Abstract:
A fastening assembly includes a fastener having a head with an underside and an elongated shaft extending therefrom. The fastener constructed of at least one of a refractory metal and a superalloy. A washer includes a body with an upper surface and an opposing lower surface which defines opening portion for receiving the elongated shaft of the fastener therethrough. The upper surface of the washer forms diffusion bonds with the underside of the head of the fastener when the washer and the fastener are held in contact at temperatures in excess of 500° C.

Description:
BACKGROUND OF THE INVENTION 
     High energy x-ray tubes are used in medical device applications to provide an x-ray source. The materials in the x-ray tube are subject to high temperatures during the operation of the x-ray tube. The x-rays generated by the x-ray tube are directed out of a window toward a target such as portion of a patient. The x-ray tube is subject to high temperatures when the x-ray tube is generating x-rays and then cools. A heat shield may be secured to a portion of the x-ray tube to shield the window from backscattered electrons. 
     SUMMARY OF THE INVENTION 
     A fastening assembly includes a fastener having a head with an underside and an elongated shaft extending therefrom. The fastener constructed of at least one of a refractory metal and a superalloy. A washer includes a body with an upper surface and an opposing lower surface which defines opening portion for receiving the elongated shaft of the fastener therethrough. The upper surface of the washer forms diffusion bonds with the underside of the head of the fastener when the washer and the fastener are held in contact at temperatures in excess of 500° C. 
     In another embodiment, an assembled structure suitable for use at high temperatures includes at least two bolts. Each bolt includes a head with an underside and an elongated shaft extending from the underside. Each bolt is constructed of at least one of a refractory metal and a superalloy. A washer includes a body with an upper surface and an opposing lower surface defining at least a first aperture and a second aperture respectively receiving the shaft of at least the first bolt and the shaft of the second bolt. The assembled structure also includes a first high temperature material into which the at least two bolts have been threaded and a second high temperature material which has been secured to the first high temperature material by the at least two bolts. The underside of the head of each bolt has mechanically measurably diffusion bonded to the upper surface of the washer. 
     In yet another embodiment a process for securing a heat shield for the insert window of a high energy X-ray tube to a collector for back scattered electrons includes providing a fastener having a head with a member extending therefrom, the member comprising at least one of a refractory metal and a superalloy. The process also includes providing a washer having a body with an upper surface and an opposing lower surface which defines at least a first opening for receiving the member of the fastener therethrough. The process further includes passing the fastener through the first opening of the washer and securing the member to a threading the fastener into a first member. The process also includes subjecting the assembled structure to a high temperature to cause the fastener to diffusion bond to the washer to a mechanically detectable degree. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic of the internal structure of the insert of a high energy X-ray tube with a rotating anode target. 
         FIG. 2  is a perspective view of a backscatter electron collector with a heat shield bolted to it across the X-ray exit path. 
         FIG. 3  is a front elevation view of a backscatter electron collector with a heat shield bolted to it. 
         FIG. 4  is a cross section of a backscatter electron collector with a heat shield bolted to it. 
         FIG. 5  is a cross section of the portion of a backscatter electron collector where a heat shield has been bolted to it. 
         FIG. 6  is a front elevation of a heat shield in position to be bolted to a collector. 
