Background

Aircraft turbine engines are high performance machines in which components operate under conditions at the edge of their materials' capabilities. Continuing advances in modern engine design and technology are placing ever increasing demands on these materials, forcing the development of superalloys to meet new applications. As well as improving the technical performance of the specialty materials, designers are also under pressure to reduce their cost.

Performance improvements within turbine engines provide the traditional avenue for major contributions to life cycle cost savings. Typically, advances in performance hinge on a new engine's ability to operate at higher temperatures, since at higher temperatures combustion is more efficient, which means lower specific fuel consumption. However, engines that run hotter place greater stress combinations on critical engine parts, particularly hot section turbine discs, figure 1.

Figure 1. Schematic of a jet engine, showing hot zones.
Powder Metallurgy

Many components produced using established materials and process strategies are becoming obsolete as they cannot measure up to the increasingly demanding operating environments of today's larger, hotter burning engines. Fortunately, replacement materials and processing methods are being identified that will allow the manufacture of complex components that meet the requirements of the newest, most advanced and efficient engines. Powder metallurgy (PM) materials, for example, were once thought to be expensive, exotic materials to be specified as a last resort, but their use is spreading because they improve life cycle costs. New, highly alloyed materials are being developed and incorporated into more and more applications. Ladish Co Inc is working with OEMs around the world to develop and optimise new PM alloys for turbine engine applications. The two most important technical benefits offered by these advanced materials are improved material properties and grain size control capabilities.

Conventional cast and wrought (ingot metallurgy) materials are typically limited to fine grain size applications. In contrast, PM materials can be tailored to various grain sizes to further enhance component performance. Intermediate grain size development optimises damage tolerance and crack growth resistance, while coarse grain size development optimises creep capabilities. Material and process engineers can use this ability of advanced performance PM nickel-base superalloys to achieve improved life cycle affordability.
Nickel-Based Superalloys

Next generation nickel base superalloys are being designed with alloy chemistries that are effective in balancing tensile strength with creep and crack growth rate resistance. Solid solution strengthening, gamma prime forming and grain boundary strengthening elements are other areas now undergoing careful engineering to enable further improvements in nickel base superalloy performance.

Ladish is exploring ways to improve the material efficiency of powder metallurgy nickel-base superalloys. PM mill products are well established and are manufactured using highly stable methods. Also, the yields within PM billet processing are well known and are much less sensitive to changes in alloy, particularly when compared to cast and wrought processing. The restriction on using highly alloyed nickel base superalloys is the extreme difficulty of converting them from ingot to billet. For this reason yield is typically lower.

Increasing the near net shape capability of turbine engine components is an important area of interest, since reducing input material directly affects component affordability. Isothermal forging is a viable technique for near net processing of PM superalloys due to the very fine and uniform grain size that allows superplastic deformation during processing.

Work has shown that nickel-base materials can benefit significantly from the use of multi zone ultrasonic inspection for cast and wrought nickel-base billet material. Improved inspection methods early in the manufacturing process have allowed diversion of material that would not have been able to meet final component requirements. This ultrasonic inspection method, originally aimed at titanium materials, has been able to interrogate cast and wrought billet material to levels previously not possible, for both traditional and more subtle microstructural defects. Tremendous time and cost savings have been made by increasing the efficiency of value added manufacturing and inspection operations.
Computer Simulation

The benefits of computer simulation of manufacturing operations has led the aerospace forging industry to fully embrace this technology, and computer process modelling is becoming common place for many manufacturing processes. Ladish is developing modelling tools that accurately predict microstructural outcomes. These advanced computer simulation tools save process development time and costs, as well as manufacturing costs, by allowing the most stable and economical process to be designed with the minimal amount of input material. Microstructure predictive tools for Waspaloy IN718 and R88DT have been developed and are being used in production to develop optimum thermo-mechanical processing routes. Figure 2 shows a computer process model prediction of an IN718 forging.

Figure 2. Microstructural prediction for a part processed by hammer forging.
Process Control Systems

Advanced control systems for forging processes are also increasing affordability by giving improved process control and repeatability, and process automation. The use of computer controls on large forging hammer facilities allows greater forging flexibility and the development of lean forging cell methods. Advanced sensor technologies, coupled with computer data collection and controls, allow further understanding and description of hammer forging processes. The computer process models become more sophisticated and more accurate as the database is built up over time, allowing the implementation of advanced modelling tools such as microstructural prediction, such as that shown in figure 2.

Today, modelling and predictive tools can be used effectively to optimise processing parameters to give the best combinations of component properties and residual stresses. Component residual stresses are of considerable importance for aircraft engine components, especially for high strength nickel-base superalloy turbine discs. High residual stresses result in large distortions during machining and increase the difficulty of holding component tolerances. Extremely tight machining tolerances are required for optimum high efficiency turbine engine performance, and obtaining such dimensional tolerances during machining of superalloy discs with high residual stresses requires added costly operations. However, new technologies are being implemented at Ladish that allow the engineering of final components with minimum residual stresses, streamlining and reducing the costs of machining operations.

Ladish has also simplified the manufacturing route via a novel thermo-mechanical process called Iso-Con, which allows the enhancement of Waspaloy disc mechanical properties. Iso-Con is a combination of isothermal press forging and conventional hammer forging which has recently been extended to more highly alloyed nickel-base materials, such as U720. This process has allowed the development of a manufacturing route which involves only two or three deformation steps, as opposed to as many as five steps required by alternative processing routes for comparable turbine disc components.
Summary

Materials engineers see superalloy affordability as a key factor in all fixture turbine engine disc designs and applications. Improvements in overall affordability are being designed and implemented at Ladish, in cooperation with numerous OEMs, by pursuing improvements to engine performance through the use of advanced materials, and through improvements to the costs and efficiencies of different manufacturing methods. The future for continuous improvement in these areas is bright, because material, process and design engineers have not yet reached the developmental limits of new material and process technologies.