Microstructural and mechanical characterisation of friction stir welded 15-5PH steel

Martensitic precipitation hardening steels are characterised 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 (e.g. post-weld heat treatment) in order to achieve satisfactory results. In this contribution, the solid state welding process - friction stir welding was used to weld 15-5PH and the results of the investigations are shown. The butt welds for 2.6 mm thick steel sheets have been carried out at Institute for Materials Science and Welding at Graz University of Technology using tungsten based tools, different welding speeds and tool rotational rates. Temperature measurements using thermocouples have been performed on the advancing and retreating sides of the weld. Detailed microstructural observations were performed for base material, heat affected zone, thermomechanically affected zone and stir zone. The appearance of retained austenite, which reduces the strength of the material, has been studied for the distinct regions of the friction stir weld. A quantitative spot analysis by energy dispersive spectroscopy was performed to identify tool remanents in the stir zone of the weld. For further characterisation, hardness profiles of the weld have been created. Tensile tests and surface fracture analysis using scanning electron microscopy have been performed. Welds with low energy input have shown better results than welds with high energy input. Additionally, effects of post-weld heat treatment on microstructure and properties of the joint have been analysed.