Properties of friction-stir-welded 7075 T651 aluminum

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.

Characteristics and Formation Mechanisms of Welding Defects in Underwater Friction Stir Welded Aluminum Alloy

“…During the FSW of heat-treatable aluminum alloys, the resulting thermal cycles tend to coarsen or dissolve the strengthening precipitates in the alloy, creating a softened region and lowering the mechanical properties of the joints [6][7][8][9]. Therefore, several researchers have applied external liquid cooling during the FSW process to improve the mechanical properties of the joints by controlling the thermal cycling.…”

Effect of Welding Speed on Microstructure and Mechanical Properties due to The Deposition of Reinforcements on Friction Stir Welded Dissimilar Aluminium Alloys

“…FSW has proven to be an alternative joining technique in which less distortion, less plastic deformation and low residual stresses resulted when compared with other conventional joining methods [2]. Therefore, the weld defects in FSW are less than defects in fusion welding process.…”

Microstructure and precipitated phase in the thermo-mechanically affected zone of friction stir welded joint for 2024 aluminium alloy

“…Usually, in the FSW process, no melt of the joining parts occurs and the weld forms through solid-state plastic flow at elevated temperature [3]. Therefore, FSW can assure the absence of porosity, hot cracking and rather a large distortion that are typical defects of the fusion processes [4]. Moreover, it assures the possibility to operate in all positions with no protective gas.…”