Relationship between grain structure evolution and tensile anisotropy in Al-Zn-Mg-Cu cylindrical part formed by additive friction stir deposition

Abstract

The thick-walled Al-Zn-Mg-Cu cylindrical part was obtained by additive friction stir deposition, and the relationship between grain structure evolution and tensile anisotropy was investigated in detail. The intra-layer grains are significantly refined due to continuous dynamic recrystallization (CDRX), and the grains at the interface are further refined by 30%–45 % due to geometric dynamic recrystallization (GDRX). The early cracking and ductility deterioration under Z direction loading are caused by strain localization due to coarse and fine grain distribution and pre-existing strain at the interface. The grain growth rates for specific orientations (Cube, P, Q, and Rotated Goss) are higher than average, and the mismatch in elastic modulus between these grown grains and the matrix results in ductility differences in the X and Y directions. Texture components and residual stresses affect yield strength in the X and Y directions. Meanwhile, dissolution of η/η′ and the coarsening of the particles due to the thermal cycling effect leads to a decrease in the yield strength in the building direction. The average ultimate tensile strength (UTS) and elongation (El) of the part could reach 370 MPa and 20 %, respectively. The tensile properties of the as-deposited Al-Zn-Mg-Cu cylindrical part are generally higher than those of melt-based additive manufacturing, but lower than those of the extruded and forged states due to the dissolution and coarsening of the strengthening precipitates during thermal cycling.