Preliminary study on the microstructure and mechanical properties of dissimilar friction stir welds in aircraft aluminium alloys 2024-T351 and 6056-T4

Abstract

Aircraft aluminium alloys generally present low weldability by traditional fusion welding process. The development of the friction stir welding has provided an alternative improved way of satisfactory producing aluminium joints, in a faster and reliable manner. In the present work dissimilar Al alloys (AA2024-T351 and AA6056-T4) were friction stir welded. Butt joints were obtained by varying process parameters, namely the rotational speed (500–1200 rpm) and the welding speed (150–400 mm/min), while axial force and tool geometry were kept constant. Parameter optimisation, which has been based on the results of macrographic analysis and microhardness testing, indicated that sound joints can be obtained in the parameters range of rotational speed equal to 800 rpm and welding speed of 150 mm/min. Light and scanning electron microscopy in several positions revealed the presence of a lamellar material flow pattern due to the differential flow, suggesting material mechanical mixing within the stirred zone. High level of strain and temperatures usually over 400 °C, resulted in a dynamically recrystallised stirred zone with refined grains. Tensile testing (standard flat L-T samples) has shown that strength is up to 90% of the weakest joining partner 6056-T4. Fracture took place in the thermo-mechanically heat affected zone of the alloy 6056-T4, where annealed structure led to decrease in microhardness. This behaviour was confirmed by microflat tensile testing, which indicated the drop in tensile strength and the associated increase of strain, in the regions of microhardness drop. This study has shown that in a dissimilar friction stir weld, the weaker component dictates the performance of the joint, where failure happens in the region of the greatest strength reduction related to annealing phenomena. Microscopic investigation as well as the evaluation of local mechanical properties has suggested that mechanical mixing is the major material flow mechanism in the formation of the stirred zone.