Metal additive manufacturing technology based on layer-by-layer fabrication is one of the newfound manufacturing methods promoted to produce complex parts, reduce production sequences, and fabricate high-quality sectors. This research aims to investigate the effect of microstructure type on the elevated temperature mechanical properties of laser powder-bed fusion (LPBF) Hastelloy X Ni-based superalloys. Hence, cubic-shaped samples were built to determine the optimized processing parameters of the LPBF process. After reaching the optimum sample regarding the relative density, the effect of using the two scanning patterns, namely the Meander and the Island, was elucidated. The mentioned samples were then subjected to heat treatment at 900 °C and 1175 °C. The microstructure of the fabricated samples was eva‎luated utilizing a scanning electron microscope, field emission scanning electron microscope equipped with an electron backscatter diffraction detector, and scanning transmission electron microscope. In addition, the tensile properties of the samples were investigated at 25 °C, 560 °C, 760 °C, and 860 °C. The microstructural eva‎luations revealed the formation of submicron solidification cell and dislocation-cell structures within the columnar grain of austenite phase for Meander and Island samples. In the dislocation-cell structure, segregation of Cr and Mo to the cell walls containing dense dislocation tangles was characterized. In the samples heat-treated at 900 °C for 120 min, Cr- and Mo-rich particles were characterized in the austenitic matrix, while the morphology of the grains remained columnar. On the other hand, heat treatment at 1175 °C for 120 min resulted in a change in the morphology of grains to nearly equiaxed ones, and a significant reduction in the dislocation density, due to the occurrence of the recrystallization transformation. The investigation results on the kinetics of recrystallization transformation revealed that as a result of having the Avrami exponent of 3.49 and 3.76 for the Meander and Island samples, respectively, the growth of recrystallized grain took place at 3D. The mechanism of the recrystallization transformation was bugling of the high-angle grain boundaries. Moreover, the microstructural investigations illustrated that the twin boundaries played an essential role in developing the recrystallized region. Study on tensile behavior of the samples indicated at room temperature and as-built condition, the Meander samples showed slightly higher strength and ductility than the Island ones. Increasing the tensile test temperature decreased the samples' strength and ductility. And the samples underwent brittle fracture due to the formation of intergranular cracks. In contrast to tensile results at room temperature, the Island sample showed better tensile behavior at elevated temperature owing to a larger grain size than the Meander sample. Heat treatment at 900 °C resulted in a slight strength improvement and ductility reduction at room temperature. However, at elevated temperatures, it had no positive impact on the tensile properties, especially ductility. On the other hand, performing the heat-treated at 1175 °C for 120 min caused a slight change in strength and significant improvement in ductility at all the experienced temperatures. According to the microstructural studies and fractography results, reduction in the dislocation density and formation of the especial high-angle grain boundaries (twin boundaries) was known as the main factors resulting in improvement in elevated temperature tensile properties of laser powder bed fusion Hastelloy X heat treated at 1175. Overall, it was revealed that the Island sample heat-treated at 1175 °C had better tensile behavior than the Meander sample under the same condition, resulting from the formation of more fraction of recrystallized grains and presence of less dislocations' density.

مرتضي شمعانيان اصفهاني , احمد كرمانپور

احسان فروزمهر

مسعود عطاپور، احمد رضائيان، همام نفاخ موسوي