CD-Laboratory for Materials Modelling and Simulation, Module 2: Microstructure modelling of the alpha-beta-titanium alloy Ti-6Al-4V during thermo-mechanical heat treatment

Böhler Schmiedetechnik GmbH & Co KG delivers forgings made of alpha-beta-titanium alloys to all major aircraft companies like BOEING, AIRBUS, EMBRAER, etc. and some helicopter and engine producers. Due to the extremely high price of titanium prematerial all those companies try to reduce costs for their final parts. Two possibilities to reach this goal are the use of special processes like beta-forging and a consistent design of the thermomechanical process to a near-net-shape forging. The use of these special processes implies a complete simulation of the whole thermomechanical process in order to guaranty that the customer specified mechanical properties can be achieved. The goal of this project is therefore to model the microstructural changes during pre- and reheating, forging and especially heat treatments in a way that it is possible to predict the microstructure in the final part. Coupling these models with special purpose finite element programs offers the possibility to directly draw conclusions on mechanical properties like ultimate tensile stress and to deliver important input parameters for further part life time simulations. Similar to other previous projects it is necessary to perform a broad experimental program. Thermal processes like heating of the prematerial, reheating between two forging steps and particularly several different heat treatments as well as forging processes have to be physically simulated in small scale in the lab varying all influence parameters like temperature, time, effective strain and strainrate. Furthermore it is necessary to analyse the change in microstructure using light, scanning and transmission electron microscopy as well as special 3D-imaging methods along the thermomechanical process. These generate a huge microstucture matrix, which has to be linked with the process parameters using adequate semi-empirical and physical models. These mathematical equations can than be programmed and coupled with the finite element software in order to simulate the microstructure development in dependence of the whole thermomechanical process. In order to check the reliability of such models it is necessary to produce special forgings with a broad variation of the process parameters. Comparing the micrustructure of these forgings with the output of the appropriate simulations offers the possibility to directly verify the quality of mathematical models.