<figure><img src="image_1.png" /></figure>

# Quality Control of Additively Manufactured Large Components using Ultrasound Phased Array Method and Digital Radiography

Antje ZÖSCH, Christopher SEIDEL

imq - Ingenieurbetrieb GmbH, Gewerbering 30, D-08451 Crimmitschau, Germany

Phone: +49 3762 9537-0; email: a.zoesch@imq-gmbh.com; ch.seidel@imq-gmbh.com

**Abstract**

The publication presents quality control in the reconstruction of large components using additive processes instead of new production by casting. The investigations on an additively manufactured demonstrator component with various inhomogeneities are described. The material properties were determined and compared with the properties of an adequate cast component. Non-destructive testing methods were used to detect and evaluate irregularities. The results were verified using reference methods. The focus was on the ultrasonic phased array method. The concept and test configuration were developed through prior sound field simulations. The test results show the possibilities and limitations of the method and provide approaches for further improving the use of the UT-PA method for evaluating component quality.

**Keywords:** Ultrasonic phased array, total focusing method, additive manufacturing, roll crusher, sizer, digital radiography, wear simulation

## 1. Introduction

Additive manufacturing is becoming an increasingly attractive alternative to traditional manufacturing processes for an ever-growing number of companies. Especially for complex components and customized products, the use is economical. For repair work in particular, restoring the component offers significant advantages over producing a new one. This application involves extending the service life of large components with high material usage and complex geometries by repairing wear damage using additive manufacturing, thereby saving resources.

In the industry, components must meet high quality and safety standards. Currently, the absence of product- or process-specific norms and standards means that production processes must be defined from scratch for each individual case. The DIN EN ISO/ASTM 52920:2023-12 standard (replacing DIN SPEC 17071:2019-12) provides a comprehensive set of requirements for designing processes for producing high-quality additively manufactured components [1, 2]. Key activities include preparing for additive manufacturing using digital tools such as simulations and ensuring component quality throughout the process. In this specific case, the aim was to repair worn bucket teeth on roll crushers (sizers) using additive manufacturing (see Fig. 1). At present, the parts are completely replaced with new cast parts, which requires a significant input of material, energy and time.

<figure><img src="image_2.png" /><figcaption>Figure 1. Cast component crusher segment (left) / crusher tooth new print (right)</figcaption></figure>

Test plans were drawn up in advance to assess the technical and material suitability of the additive material, and to compare it with components produced using conventional methods.
Both destructive and non-destructive testing methods were employed to evaluate component quality.

# 2. Experimental Design and Execution

## 2.1 Manufacture of the Test Specimen

The test blocks were manufactured using wire powder arc additive manufacturing (WPAAM) at BTU Cottbus-Senftenberg and TAKRAF GmbH [3, 4]. For restoration, the worn teeth must be applied to a substrate sheet either fully (new print) or partially (repair print). As a first approximation, the crushing teeth can be modelled as cuboids of comparable dimensions to optimise the printing process in terms of application rate and material quality (see Fig. 2). The steels S355J2+N and AM70 (S700) were selected as the materials.

<figure><img src="image_3.png" /><figcaption>Figure 2. Test block, manufactured using wire powder arc additive manufacturing (WPAAM)</figcaption></figure>

<figure><img src="image_4.png" /><figcaption>Figure 3. Overview of the tests carried out by all project partners (left) and a sampling plan for mechanical-technological parameter determination</figcaption></figure>

## 2.2 Investigation Program

First, the manufactured cuboids were tested non-destructively using ultrasonic phased array techniques. The tests were performed using an Omniscan MX (32/128 PR) from Evident Scientific and a SIUI Syncscan 3 (64/128) using high-frequency probes, such as the 5L64-A12 and 7.5L64. Reference samples were then taken according to sampling plans (see Fig. 3). Tensile tests, impact bending tests, hardness measurements and microstructural analyses were then performed to evaluate the suitability of the material. In the second step, fatigue specimens were designed and manufactured in accordance with DIN EN 6072. The reference samples were examined using digital radiography by a CP200 X-ray tube. The exposed imaging plates were digitised by the help of a Carestream scanner HPX-1 model.

