Fraunhofer IPT’s Angela Niedermeyer, Thomas Vollmer, Prof. Dr Robert Schmitt and Access eV’s Dr Matthias Bünck examine the novel process route for the manufacturing of TiAl turbine blades in order to enable material-efficient serial production.
High performance inter-metallic alloys based on titanium and aluminium (TiAl) have been attracting interest among aircraft engine manufacturers for some time. In aircraft manufacturing, lightweight materials always have been of great importance. Today, this issue has gained even more importance, not least in regard to the environmental goals set in Flightpath 2050, namely a 75% reduction in CO2 emissions and a 90% reduction in NOx. In this regard, a decrease of the aircraft thrust-to-weight ratio is desirable, which can be achieved by the use of lightweight materials. TiAl exhibits a comparably low density and at the same time attractive properties, such as high mechanical strength and good corrosion resistance. It qualifies therefore as a replacement material for turbine or compressor blades that are traditionally made of nickel-based alloys, which are nearly twice as dense. Several manufacturers such as General Electrics and MTU have introduced TiAl turbine blades. However, a current drawback is the poor machinability of TiAl-alloys. Casting and machining as well as the required product quality of aircraft parts are particularly demanding. Manufacturing therefore is material- and time-consuming. Each process step incurs a considerable loss of material and therefore causes a high buy-to-fly ratio. High costs and low throughput are the consequence of the process inefficiencies. As the demand for titanium aluminide parts is continuously increasing, companies and research institutes make efforts to achieve serial or mass production, to improve material efficiency and to reduce cost.
In the BMWi (Federal Ministry for Economic Affairs and Energy) funded research project IDEAL, Fraunhofer IPT and research centre and casting expert Access eV have developed a novel route for the continuous-flow manufacturing of TiAl turbine blades. Throughout the project duration, both technological improvements as well as improvements regarding process optimisation and quality assurance have been achieved.
Finishing allowances and machining
The extraordinary material properties of TiAl-alloys, which make the material so interesting for aerospace applications, in turn cause difficulties in manufacturing. Poor machinability results from its brittleness, low fracture strain and low thermal conductivity. Manufacturers therefore often work with large finishing allowances of cast parts or even machine out of a full material block to be on the safe side. In contrast, Access eV has developed a process for manufacturing turbine blades using near-net-shape investment casting, achieving finishing allowances below 1mm. However, the highly complex geometry of turbine blades and as-cast geometry still require a certain, yet significantly reduced, amount of individual and iterative finish by machining. The machining can be realised by grinding on the one hand, which can be time-consuming due to small removal rates, or milling on the other hand which is accompanied by higher removal rates, but can result in a reduced surface quality. Access eV and Fraunhofer IPT therefore developed a time-efficient, automated and integrated finish machining strategy, resulting in reduced processing time. By the combination of optimised casting and machining processes, cost-efficient serial production has been developed by the project partners.
Enabling serial production
As the ability of the process to manufacture turbine blades cost-efficiently and in large quantities is essential, a focus was put on the development of an efficient continuous-flow manufacturing system. With this aim, the automation and coordination of individual process stages as well as the line balancing and investment and layout planning were subject of optimisation. The results of this work were validated with special regard to process stability and achieved part quality on the technology readiness level 6 production environment developed and realised by Access eV. Overall, a substantial reduction of the processing time was achieved while at the same time the output of high quality parts meeting the requirements of the aircraft industry was significantly increased.
As the components are safety critical, the aerospace manufacturing industry is highly regulated and safety requirements are very high, demanded by both OEM and international regulations stemming from the European Union or the Federal Aviation Administration (FAA), among others. Several approaches to meet these requirements exist, e.g. tracking of batches to reduce failures and increase quality or standardised documentation. To ensure the compliance with the latest safety regulations, the European standard EN 9100, which covers the basic principles for quality management systems in the aerospace industry, was implemented.
Until today, this implementation has been difficult and time-consuming for suppliers, as the list of criteria is extensive. Therefore, within the scope of the joint project, this standard has not only been implemented at Access eV, which is now certified according to EN 9100 for the production of TiAl cast turbine blades, but a systematic and general approach has been developed in parallel by Fraunhofer IPT to support the indispensable certification process not only at Access eV, but in general at other companies as well. Specific guidelines guide companies to identify suitable suggestions and core aspects for each relevant testing or production procedure and support the transfer of those suggestions into precise and tangible on-site instructions for each procedure to ensure a person-independent and reproducible result with regard to the product quality. The developed approach assists companies comprehensively in six steps: the analysis of the status quo, comparison to a desired status, development of recommendations for action, prioritisation of these, documentation and audit.
While the final assessment is still ongoing, the success of the project has already manifested itself in the increased output of turbine blades of impeccable quality. Not only has the throughput been increased, but also has the buy-to-fly ratio been reduced. The developed process enables the material- and time-efficient production of TiAl turbine blades and can surely be transferred to the manufacturing of other TiAl components, which are similarly challenging due to complex geometries and poor machinability of this high-performance material. Overall, a significant progress improvement has been achieved to meet the rising demand for low cost TiAl turbine blades in the interest of the OEMs, while eventually the total production cost will decide about the adoption of this technology. Therefore, the succeeding BMWi-funded research project NEXT specifically addresses the process’s economic efficiency. To this end, an Industry 4.0 environment, especially addressing process digitalisation and data analytics, will be tailored to the special requirements of the aircraft manufacturing industry.
This project was funded by the Federal Ministry for Economic Affairs and Energy (20T1509C).