On visualizing material flow and precipitate evolution during probeless friction stir spot welding of an Al-Li alloy

Highlights: • Severe thermal cycle and deformation refined grains and intermetallic particles in SZ. • T₁ phases fully dissolved in SZ followed by re-precipitation of GP zone and δ′ phases. • The predominant flow direction remained in spiral-like pattern during P-FSSW. • Local abrasion existed in the lap-weld interface due to flow difference, forming joining. - Abstract: The precipitate evolution and material flow visualization during probeless friction stir spot welding (P-FSSW) of an Al-Li alloy has been investigated experimentally and numerically in this work. Due to severe heat input during welding, the main strengthening phases of T₁ fully dissolved in the stir zone followed by the re-precipitation of GP zone and δ′ phases, assisted with the refinement of the intermetallic particles and grains. The history of material flow was three-dimensionally visualized with a tracer, studied with X-ray tomography. The experiments and the simulation results provided a consistent interpretation of a circumferential material flow which moved inwards radially near the top surface. Upward flow in the bottom sheet improved with dwell time, causing the interface to bend. Local abrasion between the two sheets due to flow difference led to the disruption of oxide layer, and formed a connection at the interface.