comprising approximately 33 pounds (15 kg) of molten Ti 6-4 alloy was prepared and 38 tungsten wire clippings of diameter 0.004 inch (0.1 mm) length ranging from 0.03 inch (0.76 mm) to 0.28 inch (7.1 mm) were added to the melt. The melt was filtered through the filter (over 4 seconds) into a mold to form a casting.  
         [0066]    X-ray radiography of the filter (FIG. 5) revealed that the filter was intact and all of the wire clippings were retained on the filter. The casting contained no inclusions detectable by X-ray radiography.  
       EXAMPLE 8  
       [0067]    This example relates to filtering molten Ti 6-4 alloy through a filter comprising GdNO 3  as an imaging agent. Such a filter may be produced by the method of Example 3.  
         [0068]    A filter according to Example 3, comprising gadolinium and having 10 pores per inch is placed in the gating of a mold. A titanium 6-4 alloy melt is filtered through the filter and into the mold to produce, after cooling, a metal article. The article is subjected to at least N-ray radiography to determine if any material used to construct the filter flaked, fragmented, leached or otherwise became incorporated into the casting.  
       EXAMPLE 9  
       [0069]    This example concerns forming a metal alloy comprising an imaging agent such that inclusions of the metal alloy may be imaged in cast metal article. In this example, the alloy comprising an imaging agent was used to evaluate the effectiveness of a filters for filtering molten metals.  
         [0070]    In this particular example a titanium alloy comprising 6% gadolinium (Ti 6-6-4) was produced. To prepare the alloy, a hole was drilled in an ingot of Ti 6-4 alloy to provide an ingot weighing 20 lbs (9.1 kg). Gadolinium, 1.2 lbs (0.55 kg) was placed in the drilled hole. The ingot was melted and the melt was allowed to solidify in a mold to form an ingot. The solid Ti 6-6-4 ingot was sliced and the slices were heated in an oxyacetylene flame to form nitride and oxide slag from the Ti 6-6-4. The slag was then machined to form roughly spherical shapes having diameters ranging from about 0.010 inch (0.0254 cm) to about 0.15 inch (0.381 cm). The Ti 6-6-4 slag spheres were added to a Ti 6-4 melt, and the melt was filtered through the ceramic filter of Example 4. Following filtration, the filter was removed and subjected to N-ray radiography. The radiograph (FIG. 6) shows Ti 6-6-4 inclusions retained by the filter.  
       EXAMPLE 10  
       [0071]    This example concerns forming a metal alloy such that inclusions of the metal alloy may be imaged in a cast metal article. In this particular example, a titanium alloy comprising 6% samarium is produced. The alloy may be used to evaluate filters for filtering molten metals.  
         [0072]    To prepare the alloy, a hole is drilled in an ingot of Ti 6-4 alloy to give an ingot weighing 20 lbs (9.1 kg). Samarium, 1.2 lbs (0.55 kg), is placed in the drilled hole. The ingot is melted and the melt is allowed to solidify in a mold to form an ingot. The solid ingot is sliced and the slices are heated in an oxyacetylene flame to form nitride and oxide slag. The slag is then machined to a roughly spherical shape with a diameter of from about 0.010 inch (0.0254 cm) to about 0.15 inch (0.381 cm). The slag spheres were then added to a Ti 6-4 melt. The melt is filtered through a ceramic filter of about 20 ppi. Following filtration, the filter is removed and is subjected to at least N-ray radiography.  
       EXAMPLE 11  
       [0073]    This example concerns forming a metal alloy such that inclusions of the metal alloy may be imaged in a cast metal article. In this particular example, a titanium alloy comprising 6% Europium is produced. The alloy may be used to evaluate filters for filtering molten metals.  
         [0074]    To prepare the alloy, a hole is drilled in an ingot of Ti 6-4 alloy to give an ingot weighing 20 lbs (9.1 kg). Europium, 1.2 lbs (0.55 kg) is placed in the drilled hole. The ingot is melted and the melt is allowed to solidify in a mold to form an ingot. The solid ingot is sliced and the slices heated in an oxyacetylene flame to form nitride and oxide slag. The slag is then machined to form roughly spherical shapes having diameters ranging from about 0.010 inch (0.0254 cm) to about 0.15 inch (0.381 cm). The slag spheres were then added to a Ti 6-4 melt. The melt is filtered through a ceramic filter of about 20 ppi. Following filtration, the filter is removed and is subjected to at least N-ray radiography.  
         [0075]    The present invention has been described with respect to certain preferred embodiments. However, the present invention should not be limited to the particular features described. Instead, the scope of the invention should be determined by the following claims.