In-depth microscopic characterisation of the weld faying interface revealing stress-induced metallurgical transformations during friction stir spot welding

Friction stir spot welding (FSSW) is a solid-state process, wherein the properties of weld joint are influenced by state friction and localised thermodynamic conditions at tool-workpiece interface. An issue well-known about FSSW joints their lack reliability since they abruptly delaminate weld-faying interface (WFI). This study explores origins delamination multiple lap welded aluminium alloy (AA 5754-H111) sheets joined different rotational speeds typically used in industry. Experimental techniques such as small punch test (SPT), Vickers hardness test, Scanning Electron Microscopy (SEM), Acoustic Microscope (SAM), Transmission (TEM), Energy-dispersive X-ray spectroscopy (EDX) Frequency-Modulated Kelvin Probe Force (FM-KPFM) were employed. The experimental results revealed that complex interplay stress-assisted metallurgical transformations intersection WFI recrystallised zone (RSZ) can trigger dynamic precipitation leading to formation Al3Mg2 intermetallic phase, while metallic oxides nanopits remain entrapped WFI. These surrounded pits, precipitates induces process instability which turn paves way for fast fracture become responsible delamination.