Manufacturing is the "fatal weakness" in the application of superalloys such as nickel-based superalloys. If there is no lengthy and expensive subtractive manufacturing through casting machining, it will not be possible to obtain structurally good mechanical properties. And 3D printing can effectively manufacture complex structures, which are usually difficult to achieve. For example, in the article "Honeywell's Multi-Dimensional Testing of 3D Printing Nickel-based Superalloy Materials", the blisk, the components of the internal integrated cooling channel, and the lattice structure.
However, most conventional nickel-based superalloys cannot be used in the transition from precision casting process to 3D printing technology, because these materials are optimized for traditional processes such as casting. Due to the rapid repetitive thermal cycle of the 3D printing process, a new composition for 3D printing process parameters can be designed in a data-driven way of composition calculation, so as to adjust the microstructure and performance for the high cooling rate of additive manufacturing.
Therefore, optimizing nickel-based superalloy materials for additive manufacturing processes, reducing their metallurgical defects, and introducing alloy materials suitable for 3D printing, play an important role in promoting the application of superalloy additive manufacturing.
A research team from the University of California and other institutions has developed a cobalt-nickel-based superalloy based on the two powder bed fusion 3D printing processes of EBM and SLM, which is said to be able to resist defects generated in 3D printing [2].
Alloys optimized for additive manufacturing
Block Additive Manufacturing Challenges for Nickel-Based Superalloys
Metal 3D printing technology based on powder bed melting makes it possible to manufacture metal parts with complex geometric shapes. The degree of freedom this process brings to product design optimization has aroused interest in the fields of medical implant equipment, automobiles, and aerospace. However, the types of alloy materials that can be applied to this type of metal 3D printing process are still very limited.
Electron beam melting (EBM) and selective laser melting (SLM), two powder bed metal 3D printing technologies, realize the layer-by-layer "growth" of components through the partial melting of metal powder. In essence, this is a process of repeated welding. In this process, the printing device uses directional energy to locally melt and join materials. Therefore, candidate materials for additive manufacturing tend to be weldable alloys, which are not susceptible to cracking mechanisms generated by liquid phases (such as liquefaction cracking or hot tearing) or solid-state stresses (strain-aging cracking and high temperature demolition cracking). .
Due to its excellent mechanical properties at high temperatures, nickel-based superalloys are the material of choice for structural components such as aircraft engines and gas turbine single crystal turbine blades. These alloys consist of a high volume fraction (> 0.6) of γ'phase (Ni3 (Al, Ti), L12) submicron-sized cubic precipitates, which are combined with solid solution strengthening matrix or γ phase (Ni, A1) coherent. However, many high-performance nickel-based superalloys are not weldable. This is because the γ'phase precipitates out soon after solidification, which hinders the relaxation of thermal stress by strengthening the newly solidified material, resulting in strain aging cracks.
When the γ phase solidifies, the liquid becomes locally enriched due to the repelling of γ'forming elements such as Al, Ti and Ta12. This solute segregation reduces the local liquidus temperature and produces a solute-rich liquid film between solid dendrites. During cooling, they shrink at different rates in the molten pool, causing tensile stress and fracture.
Liquid-mediated cracking sensitivity can be influenced by controlling the alloy composition and the liquid composition and liquid fraction at a given temperature. Just below the melting point, it can adapt to the stress through solid plastic deformation. At this time, it is very sensitive to the temperature of the strengthening precipitation. Therefore, many ideal nickel-based superalloys with high γ'volume fraction are prone to cracking in the near solidified state and in the solid state. In principle, the segregation and precipitation process of solutes can be changed by overall changes in composition.
The crack sensitivity of high-performance engineering alloys, including nickel-based superalloys with high γ'volume fraction, high-strength aluminum alloys, and refractory alloys, represents a major obstacle to the use of these alloy materials for additive manufacturing in critical applications. For high-strength aluminum alloys and other alloys that operate at lower temperatures, controlling the nucleation of crystal grains in the molten pool through the functionalization of the powder surface can reduce the problem of cracking. However, this method will result in a smaller grain size, which is disadvantageous for materials used in high temperature environments. Therefore, the additive manufacturing of superalloys still requires innovative alloy designs.
