Resistance spot welds subjected to torsion testing according to ISO 17653 exhibit two predominant fracture modes, namely interfacial fracture and button-pulled fracture. Their occurrence is governed by the interplay of weld diameter, sheet thickness, local microstructural condition, and, in conventional welds, the effective sheet thickness reduction caused by electrode indentation. In this study, a theoretical fracture mode model is developed that combines these geometrical and mechanical influences and describes the fracture transition as a finite transition zone rather than as a single critical weld diameter. The model is based on theoretical considerations of the torsional section modulus and incorporates a reduction factor χ _BH to account for locally different heat-treatment states in the weld region. Its predictive capability is validated experimentally using two investigation series with different steel sheet combinations. In series S1, a four-sheet configuration is used to eliminate the influence of electrode indentation, whereas series S2 represents conventional two-sheet spot welds in which indentation effects are present. For the statistical evaluation, the reduction factor is varied. The results show that the predictive performance exhibits a distinct optimum and that the fracture transition is best represented by a parameter-dependent interval of the reduction factor in the range χ _BH=0.7 … 0.85 . For series S1, the highest mean agreement between predicted and observed fracture modes is obtained at χ _BH=0.75 with 97.3 χ _BH=0.70 , whereas inclusion of e_max increases the mean agreement to 92.2 χ _BH=0.75 . These findings confirm that reliable prediction requires both a micro-structure-related correction by χ _BH and the consideration of indentation-induced geometric effects. The proposed model provides a physically interpretable and statistically substantiated basis for fracture mode prediction in torsion-tested spot welds. In addition, the study highlights the value of torsion testing as a destructive reference method for the validation of modern imaging non-destructive testing approaches, particularly ultrasonic methods that provide spatially resolved information in the joining plane.

In order to ensure the quality and sufficient clamping force of a bolted joint, real-time monitoring using measurement data is essential. Traditionally, expert systems or control windows analyze process signals such as torque and rotation angle. However, due to the large volume of data, some anomalies or faulty screw connections may go undetected. This article introduces a new hybrid method that combines machine learning with control windows. The method works by scaling both torque and rotation angle, making it applicable to all types of bolted connections. The algorithm requires only 20 training curves, significantly reducing the time needed for setup while still maintaining high accuracy.

Torsion testing of spot welds offers ideal prerequisites for evaluating the weld diameter, which is the most important quality criteria in resistance spot welding. However, torsion testing plays until yet a minor role in quality assurance and strength testing of spot welds. The reasons for this lie in the application areas and the resulting stresses on spot welds. In general, spot welds are not subjected to torsional stress, as large torsional moments cannot be transmitted through spot welds, or torsion at all is hindered when more than one spot weld is set into the component. In addition, the relevant standards for torsion testing are outdated. Extensive scientific investigations of torsion testing in resistance spot welding date back more than 50 years to Kunsmann’s dissertation. These investigations are still the basis of ISO 17653:2012 and DVS 2916-1, which provide detailed information on the procedure for torsion testing of spot welds. Nevertheless, torsion testing is used at the Technische Universität Dresden to assess the quality of spot welds. The reason for this lies in the fracture behavior that occurs, which offers enormous advantages in evaluating the weld diameter and the fracture surface. This contribution presents the results of the influence of the test rate during the torsion test on two different steel grades representing higher and lower ductile material behavior. The experiments are conducted with different sheet thicknesses. A special focus is set on the influence of the test rate on the strain rate dependency of the materials. On the basis of these results, suggested test rates are derived and proposed that should be used for the quasi-static torsion test and that should also be considered in applicable standards like ISO 17653.

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.

Die industrielle Verarbeitung von chemisch reagierenden Klebstoffen auf ein- und zweikomponentiger Basis (1K, 2K) wird branchenübergreifend in verschiedenen Bereichen des Handwerks sowie der Großindustrie angewendet. Für Serienanwendungen ist ein stabiler und verlässlicher Klebprozess die Grundvoraussetzung, um die geforderten Verbindungsqualitäten zu erfüllen. Entscheidende Prozessphasen sind die Klebstoffdosierung, -mischung sowie der Verarbeitung inklusive des Klebstoffauftragens. In diesen Phasen fällt die Topfzeit, die die Zeitspanne der Verarbeitbarkeit nach dem Anmischen darstellt. Mit dem Ende der Topfzeit beginnt der Klebstoff relevant auszuhärten. Die Topfzeit ist abhängig von verschiedenen Faktoren, wie der chemischen Zusammensetzung und dem Mischungsverhältnis des Klebstoffs, der Verarbeitungstemperatur und z. T. der Luftfeuchtigkeit. Bisherige industrielle Anwendungen überwachen den Aushärteprozess und somit auch den Zeitpunkt bis zur Topfzeit kaum. Hier wird meist auf die oft konservativen Herstellerangaben aus dem Datenblatt zurückgegriffen. Für eine bedarfsgerechte und dynamische Weiterverarbeitung ist jedoch die exakte Kenntnis über den Aushärtegrad von entscheidender Bedeutung. So können im Idealfall die geklebten Bauteile nach durchlaufener Topfzeit, jedoch bereits vor vollständiger Aushärtung für weitere Fertigungsschritte in Betracht gezogen werden. In diesem Beitrag erfolgt eine Bewertung des Aushärtegrads der Klebverbindung mittels transversaler Ultraschallwellen im Puls-Echo-Verfahren. Im Rahmen der vorliegenden Untersuchung werden die Schalllaufzeiten und das Dämpfungsverhalten der Ultraschallwellen an verschiedenen Klebstoffdicken analysiert. Die Ergebnisse zeigen, dass der Übergang von der flüssigen in die feste Phase des Klebstoffs beobachtet werden kann. Dies eröffnet potenzielle Anwendungsmöglichkeiten für eine Inline-Überwachung der Klebschichtaushärtung.

