Microstructural evolution and phase transformation of Inconel 718 alloys fabricated by selective laser melting under different heat treatment

The microstructure, hardness and phase transformation of Inconel 718 Ni-based alloys fabricated by selective laser melting (SLM) before and after different heat treatment including the direct double ageing (720 degrees C, 3 h/ furnace cooling plus 620 degrees C, 3 h/air cooling), 950 degrees C solid solution plus double ageing and 1050 degrees C solid solution plus double ageing are studied. Results show that the obvious weld beads structure can be observed on the X-Y surface in as-received alloys, and the alloys show seriously inhomogeneous with distributing columnar crystals and fine dendrites. After direct double ageing, the columnar and small dendritic crystals transformed into bulk crystals and the weld beads structure disappeared instantly with the X-Y surface hardness of alloys is about 490-540 HV. With the increase of solid solution temperature, the X-Y surface of alloys shows an obvious refining characterization. The hardness value on the surface is gradually reduced as well. The typical columnar crystals, dendrites, intermetallic compounds and precipitated hardening phase (such as Cr-Fe-Mo-Ni, FeNi and Ni8Nb) are also reduced or could not be found. Under the 950 degrees C solid solution plus double ageing, the fine homogeneous S phase in the grain and y, phase near the grain boundary have an important effect for improving the X-Y surface hardness of alloys.