A group of researchers from the Faculty of Metals Engineering and Industrial Computer Science at the AGH University of Krakow, SOLARIS NSRC, and the Institute of Materials Engineering at the University of Silesia in Katowice led by Beata Dubiel studied the microstructure evolution and hardness behaviour of the Inconel 625 superalloy additively manufactured by laser powder bed fusion (LPBF) subjected to high-temperature annealing. As a result of systematic study, the various strengthening and softening mechanisms acting during high-temperature annealing of Inconel 625 LPBF have been identified.

Nickel superalloy Inconel 625 is characterized by high creep resistance at a temperature of up to 800 °C and excellent corrosion resistance under harsh environments; thus, it is widely applied in chemical, aerospace, aeronautics, and energy industries. The high hardness, low thermal conductivity, and poor machinability make the fabrication of complex shape Inconel 625 parts difficult and expensive. This problem can be solved by using 3D printing, but only if the required stability of microstructure and mechanical properties are ensured during prolonged high-temperature exposure service.

Using various electron microscopy techniques, the microstructure of Inconel 625 superalloy was investigated after annealing at temperatures in the range of 600 - 800 °C  for 5 - 500 h. Four stages of microstructure evolution were distinguished.
In the initial state, Inconel 625 LPBF is characterised by a fine grain with a cellular microstructure and has a hardness of 300 HV10. In the 1st stage of the microstructure evolution, the γ'' phase particles precipitate at the cell boundaries, leading to an increase in hardness to 383 HV10 after annealing at 700 °C for 5 h. The 2nd stage involves the precipitation of the γ'' phase both on the cell walls and inside the cells, as well as the formation of dislocation networks, which contribute to the softening effect and decrease in hardness to 319 HV10. In the 3rd stage, with prolonged annealing at  700 and 800 °C, the δ phase, M₂₃C₆ carbides, and the Laves phase precipitate and grow, and the subgrain boundaries are formed, accompanied by an increase in hardness to a level in the range of 340 – 350 HV10. In the 4th stage, after prolonged annealing at 800 °C, the δ phase and M₂₃C₆ carbides coagulate, and the Laves phase particles spheroidize or partially dissolve. Very intense precipitation and growth of the hard δ phase particles provide an increase in hardness to 402 HV10. 

    Figure 1.  Evolution of the microstructure and hardness during high-temperature annealing of Inconel 625 3D printed in LPBF process. 

The research was funded by the National Science Centre, Poland, project OPUS14 no 2017/27/B/ST8/02244.

Author: Beata Dubiel

Link to the publication: B. Dubiel, K. Gola, S. Staroń, H. Pasiowiec, P. Indyka, M. Gajewska, M. Zubko, I. Kalemba-Rec, T. Moskalewicz, S. Kąc, Effect of high temperature annealing on the microstructure evolution and hardness behavior of the Inconel 625 superalloy additively manufactured by laser powder bed fusion, Archives of Civil and Mechanical Engineering (2023) 23:249, https://doi.org/10.1007/s43452-023-00787-4