Effects of trace Nb addition on microstructure and properties of Ti-6Al-4V thin-wall structure prepared via cold metal transfer additive manufacturing

To refine the coarse columnar prior-beta grains across the layers in the cold metal transfer additive manufacturing (CMTAM) of a Ti-6Al-4V thin-wall structure, the addition of trace Nb, which functioned as heterogeneous nuclei, is proposed. After the addition of trace Nb powder, intra-layer bands were obtained in each layer of the Ti-6Al-4V wall, which divided each layer into two parts according to the existing forms of Nb, including a rich unmelted-Nb-powder zone and a (beta-Ti, Nb)-phase-rich zone. The unmelted Nb powder with an average diameter of 170 nm-which acted as heterogeneous nuclei, promoting the columnar-to-equiaxed transition of prior-beta grains-mainly existed in the lower zones of the layers. A random distribution of (beta-Ti, Nb) phases was observed in the upper zones of the layers, which experienced the highest loads (398.8 mN) with indentation displacement of 2000 nm in nanoindentation tests. The trace Nb addition affected the interplanar distances of (beta-Ti, Nb) phases and changed the solidification mode from planar to cellular. Tensile tests revealed that the average ultimate tensile strength increased from 990 to 1149 MPa, as the solution strengthening of the (beta-Ti, Nb) phases played the dominant role in strengthening, while the average strain decreased from 7.69% to 5.5%. The unmelted Nb powder was the cause of the change in the fracture modes from ductile behavior to mixed behavior of cleavage and ductile fracture. During the CMTAM process, the possible thermal cycles for asbuilt and trace-Nb-addition specimens were analyzed. (C) 2020 Elsevier B.V. All rights reserved.