Fraunhofer-Institut für Produktionsanlagen und Konstruktionstechnik IPK

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Einfluss von schweißbedingten Rissen auf die Schwingfestigkeit von Widerstandspunktschweißverbindungen aus hochfestem austenitischen Stahl

2012 , Brauser, S. , Schwenk, C. , Rethmeier, M. , Noack, T. , Jüttner, S.

Unter den rauen Bedingungen in der Automobilkarosseriefertigung lassen sich schweißbedingte Imperfektionen wie Risse beim Widerstandspunktschweißen von hochfesten Stählen nicht immer vermeiden. Dabei ist der Einfluss solcher Risse auf die Schwingfestigkeit insbesondere von modernen hochfesten austenitischen Stählen derzeit nicht hinreichend bekannt. Im Rahmen dieser Arbeit wurde daher der Einfluss von Schweißrissen verschiedener Lage und Ausbildung untersucht. Die erzielten Ergebnisse weisen nach, dass Oberflächenrisse bis zu einer Tiefe von 0,8 mm (Einzelblechdicke: 1,7 mm) bei den hier geprüften Bedingungen keinen negativen Einfluss auf die Schwingfestigkeitvon widerstandspunktgeschweißtem hochfesten austenitischen Stahl haben. Weiterhin ist nach derzeitigem Kenntnisstand davon auszugehen, dass diese Ergebnisse auf andere hochfeste austenitische Stähle übertragbar sind. Demzufolge können für Bauteile und Baugruppen aus hochfestem austenitischen Stahl im Hinblick auf eine wechselnde Belastung Oberflächenrisse bis zu 0,8 mm zunächst als unkritisch bewertet werden. Inwiefern eine Übertragbarkeit der Ergebnisse auf andere Belastungszustände (Zug-Druck-Wechsel, Scherzug usw.) besteht, ist in weiteren Untersuchungen zu klären. Darüber hinaus weisen die Ergebnisse darauf hin, dass auch eine Spritzerbildung beim Punktschweißen in Kombination mit den resultierenden Schweißrissen nicht zu einer Verringerung der Schwingfestigkeit führt. Vielmehr zeigt sich, dass die Spritzerbildung zu einer Erhöhung der Versagensschwingspielzahlen führen kann, sodass bezüglich der Schwingfestigkeit eine Spritzerbildung ebenfalls nicht als kritisch anzusehen ist. Da sich die hier dargestellten Ergebnisse auf gleichartige Verbindungen beziehen, im Rohkarosseriebau jedoch überwiegend Mischverbindungen, das heißt Schweißverbindungen aus unterschiedlichen Werkstoffen auftreten, ist eine Übertragbarkeit der Ergebnisse beispielsweise auf ferritisch-austenitsche Mischverbindungen in kommenden Untersuchungen zu prüfen.

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Instrumentierte Eindringprüfung zur Bestimmung lokaler Festigkeitskennwerte an Punktschweißverbindungen

2012 , Brauser, S. , Subaric-Leitis, A. , Ullner, C. , Rethmeier, M.

Die lokalen Festigkeiten im Bereich einer Schweißverbindung sind maßgebend für deren Widerstandsfähigkeit gegenüber mechanischen Beanspruchungen. Im Gegensatz zu anderen präparationsaufwendigen Methoden, z. B. die Untersuchung von Mikro-Zugproben, liefert die instrumentierte Eindringprüfung mit deutlich geringerem Aufwand Verläufe von Festigkeiten mit sehr guter Ortsauflösung. Im Rahmen dieser Arbeit werden wahre Spannungs-Dehnungskurven aus zyklischen Eindringversuchen mittels Auswertungen auf der Basis repräsentativer Spannungen und Dehnungen und auf der Basis neuronaler Netze an Punktschweißverbindungen aus hochfesten Stählen, jeweils im Bereich des Grundwerkstoffs und der Schweißlinse, bestimmt. Die Ergebnisse werden exemplarisch für einen Stahltyp mit Ergebnissen von Zugversuchen am Grundwerkstoff und am thermisch behandelten Werkstoff mit Schweißgut ähnlicher Struktur verglichen und diskutiert.

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Potential of optical strain field measurement for the characterisation of the properties of resistance-spot-welded joints

2011 , Rethmeier, M. , Brauser, S. , Weber, G.

For the designing of safety-relevant components particularly by means of finite element calculations, great significance is attached to the determination of local material parameters in the welding region. Conventional test methods can merely provide global parameters which, however, are frequently not sufficient precisely for the calculation of resistance-spot-welded components since these are characterised by the existence of joining zones with tight local limitations. In contrast, modern test methods such as optical strain field measurement permit the determination of the characteristic material data for concrete locations and times. In order to establish the potential of optical strain field measurements for the characterisation of the local material behaviour of spotwelded joints, tensile shear tests as well as tensile tests were carried out on resistance-spot-welded high-strength steels. The results show the high potential of this procedure not only for the quanti fication of the local strain behaviour in the joining region but also for the determination of local characteristic material data.

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Characterization of microstructure and deformation behaviour of resistance spot welded AZ31 magnesium alloy

2012 , Babu, N.K. , Brauser, S. , Rethmeier, M. , Cross, C.E.

