what are the benefits of X-ray CT in 3D printing?
Unlike destructive micrographic analysis, which offers only limited 2D access to defects, and requires sample preparation including cutting and polishing, X-ray CT offers us a complete mapping of defects present in the scan area. It provides unique access to the 3D geometry of defects generated during manufacturing as the images obtained provide us with a high level of precision regarding the number and distribution of these defects in the observed volume.
When the final parts are mechanically stressed, we study in particular the influence of the geometry and position of these defects on the mechanical properties obtained. In the case of truss structures, CT allows us to easily inspect the quality of the manufactured beams, the respect to the diameters compared to the CAD. Meshes can be directly extracted from the tomographic images to be used as input data in mechanical simulation software. X-ray CT is a complete solution for the accessibility and readability of complex part inspections compared to other technologies, such as radiography or ultrasonic methods.
what are the current challenges of 3d printing? How can X-ray CT help overcome them?
One of the major challenges in additive manufacturing is the real-time tracking and correction of defects set off during manufacturing.
Although not directly integrable into additive manufacturing machines, X-ray CT offers us unique access to study the influence of defects under mechanical stress, either in situ or ex-situ. By combining tomographic images of sample tests, before tensile or fatigue tests, with post-mortem fractographic images obtained by optical or electron microscopy, we are able to identify which internal defects are responsible for the failure. These data allow us to design predictive models of material behavior by taking into account the 3D morphology, size, and position of these defects. Industrially, the knowledge of these parameters is extremely important in the perspective of the wider use of additive manufacturing technologies for more and more critical parts/components. We can thus define a size threshold beyond which defect can be considered critical with regard to the conditions of use. As for all manufacturing processes, there will always be defects, the main thing is to know and evaluate their criticality with respect to the desired use properties.
what progress do you see coming in the field of X-ray CT for the inspection of 3D printed parts?
X-ray CT is seen in the industrial world as a very relevant process, but sometimes complex to implement for serial inspection, which takes time and remains expensive. In order for CT to be considered an efficient control method like radiography or ultrasound, progress must be made in acquisition and reconstruction speeds, process standardization, and automatic defect detection. The case of metal additive manufacturing is very specific as different types of more or less complex parts can be manufactured. All stages of a part require precise controls, in particular, to detect problems linked to the choice of manufacturing parameters. CT systems capable of working at multiple scales are often necessary, but they must also be able to penetrate significant material thicknesses of iron, titanium, or even nickel-based metal alloys for the applications we are studying. High power tubes maintaining a high spatial resolution even at high energy will be very useful. Progress is also expected in the management of artifacts related to the density of the parts such as the scattering effect which can complicate the accuracy of the segmentation of the surface of the parts.
What do you think about the production of industrial parts using 3 d printing? Safran recently produced a landing gear using 3D printing, does this seem close?
The industrialization of parts made with metal additive manufacturing is a major challenge for reducing costs and manufacturing times, and it is already a reality. The space industry has demonstrated the possibilities offered and is now using parts in real-life situations that have successfully passed qualification tests, such as the combustion chamber of SpaceX's SuperDraco thruster. The aerospace industry is also following this trend by developing parts that are more or less critical in flight. Non-destructive testing on these sometimes geometrically complex parts will have to adapt.
X-ray CT remains a very effective method, whatever the shape, provided of course that it can penetrate the material and detect the most critical defects. In the development phase, it offers an exhaustive and quantified vision of these defects and their impact on the mechanical properties in service.