T. Weinberger, B. Führer, H. Schröttner, S. Khosa, N. Enzinger, TU Graz, Graz, Austria
Martensitic precipitation-hardening steels are characterized by high strength which is achieved by a martensitic matrix and precipitates. The material also shows a good ductility and toughness if properly heat treated. But welding of these steel types is often problematic and requires a special procedure in order to achieve satisfactory results. In this contribution, the solid state welding process friction stir welding (FSW) was used to weld PH15-5 and the results of the investigations are shown. The butt welds for 2.6mm thick steel sheets have been carried out with a MTS ISTR BR4 Friction Stir Welding machine at Graz University of Technology using a tungsten based tool, different welding speeds and tool rotational rates. Temperature measurements using thermocouples have been performed on the advancing and retreating side of the weld. The energy input for different parameter sets has been calculated using spindle torque and rotational rate. Detailed microstructural observations were performed for base material, heat affected zone, thermomechanically affected zone and stir zone using different methods. The appearances of retained austenite, which reduces the strength of the material and ferrite, which can reduce toughness and ductility, have been studied for the distinct regions of the friction stir weld. The influence of the energy input on the formation of the zones and evaluation of microstructure is described. A quantitative spot analysis by EDXS was performed to identify tool remanents in the nugget zone of the weld. For further characterisation, a hardness profile of the weld has been created. Tensile tests and surface fracture analysis using scanning electron microscopy (SEM) have been performed. Welds with low energy input have shown better results than welds with high energy input. Additionally, effects of post weld heat treatments on microstructure and properties of the joint have been analysed.
Summary: Parameters for friction stir welding of precipitation hardening steel were optimized and joints are characterised.
