SGER: Investigating Recursive Plastic Heating in Third Body Regions during Friction Stir Welding

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Conditions within the third body region during Friction Stir Welding (FSW) are crucial for resolving asymmetries and minimizing the inherent tendency to form voids on the advancing side of the tool. Despite being recognized as an important part of the FSW process, very little work has been done on modeling the third body to explain how it emerges from system variables or its role in generating heat during the FSW process. In this Small Grant for Exploratory Research (SGER), a theory of recursive plastic heating (RPH) is proposed to show that plastic work within the third body region sustains FSW through important inertial effects which originate from a chip forming mechanism and produces a significant portion of the heat required. The new framework suggest that friction is mainly a catalyst for recursive plastic heating which may be enhanced by hybrid FSW strategies. The goal of the investigation is to formulate and verify RPH theory within the third body and investigate hot cavity approaches for augmenting FSW to create conditions which allow the third body to appear sooner and move faster than possible with friction as the sole means of heat input. This research includes experimental verification, combining RPH theory with a framework for orthogonal cutting, and formulation and solution of a mathematical model using RPH theory to predict how the third body develops under processing conditions. RPH theory suggests that volumetric heating and thermo-mechanical hot channels can influence third body conditions and minimize demands on tool materials. As efforts continue to extend FSW to a wider range of materials, RPH theory provides an alternative to producing tools with increasing hardness to meet the demands of increasingly tough workpiece materials. The notion of recursive plastic heating as the dominant mechanism for reducing flow stresses during FSW represents a paradigm shift away from a framework where frictional heating from the FSW tool is the central mechanism. Within RPH theory, frictional energy is recast as a catalyst for plastic flow; therefore, verification offers the potential to broaden the interpretation of hybrid FSW technology to hot cavity methods like Laser Assisted Stir Welding and Plasma Assisted Stir Welding. In addition, strong interaction with the shipbuilding industry should lead to rapid technology transfer.