Research On Wall-thickness Control Method For Wax Pattern Of Hollow Turbine Blade

Hollow turbine blades are the most critical components in an aero-engine,which are manufactured by precise investment casting process supplemented by subsequent machining.For a hollow turbine blade,the wall-thickness accuracy is an important index that greatly affects the strength and cooling efficiency of the blade and the blade life.According to the investment casting process,the wall-thickness precision is mainly inherited from the wall thickness of the wax pattern.At present,the wall thickness of the domestic hollow turbine blade deviates seriously,which has become one of the main reasons for affecting the qualified rate of the hollow turbine blade.Therefore,it is of great theoretical and practical significance to study the wall thickness control method of the wax pattern for improving the forming quality of the hollow turbine blade and ensuring the development and batch production of high performance aero-engines.This dissertation mainly focuses on wall thickness deviation control technology of the wax pattern of the hollow turbine blade.Based on the knowledge of kinematics,contact mechanics,computation geometry and finite element simulation,some crucial problems involved in the fixturing process of the ceramic core in the wax pattern die,such as locating and clamping layout optimization,locating compensation and clamping process have been studied.Finally,by improving the positional relationship between the ceramic core and the wax p pattern,the wall-thickness deviation of the wax pattern of the hollow turbine blade caused by error sources can be reduced.The main contents of this study are as follows:(1)Locating layout optimization of the ceramic core: Firstly,an error transfer model,which formulates the mapping relationship between the localization errors and displacement of the workpiece,is established.Next,based on the static equilibrium theory,an optimization model for the locating layout of the ceramic core with the gravity constraint is proposed.After that,considering the discrete characteristic of locating candidate point on surfaces of ceramic core,a solving strategy for the optimization model is given by utilizing the genetic algorithm.From experimental results,it can be found that the locating layout optimized with the proposed method in this paper can improve the localization accuracy of the ceramic core,while guaranteeing the localization stability.(2)Numerical simulation of wax injection and force prediction of the ceramic core: Firstly,a numerical simulation method for the physical process of wax injection is developed based on the finite element technology.With this method,the pressure field of the contact surface between the wax pattern and the ceramic core during wax injection can be obtained.Then,by utilizing this pressure field,the load involved from the melt wax on the ceamic core can be calculated.In order to verify the accuracy of prediction results,an experimental platform is developed.The verification results show that the prediction load is basically consistent with the actual load,and can be used as the initial input for the clamping layout optimization of the ceramic core.(3)Clamping layout optimization of the ceramic core: Based on the knowledge of rigid body kinematics,a cirterion for determining the fixing integritgy of ceramic core is established.Next,a calculation method for contact forces between the ceamic core and fixture elements is proposed based on contact mechanics.Afterward,by the use of the fixing-integritgy criterion,the contact force calculation method and the the Particle Swarm Optimization algorithm,a clamping layout optimization strategy is developed.From the verification results it can found that in addition to restricting the movement of the ceramic core during wax injection,the layout optimized with the proposed method can limit the contact forces between the ceramic core and the fixture elements.Thus,the fragmentation phenomena of the ceamic core caused by the extrusion of the fixture elements can be prevented.(4)Locating compensation of the ceamic core: Firstly,based on industrial computed tomography(ICT)technique and curve matching algorithms,a model reconstruction method is developed,with which the 3D model of a trial wax pattern can be easily constructed.After that,focusing on eliminating the wall-thickness errors of the trail wax pattern,an optimization method for the pose of the ceramic core in the wax pattern is proposed.Then,by mapping the optimal pose of the ceramic core to length adjustments of the locating rods,the wall-thickness errors of the wax pattern can be greatly reduced.From the verification result it can be found that by adjusting the length of the locating rods,the maximum wall-thickness error of the wax pattern caused by the locating errors and wax shrinkage can be reduced by 89.8%,which further demonstrates the effectiveness of the proposed compensation strategy.(5)Clamping system development: To restrict the movement of the ceramic core in the die cavity,an automatic clamping system with force feedback function is developed.By using it and adjusting the clamping producers of the ceramic core,the assembling gaps between the ceramic core and the clamping rods in the traditional wax pattern die can be eliminated.From the verification results it can be found that the developed clamping system can limit wall-thickness errors of the wax pattern to ±0.05 mm,which has met the wall-thickness tolerance of the wax pattern of the hollow turbine blade.