PROPERTIES OF FRICTION-STIR-WELDED 7075-T651-ALUMINUM

Friction stir welding (FSW), a new welding technique invented at TWI, was used to weld 7075 T651 aluminum, an alloy considered essentially u nweldable by fusion processes. This weld process exposed the alloy to a short time, high-temperature spike, while introducing extensive loca lized deformation. Studies were performed on these solid-state welds t o 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 recr ystallized fine grain weld nugget was developed. In addition, proximat e to the nugget, both a thermomechanically affected zone (TMAZ) and he at affected zone (HAZ) were created. During welding, temperatures rema ined below the melting point and, as such, no cast or resolidification microstructure was developed. However, within the weld nugget, a band ed microstructure that influences room-temperature fracture behavior w as created. In the as-welded condition, weld nugget strength decreased , while ductility remained high. A low-temperature aging treatment fai led to fully restore T651 strength and significantly reduced tensile d uctility. Samples tested transverse to the weld direction failed in th e HAZ, where coarsened precipitates caused localized softening. Subseq uent low-temperature aging further reduced average strain to failure w ithout affecting strength. Although reductions in strength and ductili ty were observed, in comparison to other weld processes, FSW offers co nsiderable potential for welding 7075 T651 aluminum.