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Abstract This study employs a high-fidelity numerical framework to determine the plastic material flow patterns and temperature distributions that lead to void formation during friction stir welding (FSW), and to relate the void morphologies to the underlying alloy material properties and process conditions. Three aluminum alloys, viz., 6061-T6, 7075-T6, and 5053-H18, were investigated under varying traverse speeds. The choice of aluminum alloys enables the investigation of a wide range of thermal and mechanical properties. The numerical simulations were validated using experimental observations of void morphologies in these three alloys. Temperatures, plastic strain rates, and material flow patterns are considered. The key results from this study are as follows: (1) the predicted stir zone and void morphology are in good agreement with the experimental observations, (2) the temperature and plastic strain rate maps in the steady-state process conditions show a strong dependency on the alloy type and traverse speeds, (3) the material velocity contours provide a good insight into the material flow in the stir zone for the FSW process conditions that result in voids as well as those that do not result in voids. The numerical model and the ensuing parametric studies presented in this study provide a framework for understanding material flow under different process conditions in aluminum alloys and potentially in other alloys. Furthermore, the utility of the numerical model for making quantitative predictions and investigating different process parameters to reduce void formation is demonstrated. 

Abstract Sub-surface voids and material heterogeneities resulting from the friction stir welding (FSW) process often necessitate post-weld inspection to ensure the quality of weld obtained from this solid-state welding process. In this context, in-process void detection techniques can potentially help in optimizing the process conditions and thereby reduce expensive and time-consuming post-process inspection of welds. Current in-process void detection techniques rely on approaches that try to directly correlate the part-scale welding quality to void formation, without a fundamental understanding of the underlying mechanics and materials physics that modulate void evolution. In this work, we demonstrate an effective in-process numerical technique that uses process force signals to detect volumetric void formation and connect the variations in the force signals to interactions between the tool probe and the underlying material voids. Our approach relies on a high-fidelity finite element analysis simulation of the FSW process and on correlation of numerically obtained process force signals with the corresponding void structures. This correlation is obtained in the phase-space relating in-plane reaction forces on the tool to the tool rotation angle. We focus on the interactions of the tool geometry and tool motion with the surrounding material undergoing plastic deformation and deduce novel insights into various correlations of tool motion and void formation. Through this approach, we can identify tool-related process conditions that can be optimized to minimize void formation and demonstrate a potential in situ force-based void monitoring method that links to the underlying plastic flow and void structures during the FSW process. 

Abstract The goal of this research was to examine how altering the amount of friction stir tool eccentricity while controlling the amount of slant in the tool shoulder (drivers of oscillatory process forces) effects the generation of process force transients during sub-surface void interaction. The knowledge gained will help improve the accuracy of force-based void monitoring methods that have the potential to reduce the need for post-weld inspection. Process force transients during sub-surface void formation were examined for multiple tools with varying magnitudes of kinematic runout. The eccentric motion of the tool produced oscillations in the process forces at the tools rotational frequency that became distorted when features (flats) on the tool probe interacted with voided volumes, generating an amplitude in the force signals at three times the tool rotational frequency (for three-flat tools). A larger tool eccentricity generates a larger amplitude in the force signals at the tool’s rotational frequency that holds a larger potential to create a distortion during void interaction. It was determined that once void becomes large enough to produce an interaction that generates an amplitude at the third harmonic larger than 30% of the amplitude at the rotational frequency in a weld with no interaction (amplitude solely at rotational frequency), the trailing edge of the tool shoulder cannot fully consolidate the void, i.e., it will remain in the final weld. Additionally, once the void exceeds a certain size, the amplitudes of the third harmonics saturate at 70% of the amplitude at the rotational frequency during full consolidation. The interaction between the eccentric probe and sub-surface void was isolated by ensuring any geometric imperfection in the shoulder (slant) with respect to the rotational axis was removed. The results suggest that geometric imperfections (eccentricity and slant) with respect to the tool’s rotational axis must be known when developing a void monitoring method from force transients of this nature.