Developing Process Design Methodology for Investment Cast Thin-Walled Structures

Sammanfattning: Components for engineering systems, such as gas turbines and jet engines operating at high temperature are usually produced in superalloys. The investment casting process is most widely used for manufacturing these components due to the ability of the process to produce parts with complex geometries to close dimensional tolerances. Other processing routes are less advantageous due to high mechanical strength and hardness of these alloys, which make formability and machining difficult even at high temperature. The global requirements for lower fuel consumption and emissions are increasing the demands to lower the weight of cast components in jet engines. The ability to produce components with lower wall thickness will not only help to reduce the cost of production and resource usage but also help to improve the efficiency of engineering systems resulting in lower fuel consumption and reduced emissions of environmentally hazardous gases. However, casting of thin walled components is challenging due to premature solidification in thin sections and long feeding distances often resulting in incomplete filling, cold shuts and shrinkage porosity.The castability of thin-sections is dependent upon selection of appropriate values of casting parameters to achieve favorable conditions for the mould filling and solidification. In foundry environment, fluctuation in these targeted values of casting parameters is common due to semi-automated nature of process. The effects of casting parameters on mould filling and defect formation have been widely reported in the literature, however effect of fluctuations in targeted values of casting parameters resulting from typical variation in the foundry is not well documented. Moreover, the origin of process variation and how to manage them in foundries, especially in relation to thin-walled casting has not been well documented. In this work, the common variations in critical process parameters, originating from foundry practices and equipment are identified. The effect of variations and resulting fluctuation in targeted values of casting parameters on castability of thin-walled castings is evaluated. The casting process is simulated by defining boundary conditions which replicate the foundry conditions and properties of foundry materials in a commercial casting simulation software. The effect of fluctuation of casting parameters on castability of thin-walled castings is established by casting trials as well as simulations and the validity of simulation is evaluated. A methodology to design a casting process is established by proposing methods to minimize the process variation as well as using Design of Experiments (DoE) based simulation work to achieve reliability and repeatability in the process.It is concluded that the mould temperature, casting temperature and pouring rate are common casting parameters affected by the variation originating from equipment and the casting practices. The variation in these parameters strongly effects the castability of thin-walled sections. The significance of these variations is validated by simulation and it is concluded that the validity of simulation is not only strongly dependent upon the foundry specific material data but also depends upon setting up valid boundary conditions according to the equipment and practices used. It is also concluded that by introducing material data and accurate boundary conditions, simulation can be used as tool to facilitate process development in foundries. A systematic implementation of simulations based on DoE and optimization resulted in significant reduction in process development time.The result of this work has been further developed into a process design methodology for investment casting foundries working with casting of thin-walled castings for high temperature applications. The term process design in this work is defined as design and evaluation of gating system as well as identifying optimized values of casting parameters to cast components in foundry.