Axial force effects on the mechanical performance of friction stir welded aluminium and its alloys: A comprehensive review

In friction stir welding (FSW), axial force controls tool-workpiece contact, shoulder penetration, torque, heat generation, material flow and consolidation, but no Aluminium (Al) focused review has related these phenomena to joint performance. An OpenAlex query from 2000 to 30 July 2026 combined FSW with axial, vertical, downward, forge, plunge and process-force and was complemented by backward reference scanning. Studies that assessed, varied, modelled, or controlled axial force or used force signals for monitoring were considered; unrelated FSW records, studies with no meaningful force study, FSP/FSSW reports, and non-primary literatures were excluded. 59 dealt with Al, out of 105 qualified reports. Where data were comparable, quantitative comparisons were made; heterogeneity prevented meta-analysis. For all alloy systems, the response is non-monotonic. Both too neutron force for stable penetration and consolidation resulting in root defects, voids, weak bonding; too large force and producing sticking, flash, thinning, softening, nugget/TMAZ distortion and tensile residual stress. Thus, the optimum window will be a function of alloy composition and temper, plate thickness, tool/shoulder design, rotational and traverse speed, heat input and control mode. In linking these interplay effects to microstructure, hardness, tensile strength and joint efficiency, wear, fatigue, fracture, impact/high-strain-rate response and residual stress, this review will contribute to the establishment of an aluminium-specific force-process-property framework that can even include sensor-based monitoring, adaptive control and digital twins.