RU2556849C1 - High-strength heat-treatable aluminium alloy and article made thereof - Google Patents

Abstract

FIELD: metallurgy.

SUBSTANCE: invention relates to high-alloyed superhard alloys based on aluminium of Al-Zn-Mg-Cu system to be used as a structural material in aircraft engineering and rocketry, surface vehicles and instrument making. This alloy and article made thereof contain the following components, wt %: zinc - 8.5-9.3, magnesium - 1.6-2.1, copper - 1.3-1.8, zirconium - 0.06-0.14, manganese - 0.01-0.1, iron - 0.02-0.10, silicon - 0.01-0.05, chromium - 0.01-0.05, beryllium - 0.0001-0.005, hydrogen - 0.8·10^-5-2.7·10^-5 and at least one of elements of the group including titanium - 0.02-0.06, boron - 0.001-0.01, aluminium making the rest. Total content of zinc, magnesium and copper may not exceed 12.5-13.0%. Total content of transition metals, i.e. zirconium, manganese and chromium may not exceed 0.25-0.30%. Iron-to-silicon should make at least 1.5.

EFFECT: higher strength and crack resistance.

5 cl, 2 tbl, 1 ex

Description

The invention relates to the field of non-ferrous metallurgy of aluminum-based alloys, namely to high-strength high-alloy alloys of the Al-Zn-Mg-Cu system used as a structural material for the main (usually long) power elements of the airframe (skins and stringer set of the wing top , racks, beams, etc.), missiles, as well as for products of vehicles (mainly ground) and instrument-loaded equipment.

A series of modern widespread high-strength and heavy-duty alloys of various purposes is known for the traditional Al-Zn-Mg-Cu system, additionally and effectively alloyed with a microadditive of a zirconium transition element to increase ductility, manufacturability (including hardenability), strength.

These include, first of all, the Russian high-strength alloy (RF patent No. 2165995 C1, 10/05/1999) with high fracture toughness, containing the following components, wt.%:

Zinc 6.35-8.0 Magnesium 0.5-2.5 Copper 0.8-1.3 Zirconium 0.07-0.20 Titanium 0.03-0.10 Manganese 0.01-0.1 Chromium 0.01-0.05 Iron 0.06-0.26 Silicon 0.01-0.20 Beryllium 0.0001-0.05 Aluminum Rest

Alcoa American Alloy 7085 [New Generation High-Strenght and High Damage Tolerance 7085 Thick Alloy Product with Low Quench Sensitivity Proc. of the ICAA-9, 2004, p. 969-974] has the following chemical composition, wt.%:

Zinc 7.0-8.0 Magnesium 1.2-1.8 Copper 1.3-2.0 Zirconium 0.08-0.15 Titanium <0.06 Manganese <0.04 Chromium <0.04 Iron <0.08 Silicon <0.06 Aluminum Rest

The German company Otto Fuchs has developed a new alloy 7037 [A New High Strength Aluminum Alloy for Aerospace Application. Proc of the ICAA-11, 2008, p. 209-214] chemical composition, wt.%:

Zinc 7.8-9.0 Magnesium 1.3-2.1 Copper 0.6-1.1 Zirconium 0.06-0.25 Iron ≤0.10 Silicon ≤0.10 Titanium ≤0.10 Manganese ≤0.5 Chromium ≤0.04 Aluminum Rest

These alloys are stronger than previously introduced and used alloys with zirconium (7010, 7050), but they do not have a sufficiently high level of static strength and specific strength characteristics, which do not allow to fully achieve flight characteristics, increase the weight efficiency of products to increase fuel economy, range, speed and flight altitude, carrying capacity, etc.

Common to these alloys is that they are mainly offered for bulk (up to 150-200 mm thick) semi-finished products as applied to complex products of the internal power set (spars, fittings, etc.) and should have a low sensitivity to quenching cooling rate.

