Study On Stray Grain Formation In The Platform Of Ni-Based Single Crystal Superalloys Turbine Blade

Ni-base single crystal(SX)superalloys are the main materials used in hot-section of turbine engines for its remarkable mechanical properties in high temperature environment.Stray grain(SG)formation,however,are the weak links to reduce the yield of SX turbine blades resulting in a high cost during investment casting process.Prohibiting SG formation are thereby main concerns for metallurgist to pay attention to.The previous investigations on SG formation were based on a presumable fact that SG formation is a process of heterogeneous nucleation,which has not been demonstrated.Additionally,the thermal profiles that used in these previous reseaches were simplied to be planar or consistent concaved,which can not be used in practice since the real thermal profile displaying in platform region is more complicated.If the mechanism of SG formation in planar or consistent concaved isotherm scenarios is appliable to the more complicated isotherm case is still unknown.Further more,the influencing factors like LMC,grain orientation and alloy composition on SG formation are not comprehensively investigated.The effect of orientation was not eliminated when previous reseachers investigated a single variable on SG formation.The research on SG control is mainly for engineering application,the lack of mechanism analysis makes the systematic study of SG formation and control to be necessary.In this study,a combined technique denoted as seeding-grain selection(SGS)was designed for controlling crystal orientation and at the same time insuring the SX yield as well.A investigation by experiments and simulations is conducted aimed to fully understand the origination of SG,its dynamic formation process,its influencing factors and its control means.The main conclusions can be drawn as follows.1.By using seeds in the starter block and within the help of the blockage of the spiral passage,SGS technique realized the control of crystal orientation and preventing SGs formed in melt-back region growing into the SX body.The maximum withdrawal rate was 200?m/s when using SGS for SX fabrication.2.The origination of SG formed in the platform region was determined to be heterogeneous nucleation.SG formation experienced the nucleation and the growth processes.Nucleation is determined by the thermal cooling process.While,the growth process is influenced by the matrix dendritic branching behavior The criterion of SG formation can be described as a competition of branching time and cooling time.,if the branches of the matrix dendrites cannot reach the platform end C during the cooling time,SG forms;otherwise,no SG occurs.The local thermal profile in the platform region showed a wave-shaped feature,which causes Cricled-type SG and Tranverse-type SG formation with different morphologies.3.Increasing withdrawal rate results in SG formation much easier.In HRS case,the critical withdrawal rate for SG formation is 100 ?m/s.LMC technique can effectively inhibit the SG nucleation within the same solidification scenario comparing to HRS.Using LMC technique,the critical withdrawal rate for SG formation was promoted to about 175 ?m/s.The larger platform dimensions feature the larger SG formation tendency.4.The increase of primary orientation to 150 did not trigger SG formation.With the increasing secondary orientation to 300 and 45?,SG formed.In 300 case,the SG was originated from dendritic deformation;while in 450 case,the SG was originated from thermal-undercooled nucleation.With the crystal orientation being controlled as(?1,?2)=(0°,0°),the critical withdrawal rate for SG formation could be elevated to 200?m/s.5.The additions of Rhenium in 3.16wt.%and 6.04wt.%leveds did not affect the SG formation apparently in present study.With increasing the Carbon additions from 0.085wt.%to 0.13wt.%,the trend of SG formation decreased.With increasing the addtions of Boron from 0.000wt.%to 0.005wt.%,the occupied areas of SG decreased.Further increasing the Boron addtion to 0.014wt.%impeded SG formation.6.Withdrawal rate deceleration technique,graphite heat conductor technique and grain continuator technique can all effectively inhabit the SG formation.Quantitative analyses of the three SG control techniques indicated that grain continuator technique possessed the largest room for improvement;In terms of convenient,withdrawal rate deceleration technique is the most convenient one to operate since no extra steps needed during mold preparation;while,the graphite heat conductor technique is the most flexible one which can be used in any position to modify the local thermal profile,to control the microstructure evolution and SG formation.