Laser-arc hybrid welding of high strength steel : Weld quality and process stability

Sammanfattning: The development of high strength steel enables constructions to be made with less material than with previous used steel grades. The possibility of creating lightweight constructions with the same or even better mechanical properties is of high focus in the steel industry today. This can reduce productional costs as well as transportation related costs and have a less negative environmental impact. As steels are developed to be more durable, the processing of these steels grow more challenging and the use of conventional welding methods such as MIG/MAG-welding prove difficult (impossible for some steel grades) to obtain acceptable quality with single pass welds. The interest and use of laser-arc hybrid welding has increased during the last decade for its high welding speed and deep weld penetration. There are however still areas of the process that are not well documented. In this master thesis project, joining of laser cut high strength steel sheets with laser-arc hybrid welding has been tested to evaluate weld quality and process stability. The tests was performed on threesteel grades with increasing strength (S960QL, S1100QL and S1300QL). Steel sheets with different pre-welding preparations has been laser-arc hybrid welded and a documentation displaying resulting differences in weld geometry, microstructure, process stability and hardness has been created. For example, samples welded withthe mill scale still present was found not to have negative impact in terms of weld geometry and hardness when using laser-arc hybrid welding. Cooling rates was collected to aid the evaluation of microstructural composition in the welded area. The results of the work in this thesis project can be of aid in making the joining process of high strength steels more efficient.