Spotlight on heat treatment
How to balance heat treatment with residual stress damage, and improving quality with vacuum technology.
Major aircraft manufacturers and their supply chains are often challenged with producing quality materials that can withstand extreme conditions at high operating temperatures.
Trying to make materials at the lowest cost in the shortest time and to specification is tough. While the geometry, final shape and external loads acting on a certain part are important, there are other contributory factors such as unfavourable microstructure, pre-existing defects and flaws affecting the performance of a part in service. Unfavourable microstructure could include grain size heterogeneities and non-uniform distribution of microstructural features. This may originate from forging operations during the manufacturing process and can be susceptible to crack initiation, or favourable to crack propagation, leading to unexpected failures.
A clear understanding of microstructural evolutionary mechanisms during manufacturing processes is crucial. This facilitates the design of a microstructure that is able to withstand the critical environmental and operating conditions.
Material characterisation techniques coupled with a robust understanding of underlying physical mechanisms help to produce parts that comply with technical requirements. This also reduces manufacturing time and costs, which balances competing requirements.
The influence of temperature on parts
Typically, materials might be subjected to sequential processes of forging, quenching, heat treatment and cold working in order to tailor mechanical and microstructural properties.
Heat treatments are an essential factor in designing a tailored microstructure when specific properties are expected from a certain part. Take turbine disks in jet engines as an example – the engines usually operate at temperatures over 700°C for civil engines and more than 1,100°C for military engines. The parts must have high mechanical properties at these temperatures and be resistant to corrosion, creep and fatigue fractures.
A certain microstructure is required to achieve these properties, and that microstructure usually requires a series of thermomechanical processes, which means mechanical work followed by heat treatment.
Turbine discs typically undergo two stages of thermomechanical processing. The first is forging and solution heat treatment at temperatures around the solvus temperature of the primary strengthening phase, followed by water or polymer quenching. The second is subsequent heat treatments at lower temperatures to precipitate the secondary and tertiary strengthening phases, which are critical for high-temperature creep resistance.
Here, heat treatment becomes imperative to achieve the desired microstructures. You might ask how certain heat treatments for thermomechanical processing have been proposed for achieving a certain microstructure. Traditionally, this has been based on trial and error. Now, we have a much deeper knowledge and understanding of materials and a significantly stronger power of computation.
This allows someone to predict the thermomechanical properties or heat treatments required to achieve a certain microstructure.
Seizing the opportunity to transform the design and manufacture of high value components, it is possible to control microstructure and residual stress through appropriate design of manufacturing process parameters, such as heat treatments. The research challenge is to measure, model and control these processes from microstructure, residual stress and distortion perspectives with the aim of reducing costs, improving productivity and delivering products that are made correctly first time.
A common situation
To use the turbine example, turbine disks should have a microstructure that withstands high temperatures. To get that quality, parts are forged at 980°C, followed by water quenching to room temperature, and then subsequent ageing heat treatments at lower temperatures (e.g. 720°C). The water quenching from the forging temperatures is necessary to generate a sufficient number of nucleation sites for precipitation at a later stage of ageing heat treatment. The nucleation and growth of precipitates during ageing at lower temperatures depends on the cooling rate from the previous stage, which is why it needs quenching (i.e. in water or polymer).
While quenching is extremely important to control the size and distribution of precipitates during ageing heat treatment, it also introduces a significant amount of residual stresses to the point that it creates cracking in some materials. Trying to control and manage this during manufacturing is the challenge.
Doing the technical aspects of materials characterisation in-house can be difficult, particularly for smaller companies that may not have the access to the equipment and expertise required.
Challenges to adaptation and adoption
Most processes, particularly in aerospace, are something of a black art. Manufacturing techniques and methods have typically evolved over the last 100 years, without us ever scrutinising or thoroughly understanding their influence or necessity.
In reality, some of the steps in many processes are not necessary. By reviewing the knowledge about materials and processes, it is possible to consider removing stages to make the process more efficient and economical.
Information that may be useful to help guide this review includes characterising materials over a range of temperatures and identifying the forging and forming and heat treatment characteristics and parameters. This information could then be combined into finite element models to predict material behaviours and properties.
A collaborative approach to finding solutions
Analysing data can help manufacturers and suppliers better understand their products. In turn, they can make informed decisions about their processing, production and assembling methods to produce the best result.
In our view, it is wise for a manufacturer to work with the team who researched the data. This way, it is easy for the manufacturer to interpret the data and use it to design changes to their processes.