Microstructure and Precipitation Evaluation of Al 7075-T6 Alloy Welded by Refill Friction Stir Spot Weld (Rffsw)

In the transportation industry the usage of solid state welding techniques have been growing since the advent of linear friction welding in 1991 at The Welding Institute (TWI). Although components produced by solid state welding own a number of advantages, the need for surface repairs has become a major challenge. Localized surface corrosion; damaged through holes; and especially the repair of the inevitable hole left at the end of the process of Friction Stir Welding (FSW) are main difficulties encountered nowadays. In view of these challenges to be overcome, the keyhole closure of Al 7075-T6 alloy using Refill Friction Stir Spot Welding (RFSSW) technique was studied. Within a pre-set process parameters range, welds presented typical friction-based process regions: Stir Zone (SZ) with equiaxed grains that went through dynamic recrystallization; Thermal Mechanical Affected Zone (TMAZ) formed by elongated deformed grains; and Heat Affected Zone (HAZ) owing Base Material (BM) unchanged grains shapes. Samples under uniaxial tension have fractured at HAZ or at the outer regions of SZ, with an averaged UTS of 82% and 79% of the BM strength, respectively. Dissolution and re-precipitation of hardening phases in SZ, whereas dissolution and/or coarsening of precipitates in HAZ were evidenced. The RFSSW is suitable to refill keyholes in 3.0mm thickness sheets of AA7075-T6 aluminum alloys, opening perspectives of applications of this process to produce flawless joints with a superior surface appearance.