Alloy of aluminum with iron in hydroacoustics. Aluminum and its alloys: all about this metal

Aluminum

Due to small specific gravity(2.70) aluminum and its alloys are extremely valuable structural materials, especially in aircraft construction. The high ductility of aluminum makes it possible to process it by pressure and stamping in cold and hot conditions.
The relatively low electrical resistivity makes it possible to use aluminum in the electrical industry as wires and other products. The high corrosion resistance of aluminum, due to its characteristic property to form oxide films on the surface, makes it possible to use aluminum as a cladding material.
Thanks to these valuable properties, aluminum and its alloys have been extremely widely used in all industries and in everyday life.
In Russia, technical aluminum of nine grades is produced, the chemical composition of which and the approximate purpose are given in Table. 16.

Impurities have a strong influence on the electrical, technological and corrosion properties of aluminum.
The main impurities in commercial aluminum are iron and silicon, which enter the metal during its production.
Already insignificant amounts of such impurities as iron, manganese, copper, zinc, magnesium and others sharply reduce the electrical conductivity (Fig. 1) and the thermal conductivity of technical aluminum.
Iron almost does not dissolve in aluminum: at the eutectic temperature (655°), the solubility of iron is 0.052%, with decreasing temperature, the solubility drops sharply (Fig. 2). Iron in aluminum is present as an independent AlsFe phase.

The presence of iron insoluble in aluminum reduces the corrosion resistance and significantly reduces the electrical conductivity and ductility (machinability), although we slightly increase the strength of aluminum.
With the simultaneous presence of silicon and iron in aluminum, a new phase is formed. In technical aluminum, the ratio of silicon and iron is such that a new ternary compound is formed.
The harmful effect of iron in many alloys can be weakened if manganese or chromium is added to aluminum, which contribute to the crystallization of a skeletal or equiaxed structure.
Silicon dissolves in aluminum at the eutectic temperature (577°) up to 1.65%. With decreasing temperature, the solubility of silicon decreases, and at room temperature, several hundredths of a percent of silicon is retained in the solution (Fig. 3). A change in the solubility of silicon in aluminum with decreasing temperature causes hardening processes, but they are so weak that practical value Dont Have.
Effect of silicon on mechanical properties aluminum is similar to the effect of iron.
Impurities of calcium, sodium and other elements present in technical aluminum in negligible amounts have practically no effect on the properties of aluminum.

Oxygen reacts vigorously with aluminum and forms a refractory oxide Al2O3, the presence of which in aluminum greatly reduces the mechanical properties and impairs metal forming.
nitrogen, carbon monoxide, carbon dioxide and sulfur dioxide at high temperatures react with aluminum and form refractory compounds.
The solubility of these gases in aluminum at the temperatures of the aluminum smelting process is low, but these gases are harmful because the metal is contaminated with oxides, sulfides and carbides, which increase the solubility of gases in molten aluminum.
At high temperatures, a relatively large amount of hydrogen dissolves in aluminum (at 300 ° 0.001 cm3 per 100 g of aluminum, at 500 ° - 0.0125 cm3 per 100 g, and at 850 ° - 2.15 cm3 per 100 g). When aluminum cools, part of the hydrogen is retained in it, which makes products made from such metal porous. Therefore, the presence of hydrogen or water vapor in the furnace atmosphere in which aluminum is melted is highly undesirable.
The presence of alloying additives in aluminum dramatically changes the solubility of hydrogen in it. Copper, silicon, and tin reduce the solubility of hydrogen in aluminum, while manganese, nickel, magnesium, iron, chromium, cerium, thorium, and titanium increase it. In the presence of 2.8% manganese at 600° or 6% magnesium at 500°, aluminum is capable of absorbing hydrogen.
The mechanical properties of technical aluminum depend on the degree of its deformation and the annealing temperature.
So, the tensile strength of soft aluminum is 7-10 kg/mm2, and deformed 15-20 kg/mm2, relative elongation is respectively 30-35 and 4-6%.
On fig. 4 and 5 show the dependence of the tensile strength and elongation of aluminum grades A1 and A2 on the degree of deformation and annealing temperature.

