Development of Coupled Thermal, Mechanical, and Material Transport Models of the Friction Stir Welding Process

点击翻译按钮获取中文摘要

Optimization of and broadening of applications for the friction stir welding (FSW) process will be greatly accelerated by the development of flexible and accurate process models which can be used to predict the effects of varying FSW process parameters on porosity, mechanical properties of the weld, and, ultimately, final weld microstructure. The intent of the research effort described herein is to take a large step down the path toward development of such models. A hierarchical approach is proposed in which first, a fluid mechanics based model describing the thermal history of material in the weld and the material transport in the weld is used to enhance the understanding of the physics of friction stir welding. Next, using the knowledge and experience obtained from development of the CFD model, a solid mechanics model will be developed. Both models will be developed in combination with a comprehensive, experimental characterization of the friction stir welding process. At every step of the model development, the experimental program will verify assumptions and results and conversely, modeling results will suggest critical experiments to be performed. Initial modeling attempts will be as simple as is consonant with accurately capturing the physical phenomena. As understanding of the process increases, so will the complexity, the fidelity, and utility of the models which follow. The ultimate goal is to create a solid mechanics based model which provides a complete description of the thermal, stress/strain, time history of all the material in a friction stir weld. Experimental tasks to be undertaken include: (1) Temperature measurement during the friction stir welding process. A digital IR camera will measure surface temperatures while embedded thermocouples measure subsurface temperatures. (2) Measurement of energy input to the weld by the friction stir-welding machine (via conversion of electrical energy to mechanical). (3) Determination of material transport in the weld by mapping the movement of marker materials via destructive post weld examination. (4) Post-weld microstructural evaluation. (5) Mechanical testing of weldments and fractographic examination. Each of these experimental tasks will either help to establish proper boundary conditions for the proposed models or will aid in verification of the models or both. Details of the experimental program may be amended as needed to fully support the model development. Critical tasks for the modeling efforts include: (1) Determination of an appropriate model geometry that captures the essence of the physical process without unduly complicating the analysis. (2) Deriving appropriate thermal and mechanical boundary conditions from experimental data that are compatible with the chosen model geometry. (3) Selection of an appropriate material constitutive law for characterizing the behavior of the material in the weld zone. (4) Incorporating the constitutive law into computational fluid/therma/solid analysis packages chosen for the model.If all of the pieces outlined above come together, the result will be a predictive model which can be used to guide friction stir welding process parameter optimization and give guidance in the development of process modifications which can broaden the application of FSW.