Magnesium alloys have a number of unique physicochemical properties, the main of which are low density and high strength. The combination of these qualities in materials with the addition of magnesium allows the manufacture of products and structures with high strength characteristics and low weight.
Magnesium Characteristics
Industrial production and use of magnesium began relatively recently - only about 100 years ago. This metal has a small mass, as it has a relatively low density (1.74 g / cmαΆ), good stability in air, alkalis, gas media with fluorine content and in mineral oils.
Its melting point is 650 degrees. It is characterized by high chemical activity up to spontaneous combustion in air. The ultimate strength of pure magnesium is 190 MPa, the modulus of elasticity is 4,500 MPa, and the elongation is 18%. The metal has a high damping ability (effectively absorbs elastic vibrations), which provides it with excellent tolerance of shock loads and a decrease in sensitivity to resonance phenomena.
Other features of this element include good thermal conductivity, low ability to absorb thermal neutrons and interact with nuclear fuel. Due to the combination of these properties, magnesium is an ideal material for creating tight shells of high-temperature elements of nuclear reactors.
Magnesium is well alloyed with various metals and is one of the strong reducing agents, without which the process of metallothermy is impossible.
In its pure form, it is mainly used as an alloying additive in alloys with aluminum, titanium and some other chemical elements. In ferrous metallurgy, with the help of magnesium, deep desulfurization of steel and cast iron is carried out, and the properties of the latter are improved by means of spheroidization of graphite.
Magnesium and Alloy Supplements
Among the most common alloying additives used in magnesium-based alloys are elements such as aluminum, manganese and zinc. Using aluminum, the structure improves, the fluidity and strength of the material increase. The introduction of zinc also allows you to get more durable alloys with a reduced grain size. Using manganese or zirconium increases the corrosion resistance of magnesium alloys.
The addition of zinc and zirconium provides increased strength and ductility of metal mixtures. And the presence of certain rare earth elements, for example, neodymium, cerium, yttrium, etc., contributes to a significant increase in heat resistance and maximization of the mechanical properties of magnesium alloys.
To create ultralight materials with a density of 1.3 to 1.6 g / mαΆ lithium is introduced into the alloys. This additive allows to reduce their weight by half in comparison with aluminum metal mixtures. At the same time, their indicators of ductility, fluidity, elasticity and manufacturability go to a higher level.
Classification of Magnesium Alloys
Magnesium alloys are divided according to a number of criteria. It:
- by the method of processing - on foundry and deformable;
- according to the degree of sensitivity to heat treatment - to non-hardened and hardened by heat treatment;
- by properties and applications - for heat-resistant, high-strength and general purpose alloys ;
- according to the alloying system, there are several groups of non-hardened and hardened by heat treatment of deformable magnesium alloys.
Foundry alloys
This group includes alloys with the addition of magnesium, intended for the production of a variety of parts and elements by shaped casting. They have different mechanical properties, depending on which they are divided into three classes:
- medium strength;
- high strength;
- heat resistant.
According to the chemical composition, alloys are also divided into three groups:
- aluminum + magnesium + zinc;
- magnesium + zinc + zirconium;
- magnesium + rare earths + zirconium.
Foundry properties of alloys
The best casting properties among the products of these three groups are aluminum-magnesium alloys. They belong to the class of high-strength materials (up to 220 MPa), therefore they are the best option for the manufacture of engine parts for aircraft, automobiles and other equipment operating under mechanical and thermal loads.
To increase the strength characteristics, aluminum-magnesium alloys are alloyed with other elements. But the presence of iron and copper impurities is undesirable, since these elements have a negative effect on the weldability and corrosion resistance of alloys.
Cast magnesium alloys are prepared in various types of melting furnaces: in reflective furnaces, in crucible furnaces with gas, oil or electric heating, or in crucible induction plants.
To prevent burning during the melting process and during casting, special fluxes and additives are used. Castings are obtained by casting in sand, gypsum and shell molds, under pressure and using lost-wax models.
Deformable alloys
Compared to castings, wrought magnesium alloys are more durable, ductile and tough. They are used for the production of blanks by rolling, pressing and stamping. As a heat treatment of products, quenching is used at a temperature of 350-410 degrees, followed by arbitrary cooling without aging.
When heated, the plastic properties of such materials increase, so the processing of magnesium alloys is carried out by means of pressure and at high temperatures. Stamping is carried out at 280-480 degrees under the presses using closed dies. During cold rolling, frequent intermediate recrystallization annealings are carried out .
When welding magnesium alloys, the strength of the weld of the product can be reduced in the segments where the welding was performed, due to the sensitivity of such materials to overheating.
Fields of application of alloys with the addition of magnesium
Through the methods of casting, deformation and heat treatment of alloys , various semi-finished products are manufactured - ingots, plates, profiles, sheets, forgings, etc. These blanks are used for the production of elements and parts of modern technical devices, where the priority is played by the weight efficiency of structures (reduced weight) while maintaining their strength characteristics. Compared to aluminum, magnesium is 1.5 times lighter, and 4.5 times lighter with steel.
At present, the use of magnesium alloys is widely practiced in the aerospace, automotive, military and other industries, where their high cost (some brands contain quite expensive alloying elements) is justified from an economic point of view by the possibility of creating more durable, fast, powerful and safe equipment , which can work effectively in extreme conditions, including when exposed to high temperatures.
Due to the high electric potential, these alloys are the best material for creating protectors that provide electrochemical protection of steel structures, for example, automobile parts, underground structures, oil platforms, ships, etc., from corrosion processes that occur under the influence of moisture, fresh and marine water.
Alloys with the addition of magnesium have also found application in various radio engineering systems, where sound ducts of ultrasonic lines are made from them to delay electrical signals.
Conclusion
Modern industry places ever-increasing demands on materials with regard to their strength, wear resistance, corrosion resistance and processability. The use of magnesium alloys is one of the most promising areas, therefore, research related to the search for new properties of magnesium and the possibilities of its application do not stop.
Currently, the use of magnesium-based alloys in the creation of a variety of parts and structures allows to achieve a reduction in their weight by almost 30% and increase the tensile strength to 300 MPa, but, according to scientists, this is far from the limit for this unique metal.