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# Ultrasonic Testing of Projection Weld Nuts: A Comparison Between Laboratory and Practical Applications

Christian MATHISZIK¹, Martin WEBER², Franz HIELSCHER¹, Johannes, KOAL¹, Uwe FÜSSEL¹, Hans Christian SCHMALE¹, Jere HYVÖNEN², Ari SALMI²

¹ Technische Universität Dresden, Institute of Manufacturing, Chair of Joining Technology and Assembly, Dresden, Germany
christian.mathiszik@tu-dresden.de
franz.hielscher@tu-dresden.de
johannes.koal@tu-dresden.de
uwe.fuessel@tu-dresden.de
hans_christian.schmale@tu-dresden.de

² University of Helsinki, Electronics Research Laboratory, Helsinki, Finland
martin.weber@helsinki.fi
jere.hyvonen@helsinki.fi
ari.salmi@helsinki.fi

## Abstract

Projection welding is a highly efficient welding process that can be applied with a high degree of automation. It is used in various industries for the production of automotive bodies, rail vehicles, kitchen appliances, and in the electronics industry. These industries face several challenges, including increasing safety requirements, growing material diversity, and increasingly regulated resource efficiency in the context of the circular economy. For these reasons, reliable quality assurance of projection welds is becoming increasingly important.

Visual inspection is usually not applicable for quality assurance, as the welds are not visible from the outside. Monitoring of process parameters is used alongside regular destructive testing to assess joint quality. However, the latter contradicts good resource efficiency. No industrial standard has yet been established in the field of non-destructive testing (NDT). Obvious testing methods from resistance spot welding, such as manual ultrasonic testing, have not yet become established. The reasons lie in the variety of possible projection welds and the resulting high geometric variability of the components to be tested.

This paper presents investigation results obtained from laboratory and practical ultrasonic testing of projection welded nuts. Test results obtained using coded excitation scanning acoustic microscopy (CESAM) are compared and discussed with those from manual ultrasonic testing. Possible development steps are derived by comparing these with the corresponding results from destructive testing of the welds.

**Keywords:** Projection welding, resistance welding, NDT, ultrasound, CESAM, resolution

## 1. Introduction

Projection welded joints are used in a wide range of applications in the sheet metal processing industry. These include vehicle construction, metal construction, equipment construction, mechanical engineering, the furniture industry and electrical engineering. The material-to-material connections are produced by resistance pressure welding. The advantages are low costs and high productivity. Due to the process, the welded joint is created between the joining partners and is not visible from the outside. Visual inspection, magnetic particle testing or dye penetrant testing can therefore not be used for quality assessment. This means that only non-destructive testing (NDT) methods that provide information from inside the joints can be used. These include radiographic testing, thermography and ultrasonic testing. Radiographic testing requires specialized equipment due to radiation exposure and is therefore associated with high costs. It is therefore unsuitable for use in the series production of projection welded joints and is only used for very high-priced and safety-relevant individual parts and in research and development.

Thermography has been investigated for the testing of spot-welded joints but has only been used in isolated cases due to the different emissivity of the surfaces, the influence of material
inhomogeneities and the long measurement times in series production [1–3]. Ultrasonic testing with imaging is a widely used method for testing spot welded joints [4–7]. For the evaluation, the time of flight is mapped using false color imaging depending on the location [4,7–11]. This procedure can be used to determine the bonding area in projection welding but has not been common practice to date. The reasons given are the lack of backwall echo, holes in the sheet metal parts (e.g. for weld nuts) and the coupling agent to be applied. [12] describes the development and application of a mobile ultrasonic process that is also suitable for projection welding joints. The results appear promising and successful testing in series production is reported, although no further publications are available. In summary, it can be seen that none of the NDT methods mentioned have yet been able to establish themselves for the quality assessment of projection welded joints. There is a lack of basic research.

