With the rapid development of metal additive manufacturing technologies, the use of selective laser melting (SLM) technology to rapidly manufacture nickel-based superalloy components has made a major breakthrough, which has greatly improved the manufacturing efficiency of high-performance complex components in the aerospace field and promoted optimized and upgraded component structures. IN625 nickel-based superalloy is maturely used in the SLM technology. It has high high-temperature mechanical properties, good high-temperature corrosion resistance and high-temperature oxidation resistance. It is used in nuclear power, industrial gas turbines and key materials for hot-end components in aerospace and other fields. The unique microstructural characteristics of SLM IN625 alloys cause their solid-state phase transition characteristics under long-term high temperature conditions to be obviously different from traditional solid-state phase transitions. In this paper the evolution of the structures and properties of the SLM IN625 nickel-based superalloys during long-term thermal exposure at 700 ℃ are investigated with a view to revealing the evolution of the microstructures and mechanical properties of the additively manufactured nickel-based superalloys.
IN625 powder with chemical compositions shown in Table 1 is used. Samples with dimension of 20 mm×20 mm×200 mm are prepared by the EP-M250 SLM system in nitrogen atmosphere. The processing parameters are chosen as follows: laser power of 200 W, scanning speed of 1000 mm/s, hatch spacing of 17 μm, layer thickness of 30 μm, and spot diameter of 100 μm. The scanning strategy involves rotation of 67°of the laser between two adjacent layers. All the samples for a mechanical property test are cut from the as-built samples using wire cutting machining as shown in Fig. 1. The heat treatment schemes used in the experiment are listed in Table 2. In order to compare the influence of the non-equilibrium microstructure in the as-built alloys on the evolution of the aging microstructures, a part of the samples are treated at 1200 ℃ for 1 h and followed by water quenching to eliminate the non-equilibrium microstructures. Subsequently, the as-built samples and the solution annealed samples are subjected to thermal exposure at 700 ℃ for 500, 1000, and 3000 h. Tensile tests are performed at room temperature under quasistatic loading (strain rate of 1 mm·min-1). To observe the microstructures, all samples are first ground and mechanically polished. Then, electrolytic etching is employed at 10 V for 5-10 s in an electrolyte containing 10 mL HNO3+ 30 mL HCl + 50 mL C3H8O3. The microstructures are analyzed by optical microscope (OM) and scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS). The average size of the precipitated phases is calculated use Image-Pro Plus 6.0 analysis software. JMatPro software is used to calculate the balanced phase diagram of the IN625 alloy. The temperature range is 600-1400 ℃ and the cooling rate is set to 10 ℃/s. The isothermal transformation phase diagrams at 700 ℃ and 750 ℃ are calculated.
The microstructural morphology of the SLM deposited IN625 alloy is shown in Fig. 3. From Fig. 3(a), we can see the traces of the U-shaped molten pool on the X-Z surface of the SLM forming part. The structure is mainly composed of columnar dendrites and cellular dendrites. These are the typical non-equilibrium structural characteristics of nickel-based superalloys formed by SLM. The EDS composition analysis result in the inset shows that the inter-dendritic region [zone 1 in Fig. 3(c)] has become a Laves phase rich in Nb and Mo elements. Fig. 3(b) shows the microstructural morphology of the SLM deposited IN625 alloy after solution treatment at 1200 ℃. After a high temperature solution treatment, the traces of the molten pool, the dendritic structure, and the Laves phase completely disappear, and the structure has undergone significant recrystallization, forming a uniform equiaxed structure and a large number of annealing twins. By comparing Figs. 5(a) and 5(d), it can be seen that after the initial thermal exposure of 500 h, dense needle-like δ phases are precipitated in the interdendritic regions of the deposited alloy and the original Laves phases are significantly reduced. While no δ phase is found in the solid solution alloy, a film-like precipitated phase is formed on the grain boundary, and a large number of γ″ phase particles are precipitated in the crystal. Until thermal exposure for 1000 h, needle-like δ phases are preferentially precipitated on both sides of the grain boundary in the solid solution alloy, and the δ phase grows from the grain boundary nucleation to the intragranular growth [Fig. 5(e)]. At this time, the δ phases in the deposited alloy interlace each other at an angle of about 60° to form a network structure. It can be seen from Fig. 7 that before the thermal exposure treatment, the ultimate tensile strength (UTS) of the SLM deposited alloy is 890 MPa, the yield strength (YS) is 620 MPa, and the elongation (EL) can reach 52%. After a solution treatment, the UTS value of the alloy is 887 MPa, the YS value is 390 MPa, and the EL value is as high as 64%. After an aging treatment, the tensile strength and yield strength of SLM deposited and solid solution alloys have been improved to varying degrees, while the elongation has shown a downward trend. After aging for 3000 h, the UTS and YS of the deposited alloy are increased by 36% and 51%, and the EL is decreased by 21%. The tensile strength and yield strength of the solid solution alloy are increased by 27% and 87% and the elongation is decreased by about 28%.
In the long-term thermal exposure process, the δ phase in the SLM deposited IN625 alloy preferentially nucleates in the interdendritic region, and after a solution treatment, the δ phase in the alloy gradually grows from the grain boundary nucleation to the intragranular growth. Comparing with solid solution alloy, the δ phase nucleation rate of the SLM deposited alloy is high but the coarsening rate is low, and the γ″ to δ phase transformation speed is fast. When thermal exposure for 1000 h, the transition from γ″ phase to δ phase is basically completed, the δ phase is concentrated on both sides of the grain boundary in the alloy after a solution treatment, and a large amount of γ″ phases are still distributed in the crystal. Due to the segregation of Si element at the grain boundary, a large number of Laves phases are formed at the grain boundary in the SLM deposited alloy, which causes the depletion of the δ phases near the grain boundary, and the grain boundary precipitated phase of the alloy after the solution treatment is mainly M23C6. After long-term aging, the strengths of SLM deposited and solid solution alloys are significantly increased, while the plasticity is reduced. But the tensile strength and yield strength of the SLM deposited alloys are significantly higher than those of the alloy after a solution treatment, and the elongation rate is still relatively high.
