Classification of aluminum alloys. Analysis of the results obtained State diagram aluminum magnesium

07.01.2024 -

All industrial compositions of aluminum and magnesium alloys in terms of magnesium content are in the region of the state diagram of the Al-Mg system, corresponding to the α solid solution. The concentration of the solid solution increases with increasing temperature, which makes it possible in principle to significantly strengthen Al-Mg alloys by applying heat treatment (hardening) to them.
In the cast state, aluminum alloys containing over 9% Mg have an α+β structure; The β phase, which is a brittle intermetallic compound, contains about 35-38% Mg.
According to the equilibrium phase diagram in alloys with 10% Mg, the β-phase is released from the solid solution due to a decrease in the solubility of magnesium in aluminum with decreasing temperature (Fig. 22). Under real solidification conditions, due to intense microliquation processes and insufficient speed of diffusion processes, the β-phase is released from the mother liquor at 450° C in the form of degenerate eutectic. This was proven by experiments (the hardening alloy was quenched at different temperatures). The amount of β-phase formed as a result of the precipitation of α from the solid solution depends on the magnesium content in the alloy. According to available data, when casting in a sand mold, up to 7% is retained in solid solution.

The mechanism of β-phase release depending on the duration of aging is not well understood. The following sequence of the aging process is allowed: “zones” enriched with magnesium, nonequilibrium β" - equilibrium β.
The existence of zones is confirmed only by measuring the electrical resistance of the alloys. The structure of the β" and β phases, which precipitate in the form of small plates, is very complex. These phases were studied by X-ray diffraction analysis.
The influence of the homogenization time H of the quenching medium on the aging process was studied in this work. The longer the homogenization time, the more evenly the magnesium is distributed across the cross section of the grain. When homogenized for 16 hours, subsequent aging leads to the formation of precipitates only in zones enriched in magnesium, i.e., near grain boundaries, and the dendritic structure of the alloy is clearly revealed. With a gradual increase in homogenization time, the distribution of precipitation over the cross section of grains after aging is leveled. However, even after heating for 160 hours, with a uniform distribution of secretions, individual areas with the outline of dendrites are detected. In the latter case, in contrast to the picture observed after homogenization for 16 hours, the areas near the grain boundaries are depleted in precipitates. In all cases, the discharge is in the form of needles.

In addition to the homogenization time, the formation of precipitates is influenced by the quenching conditions. When quenched in cold water, the β-phase is released along the grain boundaries in a continuous form during subsequent aging. Quenching in boiling water or hot oil produces, after aging, precipitation of the β-phase along the grain boundaries in the form of isolated inclusions.
In discussing and analyzing the results, it is recognized that residual dendritic segregation and depletion of vacancies in zones adjacent to grain boundaries have an important influence on the conditions and nature of β-phase precipitation. Vacancies accelerate the process of β-phase separation, since its formation is accompanied by an increase in volume.
Based on the metastable diagram of alloys of the Al-Mg system (Fig. 23), a diagram of the sequence of formation of the β-phase during aging of alloys with 10% Mg is proposed (Fig. 24). Along the grain boundaries, the processes of separation and sequential transformation proceed one stage faster, since the possibility of the formation of nuclei is greater here.

Precipitate-free areas along the grain boundaries are the weak point of castings, and therefore destruction occurs along the grain boundaries, especially in the second stage, during quenching in cold water, when the β-phase forms continuous chains. The strength properties of castings are reduced. Corrosion resistance deteriorates most strongly during the transformation β"→β (Fig. 25). It can be assumed that the corrosion resistance of alloys depends on the nature of the β-phase precipitation, which is clearly visible in Fig. 25. This is consistent with the fact that alloys hardened in cold water, have reduced corrosion resistance.
In table 12-14 show the compositions and properties of industrial alloys of the Al-Mg system.
Alloys of the aluminum - magnesium system containing up to 6% Mg are not strengthened by heat treatment. Solution hardening significantly improves the mechanical properties of alloys containing more than 9% Mg.

Among double aluminum-magnesium alloys, alloys with 10-12% Mg have the greatest strength with high ductility in the hardened state. With a further increase in magnesium content, the mechanical properties of the alloys decrease, since it is not possible to convert the excess β-phase, which causes the brittleness of the alloy, into a solid solution during heat treatment. Therefore, all industrial alloys of the Al-Mg system belong to the type of solid solutions with a magnesium content of no more than 13%.
In addition to magnesium, the AL13 alloy contains silicon and manganese. Silicon additives help improve the casting properties of the alloy due to an increase in the amount of double eutectic α+Mg2Si. The mechanical properties of the AL13 alloy with the introduction of 1% Si change slightly: the strength increases slightly, and the ductility slightly decreases.
Manganese is added to the AL13 alloy mainly to reduce the harmful effects of iron, which precipitates during crystallization in the form of needle-shaped and plate-shaped crystals and greatly reduces the ductility of the alloy. When manganese is introduced into an alloy, the compound MnAl6 is formed, in which iron dissolves. This connection has a compact skeletal or even equiaxial shape.
Impurities of iron, copper, zinc, and nickel negatively affect the corrosion resistance of AL13 alloy. With a silicon content of more than 0.8%, the corrosion resistance of the alloy also decreases, and with the addition of manganese it increases.
AL13 grade alloy is not strengthened by heat treatment and has low mechanical properties. Its advantage is its relatively high corrosion resistance compared, for example, with silumins, good weldability and (due to the presence of the Mg2Si compound in the structure) increased heat resistance.
AL13 grade alloy is used to produce parts that bear medium loads and operate in conditions of sea water and slightly alkaline liquids. The alloy is used for the manufacture of parts for marine shipbuilding, as well as for parts operating at elevated temperatures (up to 180-200° C).
Alloys (AL8, AL8M, AL27-1) with a high magnesium content (9-11%) in the hardened state have very high mechanical properties. However, the mechanical properties of alloys in samples cut directly from cast parts are very uneven; The main reason for the uneven properties is casting heterogeneity, detected in the form of shrinkage looseness and porosity, as well as oxide inclusions in massive parts of the casting.
A very major disadvantage of these alloys is their increased sensitivity to natural aging. It has been established that a content of more than 10% Mg in aluminum-magnesium alloys leads to embrittlement of hardened cast parts after long-term storage and during operation.
In table Figure 15 shows the change in the mechanical properties of alloys with different magnesium contents during long-term natural aging. The data presented indicate that with increasing magnesium content, the tendency to natural aging increases. This leads to an increase in the yield point, ultimate strength and a sharp decrease in ductility.
When testing samples of alloys aged for eleven years for intergranular corrosion, it was found that alloys containing less than 8.8% Mg are not sensitive to this type of corrosion, and with a higher magnesium content, all the studied alloys acquire a greater degree of corrosion under the influence of natural aging. prone to intergranular corrosion.
The average depth of focal corrosion lesions on the surface of samples tested according to the standard method by immersion for one day in a 3% NaCl solution with the addition of 1% HCl was: 0.11 mm - with a content of 8.8% Mg in the alloy, 0. 22 mm - at 11.5% Mg and 0.26 mm - at 13.5% Mg.
Aluminum-magnesium alloys AL27 and AL27-1 have the same content of main alloying components (magnesium, beryllium, titanium, zirconium); the content of iron and silicon impurities in the AL27-1 alloy should not exceed 0.05% each.

In table 16 shows the mechanical properties of an aluminum-magnesium alloy containing impurities of iron, silicon and magnesium.
The above data first of all shows that an alloy containing less than 9% magnesium (0.1% iron and silicon each) has relatively low mechanical properties (σв = 28.5 kgf/mm2; δ5 = 12.5%). Of the alloys studied, the alloy containing 10.5% Mg (σв = 38 kgf/mm2; δ5 = 26.5%) has the highest mechanical properties. With a magnesium content of 12.2%, the tensile strength is also at a high level (38.3 kgf/mm2), but the elongation is slightly lower (21%).
When the iron content in the AL8 alloy increases to 0.38% at the same silicon content (0.07%), no change in the tensile strength is observed, and the elongation decreases slightly. With an increase in silicon in this alloy to 0.22%, both the tensile strength (up to 33.7 kgf/mm2) and elongation (17.5%) decrease significantly. Increasing the silicon content to 0.34%), even with a low iron content (0.10%), significantly reduces the mechanical properties: tensile strength decreases to 29.5 kgf/mm2, and elongation to 13%. If, in addition, the iron content in this alloy is increased to 0.37%, then the mechanical properties will further decrease, but to a lesser extent than with increasing silicon content: the tensile strength will become 27.6 kgf/mm2, and the elongation will be 10.5% .
The reason for the adverse effect of even small amounts of silicon can obviously be considered to be the formation of the Mg2Si compound due to the high affinity of silicon for magnesium. The more silicon there is in the alloy, the more this compound will be present. The Mg2Si compound crystallizes in the form of the so-called “Chinese font” and, located along the grain boundaries, disrupts the bonding of solid solution grains, and in addition binds a certain amount of magnesium.