It should be noted that among these alloys, the European alloy 7037 is more alloyed with zinc (to obtain high strength), but slightly alloyed with copper. But it has an increased limiting content of manganese and silicon, which leads to the appearance of additional coarse harmful excess intermetallic compounds and secondary dispersoids and a deterioration in service characteristics.

Alcoa's US patent (US patent No. 7097719 B2, 08.29.2006) describes a high-strength alloy for various semi-finished products, including long ones, up to 76 mm thick with improved fatigue resistance (due to regulation of impurities) of the following chemical composition, wt. %:

Zinc 7.6-8.4 Magnesium 1.8-2.3 Copper 2.0-2.6 Zirconium 0.088-0.25 Titanium <0.06 Iron 0.01-0.09 Silicon 0.01-0.06

The main disadvantage of this alloy is the high copper content (more than 2.0%), which causes the appearance of excess gross unfavorable phases: soluble (type S phase - Al ₂ CuMg) and insoluble of various compositions (as a result of the active interaction of copper with an admixture of iron). This also includes insufficient plasticity in the molten state and, accordingly, the tendency to form cracks in ingots, especially large-sized flat ones for rolled semi-finished products.

For heavily loaded parts, Russia has created a high-alloy high-strength alloy based on aluminum (RF patent No. 2164541 C2, 02/05/1999), for which the static strength characteristics are very important.

The alloy has the following chemical composition, wt.%:

Zinc 8.0-9.0 Magnesium 2.3-3.0 Copper 2.0-2.6 Zirconium 0.10-0.20 Iron 0.05-0.3 Silicon 0.03-0.15 With the ratio Fe / Si≥0.5 Beryllium 0.0001-0.002 Hydrogen 0.9-3.610 ^-5 Aluminum Rest

In accordance with the tasks, the alloy is heavily alloyed, including magnesium and copper, which provides high values of static and structural strength. However, such alloying, as well as the presence of an increased maximum content of impurities, is undesirable for an alloy with special requirements for combining strength characteristics with fracture toughness, corrosion and other properties necessary for aircraft and other structures.

The closest chemical composition to the present invention is a high-strength alloy 7056 (Recently-developed aluminum solutions for aerospace applications. Proc. Of ICAA-10, Canada, 2006, p.p. 1271-1278.), Containing, wt.%:

Zinc 8.5-9.7 Magnesium 1.5-2.3 Copper 1.2-1.9 Zirconium 0.05-0.15 Iron <0.12 Silicon <0.10 Titanium <0.08 Manganese <0.20

The disadvantages of this highly alloyed alloy (mainly for elements of aircraft structures) are as follows:

- high and ultrahigh strength is ensured by strong alloying with the main components (zinc, magnesium, copper) at an unlimited maximum amount (up to 13.9%), which is higher than their total solubility in solid aluminum solution (to obtain maximum strength), and leads to the formation of excess coarse soluble intermetallic compounds and, accordingly, to a decrease in the characteristics of crack resistance, ductility, fatigue resistance;

- insufficient restriction of impurities of iron, silicon and transition elements of titanium, manganese causes the formation of insoluble coarse eutectic and primary intermetallic compounds and secondary dispersoids, also leads to a decrease in the required operational characteristics (fracture toughness, etc.), especially without regulation of their ratio; the latter does not additionally provide large-sized ingots as a result of the formation of crystallization cracks;

- the alloy composition does not create optimal conditions for the formation of the structure and the necessary set of operational characteristics of critical structures, such as wing skins and stringers, aircraft racks, etc., required for modern and promising aviation products.

The technical task of the present invention is the creation of an alloy with improved mechanical properties, combined with the required level of performance required for the power elements of an airframe, rockets and other products, with sufficient traditional technology for the production of various deformable semi-finished products, especially long ones.

The technical result of the present invention is to increase the strength characteristics and fracture toughness.

