Heat treatment is essential for imparting the desired mechanical properties into aerospace components. The process provides a variety of benefits, such as lower manufacturing costs, improved fuel consumption and increased component durability. After forming, components are heated to achieve strength and hardness, and then rapidly cooled (quenched). However, current quenching methods, such as air, water, oil and ducted-nozzle air quenching have greater variations in the rate of cooling, which increases the problem of residual stress and distortion.
The Advantage of SuperCooler Technology
In contrast, the recently patented SuperCooler technology from Ladish Co Inc uses compressed air forced through holes and nozzles to deliver a quenching stream onto targeted surface areas, giving manufacturing engineers a way to improve the mechanical properties of complex aerospace components by controlling the cooling rates over the surface. Currently being used in the production of rotating disks for jet engines, the SuperCooler technology greatly reduces the residual stresses typical of aerospace components produced by conventional methods at the same time as providing significant cost savings.
Dealing with Residual Stresses and Distortions
Residual stresses have a tendency to create distortions when excess material is removed from the workpiece in the precision machining phase of manufacturing. Consequently, reducing residual stresses is crucial to achieving improved mechanical properties. They can even cause distortions during product use. Historically, distortions have contributed to rejection rates - an undesirable outcome when dealing with costly, highly engineered aerospace components. Therefore, the ability to refine the cooling rates over the surface of aerospace parts to control residual stresses is a useful development. The SuperCooler provides these refinements by employing a fixture that directs a flow of air at different velocities, volumes and pressures over different zones of the forging.
The SuperCooler Process
The SuperCooler technology resides in a new 510m2 computer-controlled facility, which consists of a cooling station with the capability of precisely controlling and monitoring more than a dozen distinct airflow zones on a single forging. The cellular manufacturing facility contains an automated manipulator, which transfers workpieces through a three step process. Step one involves placing the workpiece into one of up to eight furnaces for heat treating, before it is transferred to the cooling station for step two. After the cooling cycle, the manipulator moves the piece from the cooling station to pallet and conveyor for transport outside the cell to the next operation. Each furnace is capable of being controlled from 800-1230°C. The result is a production technology that allows design engineers to tailor a unique ‘recipe’ for each forged component.
Complete Computer Control
The effectiveness of a SuperCooler recipe is the result of total computer control of the process. During manufacturing, all the relevant process data is captured and stored for each phase of the process. Data can be retrieved for process verification and evaluation, as desired. The data is used to control and refine the recipe for each part number. The recipes, which provide the optimal cooling for advanced mechanical properties and performance characteristics, reside in the computer system and can be downloaded for each part and batch run. The result is a process that eliminates the opportunity for human error, while producing a part with an optimal balance of differing mechanical properties throughout the component.
Heat Treatment Processes and Cooling Rates
Heat treatment processes are complex from both a metallurgical and mechanical standpoint. Solution heat treatments are designed to dissolve phases and allow the optimal re-precipitation of phases upon cooling or after ageing for various alloys. The cooling rate and cooling path to which titanium and nickel-based superalloy components are subjected is key to the development of optimal properties. Typically, higher cooling rates allow an increase in mechanical properties, but this is often accompanied by an increase in a component’s internal residual stresses.
Cooling Rates and Thermal Gradients
During cooling of solution heat treated components, variations in the cooling rate can develop thermal gradients within the component being processed - the greater the variation in cooling rate, the larger the thermal gradients. Large thermal gradients tend to lock in high residual stresses after the cooling process is complete. In turn, these locked-in residual stresses can lead to problems during subsequent manufacturing operations.
Liquid Quenching vs. Ducted Nozzle Air Quenching
Liquid quenching methods, such as oil quenching, extract 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 mechanical properties in many materials. However, these high cooling rates result in extremely large internal thermal gradients. 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 is a useful method, but the cooling rate range is restricted.
Origins of the SuperCooler Process
Not satisfied with the current technologies considered to be state-of-the-art, the development of the SuperCooler differential air-cooling system arose out of customer demand for improved mechanical properties and capabilities. New air-quenching methods were being explored at metalworking companies built around the application of high speed fans to provide cooling. After further analysis, a team of Ladish engineers concluded that the high speed fan approach to air-quenching which basically moved homogeneous air over the face of a forging, would never deliver the advanced properties and capabilities envisaged.
The SuperCooler facility is built adjacent to the existing Heat Treatment Department since most pieces are subject to an ageing or comparable cycle after the initial solution or equivalent heat treatment cycle. In fact, it is the balancing of heat treatment and cooling cycles in light of the SuperCooler’s unique capabilities that enables the manufacture of components with preferential properties.
Flexibility and Capacity of the SuperCooler Process
The SuperCooler’s ability to refine the cooling profiles of components over a much broader range means that components, which previously could no be heat treated to the optimal strength level by ducted-nozzle air-quenching methods, can now be heat treated and cooled to achieve the best balance of increased mechanical properties and reduced residual stresses. Specifically designed for nickel-based superalloy turbine discs, the new facility has the versatility to process components that vary in size up to 1270mm in diameter weighing as much as 900kg.
SuperCooler Technology Allows Heat Treating in the As-Forged State
Currently, many components require pre-heat treat machining to develop a configuration that it acceptable for the heat treatment cooling method being employed. Using the SuperCooler facility many components can be heat treated in the as forged configuration, reducing both cycle times and manufacturing costs. In addition, engineers can forge components much nearer to net shape, reducing raw materials and manufacturing costs. The next phase in the development of the SuperCooler technology includes linking the new facility to the central ERP (enterprise resource planning) and quality systems for automatic data collection for shop floor reporting, and retrieval of recipes and related part specific process parameters.
Aerospace engineers have long sought ways to improve the mechanical properties of component destined for use in high temperature operating environments. Not only does this technology offer at improved strategy for achieving that, but it advanced capabilities offer a number of potential end-user benefits for jet engine original equipment manufacturers who want their engines to burn hotter and more efficiently.