Effect of Scan Speed and Laser Power on the Nature of Defects, Microstructures and Microhardness of 3D-Printed Inconel 718 Alloy

Inconel 718 samples were printed with varying scan speed and laser power with volume energy density (VED) in the range of 52 J/mm³ to 75 J/mm³. Samples were then subjected to solution treatment and aging as per standards. Further, samples were characterized for their microstructure and metallurgical defects using optical microscopy (OM) and scanning electron microscopy. OM revealed that columnar grains had grown epitaxially in the build direction and grain size varies from bottom to top. Microscopic analysis on the XY plane (laser focusing plane) and the XZ plane (the plane in build direction) had revealed the presence of defects like metallurgical pores, interlayer cracks, open pores, lack of fusion, balling defects and their different morphologies. The process parameters such as scan speed, laser power and VED have influenced the porosity % and pore density and their distribution in the printed material. The microstructures also revealed the presence of oxides of Cr, Nb, Ti, Al and Fe which were identified near the interlayer cracks, large open pores and clusters of pores. Microhardness measurements of the printed samples were higher than the wrought Inconel 718 by 15-18%, both in the solution treated and aged conditions. The microhardness survey along the XY and XZ planes yielded different values even after double aging heat treatment. Average microhardness on the XZ plane was 15-20% more than that on the XY plane in each sample pertaining to the difference in grain morphology on the two planes. This difference in the microhardness values was less for the sample of high VED. The results suggest that VED in the range of 65 J/mm³ to 70 J/mm³ with scan speed and laser power between 860 mm/s to 970 mm/s and 260 W to 300 W, respectively, result in samples with tolerable microscopic defects.