Excellent mechanical properties, great lightweight qualities, new innovative machining technologies, advantages for the environment: these are the keys that are leading to a faster and faster diffusion in industrial ambit.
Magnesium can be considered the metal of the future, especially in the industrial world, for the relevant advantages that it can offer in terms of environmental impact and of reduction of the toxic emissions into the atmosphere in recycling processes. During the world wars of the last century it had been largely used in the military industry (and this, indirectly, confirms its unique properties), but just that use has actually prevented its diffusion in civil ambit. Too many were, in fact, the secrets connected with magnesium and the constraints imposed by Powers that hindered its distribution on a large scale; without neglecting that the productive techniques of that age were not perfected like the current ones, and that fact made the machining of that metal complex.
Today, after over 100 years since the first uses in industrial field, magnesium is “rediscovered” and, thanks to the technological progresses, its diffusion finally proceeds without anymore the obstacles and the hindrances of the past. Certainly, there are still stereotypes to be dispelled, and it is necessary that technicians and designers start studying more in-depth the characteristics of this element and of its alloys. But we are certain that magnesium will be protagonist of the industry of the future and that its diffusion will grant enormous benefits to the mechanical industry, especially referring to some specific applications.
Lightweight and inexhaustible
Magnesium is the absolutely lightest metal, with density corresponding to 1.74 kg/cdm, the lowest in comparison with any metal material for structural uses. It is, for instance, lighter by 30% than aluminium and by 70% than steel; besides, keeping the volume unchanged, it has inferior transport costs just because it is lighter.
In addition to lightweight and inexhaustibility, being among the most widespread elements on the terrestrial surface, it features other interesting characteristics: the resistance to the dynamic and static force; the similar yield strength to aluminium alloys’; the excellent capacity of damping vibrations, of absorbing shocks, of shielding electromagnetic waves, of dissipating heat (higher thermal conductivity than plastic). Besides, it is 100% recyclable, without any degradation of physical properties, and with lower recycling energy costs than other metals. The latter characteristic reduces the environmental impact related to its production, making it much more “ecological” than other materials.
In the last 5 years the use of magnesium and of its alloys in the world industry has tripled, confirming the interest in this new material, deemed “modern” because ecologic and with notable mechanical, chemical and physical properties. Moreover, as already said, the systems to machine it have changed and are totally different if compared with those used dozens of years ago: this has opened – and will open – new applicative horizons. It is in fact well known that industries of international level, like Alenia, KTM, Ducati, Ferrari, Volkswagen, etc., already use magnesium to implement items of all kinds: missile components, gears, chassis, steering wheels, seat structures etc, ; but already several other companies have mobilized to manufacture “cases” for computers, mobile phones and cameras, accessories for bikes, tennis rackets, etc. This demonstrates that the application potentialities, for a skilled and well informed designer, can be endless.
|DANGEROUS MATERIAL? MYTH TO DEBUNK
Some sector technicians, subcontractors and experts, seem to be puzzled when it comes to magnesium: “It is flammable, dangerous and difficult to work. No, we do not treat it”.
It is a common and widespread prejudice, born in the aftermath of some serious episodes occurred some decades ago, after which magnesium was “abandoned” because branded as flammable, explosive and difficult to treat.
Actually, it is now a long time since those events, and the exponential rise of the magnesium use over these years, especially in strategic sectors such as aviation, missile and military ones, is a concrete witness of the importance and of the rediscovery of this metal, anyway widely used by the German automotive industry before and after the second war world with excellent results.
Certainly, magnesium in pure form is highly flammable, especially if in powder, and reacts rapidly and exothermically in contact with air or water. On the other hand, almost all metal materials are flammable, when they are in powder, we should just consider aluminium and zinc, widely used in pyrotechnics. Magnesium is ignited with slightly greater ease than aluminium, that’s all.
It may ignite also in the form of swarf or chip, but only an incompetent technician might start machining a metal without knowing the correct parameters of cutting, conservation and disposal of swarfs. In any case, the progresses of machining technologies and deeper knowledge of the material allow minimizing productive risks, also considering that pure magnesium is never machined but only alloys, and that modern magnesium alloys have much higher thermal resistance than in the past. Presently, the factors to be kept under control, since they can give rise to combustion during the productive phases of a magnesium component with stock removal machines, are: too low feed speeds in the machining operations with machine tools; use of inadequate tools; presence of sparks owing to collisions between chip and tool at high temperatures (480°C). To avoid risks, it is necessary to provide for big feeds in order to produce chips with big thickness, to use sharp tools with large lower clearance angles, and mineral oils to refrigerate the process; besides, it is necessary to avoid the accumulation of chips and fine dusts in the machining area or, in general, in bins or containers. In any case, if a fire should break, you must not use water, water or foam extinguishers, but powders of pig iron or of extinguishing metal.
Concerning the use of the components made of magnesium, no safety problems exist, as witnessed by the millions of Volkswagen Maggiolino and Porsche cars that have impeccably functioned for years and years, by the massive use on utility cars (steering wheels, seats, panels, head covers), on French TGV trains, on helicopters and aircrafts, for which the safety standards demanded are particularly severe.
