Non-Destructive Testing of Additively Manufactured Parts

Sebastian Preintner, summer semester 2020

Additive Manufacturing (AM) is a manufacturing process in which components are produced by applying material (e.g. aluminium alloy, titanium alloy, PA12, etc.) layer by layer. Due to this process sequence, components can be produced which are unprofitable or impossible to manufacture by subtractive machining.^[1] This makes it possible to optimize the component geometry with regard to the forces that occur (topology optimization) or the available installation space (generative design).^[2] Furthermore, assemblies consisting of several components can be combined in one component by integrating their functions. This results in a lightweight construction potential.

However, this geometric complexity makes quality assurance more difficult, which leads to problems in safety-critical industries.^[3] Due to this and the usually high component costs the non-destructive testing of additively manufactured parts gains an important role.

Selective Laser Melting (SLM)

The powder bed fusion is a complex multilayer micro welding process. The material to be processed is applied in powder form in a thin layer on a base plate. The powder material (10 to 45 μm) is partially remelted by laser radiation and forms a solid material layer after solidification. The base plate is then lowered by the amount of one layer thickness (20 to 150 μm) and powder is applied again. This cycle is repeated until all layers are remelted.^[4]

During melting of the powder and solidification, high temperature gradients occur locally. These can lead to elastic and platic deformations around the heat-affected zone, resulting in residual stresses. This can lead to delamination, distortion, cracks and geometric deviations.

Due to too high scanning speed or too low laserpower, non-welded areas may occur in the component. If the scanning speed is too low or the laser power is too high, the powder may evaporate. Pores are formed.

Overall AM is vulnerable to process variation which makes non destructive testing necessary.

Possible defects are:^[5]

• Delamination
• Distortion
• Cracks
• Geometric deviations
• Internal / external pores

Offline measurement

After the job is finished and has been freed from support and powder, it can be checked for defects. Offline measurement is characterized by the fact that it is mostly applied manually and not in the production line. It represents a further step in the production process. Offline measurement can be done destructively or non destructively.

Because SLM can be seen as a micro welding process, the non destructive testing methods are partly similar to methods used for classic welding components or castings.^[6]

Penetrant Testing (PT)

Penetrant Testing is a common, simple and low cost examination method to check surface-braking defects. It is used to detect defects such as hairline cracks, surface porosity and fatigue cracks. The inspection steps are divided into surface preparation, penetrant application, exess penetrant, developer application, post-cleaning and inspection.

One of the largest issues of this testing method regarding to additive manufacturing is the high surface roughness of AM parts, which can lead to a high backround noise. Many techniques applied to reduce surface roughness, e.g. sand blasting, close surface flaws and cracks. This can lead to the fact that the penetrant can no longer enter the flaw and a crack can therefore no longer be detected. Due to procedure an inspection of internal structures, which are an advantage of AM, is impossible. Nevertheless, this method can be combined with a visual inspection to identify cracks or high porosity on the surface ^[7]. Similar to castings, functional surfaces must be subtractive machined due to surface roughness. PT can be used to examine these surfaces for defects.

Magnetic particle inspection (MT)

Magnetic particle inspection is a simple, fast and low cost examination method to detect surface braking flaws in ferromagnetic materials such as iron, nickel and cobalt. The sample is magnetised and uses the effect of a magnetic field, which field lines run within the workpiece and parallel to the surface. The material generates a magnetic flux on the area of the discontinuity on the surface. To identify the defect area, fine ferromagnetic particles are applied to the surface which will be attracted to the flux leakage.

This testing method is often used in NDT of cast parts which suggests applicability to AM parts. Compared to Penetrant testing this method can be used to detect defects without a opening to the surface as long as its not too far from the surface. However, the depth of a crack can not be deteremined. Another limitation is the fact that it is a surface-sensitive technique that gives no information about the internal features of the AM part.^[8] Similar to PT, subtractive machined functional surfaces can be examined using MT.

