US2116387A - Means and method of constructing x-ray anodes - Google Patents

Description

May 3, 1938. F. H. DRIGGS ET A1. 2,116,387 l MEANS AND METHOD OF CONSTRUCTING X-RAY ANODES Filed OO. 30, 1954 2 Sheets-Sheet l EH. .DIP/665" ATToRN May 3, 1938.

F. H. DRIGGS ET'AL MEANS AND METHOD OF CONSTRUCTING X-RAY ANODES Filed Oct. 30, 1954 2 Sheets-Sheet 2 mill" .Immrh- SW J Vl 3% O/H R MPM. o Z EDH. j mA Patented May 3, 1938 UNITED STATES PATENT OFFiCE MEANS AND METHOD F CONSTRUCTING X-RAY ANODES Application October 30, 1934, Serial No. 750,638

7 Claims.

Our present invention relates to an improved article of manufacture and in its more specic aspects is directed to the manufacture of anode electrodes for X-ray tubes and the method of producing the same.

Devices of this type usually consist of an evacuated vitreous envelope provided with an anode and a. thermionic cathode heated to incandescence disposed therein. Upon the application of a high electrical potential from a suitable source of electrical energy electrons emanate from the cathode which impinge upon the anode and produce X-rays. This bombardment of the anode is attendant with the generation of appreciable concentrated heat and in order to prevent destruction of the anode it is customary to provide the same with a target of a refractory metal. such as tungsten. While such target will withstand considerable heat without fusion it is essential that in order to preclude the temperature from rising to the fusion point that the heat in some way be dissipated. In the usual construction the target is secured to a copper anode stem having the requisite thermal and electrical character- 25 istics. Due to the difference in thermal coeiiicients of expansion of tungsten and copper it is not an infrequent occurrence for the copper adjacent the target to crack and thus fail to conduct concentrated heat away from the target 30 with sufficient rapidity. 'I'his accordingly causes the copper adhering to the target to fuse and run down into contact with the glass envelope thus puncturing the same or loosens the target to also cause destruction of the tube.

Heretofore in the art the bond between the copper anode and target has usually been formed by casting the copper around the target. The method of cooling the casting to prevent unsoundness and porosity results in a coarse grain structure of the copper adjacent the target which may to a slight extent be made finer by stirring during casting.

However, it is known that fine grained metals are mechanically superior to coarse grained yet stirring during molding does not produce a readily ne grained structure to prevent cracking for almost invariably flat grain boundaries are produced nearly parallel to the under face of the target which provide an easy path for propagation of cracks.

Moreover, in the case of cast anodes incipient cracks are frequently produced on the face of the anode during the vacuum baking operation for seasoning purposes which indicates where serious cracking will later occur. Since copper (Cl. Z50-35) cracking occurs at grain boundaries, a iine grained copper would have much greater total grain boundary area, which results not only in a reduction in the amount of the stress at any given boundary, but also in decreased concentration at the boundaries of such impurities in the copper as might be insoluble at temperatures within the working range of X-ray tube anodes.

It is accordingly an object of our invention to provide an article of manufacture wherein one metal of a high melting point is so bonded to another of high thermal conductivity as not readily to become separated therefrom when subjected to considerable heat.

Another object of our present invention is the provision of an anode for an X-ray tube wherein an exceptionally ne grained structure is produced in the anode adjacent the face of the refractory metal target.

Another object of our present invention is the provision of an anode for an X-ray tube having a copper stem to which is welded a target of a refractory metal.

Another object of our present invention is the provision of an anode for an X-ray tube wherein a target of a refractory metal is Welded to an anode of a metal of radically different thermal coeicients of expansion with the production of a iine grained structure in the anode adjacent the target face which so distributes stresses caused by unequal thermal expansion as to substantially obviate possibility of destruction thereof at the temperatures within the working range of the X-ray tube.

A further object of our present invention is the provision of an anode for X-ray tubes which may be more economically and facilely manufactured.

Still further objects of our present invention will become obvious to those skilled in the art by reference to the following description taken in conjunction with the accompanying drawings wherein:

Figure 1 illustrates a plan view of an anode for an X-ray tube constructed in accordance with our present invention,

Fig. 2 illustrates a side fragmentary View of the anode shown in Fig. 1 and depicts the grain structure as produced by our present method as may be microscopically observed from a section taken of our anode.

