Differential cooling technology offers jet engine design engineers performance improvement options

Citation

Bunge David Furrer, G. (2003), "Differential cooling technology offers jet engine design engineers performance improvement options", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 4. https://doi.org/10.1108/aeat.2003.12775daf.001

Differential cooling technology offers jet engine design engineers performance improvement options

Differential cooling technology offers jet engine design engineers performance improvement options

Keywords: Coolers, Jet engines, Design

An air-quenching system developed and patented by Ladish Co., Inc., Cudahy, Wisconsin, USA, makes available to jet engine design engineers a new technology for developing unique microstructures and properties within forgings. The metallurgical features that this system can produce within cold or hot section turbine engine disks offer engineers a new way to expand the operating efficiencies and performance range of jet engines.

Called the "SuperCooler", this heat-treating system allows engineers to design jet engine components with a better balance of mechanical properties and reduced residual stresses than that has been possible previously. Heat treatment is a critical step in imparting preferred mechanical properties into aerospace components. Yet traditional methods of heat treatment, such as air, water, oil and ducted- nozzle air-quenching, have limitations on the range of mechanical properties that engineers can design into end products. This is primarily due to the residual stresses that remain in the work piece after quenching.

In contrast, the SuperCooler has been designed to control the cooling rates over the surface of complex-shaped aerospace components using customised heat-transfer coefficients. The SuperCooler enables design engineers to significantly improve their control of the residual stresses and mechanical properties in each specific area of the forging. The SuperCooler can also help to reduce component costs because engineers can design parts in more near-net configurations. Most critical jet engine disks are manufactured from nickel-based wrought alloys or powder metallurgy alloys, which are expensive. Therefore, every kilogram of weight reduction saves not only the cost of the raw material, but also the cost of removing the material in subsequent finish-machining operations.

Capabilities of the SuperCooler

The cooling rate of heat treatment cycles, for example, during solution heat-treating operations, governs the microstructural evolution of secondary phase precipitates and subsequent mechanical properties. SuperCooler processing can be engineered to allow highly uniform cooling rates that achieve a single, consistent product result. However, SuperCooler technology can be taken one step further to allow tailored cooling rates in different areas of the work piece to produce a combination of fine, intermediate and/or coarse microstructural features, as desired. This system capability expands the potential window of opportunity for designers working with existing turbine disk material. This alternative heat-treating approach can allow broader ranges in properties and potential local optimization based on component design and requirements.

Ti-6-2-4-2 is an example of an alloy where the resultant properties are greatly influenced by the cooling rates (Helm). Rapid cooling rates will allow increased strength capability, while intermediate cooling rates can develop peak creep resistance. Thus, compressor disks within turbine engines could be processed in the SuperCooler employing differential cooling rates to produce creep- resistant rim and high-strength bore locations.

Nickel-base superalloys, such as R88DT (Krueger) and U720 (Furrer) can be processed by stepped cooling profiles to obtain enhanced damage tolerance and reduced fatigue crack growth rates. For these materials, however, maximum strength is obtained from a single, rapid quenching operation. These materials, like Ti-6-2-4-2 could be processed by a controlled SuperCooler technique to achieve selective properties, such as maximum strength in the bore and maximum damage resistance at the rim.

The problem of residual stresses

During solution heat treatment cooling, variations in cooling rate determine the thermal gradients within the component being processed. The greater the variation in the cooling rate, the larger the thermal gradients that are developed. Large thermal gradients tend to lock in high residual stresses after the cooling process is complete. Consequently, these locked-in residual stresses often lead to distortions in the work piece during subsequent manufacturing operations. This phenomenon can lead to high rejection rates for costly work-in-process components.

Unacceptably high rejection rates sometimes accompany liquid quenching methods, such as oil-quenching. Oil quenching extracts heat from all surfaces in a rapid and relatively uniform manner, regardless of component cross-section or cross-section variation. Oil-quenching methods permit very high cooling rates, which facilitate the increase of desirable mechanical properties in many materials. However, these high cooling rates can also result in extremely large internal thermal gradients and residual stresses. Ducted- nozzle-air-quenching methods have been developed that attempt to moderate the cooling rate profile within the component being processed to achieve more uniform cooling rates. This method is useful, but is limited to a restricted cooling-rate range.