The main alloying additives in wrought and cast aluminum alloys are copper, magnesium, manganese, silicon, zinc, titanium, and in some cases tin, nickel, etc.
Additives introduced into aluminum during the production of alloys significantly increase the strength of the metal, but lower its ductility, electrical and thermal conductivity, and weaken the protective effects of the aluminum oxide film, since new phases formed disrupt the continuity of the aluminum oxide layer.
Copper with aluminum forms a solid solution. At the eutectic temperature (548°C), the solubility of copper is about 5.7%; as the temperature is lowered, the solubility decreases, reaching about 0.5% at 200°C.
In the state of solid solution, aluminum-copper alloy tolerates pressure treatment well. Upon slow cooling, the chemical compound СuAl2 begins to separate from these alloys. Rapid cooling, i.e., quenching, makes it possible to prevent the decomposition of the solid solution and to obtain a solution that is unstable at room temperature. In the process of decomposition of the solid solution, the hardening of the alloys occurs, i.e., their hardness and tensile strength increase.
The hardening process begins after quenching with prolonged holding at room temperature, but stronger hardening is obtained with artificial aging (holding alloys at 100-150°). For example. an aluminum alloy with 4% copper after quenching and tempering has a tensile strength of 35-37 kg / mm2 instead of 27 kg / mm2 in a freshly quenched state and 13 kg / mm2 in an annealed state.
At present, double aluminum-copper alloys are rarely used; the most widely used alloys containing, in addition to copper, magnesium, manganese, zinc and other elements.
Magnesium, like copper, forms a solid solution region with aluminum, which decreases with decreasing temperature due to a decrease in the solubility of magnesium in aluminum.
At 451°, the solubility of magnesium in aluminum is 14.9%, at 150°-2.95% (Fig. 6).

The decrease in the solubility of magnesium in aluminum with decreasing temperature makes it possible to apply quenching and subsequent hardening tempering; in Al-Mg alloys, the hardening phenomenon is not as pronounced as in Al-Cu alloys.
A significant strengthening effect is given by aluminum alloys with the addition of the Mg2Si compound. For example, the tensile strength of a heat-treated alloy containing 1.85% Mg2Si increases by more than three times.
Zinc with aluminum forms a large area of ​​solid solution β, which sharply narrows with decreasing temperature. However, the use of zinc as a hardener aluminum alloys not found practical application. A large strengthening effect is given by aluminum alloys with the addition of the MgZn2 compound. These additives will make it possible to obtain alloys after heat treatment with a tensile strength of up to 60 kg/mm2.
Manganese does not participate in the aging process of alloys such as duralumins, but increases their strength and corrosion resistance. In the presence of manganese, a manganese component appears in the structure of the alloy. In alloys that contain magnesium and silicon, manganese gives a hardening effect that is much superior to that of copper.
Heat treatment of multicomponent aluminum alloys makes it possible to obtain alloys with a high tensile strength (over 60 kg/mm2) with sufficiently strong elongation and other high mechanical and physical properties.

Aluminum alloys

Industrial aluminum alloys are divided into wrought and cast.
wrought alloys. As a deformable age-hardenable alloy, duralumin, discovered in 1909, has become the most widely used, the composition of which has since undergone only minor changes.
Duralumin is an alloy of at least five components, with copper, magnesium and manganese added as additives, while silicon and iron (about 0.5% each) are common impurities that enter the alloy with technical aluminum, which already contains these impurities.
In table. 17 provides some data on the chemical composition and mechanical properties of wrought aluminum alloys.