This article addresses this knowledge gap and compares results from laboratory tests with practical measurements using ultrasonic testing on projection welded joints. The test results obtained using coded excitation scanning acoustic microscopy (CESAM) are compared with manual ultrasonic testing and discussed. Finally, possible further development steps are derived by comparing them with the corresponding results of the destructive tests of the welded joints.

## 2. Materials and test methods

### 2.1 Material combinations

The tests are carried out on M6 square nuts to DIN 928-M6-ST and M8 ground studs with three solid segment bosses (Figure 1). These are welded onto square, zinc-coated sheets made of CR300LA+GI50/50 and 22MnB5+AS150, each with a sheet thickness of 1.5 mm. The sheets for the square nuts have a central hole with a diameter of 7 mm. The edge length of all sheets is 48 mm. The samples are produced in a gantry welding system equipped with power sources for medium-frequency direct current (MFDC) and capacitor discharge welding (CD). Both types of welding power sources are used. The system has the advantage that the different MFDC and CD welding systems use the same mechanical structure to weld the samples. This enables comparisons to be made between the two welding systems. Electrodes according to ISO 5821-C0-20-22 of the alloy composition CuCr1Zr are used. For the M6 square nuts, the lower electrode has a hole of 7.0 mm for the centering pin for secure and reproducible positioning of the weld nuts. For the ground studs, the lower electrode has a blind hole with a diameter of 17 mm and a hole depth of 14 mm.

The hole is used for secure and reproducible positioning of the M8 ground studs and for holding the threaded stud. No welding current flows through the threaded stud, which means that it remains unaffected by the welding process. The gantry welding system and the experimental setup for producing the projection welding samples can be seen in Figure 2.

<figure><img src="image_2.png" /><figcaption>Figure 1. M6 square nut (DIN 928-M6-ST, left) and M8 ground stud (right)</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Figure 2. Welding system for CD and MFDC welding as a gantry system at the Technical University of Dresden (left) and detailed illustration of the work area with inserted M6 square nut to DIN 928-M6-ST and sheet metal (right)</figcaption></figure>

Two parameter sets (PS1, PS2) are selected for each joint combination to produce different quality levels. **Table 1** shows the test combination with the respective welding parameters for producing the weld specimens. These specifications are based on previously determined welding ranges in [13]. A welding time of 20 ms and a respective pre- and post-welding time of 400 ms are selected for the MFDC welds. These parameters are based on [14]. The CD-welds are produced with a capacitor capacity of 7.26 mF at a transmission ratio of 1:20 with a single pulse. The weld samples are welded in randomized order of the different parameter sets. This minimizes external influences such as electrode wear or variation of the cooling water temperature. For each parameter set, 30 samples are produced, from which two samples are randomly selected for the further investigations in this article.

<table><caption>**Table 1: Material combination with sheet metal and welded joining element of the individual test series (TS) and welding parameters of the parameter sets (PS)**</caption><thead><tr><th rowspan="2">TS</th><th rowspan="2">Joining element (strength)</th><th rowspan="2">Sheet metal (thickness, tensile strength R<sub>m</sub>)</th><th rowspan="2">Force</th><th rowspan="2">Welding time</th><th colspan="2">MFDC: Welding current/ CD: Charging energy</th></tr><tr><th>PS1</th><th>PS2</th></tr></thead><tbody><tr><td>VKM1</td><td>M6 square nut DIN 928-M6-ST (460 MPa)</td><td>CR300LA+GL50/50 (1,5 mm, 447,6 MPa)</td><td>3,0 kN</td><td>20 ms</td><td>11 kA</td><td>17 kA</td></tr><tr><td>VMK2</td><td>M6 square nut DIN 928-M6-ST (460 MPa)</td><td>22MnB5+AS150 (1,5 mm, 447,6 MPa)</td><td>3,0 kN</td><td>20 ms</td><td>12 kA</td><td>18 kA</td></tr><tr><td>VKM3</td><td>M6 square nut DIN 928-M6-ST (460 MPa)</td><td>22MnB5+AS150 (1,5 mm, ~1500 MPa)</td><td>10,0 kN</td><td>Single pulse</td><td>1,10 kJ (550 V)</td><td>2,04 kJ (750 V)</td></tr><tr><td>MB1</td><td>M8 ground stud (460 MPa)</td><td>CR300LA+GI50/50 (1,5 mm, 447,6 MPa)</td><td>6,0 kN</td><td>20 ms</td><td>12 kA</td><td>22 kA</td></tr></tbody></table>

