Microstructure And Mechanical Properties Of GH4169 Superalloy Fabricated By Electromagnetic Stirring Assisted Laser Solid Forming

GH4169 nickel based superalloy was used as experimental material in this paper, and microstructures and mechanical properties of GH4169 superalloy fabricated by electromagnetic stirring assisted laser solid forming(EMS+LSF) were investigated. The solidification microstructures of LSF GH4169 alloy with different magnetic field intensity were observed by metallographic microscope(OM), field emission scanning electron microscope(FESEM), energy specturm(EDS), electron backscattered diffraction(EBSD), and its mechanical property were tested. Furthermore, the strengthening mechanism of LSF GH4169 superalloy with different magnetic field intensity was studied. The research results are as follows:The dense microstructure of EMS+LSF possesses the dendrites feature with any signification metallurgical defect, which growing epitaxially along the deposition directure. The intense convection of liquid metal in molten pool under magnetic field can reduce the amount of Laves eutectic phase formed in interdendritic area and change its morphology, the element micro-segregation becomes weaker and the microhardness of that material is somewhat increased. Furthermore, electromagnetic stirring can decrease the differences of residual stress in the overlapping region and the interior region.EBSD analysis shows that electromagnetic stirring can improve the uniformity of grain size of as-deposited LSF GH4169 to some extent, but the grain boundaries of that are still characterized by a large number of anisotropic columnar crystal boundaries and a tiny amount of the sub-grain boundaries. Thus, the microstructure can not achieve dendrite fragmentation and fine equixed grains used by electromagnetic stirring in this experimental parameters.Recrystallization process indicates that EMS+LSF GH4169 alloy requires a longer incubation period and higher recrystallization temperature, whose nucleation position is more randomness and is not a priority in the overlapping area. Sub-grain and grain-boundary migration nucleation are the main nucleation mechanisms at the beginning of the recrystallization. Whether the EMS+LSF GH4169 alloy samples with or without electromagnetic stirring were heat-treated at 1100℃ for 60 min, the grains also transform the original columnar crystal grains into equixed grains, thus eliminating the anisotropic microstructure of this material. When the magnetic field strength reached up to 50 mT, the size of recrystallization grain becomes finer. Furthermore, nucleation related to twin is one of the main mechanisms in the later period of the recrystallization.Electromagnetic stirring can greatly improve the strength and plasticity of as-deposited LSF GH4169 alloy. When magnetic field strength reached up to 50 mT, the tensile strength of as-deposited LSF GH4169 alloy reaches up to 1048 MPa, which is increased by 100 MPa and its elongation is increased by 22%. After solid solution and aging treatment, the effect of electromagnetic stirring on LSF GH4169 alloy is eliminated, the difference of the strength and plasticity of LSF GH4169 alloy becomes less. Furthermore, the standard of strength and plasticity is still better than that of forging standards.Electromagnetic stirring can improve fatigue performance of as-deposited LSF GH4169 at room temperature to some extent. The crack sources of all samples locate in near surface region, which are characterized by fish-eye-like structure. When magnetic field strength reached up to 50 mT, fracture surface of EMS+LSF GH4169 superalloy has obvious stripes. However, fracture surface of EMS+LSF GH4169 superalloy without electromagnetic firring has not obvious stripes and secondary cracks, which is attributed to the improvement of plasticity of LSF GH4169 superalloy used by electromagnetic stirring. The samples were treated by solid solution and two-stage aging heat treatment, whose fatigue properties show a similar trend. All the fatigue test results show that electromagnetic stirring is helpful to the improvement of fatigue properties of LSF GH4169 superalloy.