How the SuperCooler works

SuperCooler technology, housed in a 600 m² facility within Ladish's 190,000 m² forge shop in Cudahy, Wisconsin, is designed to reduce residual stresses and improve the balance of mechanical properties in cold and hot-section turbine disks. The computer- controlled air-quench cell consists of a cooling station with the capability of precisely controlling and monitoring more than a dozen distinct air-flow zones on a single forging.

An automated manipulator transfers pieces into one of the eight adjacent furnaces, from each furnace to the cooling station and from the cooling station to pallet and/or conveyor for transport to the next operation. Each furnace is capable of being controlled from 1,500 to 2,250°F. The single-piece furnaces ensure maximized process control and manufacturing effectiveness for each individual forging.

The SuperCooler facility was constructed to be contiguous with the existing Heat Treat Department, because most work pieces are subject to an age or comparable cycle after the initial solution or equivalent heat treat cycle.

Advanced quality-control features were incorporated in the SuperCooler facility. All relevant process data are captured and stored for each phase of the process. The data can be retrieved for process verification and evaluation as needed.

The SuperCooler is also linked to the Central ERP and Quality Systems for automatic data collection for shop floor reporting and retrieval of "recipes" and related part specific process parameters.

Ladish's SuperCooler offers advantages over other technologies currently still considered to be state-of-the-art. These older technologies incorporate ducted-nozzles that provide variations in local cooling rate as desired by the manufacturing engineers. The range of cooling rate achievable by the ducted-nozzle, air-cooling method, however, is limited, as are its maximum and minimum cooling-rate capabilities.

System benefits

Manufacturing engineers have been seeking an enhanced method for tailoring cooling rates over the surface of complex aerospace components. Ladish's response was to develop specific, highly engineered heat transfer coefficient patterns that enable the control of residual stresses that are prone to develop during the heat-treat cooling process. Ladish's SuperCooler technology affords a considerably expanded cooling-rate range than older ducted-nozzle, air-cooling methods. Much higher heat transfer coefficients can be developed in areas that have greater thermal mass, and much lower heat transfer coefficients can be developed in areas that have less thermal mass, which results in an increased cooling-rate uniformity.

The Ladish SuperCooler's ability to tailor the cooling profiles of work-in-process, jet engine forgings over a much wider range allows numerous advantages. Components that were previously not heat-treated to the optimal strength level by ducted-nozzle technologies can be heat-treated and cooled to achieve a better balance of increased mechanical properties and reduced residual stresses.

In addition, SuperCooler technology facilitates the achievement of a near-net forging process. As engineers design increasingly complex part configurations, the range in cross-sectional areas and subsequently the local thermal masses are increasing. SuperCooler technology makes it easier to heat-treat these types of configurations. Previous heat-treat cooling technology would not be able to respond effectively to the extreme variations being designed into very near-net forged configurations. Another SuperCooler benefit is that the system supports manufacturers, efforts to adopt "lean manufacturing" approaches, because it enables near-net forging, which reduces raw material and machining requirements.

Moreover, SuperCooler technology can provide significant cost savings over and above the reduction in raw material and machining requirements. Currently, many components require pre-heat-treat machining to develop a configuration that is acceptable for the heat-treatment cooling method being employed. With the SuperCooler facility, many components can be heat-treated in the as-forged configuration, reducing both cycle time and manufacturing costs.

Additional potential end-user benefits that derive from increasing the performance capabilities of jet engine disk components are reduced component weight and higher- temperature engine operating environments, which translate into improved fuel consumption.

The SuperCooler facility has the capability of processing components that vary in size up to 1.3m in diameter and weighing as much as one metric ton.

Examples of results

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  Part - A: SuperCooler vs Fan cool

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  Range of average cooling rates = 100–244°C/min vs 80–300°C/min.

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  Cooling rate gradient = 145°C/min vs 222°C/min.

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  Part - B: SuperCooler vs Oil quench

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  Range of average cooling rates = 125–320°C/min vs 280–780°C/min.

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  Cooling rate gradient = 195°C/min vs 500°C/min.

Gene Bunge VP Engineering Ladish Co., Inc. David Furrer Manager, Advanced Materials and Processes Ladish Co., Inc.