Cold Flow Forming of Ti-6Al-4V – FLUOTI

The principal objective of project FLUOTI consists in simulating the spin forming process of TA6V at room temperature to produce long tubes by successive work hardening and local large deformation stages. The advantage lies in the high geometrical precision for obtaining thick tubes used in aeronautics for instance.
The material behaviour of the a+ß microstructure of TA6V under large "cold" deformation is still a challenge. However, and this is how the project is original in its purpose, the spin forming process could allow, for successive incremental deformations, to push the limits of the standard formability of the material. Indeed, preliminary tests at ROXEL with repeated deformations up to 30% have already been obtained but the industrial interest is above 70%.

Despite this, there is no certainty that an industrial solution exists. Today there are no cold scale spin formed tubes in TA6V produced at an industrial scale. However, room temperature is certainly less favourable to deformation but allows for better reproducibility and less distortion and oxidation during step by step operations. Accordingly the quality of the tubes will be better assured during an easier cycle and the processing cost to form the tube will be less expensive. Furthermore the quality of the tube at the end of the processes must be known throughout its whole length and thickness because many of theses pieces are parts of class 1 for aeronautics. Therefore the company ROXEL wishes to continue these preliminary investigations and implement the means necessary to obtain a 1st prototype tube realised by cold spin forming of TA6V.
If one combines the high cost of material, multiple settings of spin forming conditions and the many possibilities of heat treatments, we soon realize that the number of trial and error would be prohibitive for ROXEL. Therefore the numerical simulations of the process are then proposed to try to minimize the number of tests on industrial site by identifying and optimizing the operating conditions and status of the tube after the forming operation. The simulation will also incorporate changes in the material behaviour from its raw supply state to the finished piece.

A blocking point of numerical simulations of flow forming is computing time. Indeed, like all processes of incremental forming, spin forming is an unsteady process which leads to time-intensive computing. The flow zones are located in contact with the wheels and are always in motion. We must therefore optimize the size of the mesh in adequacy with both the geometry reproduction and the level of distortion experienced by the material. The detailed managements of free surfaces and the contact are also important numerical issues. More accurate representation of the kinematics of tools (mandrels and rollers) is complex. Its influence on the quality of simulation results is very important.

Many parameters come into play when evaluating the processing of the tube during the cold spin forming operation. There is of course the influence of operating conditions (kinematics and geometries of the tools) and geometries (initial, intermediate and final) of the tube on the rheology and behaviour of the material. There is also the influence of thermal treatments between two pass of spin forming that will help to push the material to its very far limit of deformation. Improving the cold formability of TA6V is going through a series of cross linked steps of analyses and characterizations of the influence of strain paths with respect to evolutions of mechanical properties and microstructure.