Ni-based superalloys are the ideal high-temperature materials due to their excellent oxidation resistance and microstructure stability at the elevated temperature. But the technical characteristic and limitation of traditional manufacturing techniques restrict the development and product of superalloy components. Recently, the laser additive manufacturing (LAM) technology provides an effective tool to fabricate the integral and complex components. However, the compositions of heritage alloys are designed based on the traditional techniques without considering the specifications of the LAM process. During the LAM process, the non-equilibrium characteristics of multiple thermal cycles, rapid heating and cooling rate and the localized microstructural evolution result in the metallurgical defects such as cracks, pores, and lack of fusion, which are difficult to be completely eliminated by optimizing the process parameters. Note that the Ni-based superalloys are developed from the Ni-20%Cr alloy, and the Ni-Cr-Al system can be regarded as the basic composition of the Ni-based superalloys. The basic composition plays a significant role in the further design of multicomponent alloys owing to the correlation to the microstructural stability, mechanical properties and weldability of Ni-based superalloys. Therefore, it is necessary to optimize the compositions by investigating the influence of Cr and Al contents on microstructures and properties of the basic alloys. In this paper, five representative basic alloys are designed based on alloying of binary Ni-20%Cr alloy with Al element, and the influence of composition on microstructures and properties of as-deposited alloys is systematically investigated. This research can be helpful to design the Ni-based superalloys which are fit for the LAM process.
Five basic alloys are first designed through the "cluster-plus-glue-atom" model, and then fabricated by the LAM process. The pure Ni plate is chosen as the substrate. The elemental powders of Ni, Cr and Al with purity (mass fraction) of 99.90%-99.99% and particle size of 50-150 μm are chosen as feedstock materials, and the powders are blended by a ball grinder for 10 h. The specimens are built on the LDM-800 additive manufacturing system using a strategy of bidirectional scanning, and the process parameters are optimized as laser power of 2 kW, scanning speed of 5 mm/min, laser beam diameter of 2 mm, overlapping rate of 50%, and powder feeding rate of 6.8 g/min. The LAM specimens are cut along the build direction for the microstructural and mechanical property analysis. The crystal structures of as-deposited alloys are identified through X-ray diffraction. The microstructural evolution and elemental distribution are analyzed by scanning electron microscope (SEM) and electron probe microanalyzer (EPMA), respectively. The precipitated phase is identified and investigated by transmission electron microscope (TEM) equipped with the selected-area electron diffraction. The microhardness is measured by the hardness tester, and the room compressive test is tested on a material testing machine. The continuous variable-temperature oxidation is performed on the thermal analyzer. In order to evaluate the weldability of alloys, three cross-sections of each alloy along the build direction are observed by optical microscopy and the solidification temperature range of alloy is measured by differential scanning calorimetry (DSC).
The matrices of Ni75.0Cr25.0 and Ni75.0Cr18.75Al6.25 alloys are composed of the γ-Ni solid solution, while the γ′ phase begins to precipitate in γ-matrix when the Al content (atomic fraction) is higher than 6.25% (Fig. 1). Additionally, the α-Cr solid solution distributes between the columnar grains of the Ni75.0Cr25.0 alloy, and the amount of α-Cr solid solution increases with the increase of Al content. In the Ni75.0Al25.0 alloy, the α-Cr solid solution is replaced by γ′-Ni3Al+ γ-Ni divorced eutectic (Fig. 2). The microhardness and strength of as-deposited alloys first slightly increase with the increase of Al content, and then sharply increase when the Al content is higher than 6.25%, owing to the precipitation of γ′ phase (Figs .4 and 5). But the ductility significantly decreases with the increase of Al content (Table 4). The continuous variable-temperature oxidation curve and the oxidation kinetics data show that the initial temperature of vigorous oxidation overall increases with the increase of Al content, while the total mass gain and mass gain rate change in an opposite trend (Fig. 7 and Table 5), and the improved high-temperature oxidation resistance can be attributed to the formation of Al2O3 oxide scale. However, the excessive Al content enlarges the solidification temperature range and deteriorates the weldability of alloys, and the large amount of pores and lack of fusions are formed in Ni75.0Al25.0 alloy (Fig. 10 and Table 7).
In this paper, five representative basic alloys are designed using the cluster model based on alloying of the binary Ni-20%Cr alloy with Al element. The influence of composition on microstructures and properties of the as-deposited alloys is investigated. The results show that the matrix structures of as-deposited alloys evolve from γ-Ni solid solution to γ′-Ni3Al strengthening phase with the increase of Al content. Meanwhile, α-Cr solid solution distributing along grain boundaries changes from granule to long-chain in morphology while increasing its fraction, and is replaced by γ′-Ni3Al+ γ-Ni divorced eutectic at 25.0% Al. The hardness and strength of as-deposited alloys increase with the increase of Al content due to the change in the strengthening mechanism from solid solution strengthening to precipitation strengthening, whereas the ductility decreases. The increase of Al content is beneficial for improving the high temperature oxidation resistance, but the excessive Al is deleterious to the weldability. Therefore, the Al content should be confined in a range of 12.5%-18.75% to make the basic alloys have a good match of mechanical properties, high temperature oxidation resistance and weldability.