In Fig. 26, a, b are shown to compare the microstructure of aluminum alloys with 10% Mg in the cast state, prepared from materials of different purities. The structure of the alloy, cast from high-purity materials, consists of grains of a solid solution of magnesium in aluminum, along the boundaries of which the Al3Mg2 phase is located. In the structure of the alloy prepared on low-purity materials, in addition to the Al3Mg3 phase, one can see the Mg3Si compound in the form of a “Chinese font” and the FeAl3 compound in the form of two types of plates - flat and star-shaped (these are apparently different sections of the same shape ). The Mg2Si compound is located along the grain boundaries, and the FeAl3 plates are located inside the grains or intersect their boundaries. In some cases, FeAl3 plates intersect Mg2Si crystals, which indicates their primary crystallization from the melt. After heat treatment, the Mg2Si phase goes into a solid solution, and the microstructure of the alloy prepared from high-purity materials represents grains of a solid solution (Fig. 26c).
A sharp limitation of harmful impurities of iron and silicon, as well as the introduction of beryllium, titanium and zirconium additives into aluminum-magnesium alloys (AL27 and AL27-1) contributes to a significant increase in the corrosion resistance and mechanical properties of these alloys compared to CO alloy AL8.
The effect of additional alloying of high-purity Al-Mg alloys with additives of various elements can be traced using the example of the AL8M alloy. One of the disadvantages of Al-Mg alloys (AL8, AL27) with a high (up to 11.5%) magnesium content is their tendency to natural aging, a decrease in plastic properties and the possibility of cracks in castings. However, it can be assumed that ways to stabilize the properties of the AL8 alloy can be found. One of them is to reduce the degree of magnesium supersaturation of the α solid solution, i.e., to reduce the magnesium content in the alloy. At the same time, the speed of the aging process will sharply decrease. It should be noted, however, that as the magnesium content in the alloy decreases, the mechanical properties of the alloy deteriorate. To improve the mechanical properties of alloys in this case, it is necessary to apply alloying and modification.

In table Figure 17 presents the results of the influence of molybdenum and treatment with potassium fluorozirconate salt on the properties and grain size of the Al-Mg (10.5% Mg) alloy according to the work.
If the melt is treated with potassium fluorozirconate, the introduction of molybdenum in tenths of a percent contributes to a very strong refinement of the crystalline grain of the alloy; the greatest grinding effect is obtained by introducing 0.1% Mo into the AL8 alloy.
Stronger grain refinement with the combined addition of zirconium and molybdenum than with the addition of each of these elements separately is apparently explained by the fact that the solubility of each additive in the presence of the other decreases. This should lead to the formation of a significantly larger number of intermetallic particles, i.e., nucleation centers. Crystallization from many centers provides a finer grain structure.
In full accordance with the effect of grain refinement there is a change in mechanical properties. The presented results of mechanical tests show that treating the melt with potassium fluorozirconate and introducing 0.1% Mo makes it possible to increase the strength properties of the alloy from 29.9 to 43-44 kgf/mm2, the yield strength from 18 to 22 kgf/mm2 and the relative elongation from 14 to 23%. When the molybdenum content exceeds 0.1%, the mechanical properties deteriorate.
In table Figure 18 shows the comparative properties of the AL8, AL8M and AL27-1 alloys.

As noted earlier, reducing the magnesium content in Al-Mg alloys, as well as alloying with various additives, can significantly reduce the rate of decomposition of a supersaturated solid solution, as well as change the rate of general corrosion and the susceptibility of alloys to intercrystalline corrosion.
In order to clarify this effect, the work presents the results of tests in a wet chamber of alloys with different contents of magnesium and alloying additives (Table 19).
The studies also showed that the change in relative weight gain over time obeys a parabolic law. This suggests that a dense oxide film with good protective properties is formed on the surface of samples from all alloys. The most intensive growth of the oxide film occurs during the first 500 days. Subsequently, the oxidation rate stabilizes. It should be noted that the film of modified alloys apparently has better protective properties.

A study of the microstructure showed that the process of intercrystalline corrosion in alloys containing during the entire period of corrosion tests did not receive any noticeable development.
Alloys containing 11.5% Mg behave differently. The nature of the change in the relative weight gain of samples of modified alloys also obeys the parabolic law. However, the oxidation rate increases noticeably compared to the oxidation rate of alloys containing 8.5% Mg, and the oxide film acquires protective properties at a noticeably greater thickness.
In the original alloy, the nature of the change in relative weight gain also obeys the parabolic law. However, in the time interval from 300 to 500 days, a sharp increase in the growth rate of the oxide film is observed. This phenomenon, apparently, can be explained by the cracking of the oxide film during this period of time due to the occurrence of significant internal stresses in it.
After the newly formed oxides heal the cracks in the oxide film, the oxidation rate will decrease and will remain virtually unchanged in the future.
A study of the microstructure of alloys containing 11.5% Mg showed that in the original alloy, after 300 days of corrosion tests, the grain boundaries become greatly thickened due to the precipitation of the β-phase, and the alloy becomes prone to intercrystalline corrosion. Obviously, during this period of time, the formation of corrosion cracks begins, since by the 500th day of testing, corrosion cracks penetrate very deeply into the metal, capturing quite a lot of grain boundaries.
Unlike an unmodified alloy, in modified alloys the process of intercrystalline corrosion is limited to the surface layer of the metal and does not develop strongly even after 1000 days of corrosion tests. It should be noted that the process of intercrystalline corrosion is least developed in the alloy modified with zirconium and molybdenum.
In full accordance with the structural changes are changes in the mechanical properties of alloys.
As the data in table shows. 19, the tensile strength of modified alloys is constantly increasing, which is explained by the natural aging process. In the original alloy, two processes occur in parallel: natural aging, which strengthens the alloy, and the process of intercrystalline corrosion, which softens it. As a result, the tensile strength of the original alloy even decreases somewhat by 1000 days of corrosion tests.
Even more indicative is the change in the relative elongation of the alloys: for the original alloy, a sharp drop in plastic properties begins after 100 days of corrosion tests, while for modified alloys only after 500 days. It should be noted that the decrease in the ductility of modified alloys after 500 days of corrosion tests can be more likely explained by the process of embrittlement of the alloy as a result of natural aging than by the process of intercrystalline corrosion.

The disadvantages of Al-Mg alloys with a high magnesium content (AL8, AL8M, AL27-1, AL27) also include sensitivity to intergranular corrosion and stress corrosion that appears as a result of prolonged heating at temperatures above 80 ° C (Table 20). Therefore, these alloys are recommended for the manufacture of power parts that operate for a short time at temperatures from -60 to +60 ° C, and in some cases they can be successfully used instead of scarce bronzes and brass, stainless steels and deformable aluminum alloys when operating components and parts with large applications. (including shock and alternating loads) under various conditions (including sea water and fog).
To reduce the tendency for cracks to form in castings made of these alloys during long-term operation, it is necessary to limit the magnesium content in the alloys to 10%, and quench the parts in oil heated to 50-60 ° C.
Alloys AL23 and AL23-1 in the hardened state are not prone to intergranular corrosion. In the cast state of these alloys, when tested for intergranular corrosion, the development of corrosion along the grain boundaries is observed, which is caused by the presence in the cast structure of this alloy of excess β-phase along the grain boundaries, released during the crystallization process.
Typical properties of AL23-1 and AL23 alloys are given in table. 21.

