Effect of defects in laser selective melting of Ti-6Al-4V alloy on microstructure and mechanical properties after heat treatment

In this paper, the selective laser melting (SLM) Ti-6Al-4V sample with defects is heat treated (HT). The heat treatment temperatures are 750 °C, 850 °C, and 950 °C. The three temperatures are kept for 2 h, and then air cooling (AC) is used for treatment. Observe the changes in microstructure, phase composition, hardness, and tensile properties. The results show that the existence of defects will not affect the overall change trend of the microstructure, hardness and phase composition of the Ti-6Al-4V sample after heat treatment. However, there are obvious changes in the surrounding of two kinds of defects, which are the surface defects with large area and the lack of fusion (LOF) defects that are communicated with the outside world and extend to the inside of the sample. These two types of defects water, oil and air are easy to enter, and the defects are not easy to remove, heating will produce a large amount of hydrogen. Hydrogen enters the sample through defects, resulting in a large number of β phase around the two types of defects, and white around the defects. The hardness near the defect decreased significantly, and the closer the distance, the more the hardness decreased. A large amount of hydrogen entered into the Ti-6Al-4V sample, resulting in the appearance of new phase δ phase in the phase composition, resulting in the decrease of tensile strength（TS）and yield strength (YS) of the tensile properties after heat treatment, and the obvious decrease of elongation (EL). Moreover, among the various defects, the most important influence on the phase composition and tensile properties is the LOF defect that communicates with the outside world and extends to the inside of the sample.