Friction stir welding of constrained groove pressed Al1050 using Al2O3 nanoparticles

In this investigation, two passes Constrained Groove Pressed (CGPed) Al1050 sheets went through Friction Stir Welding (FSW) by adding nano-sized Al2O3 particles. Then, the microstructure and mechanical properties of the welded joints, undergone various pass numbers and rotational to traverse speed (ω/ν) ratios, were presented. Findings showed all joints were fractured from the SZ, although HAZ softening happened due to grain growth in this region. The presence of the Al2O3 particles in the stir zone led to a fine and equiaxed grain structure, resulting from continuous dynamic recrystallization (CDRX) during the high-temperature welding process. This grain refinement contributed to an improvement in the mechanical properties of the joints, such as strength and microhardness. Employing two different ω/ν ratios resulted in a variety of thermal cycles during the welding procedure such that the peak temperature in the ω/ν of 70 and 25 r/mm reached ∼423 °C and ∼285 °C, respectively. Microstructure studies via optical observation and EBSD analysis illustrated that employing a higher ω/ν ratio and weld pass number resulted in more uniform particle distribution in the stir zone thanks to facilitating materials flow and incorporation during FSW. As a result, the three passes FSWed sample at the higher ω/ν ratio of 70 r/mm showed the most homogeneous particle distribution and finest grain structure in the SZ. This microstructure evolution was in reliable agreement with the mechanical performance of the joints. The three passes FSWed sample at the higher ω/ν ratio of 70 r/mm demonstrated the highest weld strength, microhardness, and %elongation compared to other FSWed joints. This sample showed an ultimate tensile strength (UTS) of ∼97 MPa and a %elongation of ∼16%, and its mean microhardness value had a ∼3.5% increase compared to the CGPed base metal. Conversely, the one pass FSWed sample at the lower ω/ν ratio of 25 r/mm represented the weakest mechanical properties due to cavities, defects, and agglomerated Al2O3 nanoparticles. The presence of nanoparticles was a decisive factor in microstructural evolution during the welding, thereby increasing the mechanical quality of the joints dependent upon their uniform dispersion, obtained by increasing the pass number and ω/ν ratio.