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A series of micro-mechanical experiments have been performed to quantify how the friction stir welding (FSW) process affects the material response within the periodic bands that have been shown to be a common feature of FSW joints. Micro-mechanical studies employed sectioning of small samples and micro-tensile testing using digital image correlation to quantify the local stress–strain variations in the banded region.Results indicate that the two types of bands in 2024-T351 and 2524-T351 aluminum FSW joints (a) have different hardening rates with the particle-rich bands having the higher strain hardening exponent, (b) exhibit a periodic variation in micro-hardness across the bands and (c) the individual bands in each material have the same initial yield stress. [Copyright &y& Elsevier]

Results from careful investigations of the banded microstructure observed on horizontal transverse cross-sections in all AA2024-T351 and AA2524-T351 aluminum friction stir weld (FSW) joints indicate the presence of periodic variations in (a) the size of equiaxed grains, (b) micro-hardness, and (c) concentration of base metal impurity particles (i.e. constituent particles) that correlate with the observed band spacing. The latter trend is more distinct in AA2024-T351, which has a higher volume fraction of constituent particles resulting in easily recognizable particle rich regions on horizontal cross-sections near the mid-thickness of the joint and well-defined variations in hardness. In AA2524, the trends are more muted but clearly visible.Results from recent numerical simulations of the FSW process enable interpretation of the trends in grain size along the weld centerline in terms of the time–temperature cycle experienced by the material. Specifically, the AA2524 FSW joints having low power and high input energy (i.e. the slow FSW), exhibit micron-size grain structure across both bands. Conversely, the fast and medium FSW in AA2524 have higher maximum temperatures, and a corresponding six-fold increase in grain size. [Copyright &y& Elsevier]