To achieve a technical result, a high-strength aluminum-based alloy is proposed, including the main components zinc, magnesium, copper with their regulated maximum quantity, additives of transition metals zirconium, manganese, chromium, also with regulated restrictions, the ratios of impurity elements of iron, silicon, at least one an element from the group comprising titanium and boron, characterized in that it additionally contains beryllium and hydrogen in the following ratio of components, wt.%:

Zinc 8.5-9.3 Magnesium 1.6-2.1 Copper 1.3-1.8 Zirconium 0.06-0.14 Manganese 0.01-0.1 Iron 0.02-0.10 Silicon 0.01-0.05 Chromium 0.01-0.05 Beryllium 0.0001-0.005 Hydrogen 0.8 · 10 ^-5 -2.7 · 10 ^-5 Aluminum Rest

And at least one element from the group:

Titanium 0.005-0.06 Boron 0.001-0.01

Preferably, the sum of the main alloying elements of zinc, magnesium, copper should not exceed 12.5-13.0%.

Preferably, the total content of transition elements of zirconium, manganese and chromium should not exceed 0.25-0.30%.

Preferably, the ratio of iron to silicon should be at least 1.5 with a strong limitation of the content of both impurities, especially silicon.

Along with the main element, the zirconium anti-recrystallizer, the presence of small amounts of chromium and manganese in the proposed alloy, while regulating the total amount of elements not exceeding 0.25-0.30%, contributes to the formation and stabilization of the unrecrystallized grain structure, the nucleation of hardening phases and, accordingly, an additional increase in strength , and also positively influences resistance to stress corrosion cracking and delaminating corrosion.

The introduction of beryllium reduces oxidizability and improves fluidity during melting, increasing the quality of ingots and semi-finished products (especially large ones). The presence of hydrogen in microdoses contributes to the formation of a fine-grained structure, a uniform distribution of the inevitable non-metallic microinclusions in the volume of ingots and semi-finished products, and an increase in their ductility.

Small additives of titanium and / or boron, which have a modifying effect, lead to heterogeneous crystallization of the alloy and grain refinement and, accordingly, to improve the ductility of ingots and semi-finished products and to expand the possibility of increasing their size and improving quality.

The excess of the content of iron impurities over the content of silicon impurities (more than 1.5 times) with strict control and regulation (to limit the appearance of coarse insoluble intermetallic compounds and adversely affect the strength and performance properties) is necessary to improve the casting properties of highly alloyed alloys in order to obtain large ingots for long semi-finished products.

Maintaining a moderate amount of copper (up to 1.8%) and magnesium (up to 2.1%) while increasing the zinc content (up to 9.3%) and maintaining the overall degree of alloying with the main components in the alloy provides increased strength characteristics. At the same time, the possibility of the formation of excess copper-containing intermetallic phases and their negative effect on the characteristics of fracture toughness, ductility, and fatigue is limited.

Corrosion resistance to dangerous types of corrosion - corrosion cracking (CR), delaminating corrosion (RAC), is mainly regulated by artificial aging.

Examples of implementation

In conditions of pilot production, ingots were cast, the chemical compositions of which are given in table. 1. The ingots had a diameter of 110 mm, obtained by a semi-continuous method with surface cooling with water. Smelting was carried out in an electric furnace. After homogenization at a temperature of 460 ° C for 24 hours, a microanalysis of the structure of the cross-section of the ingots was carried out in detail using optical and electron microscopy, X-ray spectral analysis (MRSA), and differential thermal analysis (DTA).

The values of the average grain d _cp in the studied ingots and semi-finished products were estimated by the method of quantitative metallography in polarized light on oxidized microsections; quantitative metallography was widely used in the analysis of the volume fraction and form of intermetallic phases. To study the nature and ductility of fracture, we used fractographic analysis using an electron scanning microscope.

