Effect of volume energy density on microstructure and mechanical properties of TC4 alloy by selective laser melting

Many factors can affect the formability of TC4 alloy in laser additive, and the effects of their interaction are complex. The essence lies in the energy input of the molten pool. In this study, the energy input of the molten pool was characterized by the laser volume energy density under the coupling of multiple process parameters. The effects of volume energy density on the microstructure evolution and mechanical properties of TC4 alloy were studied, and the fracture mechanism was discussed. The results showed that the microstructure of TC4 alloy prepared by selective laser melting (SLM) was composed of α phase, metastable α′ phase, and a small amount of equiaxed nano-β phase. When the TC4 powder was irradiated by low volume energy density, fine columnar β grain with low texture intensity and random grain orientation distribution appear. Meanwhile, a large number of fine acicular martensite α′ phases generated inside the β grains, but fewer nano-β phases precipitated. At the same volume energy density, compared with increasing laser power, reducing the scanning speed could lead to the coarsening of α′ martensite, and more nano-β phase precipitated at the martensite boundary. The volume energy density greatly influenced the elongation (EL) but had little effect on the ultimate tensile strength (UTS), and the UTS could reach up to 1507 MPa. The decreasing volume energy density was beneficial to improve the plasticity, mainly attributed to the increasing deformation coordination ability caused by the refinement of α/α′ martensite and the reduction of the nano-β phase. When the volume energy density was 36 J/mm3, the fine columnar β grain precipitated, and a ductile fracture displayed with high-density dimples shown in fracture morphology. The mapping relationship between process, microstructure and properties was established, which provided a theoretical basis for process, microstructure optimization and performance improvement.