| When Ni-based superalloys were used to cast turbocharger turbine wheels,hot cracks occurred in the blade of turbine.In order to solve the hot tearing problem,reduce the hot tearing susceptibility of turbine and increase the finished products rate,alloys including K418,K419 and K424 with different hot-tearing susceptibility are studied by means of experiment and finite element simulation.Hot tearing phenomena and factors that influence hot tearing susceptibility are studied.Thus,the influence of microstructure,solidification characteristics and pouring process on hot-tearing susceptibility and its control are obtained.The hot tearing phenomenon of multiple batches of turbocharged turbines was studied by microstructure observation and finite element simulation.The result clarified that eutectic participated in interdendritic and the thermal stress during solidification are two main causes of hot tearing.The microstructures of three alloys with different hot-tearing susceptibility were studied and a positive correlation between eutectic characteristics and hot-tearing susceptibility was obtained.The large size and amount of eutectic in the interdendrite resulted in the high hot tearing susceptibility.Thus,eutectic influence factor(Ef)which is the product of eutectic size and volume fraction of eutectic is defined.The eutectic influence factor which considering the eutectic size,the volume fraction of eutectic and the size of eutectic influence zone can evaluate hot-tearing susceptibility well.The effect of Al and Ti and cooling rate on eutectic characteristics was obtained.For tested alloys,with the increase of Ti contents,the size and volume fraction of eutectic increase significantly.The influence degree of Al on eutectic is lower than Ti.There is a significant inflection point in the effect of cooling rate on eutectic size.As the cooling rate increases,the number and size of eutectic increase as well.However,when the cooling rate is too high,fine dendrites formed,which lead to the formation of small size eutectic.It was concluded that the content of Al,Ti(especially Ti)and cooling rate should be controlled to reduce the amount and size of eutectic,which is in favor of reducing the hot tearing susceptibility.The solidification process and element segregation behavior of the alloy were studied.The results show that with the stronger the segregation of the y' phase forming elements in the liquid pools,the liquid pools composition reach the eutectic composition point when the residual liquid pools sizes are large.The residual liquid pools with segregation zone are the nucleation sites for eutectic.The eutectic characteristics reflect the segregation behavior and the morphology of the residual liquid phase.The large size and amount of eutectic in the interdendrite resulted in the smaller dendrite bridging area,causing the weak cohesion of interdendrite,thus the high hot tearing susceptibility.The essential reasons for the effect of eutectic properties on hot-tearing are revealed.The casting simulation software ProCAST is used to simulate the thermal stress during solidification.Further,the effect of pouring process on thermal stress was obtained.K424 alloy which shows high hot tearing susceptibility should avoid low pouring temperature and mold temperature.Besides,control strategies for reducing hot-tearing are proposed by the comprehensive consideration of solidification time and thermal stress.Besides,a model of grain structure calculation of directionally solidified/equiaxed double-properties integral turbine wheel is constructed.And the grain structure of directionally solidified/equiaxed double-properties integral turbine wheel is simulated by CAFE module belongs to ProCAST.In summary,this paper proposes the essential reasons of the effect of eutectic characteristics on hot tearing through experimental and finite element simulation.Theoretical analysis methods and experimental basis for hot-tearing prediction and quality control of Nickel-base superalloys turbocharger turbine wheels are proposed. |
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