EP0874385A1 - Method of manufacturing an anode for x-ray tubes - Google Patents

Abstract

The anode manufacture has a base body provided with a coating for emission of different X-ray radiations via an inductive plasma spray coating process. The coating may be built up from a number of thin coating layers with an overall thickness of between 0.4 and 0.6 mm. A recess may be formed in the surface of the base body with a depth corresponding to the coating thickness, before application of the coating in the base of the recess, so that the surface of the applied coating is flush with the surface of the remainder of the base body.

Description

The invention relates to a method for producing an anode for X-ray tubes consisting of a base body and one of these different X-ray emitting coating.

Such materials are used to generate X-rays, when subjected to a focused electron beam Emit x-rays. The refractory metals tungsten and Molybdenum and its alloys are, for example, such materials that depending on the desired type of X-ray radiation.

In medical diagnostics, rotating anodes are often used for X-ray tubes in the form of axially symmetrical blanks for the production of X-rays used. In most cases, only a part of the Surface in the form of an annular path, the so-called focal path, in area directly affected by the electron beam from the X-ray generating material as a comparatively thin coating executed while the main body of the rotating anode made of others high-melting materials.

His are responsible for the behavior of the burning track covering in tube operation specific material properties such as structure, heat conduction, Thermal expansion, mechanical properties and the density of the covering authoritative. Because there is residual porosity the heat conduction Fatigue crack resistance and the outgassing behavior in the X-ray tube Adversely affected are the highest possible density values for the focal track covering to strive for. A reduced fatigue crack resistance manifests itself primarily in a roughening of the Focal path and an associated reduced x-ray dose yield.

To date, the focal track covering is mainly made using powder metallurgy Process made by pressing, sintering and forging. With metallic He is preferably materials for the base body in one operation produced with the base body by layering the powder mixtures, with which Density values from 96% to 98% of theoretical density achieved as standard will. Such a manufacturing process for the focal track covering is inexpensive, but leads to properties, particularly with regard to his fatigue crack behavior are not yet optimal.

Especially where graphite is used as the material for the base body and the connection of the main body with one independently is produced by powder metallurgy focal track covering is difficult Brennbahnbelag also by deposition using known Coating process preferably by chemical vapor deposition or also applied by physical vapor deposition. With these Although processes are based on densities of almost 100% of the theoretical density reached the focal track surface due to the much higher However, these manufacturing methods are essentially production costs remained limited to the production of rotating anodes with graphite body and were able to use the powder metallurgical manufacturing process of Do not remove the focal track covering.

As a somewhat cheaper coating process with a number of The advantages of process technology are conventional plasma spraying out. This is especially true if the process is carried out in a controlled atmosphere, i.e. is used under negative pressure or in a protective gas atmosphere. With conventional plasma spraying, the material for the Burning track coating as a powder in a DC arc discharge Plasma jet introduced radially, melted in the plasma jet and the melted droplets deposited on the base body. Height Order performance per unit of time, adjustable over a wide range Coating temperature, as well as avoiding problematic Disposing chemical compounds are important advantages, for example this procedure. With the conventional plasma spraying process however, despite intensive global development efforts in recent years Years only focal track coverings with a maximum density of 93% of the theoretical density can be achieved, leading to unsatisfactory results with regard to the fatigue crack and outgassing behavior when used in this way coated rotating anode. The well-known thermal Aftertreatment process, which theoretically increases the In practice, density can only be achieved to a limited extent or also due to the effects on the material of the base body or on the Compound behavior can only be used to a limited extent. This applies particularly to Use of graphite as the material for the base body so that it is under these restricted conditions are not sufficient Post-compression and complete degassing of the burning track surface comes. Because of these disadvantages are rotating anodes, in which the focal track coating has been applied with plasma spraying, so far not to a large extent Commitment came.

In recent years, a new variant of plasma spraying, the so-called inductive vacuum plasma spraying, has been developed. The difference between this special plasma spraying process and the conventional plasma spraying process is that the plasma is generated by inductive heating, which means that the spray powder can be introduced axially in a simple manner even before the plasma jet is formed. As a result and due to the lower expansion speed of the plasma as a result of inductive heating, the powder particles remain in the plasma jet for considerably longer. This improves the energy transfer from the plasma to the individual particles of the wettable powder, so that even larger powder particles are completely heated above their melting temperature and can be separated as fully melted droplets. This makes the inductive vacuum plasma spraying process, in comparison to the conventional plasma spraying process, suitable for the use of cheaper spray powder with a wider particle size distribution.
An application of inductive plasma spraying for the production of X-ray-emitting coatings for anodes for X-ray tubes has so far not occurred due to the experience of conventional plasma spraying with regard to the unsatisfactory results of the fatigue behavior.

