Abstract:
A structure and associated process for joining dissimilar materials to form various components of an x-ray tube is illustrated that utilizes one or more intermediate or interfacial filler material members positioned between the primary welding or mating surfaces of the base material components to be joined. The use of the interfacial or intermediate filler material preserves the multiple benefits of friction welding, as well as enabling the joining of highly dissimilar material components, decreasing the required joining temperature, and providing increased microstructural control of the resulting weld or joint.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates generally to X-ray tubes, and more particularly to structures and methods of assembly for the shaft of the anode utilized in an X-ray tube. 
     X-ray systems may include an x-ray tube, a detector, and a support structure for the x-ray tube and the detector. In operation, an imaging table, on which an object is positioned, may be located between the x-ray tube and the detector. The x-ray tube typically emits radiation, such as x-rays, toward the object. The radiation passes through the object on the imaging table and impinges on the detector. As radiation passes through the object, internal structures of the object cause spatial variances in the radiation received at the detector. The detector then emits data received, and the system translates the radiation variances into an image, which may be used to evaluate the internal structure of the object. The object may include, but is not limited to, a patient in a medical imaging procedure and an inanimate object as in, for instance, a package in an x-ray scanner or computed tomography (CT) package scanner. 
     X-ray tubes include a cathode and an anode located within a high-vacuum environment. The anode structure is typically supported by one or more bearing members, such as ball bearings, and is rotated for the purpose of distributing the heat generated at a focal spot. Typically, an induction motor is employed to rotate the anode, the induction motor having a cylindrical rotor built into a cantilevered axle that supports a disc-shaped anode target and an iron stator structure with copper windings that surrounds an elongated neck of the x-ray tube. The rotor of the rotating anode assembly is driven by the stator. An x-ray tube cathode provides a focused electron beam that is accelerated across an anode-to-cathode vacuum gap and produces x-rays upon impact with the anode. Because of the high temperatures generated when the electron beam strikes the target, it is necessary to rotate the anode assembly at high rotational speed. This places stringent demands on the bearings and the material forming the anode structure, i.e., the anode target and the shaft supporting the target. 
     In other constructions, a liquid metal bearing may be employed in lieu of ball bearings. Advantages of liquid metal bearings include a high load capability and a high heat transfer capability due to an increased amount of contact area as compared to a ball bearing. Advantages also include low acoustic noise operation as is commonly understood in the art. Gallium, indium, or tin alloys are typically used as the liquid metal, as they tend to be liquid at room temperature and have adequately low vapor pressure, at operating temperatures, to meet the rigorous high vacuum requirements of an x-ray tube. However, liquid metals tend to be highly reactive and corrosive. Thus, a base metal that is resistant to such corrosion is desirable for the components that come into contact with the liquid metal bearing, such as the shaft of the anode assembly. 
     As a result, in either construction, the structure of the shaft to which the anode is connected must be capable of withstanding the high temperatures and mechanical stresses created within the x-ray tube, as well as be able to withstand the corrosive effects of the liquid metal bearing. As such, a refractory metal such as molybdenum or tungsten is typically used as the base material for the construction of the shaft as well as for the bearing components. Not only are such materials resistant to corrosion and high temperatures, but they tend to be vacuum-compatible and thus lend themselves to an x-ray tube application. 
     However, rather than have a shaft formed of a single material, it is desirable in many situations to form the shaft of different materials, each material having properties suited to the particular application or position of the shaft portion within the x-ray tube. For example, the material used for the thrust and journal surfaces of the bearing shaft must exhibit minimal reaction with the any liquid metal bearing fluid at the temperatures experienced during bearing processing and operation. However, the optimum materials used for the bearing surfaces are different from those used for the welding to the x-ray tube assembly. As a result, the dissimilar materials need to be hermetically joined in order to form the shaft. 
