Coaxial laser metal wire deposition of Ti6Al4V alloy: process, microstructure and mechanical properties

Abstract

Wire-based additive manufacturing (AM) shows a promising prospect in fabricating large-scale titanium aerospace components at a high deposition rate and low cost. The currently used wire-based AM processes commonly utilize a lateral feeding method with an acute angle to form a deposit, which reduces the flexibility and increases the difficulty of control in making complex components. In this work, a novel coaxial laser metal wire deposition (LMWD) process was used to fabricate the most widely used titanium alloy, Ti6Al4V. The coaxial wire feeding was achieved by a coaxial laser head through a vertically fed wire surrounded by an annular shape beam at focal plane. The surface quality, microstructure and mechanical properties of the deposited Ti6Al4V were examined. To show the characteristic of the coaxial LMWD process, conventional lateral LMWD was also conducted for comparison. The deposited Ti6Al4V wall showed a high surface quality and a microstructure with a mix of equiaxed β grains and columnar prior-β grains rather than coarse prior-β grains. The refined microstructure resulted in a low anisotropy in mechanical properties. The mechanism of the equiaxed β grains formation was also discussed. The combined effect of the reduced thermal gradient and the heterogeneous nucleation from the coaxial wire feeding played an important role in determining the refined microstructures in coaxial LMWD.