Abstract:
Embodiments of methods and systems for inspecting a structure for a crystallographic imperfection are provided. In the method, an X-ray wavelength that is particularly susceptible to diffraction by the crystallographic imperfection is identified. Then an X-ray source is provided to emit X-rays in the identified X-ray wavelength. While placing the structure at a sequence of positions relative to the X-ray source, X-rays are directed at the structure in multiple, non-parallel arrays to create sequential patterns of diffracted X-rays. The patterns of diffracted X-rays are digitally captured and communicated to a computer that compares them to locate the crystallographic imperfection. For a surface imperfection, the imperfection may be marked with a target to allow for physical removal.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to methods and systems for inspecting materials possessing a crystallographic structure, and more particularly relates to methods and systems for locating crystallographic imperfections in materials with crystallographic structures. 
     BACKGROUND OF THE INVENTION 
     Currently, superalloys are widely used for applications in which high stresses must be endured at elevated temperatures, for instance in the components of gas turbine engines, such as blades and vanes. Improvements in manufacturing methods have led to casting of components in single-crystal form, resulting in improved high-temperature lives and strength over conventionally prepared metallic materials that included a plurality of grains separated by grain boundaries. 
     Due to the improved performance of single-crystal superalloy components, the ability to withstand severe operating conditions is expected. However, one or more significant departures from single-crystal perfection may seriously limit the ability of a single-crystal superalloy component to perform under severe operating conditions, and may shorten the service life of the component. Because, the likelihood of fracture and separation along crystallographic boundaries around imperfections is increased, castings for turbine blades and vanes require close inspection for spurious grains and other crystallographic imperfections. The current industry practice is to use an etching process to reveal the spurious grains and crystallographic imperfections on the surface of single-crystal castings. After etching, the casting is visually inspected to evaluate the etched surface relative to the appropriate acceptance criteria for the intended use of the casting. 
     While etching processes have historically provided good grain contrast for revealing the external grain structure of equiaxed and polycrystalline directionally solidified superalloy castings, these etching processes tend to be inspector dependant, are time-consuming, and may result in dimensional nonconformance due to excessive stock loss, especially given the relatively thin walls of internally cooled components. Stock loss can be very significant if the overall etching process has to be repeated due to insufficient ‘readability’ of grain. Further, the etching processes may suffer issues with the presence of scale, with a lack of reflectivity, or with various confounding or masking effects such as anodizing iridescence (aka bluing), which may result in failure to reveal, identify, or locate imperfections, or in difficulty in revealing, identifying or locating imperfections. 
     Accordingly, it is desirable to provide methods and systems for inspecting single-crystal superalloy castings without etching. Also, it is desirable to provide methods and systems for inspecting single-crystal superalloy castings that use X-ray diffraction (XRD) to locate surface and subsurface imperfections. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, when taken in conjunction with the accompanying drawings and this background of the invention. 
     SUMMARY OF THE INVENTION 
     Methods and systems for inspecting castings are provided herein. The methods and systems may be used to inspect or characterize the external and internal grain structure of any material possessing a crystallographic structure in which satisfying the Bragg angle geometry would cause X-rays of suitable wavelengths to be diffracted. Such materials include conventional (i.e., equiaxed) castings, polycrystalline directionally solidified (i.e., DS) castings, single-crystal superalloy castings, wrought material (e.g., duplex or large grain in forgings, excessive grain growth resulting from improper cold work and/or heat treatment) and crystalline non-metallic materials. 
     The methods and systems reverse an important signal-to-noise relationship in conventional radiographic inspection technology. Namely, the ratio of the desired effect of relative X-ray absorption (density/thickness of sound metal versus density/thickness of discontinuities) compared to an undesirable effect called ‘grain diffraction’ (i.e., mottling). Mottling of an X-ray image appears as blotches corresponding to where certain grains have diverted (by diffraction) the otherwise straight-line path of the local X-ray beam. 
     A grain which diverts a significant portion of the X-ray beam will tend to appear on film (or sensor image) as having a higher density (as if the diverted beam was absorbed). If the diverted X-rays happen to be superimposed on an area of sound metal, that area or blotch (receiving extra X-rays) will appear to have a lower density, which is similar in appearance to porosity. This phenomenon results in false positives for shrinkage porosity and requires great effort in conventional radiography to minimize XRD. 
