Corrosion-resistant aluminium-based deformable alloy and articles made of same

Abstract

The invention relates to the field of non-ferrous metallurgy, and more particularly to heat-treatable aluminium alloys based on an aluminium-zinc-magnesium system, which can be used in the production of construction materials using forming techniques, and which are able to withstand high loads and environmental corrosion. A corrosion-resistant aluminium-based deformable alloy and articles made of same contain the following components: 4.2-5.2 wt.% zinc, 2.0-3.0 wt.% magnesium, 0.21-1.0 wt.% copper, 0.3-1.0 wt.% manganese, up to 0.3 wt.% iron, 0.05-0.3 wt.% chromium, 0.08-0.15 wt.% zirconium, and 0.05-0.15 wt.% scandium, and further contain at least one element from the group consisting of strontium and calcium in a combined or individual amount of 0.005-0.02 wt.%. The technical result is the creation of a deformable material for the manufacture of forged or formed parts and other types of semi-finished parts, which exhibit improved mechanical characteristics.

Description

[0001] DEFORMABLE CORROSION-RESISTANT ALLOY

[0002] ALUMINUM-BASED AND PRODUCTS MADE FROM IT

[0003] Field of technology

[0004] The invention relates to the field of non-ferrous metallurgy, namely, to thermally hardenable aluminum alloys based on the aluminum-zinc-magnesium system, used in the production of structural materials using volume stamping technology, operating under the influence of high loads under the corrosive effects of the environment.

[0005] State of the art

[0006] Al-Zn-Mg alloys are known to be the strongest wrought aluminum alloys. However, these alloys are susceptible to corrosion and, in particular, stress corrosion cracking (SCC) (also referred to as SCC). Various methods are known for alloying Al-Zn-Mg alloys with additional elements to improve corrosion resistance without compromising strength.

[0007] Aluminum alloy 7017 (the composition is described in the International Alloy Designations and Chemical Composition Limits for Wrought Aluminum and Wrought Aluminum Alloys, The Aluminum Association, Inc., August 2018, p. 13) is known for its average strength for this series of alloys, but also for its good wear and corrosion resistance. This alloy has the following composition:

[0008] Zn 4, 0-5, 2

[0009] Mg 2.0-3.0

[0010] Fe < 0.45

[0011] Si < 0.35

[0012] Si < 0.20

[0013] Ti < 0.15

[0014] Zr 0.10-0.25

[0015] Ni < 0.10

[0016] Cr < 0.35

[0017] Mn 0.050-0.50

[0018] in this case Cr + Mn > 0.15

[0019] inevitable impurities in total no more than 0.15, each 0.05

[0020] aluminum the rest. The specified alloy grade 7017 has an average level of strength characteristics in the Tb condition (heat treatment to obtain maximum strength in accordance with the standard EN 515:1993-12) with a tensile strength (with в ) equal to 460 MPa, conditional yield strength (σ, 2) equal to 400 MPa and elongation at break (δ) equal to 11%. In the T73 condition (in accordance with the EN 515:1993-12 standard, a more corrosion-resistant condition), its strength characteristics are lower, with в= 420-440 MPa, oo,2 = 350-370 MPa, 5 = 9-10%. The disadvantage of this alloy grade 7017 is the insufficient yield strength (350-370 MPa) in the T73 condition, which does not allow the production of lighter wheel rims by reducing the cross-section of the loaded material of the rim (spokes, hubs, rims, etc.).

[0021] An aluminum alloy described in application JPH08120386A is known, with the following chemical composition (mass%):

[0022] Zn 3.0 - 8.0

[0023] Mg 0.5 - 3.0

[0024] Bi 0.01 - 0.5

[0025] C 0.1 - 3.0

[0026] Ti 0.05 - 0.3

[0027] MP 0.1 - 1.5

[0028] Cr 0.05 - 0.4

[0029] Zr 0.05 - 0.4

[0030] V 0.05 - 0.4.

[0031] According to the description, the alloy has high resistance to intergranular corrosion, but no characteristics are given for strength and corrosion resistance under stress, which makes it impossible to evaluate these characteristics.

