Metal AM Vol 1 No 3

Inspection and quality control with X-ray CT

The inspection and quality control of metal AM parts with X-Ray CT X-ray computed tomography (micro CT) is just one option for the inspection of metal AM parts. Other options include using eddy current, ultrasonic technology, white-light interferometry and non-interferometric optics. However, given recent developments, it is micro CT that has the most potential in view of its unique capability for inspecting internal structures and geometries without destroying the part. The capabilities of this inspection method are presented by Andrew Ramsey and Herminso Villarraga-Gomez of Nikon Metrology Inc.

X-ray computed tomography (micro CT) is the only method able to inspect properly (with measurement strategies from coordinate dimensional metrology) volume defects and complex geometry inside a volume. Eddy current can only inspect local defects near the surface of the part, while ultrasonics can inspect only simple geometries near the surface with some reach inside the volume. Optical and interferometric methods can only inspect features at the surface of the part. While the latter (interferometric) techniques are very good at achieving higher resolutions (up to a few nm), the X-ray CT technique can cover, in a single scan, external and internal surfaces, with micrometre-level resolution and, in some cases, at higher resolutions below the micrometer level (on the order of a few hundred nanometers). This all comes at a time when interest in Additive Manufacturing is redefining the manufacturing landscape. Consulting firm IDC states that global spending on AM equipment, both desktop and industrial, reached about $11 billion in 2015 and is forecast to reach $27 billion by

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2019. Another company, MarketsandMarkets, is predicting that Additive Manufacturing will experience 30% compound annual growth and reach $30 billion by 2022. In its April 2016 study, “3D Printing Comes of Age in US Industrial Manufacturing,” Price

Waterhouse Coopers (PWC) stated that, compared to two years ago, more manufacturers (52% this year compared to 38% in 2014) expect Additive Manufacturing to be used in high-volume production in the next three to five years.

Fig. 1 A Nikon Metrology XT H225 ST X-ray CT system

Metal Additive Manufacturing | Summer 2017

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Fig. 2 Scans can reveal external and internal features for checking

Metal AM parts are increasingly being considered for the reduction of component weight without compromising strength, for example in aerospace applications where decreased weight leads to increased efficiency. For such safety-critical aerospace components, as well as applications in automotive, energy and medical devices, it is essential to know whether voids or inclusions

are present, how large they are (both individually and in total) and where they occur. Additionally, it is critical to know whether the dimensions of the part conform to those of the design. In such cases, X-ray computed tomography is a powerful answer. By supplying a full 3D density map of a sample, micro CT gives all this information in an easy-to-read visual format.

Welds need to be inspected, so why not AM parts too? Using conventional manufacturing processes, one would always inspect a weld for voids and inclusions. In metal Additive Manufacturing, the whole sample is essentially one large weld, so not to inspect it for voids, inclusions and for dimensional accuracy would be a huge leap of faith. Because of the complex nature of metal Additive Manufacturing processes such as Powder Bed Fusion, where for example there is the risk of risk of loose, partially melted powder in the build chamber, the position and nature of defects is often totally random. With traditional manufacturing processes, a few radiographs at specific orientations can often give peace of mind. However, with layer-based Additive Manufacturing processes the whole part needs to be inspected. When checking the structural integrity of these parts, it is primarily the following issues that are of concern: • Powder residues blocking channels

Fig. 3 Blocked channels are revealed without destroying the part [1]

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• Defects (voids and inclusions) – porosity, contamination, cracking

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Part to CAD comparison for AM1 and AM2 Variance [mm]

0.25

Internal spatial deformations in the flexures Z = -18.89 mm

0.20 0.15

AM1

0.10 0.05

0.00

CAD

-0.05 -0.10

AM2

-0.15 -0.20 -0.25

AM1

AM2 100 mm

Fig. 4 Left: part-to-CAD comparison for FDM and STL versions of the same part, right: Flexure deformations revealed by CT scans • Departure from the CAD model – dimensional analysis, wall thickness measurements, warping. As an example, a mould made by Selective Laser Melting was designed to make a small knocker for a watch mechanism. Micro CT could determine the cooling and flow channels built in by the metal AM process to an accuracy of 5-10 µm, depending on acquisition parameters. From flow and cooling simulations, this is known to be of sufficient accuracy for the purpose. In fact, micro-CT can find defects within samples down to a resolution given by the number of pixels across the detector. Given a sample 100 mm across and a detector 2000 px across, the limiting resolution would be 50 µm. Resolution is also limited by the focal spot size of the X-ray source, which may range from 80 µm for high energies down to less than 1 µm for low energies. Defects below the nominal resolution may also be spotted if the contrast with the surrounding material is great enough. For example, given a 3 µm X-ray focal spot, we can still see a 0.5 µm gold foil edge-on. The size of sample which can be scanned with CT depends on the