Ultrasonic Testing (UT)

Ultrasonic testing is based on the propagation of ultrasonic waves which are coupled into a sample to detect internal flaws or to characterize the material like the elastic modulus. Ultrasonic testing enables the detection of cracks and inclusions. Regarding to additive manufactured parts ultrasonic testing is limited due to the high surface roughness of AM parts that masks defects close to the surface. Another issue is the complex geometry of AM parts that can include air gaps, “microstructure” scattering or changing density which can be achieved by AM, that complicates inspection.^[9]

Active thermography

Active thermography uses an external heat source to stimulate non stationary heat diffusion inside the test sample for inspection. Lock-in Thermography, Pulsed and Stepped Thermography are the most common thermography based NDT methods used for defect detection. They differ in the technique used for heating the material. The thermal excitation can be done as follows:^[10]

• Flash lamps or Laser radiation
• Ultrasonic
• Inductive heating

A defect can be revealed by monitoring the surface temperature decay of the sample. Due to local change of heat flow, because of deviating thermal conductivity, the defect appears as an area of different temperature with respect to a reference area. Cracks, regions with lack of fusion and buried regions with keyhole defects can be detected. It has been shown that the detectable dimension range of these defects is up to 0,25 mm in depths of 0,4 mm below the surface.^[11] Thermography offers the advantage, compared to ultrasonic testing and eddy current testing, of being relatively rapid. This method is also less sensitive to the high surface roughness of AM parts.^[12] The limitation of this NDT method regarding to AM is the fact that only near-surface defects can be detected. Internal or hidden structures can not be investigated. Therefore it is not suitable for safety critical applications.

Process Compensated Resonance Testing (PCRT)

Process compensated resonance testing (PCRT) combines the advantages of the Resonant Ultrasound Spectroscopy (RUS) with the Vibration Pattern Recognition. It is based on the analysis of the resonant vibration pattern of each part produced in serial production, which allows an evaluation of function and damage severity. The PCRT system requires a tool to measure, record and analyze the resonant vibration frequencies in a sample with adequate accuracy to reflect the structurally significant characteristics of that sample. To gather information the PCRT system requires a sample set consisting of enough parts to enable an evaluation of similarities and differences among the parts. Each part of the sample set must be classified as acceptable or unacceptable. A full range of acceptable process variations and unacceptable process variations which can lead to structural failure is key for the success of the system. The difference to other NDT methods based on resonance is the fact that PCRT can compensate process variations due to the production process which allows a precise detection of defects and flaws.^[13]

Regarding to AM the system can be taught to detect printed samples with specific defect types like lack of fusion, cracks or porosity. PCRT combines internal and external inspection in one NDT method. PCRT is currently not able to localize, size or characterize defects. Nor can it detect cosmetic defects. It can be used to detect and segregate parts within a job that show an unacceptable level of porosity, lack of fusion or cracking. This NDT method allows a precise, quick and fully automated quality inspection of AM parts. For an accurate measurement the components should be removed from the built plate. Subsequent measurements can take from 30 seconds to three minutes per part.^[14] A limitation of PCRT regarding AM is the batch size of structural and geometric identical components. To be able to make an accurate statement, the system requires a certain number of identical components. Some of them have to be deliberately faulty and have to be destroyed or checked by a CT. Therefor PCRT is most effective in applications where multiple, ongoing builds produce parts in a range from hundreds to thousands annually. ^[14] In some applications of AM, however, these numbers are not reached.

X-ray Computed Tomography (XCT)

Computed Tomography (CT) is a procedure based on X-rays. It is defined as an „imaging method in which the object is irradiated from different directions and mathematical algorithms are used to determine the distribution of the specific material properties of the tomographic object in the determined volume.“^[15] An detailed article about Computed Tomography can be found here.

Additive manufactured parts have a high surface roughness due to the process. In addition, internal structures such as cooling channels can be realized. These characteristics of AM parts make it difficult for conventional non destructive testing (NDT) methods to achieve a comprehensive quality check on AM parts. X-ray CT is adressing to most of the quality control needs of AM industry and is used as a tool for AM quality check. X-ray CT can be used for structural characterization of internal and external geometries. Therefor it is the only currently NDT method to extract component dimensions of internal features. X-ray CT is mostly used to dedect cracks and porosity, internal powder residues or inclusions inside the AM part and dimensional deviations from CAD models.

