CT Metrology simply just in between – the Test

By Gina Naujokat / March 02, 2022
High-resolution computed tomography of a 3D-printed plastic impeller by the YXLON FF35 CT Metrology system with a following actual-target comparison to the CAD model provides clear insights into the inner structures and reliable quality results. This could be achieved within the short time of 2:18 minutes and an automated evaluation algorithm.


CAD model (left) - plastic impeller photo (middle) - actual-target comparison (right)


In-depth analysis of inner structures

What about the capabilities of the universal YXLON UX20 X-ray system with the new MesoFocus tube?
We started a test project with the same 3D-printed impeller made of Aluminum.


X-ray technology offers concrete insights into the interior of objects, and computed tomography provides three-dimensional virtual twins of the scanned specimens allowing detailed inspections and measurements at any designated point within their inner structures. Formerly, industrial X-ray technology was mainly used to detect defects inside components and thus ensure product quality. Today's computed tomography allows for detailed material analyses supporting production as well as research and development. It offers solutions for nearly all inspection needs due to the extensive range of components and features available on the market: modern X-ray tubes up to 600 kV with different focal spot sizes; high-resolution flat-panel detectors for cone-beam scans and line detectors for fan-beam scans; numerous manipulation axes for optimal radioscopy; various field-of-view extensions for larger objects; image enhancement software for the reduction of CT artifacts. State-of-the-art technology on both the mechanical and software side ensures reliable and repeatable results even in the nanometer range.
 
Analyses and measurements of the internal structures of an object have become very important, especially in the context of new additive manufacturing methods. With 3D printing, geometries can be created that are not possible with any other manufacturing technology. Originally used in the aerospace and automotive sectors to save material, weight, and thus fuel, structural diversity is now nearly, if not more relevant. To check the quality and safety of these structures, it is necessary to look inside them, preferably without destroying it. The same applies to the evaluation of manufacturing methods. Here, AM and CT go hand in hand from research to development to production. And in addition to failure analysis, detailed measurement tasks are the basis for checking stability and functionality.
 
Regardless of the technology, the highest precision standards apply to every metrology system. The systems YXLON FF20 CT Metrology and FF35 CT Metrology are constructed for optimal stability and temperature resistance to achieve the most accurate and repeatable results. They conform to the VDI/VDE 2630 ‘Guideline for the application of DIN EN ISO 10360 for coordinate measuring machines with CT-sensors. The FF35 CT Metrology system used in this project has a maximum permissible error of MPESD = 5.9 µm + L/75. We wanted to know how, in comparison to FF35 CT Metrology, our universal X-ray and CT system YXLON UX20 performs when it comes to measurement tasks. UX20 is primarily designed for production-related use in harsh environments, but with its compact design and user-friendly Geminy operating software, it offers many advantages for laboratory applications in the medium price segment.

The Test

The test part chosen as an example is this 3D-printed impeller like those used in the aerospace sector. The material, AlSi10Mg, is a fine powder aluminum alloy, combining good strength and thermal properties with low weight and flexible post-processing capabilities.

We split our project into three steps. In the first step, the impeller was scanned five times in the FF35 CT Metrology. Then we calculated the Golden Surface Mesh, which would serve as a reference for our measurements.


In step two, the same impeller was scanned ten times, each time with the same settings in the UX20 equipped with the new Comet MesoFocus 225 kV tube, which provides resolution down to 25 µm with its three focal spot sizes of 50 µm, 100 µm and 200 µm. The optimal focal spot for the impeller was 100 µm. Each 3D volume was then compared to the Golden Surface Mesh using a proven algorithm. To determine the best possible result, the comparisons were geometry-based and weighted. The bright green volume shows the so-called 'best fit'.







In the third step, 15 regions of interest (ROI) were defined, each with an area of 5 x 5 mm, where wall thickness measurements were performed. The smallest and the largest wall thickness of each area taken, averaged the respective results, and compared them with the Golden Surface Mesh.



The following diagram was generated, over which the curve of the measurements of the Golden Surface Mesh from the FF35 CT were superimposed serving as our reference. The small deviations were indeed within the range expected.



For verification, three more scans were performed with the line detector in the YXLON FF85 CT system. The slice-by-slice scans with a line detector produce minimal artifacts and are thus considered more reliable when it comes to measurements. Though, a complete scan for a 3D volume is much more time-consuming, however if the aim is to capture only specific regions of interest, scanning a one-pixel slice would be the fastest alternative.
 











 

CT-technologies in comparison:






Pictured here are the three scanned planes of the impeller: the top edge, the layer of our Regions of Interest, and the bottom edge. The top and bottom edges of the scan row are used for the model's alignment.






The measurements of the regions of interest indeed show a flattened waveform, deviating from the reference measurements by about 10 µm on average.


Further metrology applications with YXLON UX20 on the 3D-printed aluminum impeller

[analysis software VGSTUDIOMAX 3.5]


Porosity Analysis





Roughness Analysis



[analysis software MountainsMap]

GD&T Geometrical Dimensioning and Tolerancing



Nominal-Actual Comparison


In this nominal-actual comparison, the 3D volumes of the FF35 CT and the UX20
are superimposed achieving the expected deviation-free result.
 

Conclusion:

This project provides good insight into the challenge of measuring additively manufactured components and allowed for the demonstration of a wide range of options available to the users of our X-ray technology. Evaluation according to shape and position, pore and cavity analysis, the determination of surface roughness, delamination and crack analysis and the inspection for powder residues are the relevant inspection routines.
 
Difficult or impossible to access areas which would otherwise be measured using conventional measuring equipment such as a tactile coordinate measuring machine or optical 3D-scanners, can be recorded with the X-ray CT in a time-saving manner, eliminating some of the typical probing errors. Pre-treatment of the components or complex positioning is not necessary.

The reference formation with our precise YXLON FF35 CT metrology system and the comparison to our universal YXLON UX20 X-ray and CT system showed that UX20 is perfectly suited for metrology tasks depending on the customer specifications. Though UX20 has been developed primarily for radioscopic inspections in rough production environments, it is equipped with outstanding CT capabilities and can reliably perform metrology tasks which further increases the range of possible applications. 
 
by Christian Johanns and Christopher Zepp