Joakim Wahlsten, Pontus Rydgren, Mattias Lindén, 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 High Strengths Steels, UHSS, are employed to fulfil those needs. The majority of the materials intended for the body-In-white structure are joined by resistance spot welding, 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 was investigated in the previous project and a methodology for developing weld schedules for improved ductility was developed, including metallography and mechanical testing.

Two different post pulse heat treatments were developed; post pulse A, which increases both 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 high speed tensile shear testing.

The current work was aimed to further optimizing the post pulse technique developed in part 1. and to get a better understanding of the relation between soft zone and the fracture mode. The position and width of the soft zone have shown to be critical to which fracture mode will be received. Furthermore, additional material combinations have been introduced, Dogal 1000 DPX welded to itself and to Usibor. The improvements have been verified through regular tensile shear testing and for the most promising weld parameters also through high speed tensile shear testing. The process time for post pulse B on Usibor has been decreased by 33% with good results in mechanical testing, both static and high speed. The results from the mixed material joint were also good. However, the post pulsing technique developed here was not directly applicable to Dogal 1000DPX, the DP material used for the tests. More work is needed in this case.