Effect of intrinsic heat treatment on microstructure and hardness of additively manufactured Inconel 625 alloy by directed energy deposition

[Display omitted] •The intensified intrinsic heat treatment promotes the abnormal δ phase precipitation.•Laves and δ are heterogeneously distributed within the as-built specimens.•Longer interlayer dwell time was introduced to uniform the distribution of precipitation.•Hardness fluctuation within the as-built specimens was decreased by 47%. The effect of intrinsic heat treatment (IHT) induced by cyclic heat input in laser additive manufacturing (LAM) has been receiving increasing attention in recent years. Herein, the feasibility of tailoring the microstructure and mechanical properties of Inconel 625 alloy to improve material properties and help achieve part quality consistency by controlling IHT in directed energy deposition (DED) is demonstrated. In this study, thermal monitoring was used to obtain thermal histories of as-built specimens to reveal the differences in thermal characteristics under different IHT. The results show that the variation of IHT has an important effect on the primary dendrite arm spacing (PDAS), the temperature and cooling rate of the molten pool, the morphology and content of Laves, and can initiate unusual δ precipitation in the interdendritic region which may be related to the dissolution of Laves. And this time-position-dependent IHT effect results in a hardness fluctuation of about 23.6 % in the bottom and top of the as-built specimen with a dwell time of 1 s, while the hardness is relatively uniform in the specimen with a dwell time of 10 s. Furthermore, by simulating the IHT variation process, its effect on precipitate and hardness was further confirmed.