棒阳极X射线管的优化 - 飞机引擎焊接的X射线检测.doc

棒阳极X射线管的创新技术– 应用于航空发动机焊缝的X射线无损检测
作者：Thorsten FROBA, Dr. Jens Peter Steffen
地址：X-RAY WorX GmbH, Siemensstraße 26, D-30827 Garbsen, Germany,
www.x-ray-
摘要：根据工业标准（如ASTM [1]）的要求，航空发动机焊缝的X射线无损检测需要使用高强度长时间曝光的X射线，棒阳极X射线管的最大挑战在于其耐用性，为了增强棒阳极X射线管可靠性和耐用性，不仅要改进其机械设计，而且要引进创新的冷却技术。
此文献概述了微焦点棒阳极X射线管的安装及应用，特别是在航空航天工业中。文中讨论了不同的技术难点，如散热、冷却的改进、以及密闭的冷却回路，并讲述了X-RAY WorX公司的研发团队是如何解决这些关键问题，最终为航空发动机检测领域集成了一套紧凑、耐用的棒阳极X射线管。新的棒阳极X射线管的技术，使设备更稳定，减少故障率并为发动机制造商提供了优化的X射线检测方案。
1. 应用领域
射线照射及射线透视技术广泛应用在无损检测领域中，在普通的射线检测系统中，一般将X射线源放置在被检测物的前面，一个X射线感光胶片或探测器放置在被检测物的后面，用以接收穿透被检测物后残余的射线，但是许多被检测物的缺陷所处的位置非常不适用于这种检测方式。
那些带有很多孔腔或很厚的管璧的检测样品，由于射线穿透后分辨率以及对比度的问题，导致检测结果非常不理想。例如中空的样品[2]和热交换器[3]，管板上焊接许多管子，这种焊缝的检测就需要一个狭长的X射线源，伸进管子内部，如热交换器的管子内部，标准的X射线管都不能满足这种检测方式。

图1. 使用普通的X射线管进行焊缝检测
类似的难点在航空发动机零部件的检测中很常见，X射线源必须非常靠近焊接点（the weld），同时胶片及探测器需要放置在焊接处的另一侧，使用普通的X射线管，不可能将X射线源靠近缺陷处，而且不可避免的需要穿透很厚的管壁（如图1），穿透双壁厚度导致分辨率和对比度降低，而且会产生重叠分辨被检测区域的现象。
为了解决上述检测难点，研制了棒阳极X射线管，X射线窗位于阳极棒的顶端，阳极棒可以探入孔洞内部，图2展示了使用棒阳极X射线管检测的基本原理，相对于普通的从被检测物外部透照检测方式，使用棒阳极X射线源，可以减少透照壁厚，可以获取更高的分辨率。

图2.使用棒阳极检测焊缝
随着产品质量及安全标准的越来越严格，允许焊缝的缺陷大小也越来越小，电子束焊接可能出现的漏焊深度必须小于50μm[4]，这会直接影响到在检测中所使用的X射线源，要求X射线焦点尺寸更小，焦点尺寸大小指的是X射线源内部辐射的区域直径，针对高放大倍数及高分辨率的X射线图像是关键要求的领域，微焦点X射线源为此提供了解决方案。
2. 微焦点棒阳极X射线管
微焦点棒阳极X射线管结合了微焦点和阳极棒两个特点，如图3先进的微焦点棒阳极X射线管，可以生成非常小的焦点尺寸，同时可以探入孔洞内部非常靠近缺陷的位置进行检测。

图3先进的微焦点棒阳极X射线管
棒阳极X射线管的焦点在靶极生成，如图5，阳极靶位于阳极棒顶端，根据靶的不同形状，可以产生不同形状的X射线束，参考图表1表示了4种不同特点的X射线束。

周向靶（Panoramic）通常被用在管状结构或航空发动机的检测中，反射式靶(Reflection)可用在管板的焊缝检测中，透射式靶（Transimission）可以用在容器的检测中。
3. 带棒阳极X射线管的微焦点射线检测难点
优化的生产工艺意味着保证高质量的同时节省更多的时间，在检测焊缝时，减少检测时间，意味着减少胶片或探测器的X射线曝光时间，为保证焦点尺寸在一定范围内，可以通过增加X射线轻度来实现。
增加X射线的强度，就需要增加射线管的运行电流，电流越高，作用到棒阳极和靶上的热量就越大，这就引起了微焦点X射线技术的核心问题，即靶和密封件的散热问题。
O-型密封垫圈是为了保证射线管内的真空度，热量过高会引起钨层靶以及密封垫圈的老化，所以热量是影响微焦点X射线源稳定及耐用程度的关键。
普通的微焦点X射线管的靶冷却是在棒阳极管头顶端带有冷却介质，这样使得操作很不便利，因为冷却介质是作用于靶的背面（如图4），所以靶和O型垫圈的冷却效果很不理想，而且容易引起连续运行时真空度的不稳定。

