Influence of welding parameters on the microstructure, thermal fields and defect formation in AA7075-T6 friction stir welds

This study focuses on the influence of the welding parameters on defectiveness, thermal field and grain size in friction stir welding of aluminium alloy 7075-T6. Different welded specimens were produced employing various rotational speeds and welding speeds. In the first part of the analysis, the quality of the joints was evaluated by microscopic observation of the jointed cross-sections to investigate the modification of the shape of the stirred zones and the occurrence of internal voids. Afterwards, temperature and grain size of welds obtained at welding speed of 60 mm/min have been measured to study the influence of the rotational speed on the heat input. Temperature measurements have been carried out with thermocouples and infrared camera. Grain size evolution of the material was analysed by optical observations of the jointed cross-sections and the average grain size in the nugget zone (NZ) has been estimated through electron backscatter diffraction. The results globally show that the first internal defects arose at the same time as an unexpected zone on the advancing side between the shoulder affected zone (SAZ) and NZ. In sound welds, from 800 to 1400 rpm, the peak temperature showed an increase between 5 and 15%, depending on the considered zone, while the grain size in the stirred zone increased from 2 to 3.6 mu m. Overall, it has been shown that the approach to the defective condition is due to the low heat input which is responsible for improper material flow during the process.