After homogenization, ingots were pressed at 390–410 ° C into strips with a cross section of 15 × 70 mm with a drawing coefficient> 8.0. According to DTA, the crystallization temperature of the eutectics of the studied alloys was in the range 473–476 ° C. The blanks from the pressed strips were quenched from a temperature of 470 ° C, taking into account the differences in the furnace (after a long exposure of 90 minutes) in cold water (20-25 ° C). In the freshly quenched state, the preforms were stretched with an average degree of deformation of ~ 1.5%. Within 4 hours after quenching, the strips were subjected to various artificial aging: variant T1 in a single-stage (for maximum strength) mode 120 ° C, 24 hours and a variant in a two-stage mode of type T22 (in the first stage at 120 ° C, 1.5 h + on second stage at 150 ° C and a small degree of overcooking - 10-20 MPa).

The complex of mechanical and corrosion properties was investigated on samples cut from pressed strips.

The tensile mechanical properties (tensile strength, yield strength, elongation) were determined on round samples with a working part diameter d ₀ = 5 mm according to GOST 1497. Fracture resistance was evaluated by the specific fracture work (CCT) under shock bending of a specimen with a fatigue crack in V- shaped notch, as well as impact strength (KCU) of specimens with a U-shaped notch according to GOST 9454.

) и частоте f=3 Гц.The resistance to low-cycle fatigue (MCU) was estimated by the time until fracture of circular longitudinal specimens with an annular notch (K _t = 2.2) at high voltage ( $${\sigma }_{\max }=0.7{\sigma }_B^n$$

) and frequency f = 3 Hz.

Corrosion properties were studied by:

- resistance to delaminating corrosion (RSK) of flat longitudinal samples according to a 10-point system in accordance with GOST 9.904;

- resistance to stress corrosion cracking (CR) in time to fracture of transverse samples at a stress of σ = 0.75σ _0.2 and other conditions in accordance with GOST 9.019;

- electrical conductivity by eddy current non-destructive method according to OST 1 92133.

In the table. 2 presents a complex of mechanical (including fracture toughness indices) and corrosion properties of pressed strips of declared and known alloys, volumetric content of excess intermetallic compounds in alloys.

As can be seen from the obtained and presented results, the composition of the proposed alloy made it possible to obtain a high level of strength properties and fracture toughness indices (with transcrystalline fracture) with high ductility (relative elongation) and acceptable corrosion resistance to delaminating corrosion and corrosion cracking.

Thus, the proposed high-strength alloy provides an increase in weight efficiency while ensuring the resource and reliability of operation of the products.

The alloy is intended as a structural material for the basic elements of an airframe, especially in compressed areas (casing and stringers of the upper wing, power struts, beams, etc.), rocket technology and other products.

Rolled (sheets, plates), extruded (profiles, panels, etc.) semi-finished products, including long ones from large ingots with an increased level of strength and operational (including high fracture toughness) characteristics, are made of an alloy.

Claims (5)

1. High-strength aluminum-based alloy containing zinc, magnesium, copper, zirconium, manganese, iron, silicon, chromium, at least one element from the group comprising titanium and boron, characterized in that it additionally contains beryllium and hydrogen in the following the ratio of components, wt.%:
zinc 8.5-9.3 magnesium 1.6-2.1 copper 1.3-1.8 zirconium 0.06-0.14 manganese 0.01-0.1 iron 0.02-0.10 silicon 0.01-0.05 chromium 0.01-0.05 beryllium 0.0001-0.005 hydrogen 0.8 · 10 ^-5 -2.7 · 10 ^-5 at least one element from the group:
titanium
boron
aluminum 0.005-0.06
0.001-0.01
rest 2. An aluminum-based alloy according to claim 1, characterized in that the total content of zinc, magnesium and copper does not exceed 12.5-13.0%. 3. An aluminum-based alloy according to claim 1, characterized in that the total content of zirconium, manganese and chromium does not exceed 0.25-0.30%. 4. An aluminum-based alloy according to claim 1, characterized in that the ratio of iron to silicon is at least 1.5. 5. The product is made of a high-strength alloy based on aluminum, characterized in that it is made of an alloy according to claim 1.