Crack inhibition of non-weldable Inconel 738 alloy in ultrasound-assisted laser directed energy deposition

High crack susceptibility restricts reliable laser directed energy deposition (L-DED) and repair of γ′ precipitation-strengthened Nickel-based superalloy Inconel 738. This study investigates the effects of ultrasonic assistance synergized with reduced heat input on microstructures and mechanical properties of the L-DED Inconel 738 alloy. The mechanisms of inhibiting crack initiation and propagation of are studied. Results demonstrate that grain refinement and suppression of epitaxial growth of L-DED Inconel 738 alloy are achieved by ultrasonic assistance. The microstructures evolve from coarse columnar grains (∼82.6 μm) to a bimodal structure of shorter columnar and finer equiaxed grains (∼40 μm). The morphologies of MC carbides transform from a continuous distribution of larger sizes to a discrete distribution of finer sizes. Cracks are highly susceptible to propagate along the long, straight grain boundaries of the coarse columnar grains in deposit without ultrasonic assistance, while tortuous grain boundaries induced by ultrasonic assistance enhance crack resistance and inhibit crack propagation. As a result, crack-free L-DED Inconel 738 alloys with excellent ultimate tensile strength of 1478–1490 MPa and elongation of 15.3–17.8 % are achieved via ultrasonic assistance and are significantly higher than cast Inconel 738 (945 MPa, 7.5 %), indicating the feasibility of utilizing ultrasonic assistance to inhibit cracking in L-DED Inconel 738 alloy.