The object of the invention is therefore to provide a method for producing anodes for X-ray tubes, by means of which the cost-effective production of the X-ray-emitting coating is made possible, the coating with regard to its usage behavior, in particular its fatigue crack resistance, at least fully corresponds to the standard which is still common today or even exceeds it.
According to the invention this is achieved in that the X-ray emitting coating is applied by inductive plasma spraying.

Coverings produced in this way show a fatigue crack resistance and with it a roughening behavior, that of rubbers that with conventional plasma spraying processes have been produced and even that of coatings produced by powder metallurgy significantly exceeded. This result is completely surprising for the expert to this extent, especially because the Density values of the coverings produced according to the invention at best to the Reach the density values of powder metallurgically produced coverings in the Usually, however, are even lower. The causes of this unexpected Fatigue crack resistance are also enhanced from the plethora of material science findings on rotary anodes produced according to the invention in different states (immediately after coating, after Heat treatments, after tube tests) cannot be clearly explained. A possible explanation could be that the inductive Vacuum plasma spraying, especially using the Process parameters for the deposition of for the focal track covering suitable refractory metals are necessary, special crystalline Structures can be achieved that differ significantly from the lamellar Differentiate solidification structures, usually with conventional ones Plasma spraying process, especially with the intensively examined Vacuum plasma spraying can be achieved.

It is advantageous if the coating emitting X-rays passes through repeated overlaying of individual spray layers with a total thickness between 0.4 mm and 0.7 mm is applied. As a rule, this is a 20 - 50 overlaying of individual layers of the spray layer is recommended be.

A particularly favorable variant of the method according to the invention is reached when emitting X-rays before application Covering in the area of this covering in the base body of the anode Recess of a little more than the depth of the desired covering thickness is incorporated. In this way, the surface of the covering can simple smoothing to a level with the adjacent surface of the Anode body are brought.

Furthermore, it is advantageous for the method according to the invention if the Separation under an inductively coupled power between 50 kW and 100 kW and under a delivery rate of the wettable powder between 10 g / min and 50 g / min takes place. Under these conditions there is a total Melting and sufficient overheating of the melt droplets.

Furthermore, one has compared to usually for the conventional Plasma spraying parameters are comparatively low Relative speed at which the plasma jet is to be coated Surface covered, proven to be advantageous. This will be beneficial chosen that in the area of the point of impact of the core zone of the plasma Set maximum temperatures from 1,400 ° C to 2,400 ° C. The too Coating base body itself is advantageously on a Pre-temperature heated from 1,000 ° C to 1,500 ° C.

In a further advantageous variant of the method according to the invention the plasma jet and the base body are used to produce rotating anodes the rotating anode moves against each other in such a way that the central point of impact of the plasma particle stream on the anode surface and the Rotating anode axis concentric center line of the active Focal path area coincide at least approximately, the Particle flow of the plasma jet is set so that the within the particle stream of the plasma jet that only hits the active focal path region includes that area which is within the half-width of the Gaussian Particle distribution of the generated complete plasma jet lies. Thereby it is achieved that the edge region of the layer, which is less favorable for the layer construction Plasma rays largely shifted to areas of the anode surface that are outside the active focal path area. Under active The focal path area is that for the generation of X-rays directly from Understand the electron beam area.

Furthermore, it can be advantageous if the rotating anode is finally one Is subjected to annealing treatment. The purpose of this annealing is both one further improvement of the structural properties through diffusion processes as also degassing the anode. The type of annealing treatment is among others depending on the material from which the base body of the rotating anode is made has been. In the case of a base body made of high-melting metal, the Annealing treatment at temperatures between 1,200 ° C and 1,600 ° C during 1 to 20 hours while rotating anodes where graphite is used for the body was used, usually at temperatures up to 1,300 ° C is carried out for up to about 10 hours. in case of an Graphite base body is the possible formation of disadvantageous carbides in the border area by known diffusion barrier layers, e.g. Rhenium, advantageously delayed.

The method according to the invention can be used in a particularly advantageous manner be applied when the base body of the anode is made of graphite, molybdenum or a molybdenum alloy and the X-ray emitting coating consists of a tungsten-rhenium alloy.

The method according to the invention is described below with reference to Manufacturing examples and explained in more detail with reference to figures.

