Inconel 625 superalloy is extensively used in combustion turbines because of its superior tensile, fatigue, and creep strengths at elevated temperatures. Selective laser melting (SLM), which is one of the most promising additive manufacturing (AM) methods for high-temperature superalloys, has attracted considerable attention in many fields, including aerospace, owing to its special technical advantages. However, plastic loss occurs in the service temperature range of nickel-based superalloys, which poses a serious threat to the stability of their mechanical properties during service. To ensure the stability of SLM-fabricated Inconel 625 superalloy in high-temperature service environments, the different types of pore defects should be studied to understand the high-temperature plastic deformation behavior of this superalloy for engineering applications.

Different types of pore defect samples are prepared by adjusting the process parameters, and the samples are subjected to room- and high-temperature tensile tests. The influence of different types of pore defects on the high-temperature plastic loss of the superalloy is analyzed using fracture and fracture longitudinal sections, and the fracture mechanism is analyzed.

When the laser power is set to 300 W, with increasing scanning speed, the pore defects transit from large regular circular pore defects in “key-hole” mode to small regular circular pore defects in “conduction” mode. When the scanning speed is further increased, the laser energy input decreases, and irregular pore defects with typical tip shapes are obtained (Fig.3). In the tensile test, the influence of different pore types on the room- temperature plasticity and high-temperature plasticity of the alloy becomes more apparent. Among these pore types, irregular incomplete fusion defects exert the greatest impact on plasticity, with room-temperature plasticity and high-temperature plasticity of 6.47% and 4.85%, respectively. The second most influential defects are the large circular pores in the “key-hole” mode, which exhibit room-temperature and high-temperature plasticity of 20.45% and 11.5%, respectively. However, the regular small circular pore defects produced in the “conduction” mode have the least effect on plasticity, with room-temperature plasticity of 39.19% and high-temperature plasticity of 36.84% (Fig.6). The stress–strain curve of the SLM-formed Inconel 625 alloy at high temperatures shows a significantly different trend from that at room temperature. After the alloy stress reaches its peak value, the alloy softens with an increase in strain (Fig.5). In room- and high-temperature tensile fracture surfaces, keyhole-mode pores can be observed in a small number of larger pores, which are embedded with unfused powder particles; there are a small number of secondary cracks along the outer edges of the pores, and the fracture mode is the typical brittle fracture. In the conduction-mode pores, along the outer edges, there are smaller, tough nests, which exhibit more evident ductile fracture characteristics. Irregular pores exhibit brittle fracture characteristics because of their larger number and sizes (Fig.8). The three different pore types exhibit more plastic cracks at high temperatures (Fig.9). With the movement of dislocations, the tips of the irregular pores easily cause dislocation accumulation, which affects the plastic deformation of the superalloy and leads to its rapid fracture. Compared with irregular pores, the circular pore boundary of the “key-hole” mode can reduce the degree of dislocation pile-up; therefore, the high-temperature plasticity of these pores is higher. The circular pores in the “conduction” mode are very small, and their ability to weaken the dislocation pile-up is the strongest; therefore, their high-temperature plasticity is the highest.

The results of static load tensile tests show that the stress-strain curves of tensile specimens with different pore types at high temperatures are obviously different from those at room temperature. After the superalloy stress reaches its peak value, the superalloy appears to soften with an increase in strain. The load fluctuates continuously and periodically in the softening stage and gradually decreases until fracture. The effects of different pore types on the high-temperature plasticity of the superalloy are significantly different; the high-temperature plasticity of the irregular pore sample is 4.85%, and those of large circular pores in the “key-hole” mode and small circular pores in the “conduction” mode are 30.69% and 36.84%, respectively. Although the high-temperature plasticity of the “conduction” mode pores is improved compared to the other types, it is still lower than that of the as-cast Inconel 625 superalloy. Therefore, it is believed that the type of pore defect does not significantly improve the high-temperature plasticity of the superalloy.