Microstructural Evolution And Performance Control Of GH4169 Alloy Produced By Selective Laser Melting

As one of the most promising additive manufacturing technologies,selective laser melting(SLM)has been widely used in the manufacturing of complex and precise GH4169 alloy components.At present,SLMed GH4169 alloy has high plasticity and low strength,which seriously restricts its application in the engineering field.The main reason is that GH4169 alloy has a variety of phase transition behaviours under the action of high-frequency and unsteady thermal cycle during SLM process,and the microstructural evolution mechanism is still unclear,which is difficult to provide theoretical guidance for the regulation of mechanical properties.Therefore,the main contents and conclusions are shown as follows after detailed researches.(1)The influence of precessing parameters on the morphology,melting mode and microstructure of the molten pool was studied.With the increase of laser power or the decrease of scanning speed,the morphology of the molten pool changes from shallow U,deep U,shallow V to deep V,and the melting mode changes from conduction to keyhole mode.The microstructure of SLMed GH4169 alloy molten pool is composed of ?-matrix,interdendritic Laves phase and a small number of ?? phase,which is not affected by the processing parameters.Besides,the uniformity of microstructure and microhardness of the SLMed GH4169 alloy molten pool under conduction mode is better than that of keyhole mode.(2)The microstructural evolution of GH4169 alloy under the SLM thermal cycle was studied.The influence of SLM vertical thermal cycle on the microstructure of GH4169 alloy is greater than that of horizontal thermal cycle.The peak temperature(TP)and times experienced by the alloy during vertical thermal cycle play a decisive role in the microstructural evolution of GH4169 alloy.When the peak temperature is higher than the liquidus temperature(TL),the cooling rate of the vertical thermal cycle with TP>TL influences the size of dendrites and the volume fraction of the Laves phase.When the peak temperature is between the precipitation temperature of the strengthening phase(TPre)and the liquidus temperature,the sequential precipitation and growth of ??,composite phase and ?? occur as the times of vertical thermal cycle with TPre<TP<TL increases.When high-frequency horizontal and vertical thermal cycles are applied,the microstructure of the SLMed GH4169 parts is composed of ?-matrix,interdendritic Laves phase and three types of strengthening phases(??,composite phase and ??)on the dendrite arm.(3)The microstructural evolution mechanism of SLMed GH4169 alloy was revealed.According to the magnitude of peak temperature and cooling rate,the thermal cycles of SLM can be divided into four types including TP>TL,T?+Laves<TP<TL,TPre<TP<T?+Laves,and TP<TPre,respectively.During these four types of thermal cycles,GH4169 alloy will undergo different phase transition behaviors including melting and solidification,solid solution,aging,and cooling.In the high-frequency iterations of these four types of thermal cycles,the redistributed solute atoms formed under the non-equilibrium sodilification thermodynamics of the thermal cycle with TP>TL are released during the thermal cycle with T?+Laves<TP<TL,and then migrate and accumulate under the short aging precipitation kinetics of the thermal cycle with TPre<TP<T?+Laves through a migration channel composed of high density lattice defects in the matrix to precipitate three types of strengthening phases.Finally,the polyphase structure of ?,Laves phase and three types of strengthening phases(??,composite phase and??)forms.(4)Based on the thermodynamic/kinetics and microstructure analysis,the effect of laser energy density on the microstructure and mechanical properties of GH4169 alloy was studied.With the increase of the laser volumetric energy density,the extended residence time of the thermal cycle with TPre<TP<T?+Laves is beneficial to the precipitation of strengthening phase.However,the reduced cooling rate of the thermal cycle with TP>TL makes the ?-dendrite and Laves phase coarse,and reduces the solid solubility of the matrix to suppress the precipitation of the strengthening phase.Therefore,with the change of laser volumetric energy density,the cooling rate of the thermal cycle with TP>TL and the residence time of the thermal cycle with TPre<TP<T?+Laves show an opposite change trend.The amount of the strengthening phase is limited and the mechanical properties of SLMed GH4169 alloy cannot be improved greatly.After adjustment,the maximum tensile strength and yield strength are 1122 MPa and 828 MPa,which can only meet the traditional casting standard.(5)The effect of subsequent heat treatment on microstructure and mechanical properties of SLMed GH4169 alloy was studied.For the SLMed GH4169 alloy samples with large difference in the size of Laves phase,two groups of phase change kinetics equations in the complete solid solution process are presented.According to the above equations,two sets of solid solution systems are selected to match the traditional double aging system to optimize the mechanical property of GH4169 alloy.The tensile strength,yield strength,and elongation can reach 1507.07 MPa,1398.17 MPa,15.23% and 1531.18 MPa,1356.02 MPa,17%,respectively.Those results greatly exceed the traditional forging standard of GH4169 alloy.The research of this paper is of great significance for the further application and development of SLMed GH4169 alloy.