The main properties of the ultra-light metal and of its most widely used casting and wrought alloys
by Giuseppe Giordano
Magnesium currently faces possible important consumption developments, even in mass production sectors and particularly in the automotive industry. The fact that some car manufacturers are open to the development of magnesium in popular models is not new, but this opening usually met with obstacles which were hard to overcome, such as, low resistance to corrosion and production issues linked to the material’s flammability under certain conditions. By all means, a better knowledge of magnesium’s metallurgy would allow to overcome some of the hurdles described above, starting off from the assumption that magnesium castings and semis may be the main players of a project when intense lightweighting is required: from this standpoint, they are candidates to replace similar parts in aluminium, but even details obtained by means of plastic moulding in applications where lightness is very important such as aerospace or in niche sectors such as sports equipment, where extreme behaviour is required of the materials used. In these segments magnesium has actually always been carefully considered, but concrete results almost always resulted in rather small amounts. The situation could change very rapidly due to possible applications in mass production industries ad in particular in the automotive sector. The quest for lightness indeed becomes an increasingly important choice factor to reduce consumption and to counterbalance the increase in weight of vehicles due to the addition of new components, enforced on account of security and marketing reasons.
The profile of magnesium
Metallic magnesium, silvery in colour, has a volume density (specific weight) of 1.74 g/cm3, that is, about 40% less than aluminium; this and other chemical and physical properties of the ultra-light metal are shown in Table 1. It should be noted that a remarkable difference with respect to aluminium and steel may also be seen in Young’s modulus (elasticity modulus: magnesium about 45 GPa; aluminium 70 GPa; carbon steel 200 GPa).
In general terms, metallic magnesium can count on practically endless sources: besides being the eighth element in the composition of the earth’s crust, it is the third solid element dissolved in sea water (see Table 2).
Every litre of sea water contains on average roughly 1.2 grams of magnesium; as a comparison it should be noted that the same litre of water contains about 0.5 mg of aluminium. There are also vast territories made up mainly of minerals containing magnesium, such as dolomite, or very rich in magnesium salts dissolved in natural saline solutions. Like aluminium, magnesium is a strongly electronegative metal and therefore a high quantity of energy is needed to bring it down to its oxides (roughly three times more than aluminium). Besides, while aluminium oxide is compact and strongly adherent to the metal, in the case of magnesium we are faced with a non-homogeneous and porous oxide which does not have remarkable properties of corrosion resistance. This scarce resistance was one of the main obstacles to the widespread use of magnesium. During the first world war a system of alloys had been developed for a range of possible applications in the young aeronautical industry, but after a short while it was necessary to cut down on the use of magnesium alloys on account of the scarce resistance to corrosion attacks, even if just in humid conditions. Further studies led to the change of the composition of these alloys with the addition of a reduced quantity of manganese, capable of forming, with the iron present in the alloy, less active intermetallic compounds, from the standpoint of the corrosion of the free iron.
Magnesium alloys, just like aluminium alloys, are classified to begin with according to their final use. Foundry alloys are therefore distinguished from those for plastic machining, the former being particularly appropriate for casting processes and the latter used for the production of extrusions and rolled products.
A second distinction is made between work hardening alloys and heat treatment alloys, since with some elements magnesium forms compounds with a precipitation hardening process similar to the analogous process found in heat treatment aluminium alloys. Even in this system the precipitation hardening process is possible: the alloy, having formed a solid solution at high temperature, is hardened and aged at room temperature (T6 temper). In both cases, the hardened solid solution is unstable and a precipitation hardening process develops, with the initial presence of areas with extremely high coherence with the lattice of the solid solution and the successive transformation in precipitates with increasingly low coherence right up to the balanced hyper-aged structure.
Magnesium alloys: main alloying elements and their effects
Aluminium: Almost all commercial magnesium alloys contain aluminium; Figure 1 shows the Al-Mg state diagram. Al and Mg are completely soluble at the liquid state and at 437 °C a eutectic reaction occurs with three stages, liquid, a solid solution of Al in Mg and b intermetallic with roughly 40% of aluminium. Commercial alloys almost never have a concentration of aluminium above 10% to limit the presence of the b which reduces the tool machinability and deformability of semis.
