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Effect of Substitutional Solid Solution Plus Double Ageing Treatment on the Microstructure of the X-Y Surface of Inconel 718 Alloy Fabricated with Selective Laser Melting (SLM)
P. Liu, S-Y. Sun1, J-H. Gong, S-B. Xu, X-N. Liu, C. Xuan and C-M. Wu

Selective laser melting (SLM) could be a prospective candidate to replace certain conventional production processes. Microstructure, hardness, precipitates and phase transformation on the X-Y surface of Inconel 718 Ni-based alloy fabricated by SLM are studied herein before and after a suitable heat treatment. The test results show that the obvious weld beads structure can be observed on the unheated X-Y surface of the alloy and the microstructure shows serious inhomogeneity with distributing columnar crystals and fine dendrites. With the increase of solid solution temperature, the microstructure on the surface of the alloy shows an obvious refining characterization. The hardness value on the surface is also reduced gradually. The typical columnar crystals, dendrites, intermetallic compounds and precipitated hardening phases (FeNi and Ni8Nb) are also reduced or could not be found. Under the solid solution process of 950°C, the fine homogeneous δ phase and γ′phase in the grain or near the grain boundary have an important effect on the X-Y surface hardness value (476 to 500 HV) of the alloy.

Keywords: Yb-fibre laser, Inconel 718, Ni-based alloy, selective laser melting (SLM), heat treatment, additive manufacturing (AM), microstructure, hardness

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