Optimization of the Post-Heat Treatment of Additively Manufactured IN625

Sadek, Alber

With the molten pool’s rapid solidification during laser powder bed fusion (LPBF), the resulting microstructures differ from those expected in equilibrium conditions. Residual stresses, microsegregation, anisotropy, undesirable phases, layered structure, and lower mechanical properties are the challenges that must be addressed before LPBF-ed Inconel 625 parts can be industrially implemented. Heat treatment of Inconel 625 after the LPBF process is widely discussed in the literature, and the proposed heat treatment processes do not address all the challenges mentioned above. For this reason, specific heat treatments should be designed to achieve desired mechanical properties.

The following heat treatment procedures were designed to study the effect of heat treatment parameters and sequences on the microstructure, grain size, room, and elevated temperature mechanical properties, and to develop an elevated-temperature tensile curve between room temperature (RT) and 760^oC (1400^oF) of LPBF-ed Inconel 625.

Two different grain sizes could be obtained, starting with the same as-built microstructure by controlling post-process heat treatment parameters. The first type, coarse grain size (ASTM grain size No. G 4.5), suitable for creep application, was achieved by applying HIP followed by solution annealing. The second type, fine-grain size (ASTM grain size No. G 6), which is preferable for fatigue properties, was achieved by applying solution annealing followed by HIP.

The heat treatment with finer equiaxed grain size and similar hardness of wrought alloy was selected for mechanical testing. The mechanical properties at room and elevated temperature 538^oC (1000^oF) are higher than the available properties in the AMS 5599 for wrought Inconel 625 while maintaining a higher ductility above the average level found in the standards. It can be concluded that the performed heat treatment achieves higher isotropic mechanical properties. revealing the presence of isotropic microstructure.