Correlation of local microstructures and mechanical properties of Al–Zn–Mg–Cu alloy build fabricated via underwater friction stir additive manufacturing

Abstract

Abstract This study used underwater friction stir additive manufacturing (FSAM) to fabricate a multilayered Al–Zn–Mg–Cu aluminum alloy build. The relationship between the local microstructures and mechanical properties for the water-cooled build was established. Consequently, the underwater FSAM effectively reduced the thermal cycle effect on the former FSAM pass during the FSAM process. The common phenomenon of microhardness decrease from top to bottom in the air-cooled build was suppressed. However, after aging treatment, a low-hardness zone (LHZ) was observed in the bottom of the pin-driven zone + pin-driven zone (PDZ + PDZ). A high-hardness zone (HHZ) was located in the shoulder-driven zone + pin-driven zone (SDZ + PDZ). Furthermore, the tensile properties in the bottom of the PDZ + PDZ were inferior to those in the SDZ + PDZ. The microstructural results show that, compared to the SDZ + PDZ, the higher density of the T (AlZnMgCu) and η (MgZn2) phases precipitated in the bottom of the PDZ + PDZ, which was attributed to the finer grains and the higher density of subgrains and dislocations in this region. In addition, the peak temperature in the bottom of the PDZ + PDZ was in the range of the η-phase precipitation temperature, which also resulted in the increasing in the number of η phases. The high-density T- and η-phase precipitation in the bottom of the PDZ + PDZ implied a low-degree supersaturation, directly leading to the decrease of the aging-strengthening ability. In conclusion, low hardness and strength were observed in the bottom of the PDZ + PDZ after the aging treatment, whereas high hardness and strength were observed in the SDZ + PDZ because of the more efficient dissolution in this region.