     In other words, the method and system herein optimize the otherwise undesirable X-ray diffraction effect, yet may retain some aspects of a conventional X-ray image of the casting to serve as a reference to assist in locating the crystallographic imperfection on a particular casting. In some instances, the method and system can sufficiently detect some of the larger size conventional discontinuities (e.g. porosity, inclusions, separations, etc.) to serve as an early screening inspection for such conditions. 
     Because mottling can masquerade as porosity or otherwise interfere with proper interpretation of an X-ray image, industrial X-ray machines considered for foundry uses have special wavelength filters or are operated at voltages or with special X-ray emitting tubes to reduce mottling. 
     Unlike other methods that utilize X-ray diffraction and require highly-collimated, narrowly-focused X-ray beams or highly-parallel X-ray beams, the method and system herein do not. In fact, the present method ands system utilize X-rays that fan out from the X-ray source in multiple, non-parallel arrays to enable multiple opportunities to satisfy Bragg angle conditions, thus achieving efficient inspection of very large castings or possibly multiple castings. 
     While the analogy to conventional X-ray may suggest that only the transmission mode of capturing XRD information is used, it is envisioned that the back-reflection mode may also provide vital complementary data. 
     In accordance with an exemplary embodiment, a method for inspecting a single-crystal superalloy casting comprises the initial step of identifying an X-ray wavelength susceptible to diffraction by a crystallographic imperfection. During this step, an X-ray wavelength that exhibits significant diffraction upon encountering a crystallographic imperfection in a casting is identified. The diffraction of X-rays of specific wavelength by satisfying the Bragg angle crystallographic geometry is well known in the industry and needs no further discussion beyond awareness of those X-ray wavelengths likely to be most useful. The Laue method, which may either be back-reflection mode or transmission mode enables measurement of the specific crystallographic nature of the detected grain imperfection. While use of a monochromatic X-ray may result from the identification of the X-ray wavelength, it is also envisioned that the identified wavelength may include a defined band of X-ray wavelength, or even a plurality of non-continuous bands of X-ray wavelengths. Further, distinct bands of X-ray frequencies may be identified, with each band exhibiting significant diffraction for a different type of crystallographic imperfection. The use of multiple x-ray wavelengths may shorten the inspection time or provide a diagnostic tool by which certain crystallographic imperfections may be better characterized. 
     In order to maximize diffraction by the crystallographic imperfection during inspection, the method further provides for limiting or tuning the X-ray source so that it emits beams of X-rays within the identified wavelength or beams having a high fraction of X-rays within the identified wavelength. Further, the X-ray source may be enhanced so that it emits an enhanced beam having a selected profile of X-rays in the identified X-ray wavelength, such as a selected percentage of X-rays at one wavelength and a selected percentage of X-rays at another wavelength or multiple wavelengths. 
     After the X-ray wavelength is identified and the X-ray source readied to produce the desired beam, a casting to be inspected is placed at an initial position relative to the X-ray source. The relative position of the casting includes both its relative location in the x-, y-, and z-directions and its relative orientation about the x-, y-, and z-axes. Typically, the casting is placed on a mount or nest that may be automatically moved to the initial position by a computer. More than one casting may be simultaneously inspected depending on creation of a suitable multi-nest design. 
     For inspection, the X-ray source directs a divergent beam or stream of X-rays in the identified X-ray wavelengths at the casting for a selected exposure time for diffraction by any crystallographic imperfection therein to create a pattern of diffracted X-rays. For those rays passing through the casting (i.e., transmission X-rays), the pattern of diffracted X-rays is created behind the casting (relative to the X-ray source). For the rays diffracted back from the casting (i.e., back-reflected X-rays), the pattern of diffracted X-rays is created before the casting (again, relative to the X-ray source). 
     For either or both transmission and back-reflected X-rays, the pattern of diffracted X-rays is captured by a digital image capture device. As is understood, a capture device capable of detecting or sensing relevant X-rays to sufficient resolution is positioned behind the casting for transmission X-rays, and a capture device is positioned between the X-ray source and the casting for back-reflected X-rays. 
     After the pattern of diffracted X-rays is captured, the capture device communicates the pattern to a computer. Then the computer causes the casting to be placed at a second position and the X-ray source directs a beam at the casting to create a second pattern of diffracted X-rays. The repositioning may involve ‘centering’ or lateral movement of part relative to the X-ray beam or may involve tilting the part in one or more planes. This process is repeated for sequential positions for a predetermined number of patterns or until the computer determines that an adequate number of patterns has been captured for analysis. During or after the sequential capture of patterns, the computer compares the patterns to locate any crystallographic imperfections on or within the casting. Further, the computer may identify a volume on or in the casting with possible imperfections and may thereafter control placement of the casting to focus on that volume for further inspection. 