As follows from the data in Table. 17, with an increase in the percentage of alloying additives, the tensile strength increases and the ductility of the alloy decreases.
Duralumins are used mainly for the manufacture of sheets, profiles, wire, rods, pipes and rivets. Sheets are produced both unclad and clad with pure aluminium.
Alloys based on Al-Mg-Si, used for the production of forgings and stampings, are also widely used - a group of alloys referred to in GOSTs as alloys of AK grades. These alloys contain an increased amount of silicon compared to duralumin (up to 1.2%). In addition, in these alloys of some grades (AK2 and AK4), manganese is replaced by nickel.
High-strength alloys include AK8 alloy containing 3.9-4.8% copper, 0.4-0.8% magnesium, 0.4-1.0% manganese and 0.6-1.2% silicon. This alloy has a high tensile strength (up to 50 kg/mm2), but the alloy's tendency to intercrystalline corrosion limits its scope.
In terms of properties at room temperature, some alloys of the AK type (for example, AK2) are close to duralumin, but surpass it in resistance at high temperatures.
In recent years, the B95 alloy has begun to be introduced, which is subjected to artificial aging and has a tensile strength of over 65 kg / mm2, a hardness of 190 kg / mm2 and a relative elongation of about 7%.
Cast alloys. Among cast aluminum alloys, silumins are the most common - alloys with a high silicon content.
In addition to silumins, aluminum alloys with copper and magnesium are used, though much less frequently.
Cast aluminum alloys are alloyed with more additives than wrought alloys.
The content of additives in casting alloys is such that a eutectic is formed in the cast alloy, which, as a rule, increases fluidity, casting density and increases the resistance of the alloy to shrinkage stresses.
Alloys with a large amount of silicon usually have an acicular eutectic, but when a small amount of a modifier (sodium metal, a mixture of sodium fluoride and sodium chloride) is added to the liquid alloy, the structure of the alloy is significantly improved, since the eutectic becomes fine-grained.
Silumins lend themselves well to welding and almost do not crack from shrinkage stresses, which is explained by a small crystallization interval. A big drawback of silumins is the tendency to form oxide films (which increases the rejection of castings), as well as low mechanical strength and poor machinability. Like many casting alloys, silumin is very sensitive to iron contamination: even a slight increase in the iron content in silumin (by 0.1-0.2%) leads to a sharp decrease in relative elongation (by 2-3 times).
In table. 18 shows the composition and mechanical properties of some casting alloys.
As follows from the table, the mechanical properties of silumins are significantly lower than the mechanical properties of wrought alloys, which is a consequence of the coarser structure of silumins.
Aluminum-uranium alloys are relatively cheap, strong, and easy to process, and aluminum-clad alloys resist corrosion in water very well.

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Studying iron-aluminum alloy started after World War I. Work carried out in the Union, Germany, England and other countries showed that aluminum significantly increases the heat resistance of cast iron. It has also been found in research that these alloys have high carburization, scale resistance and good resistance in an oxidizing environment.

The scale formation rate at high temperatures depends on the properties of the oxide film formed on the metal surface. The denser and more uniform it is, the better it protects the surface from oxidation. The oxides that make up the film must not sublimate, must be refractory and must not form low-melting eutectics. The film must have low ionic conductivity. A heat-resistant alloy is considered to be one in which the loss with scale does not exceed 0.0002 - 0.0004 g / cm2 / hour. This condition applies to alloys of iron with chromium and silicon and remains valid for alloys of iron with aluminum.

It can be said that until now the most common alloy for castings subjected to thermal and chemical attack was ferchromite and similar iron-chromium alloys. Iron-silicon alloys are more often used as corrosion-resistant materials. Despite the fact that iron-aluminum alloys have been studied for a number of years, they have not found wide application. Most studies of these alloys were limited to laboratory determinations of mechanical, physical and other properties. When obtaining high-quality castings from these alloys, difficulties were encountered related to the high gas saturation of the metal, the formation of oxide films in the thickness of the metal, the waste of aluminum during melting, the destruction of castings at normal temperatures, etc., which forced the researchers not only to stop working, but also to come to the conclusion that such alloys cannot be applied in practice. Alloys containing 16 - 20% A1 and 3% C were studied most fully. It is known from the literature that such alloys, called "chugal" (cast iron + aluminum), began to be smelted in the former Soviet Union.

It can be said that, despite the exceptional properties iron-aluminum alloy, it was nowhere (since this can be judged from the literature) was not produced in large quantities. However, both iron-aluminum alloys and pyroferal needed further development of a technology for the production of castings, which could provide high quality products with minimal production costs. At the request of the inventors, one of the authors, Z. Eminger, with his working team developed a technology for smelting pyroferal, which makes it possible to establish the production of castings on a large scale. The team obtained new data on this alloy, on the basis of which the technology for its production was developed.

A rare metal so often rises into the air, is involved in the construction of houses, cars and sea ​​vessels like aluminum. It would seem - not the most durable, not the most resistant, rather soft ... What is so special about aluminum, thanks to which it is called the "metal of the future"?