Figur 1

die Prinzipskizze einer Variante des erfindungsgemäßen Verfahrens nach den Ansprüchen 3 und 6

Figur 2

die graphische Darstellung der Mittelwerte der Brennbahnaufrauhung Ra von erfindungsgemäß und pulvermetallurgisch hergestellten Brennbahnbelägen von Drehanoden

Figur 3

die Schliffaufnahme der Brennbahn einer erfindungsgemäß hergestellten Drehanode im Schnitt in 200-facher Vergrößerung

Figur 4

die Schliffaufnahme der Brennbahn einer Drehanode mit pulvermetallurgisch hergestellten Brennbahnbelägen im Schnitt in 200-facher Vergrößerung

Figur 5

die Schliffaufnahme der Brennbahn einer Drehanode, die durch konventionelles Plasmaspritzen hergestellt wurde, im Schnitt in 200-facher Vergrößerung

Show it:

Figure 1

the schematic diagram of a variant of the method according to the invention according to claims 3 and 6

Figure 2

the graphical representation of the mean values of the roughening of the firing track Ra of firing track coverings of rotating anodes produced according to the invention and powder metallurgy

Figure 3

the polished section of the focal path of a rotating anode produced according to the invention on average in 200 times magnification

Figure 4

the grinding of the focal path of a rotating anode with powder-metallurgically produced focal path coverings on average in 200 times magnification

Figure 5

the cut of the focal path of a rotating anode, which was produced by conventional plasma spraying, on average in 200 times magnification

PRODUCTION EXAMPLE 1

Disc-shaped base bodies for rotating anodes made of TZM, a molybdenum alloy with 0.5% titanium, 0.08% zirconium, up to 0.04% carbon, the rest molybdenum, with a diameter of 120 mm and a frustoconical outer area with a 20 ° opening angle mounted on a shaft with a rotary drive and installed in a vacuum chamber. The individual base bodies were coated by means of an inductively heated plasma cannon with an inner diameter of 50 mm and a power of 65 kW with spray powder made of a tungsten alloy with 5% rhenium in a powder fraction between 15 and 63 µm. The wettable powder was introduced axially at a rate of 30 g / min using Ar carrier gas. Before starting the powder injection, the base bodies were heated to 1500 ° C. The speed of rotation of the base body was 10 rpm. The plasma cannon was moved laterally to the center line of the focal track covering concentric to the rotating anode axis, in such a way that the axis of the plasma cannon continuously exceeded this center line alternately on both sides up to a maximum of 5 mm at a speed of 2 mm / sec. In a coating process lasting about 4 minutes, a focal track covering with a total thickness of about 1 mm and a width of 25 mm was applied in this way through approximately 50 individual layers deposited one above the other. After the coating process had ended, the rotating anodes, which had cooled to below 100 ° C., were removed from the vacuum chamber and then the focal track covering was ground to a thickness of 0.7 mm. Finally, the rotary anodes finished in this way were subjected to high vacuum annealing at a temperature of 1600 ° C. for 90 minutes.
For comparison purposes, the same disk-shaped base bodies as were used for the application of the method according to the invention were produced with a 0.8 mm thick focal track covering from a tungsten-5-rhenium alloy by powder metallurgy. For this purpose, a layer of the TZM powder mixture for the base body on the one hand and the tungsten-rhenium alloy for the focal track coating on the other hand was produced and pressed, the compact was sintered and the final shape was produced by forging and mechanical processing. Finally, the rotating anodes were subjected to the same high vacuum annealing as those produced according to the invention.

With the help of the buoyancy method, the density of the focal track coverings certainly. The focal track coverings applied according to the invention had a Density of 97.2% of the theoretical density, while the Powder metallurgy produced focal track coverings a density of 97.4% had theoretical density.

PRODUCTION EXAMPLE 2

In another manufacturing example, a similar Rotating anode base body as in production example 1 in the area of Burning path incorporated an annular groove of 0.8 mm depth. After that was the rotating anode body with the substantially similar Coating conditions according to the inventive method coated according to production example 1. The only difference is this Coating variant was that the plasma gun during the coating was not moved laterally, but was fixed in such a way that the axis -L- of the plasma cannon -3- with that of the rotating anode axis concentric center line -4- of the focal track covering -2- coincided and that the plasma jet and thus the particle flow was set so that the Half-width of the particle distribution -HW- with the width of the active Area -B- of the focal track covering -2- matched as in FIG. 1 is sketchy. For a better overview, the current one is in FIG local distribution of the particles in the area of the focal path is not directly on the Rotating anode body -1- but above and not to scale, but rather exaggerated. After the application of the Burning surface covering -2- was the surface of the rotating anode except for one Level -S- mechanically removed by 0.7 mm and thus the final thickness and clean lateral delimitation of the focal track covering for Rotary anode body manufactured. The one with this coating variant The manufactured anode showed that of the manufacturing example 1 Rotary anodes produced according to the invention have a somewhat improved density of 97.8% of theoretical density, resulting in a reduction in residual porosity by more than 20%.

