Forged gamma titanium aluminide (γ-TiAl) is regarded as the material of tomorrow for rotor blades in jet engines. This low-density material weighs approximately half as much as nickel-based alloys, the current conventional material for rotor blades. With its new isothermal forging press and an innovative manufacturing process from Siempelkamp, Leistritz Turbinentechnik (Germany) is able to produce rotor blades for a new generation of jet engines.

To achieve significant fuel savings in air travel, aircraft of the future are being designed increasingly lighter. This is particularly true of the complex turbine technology inside the nacelle, which is a heavy load under aircraft wings. The highest degree of expertise is required to manufacture drive-technology components from lighter materials. A weight-saving design must, however, maintain the same operational reliability as before. 

Such is the situation at Leistritz Turbinentechnik, a business unit of the Leistritz Group of Nuremberg, Germany, which offers this type of expertise. At its factory in Remscheid, the company manufactures 50% lighter turbine blades made of titanium aluminide for the latest generation of aircraft engines. To do so, the company installed a Siempelkamp 8-MN isothermal forging press in March 2016. Two identical presses will start operation soon. 

Jet-Engine Rotor Blades

Leistritz’s decision to use these presses was not a matter of chance. The turbine manufacturer has been operating a 50-MN Siempelkamp isothermal forging press since 1984 to produce components that have to comply with the strictest standards of the ICAO (International Civil Aviation Organization). Because component parts made from new alloys require new and extremely precise production methods, the time had come to invest in new production technology.

The limiting factor in the further development of innovative, fuel-saving and quieter jet engines for civil air traffic is the rotor blades that drive the turbine. Due to the extremely hot and compressed air that surrounds these blades, the components are subject to high thermal stresses. To the thermal stresses must be added the stresses resulting from extreme centrifugal forces caused by the high-speed rotation of modern turbofan engines. After a while, this combined operating load on the rotor blades results in a material flow that is moving radially outward toward the turbine housing. 

The physical result of these stresses is that the rotor blades become longer as their operating life increases. In the worst-case scenario, they can touch the turbine housing and cause the total failure of the engine. Conventional rotor blades are currently still made of high-alloyed nickel-based alloys – a material with high thermal stability due to a high inner mixed-crystal strength and the related strengthening of the grain boundaries within the material structure. 

The required thrusting power of the engines has continuously increased from 4,500 N to more than 40,000 N since the aircraft industry started using them more than 75 years ago. Consequently, any further development in the quest for more economical and quieter aircraft engines requires concurrent weight reduction. The objective, in a nutshell, is to increase efficiency and fuel savings, reduce noise emissions and maximize operational safety.

Material of the Future: Titanium Aluminide

Forged gamma titanium aluminide (γ-TiAl) is regarded as the material of tomorrow for the rotor blades in jet engines. This material is already used as a casting alloy for housing components and guide vanes. For its future application in rotor blades, it makes sense to forge γ-TiAl. 

The advantage of this material is that, with a density of less than 3.8 g/cm³, titanium-aluminum alloys (with 50% titanium content) weigh approximately half as much as nickel-based alloys. Additionally, this class of material offers excellent ductility that counteracts the centrifugal powers that occur at high turbine speeds. Using rotor blades with a significantly reduced weight leads to drastically reduced centrifugal forces, a welcome effect. 

And yet, despite all the material properties that favor the material’s use in jet-engine applications, γ-TiAl also has some properties that make it difficult to form. Unless specific parameters are meticulously kept during material forging, the material is simply not forgeable. 

A Forging Challenge

A quick forging process, as used for forging other metals, does not work in this instance. The forging of γ-TiAl requires a slow process and involves plastic forming with dynamic recrystallization. Only with isothermal forging presses can the inter-metallurgical cast structure – hard and brittle like ceramics –be changed into a fine microstructure that will withstand future stresses. If the process is carried out any faster, cracks in the material inevitably develop.

Isothermal Forging Press

This is where isothermal press technology comes in. Since the beginning of 2014, Leistritz Turbinentechnik and our technical staff have developed the requirements profile to implement a new process technology for the forging of γ-TiAl through suitable press technology. In November 2014, the first two isothermal forging presses were ordered. With this press technology, press speeds can be controlled while an optimum workpiece temperature balance is maintained.

While γ-TiAl is not forgeable at low temperatures, it shows excellent formability in a temperature range between 1150 and 1300˚C. This has to be given special attention, however, because the forging process has to take place without the unfavorable slip-stick effect.

The problem was solved with a slip-stick-free hydraulic-axis control in a symmetrically built one-piece press frame that ensures the exact alignment of top and bottom dies with its cross-thread guidance system. The continuous problem-free forging process is guaranteed by special thermally isolated guiding and sealing systems. A Siempelkamp-developed process control uses visual process mapping, which controls the individual phases of the press process. The process parameters are reliably monitored and documented.

The entire press is enclosed in a housing since the press process has to take place within an inert-gas atmosphere. Automated ejectors ensure the safe removal of the rotor blades. Afterward, an integrated cleaning system removes possible deposits from the die surfaces. The production system also includes a rotary-hearth furnace as well as an automated loading and unloading manipulator. Siempelkamp developed all electrical and hydraulic drive systems as well as the system control for monitoring of all components. 

After installation and start-up – beginning in December 2015 and concluding in February 2016 – the plant passed its acceptance test following the first forgings.

Conclusion

With the new isothermal forging press and an innovative manufacturing process, Leistritz Turbinentechnik is able to produce rotor blades for a new generation of jet engines. The new press and process uses approximately 17% less fuel during operation, lowers operational costs by 20% and significantly reduces noise. To meet the increasing demand of such components in the future, a third press will be commissioned next year. 

Carsten Daub, Dipl.-Wirt.-Ing., M.Sc., is an engineer with Siempelkamp. For additional information please contact Amir Tanbakouchi, Siempelkamp Maschinen-und Anlagenbau GmbH at amir.tanbakouchi@siempelkamp.com.