Aluminum undoubtedly has several advantages that are hard to argue with:

Ease;
- prevalence - aluminum is the most common metal on planet Earth;
- ease of processing;

Even aluminum does not emit harmful substances when heated and conducts heat well. But the most important thing is to add a little to pure aluminum, just a few tenths of another element, and .... voila! You get material with diametrically opposed physical and chemical properties. Some aluminum-based alloys are so strong that at temperatures up to -200 degrees Celsius they are comparable to titanium and steel!

Obtaining and classifying aluminum alloys

The process of obtaining aluminum alloys is called alloying. However, doping is rather not one, but several interrelated processes. Its essence lies in the fact that auxiliary (alloying) elements are introduced into molten aluminum in an amount from several tenths to several thousandths of a percent.

The proportion of excipients directly depends on the result to be obtained. It is important to bear in mind that aluminum usually already contains iron and silicon. Both elements are not better side affect the quality of the future alloy: they reduce its resistance to corrosion, electrical conductivity and ductility.

Due to the fact that aluminum and aluminum alloys are used in strategically important areas, they are subject to mandatory state certification and labeling. In Russia, the quality of alloys is determined on the basis of two GOSTs: No. 4784-97 and No. 1583-93.

Aluminum alloys can be classified in several different ways. According to the type of auxiliary (alloying) elements, alloys are:

With the addition of additives (individual elements - zinc, magnesium, manganese, chromium, silicon, lithium, etc.);

With the addition of intermetallides (compounds of several metals - magnesium + silicon, copper + magnesium, lithium + magnesium, lithium + copper, etc.).

Depending on the chosen method of further metalworking, they are divided into:

Deformable aluminum alloys (the alloy does not turn into a liquid, but simply becomes very plastic) - it is convenient to stamp them, subject them to forging, rolling, extrusion, pressing. To achieve greater strength, some of the alloys are processed at elevated temperatures (annealing, hardening and aging), while others are processed under pressure. As a result, such aluminum blanks as sheets, profiles, pipes, products of more complex shapes, etc. are obtained.

Cast aluminum alloys (the alloy comes into production in a very liquid state so that it can be easily poured into any form) - such alloys are easy to cut, they are cast shaped (obtained under pressure) and molded products.

All aluminum-based alloys can also be divided according to the degree of strength into:

Heavy-duty (from 480 MPa);
- medium strength (from 300 - 480 MPa);
- low-strength (up to 300 MPa);

Alloys resistant to high temperatures and corrosion are classified separately.

In order to make alloy products easy to distinguish, each alloy is assigned its own number, consisting of letters and numbers. This number indicates the brand of aluminum alloy. A letter or several letters are placed at the beginning of the brand name, they indicate the composition of the alloy. Then comes the digital serial number of the alloy. The letter at the end shows how the alloy was processed and in what form it is at the moment.

Let's analyze the principle of marking on the example of D16P alloy. The first letter in the brand "D" means duralumin, i.e. an alloy of aluminum with copper and magnesium. "16" - serial number of the alloy. "P" - semi-work-hardened, that is, the alloy has been cold-worked to a strength value half that of the maximum.

The production of aluminum alloys and their application vary greatly depending on the type and brand. Each alloy has its own, very specific set of physical and mechanical properties. Among these properties there are those on which the further fate of the alloy depends - where it will go from the factory: to the air base, to the construction site and to the manufacturing workshop kitchen utensils. These properties are as follows: strength level, corrosion resistance, density, ductility, electrical and thermal conductivity.

Basic properties of various aluminum alloys

Let's look at the main aluminum-based alloys in terms of their acquired properties.

An alloy of copper and aluminum can be of several types - "pure", in which the main active elements are Al and Cu, "copper-magnesium", in which, in addition to copper and aluminum, a certain proportion is occupied by magic and "copper-manganese" alloyed with manganese. Such alloys are often also referred to as duralumin and are easy to cut and "spot" weld.

A characteristic feature of duralumins is that aluminum is taken for them with impurities of iron and silicon. As we have already said, usually the presence of these elements degrades the quality of the alloy, but this case is an exception. Iron during repeated heat treatment of the alloy increases its heat resistance, and silicon acts as a catalyst in the process of "aging" of duralumins. In turn, magnesium and manganese as alloying elements make the alloy much stronger.

