Additive manufacturing (AM) provides enormous processing flexibility, enabling novel part geometries and optimized designs. Access to a local heat source further permits the potential for local microstructure control on the scale of individual melt pools, which can enable local control of part properties. In order to design tailored processing strategies for target microstructures, models predicting the columnar-to-equiaxed transition must be extended to the high solidification velocities and complex thermal histories present in AM. Here, we combine 3D characterization with advanced modeling techniques to develop a more complete understanding of the solidification process and evolution of microstructure during electron beam melting (EBM) of Inconel 718. Full calibration of existing microstructure prediction models demonstrates the differences between AM processes and more conventional welding techniques, underlying the need for accurate determination of key parameters that can only be measured directly in 3D. The ability to combine multisensor data in a consistent 3D framework via data fusion algorithms is essential to fully leverage these advanced characterization approaches. Thermal modeling provides insight on microstructure development within isolated solidification events and demonstrates the role of Marangoni effects on controlling solidification behavior.
As a new class of promising high-temperature materials, Co–Al–W-base alloys have been developed by alloying additions to improve the microstructure stability and other properties. However, the optimization of Co–Al–W-base alloys becomes more complicated with increasing variety and content of alloying elements. In this study, an accelerated approach to design γ′-strengthened Co–Ni-base superalloys with well-balanced properties was developed, by integrating the diffusion-multiple approach and machine-learning tools. A large amount of experimental data was obtained using the diffusion-multiple approach and fed into machine learning tools to establish the relationship between alloy compositions and important thermodynamic and microstructural parameters such as the phase constituent, the γ′ phase fraction (Fγ′) and the γ′ solvus temperature (Tγ′). The established machine-learning models were then employed to predict the characteristic parameters of multicomponent Co-Ni-base superalloys containing up to nine elements (Co, Ni, Al, W, Ta, Ti, Cr, Mo, Nb), even though most of the collected compositions from experiments were quinary to septenary alloys. Using the predicted results from the models and the computational thermodynamics tools, a multicomponent Co–Ni-base superalloy aimed at the application as single crystal blades was designed and characterized to test the reliability and robustness of the novel design approach.
This work demonstrates on flat test specimens that orbital friction welding is a suitable process to join a typical disk alloy 718 and a typical blade alloy 713LC. The capability to join nickel-based blades on disks enables the manufacture of blisks instead of the typically used bladed disks. Blisks have the potential to save weight within an aero engine. Orbital friction welding is an alternative process to linear friction welding to enable blade and disk joints. The advantage of orbital friction welding is that the movement is homogenous and the amplitude (or eccentricity) is smaller compared to linear friction welding. This allows joints in areas which are geometrically challenging and impossible to join for linear friction welding. Due to the solid state joining process, there is basically no material mixing within the weld, but within both alloys, recrystallization can be seen near the weld line. The result is a fine-grained microstructure within both alloys adjacent to the bond line. While this means only a small microstructural change for alloy 718, it is a major microstructural change for the cast coarse grain alloy 713LC. Consequently, the mechanical properties are changed compared to the base material due to the friction welding process. The microstructural changes of orbital friction welding and their influence on the material properties are presented within this work.
The cast and wrought nickel base superalloy M647 has been developed with high mechanical performance and deformability. This study describes an overview of the microstructure and the properties of M647. A full scale forging process has been developed with uniform fine grain distribution. The detrimental effect of thermal exposure at 700 °C ~ 800 °C on mechanical properties was evaluated. M647 showed good mechanical properties and equivalent thermal stability to conventional turbine disk alloys. M647 offers good balance of deformability and mechanical properties with lower cost than powder metallurgy alloys.
Alloying effects on the competition between discontinuous and continuous precipitation of δ-Ni3Nb (D0a) phase and η-Ni3Ti (D024) phase were investigated in model Ni-Cr-Fe-Nb and Ni-Ti-based alloys, respectively, to aim at designing polycrystalline Ni based superalloys with better temperature capability. Discontinuous precipitation tends to occur at lower temperatures, while continuous precipitation dominates at higher temperatures in the two alloy systems. The addition of Mo promotes continuous precipitation with respect to discontinuous precipitation while that of Ti promotes discontinuous precipitation rather than continuous precipitation in δ phase precipitation alloys. A replacement of Ti with Mo promotes continuous precipitation with respect to discontinuous precipitation in η phase precipitation alloys at 800 °C. The observed alloying effects are discussed in terms of chemical driving force, interfacial energy between the matrix phase and the δ(η) phases, and coherency strain caused by the formation of the metastable phases prior to the formation of δ/η phases.
