Cyclic Heat Treatment Induced Spheroidization of α Phase in Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe Alloy

Abstract

In the directed energy deposition (DED) process with high heat input, repeated heating and cooling cycles in the deposited layers have a significant effect on the microstructure. Because of the differences in the cyclic numbers and peak temperatures from the lower layer to the upper layer, inhomogeneous microstructures are formed in the as-built components. In this work, a cyclic heat treatment (CHT) with gradual cooling was used to simulate the thermal process during the DED process of Ti-5Al-3Mo-3V-2Cr-2Zr-1Nb-1Fe (Ti5321) near-β Ti alloy. The effect of CHT on the microstructural evolution, especially the spheroidization of α phase, was investigated. As the CHT cycle increased, the volume fraction of α phase gradually increased from 35.9% after 1 cycle to 60.9% after 100 cycles, and the length of α phase first increased and then gradually decreased, while the width of α phase increased slowly. The aspect ratio of α phase decreased from 9.90 ± 3.39 after 1 cycle to 2.37 ± 0.87 after 100 cycles, implying that CHT induced α phase spheroidization. This phenomenon resulted from both the boundary splitting mechanism and the termination migration mechanism during CHT. The evolution of microstructure affects its mechanical properties. As the CHT cycles increased, the hardness increased overall, from 342.8 ± 5.3 HV after 1 cycle to 400.3 ± 3.4 HV after 100 cycles. This work provides a potential method to tailor the microstructure of near-β Ti alloys by heat treatment alone, especially for non-deformable additively manufactured metal components.