# 3. Results and Discussion

## 3.1 Reference Investigation and Digital Radiography

The required material parameters were determined based on the initial material and wear simulations. Based on ANSYS ROCKY, TAKRAF developed a fracture model (the TAKRAF Breakage Model, or TBM) and used it to carry out discrete element method (DEM) simulations. Based on the wear and fracture simulations, the minimum requirements for crusher teeth used to comminute "medium-hard" bauxite can be confirmed and specified as follows:

* Rockwell hardness ≥ 55 HRC

* Yield strength ≥ 600 N/mm²
* Impact energy ≥ 27 J at -40 °C

* Shear force ≥ 600 kN

The reference tests carried out showed comprehensively that the material parameters, such as tensile strength, impact energy and hardness, met the requirements [4, 5].

In order to better evaluate and interpret the results of the tensile tests and notch impact tests, the reference samples were first examined using X-ray radiation. However, additively manufactured materials cannot be considered homogeneous. There are fluctuations in their mechanical and technological properties. These fluctuations are generally dependent on the build-up height. Metallographic investigations have shown that porosity increases slightly as the build-up height increases (from 0.6% to 1.2%). At the same time, hardness also increases slightly. Based on the correlations found, digital radiography of reference samples was employed. This method provides fast digital imaging and reliable information about volume defects within the reference samples (see Fig. 4). The tensile specimens were extracted from various build-up heights, and the same applies to the Charpy impact specimens. A total of seven tensile specimens were produced at intervals of 10 mm in build-up height. For the Charpy impact specimens, six build height levels were realized. Figure 4 shows the radiographic images of the tensile specimens taken from a build height range of 10 mm to 40 mm. The Charpy impact specimens cover a range from 10 mm to 70 mm. A pronounced concentration of pores was detected at an approximate build height of 40 mm.

<figure><img src="image_5.png" /><figcaption>Figure 4. Tensile specimens form different build-up heights and notched bar impact specimens with protruding pores and defects in the volume</figcaption></figure>

Images from digital radiography were scanned at a resolution of 25 µm, revealing even the smallest pores and material separations. Interpreting the results of the mechanical-technological test is significantly easier, and the influence of porosity on material properties can be shown. An increased number of pores leads to a significant reduction in impact energy, which can be up to 50% of the initial value [4]. Samples with significant inhomogeneities were excluded from the results evaluation. Extensive examinations of the reference specimens using digital radiography demonstrated that defects with a size of 200 µm can be detected reliably. The identified defects were predominantly porosities. Due to the nature of the process, the maximum wall thickness is severely limited when using high-resolution radiography. Therefore, this method is not suitable to inspect the additively manufactured crusher teeth and a different volumetric method had to be used.

## 3.2 Ultrasonic Testing - Preliminary Considerations

The ultrasonic phased array method has several advantages over traditional techniques. These include the imaging display and the fully digital inspection process. Unlike conventional
methods, phased array systems can generate A-scans and high-resolution cross-sectional images (C-scans and B-scans) of the material. These facilitate the identification and analysis of defects within the test volume. The ability to electronically adjust the beam angle and focal depth allows for better adaptation to inspection requirements. Reducing the number of test mechanics makes components with complex outer contours easier to test. Fewer probes are required, which simplifies and speeds up the test sequence and data acquisition.

For this test task, probes with a large active aperture (5L64 and 7.5L64) were used. Due to the resulting large near field, these apertures provide more leeway when aligning and focusing the sound fields. In the first step, two parameter sets were implemented: one for testing bonding defects (lack of fusion between welding beads and the substrate) and one for testing volume defects (see Fig. 5). Incomplete fusion is common in additive structures printed on a substrate. In this case, the substrate was made of S355J2+N with a wall thickness of 40 mm. The bonding of the printed crusher teeth must be as flawless as possible, as considerable shear and compressive forces occur during use [5].

<figure><img src="image_6.png" /><figcaption>Figure 5. Sound field simulation for various focal parameters for lack of fusion (left, focus on 40 mm) and volume defects (right, high sensitivity in the range of 35 mm to 60 mm); sensor type 5L64 0°</figcaption></figure>

## 3.3 Phased-Array Ultrasonic Testing

Testing of the components was carried out using two sets of parameters, revealing bonding and volume defects, particularly in the edge areas of the test blocks (see Fig. 6). The greatest temperature gradients occur in these areas during the welding process, which increases the probability of material separation and gas inclusions.