An alloy of aluminum and magnesium has different strength and ductility, depending on the amount of magnesium. The less magnesium, the lower the strength of the product from such an alloy and the higher the resistance to corrosion. An increase in the magnesium content by 1% leads to an increase in strength up to 30,000 Pa. On average, alloys based on magnesium and aluminum contain up to 6% of the first. Why not more? If there is too much magnesium in the alloy, the product from it will quickly become rusty, and in addition, such products have an unstable structure, they can crack, etc.

Heat treatment of magnesium alloys with aluminum is not carried out, since it is ineffective and does not give the necessary effect of strength enhancement.

An alloy of aluminum with zinc and magnesium is considered the most durable of all aluminum alloys known today. Its strength is comparable to titanium! During heat treatment, most of the zinc dissolves, which is what makes this alloy so strong. True, it is impossible to use products from such alloys in the electrical industry, they are not resistant to stress corrosion. You can slightly increase the corrosion resistance if you add copper to the composition, but the indicator will still remain unsatisfactory.

Aluminum silicon alloy is the most common alloy in the foundry industry. Since silicon readily dissolves in aluminum when heated, the resulting molten composition is excellently suited for mold and shaped castings. Finished goods are relatively easy to cut and have a high density.

An alloy of aluminum with iron, as well as alloys of aluminum with nickel, is practically never found "live". Iron is added solely as an auxiliary element so that the casting alloy can easily come off the mold walls. Nickel, in turn, is best known in the manufacture of magnets and is present as one of the elements in the aluminum-nickel-iron alloy.

An alloy of titanium and aluminum is also not found in its pure form and is used only to increase the strength of products. For the same purpose, welding of steel and aluminum alloys is carried out.

INTRODUCTION

Among metals, aluminum is the most abundant in nature. practical use- the second (after iron). Aluminum is a chemical element that is in the third group of the periodic system of D.I. Mendeleev. Aluminum atomic number 13, atomic mass 26.98, melting point 660 °C, density 2.7 g/cm 3, has no polymorphic transformations, has a lattice of a face-centered cube with a period A = 0,4041 nm.

Aluminum differs from other metals in its low density, high plastic and corrosion-resistant properties, high thermal and electrical conductivity, and reflectivity.

Thanks to these properties, aluminum is used in almost all industries - aviation, construction, chemical, etc.

Aluminum is a corrosion resistant metal. The dense film of oxide Al 2 O 3 formed on its surface has very good adhesion to metal, is poorly permeable to all gases and protects aluminum from further oxidation and corrosion in atmospheric conditions, water and other media. Aluminum is resistant to concentrated nitric acid and some organic acids (citric, acetic, etc.). Mineral acids (hydrochloric, hydrofluoric) and alkalis destroy the oxide film.

Permanent impurities (Fe, Si, Ti, Mn, Cu Zn, Cr) reduce the physicochemical characteristics and plasticity of aluminum. Depending on the content of impurities, there are grades of primary aluminum A999, A995, A99, A97, A95.

Iron and silicon are the main unavoidable impurities that enter aluminum during its production. Their presence adversely affects the properties of aluminum. Iron it is practically insoluble in aluminum, therefore, even at its smallest content, a fragile chemical compound FeAl 3 is formed. Crystallizing in the form of needles serving as cuts in the metal, it reduces the plastic properties of aluminum. Iron reduces the corrosion resistance of aluminum due to big difference electrochemical potentials of the Al and FeAlg phases, the occurrence of microgalvanic pairs at the boundary of these phases, and the development of intergranular corrosion.

Silicon does not form chemical compounds with aluminum and is present in aluminum alloys in elemental form. The solubility of silicon in aluminum at room temperature does not exceed 0.05%. Even at small amounts of silicon, inclusions of the Al-f Si eutectic are formed in the aluminum structure. Silicon crystals are similar in properties to chemical compounds, have high hardness (HB 800) and brittleness. The main negative effect of silicon impurities is expressed in the deterioration of the casting properties of commercial aluminum. Silicon sharply reduces the solidus temperature, increases the crystallization interval (at = t n -- -- t0), and hence, reduces the fluidity and increases the tendency of the alloy to cracking.

