The X-ray microscope tomography results relied on the differences in density and atomic number of the particles and the aluminium alloy base.
The results can be seen in Figures 4 and 5 which show the cross section of the weld in two directions (along the weld XZ and across the weld XY). The particles are seen as bright dots due to their higher density, and the atomic number in comparison to the aluminium base material. The particles scattered and absorbed more X-ray energy than the base metal, therefore, less energy was received which manifested itself in the brighter dots. The parameters of the microscope were set to be able to visualise particles down to 5μm.
An advantage of computed tomography in comparison with microscopy is that it has the capacity to provide features in 3D. During the project, the largest particle detected was around 26μm.
X-ray microscope tomography also allowed detection of the voids in the weld as shown in Figure 5. The voids are seen as darker areas due to the air captured inside, which has a much lower density and atomic number than the metal. Therefore, much more energy passed through here. The results generated revealed that the particles were correctly dispersed in the weld. No clustering or cracking on the boundaries between the particle and the metal were detected.
Higher magnification will provide more precise views of the particles and can be achieved by focussing on a smaller volume of the material and applying optimised machine settings. The X-ray microscope tomography system has the potential to locate particles down to 1μm.
Other methods such as electron microscopy and spectrometry can be also applied to observe, and identify the components of, the particles. The plan is to carry out these levels of magnification and characterisation in future work.
TWI Knowledge summary: Computer tomography – this article is available to TWI Member companies and organisations only.
New X-ray microscope strengthens TWI image capability
This project was sponsored by Innovate UK.
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