For the advancement of the test system, an individual wedge and a reference specimen made of a similar material (application material AM70-8MnNiMoCrSi7-6-5) were developed. This step extended the resulting test range to the entire height of the structure (approximately 100 mm). Adjustments were made over the entire depth range using cross-drillings with diameter of 3.0 mm at defined depths. Depth-dependent gain curves (TCG - time-corrected gain) were created from the echo heights of the cross-drilled holes at different depths. This procedure was performed for all phased array probe elements and focal laws to ensure consistent amplification and sensitivity throughout the test volume. The divergence of the sound field outside the focus area has a negative effect on the achievable spatial resolution. However, phased array focusing significantly improves sensitivity in the focus area, thereby enhancing detection performance. However, this increased sensitivity is limited to a programmable, fixed depth within the test piece. Reflectors outside the focus area appear less sharp and larger than the same reflector within the focus area. The enlargement of reflectors outside the selected focal zone was not determined empirically. To estimate this effect, calculations based on the PSF model were applied. Considering the transducers used and the component parameters, the displayed
reflectors exhibit an enlargement of approximately 100% compared to their actual size within the acoustic focal region.

<figure><img src="image_7.png" /><figcaption>Figure 6. Ultrasonic scans of a test cuboid with fault indications in the edge area</figcaption></figure>

Using the optimised testing system, all the demonstrator components manufactured were tested and fully documented (see Fig. 7 for an example). Compared to components from preliminary tests, the number of defects was significantly reduced. Defect indications on the demonstrator components can be divided into two groups. Those between the substrate sheet and the built-up material are classified as bonding defects and account for 60% of all indications. The remaining 40% are defects in the volume of the built-up material.

<figure><img src="image_8.png" /><figcaption>Figure 7. Ultrasonic scans of a demonstrator component with isolated fault indications</figcaption></figure>

Comparative investigations and material analyses were conducted to determine characteristic values, and these confirmed the defect indications with a high degree of agreement. However, porosities were found within the built-up material that were not detected in the UT-PA test at a sensitivity level corresponding to a 3.0 mm cross-drilling. These pores and pore nests could be documented using light microscopy at various build-up heights. Based on measurements taken from microscopic images, the majority of pores were smaller than 1.5 mm. To reliably detect these defects, the sensitivity of the test area was increased. The sensitivity was increased to the equivalent of a 1.5 mm cross-drilling and ultrasonic scans of the demonstrator components were re-evaluated. C-scans and B-scans (see Fig. 8) were employed to visualise the test volume, providing a detailed view of the interesting depth ranges. Individual pores and pore nests with sufficient amplitude could be detected. The signal-to-noise ratio was between 15 dB and 21 dB for all defect indications, which is well above the requirements of current regulations.
<figure><img src="image_9.png" /><figcaption>Figure 8. Ultrasonic Phased-Array scans of a test cuboid showing defect indications on the edges.</figcaption></figure>

## 3.4 Total Focusing Method TFM

Synthetic sound fields were also considered as an alternative approach to non-destructive testing of demonstrator components. These modern reconstruction methods (Total Focusing Method, or TFM) adopt a different approach to sonication and evaluation of reflected sound waves. The basis of this approach is the acquisition strategy known as Full Matrix Capture (FMC). The FMC dataset contains all information regarding sound propagation between all sensor elements, including reflections at interfaces and sound scattering due to faults. Any type of PAUT test data can be reconstructed using FMC. Currently, TFM is a highly effective method for generating an image of the test area with focus at any depth.

The measurements of the test components clearly represent the test volume (see Fig. 9). The enlargement or spreading of detected reflectors is less pronounced when using the TFM technique, as the focal zone can be extended across the entire component height. Nevertheless, a reduction in inspection sensitivity in regions with build heights exceeding 70 mm is still observed. Due to the longer sound paths, beam divergence and material attenuation play a significant role in this effect. The drop in sensitivity is less than 4 dB across the entire depth range, which is consistent with the simulation results. Thanks to the synthetic focusing of the sound field, all types of defects, such as bonding defects and porosities, can be clearly depicted within the volume. This makes it much easier to assess the geometric expansion and surface quality of the defect indications. Unlike with phased array testing, an expansion of the reflected indications cannot be observed in the tested region (Fig. 9). In the present application, the TFM method provides clear and measurable advantages in terms of spatial resolution and the presentability of defect indications. There is great potential for further development and optimization of reconstruction methods. These investigations form part of ongoing work that will be realized as part of a follow-up project.