[0032] Domestic aluminum alloy grade 1941 (GOST 4784-2019) is a well-known high-strength, corrosion-resistant, heat-hardenable aluminum alloy of the Al-Zn-Mg-(Cu) system. This alloy was developed and used to manufacture various products in the form of profiles, sheets, and stampings for the oil and gas industry, equipment and structures for offshore platforms, deepwater risers, drill pipes, and more. According to GOST 4784-2019, this alloy has the following composition (wt%):

[0033] Zn 5, 0-5, 6

[0034] Mg 2, 1-2.7

[0035] Si 0.15-0.30

[0036] Mp 0.3-0.5Cr 0.12-0.22

[0037] Si 0.2

[0038] Fe 0.4

[0039] Zr 0.1 -0.2

[0040] other elements each 0.05, total 0.1,

[0041] Al the rest.

[0042] Products made from alloy grade 1941 after heat treatment in the T1 condition (in accordance with the domestic classification of hardening and artificial aging for maximum strength, equivalent to the foreign T6 condition according to EN 515:1993-12) have the following strength characteristics: tensile strength of at least 460 MPa, yield strength of 390 MPa, good processability, and high corrosion resistance. A disadvantage of this alloy is the insufficiently high yield strength of semi-finished products made from it, which limits its use.

[0043] A series of medium-strength aluminum alloys of the Al-Zn-Mg-(Cu) system, additionally alloyed with scandium, is known. The closest in chemical composition to the alloy proposed in the present invention, i.e., the prototype, is the aluminum-based alloy described in patent RU2293783C1, with the following chemical composition (wt%):

[0044] Zn 3.5-4.85

[0045] B 0, 3-1, 0

[0046] Mg 1.2-2.2

[0047] MP 0.15-0.6

[0048] Fe 0.01-0.15

[0049] Si 0.01-0.12

[0050] Sc 0.05-0.4

[0051] Cr 0.01-0.3

[0052] at least one element from the group:

[0053] Zr 0.05-0.15

[0054] Ce 0.005-0.25

[0055] A1 the rest.

[0056] The alloy of the specified composition has good weldability and resistance to exfoliation (RCC 2 points) and intercrystalline corrosion (ICC absent), however, the KR value of 300-350 MPa given in the patent is insufficient for highly loaded products operating in a corrosive environment and can be increased through additional alloying. Disclosure of the invention

[0057] The technical objective and technical result of the present invention is the creation of a deformable material intended for the production of forgings, stampings and other types of semi-finished products with an increased level of mechanical characteristics, in particular, with a yield strength value of at least 430 MPa in the T3 condition (a more corrosion-resistant condition in accordance with the domestic classification of types of heat treatment, similar to the foreign designation T73 according to the EN 515:1993-12 standard), as well as an increased KR value of at least 400 MPa.

[0058] To solve the stated problem and achieve the technical result, an aluminum-based alloy containing zinc, magnesium, copper, manganese, iron, chromium, zirconium, and scandium is proposed. The alloy is cerium-free and additionally contains at least one element from the strontium and calcium group, in the following component ratios (wt%):

[0059] Zn 4.2 - 5.2

[0060] Mg 2.0 - 3.0

[0061] C 0.21 - 1.0

[0062] MP 0.3 - 1.0

[0063] Fe up to 0.3

[0064] Zr 0.08 - 0.15

[0065] Sc 0.05 - 0.15

[0066] Cr 0.05 - 0.3

[0067] at least one element from the group containing:

[0068] Sr, Ca 0.005 - 0.02 (total or separately)

[0069] inevitable impurities in total no more than 0.15

[0070] aluminum is the rest.

[0071] Drawings

[0072] The invention is explained by drawings.

[0073] Fig. 1 - Ingots with a diameter of 215 mm from the proposed alloy.

[0074] Fig. 2 - Blanks of small wheel disks made on the basis of the proposed alloy.

[0075] Fig. 3 - Blanks of large wheel disks made on the basis of the proposed alloy.

[0076] Fig. 4 - Macrostructure of the cross-section of stampings with the allocation of sampling locations from the most loaded zones of wheel disks during operation: the hub zone (height direction) and the spoke zone (length direction). Detailed description of the essence of the invention

[0077] All components of the proposed alloy, concentrations and ratios were selected experimentally, taking into account the achievement of the required properties of the finished alloy to solve the problem and achieve the technical result.

