Additive manufacturing of Ti-6Al-4V alloy by hybrid plasma-arc deposition and microrolling: Grain morphology, microstructure, and tensile properties

To address the problems of coarse columnar grains, inhomogeneous microstructure, and anisotropic mechanical properties of Ti-6Al-4V manufactured by wire and arc additive manufacturing (WAAM) and hybrid additive manufacturing with plasma-arc deposition, synchronous microrolling (HDMR) was examined in this study. HDMR leads to significant grain refinement and isotropy improvement. Unrolled additive manufacturing alloys show typical columnar grains, while the microrolled ones show a transition from columnar to equiaxed grains with the fraction depending on the microrolling force. The microrolling-induced formation of equiaxed grains is caused by both dendrite fragmentation and prior β recrystallization in the subsequent deposition. Interestingly, the rolling force required for good grain refinement in HDMR is much lower than that in WAAM with subsequent cold rolling. Microstructure characteristics are present near the grain boundaries due to the recrystallized α lamellae distribution. The width of the basketweave α lamellae decreases with the increasing microrolling force. The yield strength and ultimate tensile strength of HDMR samples increase with a decrease in elongation anisotropy. This study shows that HDMR can effectively refine grains and improve the tensile properties of titanium alloys, providing a broad prospect for the rapid formation of large titanium alloy parts.