Microstructure evolution and mechanical properties of Ti-6Al-4V alloy fabricated by directed energy deposition assisted with dual ultrasonic vibration

During the additive manufacturing (AM) of Ti-6Al-4V alloy components, the coarse columnar primary beta grain structure is more likely to form due to the low nucleation rate of titanium alloys, rapid solidification, and a high- temperature gradient. However, the effect of the size and morphology of the primary beta grains on alpha phase is not well understood. To eliminate large coarse columnar grains in Ti-6Al-4V alloy, a dual ultrasonic vibration (UV) method was proposed and implemented in laser and wire AM to fabricate thin-walled parts. The mechanisms of microstructure evolution and the effects of ultrasound under dual UV were simulated using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), and COMSOL software. It was observed that coarse columnar primary beta grains were transformed into equiaxed primary beta grains (CET),with the average grain size decreasing from 1678.4 mu m to 467.9 mu m after the simultaneous application of dual UV on the top and the bottom of the deposited layer. The alpha phase showed an obvious radial growth pattern attributed to the uniform energy field produced by the dual ultrasound. Compared with the samples without the UV treatment, the ultimate tensile strength (UTS), yield strength (YS), and elongation of the Ti-6Al-4V alloy subjected to dual UV treatment increased by 26.7 %, 24.7 %, and 104 %, respectively. Furthermore, the ultrasonic mechanism associated with the Ti-6Al-4V alloy was discussed.