Study On Process Optimization And Properties Of Modified Shells With Composite Mineralizers For Single Crystal Blades

With continuous improvement of the front temperature of aero-engine turbine,the high temperature performance of blades are increasingly demanded,nickel-based single-crystal turbine blades are widely used in advanced international aircraft engines,but the successful preparation of such precision blades is based on ceramic shells with excellent performance.However,ceramic shells for directional solidification often have problems such as softening deformation,bulging,cracking,and poor heat dissipation at high temperatures presently.Therefore,this article uses a composite mineralizer modification technology to improve high temperature mechanical properties of ceramic shell,then thickness of the shell is reduced to improve heat dissipation,so that shells can achieve high strength with thinner wall than bedore.Firstly,three different proportioning schemes of kaolin,MgO and Y₂O₃ are designed,the optimal ratio scheme is determined from influence of composite mineralizer under three different proportions on slurry viscosity,coating thickness,drying time and strength after roasting.Secondly,depend on the best ratio of composite mineralizer,the effect of composite mineralizers on mechanical properties of shells such as green strength,roasting strength,high temperature strength,thermal expansion,high temperature self-loaded deformation and thermal shock resistance was investigated.Finally,directional solidification experiments were carried out to study the effect of modified thinned shell on blade interface behavior and microstructure.It provides theoretical and experimental basis for the preparation of high strength thin-wall ceramics for single crystal blades,and the main research results are as follows:The optimization experiment results of composite mineralizer modified shell process show that,composite mineralizer can increase viscosity and coating thickness of slurry,and reduce drying time,among them,the ratio of composite mineralizer kaolin,MgO and Y₂O₃ to 98:1:1and 95:2:3 on slurry viscosity,coating thickness and drying time are more stable than 92:3:5,composite mineralizer can effectively improve the strength after roasting,among them,the ratio of composite mineralizer kaolin,MgO and Y₂O₃ to 95:2:3 on strength after roasting is more obvious than 98:1:1 and 92:3:5.Comprehensive analysis shows that the optimum ratio of kaolin,MgO and Y₂O₃ is 95:2:3,and viscosity range of surface layer,transition layer and back layer are optimized to 13¹4s,11¹2s and 9¹0s respectively,drying time range of back layer can be reduced to 9¹0h properly,and roasting temperature of the shell is 1200.Study on the mechanical properties of composite mineralizer modified shell show that,with the increase of composite mineralizer content,green strength of the shell first increases and then decreases,with the addition of 6%,the value reaches a maximum of 3.86 MPa,which is increased by 2.93%.roasting strength increased continuously.When the addition amount was 8%,the value reaches the maximum value of 5.67MPa,and increased by 19.1%,high temperature strength increased first and then decreased,when the addition amount was 6%,the value reaches the maximum value of 5.06MPa,which increased by 32.1%,when the roasting temperature below 1350?,linear expansion rate is increasing,when it is at 1350¹500?,linear expansion decreases first and then increases,the value of the composite modified shell with 6%addition at1500?is reduced to a minimum of 0.64%,high temperature self-loaded deformation first decreases and then increases,with the addition amount is 6%,it decreases to a minimum of 0.58%and decreases by 28.3%,the thermal shock resistance increases first and then decreases,when the content is 6%,strength loss rate decreases at a minimum of 49.8%.Experimental results of directional solidification of single crystal show that,the anti-deformation ability of modified shell surface and cohesive strength between surface layer and transition layer have been improved,so that surface peeling phenomenon is relieved,at the same time,the modified shell layer is smoother and more dense,which hinders diffusion and penetration of alloy liquid and shell material to a certain extent,reduces the degree of interface reaction,and improves the surface finish of blade.The thinning and uniform thickness of modified shell can effectively improves heat dissipation performance of shells,improves single crystal forming rate of the blade,and reduces generation of hybrid crystal at crown and margin,it also reduces the average value of primary dendritic and secondary dendrite to 271.8?m and83.5?m,respectively,which resulting in a more dense dendritic structure.It is proved that the modified shell after thinning can not only meet the actual production requirements,but also be benefit to improve forming rate and microstructure of the single crystal blade.