Differences of microstructure and mechanical properties of Ti64 alloy fabricated via laser powder bed fusion under the same VED value with different process combinations

When fabricating metals via Laser Powder Bed Fusion (LPBF) additive manufacturing, Volumetric Energy Density (VED) is often adopted as a key process parameter for optimization. However, whether different combinations of process parameters under the same VED would lead to differences in the microstructure and mechanical properties of the fabricated alloys remains to be investigated. In this study, the commonly used Ti64 alloy was selected as the research object. The alloys were printed with a fixed VED of 50 J/mm³ but different P-V combinations (S1: P = 150 W, V=1000 mm/s; S2: P = 210 W, V=1400 mm/s). Subsequently, the samples were subjected to heat treatment at 900 °C for 2 h, aiming to systematically investigate the differences in microstructural characteristics and mechanical properties of Ti64 alloys in both as-built and heat-treated states under the same VED with distinct P-V combinations. The results show that the Ti64 alloys prepared by both processes are composed of α phase, metastable α′ phase and a tiny amount of β phase. In the as-built state, the ultimate tensile strength (UTS) and elongation of S2 sample reach 897.91 MPa and 3.03% respectively, exhibiting superior properties to those of S1 sample. After heat treatment, the metastable α′ martensite in S1 sample is fully decomposed into α + β duplex phase, and the content of β phase surges to 9.3%. The lack-of-fusion micropores and dislocation accumulation provide sufficient nucleation sites for phase transformation, thus resulting in the heat-treated S1 sample with mechanical properties significantly superior to those of heat-treated S2 sample. Specifically, the elongation of S1 sample increases from 3.12% to 19.12%, representing an increase of 513%. In contrast, the decomposition degree of α′ martensite in S2 sample is limited, with the β phase content only increasing to 5.8%, leading to a relatively low improvement in mechanical properties. This study demonstrates that VED has inherent limitations as the core basis for process optimization, since it ignores the independent regulatory effects of P and V. The findings provide a reference for the precise regulation of individual process parameters in LPBF to fabricate high-performance metal components.