## 2.2 Ultrasonic testing

### 2.2.1 Laboratory-based ultrasonic testing using ultrasonic microscopy

The samples are examined non-destructively using ultrasound microscopy. A coded excitation scanning acoustic microscope (CESAM) is used for this purpose, as shown in Fig.3. In Fig 3a
the measurement setup is shown. The sample is immersed in water and the transducer is positioned over the sample. Fig. 3b shows the schematic of the microscope. It is controlled via the user interface. The desired chirp signal is generated by the signal generator and electrically amplified. It is then fed to the transducer via a transmit/receive switch. There it is converted into an ultrasonic signal, which is partially reflected by the sample and then returns to the transducer. From there the signal reaches the receiving amplifier and is then digitized. Pulse compression is performed using GPU acceleration. A trigger signal is derived from the position information of the XY platform to ensure synchronous data acquisition. Fig 3c displays the determination of the welded area within the analysis range (ROI) using threshold value analysis. The measuring system is developed and operated at the University of Helsinki. Detailed information on the measuring system used can be found in the literature [15,16]. A 30 MHz transducer (Panametrics V375 30/.25 F=.75 "OLF) is used in the setup. The excitation signal is a chirp signal with a Gaussian envelope (start frequency: 10 MHz, end frequency: 40 MHz, duration: 20 µs, linear frequency increase). The average wavelength is 60 µm. Pulse compression is used to convert the received reflected electrical signal into a pulse to achieve a high time resolution [16]. During the measurement, the samples are immersed in deionized water, which is necessary for sound transmission. The area is measured with a step size of 25 x 25 µm². The measurements were taken from the sheet side through the 1.5 mm thick sheets. The focal plane is placed on the back of the sheet, as the contact zone of the welded joint is also located in this plane. The amplitude of the reflected signal within the focus zone is displayed as a C-scan. At the transition from the sheet to the water, part of the ultrasonic signal is reflected, which can be seen as a gray background in the diagrams. Air inclusions and other inhomogeneities show a strong reflectivity and are shown as bright.

<figure><img src="image_4.png" /><figcaption>Figure 3.a) Experimental test setup of the CESAM with b) the schematic measuring workflow and c) the determination of the welded areas</figcaption></figure>

### 2.2.2 Practical ultrasonic testing using an industrial mobile ultrasonic testing device

The ultrasonic testing system used is the PHAsisBLU imaging ultrasonic testing system from VOGT Ultrasonics GmbH. The probe is equipped with a US phased array transducer with 121 individual transducers, 1.0 mm pitch. The measurements are carried out in a fixture. It fixes the otherwise manually guided test head and lowers it vertically onto the weld samples by means of linear guides. A plastic delay line is used. The welding samples are positioned in a water bath (coupling medium: water). The water bath can be moved manually. This is necessary because the surface of the test head does not cover the entire surface of the joining elements/joints of the test series with one measurement. Once the samples have been positioned and the test head has been lowered onto the samples, the measurement is triggered manually. The actual measurement
process is carried out automatically. This means that it is not dependent on the user and provides reproducible and objective results [17]. Each weld projection is measured with three repetitions from the sheet side. The sheet thickness or the plane on the joining element side is set in the system as an aperture. Reflections therefore indicate unconnected areas. Reflections from the interface do not occur in welded areas. This also applies to the area of the hole in the sheet metal. This must therefore be blanked out. The test system and the associated evaluation software have been specially developed for spot-welded joints. It can therefore only be used to a limited extent for determining the connection surfaces. For this reason, the D-scans generated by the test system are evaluated using a self-developed evaluation. Fig. 4 shows the test device on the left and an example of the area determination on the right. The areas are only determined in the analysis area (ROI). Edge detection of the D-scans is carried out for this purpose. The inner surface can thus be determined.