     One technique for minimizing base material expense and improving functionality is to include the preferred base metal (i.e., tungsten or molybdenum) only in regions that require the characteristics of the particular base metal. An extension made of a less expensive material may then be brazed thereto, the extension serving as a mechanical connection as support for an anode. In other words, as an example, a stationary center shall may support a rotatable support structure having an anode attached thereto. The center shaft may be made entirely of the preferred base metal, or the cost thereof may be reduced by attaching a less expensive steel thereto via brazing, thus reducing the total amount of the preferred base metal. Such a design may result in cost savings because of the less expensive steel portion being used in lieu of the preferred base metal. However, cost savings achieved while using this technique are typically offset to an extent by the additional attachment processing, such as by attaching the extension thereto having a hermetic seal. 
     However, when using brazing as the method for joining the dissimilar materials together very high temperatures are required, which can negatively affect the mechanical properties of the materials being joined, thereby reducing the tube life. 
     In an alternative method, the shaft can be formed by friction welding where the primary weld surfaces of the dissimilar materials are brought together under intense pressure and relative motion creating conditions (heat, mechanics, etc.) allowing these surfaces of the differing materials to metallurgically bond in solid state. An example of this is shown in U.S. Pat. No. 5,592,525. Traditional friction welding requires specific localized conditions that drive the bonding in the materials making up the primary welding surfaces. As the material properties of the primary surface materials diverge, the complexity of process design becomes complex and often infeasable and ultimately uneconomical. In short, friction welding is limited to particular material combinations and requires significant mechanical energy input to achieve quality joints and can degrade the properties of the base materials due to phenomenon such as grain growth. 
     As a result, it is desirable to develop a structure and method for the formation of a bearing shaft for an x-ray tube anode that can be formed with dissimilar materials but without degrading the desirable properties of the materials being joined. 
     BRIEF DESCRIPTION OF THE INVENTION 
     In the present invention a structure and associated process for joining dissimilar materials to form various components of an x-ray tube is illustrated that utilizes one or more intermediate or interfacial filler material members positioned between the mating or welding surfaces of the base materials to be joined. The use of the interfacial member preserves the multiple benefits of friction welding, as well as enabling the joining of dissimilar or highly dissimilar materials, decreasing the required joining temperature, and providing increased microstructural control of the resulting weld. The incorporation of the intermediate material members enables friction joining/inertial welding of highly dissimilar materials as well as significant process simplification of traditional materials systems due to the ability of the interfacial material member to interact with each of the base materials to be joined in a manner that provides a strong connection between the interfacial material member and the base materials. 
     In the method, the intermediate or interfacial filler material member is positioned between the mating or weld surfaces of the base materials to be joined. This intermediate material member can be applied to either or both surfaces prior to welding or can be a stand alone preform or preforms. This filler material can interact with the base materials in the molten or solid state either directly or by augmenting the thermodynamics of the primary weld surface materials. The interaction between filler material(s) and base materials is generally driven thermally, with heat provided to the system of the base materials and the intermediate material(s) in the method by any combination of preheating, direct heating during joining, or by exothermic reactions of the constituent materials forming the resulting weld or joint. 
     The addition of an intermediate filler material into a traditional friction welded joint provides several benefits. These benefits include overall manufacturing process simplification, the expansion of the possible base materials to be joined, and the enhancement of the material structure of both the joint and the resulting structure formed by the joint and the base materials. With regard to the overall process simplification, the use of friction or inertia welding results in a lower required joining temperature, and consequently a decreased cycle time for the process, lower equipment costs, and decreased stress applied to the base materials when joined. Concerning the expansion of the dissimilar materials that may be joined, the intermediate filler material enables less expensive alloys to be used as the base materials and base materials with wider differences in material properties of the primary surface materials due the ability of the intermediate material to effectively join the various base materials. Also, with regard to the resulting structure of the materials joined using the intermediate material, the structure and performance of the joint can be tailored to provide enhanced joint properties such as the avoidance of the creation of grain growth or unwanted phases in the joint, greater joint texture control provided the greater temperature flexibility in the friction/inertia welding process, and the ability to tailor the joint to increase hermetic seal length, leading to a more torturous gas leak path, among others. 
     In another exemplary embodiment of the invention, the invention is an assembly adapted for use with an x-ray tube, the assembly comprising a first component formed of a first material and having a first mating surface thereon, a second component formed of a second material and having a second mating surface thereon, the second mating surface defining a space with the first mating surface, and an intermediate member disposed within the space between the first mating surface and the second mating surface. 
