Numerical and experimental investigations of friction stir welding of tube-tubesheet joints

In the present work, a novel tube-tubesheet seal welding method using friction stir welding (FSW) and experimental setup was developed. The development involved a number of design and manufacturing aspects such as, tool and fixture design, and FSW process parameters. The influence of tool welding speed and plunging control method on tool reaction loads as well as weld quality was experimentally investigated on Al 6xxx. It was found that better weld quality could be achieved at lower welding speeds by force control condition. Higher welding speeds required increasing the plunging force limit.;The friction stir seal welding process was modeled using three finite element modeling (FEM) techniques (3-D Thermo-mechanical non-flow based, Arbitrary Lagrangian Eulerian (ALE) formulation, and Coupled Eulerian Lagrangian (CEL) Analysis) using Abaqus. The first model was developed to predict temperature distribution and residual stresses developed in tube-tubesheet. The model considered the FSW tool as a moving heat and pressure source ignoring the metal flow, while taking into account the temperature dependency of thermal and mechanical properties. The computed temperatures were validated by experimental measurements using thermocouples. The maximum predicted temperature was found to be lower than the annealing temperature. The residual stresses distribution results indicated that the expanded joint strength would be enhanced by FSW. The contact stress was found to improve by about 50%.;Material flow and void formation were predicted using two localized models, which were developed based ALE formulation and CEL analysis. In both models, the tool was considered as an isothermal rigid body. The tool and workpiece interaction was modeled using Coulomb's frictional law and the material was modeled using Johnson-Cook's approach considering temperature and strain rate dependency and 90% inelastic heat generation. The influence of tool rotational speed was investigated during plunging. It was found that the lower the rotational speed, the wider the plasticized zone would be. The welding phase results showed that the maximum temperature was around 330 °C.;In the CEL model, the effect of coefficient of friction was investigated and results showed that the higher the coefficient of friction the smaller the defect size would be. Moreover, the influences of tool welding speed and plunging method were investigated, revealing similar results to those obtained experimentally.