Nickel-based superalloys are typically used as turbine disks and turbine blade materials in aeroengines owing to their excellent high-temperature performance. Because of the different service conditions of the turbine disk and turbine blade, the premature failure of joints can be avoided using gradient materials. However, a change in the composition of the gradient transition zone can change the microstructure, which significantly affects the properties of the alloy. Therefore, the evolution of the microstructure and the hardness of laser melted nickel based superalloys must be investigated, providing a basis for the laser additive manufacturing of gradient nickel-based superalloys.

The materials used in this experiment are IC10 directional superalloy and FGH9X powder superalloy with a particle size of 0.15-0.25 mm. IC10 and FGH9X powders are prepared as nickel-based superalloy powders with different compositions, and alloy ingots with 11 typical gradient components are prepared by changing the ratio of the two alloys. (From samples F100 to F0, the mass fraction of FGH9X decreases by 10% intervals, and the mass fraction of IC10 increases by 10% intervals. For example, the FGH9X mass fraction of the F80 sample is 80% and the IC10 mass fraction is 20%). A 50 g nickel-based superalloy mixed powder is prepared and placed in a copper crucible. The laser power and opening time are set to 5 kW and 2 s, respectively. A melting experiment is performed in an argon atmosphere, and the alloy ingot samples are obtained after air cooling. The microstructures of the samples are observed using an optical microscope (Leica DM4M) and scanning electron microscope (Apreo S LoVac). The primary dendrite spacing, γ' phase size, and γ' phase content are measured using the Image Pro Plus software. Thermo-Calc 2020b is used to simulate the nucleation driving force of the γ' phase in the alloys. A hardness test is performed using a micro Vickers hardness tester (MH-6) with a load of 4.9 N and a holding time of 15 s.

The microstructure of the 11 nickel based alloy samples prepared via laser melting is composed of dendrites, and primary and secondary dendrites. When the sample changes from F100 to F0, the dendrite morphology of the alloys remains almost unchanged, and the primary dendrite spacing is in the range of 100-120 μm. Based on literature review, the dendrite morphology and dendrite spacing of the alloys are primarily affected by the cooling conditions, and the effect of alloy composition is insignificant. An analysis of the alloy microstructure shows that the 11 types of nickel-based alloys are composed of the γ and γ′ phase, carbides, and the γ/γ′ eutectic phase (Fig. 3). When the alloy composition changes from F100 to F0, the content and size of the γ′ phase increase continuously (Fig. 5), which is due to the gradual increase in Al and Ta contents in the alloys. The nucleation driving force of the γ' phase increases and more γ' phases precipitate. In addition, owing to element segregation, the content and size of the interdendritic γ' phase differ significantly from those of the dendritic trunk γ' phase. The content of γ' phase in interdendritic zone is more than that in dendritic trunk and the size of γ' phase in interdendritic zone is larger than that in dendritic trunk. In addition, the results show that the change in alloy composition does not significantly affect the microhardness, and that the overall hardness value is approximately 500 HV. This is because as the alloy composition changes from F100 to F0, the content of solid-solution strengthening elements and the content of carbides with high hardness and brittleness in the alloys decrease gradually, whereas the hardness and size of the γ' phase increase gradually; therefore, the hardness of the 11 types of nickel-based alloys is similar.

Primary dendrite and secondary dendrite arms are developed in the microstructures of the 11 types of nickel-based superalloy ingots prepared via laser rapid melting. The change in alloy composition does not significantly affect the dendrite morphology, and the average primary dendrite spacing is 110 μm. The microstructures of the alloys are composed of the γ phase, γ′ phase, carbides, and the γ/γ′ eutectic phase. As the powder nickel-based alloy content in alloy ingots decreases, the content and size of the γ 'phases increase continuously. As a result of element segregation, Al, Ti, Ta and Nb, which are the constituent elements of the γ 'phase, segregate in the interdendritic regions, thus causing the content and size of the γ' phase in the interdendritic regions are greater than those in the dendritic trunk. The microhardness values of the 11 samples are similar, and the overall hardness value is approximately 500 HV.