Magnesium alloys for aerospace structures
Magnesium is the lightest of all the metals used in aircraft. The density of magnesium is only 1.7 g cm−3, which is much lower than the specific gravity of the other aerospace structural metals: aluminium (2.7 g cm−3), titanium (4.6 g cm−3), steel (7.8 g cm−3). Only carbon-fibre composite material has a density (~ 1.7 g/cm3) that is similar to magnesium. Magnesium alloys have lower stiffness and strength properties than the aerospace structural materials, but because of its low density the specific properties are similar. However, there are several problems with using magnesium alloys, including higher cost and lower strength, fatigue life, ductility, toughness and creep resistant properties compared with aluminium alloys. Poor resistance against corrosion is one of the greatest problems with magnesium. Magnesium and its alloys are one of the least corrosion-resistant metals, with its corrosion performance being vastly inferior to the other aerospace metals. There is a widespread belief that a serious safety concern with using magnesium is flammability. Magnesium burns when exposed to high temperature, and therefore may pose a major fire risk. However, there have been no cases of aircraft accidents caused by the ignition of magnesium. Magnesium meets all the aerospace standards for material flammability resistance. The main reason why magnesium is used sparingly in modern aircraft, typically less than 1% of the structural mass of large passenger aircraft, is poor corrosion resistance.
Although the use of magnesium in aerospace structures is now extremely limited, for many years it was used extensively in structural components in aircraft, helicopters and spacecraft because of its light weight. Magnesium was originally used in aircraft during the 1940s and for the next thirty years was a common structural material. Magnesium passed through a boom period between the 1950s and early 1970s when military and civilian aircraft were built using hundreds of kilograms of the material. Magnesium was used extensively in airframes, aviation instruments and low-temperature engine components for aircraft, especially fighters and military helicopters, and semistructural parts for spacecraft and missiles. Since the 1970s, however, the use of magnesium has declined owing to high cost, poor corrosion resistance and other factors, and it is now rarely used in aircraft, spacecraft and missiles. The use of magnesium alloys is now largely confined to engine parts, and common applications are gearboxes and gearbox housings for aircraft and the main transmission housing for helicopters (Fig. 10.1). Magnesium has good damping capacity and therefore is often the material of choice in harsh vibration environments, such as helicopter gearboxes.
Despite the general decline in the use of magnesium, it remains an important material for specific aerospace applications. In this chapter, we study the metallurgy, mechanical properties and aerospace applications of magnesium. The following aspects of magnesium alloys are discussed: advantages and drawbacks of using magnesium in aircraft and helicopters; classification system for magnesium alloys; types of magnesium alloys used in aircraft and helicopters; and the engineering properties of magnesium alloys.
An international standardised system for classifying magnesium alloys, similar to the International Alloy Designation System for aluminium, does not exist. However the American Society for Testing and Materials (ASTM) has developed a coding system that is widely used by the magnesium industry. The system is a ‘letter-letter-number-number’ code. The letters indicate the two principal alloy elements listed in order of decreasing content. When the two alloying elements are present in an equal amount, then they are listed alphabetically. The code letters used to identify the alloying elements are given in Table 10.1. The two numbers specify the weight percent of the two principal alloying elements, rounded off to the nearest whole number and listed in the order of the two elements. For instance, the alloy AZ91 signifies that aluminium (A) and zinc (Z) are the two main alloy elements, and these are present in weight percentages of about 9 and 1%, respectively. As another example, WE43 identifies the alloy as containing 4% yttrium and 3% rare earth elements. In addition to the alloying elements specified in the code, magnesium alloys often contain other elements in lesser amounts. However, the coding system provides no information about these elements. In exceptional circumstances when an alloy contains three main alloying elements, then a ‘three letter–three number’ code system is used. For example, ZMC711 contains about 7% zinc, 1% manganese and 1% copper.
Magnesium alloys are classified as wrought or casting alloys. Wrought alloys account for only a small percentage (under 15%) of the total consumption of magnesium, and these alloys are not used in aircraft. A problem with wrought alloys is their low yield strength (typically less than 170 MPa). Most magnesium alloys that are used commercially, including those in aircraft and helicopters, are casting alloys. The casting alloys are often used in the tempered condition; that is heat-treated and work hardened, under conditions similar to the tempering of aluminium alloys. For this reason, the system used to describe the temper of aluminium alloys is also used for magnesium alloys (see Table 8.2). The temper conditions most often applied to magnesium are T5 (alloy is artificially aged after casting), T6 (alloy is solution treated, quenched and artificially aged), and T7 (alloy is solution treated only).
