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 r/min) 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 was −15°C and the upper shelf energy increased to ~402 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.