JPH05151891A - Manufacture of x-ray tube rotary anode - Google Patents

Abstract

PURPOSE: To reduce porosity and impurity by an after treatment of a focal track area with partial and superficial fusion at a specified depth. CONSTITUTION: A rotating anode body with a tangsten-rhenium focat track area is rotatably fixed to a supporting axis, and inserted in a piston which is highly vacuumed and can exhaust. First a temperature of the rotating anode rotating slowly is raised to a specific temperature by diffusion light elecrtron beam. The rotating anode is then degassed. In addition an output of the electoron beam is raised, so that a surface can be fused by 3 continuous rotations of the rotating anode. Thereby a fused area with a specified width mm and a depth of 1.5mm is produced. The fused area flatly placed each time by an arrangement is condensed smoothly when cooled subsequently, and as a result, when cutting to a specified mm by next polishing a smooth focal track coated film in compliance with requirements can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing an X-ray tube rotating anode having a ring-shaped focal track region produced by a powder metallurgy method or a CVD or PVD method, which is made of a highly molten metal such as tungsten or tungsten-rhenium. Regarding

[0002]

2. Description of the Related Art High melting metals or graphite, or a composite material of both materials, is used as a basic material for an X-ray tube rotating anode. The original generation area of X-rays, that is, the focal track area is made of tungsten, molybdenum, or an alloy thereof. The metal X-ray tube rotary anode is manufactured based on the powder metallurgy method for each reason of shape, material used and required properties. The focal track area itself is also produced by powder metallurgy or by the more and more recently used CVD or PVD coating methods. Rotating anodes or focal track areas of this kind have a residual porosity in the range of 0.1 to 10% in the final finished state, measured by theoretical density. An X-ray tube rotary anode of this kind is described in EP-A-0116385, in which the rotary anode is selectively post-coated after application of the focal track layer according to the method described therein. And heat treated.

This residual porosity has a series of detrimental drawbacks for the operation of X-ray tube rotating anodes, which are in principle carried out in high vacuum. This porosity causes the release of gas trapped in the pores. This also leads to the release of gas in the vacuum of the tube as well as undesired shorting of the tube which causes the anode to seal. The thermal conductivity, which is very important for the power rating of an X-ray tube, decreases with the square of the porosity. The porosity of the focal track surface increases the surface roughness and reduces the X-ray yield due to self-absorption. However, a porous surface also means the risk of particles expelling from the surface, which further aggravates the negative effects of degassing.

The mechanical joining of the individual crystallites at the joint is influenced, on the one hand, by the porosity and, on the other hand, by the metallurgical conditions at the grain boundaries, especially impurities at the grain boundaries.
However, in the course of the powder metallurgy manufacturing method, metal-insoluble impurities at grain boundaries are inevitably condensed. This represents another impediment to operating a rotating X-ray tube anode.

Focal orbital coatings (particularly composed of tungsten-rhenium) produced by powder metallurgy have in some cases a partially brittle intermetallic tungsten-rhenium phase, the so-called sigma phase. This causes inhomogeneities due to poor mixing of the individual alloy components in the powder batch. In addition, the unavoidable thermal shock loading of the rotating anode during operation leads to this and also in the region which starts from this, a very high undesired crack which will reduce the X-ray yield in the focal track region.

The various frequent failures mentioned above limit the endurance time and lead to premature failure of the X-ray tube rotating anode in each case.

[0007]

The object of the present invention is therefore to eliminate or even significantly reduce the abovementioned drawbacks. Especially to reduce porosity and impurities, especially at grain boundaries in the focal track region. Conventional manufacturing methods (powder metallurgy and CVD or PVD methods) should be retained because of their economics and the good material properties they provide.

[0008]

This problem is solved according to the invention by a method of post-treatment of the focal track region of an X-ray tube rotating anode by a local and superficial melting at a depth of less than 1.5 mm. ..

[0009]

[Advantageous effects] The post-treatment by surface melting according to the present invention is
Depending on the method actually chosen, it is carried out on the surface of the focal track region of the X-ray tube rotating anode by the action of a focused beam of energetic electrons or photons to a certain working depth. Degraded metal joints are formed in this region with melting and the porosity and the amount of impurities are decisively reduced, especially in the grain boundary range. The grain structure remains relatively orderly, unlike conventional melt metallurgy processes, due to the significant partial melting and the very rapid cooling after melting. Achievable particle sizes correspond to those typical in focal track regions produced by powder metallurgy or coating processes.

The melting can be carried out once or several times in succession and affects the metallic structure of the focal track region which can be obtained in the final state. Eliminating the residual porosity also eliminates the previous obstacles in operating the rotating X-ray tube anode described above.

Suitable focusable energy sources for the melting process include lasers, devices for producing particle beams, especially electron beams, and high power lamps capable of high focusing. The degree of phase transition, which is limited by the energy / heat material emitted, is important for the energy source chosen in each case. Furthermore, the expense and processing of the equipment, such as processing under protective gas or in high vacuum, is a problem. Due to the high reflectivity of high melting metals for electromagnetic waves in the spectral region of 0.3-20 μm (> 80%),
It is advantageous to use an electron beam, which usually has an activity of ≧ 60%.