AL23-1 and AL23 alloys can be welded satisfactorily by argon-arc welding. The strength of welded joints is 80-90% of the strength of the base material. Good results were obtained when welding cast parts made of AL23-1 alloy with parts made of wrought alloy AMg6.
Alloys of grades AL23-1 and AL23 can be used both in cast and hardened states. In the cast state, AL23 and AL23-1 alloys are intended for the manufacture of parts bearing medium static and relatively small shock loads. In the hardened state, AL23-1 alloy is intended for the manufacture of parts operating under medium static and shock loads. AL29 grade alloy is designed to work in various climatic conditions. AL29 alloy castings are used without special heat treatment. AL29 alloy in the cast state has satisfactory corrosion resistance. In order to further increase corrosion resistance, parts made of AL29 alloy are anodized in chromic acid. AL29 alloy, intended for injection molding, differs in chemical composition from AL13 alloy in its higher magnesium content, as well as a lower permissible impurity content. The alloy is used in a cast state. In terms of mechanical and casting properties, alloy AL29 is superior to alloy AL13, and in all other characteristics it is similar to it and is used for the manufacture of parts operating under medium static and shock loads, as well as in devices operating in subtropical climates. Parts made of AL29 alloy can operate for a long time at temperatures up to 150° C.
AL22 alloy has been developed for injection molding, which has found some application for the manufacture of parts operating in installations and assemblies at elevated temperatures for several minutes, and sometimes several tens of minutes. AL22 alloy contains a large amount of magnesium (10.5-13%), which allows the use of castings from it in a hardened state. Alloying the alloy with small additions of titanium and beryllium helps improve its casting and strength properties. Alloy AL22 is superior to alloy AL13 both in technological properties, strength characteristics, and heat resistance. For the greatest strength of the alloy, it should contain magnesium content at the upper limit (up to 13%), and silicon at the lower limit; for casting parts with complex configurations, the magnesium content should be at the lower limit, and silicon at the upper limit.
The disadvantage of the alloy is reduced ductility. AL22 alloy is used for casting parts with complex configurations that operate under medium static loads (aggregate and instrument type parts) under corrosive conditions of the atmosphere and sea water. The alloy is most widely used for injection molding of parts. In this case, the castings are used in a cast state. Parts made of AL22 alloy can operate for a long time at temperatures up to 200° C.
The new casting alloy grade AL28 is used in a cast state (without heat treatment) for the manufacture of fittings for fresh water pipelines, oil and fuel systems, as well as for parts of ship mechanisms and equipment, the operating temperature of which does not exceed 100° C. At higher temperatures, intense decomposition of the solid solution and precipitation of the β-phase along the grain boundaries occurs, which causes embrittlement of the alloy.
In table 22 shows the mechanical properties of the AL28 alloy depending on the content of the main alloying elements within the grade composition.
The introduction of 0.1-0.2% Zr into the AL28 alloy increases the strength properties by 2-3 kgf/mm2 and the density of the castings due to the formation of a zirconium hydride alloy that is stable at the melting temperature. When using high-purity starting materials as a charge, a significant increase in the strength and ductility of the alloy is observed.

Alloy LL28 has high corrosion resistance in fresh and sea water, as well as in the marine atmosphere. The corrosion resistance of the alloy under these conditions approaches that of pure aluminum.
In Fig. Figure 27 shows the results of testing the corrosion resistance of the AL28 alloy in a 3% NaCl solution acidified with 0.1% H2O2. The test duration was 1000 hours. For comparison, AL8, AL13 and AL4 alloys were tested under the same conditions.

In table Figure 23 shows the results of tensile testing of samples from alloys AL28, AL4 and AL13 before and after exposure to an aqueous solution of 3% NaCl + 0.l% H2O2, which confirm that the corrosion resistance of the AL28 alloy is superior to that of other aluminum alloys studied.
The mechanical properties of the AL28 alloy remained unchanged after exposure to a corrosive environment for 10,000 hours, while the AL4 alloy showed some deterioration in strength properties and a significant (more than 50%) decrease in elongation.

The increased corrosion resistance of AL28 alloy is explained by the presence of a manganese additive, which has a beneficial effect on the corrosion properties of pure aluminum and some aluminum alloys. AL28 alloy does not show a tendency to corrosion under stress at normal temperatures, as well as when heated to 100 ° C and held for a long time (up to 1000 hours). However, even relatively short-term exposures at temperatures above 100° C sharply reduce the performance of this alloy in a corrosive environment, which makes it practically impossible to use it at elevated temperatures.
Corrosion tests of experimental castings under natural conditions (in the Black Sea) for 2-3 years showed that the AL28 alloy is not prone to pitting corrosion. AL28 alloy has proven itself to be one of the most resistant aluminum alloys when tested in sea water moving at a speed of 10 m/s. The operation of crankcases of sealed freon compressors of ship air conditioners for a number of years has confirmed the feasibility and reliability of manufacturing them from AL28 alloy as a material resistant to the action of freon-22.
It should be said that recently great importance has been attached to stress corrosion, since increased demands are placed on the strength and performance of materials in modern mechanical engineering, and especially shipbuilding, under conditions of tropical temperatures, high humidity and sea water. Of interest is the work that describes the study of the susceptibility of cast aluminum alloys to stress corrosion cracking.
The tensile force was created using a pre-calibrated coil spring. The load was transferred to a sample with a diameter of 5 mm. The shape of the sample made it possible to attach baths with a corrosive environment to it. To avoid contact corrosion, the grips of the installation are removed from the bath. An aqueous solution of 3% NaCl + 0.1% H2O2 was used as a corrosive medium.
To determine the time to failure depending on the magnitude of the stress, the samples were placed in an installation in which a force corresponding to 1.2-0.4 of the conventional yield strength was created. The results obtained are shown in Fig. 28, 29, 30.

Thus, for all the alloys studied, the time dependence of the “life” of samples on stress in air (i.e., long-term strength at room temperature) in the coordinates stress - logarithm of time to failure is expressed by a straight line, which is characteristic of most metallic materials: with increasing load, time before the destruction of the samples decreases. However, the stress-time-to-fracture relationship for magnaliums (AL28, AL8 and AL27-1) is expressed by a broken curve, consisting of two almost straight branches. The left branch of the curve shows that the corrosion resistance of these alloys under stress depends largely on the stress level; an increase in load leads to a sharp reduction in the “life” of the sample. At lower loads, the dependence of the time to failure on stress disappears, i.e., at these stresses, the “lifetime” of the samples does not depend on the stress level - the right branch is a straight line, almost parallel to the time axis. For these alloys there appears to be a limit or "threshold" for stress corrosion resistance.
It should be noted that the corrosion resistance limit of the AL28 alloy under stress is a significant value, approximately equal to the conditional yield strength. As is known, the level of structural stresses usually does not exceed the yield strength, i.e., we can assume that corrosion cracking of castings made from this alloy is practically excluded.
For an AL8 alloy, the stress corrosion resistance limit does not exceed 8 kgf/mm2, which is approximately 2 times less than the yield strength of this alloy and indicates its low stress corrosion resistance.
The stress corrosion resistance limit of the AL27-1 alloy can be considered equal to its conditional yield strength. The AL27-1 alloy, like the AL8 alloy, contains about 10% Mg, however, its additional alloying with small amounts (0.05-0.15%) of beryllium, titanium and zirconium leads to a decrease in its susceptibility to corrosion cracking.
The study of the susceptibility to corrosion cracking under the influence of heat was carried out in order to determine the temperatures at which aluminum-magnesium alloys of the AL8, AL27-1 and AL28 grades are able to maintain resistance to stress corrosion for a long time, as well as to establish the admissibility of short-term heating of parts made of these alloys during the process. their manufacture (for example, during impregnation, application of protective coatings, etc.). Samples of these alloys were subjected to aging at 70, 100, 125 and 150 ° C from 1 to 1000 hours depending on the heating temperature and then tested under stresses equal to 0.9-0.8 of the stress level at which corrosion cracking does not occur , defined for the initial state.
Shown in Fig. 31 data show that the stress corrosion resistance of the AL28 alloy does not decrease when heated to 100°C for a long period of time, and short-term heating to 150°C is allowed without loss of performance in a corrosive environment.

The results of testing the corrosion resistance under stress of AL8 and AL27-1 alloys subjected to preheating showed that the use of parts made of these alloys at elevated temperatures under conditions of corrosion is practically unacceptable. The obtained results of studying the susceptibility of aluminum-magnesium alloys AL8, AL27-1 to corrosion cracking both in the as-received state and after artificial aging allow us to conclude that their corrosion behavior under stress is determined primarily by the stability of the solid solution structure.
A comparison of the stress corrosion resistance of AL8 and AL27-1 alloys containing the same amount of magnesium shows that the AL27-1 alloy, the structure of which is stabilized by additional alloying, has higher stress corrosion resistance. Alloy AL28, containing 4.8-6.3% solid solution stability of which is higher than alloys with 10% Mg, is more resistant to corrosion cracking.

Question 1. Draw a phase diagram of the aluminum-copper system. Describe the interaction of components in the liquid and solid states, indicate the structural components in all areas of the phase diagram, and explain the nature of the change in the properties of alloys in a given system using Kurnakov’s rules.

The most important impurity in duralumin is copper.

The phase diagram of A1-Cu alloys (Fig. 1.) refers to phase diagrams of type III, when the components form a solid solution with

limited solubility, decreasing with decreasing temperature. In alloys having a phase diagram of this type, a secondary

crystallization associated with partial decomposition of a solid solution. Such alloys can be subjected to heat treatment of groups III and IV, i.e. hardening

State diagram of aluminum - copper alloys.

and aging. From the phase diagram A1 - Cu it follows that the highest solubility of copper in aluminum is observed at 548°, when it is

5.7%; As the temperature decreases, the solubility of copper in aluminum decreases and at room temperature is 0.5%. If alloys with a copper content from 0.5 to 5.7% are subjected to quenching with heating above the temperatures of phase transformations (for example, above point 5 on the phase diagram of A1 - Cu alloys), then the alloy will transform into a homogeneous solid solution a. After quenching, the solid solution will decompose in the alloy, accompanied by the release of an excess phase with a high degree of dispersion. Such a phase in Al - Cu alloys is the hard and brittle chemical compound CuAl 2 .