         FIG. 7  is a photomicrograph of a cross section of the joint between the underside of a bolt head and a washer after diffusion bonding. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring to  FIG. 1 , the internal structure  10  of the insert of a high energy X-ray tube has a rotating anode target  40  mounted on a bearing  30  that is supported by a frame  20 . A cathode  100  in a housing  90  supplies electrons which are accelerated by a high electrical potential and strike a focus area on the target  40  causing the generation of X-rays. These X-rays are directed out of a window  60  constructed of a material translucent to X-rays such as beryllium. However, not all of the accelerated electrons are absorbed by the target  40  and a collector  80  is provided to absorb many of these backscattered electrons. Some of these backscattered electrons follow the path of the exiting X-rays and strike the window  60 . This subjects the window  60  to thermal stresses that can reduce the operating life of the insert. The window  60  is sealed to the envelope of the insert to maintain an effective vacuum inside the insert. However, the heating and cooling of the window  60 , as the X-ray tube is cycled through a duty cycle of generating X-rays and then being off until the next exposure is called for, causes the window to expand and contract. This expansion and contraction is not precisely matched to that of the wall of the envelope for various reasons and the mismatch causes stress upon the seal that over time can cause it to fail. The heating of the window  60  is ameliorated by interposing a heat shield  50  in the path of backscattered electrons to absorb some of them that would otherwise strike the window  60 . The heat shield is constructed of a material, such as graphite, beryllium, or titanium, which can absorb these backscattered electrons without unduly interfering with the transmission of X-rays. This material, for instance graphite, may lack the ability to undergo much elastic compression and, in fact, may be subject to crushing upon tightening of the bolts  70 ,  72 . Heat shield  50  is constructed of a material that is capable of operating in a high temperature environment such as over 500° C. and has high heat conduction. Heat shield  50  absorbs heat and radiates it out. Heat shield  50  is made with graphite or another material that permits x-rays to be transmitted therethrough minimizing effects on image quality as compared to a heat shield made of metals that deflect and/or absorb x-rays compromising image quality. The heat shield is secured in place by bolts  70  and  72  that are threaded in to the collector  80 . The bolts  70  and  72  and the collector  80  are constructed of a material, such as a molybdenum alloy like TZM (A well known and commonly used alloy of titanium, zirconium and molybdenum), or other refractory metals that are used at operating temperatures in excess of about 500° C. and in a temperature region between 500° C. and 1500° C. Other materials for the bolts that have similar operating characteristics to the refractory metals as noted herein are also contemplated. For example, austenitic nickel-chromium based superalloys or other high temperature superalloys are contemplated as well. Superalloys may include certain nickel alloys. Of course other temperature ranges are also contemplated. In one embodiment the operating temperatures will be excess of about 400° C., and in another embodiment the operating temperatures will be in excess of about 300° C. In a further embodiment the operating temperature will be between 300° C. and 1500° C. and in another embodiment the operating temperature will be between 400° C. and 1500° C. In still another embodiment, the operating temperature range will be between 600° C. and 1200° C. In another embodiment the operating temperature range will include the range of 700° C. and 900° C. All of the temperature notations used herein are in degrees Celsius. 
     Referring to  FIG. 2 , the heat shield  50  has been bolted to an appropriate place on the collector  80  with bolts  70  and  72 . The bolts  70  and  72  have been passed through a common washer  110  that has an aperture for the elongated shaft of each bolt. However other types of washer designs are also contemplated, such as a washer that has a region with a first opening and a second opening, where the openings are connected to one another, separated from one another and/or are completely surrounded by the washer body or only partially surrounded by the washer body. The head of each bolt has been snugged against the washer  110  by threading the bolt into the collector  80  such that the underside of the head is in firm contact with the top surface of the washer  110 . The bottom surface of the washer  110  then abuts the top surface of the heat shield  50 . The bottom surface of the heat shield  50  in turn abuts the collector  80 . The washer  110  is constructed of a material, such as nickel, cobalt or iron or alloys thereof, that provide diffusion bonds to the underside of the heads of the bolts  70  and  72  under appropriate conditions of time and temperature such as temperatures in excess of about 500° C. and times in excess of thirty minutes. Of course other temperature and time combinations that provide diffusion bonds are contemplated. In one embodiment the washer material is different than the bolt material. However, the washer material may be the same as the bolt material if diffusion bonds are created as described herein. In an alternative embodiment an intermediate material may be placed between the washer and the bolt to assist in the creation of a diffusion bond. In yet another embodiment, a material may be provided on the threaded portion of the bolts and/or within the threaded region of the collector to provide a diffusion bond between the threaded region of the bolts and the threaded region of the collector. In one embodiment nickel is the intermediate material applied to the threads of the bolts and/or the collector. Other materials that would provide for diffusion bonds between the bolts and collector are contemplated in this alternative embodiment. The inclusion of diffusion bonds between the bolt threads and the threads of the collector provide for additional torque retention between the bolts and collector and make removing the bolts from the collector more difficult if there is a need to repair the x-ray tube structure that requires removal of the bolts. In another embodiment described herein no diffusion bonds are created between the threaded portion of the bolt and the threaded portion of the collector to make removal of the bolts from the collector easier. 