The growing prevalence of aluminum sheets in future body-in-white concepts highlights the necessity for unceasing developments in joining techniques. This necessity arises from the distinctive characteristics of aluminum in comparison to conventional steel components. A significant challenge is the optimization of electrode lifespan in resistance spot welding, a process that is widely employed due to its reliability. The brief operational lifespan of electrodes during aluminum welding can be attributed to the insulating properties of the aluminum oxide layer that forms on the surface of the metal. One potential solution to this problem is to mechanically fracture the oxide layer prior to welding. The study investigates the influence of different relative movement configurations between electrodes and sheets on electrode wear and contact resistance during spot welding processes. Experimental setups with varying movement types-rolling frictionless, rolling with friction, translational, and stationary-were analyzed to determine their impact on sheet surface, surface shear stresses, and electrode wear. Confocal microscopy revealed noticeable surface changes depending on the movement configuration, with translational and rolling with friction movements exhibiting noticeable abrasive effects. Simulations supported experimental findings, highlighting variations in shear stress distribution and oxide layer disruption.

Das Buckelschweißen ist ein sehr effizientes Schweißverfahren mit einem hohen Automatisierungsgrad angewendet werden kann. Es wird in verschiedenen Branchen zur Herstellung u.a. von Automobilkarosserien, Schienenfahrzeugen, Küchengeräten sowie in der Elektroindustrie eingesetzt. Die Industriezweige sehen sich mit einer Reihe von Herausforderungen konfrontiert, darunter steigende Sicherheitsanforderungen, zunehmende Materialvielfalt, sowie eine fortschreitend regulierte Ressourceneffizienz im Sinne der Kreislaufwirtschaft. Aus diesen Gründen wird eine zuverlässige Qualitätssicherung der Buckelschweißverbindungen immer wichtiger. Eine Sichtprüfung ist für die Qualitätssicherung meist nicht anwendbar, da die Schweißverbindungen von außen nicht sichtbar sind. Die Überwachung der Prozessparameter wird neben regelmäßig zerstörenden Prüfungen zur Bewertung der Verbindungsqualität angewendet. Letzteres steht einer guten Ressourceneffizienz entgegen. Im Bereich der zerstörungsfreien Prüfverfahren (ZfP) wurde noch kein Industriestandard festgelegt. Naheliegende Prüfmethoden aus dem Bereich des Widerstandspunktschweißens, wie die manuelle Ultraschallprüfung, konnten sich bisher nicht etablieren. Die Gründe dafür liegen in der Vielfalt möglicher Buckelschweißverbindungen und der daraus hohen geometrischen Variabilität der zu prüfende Bauteile. Im Beitrag werden Untersuchungsergebnisse präsentiert, welche in labor- und praxisnahen Ultraschallprüfungen an Buckelschweißmuttern gewonnen wurden. Dabei werden mittels coded excitation scanning acoustic microscopy (CESAM) gewonnene Prüfergebnisse denen der manuellen Ultraschallprüfung gegenübergestellt und diskutiert. Mögliche Weiterentwicklungsschritte werden abschließend durch einen Vergleich mit den entsprechenden Ergebnissen der zerstörenden Prüfungen der Schweißverbindungen abgeleitet.

The weld diameter is the most important quality criterion in resistant spot welding and is unsually determined after destructive testing. Several standardized destructive testing methods are available for this purpose. The determination of the spot weld diameter is influenced by numerous factors. These include the measurement conditions, such as the lighting conditions, the measurement equipment used, and the person performing the measurements. The human aspect is influenced by experience and is therefore subjective. To manually evaluate a resistance spot weld, the weld diameter can be measured according to ISO 17677-1, AWS D8.1M, or DVS 2916-1, among others. The challenge here is that the statistical variation in the destructive evaluation of the spot weld diameter due to human experience is still insufficiently researched. In addition, the results of destructive testing are not statistically validated or are questioned in many scientific publications. The suitability of the destructive test methods used is often not justified or compared with other test methods. The purpose of this contribution is to statistically evaluate the human experience on manual weld diameter measurements and give recommendations, that should be taken into account in the future of some kind of issues. For this purpose, more than 180 spot welds were destroyed by torsion testing and the weld diameters were measured by 20 different participants with different levels of experience. The results show that the measurements are influenced by a number of factors, including the level of experience of the participants, the size of the weld diameters measured, the failure mode, and the duration of the measurements on a sample.