The rotating anodes produced according to the invention or according to the prior art according to production example 1 were installed in a test bench for X-ray rotating anodes and cyclically tested under the usual conditions with the following parameters: Tube voltage 90 kV Tube current 400 mA Shot duration 2 sec break time 58 sec

The test was randomly interrupted at specified times the furrow roughening that had occurred up to that point as a measure of the Fatigue crack resistance and the associated reduction in X-ray dose yield to be determined.

The respective mean values of the roughness depths Ra of three rotating anodes each Data determined for each variant are shown in FIG. 2.

The much more favorable course of roughening -B- in the case of Rotary anodes produced according to the invention are clearly recognizable. After The average roughness depth Ra in the circumferential direction was 100 hours of test duration measured by 24% lower than the corresponding comparison value of the Roughening -A- of the comparative rotating anodes produced by powder metallurgy. In Another consideration is the roughening of the invention Rotating anode, as it was at the end of the test after 100 hours, in the Comparative anodes reached on average after about 20 hours of testing. After completion of the comparative test, one according to the invention coated rotating anode and from a powder metallurgy Rotating anode each made a cut. An inclusion of these cuts in 200x magnification can be seen in FIGS. 3 and 4.

The structure of the induction plasma-sprayed combustion path according to FIG. 3 shows one fundamentally different morphology than that of powder metallurgy produced focal path according to Figure 4. The inductive plasma spraying striking enamel droplets crystallize transcrystalline as they solidify from, which in turn serve as a crystallization surface for the subsequent melting droplets. This will do it once present growth direction of the layer at least over many Maintain melt droplets largely and they do not form lamellar usually observed in conventional plasma spraying Structural structures with poorly bound grain boundaries as shown in FIG. 5 clearly using the example of one produced by conventional plasma spraying Focal path coating of a rotating anode can be seen. With inductive Plasma spraying is the original line between successive Frozen droplets only partially through intra-crystalline hems of To detect micropores, which, however, are also transcrystalline continue to be crystallized grain. In total this results the dense, predominantly columnar structure shown in Figure 3. In the Grain boundaries between these columnar growth direction Crystallites are well formed and free of micropores. In contrast, the grains lie in the powder metallurgical structure Figure 4 largely isotropic. The residual porosity occurs here in the form of coarser Pores appear clearly. Fatigue of the focal track covering at Rotary anode produced according to the invention occurs in the form of essentially microcracks running perpendicular to the surface appear however less harmful in terms of roughening the surface impact as those cracks in the powder metallurgically manufactured rotating anode. The greater roughening and the destabilization of the surface structure in the case of the powder anode made rotating anode by failure of the Grain boundaries can be clearly seen in FIG. 4.

The manufacturing examples describe particularly advantageous variants of a Manufacturing method according to the invention, but the invention is in no way on this limited. For example, it is also conceivable that Focal track covering not through several superimposed spray layers, but to apply in a single layer.

Claims (8)

Method for producing an anode for X-ray tubes, consisting of a base body and a coating that emits X-radiation different from it,
characterized by
that the X-ray emitting coating is applied by inductive plasma spraying. A method of manufacturing an anode for X-ray tubes according to claim 1, characterized in that the X-ray emitting coating by overlaying individual spray layers with one Total thickness between 0.4 mm and 0.6 mm is applied. Method for producing an anode for X-ray tubes according to one of the Claims 1 or 2, characterized in that before the application of the X-ray emitting coating in the area of this coating Recess with approximately the depth of the desired covering thickness in the Base body is incorporated. A method of manufacturing an anode for X-ray tubes according to claim 1 or 3, characterized in that the deposition under an inductive coupled power between 50 kW and 100 kW and a delivery rate of the wettable powder between 10 g / min and 50 g / min. Method for producing an anode for X-ray tubes according to one of the Claims 1 to 4, characterized in that the anode before the Application of the covering to a temperature between 1,000 ° C and 1,500 ° C is preheated and the local separation temperature in the area of the covering is between 1,400 ° C and 2,400 C. Method for producing an anode for X-ray tubes according to one of the Claims 1 to 5, characterized in that the plasma jet and the Base body are moved against each other so that the central Point of impact of the plasma particle stream on the anode surface and the center line of the active one concentric to the rotating anode axis Focal path area coincide at least approximately, the Particle flow of the plasma jet is set so that the within the particle stream of the plasma jet impinging on the active focal path region includes only that range that is within the half-width of the Gaussian particle distribution of the complete plasma jet generated lies. Method for producing an anode for X-ray tubes according to one of the Claims 1 to 6, characterized in that the anode is final is subjected to an annealing treatment. Anode for X-ray tubes, produced according to one of Claims 1 to 7, characterized in that the base body made of graphite, molybdenum or a molybdenum alloy and the X-ray emitting coating a tungsten-rhenium alloy.

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