Microstructure analysis and solid mechanics modeling of friction stir welding

Friction Stir Welding (FSW) is one of the most significant developments in solid-phase welding technology in the last decade. Joining in FSW is achieved through a mixing and extruding action of a rotating pin-shoulder tool that moves between two parts being joined.; This dissertation describes a combined numerical/experimental investigation of the FSW process. In the numerical part of the investigation, solid mechanics based models have been developed for the FSW process. To perform numerical simulations of the process based on the solid mechanics models, finite element procedures have been established and demonstrated by utilizing a general-purpose commercial software. The focus of the simulations is to determine the velocity field, material flow characteristics and plastic strain distributions in FSW. An Arbitrary Lagrangian-Eulerian (ALE) finite element formulation with adaptive meshing is adopted, which considers elastic-plastic deformation, finite strain, and temperature-dependent material properties. Two interface models (the Slipping Interface Model and the Frictional Contact Model ) have been proposed to simulate the complex physical phenomenon along the tool and work-plate interface. In the experimental part of the study, material flow patterns during and after welding have been experimentally determined by extending an existing visualization technique, in which thin slices of materials are placed along the faying surface between two plates being butt-welded. The microstructures of FSW welds corresponding to a wide range of process parameter values have been examined.; The main conclusions of present investigation are summarized here. (1) Simulation predictions based on the slipping interface and frictional contact models show good consistency with each other. (2) Simulation predicted marker positions during and after welding compare well with experimental measurements. (3) Simulation results show that a thin velocity boundary layer exists around the rotating tool. (4) Simulation predictions and experimental observations both show that material particles ahead of the rotating tool pass the tool from the retreating side and not from the advancing side. (5) Simulation results show that large plastic strains exist (a) in a thin layer around the tool and (b) in the advancing side plate material of the weld behind the tool. (6) Experimental examinations of microstructures in friction stir welds reveal that an “onion ring” type pattern always occurs in the advancing-side-plate material of the welds. (7) Simulation results show that the onion ring pattern is formed due to the build-up of high plastic deformation bands at the boundary of the velocity layer, located behind the rotating tool in advancing side. (8) Comparisons of simulation predictions and experimental observations demonstrate that there exists a good correlation between effective plastic strain distributions and microstructure zones in friction stir welds.