Banded microstructure in AA2024-T351 and AA2524-T351 aluminum friction stir welds Part I. Metallurgical studies

ABSTRACT: 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 (Le. 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 (Le. 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.

AA2524-T351 FSW microstructures on horizontal transverse cross-section (Z=0mm). Rectangle indicates one of the serial areas in semitransparent region for particle distribution measurements. AA2024-T351 microstructures on vertical-transverse cross-section. Micro-hardness measured along dotted line (at the mid-thickness plane, Z=0).
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