Understanding How Aluminum Moves Around a Friction Stir Welding Tool in order to Prevent Welding Defects

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This award will support fundamental scientific research that will investigate how aluminum moves around a friction stir welding tool and how this movement influences the formation of sub-surface defects (voids). Due to its many advantages over other welding processes, friction stir welding has rapidly gained popularity in numerous manufacturing industries. However, certain limitations have slowed its mass implementation. This research seeks to address a lack of knowledge about two limitations in friction stir welding: (1) the need for costly post weld inspection of sub-surface defects in high-reliability applications (for example: defense and aerospace industries), and (2) the desire to increase processing speed to allow for high-volume production (for example: automotive industry). Faster welding speeds increase the likelihood of defect formation. A fundamental understanding of the precise mechanisms of material flow around the friction stir tool and its role in sub-surface defect formation is currently lacking. This research will generate the knowledge necessary to help U.S. manufacturers predict defect formation, hence allow them to design a friction stir welding process that produces defect-free welds. Addressing the aforementioned limitations will greatly expedite the mass implementation of this joining method, which will have a positive impact on U.S. manufacturing, the U.S. economy, and National Security. The award will also facilitate training of the future workforce as students across all levels will gain exposure to and experience in advanced joining technologies. Knowledge generated from this project will be distributed publicly through conference presentations, journal articles, industry tours, and open house engineering expositions.The goal of this research is to generate a fundamental understanding of the physics behind the intermittent movement (an extrusion-like process) of metal around the friction stir tool once per tool revolution. This phenomenon has been widely reported in the manufacturing engineering community by means of force measurements and by studying weld cross-sections after completing the weld. However, an understanding of why this intermittent material movement occurs and direct in-process observations do not exist. One leading hypothesis is that a cavity opens up in the wake of the advancing tool and that material is subsequently extruded into this cavity during each rotation of the tool. In a good welding condition, this cavity is completely filled. However, when there is a breakdown in the material flow, the cavity is not filled completely and a defect (void) remains. This hypothesis will be tested by means of novel, in-process proton radiography of the friction stir welding process of an aluminum alloy. Proton radiography at the Los Alamos Neutron Science Center provides the capability of producing a series of radiographic images (like x-rays) of the weld zone that will show whether or not a cavity is forming and filling. The knowledge gained through this physical experimentation, combined with other experiments conducted at the University of Madison-Wisconsin, will drive the numerical simulation of the intermittent flow phenomenon. Simulation of this phenomenon is largely absent from the published literature. In this work, the complex material flow and cavity formation and filing processes will be modeled and simulated using advanced Lagrangian based numerical techniques. Ultimately, this will provide the means to predict defect formation, and design measures to avoid it during production.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.