Microstructure and forming mechanism of a laser melting deposited TC4 titanium alloy after sub-critical annealing

Characteristics and evolution of microstructure in a laser melting deposited TC4 fifanium alloy after annealing treatments were investigated. Effects of annealing temperature and cooling ways on the microstructure were examined. A unusual bi-model microstructure consisting of coarse crab-like primary α and fine lamellar transformed β is obtained by sub-critical annealing in the temperature range of 15-25°C below β-transus temperature in the laser melting deposited TC4 alloy. The crab-like morphology becomes more manifest with increasing annealing temperature. The volume fraction and aspect ratio of coarse primary α decreases, while the width of coarse primary α increases with increasing annealing temperature. Decomposition of β phase during air cooling after sub-critical annealing and plenty volume faction of β phase by annealing above a certain temperature is necessary for the formation of crab-like primary α and the bi-model microstructure.