Tungsten anode targets are joined to graphite substrates by a brazing method employing a controlled hydrogen atmosphere and a suitable braze material such as platinum and an alloy of platinum and chromium.
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1. A method for joining an anode target to a graphite substrate of a disc for use in a rotating x-ray anode tube comprising the process steps of
a. preparing a graphite substrate for assembling an anode target to a preselected surface area thereof; b. disposing a layer of a metal of a predetermined thickness on the said preselected surface area of the said substrate, the said layer of metal having a predetermined thickness of at least 1/4 mil and comprising a consisting of at least one metal selected from the group consisting of rhodium, osmium, ruthenium, platinum, palladium and an alloy of platinum and chronmium chromium, wherein chromium is present in amounts up to about 1 percent by weight; c. disposing an anode target material on the said layer of metal, the said anode target material comprising a metal selected from the group consisting of tungsten and a tungsten-rhenium alloy wherein rhenium is present in amounts up to about 25 percent by weight; d. preheating the said substrate, layer of metal and anode target material in a controlled hydrogen atmosphere at a first elevated temperature of from about 600° to about 900°C for a predetermined period of time; e. heating the said substrate, the said layer of metal layer and the said anode target material to a second elevated temperature of from about 1770° to 1830°C for a predetermined period of time up to 5 minutes in a controlled hydrogen atmosphere to braze the anode target material to the said preselected surface area of the said substrate, and f. cooling the said substrate, layer of metal layer and resulting anode target in a controlled hydrogen atmosphere to a low enough temperature to permit exposure to the atmosphere.
5. The method of
gas. 6. The method of claim 3 wherein the metal of the layer of metal layer is an alloy of platinum and chromium. 7. The method of
The method of claim 1 wherein the layer of metal is plated onto the preselected surface area of the graphite substrate by electrodeposition means, and including practicing the additional process step prior to disposing the anode target material thereon of degassing the said plated substrate at a predetermined elevated temperature for a predetermined period of time. 9. The method of claim 8 wherein the plated layer is at least 0.5 mil in thickness. 10. The method of claim 9 wherein the metal of the plated layer is platinum. 11. The method of
. The method of claim 8 wherein a second layer of metal is disposed on the plated surface of the substrate and the anode target material is disposed on the said second layer, the metal of the said second layer is one selected from the group consisting of rhodium, osmium, ruthenium, platinum, and an alloy of platinum and chromium, wherein chromium is present in amounts up to about 1 percent by weight, and palladium. . The method of claim 12 wherein the plated layer of metal and the second metal layer are each at least 0.5 mil in thickness. 14. The method of claim 13 wherein the metal of the plated layer is platinum, and the metal of the second metal layer is platinum. 15. The method of
wherein the controlled atmosphere is hydrogen gas. 16. The method of claim 14 wherein the thickness of the second metal layer is about 1 mil.
17. The method of
hydrogen gas. 18. The method of claim 13 wherein the metal of the plated layer is platinum, and the metal of the second layer is an alloy of platinum and chromium wherein chromium is present in amounts up to about 1 percent by weight. 19. The method of
atmosphere is hydrogen gas. 20. The method of claim 18 wherein the thickness of the second metal layer is about 1 mil. 21. The method of
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Although the brazed regions that develop above and below layer 26 are not shown in FIG. 1, the above solubility criteria assure that, under the proper processing conditions, the metal provided to yield layer 26 as the barrier to carbon diffusion will melt and alloy with (i.e. be brazed to) the metal of layer 24 over the surface thereof contacting layer 24 and will melt and alloy with (i.e. be brazed to) carbon over the surface of graphite substrate 15 wet thereby.
Suitable materials for comprising the metal layer 26 are platinum, palladium, rhodium, osmium and ruthenium. All of these material are non-carbide formers. In addition each of the materials is soluble in tungsten and the tungsten alloy of the target 24 and has a low solubility for carbon. In particular, the solubility for carbon is practically zero at the maximum bulk operating temperature (about 1300°C) of a rotating x-ray anode tube embodying the anode assembly 10. Platinum, palladium, rhodium, osmium and ruthenium all form a simple eutectic system with carbon. For commercial applications, however, platinum and palladium are the only practical materials to be used in the metal layer 26. Rhodium, osmium, and ruthenium, although they each have a higher brazing temperature than platinum and palladium, are too expensive at this time so as to be employed as the principle material in the metal layer 26.
Palladium is suitable for the material of the metal layer 26 as it has a minimum joining or carbon-palladium eutectic temperature of 1504° C., and nearly zero solubility for carbon at temperature less than 1300°C Excellent bonds are achieved between the anode target 24 and the substrate 15. However, the maximum bulk operating temperature of the anode assembly 10 is about 1300°C, allowing only a 200°C margin of safety. Therefore, the reliability of the anode assembly 10 is less than that when platinum comprises the material of the metal layer 26.