     In still another exemplary embodiment of the invention the invention is a method for forming an assembly for use in an x-ray tube, the method comprising the steps of providing a first component formed of a first material and having a first mating surface thereon, a second component formed of a second material and having a second mating surface thereon that defines a space with the first mating surface, placing an intermediate filler material between the first mating surface and the second mating surface; and pressing the first component and the second component towards one another to form a joint between the first mating surface and the second mating surface within the space of the intermediate filler material. 
     In still a further exemplary embodiment of the invention, the invention is An x-ray tube comprising a frame, a cathode assembly disposed in the enclosure and an anode assembly disposed in the enclosure spaced from the cathode assembly, wherein the anode assembly comprises a first component formed of a first material and having a first mating surface thereon a second component formed of a second material and having a second mating surface thereon, the second mating surface defining a space with the first mating surface; and an intermediate filler member disposed within the space between the first mating surface and the second mating surface to form a joint therebetween. 
     It should be understood that the brief description above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of an imaging system incorporating exemplary embodiments of the invention. 
         FIG. 2  is a cross-sectional view of a portion of an x-ray tube according to an exemplary embodiment of the invention and usable with the system illustrated in  FIG. 1 . 
         FIG. 3  is a schematic view of an X-ray tube in accordance with an exemplary embodiment of the invention. 
         FIG. 4  is a schematic view of a shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention. 
         FIG. 5  is an exploded schematic view of a shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention. 
         FIG. 6  is a schematic view of a mating surface on one base material component of a shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention. 
         FIG. 7  is a schematic view of a mating surface on another base material component of a shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention 
         FIG. 8  is a schematic view of a weld formed between mating surfaces on base material components of a shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention. 
         FIG. 9  is a schematic view of and X-ray tube and shaft formed of dissimilar materials in accordance with an exemplary embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1  is a block diagram of an embodiment of an imaging system  10  designed both to acquire original image data and to process the image data for display and/or analysis in accordance with embodiments of the invention. It will be appreciated by those skilled in the art that various embodiments of the invention are applicable to numerous medical imaging systems implementing an x-ray tube, such as x-ray or mammography systems. Other imaging systems such as computed tomography (CT) systems and digital radiography (RAD) systems, which acquire image three dimensional data for a volume, also benefit from the invention. The following discussion of x-ray system  10  is merely an example of one such implementation and is not intended to be limiting in terms of modality. 
     As shown in  FIG. 1 , imaging system  10  includes an x-ray tube or source  12  configured to project a beam of x-rays  14  through an object  16 . Object  16  may include a human subject, pieces of baggage, or other objects desired to be scanned. X-ray source  12  may be conventional x-ray tubes producing x-rays  14  having a spectrum of energies that range, typically, from thirty (30) keV to two hundred (200) kcV. The x-rays  14  pass through object  16  and, after being attenuated, impinge upon a detector assembly  18 . Each detector module in detector assembly  18  produces an analog electrical signal that represents the intensity of an impinging x-ray beam, and hence the attenuated beam, as it passes through the object  16 . In one embodiment, detector assembly  18  is a scintillation based detector assembly, however, it is also envisioned that direct-conversion type detectors (e.g., CZT detectors, etc.) may also be implemented. 
     A processor  20  receives the signals from the detector  18  and generates an image corresponding to the object  16  being scanned. A computer  22  communicates with processor  20  to enable an operator, using operator console  24 , to control the scanning parameters and to view the generated image. That is, operator console  24  includes some form of operator interface, such as a keyboard, mouse, voice activated controller, or any other suitable input apparatus that allows an operator to control the x-ray system  10  and view the reconstructed image or other data from computer  22  on a display unit  26 . Additionally, console  24  allows an operator to store the generated image in a storage device  28  which may include hard drives, floppy discs, compact discs, etc. The operator may also use console  24  to provide commands and instructions to computer  22  for controlling a source controller  30  that provides power and timing signals to x-ray source  12 . 
