Study On Microstructures And Low Angle Boundaries In The Rejoined Platforms Of Ni-based Single Crystal Superalloys

Due to its excellent mechanical properties at high temperature,Nickel-based single crystal superalloys are indispensable structural materials in the preparation of advanced aero-engines.However,during the preparation of single crystal castings,the formation tendency of solidification defects such as low angle boundaries(LABs)increases due to more complex geometry of single crystal blades and continuous increasing in refractory elements in the alloys,which leads to reduce yield of single crystal castings.Therefore,the research on casting defects in directional solidification has attracted extensive attention.Among them,the research on the stray grains from nucleation at the blade platform is extensive,while the systematic study of LABs has not been carried out yet.The previous investigations on LABs formation mechanism pointed out that,if the dendrites deviates during their branching,LABs will be formed at the convergence interface.It is presumed that the orientation deviation is caused by dendrite deformation,whereas there is no consensus on the origin of dendrite deformation.In addition,it is easy to form LABs in the castings with rejoined structure,such as double guide vanes,nozzle guide vanes and grain continuator technology,because of their unique geometrical structures.However,it is not clear about the evolution of dendrite growth and defect formation in such rejoined platforms under different solidification conditions.It is indeterminate about the dominant factors of the LABs formation in the platforms.Based on the above problems,a mold with rejoined platforms was designed in this study to prepare single crystal castings under different solidification conditions.The dendrite structure and orientation evolution in the transversal and longitudinal section of the platforms are characterized by metallographic and EBSD techniques.The formation mechanism of LABs in the platform and its dominant factors are studied by Pro CAST simulation results of temperature field and stress field.On this basis,a thorough and detailed investigation is conducted aimed to understand the effect of different factors on the formation of LABs in the platforms.The main conclusions are as follows:(1)The temperature and stress fields in the rejoined platforms are simulated by the finite element simulation software Pro CAST.The results of temperature field show that the temperature distributions are different between the two sides and the middle region of the platform,and there are obvious differences in temperature gradient,cooling rate and solidification rate.The results of stress field show that the stress in the platform is constantly changing during solidification,and there are two stress regions that may affect the dendrite growth:one is the section expansion region where is prone to stress concentration in the platform,the other is the lower side of the middle region of the platform.(2)The dendrite growth characteristics in the rejoined platform at different withdrawal rates were studied.It is found that the growth pattern of dendrites in the platforms mainly includes two regions:the original primary dendrite zone and lateral growth zone of the secondary dendrites.With increasing of withdrawal rates,the lateral growth will become severe;the developed secondary dendrites may lead to LABs because of their orientation deviations in the lateral growth region;local dendrite fragmentation may occur due to the solute enrichment once the withdrawal rate exceeds100?m/s.(3)The orientation characteristics of LABs and the effect of solidification parameters were studied.It is found that four different types of LABs are formed in the rejoined platforms at different withdrawal rates.At lower withdrawal rate(V=60?m/s),the LABs in the longitudinal section of the platform is mainly caused by the orientation fluctuation of the original dendrites on both sides,and the grain boundary angle is small.At higher withdrawal rate(V=150?m/s),the developed secondary dendrites deformed on the lower side in longitudinal section of the platform due to solidification contraction stress,which eventually leads to the LABs at the convergence interface,and the grain boundary angles are related with the deformation degree of the dendrites.The orientation deviation in the deformed zone is accumulate.Accompanying with the fragmentation phenomenon,a very few detached dendrites can continue growing and lead to a LABs although the grain boundary angle is small.In addition,when the lateral growth are serious in the transversal section of the platform,the secondary dendrites will deviate and lead to crisscrossing LABs affected by temperature and stress fields,and this orientation deviation is without accumulation.(4)The orientations of primary and secondary dendrites are controlled by“seeding plus grain selection”technique,and the characteristics of LABs are studied.It is found that optimized primary dendrite orientation can not avoid orientation fluctuation and the LABs at lower withdrawal rate(V=60?m/s).At higher withdrawal rate(V=150?m/s),the LABs caused by secondary dendrite deformation in the longitudinal section of the platform are mainly affected by primary dendrite orientation?₁.When?₁=0°,the secondary dendrite on the lower side of the platform is easy to be effected by contraction stress and lead to LABs in the platform.With?₁increasing,the stress effect on secondary dendrites decreases,and the angle of LABs decreases.The LABs in the transversal section of the platform are mainly affected by secondary dendrite orientation?₂.When?₂=0°,only sub-grain boundaries are formed in the platforms.With?₂increasing,the growth trajectory of the dendrites is complex and the order of the dendrite is poor,and the formation tendency of transverse LABs increases.(5)The sliver defects in the rejoined platforms were found and the formation mechanism was proposed.The slivers originate from the dendrite deformation.The section expansion region of the platform is a stress concentration region.The secondary dendrites are deformed because of contraction stress in this region.After that,the deformed dendrites compete with the surrounding dendrites,and those dendrites with competitive advantages can not be eliminated.The surviving deformed dendrites can continue to grow when they are in a favorable local temperature field and have enough three-dimensional growth space,and eventually grow into the platform to form sliver defects.