Materials through the ages: Materials for aeroplane engines
Maria Felice considers the technology that propels millions of people to their holiday destinations each year – aeroplane engines.
The requirements for materials used in aeroplane engines are naturally very exacting. They must survive extremes of temperature and force, while being as light as possible and ultra-reliable because, as people working in the industry often say to those outside it, ‘there are no laybys at 30,000 feet’.
The idea for turbojet engines was first put forward by Frank Whittle, an RAF cadet, in 1928. His first engine run was in 1937 and the first flight four years later. Meanwhile, a team in Germany was doing similar work, unaware of Whittle’s achievements. Work was started by Hans von Ohain in 1935 and it was an Ernst Heinkel plane – a He178 – that was the first to fly with a jet engine in 1939. Most civil engines nowadays are turbofan engines, as pictured above. These are jet engines with a large fan at the front, which accelerates air backwards, some of which bypasses the engine core.
A turbojet engine can be simply divided into three sections – the compressor, the combustor and the turbine. The compressor pressurises the air flowing through the engine before it enters the combustion chamber, where the air is mixed with fuel, ignited and burnt. The compressor components are predominantly made from titanium, and the combustor components are typically made of a nickel superalloy such as Inconel 625. The third stage is the turbine, and this is where the effect of advances in materials and material processes can be best appreciated. Turbine blades are the components that extract work from the high-pressure, hot gases exiting the combustion system and provide power for the compressor stages and auxiliary engine systems.
The temperature of the gas that enters the turbine has a direct effect on the power and efficiency of a gas turbine. In the last 50 years, this has risen from around 600°C to more than 1,500°C, causing thrust to increase by 60% and fuel consumption to decrease by 20%. The ability to withstand this high temperature is due to advances both in materials science and in the technology of cooling passages within the blades.
The development in the 1940s of nickel superalloys such as Nimonic and Inconel was largely driven by the need for high-performing materials for jet engine turbine blades. These alloys are oxidation- and corrosion-resistant materials that are well suited for service in extreme environments with high pressures and temperatures. They all have nickel as the predominant element and chromium as the second. Nickel is chosen because it has a face-centre cubic (FCC) crystal structure, which imparts attractive properties such as toughness, ductility, low rates of thermally activated creep, and stability at elevated temperatures. Platinum group metals have similar properties but are much more expensive and dense.
Polycrystalline metals are made up of many grains, and the grain boundaries play an important role in failure mechanisms such as creep and corrosion. Directional solidification has been used in the manufacture of turbine blades since the 1970s. In directionally solidified metals, the grain boundaries are aligned in one direction so that the grains run parallel to the major axis of the part (for example, along a blade) and there are no transverse grain boundaries. The removal of these grain boundaries increases the creep resistance of the metal, and the orientation of the grain structure provides a favourable modulus of elasticity in the longitudinal direction, improving the stiffness of the part. As a result, the creep life of the part increases three to five fold.
One step up from directionally solidified materials are single crystal materials, in which all grain boundaries are eliminated. Single crystal materials have been used to make turbine blades since the 1980s, and the fatigue life of the part is increased by approximately seven fold.
For turbine blades to operate at high temperatures, internal cooling passages are essential. Relatively cool air at 650°C is drawn from the compressor and forced through the cooling passages. This air extracts heat from the blades and creates a cool film around each one that enables them to operate at temperatures more than 300°C higher than their melting temperatures.
The technique used to make the intricate system of cooling passages within each blade is investment casting. Investment casting has been around for thousands of years and idols made by this process have been found that are at least 4,000 years old. It came into use as a modern industrial process in the late 19th Century when dentists used it to make crowns and inlays. World War II increased the demand for net-shape high-precision items made with specialised alloys that could not be shaped easily by traditional methods such as forging. Investment casting was used to deal with this, and after the war its use spread. It has been used since the 1980s to make turbine blades whether their metal is in the equiaxed form (normal grains), directionally solidified or single crystal.
Investment casting offers high dimensional accuracy, excellent surface finish and no flash or parting lines. Essentially the process involves making wax patterns of the part and coating them in a ceramic, then melting out the wax and filling the ceramic with metal. The larger cooling passages are created by building the wax pattern around ceramic rods, the removal of which creates the passages. The smaller passages are laser cut.
In the case of single crystal alloys, the process of filling the ceramic mould is as follows. The bottom of the mould is placed on a water-cooled copper plate on which the alloy crystallises. The mould is withdrawn from the bottom of the furnace so that a temperature gradient forms that causes the crystals to grow up into the mould. Multiple crystals grow through the bottom of the individual blade moulds so a crystal selector must be used to ensure only a single crystal enters. The crystal selector is a constricted passage, usually in a helical shape (a pig’s tail) in which the quickest solidifying crystal will branch and block off all other crystals.
Advances in materials engineering have enabled the performance of jet engines to steadily increase since the 1940s. Composite materials are ideal due to their weight savings and improved efficiencies. Silicon carbide fibres are used in titanium compressor discs and drums to provide hoop strength and stiffness with a weight saving of 70%. Polymer matrix composites are used to make the nacelles that encase engines. In the latter case, as well as a reduction in weight, novel properties such as acoustic damping can be achieved.
So next time you take your seat on a scheduled flight and look out along the wing, the intricacies of the engine powering you on your summer holidays will be less of a mystery.