The melting depth targeted by the method according to the invention should be measured in synchronism with the thermomechanical load of the focal track region which is expected during operation. 0.05-1.5 mm
The melting depth of was found to be usable. A melting depth of 0.5-0.8 mm provides the best cost / utilization ratio in the majority of applications.

The melting and rapid cooling steps each produce an amorphous, extremely fine-grained, isotropic, columnar or coarse crystalline structural state, depending on the process method. The stress that occurs in the structure is then 900-160.
It can be eliminated by vacuum annealing in the range of 0 ° C.

The melting process produces an extremely smooth surface with a low rough surface depth in the focal track region. Nevertheless, due to the extremely high demands on the smoothness of the surface of the X-ray tube rotating anode, it is generally unavoidable to polish the surface in the focal track region after the melting process.

[0015]

EXAMPLES The method of the present invention will be described in detail below based on examples.
A rotating anode body with tungsten-rhenium focal orbital area manufactured by conventional powder metallurgy method is mounted on a rotatable bearing shaft (as it is during subsequent operation) and can be evacuated at high vacuum Insert into the piston of. At that time, the focal track region of the rotating anode is arranged with respect to the incandescent emission cathode that collects light. First, the slowly rotating rotating anode is uniformly heated to about 800 ° C. by the scattered electron beam. The rotating anode is then degassed, that is to say that foreign atoms and particles of substances which do not stick sufficiently are removed from the surface. In addition, the electron beam length is 2
A line focus of 0 mm and a width of 2 mm and an output of 6 kW are raised, and a rotating anode rotating at 3 to 6 revolutions per minute is continuously rotated three times to melt the surface. At that time, width is about 17m
m, average depth 0.7 mm melting zone occurs. Due to the arrangement, the horizontally present melted portions each condense smoothly during the subsequent cooling, so that the subsequent polishing results in 0.2-0.
When cut to 3 mm, a smooth focal track coating surface is already obtained which meets the requirements.

Structures of the fused focal track region of this kind have oriented condensed crystallites with an average diameter of 150 μm. It never exhibits porosity and certainly contributes to the excellent binding of individual particles or crystallites.

The X-ray tube rotating anode produced according to the invention was compared with the rotating anode produced according to the prior art. Both comparative rotating anodes were subjected to the following stress cycles in a so-called tube test stand, in which the stresses of the respective X-ray tube rotating anodes could be made completely identical in the simulator during later operation: electron beam power: 60kW, focus : 12 × 1.8m
m ² , irradiation cycle: 1200 total exposures were tested for 7 × 0.1 seconds and 59 seconds cooling with 0.1 second interruptions (corresponding to radiography) respectively. After the end of this test, both comparative rotating anodes were examined by scanning electron microscopy for changes in their surface texture and also by scanning pins for surface roughness. In the case of the rotating anode according to the prior art, the average rough surface depth Ra was 5.5 mm, whereas the rotating anode according to the present invention had an average rough surface depth Ra = 3.5 μm.
The material fatigue roughening of the rotating anode according to the invention was not only less than with the rotating anode according to the prior art, but was more uniform over the entire focal track region. That is, the X-ray tube rotating anode according to the present invention was more uniform than the comparative anode according to the prior art and showed a slight tight crack network with a small crack width. The rotating anode according to the invention has a very high vacuum stability. This makes it possible to significantly reduce the so-called warm-up period, during which the rotating anode in the tube is warmed under the electron beam while continuously pumping off the residual gas which can be discharged and is brought to operating conditions for the first time. Be done. The electrical stability of the rotating anode during operation is satisfactory. X of each photo taken at the end of the test
The line dose was 20% higher for the rotating anode produced according to the invention than for the comparative anode according to the prior art. The life of the X-ray tube rotating anode was clearly higher than that of the comparative anode due to the above quality improvement.

Claims (9)

[Claims] 1. A powder metallurgical method or CV made of a highly molten metal.
A method for producing an X-ray tube rotating anode having a ring-shaped focal track region produced by the D or PVD method, characterized in that the focal track region is post-treated by local superficial melting at a depth of 1.5 mm or less. And a method of manufacturing an X-ray tube rotating anode. 2. The method for rotating an X-ray tube according to claim 1, which comprises melting at a depth of from 0.05 to 1.5 mm. 3. The method for rotating an X-ray tube according to claim 1, wherein the melting is carried out at a depth of 0.5 to 0.8 mm. 4. The method of an X-ray tube rotating anode according to claim 1, wherein the melting is carried out by means of a focused electron beam. 5. A method for rotating an X-ray tube according to claim 1, wherein the melting is carried out by means of a focused laser beam. 6. A method of rotating X-ray tube anodes as claimed in claim 1, which comprises mechanically polishing the surface of the melted areas. 7. The method of rotating X-ray tube anodes as claimed in claim 1, wherein the melted areas are additionally subjected to a white heat treatment. 8. The method for rotating an X-ray tube according to claim 1, wherein the melting of the focal track region is repeated once or several times. 9. An X-ray tube rotating anode manufactured by the method according to claim 1, wherein the material of the focal track region is a tungsten-rhenium alloy.