       FIG. 2  illustrates a cross-sectional view of an x-ray source  12  incorporating embodiments of the invention. In the illustrated embodiment, x-ray source  12  is formed of an x-ray tube  40  that includes an anode assembly  42  and a cathode assembly  44 . X-ray tube  40  is supported by the anode and cathode assemblies  42 ,  44  within an envelope or frame  46 , which houses a target or anode  48 , a bearing assembly  50 , and a cathode  52 . Frame  46  defines an area of relatively low pressure (e.g., a vacuum)  30  compared to ambient, in which high voltages may be present. Frame  46  may be positioned within a casing (not shown) filled with a cooling medium, such as oil, that may also provide high voltage insulation. While the target and anode are described above as being a common component of x-ray tube  40 , the target and anode may be separate components in alternative x-ray tube embodiments. 
     In operation, an electron beam  54  is produced by cathode assembly  44 . In particular, cathode  52  receives one or more electrical signals via a series of electrical leads  56 . The electrical signals may be timing/control signals that cause cathode  52  to emit electron beam  54  at one or more energies and at one or more frequencies. The electrical signals may also at least partially control the potential between cathode  52  and anode  48 . Cathode  52  includes a central insulating shell  58  from which a mask  60  extends. Mask  60  encloses electrical leads  56 , which extend to a cathode cup  62  mounted at the end of mask  60 . In some embodiments, cathode cup  62  serves as an electrostatic lens that focuses electrons emitted from a thermionic filament within cathode cup  62  to form electron beam  54 . 
     X-rays  64  are produced when high-speed electrons of electron beam  54  are suddenly decelerated when directed from the cathode  52  to a target or focal surface  66  formed on target  48  via a potential difference therebetween of for example, sixty (60) thousand volts or more in the case of CT applications. The x-rays  64  are emitted through a radiation emission passage  68  formed in frame  46  toward a detector array, such as detector  18  of  FIG. 1 . 
     Anode assembly  42  includes a rotor  72  and a stator (not shown) located outside x-ray source  40  and partially surrounding rotor  72  for causing rotation of anode  48  during operation. Target  48  is supported in rotation by a bearing assembly  50 , which, when rotated, also causes target  48  to rotate about the centerline  70 . As shown, target  48  has a generally annular shape, such as a disk, and an annular opening  74  in the center thereof for receiving bearing assembly  50 . 
     Target  48  may be manufactured to include a number of metals or composites, such as tungsten, molybdenum, copper, or any material that contributes to bremsstrahlung, i.e., deceleration radiation, when bombarded with electrodes. Target or focal surface  66  of target  48  may be selected to have a relatively high refractory value so as to withstand the heat generated by electrons impacting target  48 . Further, the space between cathode assembly  44  and target  48  may be evacuated in order to minimize electron collisions with other atoms and to maximize an electric potential. 
     To avoid overheating of the target  48  when bombarded by the electrons, rotor  72  rotates target  48  at a high rate of speed (e.g., 90 to 250 Hz) about a centerline  70 . In addition to the rotation of target  48  within x-ray tube volume  46 , in a CT application, the x-ray source  40  as a whole is caused to rotate about an object, such as object  16  of imaging system  10  in  FIG. 1 , at rates of typically 1 Hz or faster. 
     Bearing assembly  50  can be formed as necessary, such with a number of suitable ball bearings (not shown), but in the illustrated exemplary embodiment comprises a liquid lubricated or self-acting bearing having adequate load-bearing capability and acceptable acoustic noise levels for operation within imaging system  10  of  FIG. 1 . As used herein, the terms “self-acting” and “self-lubricating” mean that the bearing lubricant remains distributed on the surfaces of the bearing due to the relative motion of the bearing components and absent an external pump. 
     In general, bearing assembly  50  includes a stationary portion, such as center shaft  76 , and a rotating portion, such as sleeve  78  to which the target  48  is attached. While center shaft  76  is described with respect to  FIG. 2  as the stationary portion of bearing assembly  50  and sleeve  78  is described as the rotating portion of bearing assembly  50 , embodiments of the present invention are also applicable to embodiments wherein center shaft  76  is a rotary shaft and sleeve  78  is a stationary component. In such a configuration, target  48  would rotate as center shaft  76  rotates. 
