Formability improvement, cracking behavior and control of Y-modified Ti–43Al–4Nb–1Mo-0.1B alloys produced by selective laser melting

Selective laser melting (SLM) was used to produce the Ti–43Al–4Nb–1Mo-0.1B (at.%, TNM) alloys with various Y additions. The mechanism of improved formability by Y addition is firstly studied and revealed. Then, the cracking sensitivity of SLMed Y-modified TNM alloys under different Y contents are systematically investigated. Finally, the cracking behavior and control mechanism are proposed. The results show that the improved formability by adding Y to TNM alloys can be ascribed to the good laser energy absorption and lower melt viscosity. Microcracks are easy to form in the SLMed Y-modified TNM alloys despite the improved formability. The crack density decreases as the laser energy density is increased, which roughly satisfies the polynomial y = 0.02x2-0.48x+2.56 (R2 = 0.97). The crack-free samples can be obtained by the addition of 2, 3 and 4 wt% Y with the laser energy density large than 7.00 J/mm2. The formation and control of microcracks mainly depend on the microstructure characteristics and phase constitution under different Y contents. The coarse columnar grains, oxygen segregation at grain boundaries, and dominating brittle B2 phase at lower Y addition (1 wt%) lead to an obvious decline in ductility (with the average microhardness of ∼596 Hv), thus to induce cracking. By contrast, the fine equiaxed grains, enhanced oxygen-scavenging effect (formation of Y2O3 particles), and decreased brittle B2 phase content at higher Y additions (2–4 wt%) significantly improve the ductility (with the average microhardness of 554–568 Hv), thus to prevent cracking.