Gestamp (Sweden)

Resistance spot welding (RSW) is widely used as a main joining technique in industry and the electrode caps are frequently replaced because of the degradation during service. In this study, the G type copper RSW electrode caps were coated with Fe and Ni based Fe3Al, FeAl, Ni3Al, NiAl alloys by Electro-Spark Deposition (ESD), providing resistance to hot deformation, oxidation and Zn evaporation from sheet metal. The ESD coated electrode caps were tested in-situ on a hot dip galvanized steel in order to assess the performance of RSW electrode caps. For this purpose, three different coating voltages were selected for each coated electrode, and 12 different cap coatings were produced in total. Fifty resistance spot welds were consecutively manufactured with the same parameters for each type of coating electrodes. Hardness measurements, macrostructural examination, Ultrasonic Testing (UT) and chisel tests were performed on welded samples produced. In addition, effects of different coatings on RSW electrode caps were investigated on microstructural development, hardness variations and deformation capacity of resistance spot welds. Results showed that chisel tests and cross section thickness values of the welded sample made with the caps that were ESD coated with the Ni3Al electrode produced better results than the other caps. The cross-sectional thickness of nuggets was lower in all 158 V coated caps. The performance of aluminide coatings on RSW electrode caps can be listed from the best to the worst in the order of Ni3Al, NiAl, Fe3Al, and FeAl.

Press Hardening offers the possibility to obtain a wide range of mechanical properties through microstructural tailoring. This strategy has been successfully applied in thin sheet components, for instance, through differential cooling strategies. The application of these added value features to truck components implies adapting the process to the manufacture of thick sheet metal. This introduces an additional layer of complexity, but also opportunity, in a process where the final microstructure and, thus the mechanical performance is generated in the press shop. This work presents a study on optimizing the crash worthiness and impact energy absorption on a press hardened thick 22MnB5 steel sheet. Different microstructure design strategies have been studied, including ferrite-Pearlite (representative of a differential heating and austenitization strategy), in-die generated Bainite (representative of differential cooling) and Tempered Martensite (generated through laser tempering), keeping a fully hardened martensite as a reference condition. The material performance has been compared in terms of the monotonic properties, useful for anti-intrusion performance, and Essential Work of Fracture, a well-suited parameter to predict the crash failure behavior of high strength steels. The results show that laser tempering offers properties similar to Bainite-based microstructures and can be a successful replacement in components where the sheet thickness does not allow for the fine control of the in-die thermomechanical evolution.

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.