Vol. 13, No. 1

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FYIVol. 20, No. 1

Additive manufacturing (AM) offers new possibilities in manufacturing and designing products. The aerospace and automotive industries are main drivers because of the possibility of manufacturing lighter structures that reduce weight and save fuel.

During the AM process, different discontinuities or defects can occur, depending on the applied technology. To ensure constant manufacturing quality of the parts, regular sampling or 100% inspection using nondestructive testing (NDT) methods is required. In particular, computed tomography (CT) allows a contactless investigation and includes different analysis techniques. As an advantage over other techniques, it can even evaluate parts that have a very complex inner structure.

Part 1 of this article, which appeared in The NDT Technician, Vol. 19, No. 4 (Schulenburg and Herold 2020), provided an overview of the AM technology, particularly selective laser melting (SLM), and commonly occurring discontinuities along with their possible causes. In

Part 2, we will provide an overview of the CT inspection setup and process, along with a case study showing some examples of the types of discontinuities that are found in AM parts.

CT Analysis of AM ProductsWith the rapid growth of AM technology, some evaluation criteria for processing techniques, processing parameter optimization, quality control, and detection of possible discontinuities should be developed (Kruth 1991). CT, with its nondestructive nature, is one of the more promising techniques for the inspection of AM parts.

The following list provides the most relevant analysis techniques that are based on CT:l Porosity analysis: with this tool, the

software detects irregularities because of a deviation of the gray values in one region compared with the local area. Pores with different sizes can be marked in different colors (as shown in Figure 1) and statistically analyzed

Inspection of Additive Manufactured Parts Using Computed Tomography: Part 2by Lennart Schulenburg and Frank Herold

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WRITE FOR TNT!Are you interested in writing for The NDT Technician (TNT)? Content for TNT is focused for NDT practitioners engaged in field applications of NDT. Typical themes include interpretation skills, methodology, problem-solving procedures for everyday challenges, practical application of NDT with data and results, and technology trends. Contributors to TNT earn three ASNT renewal points per published paper (min. 1000 words). If you have a topic you’d like to see published in TNT, contact the editor:Jill Ross, Periodicals Editor1-614-384-2484; email jross@asnt.org

Figure 2. Residual field inside a bar generated by: (a) slow break; (b) quick break transient current.

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FYI | Inspection of AM Parts: Part 2

according to the diameter, volume, surface, frequency distribution, or probability.

l Dimensional measuring: different measurement tools allow a verification of distances and angles. This is very useful if it is necessary to check values on inboard structures (Figure 2).

l Nominal actual comparison: in this analysis, the computer-aided design (CAD) model geometry of the product is compared with the result of the 3D volume obtained by CT. The deviation can be pictured in different colors or scales, as shown in Figure 3.

l Wall thickness analysis: this is a way to analyze the different material thicknesses of the part with a colorful scale, as shown in Figure 4.

Limits of the TechnologyCT is an indirect test procedure, and therefore measurements (for example, of the size of material faults) of wall thicknesses must be compared with another absolute measurement procedure. Another potential drawback of CT imaging is the possible occurrence of artifacts in the data. Artifacts limit the ability to quantitatively extract information from an image. Therefore, as with any examination technique, the user must be able to recognize and discount common artifacts subjectively (ISO 2017).

Like any imaging system, CT can never reproduce an exact image of the scanned object. The accuracy of the CT image is dictated largely by the competing influences of the imaging system, namely spatial resolution, statistical noise, and artifacts.

Another important consideration is to have sufficient X-ray transmission through the sample at all projection angles without saturating any part of the detector (ISO 2017).

In order to reach a high-resolution CT scan, preferably with small voxels (a voxel is the 3D equivalent of a pixel), one should always pay attention to the scanning time and the file size. A reduced scanning time will be detrimental to the scanning quality, but more projections mean an increased memory requirement, particularly with a high-resolution detector with small pixels.

