Effect of titanium nitride inclusions on the mechanical properties of direct laser deposited Inconel 718

Additive manufacturing (AM) of metal is attractive for its ability to fabricate complex and tailored structures. However, unusual thermal history during the manufacturing process creates unique microstructures, which leads to non-uniform mechanical behavior. In this paper, we report the room temperature and high temperature mechanical properties of Inconel 718 fabricated by laser-based directed energy deposition. Surprisingly, failure strain at all tested temperatures exhibited noticeable scatter, which is attributed to the formation of titanium nitride (TiN) inclusions. While heat treatment was conducted on AM Inconel 718 to minimize the influence of the inhomogeneous as-built microstructure, it resulted in significant increase in the volume fraction, number density, and maximum particle radius of the TiN inclusions. Consequently, dispersion of mechanical properties became more severe. Experimental investigation (strain mapping, fractography, and X-ray microscopy) supported by finite element analysis implies that premature failure of AM Inconel 718 is mainly due to transgranular cracks of the TiN inclusions that propagates into the matrix.