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Increasing the ductility of RSW joints in UHS steel through parameter adjustments, part 1

Pontus Rydgren, Joakim Wahlsten and Joakim Hedegård

Abstract

Automotive companies are continuously seeking ways to reduce fuel consumption and to improve occupant safety of their vehicles. Materials such as Ultra HighStrengths Steels, UHSS, are employed to fulfil those needs. The majority of the materials intended for the body-In-white structure are joined by resistance spotwelding, RSW. However, RSW joints in UHSS often exhibit a tendency for less ductile fracture behavior.

The ductility in terms of energy absorption is very dependent upon the fracture mode.The reason for a specific fracture mode is investigated in this project and a methodology for developing weld schedules for improved ductility has been developed, including metallography and mechanical testing.

The current work showed that heattreatment by post pulsing increases the nugget size or the width of the soft zone in the heat affected zone, HAZ, of Usibor steel and there by increases the ductility of the joint.

Two different post pulse heat treatments are developed; post pulse A, which increasesboth the weld nugget and the HAZ soft zone width and post pulse B which places the soft zone from the post pulse right inside the soft zone from the weld pulse creating a wider soft zone without enlargement of the nugget. Both weld schedules increase the ductility in terms of energy absorption compared to the reference (welding without post pulse) in both regular tensile shear testing and highspeed tensile shear testing.

The methodology can be further optimized,but the results are very promising in terms of creating ways to improve ductility,energy absorption and weld fracture type.

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Senast ändrad av: Peter Hjertsson
26 mars 2015

 


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