The decomposition of a supersaturated solid solution can occur for a long time when the alloy is kept at room temperature (natural aging) and more quickly at elevated temperatures (artificial aging). As a result of aging, the hardness and strength of the alloy increase, while ductility and toughness decrease.

According to the theory of aging, most fully developed using Kurnakov's rules, the aging process in alloys occurs in several stages. The hardening of alloys observed as a result of aging corresponds to the period of precipitation of excess phases in a highly dispersed state. The changes occurring in the structure can only be observed using an electron microscope. Typically, this stage of the process occurs in hardened alloys during natural aging. At the same time, the hardness and strength of the alloy increase.

When hardened alloys are heated to relatively low temperatures, different for different alloys (artificial aging), a second stage occurs, consisting of the enlargement of particles of the precipitated phases. This process can be observed using an optical microscope. The appearance of enlarged precipitates of strengthening phases in the microstructure coincides with a new change in properties - a decrease in the strength and hardness of the alloy and an increase in its plasticity and toughness. Aging is observed only in alloys that have a phase diagram with limited solubility, which decreases with decreasing temperature. Since a large number of alloys have this type of diagram, the aging phenomenon is very common. Thermal treatment of many non-ferrous alloys - aluminum, copper, etc. is based on the phenomenon of aging.

In the A1 - Cu alloys discussed above, this process proceeds as follows. During natural aging in a hardened alloy, zones (disks) with increased copper content form. The thickness of these zones, called Guinier-Preston zones, is equal to two to three atomic layers. When heated to 100° and above, these zones transform into the so-called Ө phase, which is an unstable allotropic modification of the chemical compound CuA1 2. At temperatures above 250°, the 9" phase transforms into the Ө (CuA1 2) phase. Further, the precipitation of the Ө (CuA1 2) phase occurs. The alloy has the greatest hardness and strength in the first stage of aging.

In D1 grade duralumin, the Ө phase is also released during the decomposition of the solid solution, and in D16 grade duralumin there are several such phases.

The technology of heat treatment of parts made of duralumin consists of hardening, carried out to obtain a supersaturated solid solution, and natural or artificial aging. For hardening, parts are heated to 495° and cooled in cold water.

Hardened parts undergo natural aging by keeping them at room temperature. After 4-7 days of aging, the parts acquire the highest strength and hardness. Thus, the tensile strength of grade D1 duralumin in the annealed state is 25 kg/mm 2 , and its hardness is equal N IN = 45; after hardening and natural aging, the tensile strength is 40 kg/mm 2 , and the hardness increases to N V = 100.

The time required for the decomposition of a solid solution can be reduced to several hours by heating hardened duralumin to 100 - 150 ◦ (artificial aging), however, the hardness and strength values with artificial aging are slightly lower than with natural aging. Corrosion resistance also decreases somewhat. Duralumin grades D16 and D6 have the highest hardness and strength after hardening and aging. Duralumin grades DZP and D18 are alloys with increased ductility.

Duralumins are widely used in various industries, especially in aircraft construction, due to their low specific gravity and high mechanical properties after heat treatment.

When marking duraluminins, the letter D stands for “duralumin”, and the number is the conventional number of the alloy.

2. STATE DIAGRAM OF IRON-CARBON ALLOYS

Alloys of iron and carbon are conventionally classified as two-component alloys. Their composition, in addition to the main components - iron and carbon, contains small quantities of common impurities - manganese, silicon, sulfur, phosphorus, as well as gases - nitrogen, oxygen, hydrogen and sometimes traces of some other elements. Iron and carbon form a stable chemical compound Fe 3 C (93.33% Fe and 6.67% C), called iron carbide or cementite. In the iron-carbon alloys used (steels, cast irons), the carbon content does not exceed 6.67%, and therefore iron alloys with iron carbide (Fe-Fe 3 C system), in which the second component is cementite, are of practical importance.

When the carbon content is above 6.67%, there will be no free iron in the alloys, since it will all enter into a chemical combination with carbon. In this case, the components of the alloys will be iron carbide and carbon; the alloys will belong to the second system Fe 3 C -C, which has not been sufficiently studied. In addition, iron-carbon alloys with a carbon content above 6.67% are very brittle and are practically not used.

Alloys Fe -Fe 3 C (with a C content of up to 6.67%), on the contrary, are of great practical importance. In Fig. Figure 2 shows a structural diagram of the state of Fe -Fe 3 C alloys, plotted in temperature - concentration coordinates. The ordinate axis shows the heating temperatures of the alloys, and the abscissa axis shows the carbon concentration as a percentage. The left ordinate corresponds to 100% iron content, and the right ordinate corresponds to 6.67% carbon content (or 100% Fe 3 C concentration).

On the right ordinate is the melting point of Fe 3 C, corresponding to 1550° (point D on the diagram).

Due to the fact that iron has modifications, on the left ordinate, in addition to the melting point of iron, 1535° (point A on the diagram), the temperatures of allotropic transformations of iron are also plotted: 1390° (point N ) and 910° (point G).

Thus, the ordinates of the diagram correspond to the pure components of the alloy (iron and cementite), and between them there are points corresponding to alloys of different concentrations from 0 to 6.67% C

Rice. 2. Structural diagram of the state of alloysFe - Fe 3 C .

Under certain conditions, a chemical compound (cementite) may not form, which depends on the content of silicon, manganese and other elements, as well as on the cooling rate of ingots or castings. In this case, carbon is released in the alloys in a free state in the form of graphite. In this case, there will not be two alloy systems (Fe -Fe 3 C and Fe 3 C -C). They are replaced by a single Fe-C alloy system that does not have chemical compounds.

2.1 Structural components of iron-carbon alloys.

Microscopic analysis shows that six structural components are formed in iron-carbon alloys, namely: ferrite, cementite, austenite and graphite, as well as pearlite and ledeburite.

Ferrite is called a solid solution of carbon intercalation in Fe a. Since the solubility of carbon in Fe is insignificant, ferrite can be considered almost pure Fe a. Ferrite has a body-centered cubic lattice (BC). Under a microscope, this structural component has the appearance of light grains of various sizes. The properties of ferrite are the same as those of iron: it is soft and ductile, with a tensile strength of 25 kg/mm 2 , hardness N IN = 80, relative elongation 50%. The plasticity of ferrite depends on the size of its grain: the finer the grain, the higher its plasticity. Up to 768° (Curie point) it is ferrimagnetic, and above it is paramagnetic.

Cementite called iron carbide Fe 3 C. Cementite has a complex rhombic lattice. Under a microscope, this structural component has the appearance of plates or grains of various sizes. Cementite is hard (N IN > 800 units) and is fragile, and its relative elongation is close to zero. A distinction is made between cementite released during primary crystallization from a liquid alloy (primary cementite or C 1) and cementite released from a solid solution of Y-austenite (secondary cementite or C 2). In addition, during the decomposition of the solid solution a (region G.P.Q. on the state diagram), cementite stands out, called, in contrast to the previous ones, tertiary cementite or C 3. All forms of cementite have the same crystalline structure and properties, but different particle sizes - plates or grains. The largest are the particles of primary cementite, and the smallest are the particles of primary cementite. Up to 210° (Curie point) cementite is ferrimagnetic, and above it it is paramagnetic.

Austenite is called a solid solution of carbon intercalation in Fe Y. Austenite has a face-centered cubic lattice (K12). Under a microscope, this structural component has the appearance of light grains with characteristic double lines (twins). The hardness of austenite is N IN = 220. Austenite is paramagnetic.

Graphite has a loosely packed hexagonal lattice with a layered arrangement of atoms. Under a microscope, this structural component has the form of plates of various shapes and sizes in gray cast iron, a flake-like shape in malleable cast iron, and a spherical shape in high-strength cast iron. The mechanical properties of graphite are extremely low.

All of the four structural components listed are at the same time also phases of the system of iron-carbon alloys, since they are homogeneous - solid solutions (ferrite and austenite), a chemical compound (cementite) or an elemental substance (graphite).

The structural components of ledeburite and pearlite are not homogeneous. They are mechanical mixtures with special properties (eutectic and eutectoid).

Perlite called a eutectoid mixture of ferrite and cementite. It is formed from austenite during secondary crystallization and contains 0.8% C. The formation temperature of pearlite is 723°. This critical temperature, observed only in steel, is called the point A±. Perlite can have a lamellar structure, when the cementite has the shape of plates, or a granular structure, when the cementite has the shape of grains. The mechanical properties of lamellar and granular perlite are somewhat different. Lamellar perlite has a tensile strength of 82 kg/mm 2 , relative elongation 15%, hardness N V = 190-^-230. The tensile strength of granular perlite is 63 kg/mm 2 , relative elongation 20% and hardness R = 1.60-g-190.