[0078] In Al-Zn-Mg alloys, zinc and magnesium are the primary alloying elements, forming strengthening phases. Experiments have shown that magnesium content of 2.0-3.0 wt.% and zinc content of 4.2-5.2 wt.% provide the required strength during heat treatment. Further increases in magnesium content lead to a tendency toward intergranular corrosion. Exceeding zinc content reduces resistance to stress corrosion cracking.

[0079] It has also been experimentally established that additional alloying of Al-Zn-Mg alloys with copper reduces their susceptibility to stress corrosion and improves their mechanical properties. However, at high concentrations, it increases the alloys' susceptibility to cracking during casting and reduces their overall corrosion resistance. At copper contents of less than 1% by weight, new phases do not precipitate, and the alloy's mechanical properties are enhanced through solid-solution strengthening.

[0080] Experiments have shown that additions of transition metals such as Mn and Cr improve strength properties and positively impact stress corrosion resistance. The use of manganese at concentrations up to 0.3 wt.% has virtually no effect on mechanical properties, while concentrations above 1 wt.% lead to a decrease in ductility.

[0081] It was also found that chromium in the proposed alloy acts as an anti-recrystallizer, while also providing additional strengthening through both solid-solution hardening and intermetallic precipitation. Combined alloying with Mn and Cr accelerates the aging process in Al-Zn-Mg-(Cu) alloys, achieving maximum strength significantly faster. At chromium contents above 0.3 wt.%, the alloy's ductility decreases due to the precipitation of large quantities of intermetallic particles.

[0082] Iron is an impurity that is virtually insoluble in aluminum and only slightly increases its strength properties. However, as the iron content increases above 0.3% by weight, the effect of quenching and aging decreases due to the formation of insoluble compounds with manganese and chromium, thereby reducing the concentration of these elements in the solid solution. Also, high iron content, especially in the presence of silicon, another common impurity, reduces impact strength and fatigue life and increases the material's sensitivity to notching, which is extremely important for wheel rims, the production of which involves turning and milling. Given the initial iron content in primary aluminum at 0.1-0.3% by weight (depending on the purity of the electrolytic aluminum), it is not economically feasible to limit its content to less than 0.3% by weight.% and use a more purified (than grades A5-A8 in accordance with GOST 11069-2019) and, accordingly, more expensive grade of aluminum in the production of the alloy.

[0083] Zirconium additives are used to increase the strength of aluminum alloys. The maximum solubility of Zr in aluminum is 0.28 wt.% at 660.8°C. Zirconium's positive effect on thermal stability and mechanical properties is due to the formation of nanoscale precipitates of the metastable AhZr phase with an average size of approximately 10 nm. These precipitates form in the material during annealing at temperatures around 450°C and, in addition to strengthening the alloy, also increase the recrystallization onset temperature. It is known that alloying with zirconium, especially in the presence of manganese, significantly increases the stress corrosion resistance of Al-Zn-Mg alloys. Moreover, adding more than 0.15 wt.% of zirconium to the melt.% when combined with other elements (such as scandium and chromium) significantly increases the liquidus temperature (above 720-730 °C), which can cause the formation of defects in the form of large crystals of refractory compounds (in accordance with GOST R 57126-2016) during the process of semi-continuous casting of ingots.

[0084] One of the most effective alloying elements, the addition of which improves mechanical properties, is scandium Sc (with a maximum solubility in aluminum of 0.38 wt.% at 660 °C). Like zirconium, scandium is a modifier and, when combined, exerts an anti-recrystallization effect on the structure of the material. Scandium forms a coherent nanodispersed phase AhSc with aluminum with an N2 structure. When combined with zirconium and scandium, two-component nanodispersed precipitates Ah(Zr,Sc) are formed with scandium distributed in the center and zirconium along the periphery of the particles. The precipitation temperature of Sc from the solid solution is lower than that of zirconium and lies in the range of 300 - 400 °C, which allows for a more complete precipitation of zirconium and a more dispersed distribution of nanostrengthening precipitates Ah(Sc,Zr) with a smaller particle size under multi-stage heat treatment conditions.Scandium, despite its high efficiency, is a significantly expensive material and increasing its concentration above 0.15 wt.% will lead to a significant increase in the cost of the alloy.