Fig. 3 discloses a fragmentary view partly in section of an apparatus employed in one step of the production of our present invention,

Fig. 4 illustrates the same apparatus as shown in Fig. 3 and depicts the completion of one of the steps employed in the production of our novel anode structure,

Fig. 5 discloses still additional apparatus utilized in the production of an anode in accordance with our present invention, and

Fig. 6 illustrates a modification which the apparatus shown in Fig. 5 may take for the manufacture of anodes in accordance with our present invention,

Fig. '7 illustrates a side fragmentary view of a cast anode and depicts the coarse grained structure resulting,

Referring now to the drawings in detail we have shown in Fig. l an anode for an X-ray tube constructed in accordance with our present invention comprising an anode stem 5 of high thermal and electric conductivity characteristics. The target end of the stem has an angular face cut at the desired angle, which may vary from about 100 to 75 to the longitudinal axis of the stem. A target 6 of a refractory metal, preferably tungsten, is affixed to the angular face of the anode stem. in accordance with our present invention by welding the same thereto.

In order to produce a ne grained structure in the anode stem, adjacent the refractory metal target as shown in Fig. 2, so that the bond therebetween will be unaffected by the heat generated during operation of the X-ray tube, despite the radically different thermal coeiiicients of expansion, we nd it desirable to continue the welding operation over an appreciable period of time depending upon the temperature employed rather than an instantaneous weld.

Moreover, we found it desirable as an initial step in our method of producing X-ray anodes to rst coat the target 6 with a layer of copper. Even though the anode stems be machined from cast copper or baked in a hydrogen furnace in an endeavor to eliminate occluded oxygen appreciable oxidation results which destroys the bond.

To prevent resulting oxidation we accordingly subject the target t to cleaning by an acid pickle or baking in hydrogen, after which it may be placed in a refractory boat 1, as shown in Figs. 3 and 4, where it is covered with copper shavings 8. This is then heated to about 1350 C. for approximately 30 minutes in an atmosphere of hydrogen and a reasonably uniform coating of copper 8 results, as shown more clearly in Fig. 4, which is not so thick as to seriously entrap hydrogen upon freezing.

While any particular form of apparatus may be utilized in the manufacture of anodes in accordance with our present invention we found an apparatus such as a treating bottle shown in Fig. 5 to be quite satisfactory. To provide for expansion during heating, contraction at welding, and adequate pressure to force the target 6 into the copper stem 5, the latter may be supported upon an iron post Il) which iioats in a mercury well I2 with a steel spring (not shown) disposed under the post for additional pressure. A graphite sleeve or cylinder i3 is arranged to closely surround the anode stem adjacent the angular face thereof which has suicient clearance to allow for expansion of the anode stem upon heating yet prevents any tendency for molten copper to run out between the sleeve and anode stem.

For the purpose of holding the copper clad target 5 at the desired angle it may be fastened by any suitable means, such for example by the friction of a number of molybdenum wires or pins Ill extending not quite to the face of the target and set into holes in a graphite form I5, with this latter having a surface which is complementary to the angular face of the anode stem 5.

The graphite form I5 carrying the copper clad target 6, also slidably engages the sleeve I3 and is held in position by an iron disc or block I6 and copper rod II, which form a continuous rod bearing against a stationary copper block I8 serving as an electrical terminal and base member. In order to reduce end cooling effects and maintain a concentration of heat around the target a pair of tungsten plates I 9 and 20 are provided in addition to the inferior heat conducting iron post I0 and iron block I6.

The copper block I8 is accordingly connected to a source of electrical energy as is the mercury well I2 and a relatively high current ranging between 1000 and 2000 amperes is passed through the assembly shown in Fig. 5. During the passage of this current the bottle is subjected to an atmosphere of forming gas preferably an admixture of approximately nitrogen and 10% hydrogen. This admixture we found more desirable than hydrogen alone because if the latter is used, although a good weld is formed, traces of hydrogen blow holes are present and in the case of the sole use of nitrogen oxidation of the target was observed which destroyed the bond. Accordingly by the use of the forming gas at approximately the proportions above indicated evidence of oxidation and hydrogen blow holes both are eliminated resulting in a perfect bond.

The high current passing through the assembly, which is carried not only by the graphite form I5 but also by the graphite sleeve I3, accordingly heats the anode stem 5 as well as the target 6. During this heating the pressure exerted by the mercury well I2 together with the spring (not shown) which is conveyed to the iron post, presses the target 6 into the anode stem 5 as the latter becomes molten.