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.

Fasteners are widely used elements for joining sheet metal. Real-time monitoring is desired to predict the quality respectively a sufficient clamping force between the joining partners. Usually, measured process signals are evaluated through expert systems or control windows. These methods are restricted to one specific use case and need to be reevaluated for every new joint combination. Often those systems can only determine significant deviations from the reference curve. The quality criteria of the joint, the clamping force, is usually not predicted and a process monitoring using ultrasonic testing is time consuming and highly cost intensive. Another possibility is quality prediction using machine learning algorithms to detect patterns in the process signals which correlate with the quality criteria. Many studies introduced machine learning algorithms to detect outlier curves or different failure types. Therefore, this paper presents an approach of unifying the shape of different torque-angle-curves from different fastener applications while simultaneously achieving high accuracy on the prediction of the clamping force with this novel method.

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.

Capacitor discharge welding (CDW) is characterized by a pulsed electrical current profile. It is primarily utilized for resistance projection welding tasks, offering high power densities and short welding times. According to the latest findings, the welding process can be divided into different phases: contacting, activating, material connection, and holding pressure. During the activation phase, high-speed video-imaging reveals the generation of metal vapor which effectively eliminates impurities and oxide layers from the contact zone. The result of this is an activated surface. The purpose of this paper is to describe the physical effects of the bonding mechanism during short-time resistance welding. The Chair of Joining Technology and Assembly at the Technische Universität Dresden has a laboratory facility that can interrupt the welding current at any desired time during capacitor discharge welding. This allows different welding current profiles with always the same current rise time to be scientifically investigated. The experimental findings were supplemented with simulative analyses to clarify the bonding mechanism in resistance projection welding. Three different surface conditions are considered to generalize the findings on the bonding mechanism. Temperature and current density distributions were assessed to provide a physical description of the activation phase. The power density in the joining zone at different interruption times is determined, which gives an indication of activation by metal vaporization. The material connection is determined experimentally for the same interruption times. The numerical simulation model can be used to describe the bonding mechanism in short-time resistance welding. In resistance welding, the bond is formed due to the molten phase (solidification structure). In short-time resistance welding, the bond is formed due to surface activation by metal vaporization.

Capacitor discharge welding is an efficient, cost-effective and stable process. It is mostly used for projection welding. Real-time monitoring is desired to ensure quality. Until this point, measured process quantities were evaluated through expert systems. This method is strongly restricted to specific welding tasks and needs deep understanding of the process. Another possibility is quality prediction based on process data with machine learning. This requires classified welding experiments to achieve a high prediction probability. In industrial manufacturing, it is rarely possible to generate big sets classified data. Therefore, semi-supervised learning is investigated to enable model development on small data sets. Supervised learning models on large amounts of data are used as a comparison to the semi-supervised models. A total of 389 classified weld tests were performed. With semi-supervised learning methods, the amount of training data necessary was reduced to 31 classified data sets.

The amount of aluminium sheets in future body-in-white concepts is still on the rise. There is a need for optimizing the joining techniques, caused by the different characteristics compared to the established steel components. Especially the electrode life for resistance spot welding as a reliable and established process needs to be improved. One reason for the short electrode life when welding aluminium is the insulating effect of the aluminium oxide layer. One possibility to reduce the electrode wear is the mechanical destruction of the oxide layer before the welding. This paper describes the influence of a translational and rotational electrode movement on the electrode wear. The oxide layer destruction is detected by resistance measurement. It could be shown that the destruction of the oxide layer already occurs at low movements. However, a homogeneous, large-area destruction is necessary for a wear reduction.

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.

Capacitor discharge welding (CDW) for projection welding provides very high current pulses in extremely short welding times. This requires a quick follow up behaviour of the electrodes during the softening of the projection. The possibilities of experimental process investigations are strongly limited because of the covered contact zone and short process times. The Finite Element Method (FEM) allows highly resoluted analyses in time and space and is therefore a suitable tool for process characterization and optimization. To utilize this mean of optimization, an indirect multiphysical numerical model has been developed in Ansys Mechanical APDL. This model couples the physical environments of thermal–electric with structural analysis. It can master the complexity of large deformations, short current rise times and high temperature gradients. A typical ring projection has been chosen as the joining task. The selected aluminium alloys are EN-AW-6082 (ring projection) and EN-AW-5083 (sheet metal). This paper presents the investigated material data, the model design and the methodology for an indirect coupling of the thermal–electric with the structural physic. The electrical contact resistance is adapted to the measured voltage in the experiment. The limits of the model in Ansys Mechanical APDL are due to large mesh deformation and decreasing element stiffness. Further modelling possibilities, which can handle the limits, are described.