Case StudyA typical defect, as described in Part 1 (Schulenberg and Herold 2020), is the lack of fusion or non-adequately heated powder in one or more layers in the SLM process. This can occur because of a wrong parameterization of the printing machine or fluctuations in laser output. These defects are not visible from the outside after printing, because the laser first drives a contour path before melting the volume. These are two different parameter sets. In this case, only the parameter set of the volume path was manipulated. It is not known what size the defects will be;

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Figure 1. Porosity analysis: (a) cross-sectional plane with and without the marked pores; (b) a transparent 3D volume of the AM part with colored pores.

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Figure 2. Example of distance and angle measurements in a CT scan.

FROM THE EDITORInterested in becoming a contributor or reviewer?Are you interested in becoming a contributor or reviewer for The NDT Technician (TNT)? Content for TNT is focused for NDT practitioners engaged in field applications of NDT. Typical themes include interpretation skills, methodology, problem-solving procedures for everyday challenges, practical application of NDT with data and results, and technology trends. Contributors to TNT earn three ASNT renewal points per published paper (min. 1000 words). Development and/or technical review of a feature article earns one renewal point. If you have a topic you’d like to see published in TNT, contact the editor:

Toni Kervina, TNT Editor(800) 222-2768 X205; fax (614) 274-6899; e-mail tkervina@asnt.org

TNT · January 2021 · 3

that is why a test with a high-resolution CT system was used to detect these kinds of defects.

The Günther-Köhler Institute for Joining Technology and Materials Testing in Jena, Germany, produced two test samples made of AlSi10Mg powder with their SLM laser beam melting system and the

standard parameter set for aluminum with 325 W laser output. For one of these two samples, the laser output was reduced to 225 W over four layers to create an area of manufactured defects. The other sample was used as a reference. For the CT scanning, a CT microfocus tube and a digital detector array (DDA) with 100 µm

pixel size was used. For the CT scan, the following parameters were used: 70 kV, 180 µA, magnification (M) = 13, and 1000 projections with eight integrations. The defect area was already visible in the 2D projections, as shown in Figure 5.

With the chosen setup and parameters, we reached a voxel size of 16 µm. Slices

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of the CT scans are shown in Figure 6. The manipulated section can be separated from the surrounding material on the side view with the naked eye. The top view shows that there are some pores in the reference sample as well, but they are smaller in number and size compared to the manipulated part.

To quantify this, we applied a porosity analysis in similar-sized regions of both samples. The software marked the detected pores, depending on the pore volume, in a color scale. The 3D view shows the shape, extension, and position of the pores inside of the volume, as shown in Figure 7. With this analysis technique, it is possible to receive statistical data about the pore size and amount.

The histogram in Figure 8 shows a count depending on the diameter of the detected pores in both samples. The reference shows an accumulation of pores with a diameter range of 0.05 to 0.3 mm and two pores at 0.4 and 0.6 mm. The manipulated sample shows a wider range with a concentration between 0.05 and 0.6 mm in diameter and an increase of the number of pores in total. As well, there are a few pores with a higher diameter up to 2.1 mm. Thus, the range of diameters and the number of pores are correlated with different process parameters. The exact correlation will be derived with a wider range of samples in future studies.

This result shows that it is feasible to use CT for detecting incorrect process parameters for AM that can lead to material failure using the porosity analysis.

ConclusionAM technology is growing, but there are some limitations, including the standardization of the process, qualification of the parts, and optimization of the manufacturing process. The increased motivation to use AM necessitates the development of qualified inspection techniques. CT offers a wide range of

Figure 5. Use of manipulated parameters: (a) 2D image reference sample without manipulation; (b) 2D image of manipulated sample with visible area of pores.

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Figure 6. Top and side view slice of 3D volume: (a) reference sample; (b) manipulated sample.

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FYI | Inspection of AM Parts: Part 2

TNT · January 2021 · 5

analysis techniques to evaluate such complex structures depending on the application.