The composition and tensile properties of pure magnesium and several alloys used in aircraft are given in Table 10.2. Most of the magnesium alloys used in aircraft are from the Mg–Al–Zn, Mg–Al–Zr and Mg–E–Zr systems. The aerospace applications for magnesium alloys are listed in Table 10.3. As mentioned, the most common application is the transmission casings and main rotor gearbox of helicopters which utilise the low weight and good vibration damping properties of magnesium. Magnesium alloys are also used in aircraft engine and gearbox components.
|RZ5||Helicopter transmission; aircraft gearbox casings; aircraft generator housing (e.g. A320, Tornado, Concorde)|
|WE43||Helicopter transmission (e.g. Eurocopter EC120, NH90; Sikorsky S92)|
|QE21||Aircraft gearbox casing; auxiliary gearbox (e.g. F-16, Eurofighter, Tornado)|
|ZW3||Aircraft wheels; helicopter gearbox (e.g. Westland Sea King)|
Pure magnesium does not have sufficient strength or corrosion resistance to be suitable for use in aircraft. Magnesium has a hexagonal close packed (hcp) crystal structure. As mentioned in chapter 4, hcp crystals have few slip systems (three) along which dislocations can move during plastic deformation. As a result, it is not possible to greatly increase the strength of hcp metals by work-hardening. For example, annealed magnesium has a yield strength of about 90 MPa, and heavy cold-working of the metal only increases the strength to about 115 MPa. Another consequence of the hcp structure is the mechanical properties of wrought magnesium alloys are anisotropic and are different dependent on the loading direction. Owing to the anisotropy, the compressive yield strength of wrought magnesium alloys can be 30–60% lower than the tensile yield strength.
There are two broad classes of magnesium alloys that are strengthened by cold working or solid solution hardening combined with precipitation hardening. As mentioned, it is difficult to greatly increase the strength of magnesium by cold working owing to the hcp crystal structure, and therefore the majority of magnesium alloys used in aerospace applications are strengthened by the combination of solid solution and precipitation hardening. The strength properties of magnesium are improved by a large number of different alloying elements, and the main ones are aluminium and zinc. Other important alloying elements are zirconium and the rare earths. Rare earths are the thirty elements within the lanthanide and actinide series of the Periodic Table, with thorium (Th) and neodymium (Nd) being the most commonly used as alloying elements.
A problem with magnesium, however, is that the addition of alloying elements provides only a relatively small improvement to the strength properties. Compared with the annealed pure metal, the increase in yield strength of magnesium owing to alloying is typically in the range of 20 to 200%. In comparison, the alloying of annealed aluminium increases the yield strength by more than 1000% and the strength of annealed titanium is improved by up to 700%. The low response of magnesium to strengthening by alloying and work-hardening, together with low elastic modulus, ductility and corrosion resistance, are important reasons for the low use of this material in modern aircraft.
The majority of alloying elements used in magnesium increase the strength by solid-solution hardening and dispersion hardening. The alloying elements react with the magnesium to form fine intermetallic particles that increase the strength by dispersion hardening. The three most common intermetallic particles have the chemical composition: MgX (e.g. MgTl, MgCe, MgSn); MgX2 (e.g. MgCu2, MgZn2); and Mg2X (e.g. Mg2Si, Mg2Sn). These compounds are effective at increasing the strength by dispersion hardening, but they reduce the fracture toughness and ductility of magnesium. For example, the Mg–Al–Mn and Mg–Al–Zn alloys used in aircraft form particles (Mg17Al12) at the grain boundaries which lower the toughness and ductility.
Magnesium alloys must be heat treated before being used in aircraft to minimise the adverse effects of the intermetallic particles on toughness. This involves solution treating the magnesium at high temperature to dissolve the intermetallic particles in order to release the alloying elements into solid solution. The material is then thermally aged to maximise the tensile strength by precipitation hardening. A typical heat-treatment cycle involves solution treating at about 440 °C, quenching, and then thermally ageing at 180–200 °C for 16–20 h. These heat-treatment conditions are similar to those used to strengthen age-hardenable aluminium alloys. However, the response of magnesium to precipitation hardening is much less effective than aluminium. Only relatively small improvements to the tensile strength of magnesium alloys are gained by precipitation hardening. This is because the density and mobility of dislocations in magnesium is relatively low owing to the small number of slip systems in the hexagonal crystal structure.