Defects that can be local-based detected by X-ray CT:^[16]

• Cracks
• Internal / external pores
• Geometric deviations
• Internal powder residues

The potential of defect detection by X-ray CT depends on material, part size and complexity of the AM part. In general X-ray CT enables high scanning metrological resolutions in the order of 5 - 10 μm. ^[16] X-ray CT is the most expensive NDT method for AM.

Inline measurement

The discontinuous or sequential AM process flow can be used to check the quality inline after each or during each process step. In this way, errors that occur, for example, at the beginning or in the middle of the process can be detected early on. Based on this knowledge, it can be decided during the building process whether it should be stopped or not. This can save time, material and energy. In addition to inline procedures, detected faults can be investigated by a subsequent offline procedure.

Optical tomography (OT)

Optical tomography is an inline system based on an off-axis CCD camera system. The advantage of an off-axis arrangement is the possibility of a holistic view of the building platform. The camera system continuously records the welding intensity of the process in a location-true manner. After completion of a layer, the individual images are combined to form a layer image. The individual layer images can then be combined and evaluated to a 3D representation. Potential defects appear as hot spots in these images.^[17]

Figure 1 left shows a flawless layer as it is produced by a correct melting process. Figure 1 right shows a faulty layer, as it is produced by wrong process parameters, which can be seen in the OT as a hot spot. The regular stripes result from the overlapping of the welding trace and must not be interpreted as a fault. Figure 2 shows the spatial representation of two tension rods, where the left tension rod was manufactured with correct process parameters and the right one with faulty ones.

With a camera detector of 5 million pixels a building platform of 250 x 250 mm (EOS M 290)^[18] or 400 x 400 mm (EOS M 400)^[19] can be resolved with a resolution of ca. 0.11 mm per pixel or 0.18 mm per pixel. This resolution as well as the continuous recording enables insitu inspection.^[17]

IQ4AP

IQ4AP is an inline quality assurance system for SLS (plastics) additive manufacturing, which was developed at the Fraunhofer Institute. It is based on machine vision. The system uses a camera, exposure and ventilation. The powder bed is continuously observed by the camera in order to detect possible defects during the layering or sintering process. Several algorithms are used for detection. In this way, both coarse and fine defects can be detected, which can occur for example due to a damaged coater. In addition, dimensions of the already sintered layer can be recorded. For example, hole diameter, hole spacing, etc. can be viewed and evaluated inline. The system is modular, cost-effective and can theoretically also be used for metal processing (SLM).^[20]

Laser-Speckle-Photometry (LSP)

Laser speckle photometry consists of a light source, a camera and evaluation algorithms. The method is based on the evaluation of the temporal change of the speckle patterns, which the specimen develops under thermal excitation. This allows to infer material properties of the specimen surface. For additive manufacturing, an LSP sensor can be integrated into the process cell. This allows each layer to be examined inline for material properties such as porosity, pores and micro cracks in addition to geometric parameters. An advantage of this method is the small amount of data to be processed, which makes a real-time evaluation possible. Furthermore, the process is not limited to metallic materials, but can also be applied to non-metallic and organic materials. ^[21]

Other processes that are being researched

• Backscatter X-ray tomography ^[22]
• Laser ultrasonic inspection ^[23]
• Eddy current for WAAM hybrid process ^[24]
• Melt pool spectroscopy for metal deposition process and in-situ qualification of melt pool ^[25][26]

Literature

[1] Dusel, K.: Overview of the Complete Process Chain of Laser Additive Manufacturing, International Laser Technology Congress, (2014), Aachen: MTU Aero Engines

[2] Walter, U.; Pütz, D.: Ringvorlesung: Additive Fertigung Teil 8-Additive Fertigung in der Luft und Raumfahrt

[3] Hassen, A. A.; Kirka, M. M.: Additive Manufacturing: The rise of a technology and the need for quality control and inspection techniques, Materials Evaluation, vol. 76, no. 4, (2018), p. 439-453,

[4] Kruth, J. et al.: Binding Mechanisms in Selective Laser Sintering and Selective Laser Melting, In: Rapid prototyping journal, 11 (1), (2005), p. 26-36.