     Center shaft  76  may optionally include a cavity or coolant flow path  80  though which a coolant  82  ( FIG. 3 ), such as oil, may flow to cool bearing assembly  50 . As such, coolant  82  enables heat generated from target  48  of x-ray source  40  ( FIG. 2 ) to be extracted therefrom and transferred external to x-ray source  40 . In straddle mounted x-ray tube configurations, coolant flow path  80  extends along a longitudinal length of x-ray source  40 . In alternative embodiments, coolant flow path  80  may extend through only a portion of x-ray source  40 , such as in configurations where x-ray source  40  is cantilevered when placed in an imaging system. 
     Referring now to  FIG. 3 , a cross-sectional view of a portion of bearing assembly  50  is shown according to an embodiment of the invention. Bearing assembly  50  includes a center shaft  76  positioned within sleeve  78 , which is configured to support an anode (not shown), such as target  48  of  FIG. 2 . A lubricant  84  is positioned in a gap  86  formed between center shaft  76  and sleeve  78 . In embodiments of the invention, lubricant  84  is a metal or metallic alloy that exists in a liquid state at operating temperature of bearing assembly  50 . 
     The lubricating fluid  84  flowing between the rotating and stationary components of the bearing assembly  50  may include a variety of individual fluids as well as mixtures of fluids. For example, multiple liquid metals and liquid metal alloys may be used as the lubricating fluid, such as an indium gallium alloy. More generally, fluids with relatively low vapor pressures that are resistant to evaporation in vacuum-level pressures of the x-ray tube may be used. In the present context, low vapor pressures may generally be in the range of 1×10 −5  Torr. In other words, fluids that are stable in vacuums are desirable for use in x-ray tube systems so as to not adversely affect the established vacuum during operation of the system. In the present disclosure, lubricant  84  may be gallium or a gallium alloy as non-limiting examples. 
     Exemplary base materials of center shaft  76  and sleeve  78  of bearing assembly  50  include ceramics, metals, and combinations thereof. In one embodiment, center shaft  76  and sleeve  78  are constructed of the same base material. Alternatively, the base materials of center shaft  76  and sleeve  78  may differ. 
     In the embodiment illustrated in  FIG. 3 , center shaft  76  of bearing assembly  50  is a stationary component and sleeve  78  is a rotatable component constructed to rotate about center shaft  76 . However, one skilled in the art will recognize the inventive concepts described herein are applicable to alternative bearing configurations. As one example, bearing assembly  50  may instead include a stationary outer component and a rotating center shaft having a target attached thereto. As another example, bearing assembly  50  may be a “straddle” bearing that is configured to support a target between a first and a second liquid metal bearing. In other words, embodiments of this invention may be incorporated into any bearing configuration utilizing a liquid lubricated bearing to support an anode or target. Such configurations may include a stationary center shaft and a rotatable outer shaft, and vice versa. Further, one skilled in the art will recognize that such applications need not be limited to x-ray tubes, but may be applied to any configuration having a rotating component in a vacuum, the rotating component being supported by a liquid lubricated bearing. Thus, this invention is applicable to any bearing configuration having a rotatable component and a stationary component, and a liquid lubricant therebetween, regardless of configuration or application. 
     As illustrated in  FIG. 3 , center shaft  76  of bearing assembly  50  includes a thrust bearing portion  88  comprising a radial projection  90  that extends from center shaft  76  and is positioned in a radial cavity  92  of sleeve  78 . Radial projection  90  of thrust bearing portion  88  includes a pair of outer race surfaces  94 ,  96  that face inner race surfaces  98 ,  100  of sleeve  78 . In cantilever mount embodiments, sleeve  78  may also include a removable endcap (not shown) to allow assembly of components. Radial projection  90  limits axial motion of sleeve  78  relative to center shaft  76 , and, as illustrated, lubricant  84  is also included between radial projection  90  and sleeve  78 . Radial projection  90  need not be limited in axial length, but may be extended in axial length to provide additional mechanical support of components. 
