Electrodes for resistance spot welding inevitably wear out. In order to extend their service life, the tip-dressing process restores their original geometry. So far, however, the point in time for tip-dressing is mainly based on experience and not on process data. Therefore, this study aims to evaluate the in-situ or inline wear during the welding process without using additional sensors, and to base the timing for tip-dressing on continuous process monitoring, extending electrode life even further. Under laboratory conditions, electrode wear is analyzed by topographical measurements deepening the knowledge of the known main wear modes of resistance-spot-welding electrodes, mushrooming and plateau forming, and characterizing an electrode length delta over the number of spot welds. In general, electrode wear results in deformation of the electrode contact area, which influences process parameters and thereby weld quality. The conducted tests show correlation between this deformed contact area and the electrode length delta. The study shows that this electrode length delta is visible in actual process data, and can therefore be used as a criterion to characterize the wear of electrodes. Furthermore, this study gives reason to question commonly used spot-welding quality criteria and suggests different approaches, such as basing spot-welding quality on the possibility of nondestructive testing.

Conventional resistance spot welds are not visible from the outside. Therefore, it is not straightforward to evaluate the joint quality non-destructively. The pulse-echo method of manual ultrasonic is widely used for non-destructive testing. Another option is the passive magnetic flux density testing, which is being developed at Technische Universität Dresden, Germany. The spot weld is magnetized in the normal direction and the residual magnetic flux density is measured on top of the surface of the joint. This method is suitable for spot welds on typical car body steels. Previous investigations show that the magnetic properties of the materials influence the test result. In order to develop this new non-destructive testing method further, it is necessary to know the magnetic properties of the different microstructure regions of a spot weld. This article focuses on methods to measure and evaluate the magnetic properties of these regions, especially of the base material and the weld. Different measuring methods and approaches are presented and compared with each other. Based on the results, recommendations for future measurements for magnetic characterizations are given.

Resistance projection welding is predominantly performed using capacitor discharge machines, known for their short welding times, rapid current rise times, and high currents compared to medium-frequency inverter technology. The resulting joints are covered up during resistance welding, so that either destructive or non-destructive testing is required to evaluate the quality. Process monitoring is therefore essential in resistance projection welding. The requirement for this is process data that can be acquired and integrated into the process monitoring easily, cost-effectively, and contactlessly. This study investigates the use of low-cost condenser microphones to utilize the airborne sound generated during welding for process monitoring. It is shown that, acoustic data processed by the fast Fourier transform can be used to evaluate the quality of the connection. Only a minor influence of the microphone position could be determined. A machine learning model was also used to detect the batch of the welding nut. The machine parameters, welding nut geometry and material were kept constant. The results show a batch prediction of more than 90% using airborne sound.

The torsion test is rarely used for resistance spot-welded joints since they are not subjected to torsion in applications. Normal, shear, and/or peel loads are usually the main stresses. Extensive scientific investigations in the context of Kunsmann’s dissertation date back more than 50 years. These investigations are still the basis of ISO 17653 and the German guideline DVS 2916-1. Recent scientific investigations only use torsion tests, but do not describe the reason for its use. A decisive advantage of the torsion test over the other standardized destructive testing methods lies in the types of fracture modes that occur and the properties of the fracture surfaces. Torsional loading results in either interfacial or button-pulled fracture modes. No material residues occur on the fracture surfaces for ductile and advanced high-strength steels. Hence, the measurement of weld diameter is achievable with minimal constraints, resulting in reduced variability and facilitating objective assessments of spot welds. This article delineates these attributes through a comparative analysis of various destructive testing methods employing statistical approaches. Additionally, the article expounds on the design concept of the developed rig for conducting torsion tests on spot welds.

The weld of resistance projection welded joints is not visible from outside. Therefore, visual evaluation is restricted, and visual inspection is not possible at all. So far, the parameters of the welding process are monitored and controlled. In addition, the welds are periodically tested destructively to determine the quality of the welds. No industrial standard has yet been established in the field of non-destructive testing (NDT) for projection welded joints. This study focuses on NDT of projection welds using two different ultrasonic imaging inspection systems and the passive magnetic ux density testing (pMFT) method. The ultrasonic inspection systems commercially available and established in the field of NDT of spot welds. Both systems are originally designed for the NDT of spot welds and not for projection welds. Unlike the ultrasonic systems, the pMFT is still in laboratory status. The method has originally been developed to evaluate spot welds and is also used in this study to evaluate projection welds. The applicability of the investigated systems to projection welding is investigated in order to derive mandatory development steps to achieve reliable results. The pMFT method shows also good results for NDT of spot welds In this contribution, the measurement and evaluation concept of the three NDT systems for projection welded joints is presented. The NDT results are discussed in the context of the corresponding destructive results in terms of tensile forces and fracture areas. Advises for further development of all investigated systems are given.

Nondestructive testing (NDT) of spot welds still poses a great challenge for industrial mass production like automotive body manufacturing or rail vehicle constructions. In these production areas, ultrasonic-based NDT methods with pulse-echo technology have become established. Hitherto, these methods require a lot of experience from the staff, which often makes the results subjective. Furthermore, these methods reach their limits when testing spot welds with deep electrode indentations or huge differences between sheet metal thicknesses. Imaging analysis of the residual magnetic flux density shows great potential especially when addressing the restrictions of the ultrasonic-based NDT methods. Typical spot welds show a characteristic electrode indentation. This indentation influences the measureable magnetic flux density, which depends on the distance between the surface of the spot weld and the sensor. This paper will show the influence of electrode indentation on the test result and presents the resulting restrictions for imaging analysis of the residual magnetic flux density when testing spot welds.