In many places, different institutes and organizations are working on standardization for AM products and their qualification. This is needed for an orientation regarding the right quality control system, but it will be difficult to develop one standard for so many different AM technologies and possible discontinuities. h

AUTHORSLennart Schulenburg: VisiConsult X-ray Systems & Solutions GmbH, Stockelsdorf, Schleswig-Holstein, Germany; l.schulenburg@visiconsult.de

Frank Herold: VisiConsult X-ray Systems & Solutions GmbH, Stockelsdorf, Schleswig-Holstein, Germany

REFERENCESISO, 2017, EN ISO 15708-2: Non-destructive testing – Radiation methods for computed tomography – Part 2: Principles, Equipment and Samples, International Organization for Standardization, Geneva, Switzerland.

Kruth, J.P., 1991, “Material Incress Manufacturing by Rapid Prototyping Techniques,” CIRP Annals, Vol. 40, No. 2, pp. 603–614.

Schulenberg, L., and F. Herold, 2020, “Inspection of Additive Manufactured Parts Using Computed Tomography: Part 1,” The NDT Technician, Vol. 19, No. 4, pp. 1–5.

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OPINION | Nondestructive Testing Exhibit

6 · Vol. 20, No. 1

IntroductionUT technology has evolved over the past decades to propose equipment with advanced capabilities in terms of productivity and detectability. PAUT technology has revolutionized the way technicians perform their inspections; from power generation to construction, the technology helps professionals assess component quality and identify risk factors quicker and more efficiently. The technology is now widely used and is described in various international standards, such as American Society of Mechanical Engineers (ASME) BPVC Section V and International Organization for Standardization (ISO) standards.

Essentially, PAUT replaces single-element probes with probes composed of multiple elements that can be controlled electronically, independently. By applying what we call delay laws to the various elements of the probe, it is possible to steer, scan, and focus the ultrasonic energy into the test piece without moving the probe. At the end of the process, technicians are offered linear or sectorial images very similar to the echography images we are used to seeing in the medical world. The familiarity with this type of imaging makes it easier for technicians to interpret data and provide an analysis of the component they are inspecting.

After this technology step, the nondestructive testing industry has been looking for the next development. One of the technologies that has been recently introduced is TFM. Very much like optical imaging, which uses a lens to focus light, TFM uses the full aperture of a phased array probe to focus the energy at a small point inside a test piece. And very much like the optical lens, focused ultrasound offers a higher spatial resolution, which allows for seeing smaller indications.

Figure 1a shows an optical lens that focuses rays of light to a focal point, and Figure 1b shows acoustic focusing using TFM at two different locations. While an optical lens can focus light at only one depth, TFM can dynamically focus the acoustic energy anywhere underneath the probe without moving it. This means that it is possible to obtain a cross-sectional image of the component with a very high spatial resolution. As an example, the size of the focal point in Figure 1 is 0.3 mm using a 10 MHz probe. This small sensing point gives technicians the ability to detect really small indications.

Data Acquisition: Full Matrix CaptureWhile this is done in real time with today’s equipment, TFM is a two-step process composed of data acquisition and data reconstruction (Holmes et al. 2004). There are many ways

Challenges and Benefits of the Total Focusing Methodby Frederic Reverdy and Casper Wassink

The total focusing method, or TFM, is an ultrasonic testing (UT) technique that has been gaining some traction over the past few years, and recently was introduced into the ASME Boiler and Pressure Vessel Code (BPVC). Portable phased array systems with real-time TFM capabilities have been available for a couple of years, making the application of the technique in the field a reality. TFM offers better spatial resolution, allowing better detection of small indications (for example, high-temperature hydrogen attack [HTHA]) and better sensitivity, allowing better sizing thanks to its ability to make tip diffraction echoes more sensitive. Furthermore, TFM is easier to set up than phased array ultrasonic testing (PAUT). This article explains the challenges and benefits of using this technique.

