Beyond X-rays, the territory of neutrons
Tomography as a tool for non-destructive material testing
Abstract
Neutron imaging can outperform the well-known X-ray radiography and tomography in many aspects, including contrast and penetration depth. Thanks to these features, it can pave the way to previously unexploited applications in non-destructive material testing and industrial process development. The 3D visualization and quality assessment of a bulky casted electro-rotor are presented here as an illustrative example.
A few words about non-destructive material testing
Probably, only a few people would correctly answer the question of what is common in the energy unit of rovers to be usedin NASA’s interplanetary space missions, the socketed axe of a Late Bronze Age warrior, and the rotor of an electric motor. The correct answer is the need for non-destructive material testing (NDT). The need for the testing emerges from several sources: in the case of a Mars rover, it will only be clear after the landing if it operates, and the chance of any malfunction has to be minimized. The quality ofthe warrior's weapon was a matter of life and death, that by now turned into avaluable artifact. In the case of the rotor of an electric motor, the motivation can be its development, the validation of the manufacturing process, and the need for quality improvement measures.
Each of these three objects can only function if their material quality and manufacturing precision reach the desired level. This must be verified, however, this testing must not damage the object itself. Due to this dual requirement, it is not easy to perform material testing of these types of objects.
An important aspect to consider is the expected outcome of the tests. If it is supposed to be 3D information, modelling, reverse engineering, and possibly virtual visualization, tomographic imaging methods stand out among the numerous NDT procedures. And here we reach the point, where the involved engineers start to contemplate using X-ray 3D imaging, which is well-known in the industry. Is this applicable to the cases listed above?
In most cases yes, but not always.If the sizes and materials of the objects exceed certain limits, X-ray imaging faces obstacles. The listed objects are all relatively large and thick (several centimetres), as well as the interaction of their materials with X-ray is either too strong (iron, copper) or too weak (e.g. hydrogen-containing organic materials). In these cases, one fails even if X-rays with hundreds of kilovolts are used. The application of neutron beams still offers a solution to these difficult cases.
Neutron imaging
The scope of neutron tests, similarly to X-ray-based NDT procedures, is very broad: from macroscopic and microscopic structural tests to the determination of material quality, and to the observation of real-time processes. Neutron-based measuring stations are less abundant, as they are installed at large-scale facilities of research centres. Nowadays, it is best to use the two types of testing procedures (modalities) as complementary rather than either one alone. Unlike optical 3D scanning, the results of neutron and X-ray imaging are volumetric digital data, i.e. not only the external surface of objects but also their internal structures are probed, by detecting the decrease in transmitted intensity of the beam as it passes through the object.
In the course of computed tomography (CT), 3D imaging is created from a series of projection (2D) images taken during incremental rotation steps. The result of the tomographic reconstruction, ideally, provides the values belonging to different points of the tested volume, the so-called attenuation coefficients. In the images, this is shown on a greyscale, where lower values (usually denoted by the dark colour) indicates materials that attenuate the radiation to a smaller extent, while the higher values (usually denoted by bright colour) refers to materials that attenuateit to a larger extent. Assuming identical raw material, these values provide us information on the local density or even the discontinuity of the material (cracking, pore).
How will the need for this test appear in the industry, including the development of electric cars?
During the development and testing of motors to be built into electric cars, developers and manufacturers encountered quite a few problems. The expert team of KÖR Group is well-informed about these problems, for example, heating, cracking, and rotor imbalance (and their indirect effect on the battery performance). The origin of the problem lies in the production technology of the rotor, namely the die casting. This is what currently poses a real challenge to rotor manufacturing companies. How can they produce a product free of defects, in this case, free of pores and inclusions? How can they control their manufacturing process to ensure that the quality of the product is reproducible - even if it contains minor material defects?
What effects can inclusions or pores have?
The dynamic effects resulting from the imbalance of a rotor rotating at tens of thousands of revolutions per second or per minute (rpm) can affect the dynamics of the engine and the driving experience of the entire vehicle. To avoid this, balancing the rotors is needed that results in an additional manufacturing cost. During the rotation of the rotor, the inclusions ‘move’ and the material ‘creeps’ towards the material-deficient part. Due to warming expansion and shrinkage, defect locations carry the risk of crack formation. A consequence of the heating, which is ofcourse not only due to the defects, is the lower energy-efficiency, and extra energy is consumed from the battery.