Ledeburite called a eutectic mixture of austenite and cementite. It is formed during the process of primary crystallization at 1130°. This is the lowest crystallization temperature in the system of iron-carbon alloys. Austenite, which is part of ledeburite, transforms into pearlite at 723°. Therefore, below 723° and up to room temperature, ledeburite consists of a mixture of pearlite and cementite. He's very hard (N V ^700) and fragile. The presence of ledeburite is a structural feature of white cast irons. The mechanical properties of iron-carbon alloys vary depending on the number of structural components, their shape, size and location.

The structural diagram of the state of Fe -Fe 3 C is a complex diagram, since in iron-carbon alloys not only transformations associated with crystallization occur, but also transformations in the solid state.

The boundary between steel and white cast iron is a carbon concentration of 2%, and the structural feature is the presence or absence of ledeburite. Alloys with a carbon content of less than 2% (which do not have ledeburite) are called steels, and alloys with a carbon content of more than 2% (which have ledeburite in their structure) are called white cast iron.

Depending on the carbon concentration and steel structure, cast irons are usually divided into the following structural groups: hypoeutectoid steels (up to 0.8% C); structure - ferrite and pearlite; eutectoid steel (0.8% C); structure - pearlite;

hypereutectoid steels (over 0.8 to 2% C); structure - pearlite into secondary cementite;

hypoeutectic white cast iron (over 2 to 4.3% C); structure - ledeburite (disintegrated), pearlite and secondary cementite;

eutectic white cast iron (4.3% C); structure - ledeburite;

hypereutectic white cast iron (over 4.3 to 6.67% C); structure - ledeburite (disintegrated) and primary cementite.

This division, as can be seen from the Fe-Fe 3 C phase diagram, corresponds to the structural state of these alloys observed at room temperature.

Question 3.

Select a tool carbide alloy for fine milling of the surface of a part made of 30KhGSA steel. Give characteristics, decipher the selected brand of alloy, describe the structural features and properties of the alloy.

Tools are divided into three groups: cutting (cutters, drills, cutters, etc.), measuring (gauges, rings, tiles, etc.), and tools for hot and cold metal forming (stamps, drawing boards, etc.). Depending on the type of tools, the requirements for steels for their manufacture are different.

The main requirement for steels for cutting tools is the presence of high hardness, which does not decrease at high temperatures that arise during the processing of metals by cutting (red resistance). The hardness for metal-cutting tools should be R c = 60÷65. In addition, steels for cutting tools must have high wear resistance, strength and satisfactory toughness.

High-speed steels are most widely used for the manufacture of cutting tools. High-speed steel is a multicomponent alloy and belongs to the carbide (ledeburite) class of steels. In addition to iron and carbon, its composition includes chromium, tungsten and vanadium. The main alloying element in high-speed steel is tungsten. The most widely used (Table 3) are high-speed steel grades P18 (18% W) and P9 (9% W).

High-speed steel acquires high hardness R C = 62 and red resistance after heat treatment, consisting of quenching and repeated tempering.

Table 1

Chemical composition of high speed steel

(according to GOST 5952-51)

steel grade
	C	W	Cr	V	Mo
R 18	0,70 – 0,80	17,5 – 19,0	3,8 – 4,4	1,04 – 1,4	≤0,3
R 9	0,85 – 0,95	8,5 – 10,0	3,8 – 4,4	2,0 – 2,6	≤0,3

Figure 3 shows a graph of heat treatment of high-speed steel R18.

We choose it as a tool grade for clean milling because... This grade of steel suits us in terms of its characteristics.

Heat treatment of high-speed steel has a number of features that are determined by its chemical composition. Heating of high-speed steel during hardening is carried out to a high temperature (1260-1280°), necessary to dissolve chromium, tungsten and vanadium carbides in austenite. Up to 800-850° heating is carried out slowly in order to avoid large internal stresses in the steel due to its low thermal conductivity and brittleness, then rapid heating is carried out to 1260-1280° in order to avoid austenite grain growth and decarburization. Cooling of high-speed steel is carried out in oil. Stepwise hardening of high-speed steel in salts at a temperature of 500-550° is also widely used.

The structure of high-speed steel after quenching consists of martensite (54%), carbides (16%) and retained austenite (30%). After hardening, high-speed steel is subjected to repeated tempering at 560°. Typically, tempering is carried out three times with a holding time of 1 hour in order to reduce the amount of retained austenite and increase the hardness of the steel. During exposure at the tempering temperature, carbides are released from the austenite, and upon cooling, the austenite transforms into martensite. It is as if secondary hardening occurs. The structure of high-speed steel after tempering is tempered martensite, highly dispersed carbides and a small amount of retained austenite. To further reduce the amount of retained austenite, high-speed steels are subjected to cold treatment, which is carried out before tempering. The use of low-temperature cyanidation is very effective in increasing hardness and wear resistance.

High-speed steels are widely used for the manufacture of various cutting tools; Tools made from these steels operate at cutting speeds that are 3-4 times higher than the cutting speeds of tools made from carbon steels, and retain cutting properties when heated during the cutting process up to 600º - 620º.

Question. 4 Select the most rational and economical grade of steel for the manufacture of a spring, which after heat treatment should obtain high elasticity and hardness of at least 44 ... 45 HRC E. Give a characteristic, indicate the composition of the steel, select and justify the heat treatment mode. Describe and sketch the microstructure and properties of steel after heat treatment.

Springs are used to store energy (spring motors), to absorb and absorb shock, to compensate for thermal expansion in valve distribution mechanisms, etc. Deformation of a spring can manifest itself in the form of its stretching, compression, bending or twisting.

The relationship between the force P and the spring deformation F is called the spring characteristic.

According to the designer's handbook - mechanical engineering, author. Anuriev. V.I., we choose the most rational and economical steel grade:

Steel – 65G(manganese steel), having elasticity and hardness equal to 42...48 HRC E. according to Requel. Heat treatment of steel: hardening temperature - 830 º C, (oil medium), tempering - 480 º C. Tensile strength (δ B) - 100 kg/mm 2, yield strength (δ t) - 85 kg/mm 2, relative elongation (δ 5) – 7%, relative narrowing (ψ) – 25%.

Characteristics – high quality spring steel with a P – S content of no more than 0.025%. Divided into 2 categories: 1 – decarbonized layer, 2 – with normalized decarbonized layer

Question 5. AK4-1 alloy was used to manufacture aircraft engine compressor discs. Give a description, indicate the composition and characteristics of the mechanical properties of the alloy, the method and nature of hardening the alloy, methods of protection against corrosion.

AK4-1 is an aluminum-based alloy, processed into a product by deformation, strengthened by heat treatment, and heat-resistant.

Alloy composition: Mg – 1.4…1.8%. Cu – 1.9…2.5%. Fe – 0.8…1.3%. Ni – 0.8…1.3%. Ti – 0.02…0.1%, impurities up to 0.83%. The tensile strength of the alloy is 430 MPa, the yield strength is 0.2 - 280 MPa.

Alloyed with iron, nickel, copper, and other elements forming strengthening phases

Question 6. Economic prerequisites for the use of non-metallic materials in industry. Describe the groups and properties of gas-filled plastics, give examples from each group, their properties and scope of application in aircraft structures.

Recently, non-metallic polymer materials are increasingly used as structural materials. The main feature of polymers is that they have a number of properties not inherent in metals, and can serve as a good addition to metal structural materials or be their replacement, and the variety of physicochemical and mechanical properties inherent in various types of plastics and the ease of processing into products determine Widely used in all branches of mechanical engineering, instrument making, apparatus manufacturing and everyday life. Plastic masses are characterized by low specific gravity (from 0.05 to 2.0 g/cm 3 ), have high insulating properties, resist corrosion well, have a wide range of friction coefficients and high abrasion resistance.

If it is necessary to obtain products that have anti-corrosion resistance, acid resistance, noiselessness in operation while simultaneously ensuring lightness of construction, plastic masses can serve as substitutes for ferrous metals. Due to the transparency and high plastic properties of some types of plastics, they are widely used for the manufacture of safety glass for the automotive industry. In the manufacture of products with high electrical insulating properties, plastics are replacing and displacing high-voltage porcelain, mica, ebonite and other materials. Finally, steam, petrol and gas permeability, as well as high water and light resistance with good appearance, ensure the widespread use of plastics in a number of industries.

Plastics are used to make bearing inserts, separators, silent gears, fan blades, blades for washing machines and mixers, radio equipment, cases for radios and watches, electrical equipment, distributors, grinding wheels, waterproof and decorative fabrics and various figurative consumer goods.

Foam plastics They are lightweight gas-filled plastics based on synthetic resins. Foam plastics are divided into two groups: 1 - materials with interconnected pores - sponges (density less than 300 kg/m3), 2 - materials with isolated pores - foams (density more than 300 kg/m3).

The properties of foam plastics are very diverse: some have hardness, like glass, others have elasticity, like rubber. All foam plastics lend themselves well to mechanical processing with carpentry tools, are easily pressed in a heated state into products of complex shapes and are glued together. In the aircraft industry, foam plastics are used as a filler between two skins in order to increase the rigidity and strength of the structure, as well as as a heat and sound insulating material.

