SPP 2086 - Model-based determination of the boundary zone properties during milling of Ti-6Al-4V

Initial situation

The continuing trend towards lightweight construction is increasingly leading to the use of high-strength construction materials. For example, high-temperature titanium alloys are used in gas and steam turbines to increase the maximum possible temperatures in the turbine - and thus its efficiency. In aerospace technology, titanium alloys Ti-6Al-4V are used for the production of structural components due to their high strength and low density. These components have to meet high quality requirements in terms of fatigue strength and corrosion resistance. In order to meet these increasing requirements, materials technology is researching new, more resistant titanium alloys. On the other hand, optimized machining of titanium also offers the possibility of further improving component quality. A decisive influencing factor, which determines the strength of a component after machining, is the state of residual stress in the edge zone. Residual stresses occur during machining due to mechanical and thermal loads and, to a lesser extent, phase transformations.

Project goal and procedure

The primary goal of the research project is the model-based control of the milling process of Ti-6Al-4V for the simultaneous adjustment of defined geometries and residual stress states. In order to achieve this goal, a real-time capable analytical model is to be developed in the first project phase, which describes the relationship between the process parameters and the residual stress state. The tool wear condition as observable disturbance variable as well as scattering material properties of the raw parts as hidden disturbance variable shall be considered. For the control, a prototype sensory tool holder for in-process measurement of temperatures and cutting forces is qualified and used. In order to be able to analytically map the complex relationship between the process control variables and the surface layer condition, a two-stage approach is chosen. The first step is to model the heat and force input in the process zone. In a second step, the resulting residual stresses and the hardness curve in the component as a result of machining are modelled. The resulting partial models are then coupled to form an overall model. The work of the first project phase provides real-time models for determining the thermomechanical component loads during Ti-6Al-4V milling. These form the basis for modelling the residual stress state and the hardness curve of the component edge layer after machining. The models are used in the second project phase for dynamic process control of the residual stress state. This includes the development and implementation of a suitable control concept and the testing of the control for robustness against perturbations.

Thanks

Our thanks go to the German Research Foundation (DFG) for funding the project within the framework of the priority program 2086 "Surface conditioning in machining processes".