Casting simulation model as a tool to obtain optimal process designs for investment casting of Rene 77 alloy

摘要

In this study, a casting simulation model for prediction the occurrence of shrinkage porosity on nozzle segments investment casting of Rene 77 alloy are investigated. The casting under investigation is a 6-vane turbine multi-nozzle segment with item dimensions of 230mmL * 180mmH * 51 mmW. The optimal gating design and process design for making this part are virtually according to the result of simulation model. The investigated process for prediction the map of macroporosity and microporosity defects, at first design a top pouring gating system with two symmetric castings by a 3D CAD drafter. Then by a pre-processor to transfer the model and mesh the 3D model, material assignment, and assign the interface, radiation, initial and boundary conditions. Then pass through a simulator to analyze the mold filling and solidification-related casting defects and display the simulation result by a postprocessor. The results of defects from simulation then compared with actual trial run and confirmed they are consistent. Then, design 4 types of gating system with assigning the same mold thickness, pouring temperature and mold preheat temperature, and assigning different insulation wrap methods to run, analyse, and display the result of simulation and finally find out the optimal gating system and casting conditions. The gating systems are include 3 types of mixing pouring method putting ingate on both outer shroud and inner shroud area, and one type of top pouring putting ingate on outer shroud in conjunction with steel chiller on the inner shroud. The study from simulation shows that the optimal gating design is to use the high emissivity material on the inner shroud area in conjunction with the top pouring method, inward TE direction, upward outer shroud direction, 9 mm of shell thickness, pouring temperature of 1425 ℃, and two layers of insulation which wrapped the mold from the bottom of airfoils to the pouring cup and from the mid of airfoils to the pouring cup by 6.35 mm thick insulation, individually. It also shows, the macroporosity defects have been completely eliminated by the optimal gating design. For this optimal design, the worst micro-porosity formation is around 1.5% for the casting. The simulated results and the experimental observations are then compared, and good agreement is found.