Investigation on microstructural evolution and property variation along building direction in friction stir additive manufactured Al–Zn–Mg alloy

Abstract

A multilayer build was fabricated by friction stir additive manufacturing (FSAM), using 4-mm thick 7N01-T4 aluminum alloy sheets in this study. The microstructural evolution along the building direction during the FSAM process and its effect on the microhardness and tensile properties were investigated using combined optical microscopy (OM), electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), and Vickers hardness and tensile tests. The stir zone with fine equiaxed grains was considered to be the effective area of the build, and inhomogeneous microstructures were observed along the building direction. It was found that the sizes of grains and the precipitates in the non-overlapping interface regions increased from the top to the bottom influenced by the multi-pass thermal cycles. Furthermore, the degree of precipitate dissolution decreased from the top to the bottom region. These phenomena were considered to be the key factors that contributed to the decrease of the hardness and the strength recovery from the top to the bottom region. The strength of the tensile samples that were extracted from the effective area of the cross-sectional build increased with the increase of the natural aging time. After natural aging for 180 days, the average strength of the build increased to 352.7 MPa which corresponded to about 73% of the strength of the base material (BM) (483.1 MPa). Moreover, the tensile strength and the elongation of the samples that fractured along the overlapping interface were lower than those of the samples that fractured away from the overlapping interface. The kissing-bond defects formed in the overlapping interface regions affected the tensile properties of the build.