<figure><img src="image_10.png" /><figcaption>Figure 9. TFM scans of demonstrator component with defects in the volume and a large-area connection defect / porosities in the volume compared with a section through the component (Figure not to scale)</figcaption></figure>
The applicability of non-destructive testing methods was demonstrated using a series of test and demonstrator components manufactured under real production conditions. These components have the same structural design as the actual crusher teeth to be produced. The majority of typical manufacturing and welding defects were reliably detected, catalogued, and correlated with the manufacturing parameters. Initially, the ultrasonic data was evaluated manually, but an automated solution was required for in-production inspection.

<figure><img src="image_11.png" /><figcaption>Figure 10. Automatically generated error table with displays from a scan section</figcaption></figure>

Automatic aperture discrimination was used to automatically evaluate and record error displays. This method was developed during project work at imq GmbH in cooperation with the device manufacturer, based on the requirements profile. This method involves recording the amplitudes in a two-dimensional aperture range (path and depth coordinates) and storing them for evaluation. The recording threshold can be set variably; in this case, 80% BSH was selected. This makes it possible to monitor a defined depth range across the entire bonding surface of one or more crusher teeth. The recorded fault areas are documented in tabular form alongside their length along the scan coordinate and the associated amplitudes (see Fig. 10). Additionally, the defect coordinates and the depth positions of the indications are saved. The inspection results for each component can be exported as CSV files and integrated into other database systems.

## 4. Summary

The project work demonstrated that the selected material, when combined with the substrate material, is suitable for additive manufacturing of crushing teeth. The material's characteristics and strength meet the requirements in terms of intended use and durability. During the course of the investigations, a number of special features associated with additive manufacturing technology were identified and addressed. These primarily relate to the critical analysis of reference samples using digital radiography and the detection of manufacturing defects after the welding process using ultrasonic methods. The use of ultrasonic phased array and TFM (Total Focusing Method) techniques is essential for inspecting the wall thicknesses of the components, as build heights exceeding 80 mm are required to fully capture the geometry of the parts. Radiographic testing was successfully employed for the evaluation of reference specimens, such as tensile and Charpy impact samples. Digital radiography proved to be an important tool for interpreting the results of tensile tests and other destructive testing methods. In particular, for additively manufactured components, it enabled better documentation of material properties and specific characteristics. Ultrasonic phased array testing has proven to be a reliable and effective method for identifying bonding and volume defects.
Real defects with an equivalent circular disk reflector size of 2.0 mm were detected at various build heights and within the bonding zone. The application of the TFM technique significantly improved the geometric resolution, enhancing the detectability of these defects. The detected defect indications were referenced destructively on sample components and confirmed with a high level of agreement. Cracks in the edge regions of the test components, bonding defects, and pores with a diameter of 2.0 mm or greater were verified. The detected crack lengths ranged from 4.0 mm to 10.0 mm.

Surface bonding defects, cracks in the built-up material, and volume defects up to macropores could be detected. However, the limitations of the method in terms of local defect resolution and process-related sound field focusing were also highlighted.

## Acknowledgements

The authors would like to thank the German Federal Ministry of Education and Research BMBF for the financial support of the project "DIGMATEST: Digitalization in materials testing". Special thanks go to the project partners at TAKRAF GmbH for their constructive and successful collaboration.

## References

1. DIN EN ISO/ASTM 52920:2023-12: Additive Fertigung - Grundsätze der Qualifizierung - Anforderungen an industrielle additive Fertigungsverfahren und Produktionsstätten (ISO/ASTM 52920:2023); Norm; Deutsche Fassung EN ISO/ASTM 52920:2023

2. DIN SPEC 17071: 2019-09 Additive Fertigung – Anforderungen an qualitätsgesicherte Prozesse für additive Fertigungszentren; Text in Deutsch und Englisch

3. R. Lorenz Bachelorarbeit: Erhöhung der Auftragsrate des WAAM-Prozesses und Analyse des Einflusses der Fertigungsparameter auf die Werkstoffstoffeigenschaften 2024

4. M. Schmidt Abschlussbericht: WEARINGGREEN: Additive Fertigung und Aufarbeitung von Verschleißteilen Teilprojekt TP3-1.2.5 von: DIGMATEST: Digitalisierung in der Materialprüfung 2024

5. Salzgitter Mannesmann Forschung GmbH, Duisburg MTU Aero Engines AG, München Kalibrierung des TFM-Verfahrens mittels additiv gefertigter Kalibrier-Reflektoren und Vergleich mit Simulationen DACH-Jahrestagung 2019