Shear Localization Modeling of Friction Stir Welding and Applications in Process Window Estimation

Abstract

Friction stir welding (FSW) has numerous advantages over conventional fusion welding processes, particular for metals or dissimilar metals that are difficult to weld with conventional welding method, which has been viewed as a key enabler for achieving structural lightweighting. However, FSW process development for a given application, i.e., establishing a process window in terms of welding travel speed and pin rotation speed, relies largely on empirical process, typically requiring detailed Design of Experiments (DoE) which can be rather time-consuming and costly. In this work, a shear localization model is presented to study the joint formation and defect generation mechanism during FSW by focusing the band structure development which is the unique feature associated with FSW process. A simplified shear localization model is first conducted to demonstrate its application in studying FSW process of different metals. With this model, some fundamental questions such as why titanium alloys are more difficult to weld than aluminum alloys or steels, can be more quantitatively addressed. Different material constitutive equations are also compared and Sellars Tegart material model is identified to be more suitable for the proposed shear localization model. A three dimensional analytical based heat transport model is further developed to provide the thermal environment within which shear localization phenomenon happens. Pin/workpiece interaction during FSW is studied in detail by introducing a contact mechanics model. The shear localization model is then refined by coupling the heat transport model and contact model. The refined shear model can provide a good estimation of peak temperature and torque information during FSW which is examined by published experimental data. Some of the major weld defect formation mechanisms (e.g. “lack of fusion”, “abnormal stirring”, “surface gilling” and “excess flash”) have been elucidated in details based on the refined shear localization model by further enforcing both mass conservation and plastic flow continuity conditions. A set of algorithms are also developed to theoretically estimate process window of FSW based on the proposed model. To demonstrate this capability, the model is exercised for three types of aluminum alloys on which process windows were determined through DoE and published by various investigators. The theoretically estimated process windows are in good agreement with experimental results.