Effect of ball milling on microstructure, porosity and mechanical properties of laser directed energy deposited Inconel 718 superalloy

Powder ball milling was employed as a novel pre-treatment method on the gas atomized powder, its effect on the microstructure, porosity and mechanical performance of the laser directed energy deposition (LDED) of Inconel 718 was studied. The results showed that the base heating and melting penetration of substrate is gradually enlarged with the increase of ball milling intensity, which is mainly attributed to the enhanced scattering laser irradiation to the substrate due to the increased surface roughness of ball-milled powders. Owing to the increased base heating, the formation of Laves phase is dramatically depressed from 17.9 vol% to 7.4 vol% at 200 rpm pre-treatment. Further increasing the ball milling intensity, the molten pool is overheated and the Laves phase formation is reversely increased. Corresponding to the depressed Laves phase formation, the tensile strength is increased from about 830 MPa to 875–877 MPa, and the tensile elongation is increased from about 22 % to 27 %, at an optimal ball milling speed of 200–300 rpm. Porosity formation is also diminished effectively from 0.6–1.14 % to 0.08–0.48 % due to the compaction and collapse of gaseous voids within the hollow feedstock powder during ball milling process. This work demonstrates that powder ball milling is a promising strategy to tailor the microstructure and enhance the mechanical performance of LDED-fabricated Inconel 718 superalloy.