Stress-Relief Heat Treatment Protocol and δ-Phase Formation in Additively Manufactured Inconel 625

Inconel 625 (IN625) is an alloy that has received significant attention for use in additive manufacturing technology.  In its wrought form, IN625 is a solid-solution strengthened FCC phase alloy, used for its high strength and corrosion resistance at temperatures up to about 950 °C.  The microstructure of additively manufactured (AM) IN625 in the as-built condition, although still FCC, is markedly different, consisting of a heterogeneous solidification microstructure containing significant elemental microsegregation.  A common processing step in creating AM IN625 parts via laser powder-bed fusion techniques is an industry-recommended stress-relief heat treatment for 1 h at 870 °C.  In this work we demonstrate this the stress-relief heat treatment process can have dramatic effects on the material microstructure, and ultimately the properties of the AM IN625 part.  Specifically, if the heating and/or cooling rates used to heat to and cool from the annealing temperature are too slow, the extended period of time at elevated temperatures experienced by the part, in combination with the high local concentrations of Nb and Mo arising from the solidification microstructure, allow the formation of significant volume fractions of the deleterious δ-phase which may exceed of 20 %.  Although δ is known to form in conventional wrought IN625, it is only after thousands of hours at an elevated service temperature.  Here we examine the annealing conditions required to form δ, with the intention of identifying a stress relief protocol to avoid its formation in significant volume fractions.  Finally, it is noted that even if δ-phase formation is avoided during stress-relief, prolonged exposure of the AM microstructure to elevated service temperatures will still promote its formation.  A post-build homogenization heat treatment to alleviate the solidification microsegregation is recommended to allow AM IN625 material to be used in elevated temperature applications.