Microstructure and mechanical evolution behavior of LPBF (laser powder bed fusion)-fabricated TA15 alloy

Highlights: • The microstructure of TA15 fabricated by laser powder bed fusion (LPBF) was studied and the formation mechanism of martensite classification was mainly explained. • The influence of different heat treatment temperatures on evolution of microstructure, mechanical properties and fracture mechanism were studied. • The sample after heat treatment at 940 °C obtains excellent comprehensive mechanical properties, because the quartic acicular α phases precipitate in the transformed β phase. -- Abstract: TA15 titanium alloy has been widely used in the aerospace industry due to its moderate high-temperature performance. In this research, TA15 samples with excellent mechanical properties were successfully fabricated by laser powder bed fusion (LPBF). Results show that the microstructure of TA15 sample is composed of acicular α′ martensites within the columnar prior β grain. During manufacturing process of LPBF, the non-equilibrium heating and cooling of the thermal cycles experiencing different peak temperatures result in the hierarchical structure of martensite. After heat treatment at 750 °C, 840 °C, 900 °C, and 940 °C for two hours, the fine martensitic structure has transformed into a mixture of α and β. With the increase of temperature, the volume fraction of β increases from 10.3% to 49.9%, the width of primary α increases from 0.72 ± 0.16 µm to 2.96 ± 0.17 µm because of the simultaneous fragmentation of the primary α. So the ultimate and yield strengths decrease while the elongation increases. After heat treatment at 940 °C, the specimen shows the best comprehensive mechanical properties, the elongation increases to 11.5%, the tensile strength is 1061 MPa, and the yield strength is 961 MPa. The fracture surface shows the typically ductile fracture character.