Development of a New Process to Reduce Distortion in Gas Turbine Blade Forging
Creators: name: Bai, Qian
Publisher: name: Imperial College London
Year: 2012

description: The aim of this study is to develop a new process for high precision hot forging of Ti-6Al-4V gas turbine blade. In this new process, the work-piece is hot formed and then is clamped between the dies at high pressure for a certain time in order to decrease the distortion and increase the geometric accuracy. The feasibility study of the new process has been carried out in this thesis by using experiments and finite element (FE) modelling, providing a scientific understanding of the process. From the experimental and modelling work, it has been demonstrated that the new process proposed in this thesis is an effective way to reduce distortion in gas turbine blade forging. The study can be divided into three parts: interfacial heat transfer coefficient determination, material modelling, and Ti-6Al-4V hot forming. A closed form method to determine an interfacial heat transfer coefficient (IHTC) was developed, and a one-dimensional heat transfer model was proposed and validated. Heat transfer tests were performed to study the heat transfer between Ti-6Al-4V work-pieces with an initial temperature of 920°C and H13 dies with an initial temperature of 150°C. Temperature histories measured by thermocouples were obtained, and were used as an input for the closed form method. The effects of pressure, glaze thickness and surface roughness on IHTC between Ti-6Al-4V work-pieces and H13 steel dies were studied. Thermo-mechanical properties of Ti-6Al-4V were investigated for a temperature range of 820°C to 1120°C and a strain rate range of 0.1s-1 to 10.0s-1, using a Gleeble thermo-mechanical simulator. The flow softening mechanisms of Ti-6Al-4V during hot forming were studied. A set of unified elastic-viscoplastic constitutive equations for Ti-6Al-4V during hot forming were developed. Plastic strain for alpha and beta phase, isotropic hardening, normalised dislocation density, adiabatic heating, phase transformation, and globularisation of alpha phase were described in the set of constitutive equations. The developed material constitutive model was determined by fitting with experimental strain-stress curves from uniaxial compression tests, using an Evolutionary Programming (EP)-based optimisation method. Good agreements between the experimental and computed results were obtained: the error of predicted stress is under 10%. The general trend exhibited by softening mechanism of Ti-6Al-4V during hot forging is correctly fitted. Hot forming tests were conducted to study the effects on distortion during hot forming strips used as analogous to gas turbine blades. The effect of work-piece thickness and holding time on spring-back were investigated. The unified constitutive equations for Ti-6Al-4V and IHTCs were integrated with commercial FE software DEFORM/2D to simulate material flow and heat transfer in the hot forming process. The FE model was validated by experimental results. A good agreement was obtained between experimental and FE results for temperature history and thickness distribution. The stress state, temperature field and phase volume fraction were obtained from FE simulation. The FE model can be used to predict the extent of distortion in an appropriate FE software, and thus to optimise the new hot forging process for Ti-6Al-4V high precision gas turbine blade.