Goal of the work: study of phase equilibrium diagrams and phase transformations in binary alloys of aluminum with other elements.

Necessary equipment, devices, tools, materials: muffle furnaces, hardness tester TK-2M, samples of duralumin, stand “Microstructures of non-ferrous alloys”, metallographic microscope.

Theoretical information

Aluminum is an essential metal widely used in the manufacture of a variety of aluminum alloys.

The color of aluminum is silvery-white with a peculiar dull tint. Aluminum crystallizes in the spatial lattice of a face-centered cube; no allotropic transformations were detected in it.

Aluminum has a low density (2.7 g/cm3), high electrical conductivity (about 60% of the electrical conductivity of pure copper) and significant thermal conductivity.

As a result of the oxidation of aluminum by atmospheric oxygen, a protective oxide film is formed on its surface. The presence of this film explains the high corrosion resistance of aluminum and many aluminum alloys.

Aluminum is quite resistant under normal atmospheric conditions and against the action of concentrated (90-98%) nitric acid, but it is easily destroyed by the action of most other mineral acids (sulfuric, hydrochloric), as well as alkalis. It has high ductility in both cold and hot states, is well welded by gas and resistance welding, but is poorly processed by cutting and has low casting properties.

The following mechanical properties are characteristic of rolled and annealed aluminum:  V= 80-100 MPa,  = 35-40%, NV = 250...300 MPa.

When cold-working, the strength of aluminum increases and the ductility decreases. Accordingly, according to the degree of deformation, annealed (AD-M), semi-cold-worked (AD-P) and cold-worked (AD-N) aluminum are distinguished. Annealing of aluminum to remove hardening is carried out at 350…410 С.

Pure aluminum has a variety of uses. Semi-finished products are made from technical aluminum AD1 and AD, containing at least 99.3 and 98.8% Al respectively, - sheets, pipes, profiles, wire for rivets.

In electrical engineering, aluminum serves to replace more expensive and heavier copper in the manufacture of wires, cables, capacitors, rectifiers, etc.

The most important elements introduced into aluminum alloys are copper, silicon, magnesium and zinc.

Aluminum and copper form solid solutions of variable concentration. At a temperature of 0°C, the solubility of copper in aluminum is 0.3%, and at a eutectic temperature of 548°C it increases to 5.6%. Aluminum and copper in a ratio of 46:54 form a stable chemical compound CuAl 2.

Let us consider the state of aluminum-copper alloys depending on their composition and temperature (Fig. 1). The CDE line in the diagram is the liquidus line, and the CNDF line is the solidus line. The horizontal section of the NDF solidus line is also called the eutectic line.

The MN line shows the temperature-variable solubility of copper in aluminum. Consequently, the MN line is the boundary between unsaturated solid solutions and saturated solutions. Therefore, this line is often also called the limiting solubility line.

In region I, any alloy will be a homogeneous liquid solution of aluminum and copper, i.e. AlCu.

R
is. 1. State diagram of the Al–CuAl 2 system

In regions II and III, the alloys will be partly in liquid and partly in solid states.

In region II, the solid phase will be a solid solution of copper in aluminum, and the liquid phase will be a liquid solution of aluminum and copper, i.e. Al(Cu) + (Al Cu), if we agree to designate a solid solution of limited solubility of copper in aluminum as Al(Cu).

In region III, the liquid phase will also be a liquid solution of aluminum and copper, and the solid phase will be the metal compound CuAl 2, i.e.
+ (Al Cu). The index “I” (primary) shows that CuAl 2 was formed during crystallization from a liquid state.

In other areas, fully solidified alloys will have the following structure:

In region IV there is a homogeneous solid solution of copper in aluminum, i.e. Al(Cu);

In region V – solid solution of copper in aluminum and secondary
;

In region VI - solid solution of copper in aluminum, secondary CuAl 2 and eutectic, i.e. Al(Cu) +
+Al(Cu) + CuAl 2 ;

In region VII - primary CuAl 2 and eutectic, i.e.
+Al(Cu) + CuAl 2 .

The eutectic of these alloys is a special mechanical mixture of alternating tiny crystals of a solid solution of copper in aluminum and the metal compound CuAl 2, i.e. Al(Cu) + CuAl 2 .

All alloys of the Al – CuAl 2 system can be divided into four groups according to structure and concentration:

Group 1 contains copper from 0 to 0.3%;

Group 2 contains copper from 0.3 to 5.6%;

Group 3 contains copper from 5.6 to 33.8%;

Group 4 contains copper from 33.8 to 54%.

Let us consider the structure of alloys of the Al – CuAl 2 system.

In Fig. 2, A shows the structure of the alloy of the first group, consisting of grains of a solid solution of copper in aluminum. The structure of the alloy of the second group is shown in Fig. 2, b: grains of a solid solution of copper in aluminum and crystals of secondary CuAl 2 are visible,

The structure of a hypoeutectic alloy (solid solution of copper in aluminum, crystals of secondary CuAl 2 and eutectic) is shown in Fig. 2, V. The structure of a eutectic alloy - eutectic, consisting of tiny crystals of a solid solution of copper in aluminum and CuAl 2 is shown in Fig. 2, G. In Fig. 2, d The structure of a hypereutectic alloy is shown, consisting of primary crystals of CuAl 2 and eutectic.

In alloys containing eutectic, the copper content can be determined by their structure. However, in this case, it is necessary to take into account the amount of copper present in the eutectic and in the solid solution. For example, in a hypoeutectic alloy containing 30% eutectic and 70% solid solution, the amount of copper in the eutectic

and in solid solution

Consequently, the alloy under study contains k x + k y = 14.06% copper, which corresponds to point A, which lies on the abscissa axis of the state diagram of the Al – CuAl 2 system (Fig. 1).

When determining the composition of hypereutectic alloys, the amount of copper present in the eutectic and in the chemical compound is calculated
. The sum of these quantities will correspond to the copper content in the hypereutectic alloy. The chemical compound CuAl 2 is very hard and brittle.

Duralumins are used for the manufacture of parts and structural elements of medium and high strength (  V= 420...520 MPa), requiring durability under variable loads in building structures.

Duralumin is used to make skins, frames, stringers and spars of aircraft, load-bearing frames and bodies of trucks, etc.

Alloys of Al and Si are called silumins. They have good casting properties and contain 4...13% Si. From the phase diagram of these alloys (Fig. 3) it follows that silumins are hypoeutectic or eutectic alloys containing significant amounts of eutectic in the structure.

However, when cast under normal conditions, these alloys acquire an unsatisfactory structure, since the eutectic turns out to be coarsely lamellar, with large inclusions of brittle silicon, which gives the alloys low mechanical properties.

In Fig. 4, A The structure of AL2 grade silumin containing 11...13% Si is presented. In accordance with the state diagram, an aluminum-silicon alloy of this composition has a eutectic structure. The eutectic consists of  -solid solution of silicon in aluminum (light background) and needle-shaped large and fragile silicon crystals. Acicular releases of silicon particles create internal sharp cuts in ductile aluminum and lead to premature failure under loading.

Rice. 3. State diagram of the Al–Si system

Rice. 4. Silumin: A– before modification, coarse-needle eutectic (Al-Si) and primary silicon precipitation; b– after modification, fine eutectic

(Al-Si) and dendrites of a solid solution of silicon and other elements in aluminum

The introduction of a modifier changes the nature of crystallization. The lines of the phase diagram shift so that the alloy with 11...13% silicon becomes hypoeutectic.

Excessive light grains appear in the structure  -solid solution (Fig. 4, b).

The modifier changes the shape of silicon particles: instead of needle-shaped ones, small equiaxed ones fall out, which do not create dangerous stress concentrations during loading.

As a result of modification, the tensile strength of these alloys increases from 130 to 160 MPa, and the relative elongation from 2 to 4%.

Pressure-processed alloys contain less than 1% silicon. In aluminum alloys containing magnesium, silicon binds with it into a stable metal compound Mg 2 Si; with aluminum it forms a eutectic-type phase diagram with limited solid solutions (Fig. 5).

The Mg 2 Si compound is characterized by high hardness, its variable solubility in aluminum allows it to achieve significant hardening during heat treatment.

In electrical engineering, aluminum alloys such as Aldrey, alloyed with magnesium and silicon, are used. When hardened alloys age, Mg 2 Si falls out of the solid solution and strengthens it. As a result of this treatment, it is possible to obtain a tensile strength of up to 350 MPa with a relative elongation of 10-15%. It is significant that the electrical conductivity of such an alloy is 85% of the electrical conductivity of conductive aluminum. This is due to the fact that Mg 2 Si is almost completely removed from the solid solution during aging and the alloy consists of pure aluminum and a strengthening phase (Mg 2 Si).