FOCUS

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PAUT probe PAUT probe

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Figure 1. Illustration of TFM: (a) light focalization; and (b) beam focus for two pixels using TFM.

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Figure 2. Wave propagation generated by the FMC data acquisition: (a) wave generation; (b) wave reflection.

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FOCUS | Total Focusing Method

of acquiring the data to perform TFM, but in this article we will focus only on the most common one, which is called full matrix capture (FMC). FMC is a data acquisition process for which each element of a phased array probe is fired individually. This generates divergent or spherical waves inside the test piece very similar to the wave that occurs on the surface of water when someone throws a stone in a pond. This is illustrated in Figure 2a. A PAUT probe with several elements (indicated by the small gray rectangles) sits on top of a sample with two discontinuities (orange circles). One element (indicated in red) is fired individually, which generates the divergent incident wave. We can see that this wave is spherical, sending sound in all directions.

Then, the wave interacts with the two discontinuities and some energy is reflected and travels back to the elements of the probe. The elements are now acting as receivers (indicated with blue rectangles) listening for the reflected acoustic energy, and they all record a signal.

The process is then repeated for the other elements of the probe, each of them firing individually with all of them listening. At the end of the FMC process, the system has a huge database of signals (every transmitter-receiver combination possible), all with a little bit

of information about the indications located inside the test piece. The number of signals contained in the FMC is proportional to the number of elements in the PAUT probe. For example, a 64-element probe will generate an FMC database of 4096 signals.

Data Reconstruction: Total Focusing MethodThe second step is the reconstruction one; in other words, the TFM. The idea now is to use all the information recorded during the FMC process to detect and locate potential indications. While PAUT typically displays data with a linear or sectorial view, TFM shows them within a rectangular area called the region of interest (ROI). This ROI is typically an image composed of several pixels. The TFM process uses a delay and sum algorithm and applies it to all the signals stored into the FMC database to focus the acoustic energy at every single pixel of that image. Essentially, what we obtain are the acoustic beams displayed in Figure 1, but for all the pixels in the image.

Figure 3 shows the type of results we can obtain using TFM (Figure 3c) compared to the traditional linear and sectorial scans (Figures 3a and 3b). For all cases, we use a 64-element PAUT probe and a mockup that contains multiple side-drilled holes.

TNT · January 2021 · 9

It is easy to see that TFM offers a much better resolution compared to the linear and sectorial scans. The echoes reflected from the holes are all clearly visible and have a similar size and similar amplitude. This is due to the fact that TFM focuses the energy everywhere, leading to a similar focal spot for every single pixel of the image.

While we can distinguish the two “Ms” and the number “2” with the linear scan, the spatial resolution is quite poor, especially for the holes located on the bottom where the beam spread is the most important. This is due to the fact that we are using a much smaller aperture (eight elements in this case).

For the sectorial scan, we focused the energy along the holes located in the middle using the full 64-element aperture. We can see that for these holes, the spatial resolution is identical to the one observed in the TFM image, which is normal as we used the same

aperture. However, for the other holes the resolution is quite poor, since we are outside of the focusing area.

Figure 4 shows flat-bottom holes (FBHs) along the backwall of a sample. The linear scan with the eight-element aperture (Figure 4a) is compared to TFM (Figure 4b). Figure 4b shows a clear detection of the four FBHs with TFM including the inclined one on the left. The linear scan is too narrow to contain this FBH. Also, due to beam spread, the FBHs are quite large and the one closest to the surface is barely detected. Thanks to its high focusing capability, TFM offers clear detection of the FBHs, even the one close to the backwall.

We can see that the ROI of TFM covers a larger inspection area than the linear scan. This, combined with the divergent beam generated by each element, allows the detection of the tilted FBH, even though it is not underneath the probe.

Figure 3. Example scans: (a) linear scan using PAUT; (b) sectorial scan using PAUT; (c) TFM.

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Figure 4. A mockup with several flat-bottom holes along the backwall: (a) linear scan; (b) TFM.