<figure><img src="image_5.png" /><figcaption>Figure 4. Ultrasonic testing device with a phased array probe with 121 individual transducers (left) and illustration for determining the contact area within the analysis area (ROI) using edge detection</figcaption></figure>

### 2.2.3 Equipment for destructive testing

The NDT results are compared with the results of the quasi-static destructive testing of the weld samples. The specimens from the test series containing weld nuts are screwed onto a threaded stud in a head tensile fixture (Fig. 5, left). The specimens with ground studs are screwed into the tensile fixture via the threaded stud. The tensile fixture pulls on the threaded bolt in the axial direction, which corresponds to a head tensile test. The sheets are fixed by a die that encloses the joining elements with a circumferential distance of 1 mm. The die geometry is shown as an example for the M6 square nuts (DIN 928-M6-ST) in Fig. 5, right. The joining elements are pulled until they tear away from the sheet metal. The force F is recorded via the crosshead travel s of the tensile testing machine. The test speed for all specimens is set at 10 mm/min.
<figure><img src="image_6.png" /><figcaption>Figure 5. Schematic representation of the device for the destructive testing of welded specimens using the head pull test (left) and technical representation of the die geometry used for the M6 square nut (DIN 928-M6-ST) (right)</figcaption></figure>

## 3 Results

The laboratory-based and practical ultrasonic measurements show differences in the individual test series and the various welding parameter sets. Fig. 6 shows the respective images using the example of sample VKM1-MF_PS1, representative of the samples with M6 square nut. No reflective ultrasonic waves are measured in the dark areas of the C-scan of the CESAM measurement. There is a welded joint between the joining partners. The inner round dark area is the hole in the sheet metal. This area is not considered when evaluating the joint surfaces of the projections. Reflections are detected within the joint areas of the projections. This may indicate a poor welded section within the joint area. It is to be expected that the projections are only welded together at the edges. The corresponding measurement using the phased array inspection system can be seen in the center of Fig. 6. The projections are shown in the respective image quadrants, all welded joints and the hole are shown in light gray. Dark gray are reflections from the sheet metal plane. The larger light gray area is the hole in the sheet metal. Fig. 6, right, shows the light microscope image. The outlines of the fracture surfaces are similar to those of the CESAM measurements. Differences can be seen in the comparison between the practical ultrasonic measurement and the fracture surfaces. It can be seen that the outlines do not match. Furthermore, converging areas between the hole and the projection welded joint in Q4 can be seen in the center of Fig. 6. The areas can be separated by adjusting the ROI. The inner radius of the ROI is set to a constant 3.75 mm in order to filter out the areas of the hole equally for all samples.

<figure><img src="image_7.png" /><figcaption>Figure 6. Measurements using CESAM (a), phased array testing system (b) and destructive testing using the head tensile test (c) using the example of the VKM1-MF specimen PS1-1 test series with M6 square nut (DIN 928-M6-ST)</figcaption></figure>
Fig. 7 shows the images of the ultrasonic measurements, and the projection welded joints with M8 dimensional studs taken under the microscope using the example of sample MB1-PS2-1. The weld projections are clearly visible in both ultrasonic measurements. Minor geometric deviations between the outer contours can be observed. It should be noted that the weld projections are unbuttoned from the sheet metal during destructive testing. The corresponding tensile forces therefore refer to surfaces other than the measured surfaces of the ultrasonic systems when considering strength.