R
is. 6. State diagram of the Al–Mg system

Magnesium forms solid solutions with aluminum, as well as  -phase based on the Mg 2 Al 3 compound. Most aluminum alloys contain no more than 3% magnesium, but in some cast alloys such as magnesium its content reaches 12%.

As can be seen from Fig. 6, eutectic is formed in aluminum alloys with magnesium. The solubility of magnesium in aluminum varies greatly with temperature.

An example is the AL8 alloy. In the cast state, it has a structure consisting of grains of a solid solution of magnesium in aluminum and inclusions of the brittle compound Al 3 Mg 2.

After casting, homogenization is carried out at a temperature of 430 °C for 15...20 hours, followed by quenching in oil.

During the homogenization process, Al 3 Mg 2 inclusions completely pass into solid solution. The hardened alloy acquires sufficient strength (  V= 300 MPa) and greater ductility. At the same time, the alloy acquires high corrosion resistance. Aging for AL8 alloy is harmful: ductility sharply decreases and corrosion resistance deteriorates.

Zinc is introduced into some high-strength aluminum alloys in amounts up to 9%. In binary alloys with aluminum at temperatures above 250 °C, zinc (within these limits) is in solid solution (Fig. 7).

Rice. 7. State diagram of the Al–Zn system

All high-strength alloys have a complex chemical composition. Thus, alloy B95 contains 6% Zn, 2.3% Mg, 1.7% Cu, 0.4% Mn and 0.15% Cr. Zinc, magnesium and copper form solid solutions and metal compounds with aluminum MgZn 2, Al 2 CuMg - S-phase, Mg 4 Zn 3 Al 3 - T-phase. When heated, these metal compounds dissolve into aluminum.

For example, at a temperature of 475 ºС, the solubility of MgZn 2 in aluminum increases to 18% (Fig. 8).

After hardening and artificial aging, alloy B95 has  V= 600 MPa,  = 12%. Manganese and chromium enhance the aging effect and increase the corrosion resistance of the alloy.

(wt.)

Rice. 8. State diagram of the Al–MgZn 2 system

Safety regulations

1. Observe all precautions and safety rules when preparing microsections.

2. When grinding a microsection, you should cool the sample more often to prevent burns to your fingers.

3. When etching thin sections, use rubber gloves.

4. When studying the structure of the alloy on a microscope, you should make sure that it is reliably grounded.

5. You should use only serviceable tools and equipment.

Work order

1. Study the state diagram of aluminum alloys.

2. Give characteristics of a given alloy (structure, phase transformations, composition, properties, scope of application).

3. Draw the structure of the alloy under study.

Sketches of microstructures of the studied alloys indicating phases and structural components.

Copying the phase equilibrium diagram specified by the teacher.

For an alloy of a given composition, a description of all phase transformations during heating or cooling and determination of the chemical composition of the phases.

Control questions

Why is the corrosion resistance of many aluminum alloys lower than that of pure aluminum?

Is it possible to determine the type of alloy by the microstructure of the alloy - cast or wrought?

What is the structure of wrought aluminum alloys that cannot be strengthened by heat treatment?

How is strengthening of single-phase aluminum alloys achieved?

What is the strengthening heat treatment of dual-phase aluminum alloys?

What is the purpose of hardening duralumin?

What are the main mechanical properties of duralumin?

What alloys are called silumins?

What is the specific strength of aluminum alloys?

Main alloying elements in aluminum alloys.

Goal of the work: study of phase equilibrium diagrams and phase transformations in binary alloys of aluminum with other elements.

Necessary equipment, devices, tools, materials: muffle furnaces, hardness tester TK-2M, samples of duralumin, stand “Microstructures of non-ferrous alloys”, metallographic microscope.

Brief theoretical information

Aluminum is an essential metal widely used in the manufacture of a variety of aluminum alloys.

The color of aluminum is silvery-white with a peculiar dull tint. Aluminum crystallizes in the spatial lattice of a face-centered cube; no allotropic transformations were detected in it.

Aluminum has a low density (2.7 g/cm3), high electrical conductivity (about 60% of the electrical conductivity of pure copper) and significant thermal conductivity.

The following mechanical properties are characteristic of rolled and annealed aluminum:  V= 80-100 MPa,  = 35-40 %, NV= 250...300 MPa.

When cold-working, the strength of aluminum increases and the ductility decreases. According to the degree of deformation, annealed (AD-M), semi-cold-worked (AD-P) and cold-worked (AD-N) aluminum are distinguished. Annealing of aluminum to remove hardening is carried out at 350…410 С.

In electrical engineering, aluminum serves to replace more expensive and heavier copper in the manufacture of wires, cables, capacitors, rectifiers, etc.

The most important elements introduced into aluminum alloys are copper, silicon, magnesium and zinc.

In region I, any alloy will be a homogeneous liquid solution of aluminum and copper, i.e. AlCu.

Rice. 1. State diagram of the Al–CuAl 2 system

In regions II and III, the alloys will be partly in liquid and partly in solid states.

In other areas, fully solidified alloys will have the following structure:

In region IV there is a homogeneous solid solution of copper in aluminum, i.e. Al(Cu);

In region V – solid solution of copper in aluminum and secondary
;

In region VI - solid solution of copper in aluminum, secondary CuAl 2 and eutectic, i.e. Al(Cu) +
+Al(Cu) + CuAl 2 ;

In region VII - primary CuAl 2 and eutectic, i.e.
+Al(Cu) + CuAl 2 .

The eutectic of these alloys is a special mechanical mixture of alternating tiny crystals of a solid solution of copper in aluminum and the metal compound CuAl 2, i.e. Al(Cu) + CuAl 2 .

All alloys of the Al – CuAl 2 system can be divided into four groups according to structure and concentration:

Group 1 contains copper from 0 to 0.3%;

Group 2 contains copper from 0.3 to 5.6%;

Group 3 contains copper from 5.6 to 33.8%;

Group 4 contains copper from 33.8 to 54%.

Let us consider the structure of alloys of the Al – CuAl 2 system. In Fig. 2, A shows the structure of the alloy of the first group, consisting of grains of a solid solution of copper in aluminum. The structure of the alloy of the second group is shown in Fig. 2, b: grains of a solid solution of copper in aluminum and crystals of secondary CuAl 2 are visible,

and in solid solution

Consequently, the alloy under study contains

k x + k y = 14.06% copper,

which corresponds to point A, which lies on the abscissa axis of the state diagram of the Al – CuAl 2 system (Fig. 1).

In technology, mainly aluminum alloys containing 2...5% copper, called duralumin, are used. They are well processed by pressure and have high mechanical properties after heat treatment and cold hardening. Duralumins are used for the manufacture of parts and structural elements of medium and high strength (  V= 420...520 MPa), requiring durability under variable loads in building structures. Duralumin is used to make skins, frames, stringers and spars of aircraft, load-bearing frames and bodies of trucks, etc.

Rice. 3. State diagram of the Al–Si system

Rice. 4. Silumin: A– before modification, coarse-needle eutectic (Al-Si) and primary silicon precipitation; b– after modification, fine eutectic

(Al-Si) and dendrites of a solid solution of silicon and other elements in aluminum

The introduction of a modifier changes the nature of crystallization. The lines of the phase diagram shift so that the alloy with 11...13% silicon becomes hypoeutectic. Excessive light grains appear in the structure  -solid solution (Fig. 4, b). The modifier changes the shape of silicon particles: instead of needle-shaped ones, small equiaxed ones fall out, which do not create dangerous stress concentrations during loading.

As a result of modification, the tensile strength of these alloys increases from 130 to 160 MPa, and the relative elongation from 2 to 4%.

Pressure-processed alloys contain less than 1% silicon. In aluminum alloys containing magnesium, silicon binds with it into a stable metal compound Mg 2 Si; it forms with aluminum a eutectic type phase diagram with limited solid solutions ( rice. 5).

The Mg 2 Si compound is characterized by high hardness, its variable solubility in aluminum allows it to achieve significant hardening during heat treatment.

R
is. 6. State diagram of the Al–Mg system

As can be seen from Fig. 6, eutectic is formed in aluminum alloys with magnesium. The solubility of magnesium in aluminum varies greatly with temperature. An example is the AL8 alloy. In the cast state, it has a structure consisting of grains of a solid solution of magnesium in aluminum and inclusions of the brittle compound Al 3 Mg 2. After casting, homogenization is carried out at a temperature of 430 °C for 15...20 hours, followed by quenching in oil.

Zinc is introduced into some high-strength aluminum alloys in amounts up to 9%. In binary alloys with aluminum at temperatures above 250 °C, zinc (within these limits) is in solid solution (Fig. 7).

Rice. 7. State diagram of the Al–Zn system

For example, at a temperature of 475 ºС, the solubility of MgZn 2 in aluminum increases to 18% (Fig. 8).

After hardening and artificial aging, alloy B95 has  V= 600 MPa,  = 12%. Manganese and chromium enhance the aging effect and increase the corrosion resistance of the alloy.

