Extensive Introduction of Ultra High Strength Steels Sets New Standards for Welding in the Body Shop

Abstract

In order to meet upcoming legislative demands regarding acceptable levels of CO2 emissions and to contribute to the fight against global warming, while also meeting customer expectations of reduced fuel consumption, all automotive OEMs are today focusing on lightweight engineering. Some of them, mainly low volume premium brands, have chosen to introduce fairly expensive lightweight materials such as aluminium and magnesium to meet these targets, whereas the main portion of high volume producers are trying to optimize the classic steel concept by introducing different grades of advanced high strength steels. Volvo Cars has decided upon a unique utilization of hot-formed, press-hardened, ultra high strength steel components featuring tensile strength levels in the order of 1 500 MPa, but on the other hand, producing these parts in very thin gauges for weight saving reasons. The first product launched according to this ultra high strength steel intensive concept was the 2008YM version of the Volvo V70 and its sibling, the cross-country version XC70, both built on the EuCD platform. The body contains several parts manufactured by hot-forming and press-hardening. The extensive use of this type of Boron alloyed steel challenged all welding methods commonly used in car body manufacturing, not least, traditional resistance spot welding. This paper will address the following topics: overview of the V70 body structure and utilization of materials, a brief description of the hot-forming, press-hardening process, the procedure for validating weldability before going into series production, lessons learnt from resistance spot welding trials of material combinations involving one or more boron alloyed steel parts, and recommendations for default welding data, influence on the manufacturing system, introducing electro-servo welding guns, adaptive weld timers and ultrasonic non-destructive weld quality checking, necessary revisions of existing spot weld requirements. The presentation will end with a glimpse at perspectives for future welding challenges in Volvo body shops, as the need for further weight saving will promote a considerable introduction of various new grades of advanced high strength steels.