图4. 带外部冷却的普通的棒阳极X射线管
4. 内部液体冷却以改善性能
微焦点棒阳极X射线管的创新水冷技术，采用内部液体循环通道，将冷却介质直接作用到O型密封垫圈和靶上（如图5），这样有利于操作并固定棒阳极，并且保证稳定的真空环境。

图5.优化的棒阳极X射线管的内部冷却
先进的冷却技术，可以是管功率增加至100W，可以增加X射线强度，缩短检测时间，而且可以有效改善O型密封垫圈的使用寿命，从而可以降低棒阳极射线管的维护成本，增强了棒阳极的耐用性。
如图6.，采用内部液体冷却，可以减少棒阳极的直径，可以进入更小的孔洞。

图6：棒阳极创新设计

参考文献
[1] ASTM E1032 - 06 Standard Test Method for Radiographic Examination of Weldments
[2] Schröder, G., Pauly, F., Untersuchung von Verbindung und Struktur geschweißter Aluminium-
Strangpressprofile, Berichte des Forschungszentrum Jülich, 3944, S.11 ff
(http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2590/1/Juel_3944_Schroeder.pdf)
[3] Ding, K., Chen, G., Shou, B., Zhang, X., Huang, D., Digital Radiographic Imaging Inspection System on
The Tube to Tube Sheet Welding Joints of Heat Exchanger, Proceedings of the WCNDT 2008, Shanghai.
( )
[4] Kumar, A. and Kumar, S.,
X-Ray Radiography of EB Welded Joints in India, Proceedings of the National
Seminar on Non-Destructive Evaluation, 2006, Hyderabad
(

Optimization of Rod Anode Tubes for X-ray Inspection of Aircraft Engine Weldings

Thorsten FRÖBA*, Dr. Jens Peter STEFFEN*
*X-RAY WorX GmbH, Siemensstraße 26, D-30827 Garbsen, Germany,
www.x-ray-

Abstract. One of the major challenges for rod anode tubes used in microfocus X-ray inspection of aircraft engine parts is endurance. Industry standards like ASTM [1] require long exposure times of X-ray films at high X-ray intensity. To increase reliability and endurance of rod anode tubes new mechanical designs had to be developed and new cooling techniques needed to be applied.
This paper gives an overview over the general setup and application of microfocus rod anode tubes in industry with a special focus on aerospace industry. It discusses different technical challenges like heat reduction, improvement of the cooling process as well as the integration of sealings into the cooling circuit. It shows how the development team of X-RAY WorX solved the major issues to assemble a compact and durable rod anode tube for use in aircraft engine inspection. The new design of the rod anode tube decreases down times of the equipment and thus optimizes the process of X-ray inspection of the engine manufacturer.

1. Areas of Application
Radiography and radioscopy are common and widely accepted techniques for non-destructive testing. In a standard test setup an X-ray source is placed in front of the inspected part. An X-ray sensitive film or detector is placed behind the part to collect the remaining radiation that penetrated the part. In many setups the region of interest is found at positions that are inappropriate for this procedure.

Parts that have several cavities and high overall wall thickness may only be penetrated with insufficient results in terms of resolution or contrast. Examples are hollow chamber profiles [2] and heat exchangers [3], where several tubes are welded on a tube sheet. Inspecting the welds requires a narrow X-ray source that needs to be inserted into the tubes of the heat exchanger. Standard X-ray tubes have dimensions that do not allow this insertion
.
Figure 1: Weld Inspection using standard X-ray tube
Similar challenges can be found in inspection of aircraft engine parts. Here the X-ray source has to be moved very close to the weld. At the same time the film or detector needs to be positioned at the opposite side of the weld. With a standard X-ray tube it is not possible to move the source of X-ray close to the region of interest while avoiding the doubling of the penetrated wall thickness (compare figure 1). Effects of doubling the penetrated wall thickness are loss of contrast and resolution as well as overlap of details in the inspected regions

To meet the described challenges rod anode X-ray tubes were developed. The source of radiation is located at the top of a rod and can be inserted into cavities. Figure 2 illustrates the basic principle of using rod anode tubes to decrease the wall thickness that needs to be penetrated. Welding defects can of a rod and can be inserted into cavities. Figure 2 illustrates the basic principle of using rod anode tubes to decrease the wall thickness that needs to be penetrated. Welding defects can from the outside of the part.

