Microstructure formed during selective laser melting of IN738LC in keyhole mode

Abstract

Understanding how grains grow and how growth direction can deviate considerably from build direction (BD) during selected laser melting (SLM) is necessary for SLM of the difficult-to-weld IN738LC Ni-based alloy. In the present study, samples were made with two laser power (P) values while other parameters were kept unchanged, corresponding to SLM keyhole mode with two melt penetration depths. Aspect ratios (width/depth) of tracks, dendrite arm spacing, and grain growth directions have been measured and determined. The optimal SLM condition for defect free SLM of the alloy previously identified in the literature has been found to be a stable keyhole SLM condition. High P increases the depth of keyhole, causing it to be unstable and pores to form likely due to the periodic collapse of the keyhole. In keyhole mode SLM, heat is conducted away more horizontally and thus grains grow in directions considerably different from BD. Under the stable keyhole condition, primary dendrite arm spacing (λ1) ranges from 0.32 μm at the track bottom to just over 1 μm in the top track region corresponding to cooling rates (T˙) equal to from 1 × 107 K/s to 3 × 105 K/s, respectively, during solidification. Significantly lower λ1 observed at the track bottom indicating T˙ being up to 108 K/s under the condition of flow instability will also be presented and discussed.