When the target has been suiiiciently welded to the anode stem the current is interrupted after the expiration of the appropriate period of time. As it is difficult to observe the actual welding process it is necessary that the operator have cognizance of its progress. This may be readily accomplished by the utilization of proper instruments for recording the current, voltage and time period but for the sake of simplicity we have not shown the same. Moreover, a pair of contacts, such as diagrammatically shown in Figs. 5 and 6, may be positioned adjacent the mercury well and connected in series with the source 1n such a manner that after the expiration of the desired period of time, and after complete depression of the target into the anode stem, these contacts are operated to interrupt the current from the source of supply.

In Fig. 6 we have shown a modification of the apparatus shown in Fig. 5 which differs from the latter only in that a graphite form 22 is disposed at the bottom of the assembly and the pressure exerted by the mercury well I2 and spring is applied thereto. The anode stem 5 is slidably disposed within an opening provided in the graphite form 22 so that the target f5 is disposed normal to the vertical axis of the assembly rendering it unnecessary to provide the sleeve I3 and the graphite form I5 with a surface complementary to the angular face of the anode stem 5. We also found that the temperature gradient of the target is increased by the provision of an additional graphite disc or plate 23 next to the target 5. The relatively high resistance of this disc 23 causes it to serve as a source of high temperature which is communicated to the target.

I'his arrangement of the assembly shown in Fig. 6 has the advantage of a considerably higher temperature gradient in the copper anode 5, which is contributed to a measure by the additional graphite disc 23, than may be obtained with the assembly shown in Fig. 5, as a result of which the copper melted during the welding has a tendency to solidify more quickly.

By the utilization of either modifications of the apparatus shown in Figs. and 6 a fine grained structure of the copper is produced adjacent the tungsten target, as is shown diagrammatically and on an enlarged scale in Fig. 2. Because of the way in which solidification of the molten copper takes place as a result of welding there is a marked tendency for the Welded anodes to exhibit a columnar structure, with the long axis of the grains nearly normal to the target, which resists the propagation of cracks under the target in contradistinction to cast anodes wherein the grain structure is coarse and cracks in grain boundaries may be observed parallel to the underside of the target in the manner shown in Fig. '7. T'he fine grained structure enables a perfect bond to be established between the target and the anode stem together with a more uniform distribution of the stresses rendering the anode capable of withstanding temperature changes without destruction thereof, despite radically different thermal coefficients of expansion therebetween. This results not only in obtaining longer X-ray tube life, but also considerable economy in manufacture.

It thus becomes obvious to those skilled in the art that we have provided an X-ray tubeanode wherein an refractory metal target, preferably formed of tungsten, is welded to a copper anode stem having good electrical and thermal characteristics.

Despite the difference in thermal coefficients of expansion between the two metals the stresses are so distributed that cracks are substantially eliminated and the continuity of the copper is maintained which rapidly conducts the heat away from the target thus enabling operation of the tube over a lo-nger period of time than is possible with cast anodes. Furthermore, by resorting to our novel method of manufacture a fine grained structure of the copper anode stem adjacent the target is produced enhancing the mechanical strength of the copper adjacent the target.

Although we have shown several embodiments of our present invention we do not desire to be limited thereto as various other modifications of the same may be made without departing from the spirit and scope of the appended claims.

What is claimed:

1. 'I'he method of making an electrode having metallic constituents, one of which has a high melting point and the other of which has high thermal conductivity, with the latter constituent having a fine grain structure in the end adjacent the adhering metal of high melting point with the major axes thereof substantially normal to the latter metal to substantially eliminate the tendency of the metal of high heat conductivity to cracks parallel to the interface of the metals which consists in disposing the metal of high melting point against the fiat end surface of the metal of high thermal conductivity, locally heating the metal of high thermal conductivity at the interface of the metals by the passage of an electrical current therethrough to soften the metal of high thermal conductivity for a depth corresponding to the thickness of the metal of high melting point, and pressing the metal having the high melting point while in a solid state into the softened flat adjacent surface of the metal of high thermal conductivity to form an eflicient mechanical bond therebetween.

2. The method of making an anode stem for an X-ray tube having metallic constituents, one of which has a high melting point and the other cf which has high thermal conductivity, with the latter constituent having a ne grain structure in the end adjacent the adhering metal of high melting point with the major axes thereof substantially normal to the latter metal to substantially eliminate the tendency of the metal of high heat conductivity to cracking parallel to the interface of the metals, which consists in precoating surfaces of the metal having a high melting point with a thin layer of metal of the same composition as the metal of high heat conductivity, disposing the preclad metal of high melting point against the flat end surface of the metal of high thermal conductivity, locally heating the metal of high thermal conductivity at the interface of the metals by the passage of an electrical current therethrough to soften the metal of high thermal conductivity for a depth corresponding to the thickness of the metal of high melting point, and pressing the metal having the high melting point while in a solid state into the softened fiat adjacent surface of the metal of high thermal conductivity to form a welded bond therebetween.

