Effect of Heat Treatment on Microstructure and Properties of TC4 Alloys Fabricated by Selective Laser Melting

A large number of hard and brittle needle-like α' phases are presented in the microstructure of TC4 alloys fabricated by selective laser melting (SLM) due to the extremely fast cooling rates, resulting in a significant decrease in the plasticity of SLM TC4 alloys, which seriously limits their engineering applications. To improve the plasticity of the SLM TC4 alloys, the effect of different heat treatment temperatures on microstructure and mechanical properties of SLM TC4 alloys was systematically investigated. Results show that the needle-like α' phase in the primary β grains is transformed into the α phase after heat treatments at four temperatures. With the heat treatment temperature increasing from 860 ℃ to 950 ℃, the morphology of the needle-like α phases is transformed from basket-like distribution into a colony with the same orientation. Furthermore, the grain size of α phases increases from 1.73 μm to 2.32 μm and the fraction of β phases increases from 0.4% to 2.7%. The ultimate tensile strength (UTS) of SLM TC4 alloys is slightly reduced from 961 MPa to 870 MPa while the elongation index (EI) of the SLM TC4 alloys first increases and then is reduced. Among the temperatures, the UTS of 941 MPa and the EI of 11.1% are obtained by heat treatment at 890 ℃, which increases by 126.5% compared with the unheat-treated samples.