[0085] Both strontium and calcium are effective modifiers for aluminum alloys. Small amounts of strontium additions provide a stable modification effect, refine grain size, improve ductility, and favorably influence the alloy's corrosion properties. Calcium's effect on structure and properties is primarily due to its modifying action and, secondly, to the formation of intermetallic compounds АЬСа and АЦСа, which compact grain boundaries, reduce grain growth during high-temperature heating, promote grain spheroidization, and, as a result, improve the strength and ductility of the material. Experiments have established the optimal strontium and calcium content in the proposed aluminum alloy, ranging from 0.005 to 0.02 wt.% combined.

[0086] Cerium's solubility in aluminum is negligible (0.05%), and in aluminum alloys containing cerium, intermetallic phases (AKCe) precipitate, which can reduce ductility. Cerium also reacts with magnesium to form MgCe, which in turn reduces the strengthening effect of the main alloying elements. In the prototype, the presence of cerium is justified by its modifying effect, but when alloyed with other modifying elements such as zirconium, scandium, strontium, and calcium, the addition of cerium is excessive.

[0087] The proposed alloy can be used to manufacture various stamped and other deformable semi-finished products (sheets, plates, extruded sections) that possess both high mechanical properties and high stress corrosion resistance. This alloy is particularly suitable for the production of lightweight automotive wheels, for which not only high mechanical properties but also high stress corrosion resistance are critical due to the wheels' use in corrosive atmospheres, including exposure to roadside chemicals. Implementation of the invention

[0088] Example 1

[0089] Ingots with a diameter of 215 mm (Fig. 1) were produced under industrial conditions using semi-continuous casting. The ingots were made using grade A8 aluminum (in accordance with GOST 11069-2019) as a base (raw material) and alloyed with Zn, Mg, Cu, Mn, Cr, Sc, Zr, as well as Sr and Ca in various ratios. The ingot compositions for the three heats are shown in Table 1.

[0090] Table 1. Chemical composition of ingots according to Example 1 with a diameter of 215 mm

[0091]

[0092] After casting, the ingots were subjected to multi-stage homogenization annealing with a maximum temperature of 480 °C and a holding time of up to 24 hours. Homogenization annealing is necessary for Al-Zn-Mg alloys not only to equalize the composition across the ingot cross-section, dissolve excess phases, and relieve casting stresses before cutting the ingots, but also to precipitate nanodispersed strengthening phases Ah(Sc,Zr).

[0093] Small wheel rim blanks (Fig. 2) were produced from ingots of three compositions using the hot die forging method for subsequent final milling and production of 18-inch motorcycle wheel rims. The stamped blanks (before final milling) were subjected to hardening at a temperature of up to 480°C, followed by two-stage aging in a mode providing the T73 condition according to the EN 515:1993-12 standard (to ensure high resistance to stress corrosion cracking) to obtain an optimal ratio of the mechanical properties, namely, a yield strength (ω, 2) of at least 430 MPa, and a critical stress (CSC) value of at least 400 MPa.

[0094] To control the material properties of the stampings, samples were taken from the areas of the wheel rims that were most heavily loaded during operation: the hub zone (elevation direction) and the spoke zone (lengthwise direction). The macrostructure of the stamping cross-section with the sampling locations highlighted is shown in Fig. 4. The mechanical properties were tested according to GOST 1497-84. Exfoliation corrosion tests (ECT) were conducted according to GOST 9.904-82. Uniaxial tensile stress corrosion cracking tests were conducted according to GOST 9.901.4-89 using the Signal setup.

[0095] The critical stress (CST) value, which represents the maximum stress that the test specimens can withstand (for 45 days) without failure in a corrosive environment, was used as a criterion characterizing the alloys' susceptibility to stress-corrosion cracking. The mechanical and corrosion properties of the forged specimens are presented in Table 2.

[0096] Table 2. Properties of small wheel rim stampings

[0097]

[0098] Where:

[0099] Ov - ultimate strength according to GOST 1497-84;

[0100] oo,2 – conditional yield strength according to GOST 1497-84;

[0101] 5 - relative elongation at break according to GOST 1497-84;

[0102] RSC - exfoliating corrosion in accordance with GOST 9.904-82;

[0103] GKP - critical tensile stress during long-term testing in a corrosive environment according to GOST 9.901.4-89.

