In the past decade, due to the needs of industries and the capabilities of additive manufacturing, special attention has been given to the production of various components made of superalloys, particularly the production of turbine blades made of IN738LC superalloy using laser powder bed fusion. The heat treatment process plays a key role in the properties of this superalloy. Therefore, the aim of this research is to eva‎luate the effect of heat treatment variables on the microstructure and mechanical properties at room and high temperatures of the IN738LC superalloy produced by laser powder bed fusion. Two sets of heat treatments were designed and conducted to investigate the effects of temperature and annealing time as well as pre-annealing treatment on precipitates, crystallographic texture, and recrystallization. For this purpose, cubic samples were made using the laser powder bed fusion process and microstructural and mechanical investigations were carried out after heat treatment and also in the as-built state. Microstructural investigations of as-built samples showed that due to the nature of the laser powder bed fusion process, columnar and elongated grains are formed in the building direction and a cellular structure is created. A dominant cubic crystallographic texture was also observed in the sample. By performing a solution annealing process, it was observed that with increasing annealing temperature, grain growth intensifies, especially in the building direction, so that the highest aspect ratio of the grains after annealing is observed at 1220 °C for 2 hours with the value of 3.3. With an increase in annealing time to 5 and 10 hours at 1220 °C, the aspect ratio of the grains decreases to 1.7 due to recrystallization. Studies on the effect of annealing temperature on precipitates also showed that in general, with increasing annealing temperature, the size of γ' precipitates decreases and MC carbides become coarser. Increasing the annealing time also had a similar effect on MC carbides as increasing the annealing temperature. By carrying out the aging process, an increase in the size of the strength-enhancing γ' precipitates and a change in their morphology was observed. Mechanical property investigations using hardness testing showed that generally, with a decrease in the size of γ' precipitates, hardness increases and for this reason, the sample annealed at 1220 °C for 2 hours showed the highest hardness of 496 Vickers. The strength of the samples examined by the small punch test at room temperature had similar results to hardness testing, with the difference that in this test, grain size is of greater importance compared to hardness testing, and the two samples annealed at 1180 and 1220 °C for 2 hours showed the highest strength of approximately 1830 N. In the high-temperature small punch test, the trend was opposite to the room temperature trend, so that the sample annealed at 1120 °C showed the highest strength of 2500 N at 650 °C due to the larger size of γ' precipitates. The examination of fracture surfaces also showed that the room temperature fracture of all heat-treated samples and the as-built sample was ductile, while at high temperature, the as-built sample fractured in a brittle intergranular manner and the heat-treated samples exhibited ductile fracture.

احمد كرمانپور , احمد رضائيان

عباس بهرامي , محمود منيرواقفي