<figure><img src="image_8.png" /><figcaption>Figure 7. Measurements using CESAM (a), phased array testing system (b) and destructive testing using the head tensile test (c) using the example of the MB1-MF specimen PS2-1 test series with M8 mass studs</figcaption></figure>

The individual NDT areas determined for each test series are compared with the measured areas after the destructive test in Fig. 8. Within the test series, the areas are assigned to the respective parameter sets PS1 and PS2. Good correlations can be observed in all measurements. The determination coefficients $R^2$ are calculated for this. For the CESAM measurements, they are between 0.85 and 0.97. For the practical ultrasonic test, they are higher on average between 0.90 and 0.97. For better illustration, the ideal 1:1 correlation is shown as a black dashed line. It can be seen that the straight lines of the linear regression for the CESAM measurements are more similar to the ideal correlation than those of the practical ultrasonic measurements, which show a much greater increase. It should be noted that the practical ultrasonic measurement underestimates small areas in PS1 and overestimates larger areas from PS2. For the CESAM measurements, the areas are usually smaller than those of the destructive testing.
<figure><img src="image_9.png" /><figcaption>Figure 8. Comparison of the NDT areas determined from CESAM and phased array testing system (US, practical) with the areas after destructive testing (NDT) by means of a head tensile test for all test series with specification of the respective determination coefficients $R^2$ by linear regression</figcaption></figure>

## 4 Discussion

The correlations between the results can be confirmed and emphasized in the statistical evaluation in Fig. 9. With the exception of VKM2-PS2 and VKM3-PS2, the mean values of the CESAM areas are below the ZP areas. The differences for VKM2-PS2 and VKM3-PS2 are not significant. The averaged areas for the practical ultrasonic measurement, on the other hand, deviate significantly from those of the destructive test. They scatter more than the CESAM and ZP measurements. Incorrect evaluations with larger evaluated areas represent a risk for the application. Therefore, the need for higher resolution ultrasonic systems suitable for practical use for the evaluation of small areas, such as projection welded joints, can be derived from this article. To evaluate the required resolution of a practical ultrasonic measurement system, the resolution of the CESAM measurements is first reduced. The original resolution is then reconstructed to 0.025 mm. Both steps were carried out using quadratic interpolation. Fig. 10 shows the absolute and percentage deviations of the determined total area of all weld projections over the resolution. It can be seen that the deviations increase with decreasing resolution. Deviations of up to 15% can be expected for a resolution greater than 1 mm. For deviations of less than 1% across all test series, a maximum resolution of 0.25 mm is permissible.
<figure><img src="image_10.png" /><figcaption>Figure 9. Statistical evaluation of the areas determined from CESAM, phased array testing system and the areas after destructive testing and destructive testing using the head tensile test for all test series</figcaption></figure>

<figure><img src="image_11.png" /><figcaption>Figure 10. Deviation of the determined areas after reduced and reconstructed resolution of the CESAM measurements for the different test series with the area deviation (left) and the percentage deviation (right)</figcaption></figure>

# 5 Summary

There are currently no NDT systems suitable for series production for projection welded joints with small cross-sections. Ultrasonic imaging systems, which are used for spot weld testing, appear to be the first choice for this purpose. However, the systems are not yet sufficiently prepared for the many geometric variations in the area of projection welded joints. Complex geometries of the weld projections often prevent a reproducible quality assessment. Furthermore, the resolution of the available systems is too low to reliably detect the small connection surfaces of the weld projections. The investigations carried out confirm this aspect. The comparison between high-resolution CESAM measurements and practical ultrasonic measurements showed which further development steps are at least necessary for reliable quality assessments. In particular, this includes improving the resolution to at least 0.25 x 0.25 mm².

# Acknowledgements

This article is a translation of DOI:10.58286/31254. DeepL was used during the preparation of the translation and the accuracy of the translation was validated by the authors.
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