Manganese: the presence of manganese in magnesium alloys is necessary to lower the amount of iron in the liquid leading to the formation of hardly soluble Al-Mn-Fe compounds.
Silicon: adding silicon increases creep resistance.
Zinc: zinc is added to Mg-Al alloys to increase their tensile properties
Zirconium: zirconium is added to all families of Mg alloys to exploit its properties of grain refiner and nucleant in the solidification phase
Unusual alloying elements: a significant increase in the machinability of magnesium alloys is obtained with small additions of uncommon elements such as Thorium and Rare Earths.
Magnesium alloys: classification according to ASTM B275
Magnesium alloys are classified with an alpha-numerical system defined by the ASTM B275 norm, specifically the alloying element is identified by a capital letter. In the case of binary alloys, two letters identify the main alloying agent (first letter) and the second in percentage terms (second letter), then a whole number with one or two digits appears which represents the percentage of the alloying element rounded off to a whole number. Table 3 shows the classification scheme for the main alloying elements.
Magnesium wrought alloys
Table 4 shows for some magnesium wrought alloys the percentage of the main alloying elements and the tensile properties of semis in different physical states.
Foundry magnesium alloys
Foundry technologies currently represent the main share of consumption of magnesium alloys, especially die casting is unanimously forecast as the foundry technology with the greatest development. Table 5 shows the tensile properties of some alloys for different foundry techniques.
Production of primary metallic magnesium and its main uses
In the past few years the production of primary metallic magnesium took place in only seven countries and added up to roughly one million tons. Table 6 shows the amounts produced in 2015 and 2016 by the first 5 producing countries. It is easy to note how the Chinese production makes up about 80% of the total amount. Besides the primary metal, it is also necessary to consider as part of consumption even a significant share of secondary metal. Magnesium, like aluminium, may be recycled practically without any loss in its properties, so it may be used as an alternative to primary metal in almost all types of processes, with a net and significant energy gain with respect to primary metal. The production of secondary magnesium is estimated, in the same years considered in Table 6, to be equal o 200-250,000 tons per year, among them 125,000 tons produced in the United States. Growth estimates suggest an increase of about 3.5% per year with a primary production forecast for 2020 adding up to about 1.2 million tons.
The main final uses of magnesium during the past few years may be thus summarized:
1. Reducing agent in the production of several metals such as titanium
2. Alloying element for aluminium alloys where it is worth remembering the Al-Mn 1%-Mg 1% EN AW 3004 alloy used to produce beverage cans, which is one of the most widely produced aluminium alloys in the world. Al-Mg alloys with alloying element content ranging from about 2 to about 5% are used in products in the packaging segment other than cans, in nautical construction works and in car body parts. All in all, the annual share of magnesium used as an alloying element in aluminium alloys is equal to around 35% of the consumption of primary magnesium.
3. Magnesium alloys for foundry castings, especially made using die casting. In the USA structural applications including automotive applications account for roughly 20% of consumption.
Limiting our attention to the latter type of use, the consumption of magnesium is growing in the production of protection carters, lids, oil pump bodies, seat frames and steering wheels. Even in the recent past the development of large cast parts (doors, tailgates) and of aluminium-magnesium compound engine parts was technically very interesting. Recently, a growing segment which developed ultra-light applications is e-bikes, widely used in urban bike sharing systems.
A rapid analysis of the Italian market during the past few years shows a “golden age” in the use of magnesium castings for Italian cars. At the turn of the century the Fiat Group developed different magnesium details using especially high pressure die casting technologies. Large castings have been produced exploiting the high fluidity of the ultra-light metal up to the development of entire doors for Alfa Romeo models. During the following years the group chose different development strategies.
Regarding the current consumption of metallic magnesium in Italian foundries it should be noted that it adds up to about 8,000 tons per year (source: Assofond), that is, about 1% of the corresponding consumption of aluminium (800,000 tons per year). If the European automotive industry will develop the projects linked to magnesium, the figures deriving from new requests will be very different from the ones reported above. In the new scenario it will be important for Italian foundries to acquire rapidly a much deeper scientific and practical knowledge regarding the ultra-light metal.