     Referring to  FIG. 3 , the heat shield  50  is bolted to the collector  80  by bolts  70  and  72  whose elongated shafts pass through apertures in washer  110  that has an area of weakness  112  in the region between the two apertures. When the undersides of the heads of the two bolts  70  and  72  have become bonded to the upper surface of the washer  110 , this allows one of the bolts to be retracted by fracturing the washer  110  through this area of weakness  112 . The area of weakness  110  is shown as the mere elimination of some of the web of the washer  110  but other means of enhancing frangibility such as scoring could also be used. In the absence of this area of weakness  112 , the retraction of either bolt  70  or  72  is only possible if that bolt&#39;s diffusion bond with the washer  110  is broken. It is mechanically not possible to rotate a single bolt so as to retract it so long as both bolts are attached to the washer  110  and washer is unfractured. The washer is not free to rotate with the bolt being retracted because the other bolt has been passed through it and is still engaged in the threads of the collector  80 . 
     Referring to  FIG. 4 , the heat shield  50  is held in position across the channel  120  through which X-rays pass after being generated by the collision of accelerated electrons with the rotating anode target  40  shown in  FIG. 1 . It is held in position by bolt  70  that is threaded into the collector  80 . Neither the washer  110  nor the other bolt  72  is shown in this view. 
     Referring to  FIG. 5 , the elongated shaft  73  of the bolt  70  passes through an aperture in the washer  110  and through the heat shield  50 . The threads  77  of the bolt  70  engage the threads  82  of the collector  80 . The bolt  70  is tightened by threading its threads  77  into the threads  82  of the collector  80  until the underside  75  of its head  71  come in contact with the top surface  111  of the washer  110 . If the heat shield  50  is constructed of a material, like graphite, that does not undergo much elastic compression, it is difficult to secure the bolt  70  against loosening by tightening it so far as to cause elastic compression on the washer. Heat shield  50  may be formed of other materials that provide similar x-ray transmission, electron absorption and high surface temperature to graphite. In other environments washers are adjacent to rigid materials that resist compression and so tightening of the bolt causes elastic compression forces that press against the head of the bolt and providing resistance to its retraction. The bolts  70  and  72  are subject to being loosened by vibration and thermal stresses such as not seeing precisely the same temperature profile as the portion of the collector into which they are threaded and not experiencing the same expansion and contraction as the heat shield  50  through which they are passed. However, at the service temperatures typically seen by the collector  80  and the bolts  70  and  72 , the typical materials of construction, such as molybdenum or molybdenum alloys, do not appreciably diffusion bond across their respective threads  82  and  77 . To address this situation special steps are taken so that the undersides of the heads of theses bolts, such as the underside  75  of the head  71  of bolt  70 , become diffusion bonded to the upper surface  111  of the washer  110 . In particular, the material of the washer  110  is selected so that it will readily diffusion bond to the underside of the bolt head and temperatures are used in manufacture to cause such diffusion bonding. As is readily apparent from  FIG. 2  and  FIG. 3 , this means that these bolts can only be loosened by fracture of the washer  110  through its region of weakness  112  or fracture of the diffusion bonds to one of the bolts  70  and  72 . In one embodiment the region of weakness  112  will break before the diffusion bonds break between the bolts and washer. 
     Referring to  FIG. 6 , the washer  110  has a region of weakness  112  between its two apertures  113  for bolts that secure the heat shield  50  to the collector  80 . A substantial amount of the material of the washer has been removed to facilitate its fracture upon the application of a reverse torque to a bolt which has diffusion bonded to the washer  110 . 
     Referring to  FIG. 7 , diffusion bonding has occurred between the top surface  111  of the washer  110  and the underside  75  of the head  71  of a bolt after they were placed adjacent to each other and subjected to 1100° C. for 30 minutes. 
