Influence of heat treatment on microstructure and tensile properties of selective laser melting metastable Ti55531-0.5Nb alloy

The deposition of metastable titanium alloys using additive manufacturing techniques is of increasing interest. The as-build samples generally require further subsequent heat treatment to control the microstructure and to meet the good comprehensive performance of structural components. In this study, the Ti55531-0.5Nb alloys fabricated by selective laser melting (SLM) technology after solution treatment were cooled in different modes to investigate the effect of cooling rates on microstructural evolution and tensile properties. The results indicate that the volume fraction of primary alpha decreases with the increase of cooling rate, while the secondary alpha displays the opposite trend. The high cooling rates increase the residual stresses in the sample, providing an additional driving force for phase precipitation during the aging process. In addition, all samples with different cooling rates exhibited high elongation (>10 %). The difference is that the Ti55531-0.5Nb alloys are more sensitive to changes in elongation than ultimate tensile strength (UTS) with variations in cooling rates. The UTS and elongation of the sample with solution plus water cooling and aging (STWC+AG) are 1185 +/- 9 MPa and 10.3 +/- 0.3 %, respectively, while the furnace cooling and aging (STFC+AG) sample exhibits a 71.4 % improvement in elongation at the expense of the UTS of 13.8 %. Furthermore, the fracture behavior of the samples exhibited a mixed fracture mechanism dominated by ductile fracture, and the ductile fracture is more prominent with the cooling rate decreases. The microscale shear band contributing to the elongation was found in the shear lip zone for the low cooling rate.