The precipitation processes that occur in most magnesium alloys during thermal ageing are complex. In chapter 8, it is mentioned that aluminium alloys undergo the following transformation sequence in the age-hardening process: supersaturated solid solution → GP1 zones → GP2 zones → coherent intermetallic precipitates → incoherent intermetallic precipitates. Some magnesium alloys also undergo this sequence of transformations during ageing whereas other alloys form precipitates without the prior formation of GP zones. The types of precipitates that develop are obviously dependent on the composition and heat-treatment conditions. Precipitates in Mg–E–Zr alloys, such as WE43 used in helicopter transmissions, are Mg11NdY and/or Mg12NdY compounds. Precipitates in Mg–Zn alloys, such as ZE41 that is also used in helicopter transmissions, are coherent MgZn2, semicoherent MgZn2, and incoherent Mg2Zn3 particles. Several magnesium alloys used in aircraft contain aluminium, such as QE21 that is used in aircraft gearboxes. The main precipitate formed in Mg–Al alloys is Mg17Al12, which is effective at increasing strength. Figure 10.2 shows the effect of aluminium content on the tensile properties of a fully heat-treated Mg–Al–Zn alloy. The yield and ultimate tensile strengths increase with the aluminium content owing to solid solution hardening and precipitation hardening. When the aluminium content exceeds 6–8%, the ductility is reduced owing to embrittlement of the grain boundaries by Mg17Al12 particles and, for this reason, the aluminium concentration is kept below this limit.
Two important alloying elements used in magnesium are zirconium and thorium. Zirconium is used for its ability to reduce the grain size. Cast magnesium has a coarse grain structure which results in low strength owing to the weak grain boundary hardening effect. Zirconium is used in small amounts (0.5% to 0.7%) to refine the grain structure and thereby increase the yield strength. In the past, thorium was often used to reduce the grain size and, for many years, magnesium–thorium alloys were used in components for missiles and spacecraft. However, thorium is a radioactive element that poses a health and environment hazard and, therefore, its use has been phased out over the past twenty years and it is now obsolete as an alloying element.
The biggest obstacle to the use of magnesium alloys is their poor corrosion resistance. Magnesium occupies one of the highest anodic positions in the galvanic series, and for this reason has a high potential for corrosion. There are many types of corrosion (as explained in chapter 21), and the most damaging forms to magnesium and its alloys are pitting corrosion and stress corrosion. Pitting corrosion, as the name implies, involves the formation of small pits over the metal surface where small amounts of material are dissolved by corrosion processes. These pits are sites for the formation of cracks within aircraft structures. Stress corrosion involves the formation and growth of cracks within the material under the combined effects of stress and a corrosive medium, such as salt water. Magnesium is much more susceptible to corrosion than other aerospace metals, and must be protected to avoid rapid and severe damage.
The corrosion resistance of magnesium generally decreases with alloying and impurities. The addition of alloying elements to increase the strength properties comes at the expense of reduced corrosion resistance. The most practical methods of minimising the damaging effects of corrosion are careful control of impurities and surface protection. The corrosion resistance of magnesium can be improved by ensuring a very low level of cathodic impurities. Iron, copper and nickel, which are not completely removed from magnesium during processing from the ore, act as impurities that accelerate the corrosion rate. The amount of iron, copper and nickel must be kept to very low levels to ensure good corrosion resistance. The maximum concentrations are only 1300 ppm (parts per million) for copper, 170 ppm for iron and and 5 ppm for nickel. In some aerospace magnesium alloys (e.g. AZ63), a small amount of manganese (< 0.3%) is used to improve corrosion resistance. The manganese reacts with the impurities to form relatively harmless intermetallic compounds, some of which separate out during melting. The other way of protecting magnesium alloys from corrosion is surface treatments and coatings.
Magnesium used to be a popular aircraft structural material owing to its low density. However, the use of magnesium alloys has fallen from the boom period of the 1950s and 1960s when it was commonly used in aircraft, helicopters and missiles. The use in modern aircraft is limited mostly to gearboxes and gearbox housings for fixed-wing aeroplanes and main transmission housings on helicopters which require the high vibration damping properties afforded by magnesium.
The greatest problem with using magnesium in aircraft is poor corrosion resistance. The impurities content must be carefully controlled and the surface protected with treatments or coatings to avoid corrosion problems.
Other drawbacks include high cost and low stiffness, strength, toughness and creep resistance compared with other aerospace structural materials. It is difficult to increase the strength properties of magnesium owing to its low responses to cold-working and precipitation hardening.
The magnesium alloys most used in aerospace components are Mg–Al–Zn, Mg–Al–Zr and Mg–E–Zr alloys. These materials are strengthened predominantly by solid solution and precipitation hardening. However, the maximum strengths of magnesium alloys are much lower (50% or more) than high-strength aluminium alloys.