Role of grain size in governing the mechanical response in friction stir welded high aluminum low-density ferrite steel

The susceptibility of high-Al ferritic steels to thermal cracking and microstructural coarsening during fusion welding restricts their practical implementation. To address this challenge, high-quality joints of Fe-10Al alloy were successfully fabricated by friction stir welding (FSW) under ultra-low rotation (80 rpm) and high load (40 kN) at sub-700 °C. The effects of the unique thermomechanical conditions, combined with post-weld heat treatments, on microstructural evolution and mechanical performance were systematically examined. Grain refinement (~ 6 μm) was found to markedly enhance ductility and toughness. The Fe-10Al alloy exhibited a local elongation (L-El.) of 28%, surpassing previously reported values, while the ductile-to-brittle transition temperature decreased by ~15 °C and the upper shelf energy increased to ~390 kJ/m², both due to improved crack propagation resistance. Grain refinement also shifted the dominant deformation mode from twinning to dislocation slip and raised the fraction of “effective grains” with high-angle boundaries (15–65°) to ~64%. These findings demonstrate that FSW provides an effective strategy to overcome weldability issues in high-Al ferritic steels, offering a pathway to their broader structural applications.