JPH0447019B2 - - Google Patents

Description

[Detailed description of the invention]

TECHNICAL FIELD The present invention relates to high strength aluminum alloys, aluminum/lithium (Al/Li) alloys, which are particularly suitable for aerospace vehicle structures. PRIOR ART Such alloys are attractive because they allow a notable weight reduction of less than 20% over other aluminum alloys, and these alloys have high strength and stiffness and good corrosion resistance. . However, to date, Al/Li alloys have suffered from reductions in other properties, such as fracture toughness, compared to other aircraft alloys, and have been difficult to cast and subsequently process. Most Al/Li alloys proposed so far are based on the Al/Li/Mg system (e.g. Li2.1% and Mg5.5
%) or by using relatively high levels of lithium addition to conventional aerospace alloys in powder metallurgy, e.g. 3% or more Li addition to alloy 2024. was. Recently, the addition of Mg and Cu has been proposed, for example, Li3% or more,
The additions are about 1.5% Cu, about 2% Mg, and about 0.18% Zr. This gives the alloy improved fracture toughness and facilitates hot and cold working. SUMMARY OF THE INVENTION We have made additional improvements in ease of manufacture and subsequent processing by further modifying the lithium, magnesium and copper content of the alloy and by applying specific heat treatments to hot rolled blanks made from cast ingots. I discovered that it can be achieved by applying We also provide Al/Li alloys that can be cast into large ingots using the Direct Chill Casting Process (ie, semi-continuous casting process). According to one aspect of the invention, the composition in weight percent ranges from: lithium 2.3-2.9%; magnesium 0.5-1.0%; copper 1.6-2.4%; zirconium 0.05-0.25%; and aluminum balance (including incidental impurities). The peak value of tensile strength after aging treatment is
Provided is a high-strength aluminum alloy with good formability that has a strength of 480 MPa or more and a density that is 95% or less of pure aluminum. Al/Li alloy is basically a low-density, high-rigidity alloy, and even in the Al/Li alloy according to the present invention, the density can be lowered as the lithium content increases, and there is a difference between the Al/Li alloy composition and density. has the following relationship. Density = 2.71 + 0.024Cu + 0.018Zn−0.079Li−
0.01Mg Here, the unit of density is g/cm ³ and each element is % by weight. In the composition range defined in claim 1 of the present invention, the composition that maximizes the density is
It contains 2.3% lithium, 2.4% copper and 0.5% magnesium, and the density calculated by substituting these into the above formula is about 95% of pure aluminum. On the other hand, the composition that minimizes the density is 2.9% lithium, 1.0% magnesium, and 1.6% copper, and the density at this time is 92.6% of pure aluminum. As the lithium content increases, the density can be reduced, but during processing and use
The upper limit is set at 2.9% because the Al/Li alloy becomes more brittle and becomes difficult to process. This increased brittleness is due to the brittle intermetallic phase, delta phase and C
This is consistent with the tendency of alloying phases to form as lithium increases. On the other hand, the lower limit of lithium content is
This is the amount necessary to obtain an effective reduction in density. Magnesium has a relatively small reducing effect on alloy density, but has a strength improving effect at contents up to 1%. If it exceeds 1%, the strength improvement will not be significant and the toughness will decrease. The lower limit of the magnesium content is set by the appearance of an appropriate strength-enhancing effect and the influence of the copper to magnesium (Cu:Mg) ratio on the precipitation of intermetallic phases. Copper is added to increase strength, but increases density according to the above formula. The upper limit of the copper content is set by the relationship between the lithium content and the required amount of magnesium to control the precipitation of intermetallic compounds. The lower limit amount is set as the amount that provides an appropriate strength-improving effect. The ratio of copper to magnesium is determined by the S phase (Al-Cu-
Mg compound) and T ₁ phase (Al-Cu-Li compound)
is defined in part by the appearance of two intermetallic compounds. The S phase improves mechanical properties, while
_T1 phase reduces mechanical properties. Therefore,
Setting the copper to magnesium ratio to favor S phase formation can give Al/Li alloys more favorable properties (increasing fracture toughness and strength), with ratios ranging from 1.6:1 to 4.8:1. 3:1 is preferred. Zirconium is added as a grain refining element. The lower limit of zirconium content (0.05%) is the amount at which its effective effect appears, while the upper limit (0.25%)