[5] Herold, Dr. Ing. Frank: Computertomographie und zerstörungsfreie Prüfung bei der Additiven Fertigung, (2018), Lübeck

[6] Weaver, G. J.: Additive Manufacturing and the Inspection Processes, 8 July 2019, https://www.qualitymag.com/articles/95561-additive-manufacturing-and-the-inspection-processes (Acessed on 08.08.2020)

[7] Richard, H. A.; Schramm, B; Zipsner, T. (Hrsg.): Additive Fertigung von Bauteilen und Strukturen. Springer publ., Wiesbaden (2019), p. 28 - 33

[8] Waller, J. M.; Parker, B. H.; Hodges, K. L.  et al: Nondestructive Evaluation of Additive Manufacturing State-of-the-Discipline Report, (2014)

[9] Koester, Lucas W.; Taheri, Hossein; Bigelow, Timothy A.  et al: Nondestructive Testing for Metal Parts Fabricated Using Powder-Based Additive Manufacturing, In: Materials Evaluation, (2018)

[10] Hochrein, T.; Schober, G.; Kraus, E.; Heidemeyer, P.; Bastian M.: Ich sehe was, was du nicht siehst. In: Kunststoffe. 10, 2013, S. 70.

[11] D’Accardi, E.; Altenburg, S.; Maierhofer, C. et al: Detection of Typical Metal Additive Manufacturing Defects by the Application of Thermographic Techniques, 15th International Workshop on Advanced Infrared Technology and Applications, (2019), Florence

[12] Runnemalm, A.; Broberg, P.; Fernandez, E. et al: Automatic thermography inspection of welded components with limited access, (2014), Madrid

[13] Nath, R.: The Power of PCRT, 2 October 2007, https://www.qualitymag.com/articles/89183-the-power-of-pcrt (Accessed on 08.08.2020)

[14] Vibrant Corporation: PCRT resonance solutions, Additive Manufacturing (2018)

[15] VDI/VDE-Gesellschaft Mess- und Automatisierungstechnik (Ed.): Computed Tomography in Dimensional Measurement - Fundamentals and Definitions. VDI/VDE 2630 Part 1.1. Beuth Verlag GmbH, Düsseldorf (2016), p. 6.

[16] Villarraga-Gómez, H.; Peitsch, C. M.; Ramsey, A. et al: The role of computed tomography in additive manufacturing; (2018)

[17] Zenzinger, G.; Bamberg, J.; Hess, T. et al: Online-Prozesskontrolle bei der additiven Fertigung mittels Laserstrahlschmelzen; ZfP-Zeitung; (2014)

[18] https://www.eos.info/de/additive-fertigung/3d-druck-metall/eos-metall-systeme/eos-m-290 (Accessed on 24.07.2020)

[19] https://www.eos.info/de/additive-fertigung/3d-druck-metall/eos-metall-systeme/eos-m-400 (Accessed on 24.07.2020)

[20] Walz, J. D.: Qualität im 3D-Druck automatisiert sichern; Fraunhofer-Institut für Produktionstechnik und Automatisierung IPA; (2017)

[21] Chen, L.; Cikalova, U; Bendjus, B. et al: Laser speckle photometry- Optical sensor system for condition and process monitoring, (2019), p. 213-219

[22] Moritz, T: Industrielösungen Additive Fertigung, Fraunhofer IKTS, Dresden

[23] Davis, G.; Nagarajah, R.; Palanisamy, S. et al.: Laser ultrasonic inspection of additive manufactured components, The International Journal of Advanced Manufacturing Technology, (2019), London

[24] Du, W.; Bai, Q.; Wang, Y. et al: Eddy current detection of subsurface defects for additive/subtractive hybrid manufacturing, The International Journal of Advanced Manufacturing Technology, (2017)

[25] De Beare, D.; De Pauw, B.; Smeesters, L. et al: Spectroscopic monitoring and melt pool temperature estimation during the laser metal deposition. Jornal of Laser Applications Vol. 28, No.2 , (2016)

[26] Gou, Q; Zhao, C.; Qu, M. et al: In-situ characterization and quantification of melt pool variation under constant input energy density in laser powder bed fusion additive manufacturing process. Additiv Manufacturing Vol. 28 (2019), p. 600-609