(wt.)

Rice. 8. State diagram of the Al–MgZn 2 system

Safety regulations

Work order

Sketches of microstructures of the studied alloys indicating phases and structural components.

Copying the phase equilibrium diagram specified by the teacher.

For an alloy of a given composition, a description of all phase transformations during heating or cooling and determination of the chemical composition of the phases.

Control questions

Why is the corrosion resistance of many aluminum alloys lower than that of pure aluminum?

Is it possible to determine the type of alloy by the microstructure of the alloy - cast or wrought?

What is the structure of wrought aluminum alloys that cannot be strengthened by heat treatment?

How is strengthening of single-phase aluminum alloys achieved?

What is the strengthening heat treatment of dual-phase aluminum alloys?

What is the purpose of hardening duralumin?

What are the main mechanical properties of duralumin?

What alloys are called silumins?

What is the specific strength of aluminum alloys?

Main alloying elements in aluminum alloys.

The alloys of the Al-Mg system include a large group of alloys widely used in industry: AMg0.5; ; ; ; ; ; . Almost all types of semi-finished products are made from them: sheets, plates, forgings, stampings, pressed products (rods, profiles, panels, pipes) and wire. All alloys of the group under consideration are well welded by all types of welding.

Semi-finished products from these alloys have a relatively high level of strength characteristics compared to other thermally non-hardening alloys. Thus, the minimum values of the yield strength for sheet material (thickness ~2 mm) in the annealed state for the indicated series of alloys are respectively 30, 40, 80, 100, 120,150 and 160 MPa. The tensile strength is usually twice the yield strength, indicating the relatively high ductility of these alloys. However, they harden quite quickly, which negatively affects their technological ductility. The latter decreases significantly with increasing magnesium concentration. Therefore, alloys with a magnesium content of more than 4.5% can be classified as “semi-hard” and even “hard” alloys.

The negative role of increased magnesium content is more pronounced in the manufacture of pressed products. Alloys with a high magnesium content are pressed at low speeds (tens of times lower than, for example, some alloys of the Al-Zn-Mg or Al-Mg-Si system), which significantly reduces the productivity of pressing shops. The production of rolled semi-finished products from AMg6 alloy is a labor-intensive process. Therefore, recently, highly alloyed magnesium began to be replaced with more technologically advanced alloys, for example, alloys based on the Al-Zn-Mg system (1935, 1915, 1911), which significantly exceed the AMg6 alloy in strength properties (especially in yield strength) and are not inferior to it in many corrosion characteristics.

Low-alloy magnesium with a magnesium content of up to 3% will find even wider use due to their high corrosion resistance and ductility. According to the phase diagram of Al-Mg alloys, at the eutectic temperature 17.4% Mg dissolves in aluminum. With decreasing temperature, this solubility decreases sharply and at room temperatures is approximately 1.4%.

Thus, alloys with a high magnesium content under normal conditions have a supersaturation of this element (depending on the grade of the alloy), and, therefore, they should exhibit an aging effect. However, the structural changes that occur in these alloys during the decomposition of the solid solution have virtually no effect on the level of strength characteristics and at the same time sharply change the corrosion resistance of semi-finished products. The reason for this anomalous behavior lies in the nature of the decomposition of the solid solution and the phase composition of the precipitates. Since for Al-Mg alloys the upper temperature limit for the formation of GP zones (or the critical solubility temperature of GP zones - t K) is significantly lower than room temperature, the decomposition of the solid solution occurs according to a heterogeneous mechanism with the formation of transition (B') and equilibrium (B-Mg 2 Al3) phases. These precipitates nucleate heterogeneously at interfaces (grains, intermetallic particles, etc.), as well as dislocations, and therefore their contribution to the hardening process is small and is completely compensated by the degree of softening caused by a decrease in the concentration of magnesium in the solid solution. For this reason, in practice, the effect of strengthening of alloys of this group is not observed during the decomposition of the solid solution during natural or artificial aging or under various annealing conditions.

Phase B in a neutral aqueous solution of chlorides (3% NaCl) has a negative corrosion potential equal to - 0.930 V. In the same solution, but at lower pH values, i.e. in an acidic environment, the potential difference between the phase and the solid solution, although decreases, but remains quite large: (-0.864 V) - - (-0.526 V) = 0.338 V. And, conversely, in an alkaline environment (3% NaCl + 1% NaOH) aluminum and aluminum alloys containing 1-9% Mg , become more negative than the B-phase, and the potential difference for the extreme values of the indicated region of magnesium concentration is, respectively, +0.24 and +0.18 V. The considered features of changes in the electrochemical characteristics of individual structural components of A1-Mg alloys depending on the external environment are mainly and determine the resistance of these alloys MKK, RSK and KR.

From the above it follows that alloys with a magnesium content of more than 1.4% can potentially be sensitive to one, two or all of the previously mentioned types of corrosion. However, extensive experience in operating structures and numerous experiments show that practically alloys with a magnesium concentration not exceeding 3.5% (AMrl, AMg2 and partially AMg3) do not show sensitivity to RS and RSC (Fig. 56).

Electron microscopic studies show that this is due to the discrete distribution of B-phase particles along the grain boundaries due to the low supersaturation of the solid solution. Therefore, the corrosion process in neutral and acidic environments is limited only by the electrochemical dissolution of those particles that come to the surface of the alloy in direct contact with the electrolyte.

Such alloys are corrosion-resistant even in the cold-worked state, i.e., although cold-working accelerates the decomposition of the solid solution, it does not change the nature of the distribution of precipitates at the grain boundaries. At the same time, due to the beneficial influence of structural anisotropy in this case, the resistance to corrosion pitting increases significantly. Alloys with a magnesium content of more than 3.5% (AMg3, AMg4) and especially more than 5% (AMg5, AMg6) in a certain structural state and under certain environmental conditions can be sensitive to MCC and RSC, as well as to CR.

For alloys of the Al-Mg system, electrochemical factors in corrosion cracking play a much larger role than for alloys of other systems. Therefore, preventing the formation of a B-phase film along grain boundaries is also advisable for increasing the Raman resistance. In production conditions, it is precisely this method of increasing the Raman resistance of medium-doped magnalium that has found widespread use.

For low-alloy alloys with a magnesium content of more than 1.4%, the use of thermal and thermomechanical treatment methods that promote uniform distribution of the B-phase plays a lesser role than for medium- and high-alloy alloys. However, in the semi-hardened state obtained using the LTMT effect, in addition to the appearance of structural anisotropy, which inhibits the spread of corrosion deeper, a more uniform distribution of the B-phase also appears to have a positive effect. For example, the depth of corrosion on sheets of AMg2 alloy subjected to TMT is significantly reduced compared to the depth of corrosion on conventional cold-worked sheets.

The increase in the depth of local lesions in the AMg2 alloy in the annealed state under marine atmosphere conditions can also be partially associated with the heterogeneity of the B-phase precipitates. Thus, for the AMg2 alloy it is advisable to use a technology that allows one to obtain a uniform distribution of the excess phase. However, even when using conventional technology, the low content of alloying elements turns out to be a decisive factor in determining the corrosion resistance of this alloy. This is confirmed by the fairly high corrosion resistance of the AMg2 alloy in different environments.

A typical example is the behavior of Magnalia in sea water. After 10 years of testing, the AMg2 type alloy had a corrosion resistance very close to that which it has in the marine atmosphere (Table 30).

The AMg4 type alloy has a significantly greater depth of corrosion pitting in seawater than the AMg2 type alloy. For an alloy of the AMg5 type, the maximum pitting depth increases even more sharply.

Thus, in seawater there is a clear correlation between sensitivity to structural corrosion (i.e., stress corrosion cracking and exfoliation corrosion) and normal pitting. With an increase in the degree of alloying, the supersaturation of the solid solution increases and, accordingly, the sensitivity to structural corrosion associated with the tendency to selective precipitation of the B-phase. In this regard, for the AMg4, AMg5 and especially AMg6 alloys, the role of technological factors that determine the uniform distribution of the B-phase in the alloy increases.

One of the effective ways to increase the corrosion resistance of medium-alloyed magnalium is TMT. In accordance with this, the maximum resistance of RSC and CR can be achieved only when a polygonized structure is formed in semi-finished products in combination with a uniform distribution of the second phase. Positive results can be achieved by also using annealing modes at a temperature below the solubility line of magnesium in aluminum at the final stage of processing. It should be taken into account that semi-finished products with different degrees of recrystallization behave differently. Currently, structures are made from annealed semi-finished products with a partially (pressed and hot-rolled semi-finished products) and completely recrystallized (cold-rolled sheets and pipes) structure. Since the correlations between technological parameters and corrosion properties change depending on the nature of the structure, we will consider the effect of annealing separately for cold- and hot-deformed semi-finished products.

Classification of aluminum alloys. Analysis of the results obtained State diagram aluminum magnesium

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