Elucidating the role of ordered phases and carbides in the mechanical performance of friction stir welded high aluminum low-density ferrite steels

The widespread adoption of high-Al steels is hindered by weldability challenges due to thermal cracking susceptibility and coarse-grained phase transformations by traditional fusion welding. For the first time, we demonstrate an innovative low-temperature (<700°C) friction stir welding (FSW) for processing Fe-10Al and Fe-0.1C-10 (wt%) Al alloys. The X-ray diffraction (XRD) and transmission electron microscope (TEM) observations confirmed that the ultralow-rotation/high-load FSW process induces order phase DO₃ disordering by severe plastic deformation and κ-carbide shearing by gliding dislocations, collectively relieving stress concentrations while achieving ∼6 μm grain refinement. This unique microstructural evolution promotes dislocation slip-dominated deformation and consequently superior strength-ductility synergy in high-Al steels compared to the existing methods. Quantitatively, the small sized specimens of Fe-10Al alloy exhibited a tensile strength of ∼684 MPa with a total elongation of ∼40 %, local elongation around 28 %, and an impact upper shelf energy of ∼390 kJ/m² with a DBTT of –15 °C. Meanwhile, the Fe-0.1C-10Al alloy achieved a higher strength of ∼725 MPa and a total elongation of ∼38 %, along with an upper shelf energy of 454 kJ/m². However, due to the presence of residual κ-carbides and DO₃, faster crack propagation occurred during impact testing, resulting in an increased DBTT (∼70 °C). The study establishes new FSW parameter-design principles for manufacturing high-performance high-Al steel components.