Crystal Growth And Microstructure Formation Of Nickel-based Single-crystal Superalloy By Laser Powder Deposition

Nickel-based single-crystal(SX) superalloys have been used as the material of high-pressure blades and vanes in modern gas turbines due to their superior high-temperature creep resistance and thermal fatigue behavior. The material and manufacturing of SX turbine blades are very costly. The lifetime of SX turbine blades and vanes is limited by many defects such as thermal fatigue cracks, tip erosion, surface wear and hot corrosion. The replacement of the SX components contributes largely to the maintenance cost of modern gas turbines. Life extension by inspection and repair technologies can help to reduce the operation cost of gas turbines and save a large amount of high-value SX alloys. As an effective repair technology, laser powder deposition(LPD) processing has been implemented to restore the worn tips of polycrystalline turbine blade with a near-net-shape capability. Successful blade repair technology for SX alloys without degradation of thermomechanical behaviors should retain a SX solidification microstructure which is identical to the crystalline structure of the base material. Meanwhile,investment casting technology is the main method to manufacture SX turbine blades. Nevertheless casting technology has some disadvantages such as high failure rates, casting defects,long production cycles, high costs, etc., which greatly increase the price of SX turbine blades and associated maintenance costs of modern gas turbines. LPD technology can manufacture dense parts of various kinds of material with CAD, CAM and monitoring and feedback control methods. Thus, LPD technology not only provides a way to repair SX components, but also supports a probability to manufacture SX components directly and with a low cost due to its characteristics of rapid prototyping and near-net-shape. The achievement of having continuous SX microstructure without any defects such as stray grain and cracks in multi-layer or even in multi-track and multi-layer deposited SX alloys is the key point to apply LPD technology to repair and manufacture SX components successfully. While the crystal growth and microstructure formation in LPD processing of SX alloy is affected by many factors and very difficult to control. The further study of the transport phenomena and the mechanism of crystal growth and microstructure formation in LPD processing of SX superalloy is meaningful and valuable for the development of repair and the manufacturing of SX turbine blades.For a better understanding of the fundamental transport phenomena occurring in the LPD processing of SX superalloy, a self-transient three-dimensional(3D) mathematical model was developed. Physical phenomena including laser-powder interaction, heat transfer, melting, solidification, mass addition, liquid metal flow, overlapping and remelting in multi-layer or even inmulti-track LPD processing of SX superalloy were studied quantitatively and verified by experimental approaches. The effects of processing parameters including laser power, scanning speed and powder feeding rate on geometric size of molten pool and remelting ratio are discussed.Based on the self-transient 3D transport phenomena model, a crystal growth model is established and coupled to predict the crystal growth and microstructure formation in LPD processing of SX superalloy. The mechanism of epitaxial growth of SX superalloy is explored.The effects of the governing processing parameters of LPD, i.e. laser power, scanning speed,powder feeding rate, substrate crystallographic orientation, inclination angle of the coaxial nozzle on the crystal growth and microstructure formation were studied systemically through the mathematical modeling and experimental approaches. Experiments with SX nickel-based superalloy were conducted to verify the computational results. For the LPD of SX alloy on[001]/<100> surface of the SX substrate, optimized processing parameters are summarized to achieve the continuous SX microstructure in wall-deposited structure, which can be used to repair the SX turbine blade.Furthermore, the crystal growth model is improved to study the microstructure formation in multi-layer and multi-track LPD processing of SX superalloy. The effects of overlapping ratio and scanning strategies on crystal growth and microstructure formation are analyzed to optimize the processing parameters. Based on the simulation results, a mathematical model is established to study the effects of geometric parameters of the molten pool and the overlapping ratio on the continuity of SX microstructure in multi-track and multi-layer deposited bead.An effective processing window for continuous SX microstructure is summarized. Based on the optimized processing parameters of the LPD processing of SX superalloy, the mechanism of crack appearance in multi-track and multi-layer deposited bead with SX superalloy is analyzed,and some aiding methods are developed to control the crack formation and propagation. Finally,the potential of repair and manufacturing SX turbine blade directly by LPD processing is evaluated.