Innovative Methods for Welding Ultra High Strength Steel with Resistance Spot Welding

Sammanfattning: Resistance spot welding (RSW) is the most frequently used method for welding thin sheets in manufacturing industries such as the automotive industry, because of the high productivity of RSW. In order to reduce CO2 emissions the automotive industry strives towards creating structures of light weight, this is partly achieved by the use of lightweight materials such as Aluminum and composite materials. In parts of the car body designed to protect the driver and passengers in case of a collision High strength steel is used due to its high strength and relative high ductility. High strength steels are called Ultra High Strength Steels (UHSS) with typical ultimate tensile strength of 700 up to 2000 MPa and elongation of 10-40%. Because of the strive against lighter structures and great safety demands UHSS materials is of great interest for the automotive industry in order to create strong structures of light weight. In welding of modern materials such as UHSS with RSW, achieving adequate weld quality is a challenge. Hence this thesis aims to investigate new innovative ways to broaden the area of use and include modern materials such as UHSS for the traditional method of welding such as RSW. In RSW elliptical shaped welds are created between two or more faying metal sheets by passing current through the sheets. The current is applied to the sheets by copper electrodes in contact with the sheets on each side. The geometrical shape of these electrodes will affect multiple welding parameters such as applied stress, current density, electromagnetic stirring, temperature gradients and the possibility for the welded material to thermally expand during welding. Hence the geometrical shape of the electrodes will affect the final shape and size of the weld nugget. In this thesis RSW electrode geometries are modified and tested. The weld properties from modified electrodes are compared to the weld properties from standard RSW electrodes with respect to process robustness, weld nugget shape and size, micro hardness and weld tensile strength. Various modified geometries are used, all modified geometries are designed in order to allow the welded material to expand more, compared to standard electrodes. Previous work has been done and shown that hollow electrodes that allow the welded material to expand can improve the weld quality and process robustness. However, this has been to the cost of nugget growth in the normal direction to the welded sheet, leaving a non-uniform surface. Hence the aim of this thesis is to investigate if it is possible to widen the current range in the weld lobe diagram when welding UHSS material combinations with RSW by the use of hollow electrodes without affecting weld quality negatively compared to standard electrodes. Weld quality in this thesis will be evaluated based on surface condition, mechanical strength, micro-hardness and weld nugget size. The modified electrodes have shown better weld properties with respect of current range in the weld lobe curves in most cases tested but not all of the material combinations tested compared with standard electrodes. The surface conditions of the welded specimens have been controlled by measuring any indent and raise by line laser scanning. Modified RSW electrodes has showed improved welding properties with respect to current range in the weld lobe curves compared to standard RSW electrodes when welding UHSS material combinations. However modified electrodes have shown a higher sensitivity to misalignment and angle fault. Several material combinations of UHSS that has shown non-weldable behavior with standard RSW electrodes have shown improved current range. In the best case the current range was increased to 3,9 kA for an UHSS material combination that is non-weldable with standard electrodes.