How can manufacturers get enough information about the inside of the rotors and the quality of their products without destruction?
The answer is the neutron tomography and related expert know-how that KÖR Group has developed in collaboration with the Budapest Neutron Centre (BNC). Future development of batteries may take advantage of this testing opportunity.
What do the problems listed above look like in images?
NDT testing of the electrical steel copper and electrical steel aluminium rotors (figure 1. – figure3.a) has been carried out in cooperation with KÖR Group and BNC at the BNC’s RAD imaging station. The application of the so-called filtered neutron beam allowed the scanning of large objects (10 cm size range) with a spatial resolution of 0.7 mm and an exposure time of a few seconds per image.
After a few days, the radioactivity induced during the neutron irradiation decreased to negligible levels, so the sample was available for further, even destructive tests.
It is not permitted to present the entire test data set, as it is an industrial secret, still, the images below can illustrate the developments, and open up new “ways of thinking”, based on the so-far unseen or uncertain experimental evidence.
Compared to X-rays, there are much fewer imaging artefacts in the images. In the 3D sections, any structural changes can be confidently identified. According to this, rotors are generally good-quality castings and the number of casting defects larger than the spatial resolution of the test varied.
Figure 1.
Figure 1. (b-c): Together with some porous casting defects, the fan-like casting defect found in the copper rotor is visible in the 3D section,
Figure 2.
Figure 2. (a): The tested rotor is an aluminium cast electrical steel rotor with closed channel.
Figure 2. (b): In the aluminium sealing ring of the rotor large-volume casting defects are visible.
Figure 2. (c-d): In the arrangement of the plates used in production slight geometric inaccuracies can be seen.
Figure 2. (d): Pores detected in aluminium moulded channels, which are covered by electrical steel. Finally, it was possible to show the pores formed during the pressure casting that fills the gaps between the lamellae.
Figure 3.
Figure 3. (a): The tested rotor is an electrical steel rotor cast with open channel aluminium.
Figure 3. (b): During casting, inclusions were placed in the rotor sealing rings, but in different amounts. These are probably hydrogen-containing particles (e.g. abrasives)
Figure 3. (c): In addition, near the connection area between the rotor body and the flange several pores have been formed. The molten metal probably carried the particles forming the inclusion away, and during its solidification and contraction, it was deposited in various positions to different extents.
The technology presented here is one of the many technologies, in which the expert team at the KÖR Group has competencies and can be applied to solve customer problems or perform the tests necessary for development.
About the authors
Dr. Zoltán Kis, an internationally recognized specialist in X-ray and neutron imaging, image processing, advanced visualization, as well as related non-destructive material analysis and simulation techniques. Besides his research work, he actively contributed to the success of numerous industrial NDT studies.
Dr. László Szentmiklósi is the head of the Nuclear Analysis and Radiography Department at the Centre for Energy Researchand the manager of the Budapest Neutron Centre’s element analysis and imaging facilities. His research focuses on neutron- and X-ray-based non-destructive composition analysis and imaging methods, as well as their applications.
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About the Budapest Neutron Centre
The Budapest Neutron Centre (BNC, www.bnc.hu), one of the most significant research infrastructures and knowledge centres in Central Europe, belongs to the Centrefor Energy Research (EK, www.ek-cer.hu), a member of the Eötvös Loránd Research Network (ELKH, www.elkh.org). The BNC operates an excellence-based user access program for basic and applied sciences and an industrial outreach program to promote the R&D activities. It is located in the KFKI Campus, Budapest, Hungary, where - beyond its core NDT competencies - a broad range of material testing and engineering techniques are available.
About KÖR Group
KÖR Group Plc offers industrial technical problem-solving services to its clients from problem identification through technical analysis to tangible technical problem-solving. Its services are focused on solving its clients’ competency -, capacity - , management issues with using its vast network of laboratories, universities, and scientific institutions. KÖR Group’s roots are coming from the automotive, aerospace, heavy-transportation, rail industry with its professionals working in the industry for more decades. KÖR has been founded as the spin-off of TRIGO Group in 2020.