How is it machined?
Magnesium can be machined more easily than other materials, since it can grant low resistance to cutting tools, then possibility of being machined at high cutting speeds, excellent weldability without resistance loss (in controlled atmosphere), good resistance to corrosion (with binding agents) and to ageing.
To implement a magnesium component, we can use the same processes adopted for the other metals: sand and shell casting, die casting (also for complex components with subtle wall), chip machining, extrusion, forging (post-extrusion plastic deformation), welding (in controlled atmosphere) and assembly. These features make it interesting for industry, despite some precautions to be observed since it is a more flammable material than others.
The most used technological process is die casting, especially for the high productivity, the high precision, quality and surface finishing, the possibility of obtaining fine micro-structures, also on thin walls and in complex geometries, the high casting fluidity and the possible speed increments in the casting process up to 50%. As a matter of fact, thanks to the good dimensional stability, the shrinkage is constant and the shrinkage stress is almost absent. Besides, it is possible to use steel dies with extension of their service life, and with energy saving during the process owing to the low thermal dispersion.
Besides standard foundry processes, magnesium components can also be achieved through other technologies, some of which really innovative, like semi-solid casting and/or injection systems (Thixomolding, Rheocasting) that are rapidly and constantly developing. “Thixomolding”, in particular, is an injection moulding system in which magnesium is at the semi-solid state: it allows “processing” spherical balls or powders and manufacturing metal components standing out for high mechanical characteristics, with detailed surface aspects and very subtle thicknesses.
Finally, concerning the stock removal machining, magnesium and its alloys can be machined also at high cutting speeds and with big chip thicknesses, for a reduced tool wear, with the possibility of achieving excellent surface finishing and roughness up to 0.1 micron.
Used in alloys
Like aluminium and other metals, magnesium is used in different alloys in the various casting processes or for the production of billets for the extrusion and the hot forging.
The main binding agents are aluminium, which increases the mechanical resistance, zinc, which improves the elongation, manganese, which raises the resistance to corrosion, zirconium, which refines the grain, copper, which improves the fluidity, calcium, which decreases density.
In general, in die casting processes are used high purity alloys, called “High Purity”, that’s to say with low contents of impurity like iron (within 0.004%), nickel and copper. For most of the applications it is used the AZ91 alloy, ideal for die casting due to the excellent surface finishing of the produced goods.
Magnesium alloys have a specific weight of 1.75÷1.85 kg/cdm that in the case of special or “ultra-light” alloys (for instance magnesium-lithium) can decrease to 1.3 kg/cdm.
They are under development also particular alloys “reinforced” by other elements, which are included in the category of composite materials and are aimed at a rise of the modulus of elasticity and an increase of the resistance, both to corrosion and to creep.
The name of our element derives from Magnesia, one of the prefectures of ancient Thessaly, in Greece, in whose region were extracted numerous substances, chemically different but similar by consistency and colour, used by the ancient alchemists.
In 1755 the Scottish chemist and physicist Joseph Black recognized magnesium as element, in 1808 Sir Humphrey Davy, renowned English chemist, isolated it electrolytically, while in 1831 another chemist, the French Antoine Bussy, prepared it in “coherent” way.
The mechanical characteristics of magnesium are more modest in comparison with other materials. In particular, the tensile strength of traditional magnesium alloys is slightly lower than aluminium alloys’, and the modulus of elasticity is inferior by 36%. Besides, the coefficient of thermal expansion is higher and this is another aspect that can cause criticalities in certain cases. These “inconveniences” can be solved, at least for certain types of use, by redesigning the components, that’s to say increasing sections and optimizing geometries.
In the past magnesium had gained a questionable reputation in terms of resistance to corrosion and this has partly hindered its diffusion, but the development of state-of-the-art protective treatments and coatings, coupled with the availability of high-purity or new composition alloys, permits to overcome this weak point, at least for the majority of uses.
Another aspect worth considering is that traditional magnesium alloys are affected by a drastic deterioration of their mechanical characteristics already at (relatively) low temperatures, fact that reduces their application field. Today, however, are available alloys that far exceed this limitation and that result neatly superior also under other aspects. The cost of such alloys remarkably varies depending on the elements contained; for the alloys with largest utilization is slightly higher than most common aluminium alloys’, but this is compensated by the better workability, which results also in minor tool wear and relevant energy saving.
Magnesium is the element of the periodic table of elements whose symbol is Mg and whose atomic number is 12. It is an alkaline-earth metal, it is the eighth most abundant element, it constitutes about 2% of the Earth’s crust, and it is the third by abundance among the elements dissolved in seawater. Its availability, then, is almost unlimited. It does not exist at the free state in nature, but it is present with other elements in “chemical complexes” such as magnesite, dolomite, brucite, carnallite, and so on.
Its density is equal to 1.74 kg/dm³ and its melting point to 649.85 °C.
Presently, magnesium is mainly obtained through electrolysis of the seawater, where it is present in relevant percentage (about 0.12% in weight); it is worth reminding that, as well as in rocks and seawater, magnesium is present both in the vegetal and animal world, of which it constitutes one of the essential components.