Laser Direct Metal Deposition (dmd) Of Ni-base Superalloy And Its High Temperature Oxidation Resistance

Superalloys are important materials for modern power plant, which have a wide range of applications in aviation, spaceflight, energy and power fields. In this dissertation, different wt. % Y₂O₃ was added in the Inconel738 powder and deposited by laser DMD technology on K438 superalloy substrate. The porosities and microcracks were successfully eliminated in the components by DMD. Then high temperature oxidation resistance of the DMD components and substrate were also studied.In this research the DMD components contained different proportion of Y₂O₃ or under the different laser parameters were compared. The microstructure, micro-hardness, phase composition were investigated by scanning electron microscopy (SEM), micro-hardness tester, X-ray diffraction (XRD). The isothermal oxidation behavior of the DMD components and substrate were also tested and the oxidation kinetics curves were drawn. The surface morphology and cross-sectional microstructure of the specimens oxidized for different time were observed and analyzed by SEM. The phase composition and the distribution of elements of the oxidation layer were investigated by XRD and energy dispersive X-ray spectroscopy (EDS) fixed on the SEM. The oxidation mechanisms were discussed to understand the experimental results.1. The residual thermal stress and the porosities in the Ni-base superalloy determined the types and the mechanism of the crack. The macro-cracks origined at hole cusp and extended to the top of the DMD components for the existence of thermal stress. The impurity compounds contained B, Si and other elements are prone to formation of micro-cracks.2. Under the optimized laser parameters, powder of 2.2KW, scanning rate of 1000mm/min, superalloy powder contents 0.2%wt Y₂O₃ can be formed a good surface quality component with a certain height. The microstructure of the component is dense and made up of fine dendrites, which grows epitaxially. The DMD component and substrate have the same phase Ni3(Al,Ti).3. The growth morphologies of dendrite transformed from long columnar to cellular with increment of laser power, while the grain size is also increased. With the increment of laser scanning speed, the grains are refined and the grain boundaries are also increased because of the rapid solidification rate.4. The isothermal oxidation kinetics of DMD components followed linear law at the early oxidation and parabolic law after oxidized steady state. The oxidized law between DMD component and substrate are the same.5. As the XRD results show, the main components of the oxidation scales are Cr₂O₃, Al2O3, small amounts of TiO₂ and trace of NiCr₂O₄ phase. The morphology of cross-section is investigated by SEM, the oxidation scales consist of three parts. With continuous distribution and uniform thickness, the inner layer of DMD components has good combination with the substrate. The high temperature oxidation resistance of DMD components is superior to the substrate.