Industrial aluminum contains iron and silicon at the same time, so it can be considered as a ternary alloy of the Al--Fe--Si system. In this case, two ternary chemical compounds can be formed in aluminum: a (A1--Fe--Si) and J (A1--Fe--Si), which are practically insoluble in Al. The appearance in the structure of technical aluminum of a skeletal, crab-like phase A(A1--Fe--Si) and coarse lamellar phase (3 (A1--Fe--Si) dramatically changes its properties.

Depending on the content of impurities, aluminum is divided into grades: technical, high purity and high purity.

Table 1 shows some brands, the chemical composition of wrought aluminum (intended for the production of semi-finished products by hot or cold deformation). Primary aluminum supplied in the form of ingots and ingots is subject to the GOST 11069-74 standard, examples of grade designation of which are given in Table 2. The mechanical properties of aluminum depend on its purity and condition. An increase in the content of impurities and plastic deformation increase the strength and hardness of aluminum (Table 3).

Table 1

Aluminum wrought

table 2

Aluminum primary

Table 3

Mechanical properties of aluminum various
annealed purity

Purity, %

Aluminum is characterized by high technological properties. Any semi-finished products of various dimensions can be made from it. Due to the high plasticity of aluminum semi-finished products, it is easy to deform without significant heating. Welding can be carried out by almost all methods, including fusion welding. Machinability due to the high toughness of aluminum is poor.

It is used in the electrical industry and heat exchangers. The high reflectivity of aluminum is used for the production of mirrors, powerful reflectors. Aluminum practically does not interact with nitric acid, organic acids and food products. It is used to make containers for transportation. food products, home stuff. Sheet aluminum is widely used as a packaging material. The use of aluminum in construction and transport has grown significantly.

MAIN PART

1. Classification of aluminum alloys

Depending on the method of production, industrial aluminum alloys are divided into sintered, cast and wrought (Fig. 1).

Cast alloys undergo eutectic transformation, while wrought alloys do not. The latter, in turn, are thermally non-hardened (alloys in which there are no phase transformations in the solid state) and deformable, thermally hardened (alloys hardened by quenching and aging).

Aluminum alloys are usually alloyed with Cu, Mg, Si, Mn, Zn, less often with Li, Ni, Ti.

2. Deformable aluminum alloys, not hardened by heat treatment

This group of alloys includes commercial aluminum and non-hardened weldable corrosion-resistant alloys (alloys of aluminum with manganese and magnesium). AMts alloys belong to the Al – Mn system (Fig. 2).

Rice. 1. State diagram "aluminum - alloying element":

1 - deformable, thermally non-hardening alloys;
2 - deformable, thermally hardened alloys.

Rice. 2. Diagram of the state "aluminum - manganese":

Rice. 3. Microstructure of the AMts alloy

Rice. 6. Microstructure of duralumin after:

a) quenching in water from a temperature of T 2 ;
b) hardening and artificial aging at T 3 (on the right - a schematic representation)

The structure of the AMts alloy consists of an a-solid solution of manganese in aluminum and secondary precipitates of the MnAl 6 phase (Fig. 3). In the presence of iron, instead of MnAl 6, a complex phase (MnFe)Al 6 is formed, which is practically insoluble in aluminum; therefore, the AMts alloy is not strengthened by heat treatment.

The composition of these alloys has very narrow limits: 1 - 1.7% Mn; 0.05 - 0.20% Cu; copper is added to reduce pitting corrosion.

It is allowed up to 0.6 - 0.7% Fe and 0.6 - 0.7% Si, which leads to some hardening of the alloys without a significant loss of corrosion resistance.

As the temperature decreases, the strength increases rapidly. Therefore, alloys of this group have found wide application in cryogenic technology.

AMg (magnalium) alloys belong to the A1 – Mg system (Fig. 4). Magnesium forms an a-solid solution with aluminum, and in the concentration range from 1.4 to 17.4% Mg, a secondary b-phase (MgAl) is released, but alloys containing up to 7% Mg give very little hardening during heat treatment, so they are strengthened by plastic deformation - work hardening.

