Formation and Structure Features of the Weld Joints Made by Friction Stir Welding

Abstract

The microstructure of the weld core and the heat-affected zone, as well as the zone affected by thermal strain located between them, formed during friction stir welding (FSW) of aluminum alloys, copper alloys, 12Kh18N10T steel, and OT4-1, VT-1 alloys is investigated. The core of the welded joints at FSW is formed in the structural superplasticity mode, and its structure has a shear-banded structure of the “onion pattern” type containing a large number of lamellae forming the shear bands under the influence of the movable welding tool. Attainment of the superplastic state during formation of the weld core is ensured by the gradual flow of various plastic deformation mechanisms in the mode of simple, collective, and anomalous dynamic recrystallization initiated by the dynamic recovery and polygonization processes with transition to post-dynamic recrystallization by the Bailey–Hirsch and Kahn–Burgers–Taylor mechanisms. The superplastic state is also maintained owing to twinning recrystallization (copper, steel, Ti and its alloys) and as consequence of α → γ or α → β phase transformations for a group of metals and alloys possessing multiple forms (high-alloy steel, titanium alloy).