3. The method of making an electrode having metallic constituents, one of which has a high melting point and the other of which has high thermal conductivity, with the latter constituent having a fine grain structure in the end adjacent the adhering metal of high melting point with the major axes thereof substantially normal to the latter metal to substantially eliminate the tendency of the metal of high heat conductivity to cracking parallel to the interface of the metals, which consists in surrounding one end of the metal having the high thermal conductivity While in solid form with a refractory metal for concentrating heat at the end thereof, maintaining the metal having a high melting point in Contact with an adjacent flat end surface of the first mentioned metal, passing a relatively large current through both of the metals for a period of time sufficient to locally heat the metal of high thermal conductivity at the interface of the metals to soften the metal of high thermal conductivity for a depth corresponding to the thickness of the metal of high melting point, and simultaneously applying pressure to the constituents to gradually force the metal having a high melting point into the adjacent ila-t end surface of the other for the purpose of producing a ne tortuous grain structure in the metal of high heat conductivity to form a good tenacious bond between the constituents upon cooling of the electrodes.

4. A method cf making an electrode for an X- ray tube having metallic constituents, one of which has a high melting point and the other of which has high thermal conductivity, with the latter constituent having fine grain structure in the end adjacent the adhering metal of high melting point with the major axes thereof substantially normal to the latter metal to substantially eliminate the tendency of the metal of high heat conductivity to cracking parallel to the interface of the metals, which consists in surrounding one end of the metal having high thermal conductiv ity While in a solid form With a refractory metal for concentrating heat at the end thereof, maintaining the metal of high melting point in contact With the flat end surface of the metal of high thermal conductivity, passing a relatively large current through both constituents While subjecting the same to an atmosphere of forming gas to locally heat the metal of high thermal conductivity at the interface of the metals to soften the metal of high thermal conductivity for a depth corresponding to the thickness of the metal of high melting point, and simultaneously applying pressure to the constituents to gradually force the metal having a high melting point into the adjacent softened flat surface of the other for the purpose of forming a good Welded bond between the constituents upon cooling the electrode.

5. The method of making an electrode for an X-ray tube having metallic constituents, one of Which has a high melting point and the other of Which has a high thermal conductivity, with the latter constituent having a ne grain structure in the end adjacent the adhering metal of high melting point With the major axes thereof substantially normal to the latter metal to substantially eliminate the tendency7 of the metal of high heat conductivity to cracking parallel to the interface of the metals, Which consists in surrounding one end of the metal having the high thermal conductivity while in solid form With a refractory material for concentrating heat at one end thereof, maintaining the metal having a high melting point in contact with the adjacent flat end surface of the metal of high thermal conductivity, passing a relatively large current through both constituents while subjecting the same to an atmosphere of forming gas comprising approximately nitrogen to approximately 10% hydrogen for a period of time sucient to heat the end of the metal having high heat conductivity to a molten state for an appreciable depth, and simultaneously applying pressure to the constituents to gradually force the metal of high melting point into the adjacent flat surface oi the other as it reaches a molten state, for the purpose of forming a good Welded bond between the constituents upon cooling of the electrode.

6. An electrode for an X-ray tube comprising an anode stem of good thermal and electrical characteristics, and a target of a refractory metal Welded thereto, the grain structure of said anode stem adjacent said target being of a ine tortuous columnar nature with the long axes of the grains of metal being substantially normal to the target, to obviate possible incipient cracks extending parallel With the target tending to destroy the continuity of said anode stem in the vicinity of said target and reduce the rapidity of thermal conductivity of said anode with a resultant destruction of the bond between said target and said anode stem.

7. An electrode for an X-ray tube comprising an anode stem of good thermal and electrical characteristics, and a target provided with a coating of metal of the same composition as said anode stem Welded to the latter and pressed thereinto while the end of said anode stem is in a molten state for a depth corresponding to the thickness of the target, to thereby form a ne grain structure in said anode stern adjacent said target with the major axes of the ne grain structure eXtending in a tortuous path substantially normal to the target surface and to cause a substantially uniform distribution of the stresses caused by unequal thermal expansion to prevent cracking of said anode parallel to said target Within the range of operating temperatures of said X-ray tube.

FRANK H. DRIGGS. HARRY WALTER I-IIGHRITER.

Images