Abstract:Resistance spot welding (RSW) as a predominant welding technique used for joining steels in automotive applications needs to be studied carefully in order to improve the mechanical properties of the spot welds. The objectives of the present work are to characterize the resistance spot weldment of DP600 sheet steels. The mechanical properties of the welded joints were evaluated using tensile-shear and cross-tensile tests. The time-temperature evolution during the welding cycle was measured. The microstructures observed in different sites of the welds were correlated to thermal history recorded by thermocouples in the corresponding areas. It was found that cracks initiated in the periphery region of weld nuggets with a martensitic microstructure and a pull-out failure mode was observed. It was also concluded that tempering during RSW was the main reason for hardness decrease in HAZ.
OPEN ACCESSMetals 2015, 5 1705
The objectives of the present work are to characterize the Gas Metal Arc Welding process of DP 600 sheet steel and to summarize the modelling techniques. The time-temperature evolution during the welding cycle was measured experimentally and modelled with the softwaretool SimWeld. To model the phase transformations during the welding cycle dilatometer tests were done to quantify the parameters for phase field modelling by MICRESSm. The important input parameters are interface mobility, nucleation density, etc. A contribution was made to include austenite to bainite transformation in MICRESSm. This is useful to predict the microstructure in the fast cooling segments. The phase transformation model is capable to predict the microstructure along the heating and cooling cycles of welding. Tensile tests have shown the evidence of failure at the heat affected zone, which has the ferrite-tempered martensite microstructure.Keywords: simulation / modelling / gas metal arc welding / microstructure / dual phase steel / Die Ziele der gegenwärtigen Arbeit sind, das Verfahren des Metallschutzgasschweißens von DP 600 Blech-Stahl zu charakterisieren und die entsprechenden Modellierungstechniken zusammenzufassen. Der Zeit-Temperaturverlauf während des Schweißzyklus wurde experimentell gemessen und anhand der Simulationssoftware SimWeld modelliert. Um die Phasenumwandlungen während des Schweißzyklus zu modellieren wurden Dilatometrie-Versuche, für die Quantifizierung der Parameter für die Phasenfeldmodellierung durch MICRESSm, durchgeführt. Die wichtigsten Eingangsparameter sind Grenzschichtmobilität, Keimbildungsdichte etc. Es wurde eine Methode entwickelt, mit der die Transformation von Austenit zu Bainit in MICRESSm überführt werden kann. Dies ist nützlich, um die Mikrostruktur in den schnell abkühlenden Bereichen vorauszusagen. Das Phasenumwandlungsmodell ist in der Lage, die Mikrostruktur entlang der Aufheizund Abkühlungszyklen des Schweißens abzuschätzen. Zugversuche haben gezeigt, dass das Versagen in der Wärmeeinflusszone, die aus einer ferritisch-gehärteten martensitischen Mikrostruktur besteht, stattfindet.
Dual‐phase (DP) steels are suitable candidates for automotive applications due to their high strength and ductility. These advanced mechanical properties result from the special microstructure of the DP steel with 5∼20% martensite phase in a soft ferrite matrix. However, during welding, which is an important process in automotive industry, this special microstructure is destroyed. In this research the characterization of Gas Metal Arc (GMA) welded joining zones was performed by optical microscopy and hardness mapping. Tensile tests were also performed keeping the welded portion in the gauge length. Scanning Electron Microscopy (SEM) was used for the fracture investigation. From the characterization and tensile tests, the soften zones were found, which are caused by the tempered martensite and larger ferrite grain size than that in base metal. Furthermore, GMA welding make a large Heat Affected Zone (HAZ).
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