%) is the amount that can be reliably dissolved in molten aluminum. Titanium up to 0.5%, manganese up to 0.5%, 0.5
It is preferable to optionally add at least one element of nickel of 0.5% or less and chromium of 0.5% or less for the following reasons. Titanium is added to control grain size in the as-cast product. An upper limit for titanium addition (0.5%) is set to avoid the formation of large intermetallic compound (TiAl ₃ ) particles that reduce fracture toughness. Manganese is added when producing recrystallized products (sheets) and to improve fracture toughness, albeit with a reduction in tensile strength. When annealing cold-worked products, manganese promotes recrystallization. If the amount of manganese is too large (more than 0.5%), coarse grain precipitation of MnAl ₆ and other intermetallic compounds, which adversely affect fracture toughness, tends to occur. Nickel is added for the same reason as manganese, and an upper limit is set, and the intermetallic compound contains nickel. And the reason chromium is added is to help prevent recrystallization when annealing cold-worked items. Chromium forms fine precipitates in the alloy composition that inhibit recrystallization. If the amount of chromium is too large (more than 0.5%), it will lead to intermetallic compound coarse particles that reduce fracture toughness. It has long been recognized that mechanical deformation by methods such as hot and cold rolling leads to the development of crystallographically preferred orientations in metallic materials in sheet or strip form. This has been demonstrated in several ways, many of which are quite detrimental to the properties of the product. In particular, anisotropy in the mechanical properties results in the strength and ductility of the fabricated or fabricated and annealed product being appreciably changed with direction in the plane of the sheet or strip; These properties are measured.
These results are typical for simple aluminum-based alloys such as the 1000, 3000 or 5000 series (named by the Aluminum Institute of America) alloys, but for the 2000 and 7000 series commonly used in aircraft structures. There are no harmful consequences for aluminum alloys. However, experiments in the development of aluminum-lithium based alloys have shown that alloys with 2000 and 7000
The anisotropy of properties poses a considerable challenge when processed through a similar process to that used for alloys.
In addition, anisotropy control techniques traditionally applied to 1000, 3000 and 5000 series alloys, such as controlling the Fe to Si ratio, cannot be applied to aluminum-lithium based alloys because iron levels are necessarily low. This is because it is preserved. Therefore, special thermal and mechanical processing techniques need to be developed to control the mechanical properties anisotropy and especially elongation within acceptable limits in the alloy. Our pending UK Application No. 8308907 (Japanese Unexamined Patent Publication No. 197551) filed on March 31, 1983
discloses a heat treatment technique commonly applicable to Al/Li alloys, and which is particularly suitable for the alloys of the present invention. Therefore, the method for manufacturing the alloy according to the present invention into a sheet or strip is as follows: (a) producing an ingot of the aluminum alloy according to the present invention by direct quench casting, and (b) casting the ingot in one or multiple stages. (c) holding the hot blank at a temperature and time that causes substantially all of the lithium, magnesium, and copper and zinc to be in solid solution; and (d) hot rolling the hot blank at (e) further heat-treat the cooled blank to precipitate these age-hardened phases in solid solution; (f) continue the heat treatment to create a coarse-grained overaged structure; (g) cold rolling the blank to form a sheet or strip such that the sheet or strip has a variation in its elongation properties at any location and in all directions within 2.0% of the elongation properties in the rolling direction; Contains; The sheet or strip has a variation in its tensile strength properties at every location and in every direction that varies from its properties in the rolling direction by less than 25 MPa (0.2% stress plus tensile stress). The initial holding temperature of the hot blank is between 480°C and 540°C and the time is between 20 and 120 minutes, which are dependent on the thickness of the blank and the prior thermal history of the blank. . Hot blank is 480
Once the temperature has fallen below °C, the blank is reheated to solutionize the Li, Mg, Cu, and some Zr. Preferably the hot blank is 12.5mm to 3mm
The thickness is . The sheet or strip will be less than 10 mm thick, preferably less than 5 mm thick. Desirably, the hot blank is reliably cooled. Further heat treatment ensures reliable cooling.
