Magnetic Characterization of the Nugget Microstructure at Resistance Spot Welding

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

Lehrerselbstwirksamkeit von Primarstufenstudierenden im Anleiten Forschenden Lernens

Im Beitrag wird untersucht, ob (1) Lehrerselbstwirksamkeit im Anleiten Forschenden Lernens empirisch zu trennen ist von jener, die sich auf das allgemeine Unterrichten naturwissenschaftlicher Inhalte im Sachunterricht bezieht und (2) wie sich diese im Laufe eines Semesters entwickeln. Für die Analysen liegen Daten von 266 Studierenden vor. Die Ergebnisse zeigen, dass zwischen den beiden Facetten der Lehrerselbstwirksamkeit zu differenzieren ist. Im Laufe eines Semesters zeigt sich für beide Aspekte eine signifikant positive Entwicklung, die für die Selbstwirksamkeitserwartungen im Forschenden Lernen deutlicher ausfällt und auch unter Kontrolle von Vorwissen und Interesse bestehen bleibt

General Approach for Inline Electrode Wear Monitoring at Resistance Spot Welding

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

General Approach for Inline Electrode Wear Monitoring at Resistance Spot Welding

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.</jats:p

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

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

Magnetic Characterization of the Nugget Microstructure at Resistance Spot Welding

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

Ultrasonic testing of projection weld nuts : A comparison between laboratory and practical application

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.Peer reviewe