Study On Directional Solidification Law And Microstructure Control Of Nickel-Based Single Crystal Superalloy

Turbine engine blades have a harsh working environment and stringent requirements for microstructure and blade quality.The single crystal hollow blade currently used has a complicated structure,and the difficulty of the single crystal preparation process is increased,which leads to defects such as stray grain,orientation deviation,and coarse structure in the directional solidification process.The increase of the content of refractory elements in the superalloy leads to high composition segregation of the as-cast microstructure,and there are defects such as high eutectic phase content and uneven structure,which seriously reduce the high temperature mechanical properties of the blade.It is necessary to use appropriate heat treatment to improve the as-cast microstructure of the blade.With the development of computer hardware and software technology,numerical simulation has become an important method to study the directional solidification process of single crystal blades.In this paper,the combination of simulation and experiment method was used to study the crystal selection process,directional solidification law and formation of stray grains,and the effect of heat treatment method on alloy microstructure.And dedicated to obtaining single crystal blades with stable and uniform structure and good performance.The main contents are as follows:1.Simulation and experimental studies were carried out on the grain selector required for the preparation of single crystals.The grain selector comprises two parts: a starter block and a spiral block.The main function of the starter block is to reduce the grain density and optimize the grain orientation by the barrier of the outer wall and the competition between the grains.As the height of the starter block increases,the number of crystal grains decreases,and the equiaxed crystal changes toward the columnar oriented structure.The declination of the [001] direction and the axial direction gradually decreases,and the primary dendrite spacing increases.After the height of the starter block is increased to a certain value,the optimization effect on the grain orientation is weakened.The spiral block selects a grain to grow into a single crystal structure by providing a narrow spiral channel and promoting competitive growth of the grains.The crystal selection mechanism of the spiral block includes the longitudinal selection of primary dendrites and the lateral limitation of secondary dendrites.The grains in the favorable position in the channel have more growth space,occupy the growth advantage and are easy to be selected.The simulation results show that the orientation of the grain that is won by competitive growth in the spiral block is random.2.The distribution of temperature field and solidification sequence of single crystal blade during directional solidification were studied.The formation mode and influencing factors of stray grains in variable cross section were analyzed.The solidification interface moves at different speeds in different parts of the casting and is related to the withdraw rate.The bending state of the mushy zone is different under different withdraw rate.Increasing the withdraw rate leads to a decrease in the temperature gradient of the liquid phase front and an increase in the width of the mushy zone.The heat dissipation condition of the blade back is better than that of the leaf basin,and the isotherm is advanced from the leaf back to the leaf basin.The presence of the core reduces the wall thickness of the blade,prevents the formation of a continuous lateral temperature gradient,and reduces the growth space of the lateral dendrites.The single crystal structure of the blade body is better.The dendrites of the blade edge plate extend from the blade body to the periphery,and the edge of the edge plate has a large degree of undercooling,and the dendrite grows to undercooled region then grows rapidly.When the edge undercooling exceeds the critical nucleation undercooling heterogeneity,the heterogeneous tendency of the lowest point of the inclined cross section is greatest.The withdraw rate increases the degree of undercooling of the platform by changing the degree of bending of the isotherm,thereby affecting the tendency of formation of the stray grain.A smaller withdraw rate can effectively prevent the formation of stray grains.3.The microstructure and composition segregation of as-cast and heat-treated single crystal blades were studied.In the as-cast leaf samples,the ?' phase of dendritic core is uniform and small in size,the ?' phase in inter-dendrite is coarser,and there are a large number of low-melting ?/?' eutectic phases.The W and Re elements are segregated in the dendritic core.Al and Ta elements are enriched in inter-dendrite.With the increase of solid solution temperature and solution time,the content of eutectic phase decreases gradually,and the eutectic phase melts completely when the solution temperature is 1340 °C.The solution temperature is further increased,the initial melting phenomenon occurs between the inter-dendrite and the dendritic core,and the initial melting is more likely to occur in the inter-dendrite The ?' phase size decreases with increasing solid solution temperature and tends to be uniform at 1340 °C.Solution treatment can improve the segregation of elements between dendritic core and inter-dendrite.Cr,Co and Ni can be uniformly diffused at a lower solid solution temperature.The temperature at which the Ta and Al elements are homogenized is slightly higher.The segregation of W and Re elements is the most difficult to improve.In order to avoid the initial melting in the microstructure,the segregation of the W and Re elements is not eliminated.