Figure 2: Weld Inspection using Rod Anode
Increasing requirements for quality and security are leading to lower tolerance levels for various weld defects. In electron beam welding missed joints may have depths of less than 50µm [4]. This also has an impact on the X-ray sources used for inspection. The major \impact is the need for smaller X-ray focal spot sizes. The focal spot size is the diameter of the area inside the X-ray source that emits radiation. Microfocus X-ray tubes are the solution when high magnification and high resolution of the resulting X-ray image are key requirements.

2. Microfocus Rod Anode Tubes
Microfocus rod anode tubes combine the advantages of microfocus X-ray tubes and a rod anode tubes. Figure 3 shows a modern microfocus rod anode tube. It produces a very small X-ray focal spot that can be positioned in cavities very close to very close to the area of interest.

Figure 3: Microfocus Rod Anode Tube
The X-ray focal spot of a rod anode tube is generated at the target, compare figure 5. The target is located in the head of the rod anode. Depending on the shape of the target different beam characteristics can be generated. Basically four different types of beam characteristics can be distinguished as described in table 1.

Panoramic targets are mainly used in pipe construction and aircraft engine inspection. Reflection targets are needed to control tube sheet welds whereas transmission targets have their main application in tank inspection.

3. Challenges in Microfocus Radiography with Rod Anode Tubes
Optimizing production processes means saving time while concurrently keeping the quality high. Decreasing inspection times when inspecting welds basically means decreasing exposure time of X-ray sensitive films or detectors. This can be achieved by delivering higher X-ray intensity while keeping the focal spot at a constant size.

To increase X-ray intensity an X-ray tube needs to be operated at higher current. Higher current introduces a higher amount of heat into the rod anode and the target. This line of 3reasoning leads to the core problem of microfocus X-ray technology, namely the deduction of heat from the target and sealings.

Heat leads to degradation of the Tungsten target layer as well as degradation of the O-ring sealings that keep the vacuum stable inside the tube. Thus in microfocus X-ray technology heat is the classical antagonist of stability and endurance.

Standard microfocus rod anode tubes have a target cooling where the cooling agent is lead in at the head of the rod anode. This makes handling of the rod anode inconvenient. The cooling of the target and O-ring sealings is insufficient because the cooling agent is passing on the back side of the target (compare figure 4). This may lead to unstable vacuum during continuous operation.

Figure 4: Standard Microfocus Rod Anode with external cooling
4. Improvement of Performance by Internal Cooling
The new developed microfocus rod anode tubes have a different type of cooling. Internal cooling channels lead the cooling agent through the rod towards the head. The cooling agent is directly cooling the O-ring sealings and target (see figure 5). This allows easier handling and positioning of the rod anode and generates stable vacuum conditions.

Figure 5: Optimized Rod Anode with internal Cooling
The advanced design of the cooling allows the e tube power to be increased up to 100W. This increases X-ray intensity and shortens inspection time. Furthermore the improved cooling significantly improves the endurance of the O-ring sealings which leads to lower maintenance effort and higher overall endurance of the complete rod anode tube.

An example for the improvement of handling is shown in figure 6. By cooling internally the diameter of the rod is reduced. Smaller cavities become accessible.

Figure 6: Advantages of improved design of the rod anode
References
[1] ASTM E1032 - 06 Standard Test Method for Radiographic Examination of Weldments
[2] Schröder, G., Pauly, F., Untersuchung von Verbindung und Struktur geschweißter Aluminium-
Strangpressprofile, Berichte des Forschungszentrum Jülich, 3944, S.11 ff
(http://juwel.fz-juelich.de:8080/dspace/bitstream/2128/2590/1/Juel_3944_Schroeder.pdf )
[3] Ding, K., Chen, G., Shou, B., Zhang, X., Huang, D., Digital Radiographic Imaging Inspection System on
The Tube to Tube Sheet Welding Joints of Heat Exchanger, Proceedings of the WCNDT 2008, Shanghai.
( )
[4] Kumar, A. and Kumar, S.,
X-Ray Radiography of EB Welded Joints in India, Proceedings of the National
Seminar on Non-Destructive Evaluation, 2006, Hyderabad
( )