Machining titanium alloys
There are probably no other materials more appropriate for high performance aeroengine applications than titanium and its alloys. With a density of 4.5g/cm3 titanium alloys are only half as heavy as steel or nickel-based superalloys and furthermore, they are characterised by superior strength-to-weight ratio. With the completion of the European project DUTIFRISK, engineers' efforts to advance the state-of-the-art in the design of gas turbine components have achieved a milestone. In a conventional turbine compressor, blades are manufactured separately from the disc, and then mechanically assembled together. Led by MTU Aero Engines, the DUTIFRISK project partners explored an alternative scenario in which the blades and the disc form a single component without discontinuities. During the linear friction welding process, the blade is fretted against the disc, and a load is applied normal to the junction until a joint is developed. The ultimate aim was to identify optimised machine parameters for the production of linear friction welded bladed discs (blisks) with discs made from β-forged Ti6246. The attached blades were made from α/β-forged Ti64, Ti6246 and Ti6242 with different microstructure to the discs. Besides other attractive mechanical properties, forgings of high strength metastable β-Ti alloys ensured a low level of residual stresses after machining. More specifically, the influence of microstructure and stress effects on the fatigue performance of titanium alloys was evaluated with the use of an advanced process simulation system. Based on the finite element method (FEM), DEFORM-2D allowed a detailed analysis as well as the realistic modelling of the complex metal forming process on the computer. Valuable information on key physical parameters was then compared and found to be in accordance with residual stress measurements collected during extrusion trials. The accurate characterisation of all welded materials' combinations for the disc and blades may offer the possibility of choice for intelligent design in the future.