Resistance spot welds were prepared on 3. mm thick sheets of continuous cast and rolled AZ31 magnesium alloy. The microstructure and composition analysis of weld nugget, heat affected zone (HAZ) and base metal were examined using optical and scanning electron microscopy (HR-SEM and EDS/X). The resistance spot welded magnesium alloy joints consist mainly of weld nugget and HAZ. The nugget contains two different structures, i.e. the cellular-dendritic structure at the edge of the nugget and the equiaxed dendritic structure in the centre of the nugget. The structure transition is attributed to the changes of solidification conditions. In the HAZ, grain boundary melting occurred and grain boundaries became coarse. It has been shown that hardness reduction in the weld nugget and HAZ compared with base metal is evident due to dendritic microstructure and grain growth, respectively. The results showed that spot welded joints have failed in interfacial mode under torsion and te nsile-shear loading conditions. Digital image correlation during tensile-shear testing showed that low surface strains occur in the interfacial failure mode, because fracture and deformation happened primarily in the nugget area.

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Influence of production-related gaps on strength properties and deformation behavior of spot welded TRIP Steel HCT690T

2012 , Brauser, S. , Pepke, L.-A. , Weber, G. , Rethmeier, M.

In this study, the influence of production-related gaps on the shear tension strength and fatigue performance was investigated for resistance spot welded TRIP steel HCT690. Furthermore, the local strain distribution in shear tension test was calculated by the digital image correlation technique (DIC). The static shear tension strength was found to be almost independent of gaps up to 3 mm. The maximum local strain in the spot weld region however decreases depending on which sample side (deformed or undeformed) is considered. In addition, it has been ascertained that gaps of 3 mm lead to a significant drop in fatigue life compared to gap-free shear tension samples. This fact could be attributed to decreased stiffness, higher transverse vibration and higher rotation (?) between the sheets as well as increased local stress calculated by 2 dimensional FE simulation.

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Methods to obtain weld discontinuities in spot-welded joints made of advanced high-strength steels

2011 , Gaul, H. , Brauser, S. , Weber, G. , Rethmeier, M.

Resistance spot welding is the major joining technique in mass car production. This applies in particular to high-strength steel and advanced high-strength steel (AHSS) joining of thin sheet steel components for lightweight body shell structures. Joining of AHSS in mass production might lead to weld discontinuities under certain circumstances. Those discontinuities in form of cracks might be an initial start of cracking in the spot-welded joints regarding fatigue loads. It is of great interest to figure out, if, in comparison to specimens without weld discontinuities, the crack initiating point changes and if the fatigue resistance might be reduced by the discontinuities. In this contribution, an overview of potential discontinuities is given. Their possible causes are discussed and means for their detection are highlighted. Among the possible causes of weld discontinuities, two major groups are distinguished: the welding parameters as primary influences in the welding process, and the production-specific influences as secondary ones. With emphasis on major cracks penetrating the weld nugget, these influences are analysed. Finally, a combination of extreme welding parameters with production-specific influences is chosen in order to establish a method which enables the preparation of fatigue test specimens with reproducible major cracks in different locations of the spot-welded joints. This method is than applied in order to prepare spot weld specimens for fatigue tests.

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Influence of welding-induced cracks on the fatigue strength of resistance-spot-welded joints made of high-strength austenitic steel

2012 , Rethmeier, M. , Brauser, S. , Schwenk, C. , Noack, T. , Jüttner, S.

In the rough conditions in the fabrication of automobile bodies, it is not always possible to avoid welding-induced imperfections such as cracks during the resistance spot welding of high-strength steels. In this respect, the influence of such cracks on the fatigue strength particularly of modern high-strength austenitic steels is not sufficiently well-known at present. The influence of welding cracks with various positions and formations was therefore investigated within the framework of this paper. In this case, the analysis of the standardised stiffness courses of specimens and the comparison of the numbers of failure stress cycles served to prove that the surface cracks produced without any spatter in the centre, interfacial region and peripheral region of the weld nugget do not have any negative influence on the fatigue strength of the high-strength austenitic material investigated here. Specimens which were manufactured with welding spatter and exhibit cracks in the peripheral region show considerably higher numbers of failure stress cycles than crack-free reference specimens.

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Determination of local stress-strain properties of spot welded joints using the instrumented indentation test

2012 , Brauser, S. , Subaric-Leitis, A. , Ullner, C. , Rethmeier, M.

Determination of Local Stress-Strain Properties of Spot Welded Joints Using the Instrumented Indentation Test. The local strength properties in the region of weld joints are essential for their resistance to mechanical stress. In contrast to an alternative method of determining the local stress strain behaviour i.e. the preparation intensive, destructive micro tensile test the instrumented indentation test (ITT) provides gradients of strength properties with excellent local resolution. Two methods of determining true stress strain curves from cyclic indentation test have been used in order to investigate the strength behaviour in the region of resistance spot welded joints (region of base metal and weld nugget) of two high strength steels, a TRIP steel, and a martensitic steel and on the other hand a mild steel. Exemplary for the TRIP steel the results were compared and discussed with stress-strain-curves obtained by tensile tests on specimens made of the base metal and on specimens which were heat-treated corresponding to the welding process.

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