     A radial or journal bearing portion  102  of bearing assembly  50  is located adjacent thrust bearing portion  88 . An outer surface  104  of journal bearing portion  102  of center shaft  76  faces an inner surface  106  of journal bearing portion  102  of sleeve  78 . While journal bearing portion  102  is illustrated on a first side of thrust bearing portion  88  adjacent outer race surface  94 , one skilled in the art will recognize that bearing assembly  50  may include a second journal bearing portion located on a second side of thrust bearing portion  88  adjacent outer race surface  96 . Various coatings, textures, and patterns including grooves embedded in the contacting surfaces of bearing assembly  50  may be applied to alter bearing behavior as the shall  76  and sleeve  78  rotate relative to each other. 
     Bearing assembly  50  may be referred to as a spiral groove bearing (SGB) due to the patterning of grooves along the various surfaces of the bearing. In some examples, the spiral groove may be formed from a logarithmic spiral shape. The spiral groove bearing may also be equivalently referred to as a fluid dynamic bearing and liquid bearing as well. In such spiral groove bearings, ways to contain the liquid lubricant  84  may be categorized in two general methods. The first includes providing physical barriers near the ends of the bearing where shaft seals would be placed in other applications. Rubber or other types of shaft seals in the presence of the vacuum inside the x-ray tube may function improperly, degrade quickly, and/or destroy the pressure inside the x-ray tube. For similar reasons, o-rings, grease, or other conventional means for aiding in rotational lubrication between two components may be undesirable because of the vacuum in the x-ray lube. Greases and other lubricants with lower vapor pressure than liquid metals may vaporize and destroy the vacuum. In some examples, physical walls of different shapes and sizes may be placed at different angles to capture the lubricant to reduce leakage through the bearing. 
     The second general method includes utilizing the capillary forces of the lubricant, wherein the small gap between two opposing bearing surfaces wets the fluid to retain the fluid within the gap. In other words, the anti-wetting properties of the surface (via texturing, coating, or both) aids in preventing the lubricant from flowing in between the small gaps. In some examples, the surfaces are coated and/or textured to be more wetted such that the lubricant clings in the small gap to reduce lubricant moving through the gap. In other examples, the surfaces are coated and/or textured to be more anti-wetting such that the lubricant is pushed away from the small gaps near the ends of the bearing assembly. In this context, the small gap may be in the range of 50 microns. 
     Operation of liquid bearings in x-ray tube systems, such as bearing assembly  50  of  FIGS. 2 and 3 , may be at least partially dependent on a tradeoff between load carrying capacity and fluid pumping force. In some examples, the load carrying capacity and fluid pumping force are inversely proportional and directly related to geometry of the bearing grooves. For example, given a substantially constant rotational speed of the liquid bearing, deeper grooves may provide a higher pumping force, while the increased clearance between the shaft and sleeve can reduce the load carrying ability of the bearing. Pumping force may be utilized to contain the lubrication fluid and anti-wetting coatings may be applied to sealing surfaces to further assist in containing the lubrication fluid. 
     The lubricating fluid in between bearing surfaces such as the shaft and sleeve are rotating relative to each other. As such, the lubricating fluid is moved in a number of ways, including but not limited to, shearing, wedging, and squeezing, thereby creating pressures to lift and separate the shaft and sleeve from each other. This effect enables the liquid bearing to function and provide low-friction movement between the shaft and sleeve. In other words, shearing of the lubricating fluid imparts energy into the fluid which cases the fluid to pump, wherein the pumping action into the gap between the shaft and sleeve is how the liquid bearing functions. Energy transfer from the surfaces to the fluid enables bearing functionality. In application, in the context of the x-ray tube, wetting between some bearing surfaces and the lubricating fluid allows shearing to impact energy to the fluid. However, anti-wetting between some bearing surfaces and the lubricating fluid allows friction between the bearing surfaces to be reduced, thereby reducing operating temperatures of the bearing assembly. 
