The aim of the present study is to eva‎luating effects of precipitation hardening heat treatment on microstructure and creep properties of IN 718 Superalloy fabricated by selective laser melting (SLM) technique. For this purpose microstructure of the SLMed cubes were characterized in as-built condition and after heat treatments and then creep properties of them were eva‎luated by stress-rupture test. SLMed specimens were subjected to different cycles of solution treatments at temperatures of 980℃ , 1060 ℃ and 1130 ℃ for 1 h and double aging treatment in 720 ℃ and 620 ℃ each for 8h. Microstructural investigation were done by optical microscope, scanning electron microscope, field emission scanning microscope equipped by electron back-scattered diffraction detector and transition electron microscope, while creep properties at perpendicular to the build direction were determined at 650 ℃ and 690 MPa. The microstructural results showed that the nature of fabricating sample by SLM process, leads to the formation of columnar grains along the build direction, strong cubic texture and dendrite structure with alloy element segregation in as-built specimen. Solution treatment in 980 ℃ did not provide enough activation energy for the complete recrystallization and thus the columnar structure could still be seen in microstructure of the HT 980 as well as as-built sample. In addition the δ phase were precipitated in grain and grain boundary in this specimen. Creep properties were also reduced because of the acceleration of nucleation and crack growth due to the formation of these precipitates in the grain boundary. However, with increasing the solution temperature to 1060℃, the equiaxed grains were formed among the columnar grains, δ precipitates were largely removed from microstructure and nano-scale γ, which play a major role in Inconel 718 strength, were formed larger and distribute more uniformly in this sample. As a result, the creep properties increased significantly. The non-uniform distribution of residual stresses in the as-built microstructure caused heterogeneous grain growth after heat treatment at 1130℃. Uneven distribution of grains and formation of small regions of equiaxed grains along with large grains may act as nuclei of failure. In fact, the δ and laves phases were dissolved in the matrix by the solution treatment at higher temperatures; However, creep properties were not greatly improved because of the inhomogeneous grain growth.

احمد كرمانپور، احسان فروزمهر

احمد رضائيان

مسعود عطاپور، محسن بدرسماي