The preferred material at this time for comprising the material of the metal layer 26 is platinum. The temperature of joining the anode target 24 to the graphite substrate 15 is about 1800°C The minimum joining temperature, or carbon-platinum eutectic temperature is 1705°C This provides a greater safety margin for the anode tube operation, that is 400°C Below about 1500°C, the platinum metal layer 26 has a zero solubility for carbon. Therefore, the platinum metal layer 26 provides an excellent barrier against carbon diffusion into the anode target 24 at the operating temperature range of about 1000° to about 1300°C
Alloys of platinum may also be used. However, one must not employ large concentrations of elements therein which when alloyed may cause carbide formation at the joining temperature or excessive carbon diffusion in the tube operating temperature range. Although chromium is a carbide former, platinum with up to 1% by weight chromium can be employed as the metal layer 26.
Several methods may be employed to provide the platinum or platinum alloy metal layer 26. One may plate the graphite. Preferably an electroplating process is employed. A thickness of from 1/4 mil to about 1 mil is preferred. Alternately, the platinum may be sputtered onto the graphite. The platinum deposition is followed by heat treating the plated graphite at about 1200°±20°C for a period of about 3 hours in vacuum to degas the plated graphite.
The metal layer 26 may also be provided by employing platinum or a platinum-chromium alloy in a foil form. The thickness of the foil depends solely on the need to assure one of a good bond or joint. The foil has a thickness of at least 1/2 mil. Should the foil thickness be less than 1/2 mil, an incomplete bond may result because of the lack of intimate contact between the anode target 24 and the graphite substrate 15 due to the irregularities on each surface. Preferably the foil has a thickness of 1 mil in order to assure one of having a reliable joint formed by the the metal layer 26.
The anode assembly 10 may be fabricated in several ways. In one instance the anode target 24 is disposed on the plated graphite substrate 15 and joined together at an elevated temperature of about 1800°C In a second instance, a sandwich of a graphite substrate 15, a foil of platinum or a platinum-chromium alloy and the anode target 24 is assembled and joined together at about 1800°C
A preferred method of joining the tungsten or tungsten-rhenium alloy target anode 24 to the graphite substrate 15 includes the assembly, in a sandwich configuration, of a platinum plated graphite substrate 15, a foil member and the target anode 24. The foil member is disposed on the plated surface of the graphite substrate 15. The anode target is then disposed on the foil member. The components of the "sandwich" are held together in a suitable manner so that the surfaces to be joined together are in a close abutting contact relationship with each other.
The assembled components are placed in a controlled atmosphere furnace. The preferred atmosphere is hydrogen. The hydrogen aids the platinum wetting of the surfaces to be joined together. In addition, the hydrogen atmosphere acts as a reducing agent for any oxide present on the surface of the components to be joined together.
The assembled components are initially placed in the coolest portion of a hydrogen tube furnace and preheated for a period of time up to about 30 minutes to acclimatize the component. A minimum of 10 minutes is desired. Upon completion of preheating, the assembled components are moved into a portion of the furnace where the temperature is about 1800° C.±30°C The assembled components are retained in this portion of the furnace for a period of time sufficient to join the components together by brazing by formation of the layer of metal 26. A period of time up to 10 minutes has found to be sufficient, with about 3 minutes being preferred. Upon completion of the brazing step, the assembly, now the disc 12, is moved to a "cool down zone" in the tube furnace where it remains for a sufficient time to cool the components and solidify the melt to form the metal layer 26. A time of approximately 1 hour has been found sufficient to cool the disc sufficiently from a temperature of about 1000° C. in the "cool down zone" for removal from the furnace.
A layer of platinum, 1 mil in thickness, was disposed on a surface of a block of graphite, 1 inch in thickness, by electrodeposition means. The plated substrate was degassed at 1200°C±20°C for a period of 3 hours. A tungsten anode target was prepared and one surface metallographically polished to 600 grit paper. A preform, 1 mil in thickness, was prepared from a foil sheet of platinum.
A sandwich was then assembled. The platinum preform was disposed on the platinum plated surface of the graphite substrate. The anode target was placed on the preform with the polished surface in an abutting contact relationship with the preform. The assembled components were bound tightly together, disposed in a molybdenum boat and placed in the coolest end of a hydrogen tube furnace. The assembled components were allowed to acclimatize for 10 minutes then moved into the hottest portion of the tube furnace. The temperature was measured by an optical pyrometer and found to be 1800°C±30°C The assembled components remained in the hot zone for 3 minutes to braze the components together. The assembled components were then moved to a cooler zone in the furnace, 1000° C.±20°C and allowed to furnace cool from that temperature for 45 minutes before removing them from the furnace.
Upon removal from the furnace the brazed components were examine examined visually. The braze joint appeared sound. The brazed assembly of components was then sectioned and the tungsten-platinum-carbon interface examined. The braze joint was sound throughout. Various sections were then subjected to bending loads until fracture occurred. All fractures occurred either in the tungsten anode target or in the graphite substrate but never in the platinum-tungsten or the platinum-graphite interfaces.
The new disc assembly enables one to employ radiographic techniques which require higher power outputs for either short or long durations without the fear of premature failure during use than what could be employed by the prior art disc assemblies. The capability of being able to withstand higher power outputs enables one to expose patients for a shorter time during x-raying procedures.
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| Executed on | Assignor | Assignee | Conveyance | Frame | Reel | Doc |
| Apr 05 1982 | General Electric Company | (assignment on the face of the patent) | / |
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