     Diffusion bonding of the bolts to the washer may be done as an independent operation or as a part of the manufacturing of the overall structure. In either case the bolts are passed through apertures in the same washer and threaded into a first high temperature material in such a way as to secure a second high temperature material to the first. One embodiment is when bolts are threaded into the collector of an X-ray tube constructed of a first high temperature material to secure a heat shield constructed of a second high temperature material to the collector. Typically the bolts pass through the second high temperature material after first passing through the washer and before being threaded into the first high temperature material. The bolts are threaded into the first high temperature material until the undersides of their heads contact the top surface of the washers. Then the bolts and washer are subjected to an elevated temperature for a sufficient time to cause mechanically detectable diffusion bonding between the bolts and the washer, i.e. diffusion bonding which can sustain a measurable mechanical load. The mechanically detectable diffusion can be detected and measured with a torque wrench. A typical bonding process for a nickel washer and molybdenum alloy bolts is about 1100° C. for about 30 minutes. This bonding process may be done as part of the procedure for the manufacture of an X-ray tube. The insert portion of an X-ray tube is built up. As part of this build up a collector is installed which has tapped holes. A heat shield is attached using bolts which pass through a common washer which has a separate through aperture for each bolt. The bolts are passed through the heat shield and into the tapped holes in the collector. The bolts are then drawn at least snug-tight against the washer. The vacuum envelope which is typically the outer boundary of the insert is then completed. Processing of the insert results in the collector being exposed to elevated temperatures, resulting in the diffusion bonding between the washer and bolt. The diffusion bonding provides a structure that is suitable for use in a vacuum environment and will not be the source of outgassing which would contaminate the vacuum. The diffusion bonding provides both adequate torque retention to ensure the bolts remain secured within the structure at the high temperature environment as well as ensuring that the means used to provide a secondary torque retention over the use of the threads do not contaminate the vacuum environment when the structure is at temperatures at or above 500° C. or as otherwise noted above herein. The diffusion bonding of the bolts and washer as described herein does not introduce chemicals into the vacuum environment at operating temperature above 500° C. or as otherwise noted above herein. 
     The diffusion bonding may be weak enough to allow the removal of the bolts by fracture of these bonds but alternatively the washer may be provided with a region of weakness between adjacent apertures that allows the removal of a bolt by applying a reverse torque to the bolt head which causes a fracture of the of the washer through this region. This alternative provides fairly reproducible control of the force necessary to remove a bolt, particularly if the weakness is provided by eliminating some of the material of the washer. 
     It may be of value to be able to readily remove bolts that have diffusion bonded to a washer. This facilitates to ability to do rework in the construction of an X-ray tube. If needed rework were to require the removal of the heat shield from the collector the ability to remove its securing bolts by fracturing the retaining washer through its area of weakness would be of value. It would allow one to remove the heat shield with less chance of damage. The bolts as described herein at the operating temperature retain their structural integrity in such a manner that the shape of the bolts are not compromised to the extent that the torque between the bolts and collector is not completely degraded. The bolt material provides for resistance to creep at temperatures over 500° C.; good surface stability; and/or corrosion and oxidation resistance. The materials of the bolts being selected to maintain sufficient pretension of the bolt and collector of a predetermined value. 
     Although the present disclosure has been described with reference to example embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the claimed subject matter. For example, although different example embodiments may have been described as including one or more features providing one or more benefits, it is contemplated that the described features may be interchanged with one another or alternatively be combined with one another in the described example embodiments or in other alternative embodiments. The term metal or metals as used herein with contemplates and includes alloys of the same metal. In another embodiment washer may include a member extending therefrom that is prohibits rotation of the washer by engaging a feature of a surface of the member that the washer is adjacent to. In this manner, the washer may only be removed by fracturing the washer body and or completely removing the bolt or other fastener from the opening of the washer. Because the technology of the present disclosure is relatively complex, not all changes in the technology are foreseeable. The present disclosure described with reference to the example embodiments and set forth in the following claims is manifestly intended to be as broad as possible. For example, unless specifically otherwise noted, the claims reciting a single particular element also encompass a plurality of such particular elements.