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Figure 6. TFM image obtained from a sample containing high-temperature hydrogen attack (HTHA) damage.

Application CaseTFM is already being used in various industries (oil and gas, power generation, aerospace, and more) for a variety of applications. In the following example, we will show how TFM was applied to the detection of HTHA.

HTHA is a form of damage in which hydrogen atoms diffuse into steels, react with carbon, and form methane gas, which results in microcracking. The macrography in Figure 5 shows microcracks as small as a few dozen micrometers, which is much smaller than

the wavelength. It is thus fundamental to use a technique that can focus the acoustic energy to a tiny focal point to maximize the sensitivity of detection.

Figure 6 shows the result obtained using a 10 MHz probe with TFM. The small echoes indicated by the dotted white circle are indicative of microcracks similar to the ones seen on the macrography in Figure 5. As argued by Lozev and others (2017), FMC/TFM may be used to better image small cracks and fissures resulting from HTHA in stages 2 and 3.

FOCUS | Total Focusing Method

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ConclusionTFM is now described by international standards such as ASME, and training schools are proposing courses to cover the basic theory and use of equipment. Most of the standardization tools, such as material velocity, element check, wedge delay, and time corrected gain, remain the same as the ones used with PAUT, ensuring a smooth transition for operators who would like to move from PAUT to TFM. h

AUTHORSFrederic Reverdy: Eddyfi Technologies, 1 rue de Terre Neuve, 91940 Les Ulis, France; freverdy@eddyfi.com

Casper Wassink: Eddyfi Technologies, 1 rue de Terre Neuve, 91940 Les Ulis, France

REFERENCESHolmes, C., B. Drinkwater, and P. Wilcox, 2004, “The Post-Processing of Ultrasonic Array Data Using the Total Focusing Method,” Insight: Non-destructive Testing and Condition Monitoring, Vol. 46, No. 11, pp. 677–680, https://doi.org/10.1784 /insi.46.11.677.52285.

Lozev, M.G., G.A. Neau, L. Yu, T.J. Eason, S.E. Orwig, R.C. Collins, P.K. Mammen, F. Reverdy, S. Lonne, C. Wassink, H. Cence, and J. Chew, 2020, “Assessment of High-Temperature Hydrogen Attack Using Advanced Ultrasonic Array Techniques,” Materials Evaluation, Vol. 78, No. 11, pp. 1223–1238, https://doi.org/10.32548/2020.me-04183.

CorrectionIn the article titled “Origin of AWS D1.1/D1.5 Indication Rating UT Technique” by Gary S. Martin (October 2020 TNT ), the 0.060 in. IIW side-drilled hole size was convert-ed to 0.15 mm. The correct conversion is 0.15 cm. The article has been corrected online and can be down-loaded via The NDT Technician webpage (asnt.org) or the NDT Library (ndtlibrary.asnt.org).

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Catherine Stravino, known as Cat, is currently a Level II in fluorescent penetrant testing (PT) as certified by her employer Pratt & Whitney in Middletown, Connecticut. She entered the NDT field a few years ago, after a major life change prompted her to find a new career.

Q. How did you first become involved in NDT? A. In 2016, I experienced a major disruption in my life, which

pushed me to find a career. Prior to that, I had been a homemaker raising four children. I was living in a large house I could not afford, so I started redoing it by hanging out at Home Depot and questioning the workers and other customers. I learned to replace everything from outlets to toilets with their tutelage and YouTube. While rehabbing the house, I would send “before” and “after” photos to my sister in Minnesota. We both realized I found satisfaction from working with my hands. She knew of the NDT program at Ridgewater College in Hutchinson, Minnesota, and suggested I look into it. I had never heard of such a thing! But when I visited, I found it fascinating. I applied, sold nearly everything from my life in Connecticut, and started at Ridgewater in May 2017.

