The formation of sub-surface defects during friction stir welding has limited the adoption of the transformative joining process in high-reliability applications and high-volume production. This is due to the need for costly post-process non-destructive evaluation (NDE) of welds in high-reliability applications, and a greater tendency for sub-surface void formation at greater process speeds.&nbsp; Process adoption can be increased by developing robust methods of in-process defect monitoring, along with designing the process for higher travel speeds while avoiding sub-surface defect formation. The objective of my research is to develop a fundamental understanding of how sub-surface voids form, as well as develop a fundamental understanding of how void formation alters measured process outputs. A fundamental physical understanding must be developed in order to advance the state of the art in these two areas. The fundamental understanding will form the basis for more robust methods of defect monitoring that are transferable across changes in process settings and will help drive improved numerical simulation of the sub-surface void formation process.The void formation process has been examined through multiple experimental techniques. The first of which was through an improved examination of process force transients during void interaction. Prior literature had shown that process forces oscillate at the tool rotational frequency and that material flow and void formation occur at the periodicity of each tool revolution. It was hypothesized that the eccentric motion of the tool (tool runout) was the primary driver of process force oscillations. A novel experimental setup was used to measure process forces in combination with the angular position of the eccentric points of the tool, which produced the observation that the eccentric motion of the tool applies the oscillatory force to the workpiece and thus appears to be the driver of the flow of material per revolution. Additionally, when features of the tool probe (flats) interact with void volumes there is a momentary reduction in the oscillatory force that the tool applies to the workpiece. The fundamental understanding of the force transients holds the potential to form the basis of a more robust force-based void monitoring method.The hypothesizes derived from force observations were examined further by experimentally measuring the real-time motion of the tool during friction stir welding via a laser vibrometer. The results showed that the runout of the tool is constrained by the workpiece during welding, which supports the hypothesis that the eccentric motion of the tool drives force oscillations. Additionally, the motion measurements showed that the tool is momentarily deflected into void volumes during feature interaction. This suggests that a motion-based method of void monitoring has potential for application, which would provide the option of using an accelerometer instrumented toolholder. A simple mass-spring-damper model was developed in order to describe the dynamic properties of the welding system that will govern tool motion during void interaction. This will provide a fundamental understanding of how the dynamics of a welding system will affect a motion-based method.Lastly, I am working on the realization of in situ imaging of the sub-surface void formation process. This will be achieved through the utilization of the high-speed X-ray imaging capabilities at Argonne National Laboratory’s Advanced Photon Source. Static imaging performed at the beamline has proven the feasibility of imaging sub-surface friction stir welding voids within thin sections of aluminum alloys. Subsequent dynamic imaging will be performed in order to capture the evolution of sub-surface voids within the material during welding. This will be achieved by friction stir welding thin sections of aluminum within the beamline. The resultant radiographic images will be used to test prior hypotheses on material flow and void formation physics. Additionally, it will inform the numerical simulation of the sub-surface void formation process. The fundamental knowledge gained as a result of my research will advance the state of the art in friction stir welding, allowing for increased adoption of the novel process in the U.S. manufacturing sector.

Sub-Surface Void Formation and Detection During Friction Stir Welding of Aluminum Alloys

Paper Proceedings: https://asmedigitalcollection.asme.org/MSEC/MSEC2021/volume/85062

2021 International Manufacturing Science and Engineering Conference (MSEC2021)

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