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Aluminum with iron is capable of producing solid solutions, intermetallic compounds (Fe 2 Al 4 -62.93% Al; Fe 2 Al 5 - 54.71% Al; FeAl 2 -49.13% Al; FeAl -32.57% Al, etc.) and eutectics (Al + FeAl 3, T melt = 654 ° C, iron content in metal 1.8%). The solubility of iron in the solid state is limited to 0.053% at the eutectic temperature. The solubility of aluminum in iron is about 32%, i.e., 600 times higher. During solidification in the structure of aluminum and iron alloys, crystals of the compound FeAl 5 (59.18%) precipitate.

Welding conditions are characterized by the appearance of FeAl 3 and Fe 2 Al 5 . They have a low limit of temporary resistance (15-17 MPa). The hardness of Fe 2 Al 5 , FeAl 3 and FeAl 2 lies in the range μv = 9600-11500 MPa. With an increase in the iron content and with an increase in temperature, the hardness decreases. For Fe 3 Al μv = 2700 MPa. The softening of FeAl 3 and Fe 2 Al 5 begins at a temperature of 0.45 T pl. Fe 2 Al 5 is characterized by an anomalously high electrical resistivity.

Intermetallics are chemically resistant. Subsequent heat treatment compounds can only lead to an increase in the length of the intermetallic zone. There are three characteristic areas in the compound: iron (steel) - intermetallic zone - aluminum (aluminum alloy). The mechanical properties of the compounds depend on the intermediate zone - its composition. the amount of intermetallic compounds, their shape, length, nature of location and continuity.

A chemically resistant refractory oxide film is formed on aluminum (Al 2 O 3 has Tmelt = 2047 ° C), which during fusion welding can lead to a defect in the form of inclusions of this film in the weld metal. The use of fluxes does not give positive results: fluxes for aluminum welding are fusible, fluid, poorly wet steel; fluxes for steel actively react with molten aluminum.

The nature of diffusion processes during solid-phase welding of aluminum with iron and steel at the initial stage of interaction and further differs. It is shown that diffusion of iron into aluminum takes place in the initial period. As a result, a layer consisting of a mixture of FeAl 3 + Fe 2 Al 5 phases is formed in the boundary zone. Further, at a temperature corresponding to the recrystallization of steel, an intense diffusion of aluminum into steel is observed. The speed of this process depends on chemical composition material of contacting blanks and heating conditions. For solid-phase interaction under certain temperature-time conditions of welding, there may be no continuous front of intermetallic compounds.

Reaction diffusion in the aluminum-iron system is observed at temperatures >400°C. The growth of the intermetallic layer obeys a parabolic law: y 2 = 2k 1 τ, where k 1 is a value proportional to the diffusion coefficient of aluminum through the layer.

Alloying materials aluminum billet Si, Mn and other elements, and steel - V, Ti, Si and Ni leads to an increase in the activation energy of the reaction diffusion. Their influence is associated with the difficulty of the formation of nuclei in the intermediate phase. The opposite effect is exerted by C and Mn in steel. An increased content within certain limits in steel of free oxygen and nitrogen leads to an increase in the temperature of the onset of the formation of intermetallic compounds. The appearance of an intermetallic layer for each temperature begins after a certain critical time, i.e., there is a latent period (τ 0), after which there is an intensive formation of intermetallic compounds. Its dependence on temperature has the form

τ 0 = 6.0 10 -13 exp (192.3/RT).

When the process is carried out in a solid-liquid state (with aluminum melting), Fe 2 Al 5 is formed from the iron (steel) side, and FeAl 3 from the aluminum side.

When welding chromium-nickel stainless steels with aluminum alloys, the intermetallic layer has a more complex character and Cr and Ni are involved in its formation.

The bimetallic compound has satisfactory mechanical properties only up to the formation of a continuous layer of the intermetallic phase. The operability of the connection is maintained under a certain temperature-time effect. The upper temperature threshold for bimetallic products from the considered combination of materials is 500-520 °C.

