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

灵活准确的搅拌摩擦焊工艺模型的发展将极大地促进搅拌摩擦焊工艺的优化和应用范围的扩大，这些模型可用于预测不同搅拌摩擦焊工艺参数对气孔率、焊缝力学性能以及最终焊缝微观结构的影响。本文所描述的研究工作的目的是在开发这种模型的道路上迈出一大步。提出了一种分层方法，首先，采用基于流体力学的模型来描述材料在焊缝中的热历史和材料在焊缝中的传输，以增强对搅拌摩擦焊接物理特性的理解。接下来，利用CFD模型开发过程中获得的知识和经验，建立实体力学模型。这两种模型将与搅拌摩擦焊接过程的综合实验表征相结合。在模型开发的每一步，实验程序将验证假设和结果，相反，建模结果将建议进行关键的实验。最初的建模尝试将与准确捕捉物理现象一样简单。随着对流程的理解的增加，随之而来的模型的复杂性、保真度和实用性也会增加。最终目标是创建一个基于固体力学的模型，该模型提供了搅拌摩擦焊接中所有材料的热、应力/应变、时间历史的完整描述。实验任务包括：(1)搅拌摩擦焊接过程的温度测量。数字红外相机将测量表面温度，而嵌入式热电偶测量地下温度。(2)摩擦搅拌焊机对焊缝输入能量的测量（将电能转化为机械能）。(3)通过破坏性焊后检查绘制标记材料的运动图，确定焊缝中的材料运输。(4)焊后显微组织评价。(5)焊接件的力学试验和断口检验。这些实验任务中的每一个都将有助于为所提出的模型建立适当的边界条件，或者有助于模型的验证，或者两者兼而有之。实验计划的细节可能会根据需要进行修改，以充分支持模型的开发。建模工作的关键任务包括：(1)确定适当的模型几何形状，在不使分析过于复杂的情况下捕获物理过程的本质。(2)从与所选模型几何形状相匹配的实验数据中得出适当的热力学边界条件。(3)选择合适的材料本构律来表征材料在焊接区的行为。(4)将本构律纳入为模型选择的计算流体/热/固体分析软件包中。如果将上述各方面综合起来，将得到一个预测模型，该模型可用于指导搅拌摩擦焊工艺参数的优化和工艺修改的开发，从而扩大搅拌摩擦焊的应用范围。

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.