     For purposes of correction, rework, or investigation, the method can include marking the detected crystallographic imperfection with a target such as a dab of paint or outlining circle, oval, etc. For instance, a relatively superficial imperfection on the surface of a casting may be physically removed to allow use of the casting. Therefore, marking the imperfection with the target allows for a removal operator to visually observe where the imperfection is before and during the removal process. For subsurface imperfections, a surface target also may be used, along with printed depth, periphery, or location information. The use of X-ray Computed Aided Tomography is well known method for three-dimensional mapping of internal imperfections and would be applicable for this method as well. In addition or in the alternative to marking, the method can provide for creation of a map of the casting including the location and three-dimensional periphery of the imperfection. This may be particularly relevant when reviewing crystallographic imperfections that are not in vital positions in the casting. In other words, while castings having imperfections in certain positions may be rendered unfit for use, castings with imperfections in other positions may be acceptable for use, depending on the imperfection size and type. Therefore, creation of the map may facilitate determination of whether a casting having an imperfection is still fit for use. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Embodiments of the present invention will hereinafter be described in conjunction with the following drawing figures, wherein like numerals denote like elements, and wherein: 
         FIG. 1  is a schematic illustration of a system for inspecting a structure for a crystallographic imperfection in accordance with an exemplary embodiment; 
         FIG. 2  is a schematic illustration of the system of  FIG. 1  shown in communication with a computer for automated operation of the system in accordance with an exemplary embodiment; and 
         FIG. 3  is a flow chart representing the method of inspecting a structure for a crystallographic imperfection in accordance with an exemplary embodiment; and 
         FIG. 4  is a schematic illustration depicting the results of an inspection of a structure for a crystallographic imperfection in accordance with an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following Detailed Description is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding Background of the Invention or the following Detailed Description. 
     Referring to  FIG. 1 , a system for inspecting structures such as single-crystal superalloy castings in accordance with an exemplary embodiment is shown and generally designated  10 . As shown, the system  10  includes an X-ray source  12 , such as an X-ray tube, for emitting arrays  14  of X-rays  16  along a plurality of non-parallel paths  18 . Further, the system  10  includes a testing area  20  for receiving a structure for inspection. The system  10  further includes capture devices  22 ,  24  that are positioned before and behind the testing area  20  (relative to the source  12 ) to allow for digital radiography during inspection as discussed below. As shown, the capture device  22  defines an opening  26  through which the arrays  14  of X-rays  16  pass along their non-parallel paths  18  (three paths are illustrated, though it is understood that arrays  14  of X-rays  16  can be emitted along hundreds of paths). 
     With the illustrated structures of the system  10  in  FIG. 1  defined, the placement of a structure  30 , such as a casting, for inspection for a crystallographic imperfection  32  may be discussed. As shown in  FIG. 1 , the structure  30  is placed in the testing area  20  along the paths  18  at a position  31 . Importantly, the position  31  has a measurable location relative to the X-ray source  12  in the direction along the x-axis  34   x , the y-axis  34   y , and the z-axis  34   z . Further, the position  31  includes a measurable orientation of the structure  30  about the x-axis  34   x , the y-axis  34   y , and the z-axis  34   z.    
     In  FIG. 1 , X-rays  16  are directed at the structure  30  for diffraction by the crystallographic imperfection  32  to create diffracted X-rays  36 . As shown, the diffracted X-rays  36  may include back-reflected X-rays  36   a  that are reflected back from the structure  30  to be captured by the capture device  22 . These back-reflected X-rays  36   a  may be analyzed to locate an imperfection  32  on the surface of the structure  30 . More specifically, an imperfection  32  on the surface of the structure  30  will diffract the oncoming X-rays  16  differently from the rest of the surface of the structure  30 . As a result, the back-reflected X-rays  36   a  will create a two-dimensional pattern  38  on the capture device  22 . Graphically, the pattern  38  will include lighter areas including fewer X-ray collisions per area and darker areas with more collisions per area. 