     In  FIG. 4  in an exemplary embodiment of the invention a construction of the anode assembly  42  is illustrated. In the assembly  42 , the shaft  76  is formed of a pair of component structures  200  and  202  joined to one another to form the shaft  76 . In this exemplary embodiment, the components or structures  200  and  202  are formed of different materials, such as different grades of stainless steel for the application of forming the bearing shaft  76  for use in a spiral groove bearing of the x-ray tube  40 . The reason for using different materials for components  200  and  202  to form the shaft  76  is due to the differing requirements of the different components  200 , 202  of the shaft  76 . In the exemplary embodiment of  FIG. 9 , the component  200  is welded to the frame  46 , and thus the component  200  is a material with a very low carbon level (≦0.05 wt % C and preferably ≦0.03 wt. %) to prevent the formation of grain boundary carbides that can cause loss of hermeticity. The component  202  is exposed to gallium alloy, i.e., the metal lubricant utilized, and therefore the material forming the component  202  must have a very low reaction rate with liquid gallium alloy. Referring to  FIG. 5-7 , the components  200 , 202  are formed with complementary mating surfaces  204 , 206 . The mating surfaces  204 , 206  can have any suitable profile, but in the exemplary embodiment the surface  204  on component  200  is formed on the inner diameter of the component  200 , while the surface  206  is formed on the outer diameter of the component  202 . While the surface  206  can be placed on either structure  200  or  202 , in an exemplary embodiment the surface  206  is present on the structure  200  or  202  with the lower coefficient of thermal expansion such that the resulting joint  222  will be in compression after formation. Further, the surfaces  204 , 206  are formed to be generally smooth, with a radial component  208  and an axial component  210  joined at an intersection  212 . The surface  204 , and optionally surface  206 , also includes a recess  214  (See e.g.  FIG. 7 ) located at the intersection  212  that expands into the component  200  in order to form a point of mechanical engagement between the component  200  and a joint or weld  222 . In the exemplary embodiment of  FIG. 9 , the joint or weld  222  is formed from an interfacial or intermediate member  216  (See e.g.  FIG. 5 ) positioned between the components  200 , 202 , the component  202  and the target  48 , and the component  200  and the frame  46 . In addition, in the exemplary embodiment of  FIG. 9 , the joint  222  formed between the shaft components  200  and  202  is disposed outside of the portion of the shaft  76  exposed to the gallium alloy. 
     While the surfaces  204 , 206  are generally complementary to one another, they are also shaped to leave a space  218  (See e.g.  FIG. 4 ) between the surfaces  204 , 206  when the components  200 , 202  are in the fully engaged position in order to allow for the interfacial or intermediate member  216  to be positioned within the space  218 . The intermediate member  216  is formed of a material that is capable of forming stable and structurally sound bonds capable of maintaining high vacuum hermeticity between the components  200 , 202 . In certain exemplary embodiments, the material forming the intermediate member  216  is selected from copper, copper-germanium alloy, copper-silicon alloy, copper-gold alloy, or copper-palladium alloy to facilitate bonding between the dissimilar materials of the components  200 , 202 . 
     In one particular exemplary embodiment of the formation of a shaft  76  or sleeve  78  for use in an x-ray tube  40  having a spiral groove bearing (SGB), the base materials used for the components  200 , 202  are refractory metals and alloys thereof, Kovar® (including nickel-cobalt ferrous alloy-based materials), (Kovar® is a registered trademark of Westinghouse Electric and Manufacturing Company, Pittsburgh, Pa.), tool steels, maraging steels (low carbon, ultra-high strength iron alloys known for having superior strength and toughness without losing malleability), iron-nickel (FeNi) alloys, superalloys, or stainless steels. In another particular exemplary embodiment, component  200  is formed of a molybdenum alloy and component  202  is formed of Kovar®. As opposed to the traditional method of joining or welding these materials together by brazing with copper-palladium alloy braze material, which is expensive due to the palladium content required for wetting and also requires a high brazing temperature of 1120° C. relative to the normal joint use temperature, in the exemplary embodiment of the invention a friction/inertia weld can be formed between the components  200 , 202  using a low-cost, low temperature intermediate material for the intermediate member  216 , such as a copper bushing  220 . The bushing  220  can take any desired and suitable shape, an in the exemplary embodiment is formed with a cross-sectional shape and area approximately equal to the shape and area defined by the space  218 , such that the material forming the bushing  220  can entirely fill the space  218  during the friction/inertia welding process to form the joint or weld  222  ( FIG. 8 ). The friction/inertia welding procedure can be done at room temperature and atmospheric pressure due to the near-instantaneous application of frictional heat in the joining process and results in a strong, hermetic seal between the components  200 , 202  and the intermediate member  216  without formation of brittle reaction layers. In addition, the flexibility allowed by the use of friction welding to form the joint or weld  222  enables the tailoring of the shape(s) of the surface(s)  204 , 206  and thus the resulting joint  222  formed between the surfaces  204 , 206  on the components  200 , 202  to increase the hermetic seal path length, leading to a more torturous gas leak path, and/or to design in mechanical interlocking features for increased joint strength in operation, such as the recess  214 , among other advantages. 
