Simulation of stray grain formation in Ni-base single crystal turbine blades fabricated by HRS and LMC techniques

The simulation models of the thermal and macrostructural evolutions during directional solidification of Ni-base single crystal(SX) turbine blades under high rate solidification(HRS) and liquid metal cooling(LMC) have been constructed using Pro CAST software, coupled with a 3D Cellular Automaton Finite Element(CAFE) model. The models were used to investigate the tendencies of stray grain(SG) formation in the platform region of turbine blades fabricated by HRS and LMC techniques. The results reveal that the LMC technique can prohibit SG formation by smoothing the concaved isotherm and in turn alleviating the undercooling in the platform ends to let the dendrites fill up the undercooled zone before SG nucleation. The simulation results agreed well with the experimental results, indicating that these models could be used to analyze the macrostructural evolution or to optimize process parameters to suppress SG formation. Using these models, the critical withdrawal rate for casting SX turbine blades without SG formation were determined to be around 75 μm·s^(-1) and 100 μm·s^(-1) for HRS and LMC respectively, suggesting that LMC can be used as an efficient technique in fabricating SX turbine blades without any SG defect formation.

Simulation of stray grain formation at the platform during Ni-base single crystal superalloy DD403 casting

The mechanism of stray grain formation at the platform of turbine blade simulator and the effect of withdrawal rate （V） on the stray grain phenomenon have been investigated using a macro-scale ProCAST coupled with a 3D Cel ular Automaton Finite Element （CAFE） model. The results indicate that the stray grains nucleate at the edges of platform at V=150μm·s-1 and 200μm·s-1. Using ProCAST computer simulation software, it was proven that the stray grain formation is signiifcantly dependent on the undercooling and the temperature ifeld distribution in the platform. The macroscopic curvature of the liquidus isotherm becomes markedly concave with an increase in the withdrawal rate. The probability of stray grain formation at the edges of platform can be increased by increasing the withdrawal rate in the range of 70μm·s-1 to 200μm·s-1.

Simulation of stray grain formation during unidirectional solidification of IN738LC superalloy

The influence of casting parameters on stray grain formation of a unidirectionally solidified superalloy IN738LC casting with three platforms was investigated by using a 3D cellular automaton-finite element （CAFE） model in CALCOSOFT package. The model was first validated by comparison of the reported grain structure of AI-7%Si （mass fraction） alloy. Then, the influence of pouring temperature, heat flux of the lateral surface, convection heat coefficient of the cooled chill and mean undercooling of the bulk nucleation on the stray grain formation was studied during the unidirectional solidification. The predictions show that the stray grain formation is obviously sensitive to the pouring temperature, heat flux and mean undercooling of the bulk nucleation. However, increasing the heat convection coefficient has little influence on the stray grain formation.

Effect of mould baffle technology on stray grain formation in single crystal blades by integral fabrication based on 3D printing

Stray grains,the most serious casting defect,mainly occur in the platform because of the abrupt transition of the cross-section in the directional solidification of superalloy single-crystal blades.A new mould baffle technology based on 3D printing and gelcasting is proposed herein to reduce the formation of stray grains in the platform.The influence of the proposed mould baffle technology on the temperature field in the platform during solidification was investigated by simulation and experiment.The numerical simulation results indicate that the proposed mould baffle technology can effectively hinder the radiation and heat dissipation at the platform extremities,and therefore,reduce undercooling in the platform and the formation of stray grains during directional solidification.Casting trials of a hollow turbine blade were conducted using CMSX-4 superalloy.The trial results demonstrate the potential of the proposed approach for manufacturing single-crystal superalloy blades.

Influence of platform position on stray grain nucleation in Ni-based single-crystal dummy blade clusters

Stray grains are the most severe of the solidification defects that occur in the industrial single-crystal blade preparation process.In this study,a single-crystal dummy blade cluster with different crystal orientations controlled by the seeding method was prepared,and the influence of the position of the circular platform(relative to the sample and furnace body)on stray grain nucleation was investigated.Results show that the microstructure of the circular platforms could be divided into the center,expansion,and stray grain regions.The inside of the circular platform facing the center of the cluster is more prone to stray grain formation than the outside of the circular platform facing the furnace body.With an increase in the distance between the circular platform and the bottom of the dummy blade cluster,the stray grain region expands,whereas the expansion region narrows.The stray grain is slightly aggravated with increase of the misorientation.Finally,the mechanism underlying the influence of platform position on the formation of stray grains in single-crystal dummy blade clusters is discussed based on the temperature evolution during directional solidification.

Prediction of Dendrite Orientation and Stray Grain Distribution in Laser Surface-melted Single Crystal Superalloy

A vectorization analysis technique for crystal growth and microstructure development in single-crystal weld was developed in our previous work. Based on the vectorization method, crystal growth and stray grain distribution in laser surface remelting of single crystal superalloy CMSX-4 were investigated in com- bination of simulations with experimental observations. The energy distribution of laser was taken into consideration in this research. The experimental results demonstrate that the simulation model applies well in the prediction of dendrite growth direction. Moreover, the prediction of stray grain distribution works well except for the region of dendrites growing along the [100] direction.

Temperature evolution and grain defect formation during single crystal solidification of a blade cluster

In order to investigate the asymmetry of thermal conditions during directional solidification, the temperature evolution and correspondingly developed undercooling in a simplified single crystal blade cluster were numerically simulated. Simulation results demonstrate that the temperature distribution at the blade platforms is obviously asymmetrical. On the outside of the blade which directly faces the heating element, the liquidus(TL) isotherms progress relatively smoothly. On the inside of the blades facing the central rod, however, the TLisotherms are in concave shape and the slope goes upwards to the platform extremities. The average undercooling extent ?T and undercooling time ?t at the inside are much higher than those at the outside. It was then predicted that the inside platform extremities have significantly higher probabilities of stray grain formation compared to the outside ones. A corresponding experiment was carried out and the metallographic examination exhibited the same side-and height-dependence of stray grain formation in the blades as predicted. On the inside of the blades, all platforms are occupied by stray grains, while the platforms on the outside are nearly stray grain free. The simulation result agrees very well with the experimental observation.

Experimental Study and Numerical Simulation of Directionally Solidified Turbine Blade Casting

The directional solidification process of turbine blade sample castings was investigated in the work. Variable withdrawal rates were used in one withdrawal process and compared with the other using uniform rate. A mathematical model for heat radiation transfer and microstructure simulation of directional solidification process was developed based on CA-FD method. The temperature distribution and microstructure w.ere simulated and compared with the experimental results. The stray grains were predicted and compared with the experimental results. The uneven temperature distribution of platform was the main reason of the formation of stray grains.

Effects of Substrate Crystallographic Orientations on Microstructure in Laser Surface-Melted Single-Crystal Superalloy:Theoretical Analysis

A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed.And the effect of welding conditions on the tendency of stray grains formation during solidification was researched.In the present work,these analytical methods were further extended.Combined with an original vectorization method,a 3D Rosenthal solution was used to determine thermal conditions of the welds.Afterward,the dendrite growth orientation,the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted.Finally,the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot.The results indicate that substrate orientation has some impacts on the crystal growth pattern,dendrite growth velocity,distribution of thermal gradient and stray grain.Based on the research methods proposed in this work,any substrate crystallographic orientation can be studied,and predicted stray grains distribution can be visualized.