Process-Induced Defects in Friction Stir Additive Manufacturing: Mechanisms, Detection and Mitigation

Friction Stir Additive Manufacturing (FSAM) is a layer-by-layer solid-state AM process derived from friction stir lap welding. By joining metal plates below melting, FSAM produces fine equiaxed microstructures with superior mechanical properties and minimal melt-related defects (such as porosity or solidification cracks) compared to fusion-based AM. However, the process’s thermal–mechanical lap-weld character also introduces its own defect spectrum. Major defect classes reported in the literature include geometrical distortions (macroscopic misalignment or unbonded side edges), surface irregularities (e.g. flash, roughness, wormhole marks), internal flaws (e.g. interlayer voids, tunnels and “kissing” bond cavities), and metallurgical anomalies (e.g. incomplete bonding or oxide entrapment between layers). The review critically examines these FSAM defects—discussing their root causes and how they are detected or characterized. Defect detection in FSAM relies on both destructive techniques (cross-sectional microscopy and mechanical testing) and nondestructive methods (including ultrasonic scanning and X-ray computed tomography). Recent studies highlight several mitigation strategies aimed at minimizing defect formation. These strategies include optimizing tool geometry, fine-tuning process parameters, and applying enhanced axial force or post-weld forging to improve material consolidation and structural integrity.