     Additionally or alternatively, the diffracted X-rays  36  may include transmission X-rays  36   b  that pass through the structure  30  to be captured by the capture device  24 . These transmission X-rays  36   b  allow for the inspection of subsurface or interior imperfections  32  in the structure  30 . Similar to the discussion related to surface imperfections  32 , a subsurface imperfection  32  in the interior of the structure  30  will diffract the oncoming X-rays  16  differently from the rest of the internal volume of the structure  30 . As a result, the transmission X-rays  36   b  will create a two-dimensional pattern  38  on the capture device  24 . Again, the pattern  38  will include lighter areas including fewer X-ray collisions per area and darker areas with more collisions per area. 
     For purposes of the present embodiment, a plurality of patterns  38  are captured and compared with one another or otherwise analyzed to locate crystallographic imperfections  32  in the structure  30 . Specifically, a pattern  38  is captured for each of a sequence of different positions  31  of the structure  30  relative to the X-ray source  12 . The position  31  of the structure  30  is directly related to the resulting pattern  38 , and the positional data is used in the comparison of patterns  38  to locate the imperfections  32  as is understood in radiography. 
     Referring to  FIG. 2 , the system  10  is shown to provide for automatic operation and analysis to locate imperfections  32 . In  FIG. 2 , the X-ray source  12  and capture devices  22 ,  24  are connected to a computer  40 . Also, the structure  30  is shown to be situated on a mount  42  that is connected to the computer  40 . Further, the computer  40  is in communication with a marking device  44  and a display device  46 . 
     As may be understood by cross-referencing  FIGS. 1 and 2 , the computer  40  is able to place the structure  30  at an initial position  31  and activate the X-ray source  12  to direct X-rays  16  at the structure  30  for a selected exposure time. After the X-rays  16  are diffracted and the diffracted X-rays  36  are captured by the capture device  22 ,  24 , the pattern  38  of diffracted rays  36  is communicated to the computer  40  by the capture device  22 ,  24 . The computer  40  then moves the structure  30  to a new position  31  and repeats the X-ray procedure. The computer  40  may move the structure  30  to a sequence of scripted positions or to a sequence of positions determined based on ongoing analysis of the already-received patterns  38 . Because the profile of the X-rays is enhanced and the process is automated, numerous patterns  38  may be captured in a short amount of time. 
     Upon location of a crystallographic imperfection  32 , the computer  40  may instruct the marking device  44  to mark the imperfection  32  with a target of paint, ink, resin or the like, or the computer  40  may create a three dimensional map of the structure  30  and the location of the imperfection  32  for graphic display, either electronically on the display device  46  a monitor or printed via a non-illustrated printer. 
     Referring now to  FIG. 3 , the method of an embodiment is illustrated in a flow chart. Initially, an X-ray wavelength susceptible to diffraction by a crystallographic imperfection is identified at  50 . As stated above, the wavelength may include a single monochromatic X-ray, a band of wavelengths, or a plurality of noncontiguous wavelengths. 
     After the wavelength is identified, the X-ray source  12  is limited and/or enhanced at  52  to emit arrays  14  having a high fraction of X-rays  16  of the identified wavelength, such as over fifty percent, to produce useable diffraction imaging with a short exposure time. This is done so as to maximize diffraction by any crystallographic imperfection that may be present. By maximizing diffraction, the patterns  38  of diffracted X-rays  36  are amplified so that small imperfections are more easily located. The output of the X-ray source  12  may be enhanced by changing the X-ray tube target material, changing the X-ray tube voltage, filtering to remove non-interactive X-ray wavelengths, or through other methods including the use of synchrotrons. Enhancement of the array  14  of X-rays  16  will result in a shortening of the amount of time needed for capturing a proper pattern  38  of diffracted X-rays  36  as well as improved sharpness of the pattern  38 . 
     At  54 , the structure  30  is placed at a position  31  relative to the X-ray source  12 . Thereafter, the array  14  of X-rays  16  is directed toward the structure  30  along non-parallel paths  18  at  56  for diffraction by an imperfection  32  at  56 . At  58 , the diffracted X-rays  36  are captured by the capture device  22 ,  24  and the pattern  38  of diffracted X-rays  36  is communicated to the computer  50 . 