Block strategy to reduce cracks
The research team pointed out in the paper that there are several different strategies for the development of additive manufacturing of superalloys. For example: by adding solid solution strengthening elements to the nickel-based superalloy Hastelloy X, which is within the range of the existing commercial alloy composition, it can be observed that the microcracks in the additive manufacturing components are reduced; in order to control the anisotropy of the material, some materials research Sensitivity analysis was carried out, focusing on adjusting the alloy composition to control the columnar transformation of the nickel-based alloy to the isometric transformation; there are also ways to control the columnar to isometric transformation through the control of the additive manufacturing process, and the atomic-scale grain boundary engineering was successful In addition, through additive manufacturing technology, alloy powders can be mixed before printing to produce metal-metal composites with unique microstructures, which are difficult to manufacture by other methods.
Since the γ-γ' microstructure in modern nickel-based superalloys has become an ideal material due to its excellent mechanical properties, the research team sought to design a nickel-based high temperature that contains high γ'volume fraction while maintaining good printability. alloy. According to the understanding of 3D Science Valley, they designed a cobalt-nickel (CoNi)-based super alloy that can be processed by selective laser melting (SLM) and electron beam melting (EBM) manufacturing paths. This material can produce crack-free growth. Material manufacturing components.
The lower solute segregation during the solidification process reduces the liquid-mediated cracking sensitivity, and the reduced γ'solid solution temperature can alleviate the cracking after the solidification is completed. The research team stated in the paper that the room temperature tensile test showed that, compared with other additive manufacturing nickel-based superalloys currently under study, the cobalt-nickel-based superalloy has an excellent combination of ductility and strength, which is a powder bed additive manufacturing. The application of technology in the manufacture of high-temperature parts provides a new space.
The research team prepared 136 kg of SB-CoNi-10 powder through vacuum induction melting and argon atomization. The powder size range used for SLM 3D printing is 15-53μm, and the powder size used for EBM 3D printing is 53-177μm, as shown in Figure 1a and b. In both processes, typical nickel-based alloy 3D printing process parameters are used to print rectangular blocks and leaf-shaped samples. SLM 3D printing is performed on a powder bed with a preheating temperature of 200°C, while EBM printing uses electron beams to preheat the powder to 1000°C.
The limited preheating used during SLM (200°C) does not promote phase formation and thickening during the additive manufacturing process, as in EBM. The existing molten pool can be seen in Figure 5a. The columnar crystal grains grow from the bottom of the molten pool along the construction direction, and the crystal grains grow laterally from the wall of the molten pool to the centerline of the laser track, and have a limited amount of porosity. BSE photomicrographs at different depths under the final build layer shown in the figure 5 B to E show the presence of the honeycomb-like microstructure of the entire build.
The research team finally came to the conclusion that the cobalt-nickel-based superalloy SB-CoNi-10 has been successfully additively manufactured using two powder bed metal 3D printing technologies, EBM and SLM. The composition map of the 3D printed microstructure shows that the favorable solute distribution combined with a good γ'-solid solution temperature can inhibit cracking in the range of solidification conditions encountered in the EBM and SLM processes.
Compared with the traditional processing route, the high thermal gradient and cooling rate in the additive manufacturing process can significantly improve the structure after solidification, thereby reducing the time required for solution heat treatment. The alloy can be processed by standard post-treatment and heat treatment, in which a fine dispersion of high volume fraction of γ'phase is precipitated.
Tensile testing shows that, compared with other nickel-based superalloys with high γ'volume fraction manufactured by EBM and SLM, the new cobalt-nickel-based superalloys are less likely to form defects during the additive manufacturing process, so they have excellent The ductility and higher tensile strength limit.