Processing Nickel-Base Superalloys
Nickel-base superalloys are the material of choice for the harshest operating conditions in gas turbine engines. Superalloys are used in the high temperature regions of the gas turbine, specifically the turbine blades and turbine disks. The blades lie in the gas stream and extract the work to power the aircraft and the turbine disks, which in turn constrain the blades and transmit the work to the shaft connecting the turbine to the compressor stages at the front of the engine. These two components perform wildly different roles and as such require different properties. This has led to the development of different alloys and processing routes to achieve the most efficient and best performing engines.
The turbine blades of a jet engine are cast by solidifying the liquid alloy inside a ceramic mould, which contains cores to facilitate internal cooling channels. This requires a complex mould that begins as a wax assembly with dimensions matching those of the blade, upon which layers of ceramic slurry and ceramic particles are applied to build up a shell. The wax assembly is then melted from the shell at high pressure, before the ceramic shell is fired to strengthen it. The liquid alloy is then poured into the shell and allowed to solidify before the blades are removed by shattering the ceramic mould. The internal coring is then leached out using acid baths.
The turbine blade of a jet engine operates at extreme temperatures and undergoes a process called creep deformation during service, where slow elongation of the component can lead to contact with the casing. This wears down the component and can result in replacement of the blade or in severe cases; damage to the engine itself. However, under certain conditions the crystal structure of a material has a natural resistance to creep deformation. This has led to developments in processing to maximise this natural resistance.
A standard conventionally cast blade will contain multiple crystal structures, or grains, at random orientations. This arrangement has the least resistance to creep deformation, so materials scientists sought ways to alter the grain structure in order to change the material's properties. The first refinement, developed in the 1970s, sought to align the crystal structure along the axis of greatest stress by removing grains oriented perpendicular to the stress. This was achieved through directional solidification, whereby the blade is solidified from the bottom upwards as it passes through a sharp temperature gradient at a rate of a few inches per hour. This means that the solid to liquid interface moves vertically through the blade, and hence aligns the grains along the length of the component, as they grow in the direction of the thermal gradient.
In the 1980's this process was refined to the single-crystal casting process currently employed today. Single crystal casting follows an identical procedure as directional solidification, with one difference; the crystal structure of the material is aligned prior to entering the blade section of the component through the use of a grain selector in the ceramic shell. The tight turns of the grain selector (one form is that of a pig's tail) limits the number of orientations of crystal that can grow into the blade to a single grain. This grain is oriented parallel to the length of the component, and so confers the maximum resistance to deformation.
The production of nickel-base superalloy disks begins by producing a metal ingot of correct composition for the alloy. This is achieved through a triple-melting program, whereby the alloy is re-melted three times by different methods in order to reduce segregation (the dispersion of elements), to remove 'tramp' elements (such as oxygen, nitrogen and sulphur) and to achieve an ingot with a uniform microstructure. Once the ingot has been produced in this way it is annealed (left at high temperature) to further reduce segregation prior to production of the disk.
After the ingot has been formed the disk is shaped either by the cast and wrought route or by powder processing. In the cast and wrought process the ingot is first deformed by hydraulic presses that reduce the diameter of the ingot and break up the microstructure (known as cogging or upset and drawing), before being formed into 'pancakes' prior to the final forging operations.
As the nickel-base superalloys have become more complex with more alloying elements and designed for higher strengths, deforming the alloys by mechanical methods with the cast and wrought route has become more difficult. This has led to the use of an alternative route: powder processing. Powder processing begins by melting the triple-melt ingot in a crucible shaped like a funnel, and then atomising the molten metal by directing argon gas jets through the liquid metal as it falls. The spheres of metal are then passed through a series of sieves to achieve a uniform powder size, and the powder is then compacted by hot pressing; at temperatures in excess of 1000˚C and pressures of 100MPa. This process consolidates the powder into a shape closely resembling that of the final component; it is referred to as a 'near net-shape process'.
All turbine blades and disks are subjected to a heat treatment appropriate to their application. This takes the form of a treatment to obtain a homogenous material and the required grain size in disks, followed by an aging heat treatment that enables the strengthening crystal structures known as gamma-prime precipitates to form and grow to an optimum level that will maximise the properties of the component during service.
Once the heat treatment program is complete the blades and disks are sent for final machining, where blade attachments and mechanical fixings are added to the components. The dimensions of the overall structure are then are refined with very high precision - a tolerance of a few microns. In the case of the turbine blades, ceramic coatings are then applied to confer additional resistance to the high operating temperatures, and the disks are spin tested to ensure they remain in balance.
The final stages in producing these highly sensitive components are to undergo non-destructive analysis to ensure they are within the manufacturing limits, before they are arranged into the constituent sections of the engine. These are then assembled to form one of the tens of thousands of components that together make up a modern jet engine.