Metal additive manufacturing: IRT Saint Exupéry inspects its parts using X-ray tomography.

What advantages does CT offer over conventional methods?

Destructive micrographic analysis gives only limited 2D access to defects and requires sample preparation (cutting, polishing). X-ray CT, by contrast, gives us a complete map of the defects in the scanned area. It offers unique access to the 3D geometry of the defects generated during manufacturing, most of the time without destroying the part.

Method	Dr. Lesseur’s answers
Micrography	Destructive; limited 2D access to defects; cutting and polishing required
Conventional NDT (dye penetrant, ultrasonic, dimensional inspection, optical methods)	Made ineffective by internal cavities, channels and high roughness
X-ray CT	Complete map of the defects in the scanned area: non-destructive 3D geometry

"The images are very precise about the number and distribution of these defects within the observed volume. When final parts are mechanically loaded, we pay particular attention to how the geometry and position of these defects affect their mechanical properties."
 
For lattice structures, CT makes it easy to check the quality of the struts and whether their diameters match the CAD. Finite element meshes can even be extracted directly from the CT images and used as input for mechanical simulation software.

What are the current challenges of additive manufacturing, and how does CT help address them?

One of the major challenges of additive manufacturing is real-time monitoring and correction of defects during the build. X-ray CT can’t be integrated directly into additive manufacturing machines, but it offers unique access to study the influence of defects under mechanical loading, in situ or ex situ.

"By combining CT images of specimens before tensile or fatigue tests with post-mortem fractographic images (optical or electron microscopy), we identify which internal defects are responsible for failure. This data then lets us build predictive models of material behavior that take into account the 3D morphology, size and position of the defects."

Industrially, these parameters are essential for extending the use of additive manufacturing to ever more critical parts. We can thus define a size threshold above which a defect becomes critical for the part’s conditions of use. As with any manufacturing process, there will always be defects: what matters is knowing them and assessing how critical they are for the properties required in service.

What progress should we expect from CT for inspecting additive manufacturing parts?

In industry, X-ray CT is seen as a highly relevant process, but one that is hard to implement for series inspection: it takes time and remains expensive.

For it to be considered an effective inspection method on a par with radiography or ultrasound, progress is needed on acquisition and reconstruction speed, process standardization and automatic defect detection. Metal additive manufacturing is a special case: walls are built, specimens are cut from them, then parts of components, then the final part. All these steps call for precise inspections, notably to detect problems linked to build parameters.

You often need CT systems that can work at several scales, but also penetrate thick sections of iron-, titanium- or nickel-based alloys for the applications we study. High-power tubes that keep a high spatial resolution, even at   high energy ,   will be very useful. Progress is also expected in handling artifacts linked to part density,  such as scattering  , which can make accurate segmentation of the part’s surface difficult.