Mechanisms of formation of the structure of welded joints, performed by friction stir welding

Abstract

The analysis of welded joints of structural materials is carried out. As a result of this, in addition to the traditionally formed zone of thermal influence (HAZ), a zone of thermal deformation influence is formed in the near-seam zone, located between the welded nugget and the HAZ. The welded nugget joints in FSW is formed in the regime of structural superplasticity (SP). The structure of the welded nugget has a sheared band structure in the form of an "onion pattern" consisting of a large number of lamellas forming shear bands under the influence of a movable welding tool. The attainment of the superplastic state (SPS) during the formation of the welded nugget is ensured by the stepwise flow of various mechanisms of plastic deformation in the regime of simple, collective, anomalous dynamic recrystallization, prepared by dynamic return processes, polygonization with transition to postdynamic recrystallization by the mechanisms of Bailey- Hirsch, Kahn-Burgers-Taylor. For the group of metals and alloys with polyformism (high-alloy steels of different structural classes, Ti- alloys), SPS is supported additionally by recrystallization by twinning (copper, austenitic steel and other structural classes, Ti and its alloys) and as a result of phase transformations of alpha-gamma or alpha-beta phases).