     In another exemplary embodiment of the invention, to manufacture the x-ray target  48 , refractory metal components forming the target  48  must be joined to other refractory or high temperature alloys used to form the sleeve  78 . During friction/inertia welding, the use of an intermediate filler material  216  positioned between the surface  204 , 206  of the material forming the target  48  or component  200 , and the surface  204 , 206  of the material forming the sleeve  78  or component  202 , allows joints  222  to be created at a reduced process temperature. In this exemplary embodiment, typical materials for the x-ray target  48  and sleeve  78  include TZM, TZC, MHC, Mo—V, ODS Mo and W. These materials can be joined to one another using traditional braze filler alloys as the intermediate material  216 , including Cu-ABA, Cusil-ABA, Cusin-1-ABA, Gapasil, Incusil-ABA, Palsil, Ag-ABA, Ticuni, and Tini. Additionally, this technique enables joining of refractory components such as those listed previously to low thermal conductivity alloys with thermal conductivity ≦75 W/mK or more preferably ≦50 W/mK, including but not limited to Nb or IN909. By using the intermediate filler material  216 , low thermal conductivity alloys can be used to form the sleeve  78  attached to the target  48 , thereby increasing the restriction of heat flow to the bearing assembly  50  through the sleeve  78 . Reduced heat flow to the bearing assembly  50  consequently increases the tube life, especially where the bearing assembly  50  includes ball bearings, and simplifies the bearing assembly  50  attachment for SGB architectures. Specifically, the application of a Ti filler material  216  to the TZM to TZM conventional friction weld between the target  48  and the sleeve  78  could reduce the joining temperature from 1500° C. to ˜1200° C. which would avoid or minimize the present friction weld heat affected zone (grain growth and voids) created in current friction welding processes. 
     Additional exemplary embodiments of assemblies in the x-ray tube  40  such as the target  48  and the sleeve  78  that can be joined by a joint  222  formed using the intermediate filler material  216  ( FIG. 9 ) include the joining of a molybdenum SGB shaft  76  to Kovar® shaft ends, and the joining of a molybdenum electron collector (not shown) to a copper heat exchanger (not shown), among others. In another exemplary embodiment, a component  202  can be joined at each end to a separate component  200  by a pair of friction/inertia welding joints  222  each formed from an intermediate filler material  216  positioned between the ends of the component  202  and each adjacent component  200  to form a straddle bearing construction. This exemplary straddle bearing construction can also be reversed with a single component  200  and a pair of components  202  joined to each end of the component  202  by a pair of friction/inertia welding joints  222  formed from the intermediate material  216 . In addition, the components  200 , 202  of the various assemblies can be joined by the intermediate filler material  216  in a friction or inertia welding process as disclosed herein, and can subsequently be machined, textured and/or coated as desired, such as to improve the wettability of the components  200 , 202 , as disclosed in U.S. Pat. No. 7,933,382, which is expressly incorporated by reference herein in its entirety. 
     The written description uses examples to disclose the invention, including the best mode, and also to enable any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the invention is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they have structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal language of the claims.