The main ways to obtain a workable connection of aluminum alloys with steels are as follows:

limiting the length of the layer of intermetallic interlayers. High strength can be obtained with a zone width of 10 µm;

alloying aluminum with elements that hinder the formation of an intermediate phase, primarily silicon, as well as the use of steel with a low content of carbon and manganese, which makes it possible to raise the temperature of formation of intermetallic compounds by 40-60 ° C above the temperature of steel recrystallization. This way can be successfully used in solid phase welding.

Differences in plastic properties and hardness make it possible to successfully apply wedge-press welding in the manufacture of bimetallic rods, tubular adapters, etc. for the considered combination of materials. Provide measures to protect steel from oxidation. High mechanical properties of the connection are obtained by using zinc coatings on the surface of the wedge.

Diffusion welding carried out at a temperature of 425-495 ° C (time up to 10 minutes, welding pressure 210-310 Pa). The surface of the steel billet is covered with a layer of Ni and W. The latter is capable of forming a eutectic with aluminum. In this case, the welding temperature must be below the eutectic formation temperature.

ultrasonic welding allows you to get lap, spot and seam joints on thin workpieces. Vibrations are supplied from the aluminum side. The thickness of aluminum is limited to a value of the order of 1.0-1.25 mm.

Friction welding allows to obtain high-quality joints, equal in strength to aluminum alloy in the annealed state. During the welding process, the temperature in the joint quickly reaches its maximum and then stabilizes. When welding austenitic steel 12X18H10T with AD1, the duration of the latent period for a temperature of 660 ° C, which is close to that developed at the joint, is 100-120 s. Welding time ~ 10 s. Therefore, the intermetallic phase does not have time to form in any significant quantities. On the other hand, continuously running sediment (mainly due to aluminum) contributes to obtaining a seam clean from intermetallic compounds (total sediment ~ 14 mm).

In the presence of magnesium in an aluminum alloy, the duration of the latent period is sharply reduced. Therefore, aluminum magnesium-containing alloys are welded in modes that ensure the temperature at the joint is not higher than 500 ° C.

Explosion welding such materials require the use of a barrier layer, which is applied to the steel billet. In this way laminated sheets and tapes are obtained.

Has been widely used rolling welding, which allows you to regulate the heating temperature of the connection zone. In this way, in industrial scale welded 12X18H10T + AMg6; armco-iron + AMg5 and other combinations.

At fusion welding and soldering the processes of nucleation and growth of the intermetallic layer are much more intense. When forming a joint, it is essential to wet the solid steel with aluminum. To improve wetting and thereby reduce the contact time of the melt with steel, alloying the seam and applying coatings to the surface of the steel billet (zinc, zinc-nickel - as the most technologically advanced and inexpensive) are resorted to. After wetting, the process of iron dissolution in liquid aluminum takes place. It has been established that the Fe 2 Al 5 phase formed during dissolution can pass into the melt in the form of crystals and dissolve. Moreover, the growth rate of the intermediate layer is greater than the dissolution rate, which makes it impossible to obtain a compound without intermetallic interlayers. Reducing the negative effect of this factor can be achieved by increasing the volume of the aluminum melt (preliminary cutting of the edge), optimizing the regime in order to limit the temperature of the melt, alloying the pool through the filler material with elements that affect the growth rate and composition of the intermetallic layer. The introduction of Si (4-5%), Zn (6.5-7%), Ni (3-3.5%) into the weld makes it possible to reduce the thickness of the intermetallic layer and obtain joints with a strength of 300-320 MPa.

Taking into account the noted features, two variants of the technology of joining by melting aluminum with steel have found application in practice: 1) welding-brazing with preliminary coating on the steel edge using argon-arc devices with a non-consumable electrode and 2) automatic arc welding with a consumable electrode along the AN-A1 flux layer. Coatings (zinc, aluminum) have a thickness of 30-40 microns and are applied by electroplating or aluminizing. When welding, it is necessary to conduct an arc along the edge of the aluminum sheet at a distance of 1-2 microns from the joint line and observe a certain speed (at low speeds, overheating and burnout of the coatings are observed, at high speeds - non-fusion).

In submerged arc welding, the role of the flux is reduced to improving wettability and inhibiting the formation of intermetallic compounds. It is necessary to prevent the direct impact of the arc on the edge of the steel, and the cutting of the edge on the steel should be done as close as possible to the outline of the bath profile. Thicknesses of 15-30 mm are welded in this way.