Q. Can you tell us about your certification and training?A. I received my associate of applied science degree in NDT

from Ridgewater College in 2019. I also have a bachelor’s degree in English and a master of fine arts in writing, but I earned about one quarter of what I now make with my associate’s degree. As an internship during the summer of 2018, I had the opportunity to examine pipeline on cattle ranches near the Gulf of Mexico, thanks to Acuren in Conroe, Texas. I did a lot of magnetic penetrant and spray-painted many feet of 36-in. pipe in the Texas sun. So many people said, “Who wants to live in Minnesota in the winter and Texas in the summer? You are doing it backwards.” This could sum up my education as well, since I earned by master’s degree before my associate’s degree!

Q. What is your work environment like? A. I work second shift and am often running the line by myself.

Fortunately, I have the support of men with experience who work first shift. One of my colleagues takes my texts night and day whenever I have a question. This is invaluable. So much of our work requires the handing down of tribal knowledge that people have learned with experience. As a newbie, I need that. As a team, we all benefit when it is shared.

Q. What is your involvement with ASNT? A. I have had really great experiences through ASNT; attending

the conferences as a student allowed me to meet lots of people in our field. My local section in Minnesota exposed me to great experiences, such as a behind-the-scenes tour of an amusement park. Most of all, through ASNT I have been able to meet other women who have made stellar careers in NDT. I currently help administer the Women in NDT page on Facebook and LinkedIn. Everyone needs to be on those!

Cat Stravino Practitioner ProfilePractitioner Profile

TNT · January 2021 · 15

Q. Have you ever had a mentor? A. Before landing the job at P&W, I worked at the Caterpillar large

engine facility in Lafayette, Indiana, and met a woman who had graduated from Ridgewater College 30 years prior to my start. She was and is an excellent mentor. Although she is now considered a retired Level III, I am finding there is no such thing! She just has tons of flexibility in her work life.

Q. What areas of NDT would you like to learn more about? A. Who knows where this profession will take me? It’s a constantly

changing adventure, one I never knew existed just a few years ago!

Cat Stravino can be reached at catstrav@yahoo.com. On LinkedIn, search for “Women in NDT (WIN)” and on Facebook, search for “Women in NDT.”

Read Stravino’s post titled “COVID-19 and the Working Technician” on ASNT’s new blog, ASNT Pulse!

For all the latest industry news along with first-person perspectives, visit blog.asnt.org.

Two “Cats” working on the Texas pipeline.

Volume 20, Number 1 January 2021

Publisher: Neal J. Couture, CAEDirector of Publications: Toni KervinaEditor: Jill RossAssistant Editor: Cara MarklandGraphic Designer: Synthia JesterTechnical Editor: Raymond G. MorasseReview Board: Phillip Charles Berry, Bruce G. Crouse, Anthony J. Gatti, Sr., Edward E. Hall, William A. Jensen, Edward Kang, Jocelyn Langlois, Ricky L. Morgan, Ronald T.Nisbet, Ross Russo, Richard Saxby, Angela SwedlundThe NDT Technician: A Quarterly Publication for the NDT Practitioner (ISSN 1537-5919) is published quarterly by the American Society for Nondestructive Testing Inc. The TNT mission is to provide information valuable to NDT practitioners and a platform for discussion of issues relevant to their profession. ASNT exists to create a safer world by advancing scientific, engineering, and technical knowledge in the field of nondestructive testing.Copyright© 2021 by the American Society for Nondestructive Testing Inc. ASNT is not responsible for the authenticity or accuracy of information herein. Published opinions and statements do not necessarily reflect the opinion of ASNT. Products or services that are advertised or mentioned do not carry the endorsement or recommendation of ASNT.IRRSP, NDT Handbook, The NDT Technician, and asnt.org are trademarks of the American Society for Nondestructive Testing Inc. ACCP, ASNT, ASNT Daily, Chat NDT with ASNT, Level III Study Guide, Materials Evaluation, Nondestructive Testing Handbook, Research in Nondestructive Evaluation, and RNDE are registered trademarks of the American Society for Nondestructive Testing Inc.

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