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Additive Reinforced friction stir welds

Computed tomography is a non-destructive method that allows visualisation of the interior of the specimen, by creating its 3D representation from a series of 2D images, generated from X-rays passing through the specimen under various angles.  The visualisation is possible due to the differences in both density and the atomic number between components of a specimen.

Objectives

The objective of the computed tomography scan was to detect and assess the particles in the examined specimens and verify their homogeneity. TWI applied the X-ray microscope tomography to characterise the homogeneity of friction stir welds which were reinforced using additive nanoparticles.

Work programme

The computed tomography activity was conducted on the friction stir weld, to which the silicon carbide particles were added prior to the welding (size 10µm-26µm), in 15% of the weld volume.  The weld joined together two 4mm aluminium plates (alloy AA6082-T6).

The tomography system: Zeiss Versa 520 (Figure1) was used for the tests which were undertaken at TWI.  The sample was prepared by cutting off a cylinder of the size: d=1.3mm and l=2mm from the FSW specimen (two welded plates of total length and width, 120mm, and thickness, 4mm) in order to allow a fine enough resolution for the detection of very small features and to equalise the X-Ray penetration through the sample (thickness= constant diameter).  The cut off sample can be seen in Figure 2, and the position of the sample in relation to the source and the detector in Figure 3.  In this case, the application of computed tomography cannot be classified as a non-destructive technique because it required the cutting off of a sample.

Figure 1. X-ray microscope tomography system at TWI
Figure 1. X-ray microscope tomography system at TWI
Figure 2. Cylindrical sample for X-ray microscope tomography
Figure 2. Cylindrical sample for X-ray microscope tomography
Figure 3. Schematic cylinder sample between source of X-rays and detector
Figure 3. Schematic cylinder sample between source of X-rays and detector

Project outcomes

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.

Further information

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.

For more information please email contactus@twi.co.uk

Figure 4. Part of XZ cross section with seen particles
Figure 4. Part of XZ cross section with seen particles
Figure 5. XY cross section with seen particles and voids
Figure 5. XY cross section with seen particles and voids
Avatar Grzegorz Ptaszek Project Leader, Non-Destructive Testing (NDT)

Grzegorz (Greg) Ptaszek joined TWI in November 2013. He is a doctor in mechanical engineering with 24 years’ experience in non-destructive evaluation. Grzegorz has built up his knowledge and skills in a range of different roles including working in academia for Imperial College London, at a power generation plant, for a steel manufacturer and for a titanium manufacturer. Grzegorz has also studied financial engineering and is the co-owner of a pump rotor patent. In his current role in the Non-Destructive Testing team, Grzegorz works on a variety of projects employing various techniques such as non-linear ultrasonics, phased array, eddy current and computed tomography, and he also has extensive knowledge of thermography.