     The computer  50  compares or otherwise analyzes the patterns  38  at  60 , and determines whether more data (e.g., additional patterns  38 ) is needed at inquiry  62 . If more data is needed, then the computer  50  moves the structure  30  to a new position  31  at  54  and repeats the succeeding steps until the inquiry  62 . When more data is not necessary, the location of the crystallographic imperfection  32  is performed at  64  by comparing patterns  38 . Specifically, pattern elements such as the presence and position of lighter areas (indicating fewer X-ray collisions) and darker areas (indicating more X-ray collisions) are analyzed in each pattern  38  in view of the associated casting position  31 . A comparison of these pattern elements for a plurality of patterns  38  indicates what pattern elements are caused by a crystallographic imperfection  32 , and the location and physical characteristics of that crystallographic imperfection  32 . After the location of the crystallographic imperfection  32  is performed, the marking device  44  may mark a target on the imperfection  32  or the structure  30  at  66  and/or create a map of the structure  30  showing the location of the imperfection  32  at  68 . 
     Referring now to  FIG. 4 , exemplary results of an inspection are illustrated. As shown, an X-ray source  112  is configured to emit non-parallel beams such as exemplary beams  114 ,  115 ,  116 , and  117 . Beams  115 ,  116  and  117  are separated from perpendicular beam  114  by a divergence angle, for example the divergence angle between beams  114  and  117  identified by arrow  118 . Further, each beam  114 ,  115 ,  116 , and  117  is directed at a casting  130  which contains a crystallographic imperfection  132 . In  FIG. 4 , the beams are shown passing through the casting  130 , and the resulting film  124  produced by a capture device positioned beyond the casting  130 , although such beams may be reflected back to capture device position between the casting  130  and the source  112 . In  FIG. 4 , the X-ray source  112  and casting  130  are presented in cross-section view, while the resulting film  124  is illustrated as a top view. Further, while the film  124  includes cross hatching for clarity, in actuality, shading typical of an X-ray capture would be present. 
     As shown in  FIG. 4 , beam  114  is diffracted from its path  164  to a diffracted path  166  which reaches the capture device  124 . Further, beam  115  reaches the capture device  124  along a substantially non-diffracted or slightly diffracted path  168 . As paths  166  and  168  intersect the capture device as substantially the same position, the film  124  registers a dark area in a section  125  indicative of the crystallographic imperfection  132 . Further, as beam  114  is diffracted off of path  164 , the film  125  registers a light area  171  at its intersection with path  164 . 
     This occurrence is repeated with beams  116  and  117 . As shown, beam  117  is diffracted from path  172  to path  174 . Further, beam  116  is substantially non-diffracted or slightly diffracted and remains on path  176 . As paths  174  and  176  strike the film  124  at substantially the same position, a dark area  178  is registered by the capture device  124 . Further, as beam  117  is diffracted from its path  172 , a light area  180  is registered at the intersection of path  172  and the film  124 . 
     The existence and position of the dark areas  170  and light area  171  are caused by wavelength induced diffraction. Where more X-rays reach the film  124 , darker areas are created, and where fewer X-rays reach the film  124 , lighter areas are registered. This is repeated with dark area  178  and light area  180 . Further, dark area  178  and light area  180  also exhibit divergence angle induced diffraction, as beams  116  and  117  were emitted at divergence angles from the perpendicular beam  114 . 
     In  FIG. 4 , it can be seen that the casting  130  may be pivoted to a new position indicated by dotted line  181  which is at a tilt angle of few degrees or more and represented by arrow  182 . As a result of tilting the casting  130 , the resulting pattern of dark and light areas is changed. The film produced by the capture device is shown with the changed pattern as indicated by numeral  224 . As shown, dark and light areas  170 ,  171 ,  178 ,  180  are moved as a result of tilting the casting  130 . Further, additional dark areas  184  and light areas  186  may be created, as a result of random satisfaction of Bragg angle and wavelength conditions. 
     As can be seen from  FIG. 4 , a combination of wavelength, divergence angle, and tilt angle (which may be three-dimensional) allows for the analysis of patterns of dark and light areas that indicate the impact or absence of X-ray beams. The analysis results in the identification of crystallographic imperfections as well as their location and boundaries. 
     While at least one exemplary embodiment has been presented in the foregoing Detailed Description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or exemplary embodiments are only examples, and are not intended to limit the scope, applicability, or configuration of the invention in any way. Rather, the foregoing Detailed Description will provide those skilled in the art with a convenient road map for implementing an exemplary embodiment of the invention, it being understood that various changes may be made in the function and arrangement of elements described in an exemplary embodiment without departing from the scope of the invention as set forth in the appended Claims and their legal equivalents.