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material it is made from and the energy of the X-ray source, measured in kilovolts (kV). Larger, lower density samples can be scanned, as can smaller, higher density samples. Typical largest samples are: • 225 kV – aluminium piston heads; diesel injectors • 450 kV – aluminium cylinder heads; aircraft turbine blades Maximum part size also tends to be limited by the size of the detector, but also by the penetrating power of the X-rays. This decreases as material density and atomic number increases. Much greater thicknesses of polymeric materials can be penetrated than steel, and much more steel than tungsten. CT scanning case study An investigation into the X-ray CT scanning methods for a flexure mechanism fabricated by Additive Manufacturing offers insight into the capabilities of the process [2]. Whilst this research was in relation to polymeric products, the principles and benefits apply equally to metal AM applications. A first flexure sample (AM1) was manufactured by Fused Deposition Modelling (FDM). A second sample (AM2) was manufactured by

Stereolithography (STL). For reference, the FDM and STL processes have, in general, printing resolution of approximately 100 μm and 0.5 μm, respectively. Micro CT scans show variance analysis for both AM1 and AM2 flexures against the original CAD model (Fig. 4, left). From the measurements, it can be seen that deviations from the nominal geometry (CAD model) rise up to ±0.25 mm and larger when using the AM1 process. In contrast, the AM2 process generated partto-CAD deviations mostly between ±0.1 mm, with a few exceptions, particularly around surface edges or corners. In addition to external checks, a cross section of AM1 shows residual internal and spatial deformations (Fig. 4, right). On the other hand, the manufactured part generated by the AM2 process does not reveal the presence of major deformations in the thin-walled flexure leaf structures.

Rules of micro CT and when to break them High-accuracy X-ray micro CT technology has continued to evolve over the past ten years. Applications are diverse and growing across the automotive, aerospace, energy,


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Fig. 5 CT scan of a Nikon SLR camera demonstrating what can be achieved with the technology

medical and consumer sectors, dealing with metals and exotic alloys as well as plastics and other workpiece materials. Accompanying software tools enable the analysis of part volume against the CAD model, either via direct volume-to-CAD comparisons, or through geometric dimensioning and tolerance measurements. With costs now low enough to make it competitive with other techniques, X-ray micro CT can now be considered for application in many broader metrology applications. A better understanding of the rules of X-ray micro CT not only opens the door to production cost savings and productivity improvement, but knowing when to break them can provide even further process flexibility. The rules for good X-ray micro CT are as follows: 1. Penetrate the sample from all angles 2. Minimise noise in each projection image 3. Use filters to reduce beam hardening

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4. Always use 360° rotation 5. Use the detector’s full dynamic range 6. Keep the object in the field of view X-ray basics X-rays are at the short end of the electromagnetic spectrum with an average wavelength between 10-8 and 10-12 metres, around the size of water molecules, compared to radio waves whose wavelengths could span a soccer field. There are no radioactive sources in X-ray micro CT; rather electrons are produced from a hot filament similar to a light bulb and accelerated at high voltage, reaching speeds of roughly 80% of the speed of light. They are fired at a metal target through a magnetic lens that focuses the beam energy into a spot between 1-5 µm in diameter. The sudden deceleration of the charged electrons when they hit the metal target produces more than 99% heat and less than 1% X-rays.


When the electrons hit the target, X-rays are created by two different atomic processes: 1. With enough energy, the electron can knock an orbital electron out of the inner electron shell of a metal atom. As a result, electrons from higher energy levels fill up the vacancy and X-ray photons are emitted. This process produces an emission spectrum of X-rays at a few discrete frequencies, sometimes referred to as characteristic emission lines 2. Bremstrahlung (decelerating or “braking” radiation in German): This is radiation given off by the electrons as they are scattered by the strong electric field near the high-Z (proton number) nuclei. These X-rays have a continuous spectrum. The intensity of the X-rays increases with decreasing frequency X-rays travel in straight lines through the object being inspected

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Fig. 6 CT image by Herminso Villarraga-Gómez demonstrating the capabilities of micro CT technology. From left to right: an Edison-style incandescent bulb, a fluorescent light bulb and an LED bulb

and onto a detector. The object will absorb some of the X-rays (denser objects absorbing more), leaving only a portion to reach the detector. At low X-ray energies (