Effect of building direction on anisotropy of mechanical properties of GH4169 alloy fabricated by laser powder bed fusion

Abstract

Ni Base superalloy samples were fabricated by laser powder bed fusion in vertical, oblique, and horizontal building directions. The microstructures, tensile properties, and fracture morphologies of the three specimens were studied. The precipitated strengthening phase structure, grain characteristics, and geometric necessary dislocation density of the samples were characterized. The reason for the anisotropy of yield strength for the three specimens was discussed based on the strengthening theory. The results show that the microstructures of the as-built alloys are composed of coarse columnar grains, fine dendritic structures, and high-density dislocations. Partial recrystallization occurs with the precipitation of the ellipsoidal γ″ strengthening phase in the alloys after solution and double aging heat treatment. Compared with the vertical and horizontal specimens, the oblique sample has the highest yield and tensile strength and has a good elongation of 19%. The effective grain sizes of the horizontal, vertical, and oblique samples are 11.92 μm, 15.15 μm, and 11.67 μm, respectively. There are no significant differences in the average Taylor factors of the three samples. The geometrically necessary dislocation densities of the three samples are 4.021 × 1014 m−2, 3.078 × 1014 m−2, and 4.362 × 1014 m−2, respectively. The anisotropy of yield strength for the samples with different building directions is explained by the effect of dislocation strengthening, followed by precipitation strengthening and grain boundary strengthening which is essentially related to the dislocation density, Taylor factor and effective grain size.