[0104] The results obtained on samples of stampings from ingots of the proposed composition confirm higher values ​​of tensile strength (o в with a minimum value of 490 MPa), yield strength (with a minimum value of 430 MPa) and the OKR value (with a minimum value of 400 MPa) compared to the prototype (with a minimum value of 490 MPa) в 420 MPa, 2 390 MPa, sKR 300 MPa). Example 2

[0105] Ingots with a diameter of 365 mm were produced under industrial conditions using semi-continuous casting using A8 aluminum as a base (raw material) and alloyed with Zn, Mg, Cu, Mn, Cr, Sc, Zr, as well as Sr and Ca in various ratios. The ingot compositions for the three heats are shown in Table 3.

[0106] Table 3. Chemical composition of ingots according to Example 2 with a diameter of 356 mm

[0107]

[0108] After casting, the ingots were subjected to multi-stage homogenization annealing with a maximum temperature of 480 °C and holding for up to 24 hours. As already noted, homogenization annealing is necessary for alloys of the Al-Zn-Mg system not only to equalize the composition across the ingot cross-section, dissolve excess phases and relieve casting stresses before cutting the ingots, but also to separate nanodispersed strengthening phases Ah(Sc,Zr).

[0109] Large wheel blanks (Fig. 3) were produced from ingots of three compositions using the hot die forging method, ready for final milling and the production of 20-inch diameter automobile wheels. The stamped blanks (prior to final milling) were quenched at temperatures up to 480°C, followed by two-stage aging in a mode ensuring a T73 condition according to standard EN 515:1993-12 (to ensure high resistance to stress-corrosion cracking) to achieve an optimal balance of mechanical properties and corrosion resistance.

[0110] To monitor the material properties of the stampings, samples were taken from the areas of the wheel rims that were most heavily stressed during operation: the hub zone (elevation direction) and the spoke zone (length direction). The cross-sectional macrostructure of the stampings, with sample locations highlighted, is shown in Fig. 4. The mechanical and corrosion properties of the stampings are presented in Table 4. Table 4. Properties of Large Wheel Rim Stampings.

[0111]

[0112] The examples provided demonstrate that the proposed technical solution in the form of a new aluminum alloy with the specified qualitative and quantitative composition allows for the production of products by the method of volume stamping with the required level of mechanical properties in a corrosion-resistant state (TZ in accordance with the domestic classification or T73 in accordance with EN 515:1993-12), namely with a value of the conditional yield strength (σ, 2) of at least 430 MPa, as well as an increased value of KR of at least 400 MPa, which exceeds the data for the prototype and analogues.

Claims

CLAUSES OF THE INVENTION 1. A deformable corrosion-resistant aluminum-based alloy containing zinc, magnesium, copper, manganese, iron, chromium, zirconium, scandium, characterized in that it additionally contains at least one element from the group containing strontium and calcium, in the following ratio of components, mass%: Zn 4.2 - 5.2 Mg 2.0 - 3.0 C 0.21 - 1.0 MP 0.3 - 1.0 Fe up to 0.3 Zr 0.08 - 0.15 Sc 0.05 - 0.15 Cr 0.05 - 0.3 at least one element from the group containing: Sr, Ca 0.005 - 0.02 (total or separately) inevitable impurities in total no more than 0.15 aluminum is the rest.

2. An alloy according to item 1, characterized in that it has in its structure nanodispersed precipitates with a crystal lattice of the AhSc and AhZr phases, as well as complex Ah(Sc,Zr) phases.

3. A metal product based on an aluminum alloy, made in the form of a stamping, characterized in that it is made from an ingot based on an aluminum alloy according to any of paragraphs 1-2.

4. A metal product according to clause 3, having a yield strength of at least 430 MPa and a stress corrosion cracking value of at least 400 MPa.

5. A metal product according to item 3, the ingot for the production of which, after casting from the alloy, is subjected to multi-stage homogenization annealing with a maximum temperature of 480 °C and holding for up to 24 hours, ensuring the alignment of the composition across the cross-section of the ingot, the dissolution of excess phases, and the precipitation of nanodispersed strengthening phases Al3(Sc,Zr).

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