Properties of friction-stir-welded 7075 T651 aluminum |
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Authors: | M W Mahoney C G Rhodes J G Flintoff W H Bingel R A Spurling |
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Affiliation: | (1) the Rockwell Science Center, 91360 Thousand Oaks, CA |
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Abstract: | Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered
essentially unweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while
introducing extensive localized deformation. Studies were performed on these solid-state welds to determine mechanical properties
both in the longitudinal direction, i.e., within the weld nugget, and, more conventionally, transverse to the weld direction. Because of the unique weld procedure,
a fully recrystallized fine grain weld nugget was developed. In addition, proximate to the nugget, both a thermomechanically
affected zone (TMAZ) and heat affected zone (HAZ) were created. During welding, temperatures remained below the melting point
and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a banded microstructure
that influences room-temperature fracture behavior was created. In the as-welded condition, weld nugget strength decreased,
while ductility remained high. A low-temperature aging treatment failed to fully restore T651 strength and significantly reduced
tensile ductility. Samples tested transverse to the weld direction failed in the HAZ, where coarsened precipitates caused
localized softening. Subsequent low-temperature aging further reduced average strain to failure without affecting strength.
Although reductions in strength and ductility were observed, in comparison to other weld processes, FSW offers considerable
potential for welding 7075 T651 aluminum. |
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