Material flow and intermixing during dissimilar friction stir spot welding and friction stir seam welding are investigated. During friction stir spot welding, a ribbon of contiguous dissimilar lamellae is produced during each rotation of the tool and the number of intermingled lamellae contained in the intermixed region is determined by the tool rotational speed setting and the dwell time applied. When the rotating tool moves across the component, the ribbon of dissimilar contiguous lamellae continues to be produced and the linear distance in the traversing direction between dissimilar lamellae corresponds with the pitch distance [the travel speed (mm s⁻¹) divided by the tool rotational speed (Hz)]. The material flow pattern produced when a threaded tool moves across a component is therefore a variant of that produced during the touch down period when the rotating tool is held stationary. It is suggested that the onion ring structures observed in similar and dissimilar friction stir seam welds made using threaded tools are produced by material incorporation from the locations beneath the tool shoulder and the bottom of the rotating pin and the creation of a helical vertical rotational flow within the intermixed region formed beside the periphery of the rotating pin.

Local melting of eutectic films and cracking is found in Al 2024 and Al 7075 alloy friction stir spot welds. Dissolution of melted films removes all evidence melted film formation in spot welds made using typical welding parameter settings. For this reason friction stir spot welding is carried out at a rapid plunge rate of 10 mm s⁻¹ and an extremely short dwell time of 0·05 s and after tool retraction, the welded samples are rapidly quenched using a mixture of methanol and liquid nitrogen at a temperature of −80°C. Eutectic films rich in Zn and Cu are formed in Al 7075 spot welds while melted Al₂CuMg particles promote the formation of α-Al+Al₂CuMg eutectic films in Al 2024 spot welds. Melted eutectic formation and cracking is also observed beneath the tip of the rotating pin during Al 7075 friction stir spot welding and is consistent with the occurrence of melt wear in this location.