treatment), so that certain cooling and heat treatment steps may be combined. This heat treatment will generally be for a period of 8 to 16 hours at a temperature between 300 and 400°C. This heat treatment of the blank not only controls the anisotropy of the cold rolled sheet or strip;
Facilitates subsequent cold rolling and, in the case of superplastic alloys, enhances their superplastic properties. The invention will be further described in connection with the following examples and with reference to the accompanying drawings. The copper-to-magnesium ratio has been found to be a key feature in an alloy that can achieve increased strength with satisfactory fracture toughness compared to previously proposed alloy compositions. This shows that the first differential scanning calorimetry curve for the three alloy compositions is
Illustrated in fig. 2.5%Li, 2.0%Cu, 0.7%
The first curve (a) for the aluminum alloy with Mg and 0.12% Zr shows aging peaks at about 200°C and about 325°C. The peak at 200°C is due to the Al/Li _three- phase, and the peak at 325°C is due to the S phase (Al-Cu-Mg) and the equilibrium Al/Li phase.
It is attributed to the combinatorial precipitation of Li phase. For Mg, 2.5% Li, 2.0% Cu, 0.45 lower than the limits of the present invention
In curve (b) for the aluminum alloy with %Mg and 0.12Zr, there is a flat bottom between 250°C and 285°C, indicating the lack of precipitation of the S phase. Finally, for Mg, 2.5% more than the limit of the present invention
In curve (c) for the aluminum alloy with Li, 2.0% Cu, 1.1% Mg and 0.12Zr, Al
-Cu-Mg addition precipitation mechanism exists. It was well known that Al/Li based alloys have low ductility and low fracture toughness due to Al/Li ₃ precipitation without dispersing slip during deformation.
The alloy according to the present invention disperses slippage and increases strength and strength.
Maximizes the precipitation of S phase, which increases ductility and toughness. Example 1 Alloy composition (wt%) Lithium 2.5 Magnesium 0.6 Copper 2.1 Zirconium 0.14 Chromium 0.05 Titanium 0.013 Aluminum Balance (includes incidental impurities) The alloy was cast as a 508 mm x 178 mm 300 kg ingot by a direct quench casting method. Next, the ingot
It was homogenized at 540°C for 16 hours and peeled to remove surface defects. The ingot was then reheated to 540°C and hot rolled into a 25mm plate. The plates were solution treated at 540°C for 1 hour, cold water quenched, stretched to 2% permanent elongation and the tensile strength of the material was evaluated after aging at 170°C for various periods. The longitudinal tensile properties are shown in Figure 2 in comparison to the minimum specified property levels for comparative alloys 2014-T651 and 7010-T7651. The alloy has been shown to have strength levels well in excess of the minimum requirements of comparative alloys. During aging treatment after compaction treatment (60 hours of aging at 170°C, the tensile strength reaches its peak value), the alloy has a 0.2% proof stress of 2014-
The tensile strength is about 100 MPa higher than typical for T651 plate and about 80 MPa higher than typical for 2014-T651 plate. Additionally, the alloy is 2014−T651
(both materials were tested in their fully heat-treated state). All heat treated alloys have 8-10% lower density and 10-15% higher modulus than all specified aerospace aluminum alloys. Example 2 Lithium 2.8% Magnesium 0.9% Copper 21.8% Zirconium 0.12% Aluminum Balance (includes incidental impurities) The alloy was cast as a 508 mm x 178 mm 300 kg ingot by a direct quench casting method. The ingots were then homogenized at 540°C for 16 hours and peeled to remove surface defects. The ingot was then reheated to 540°C and hot rolled into a 5 mm thick hot blank. Blank our pending UK application no.
Heat treatment was performed according to the heat treatment schedule detailed in No. 8308907. In particular, 5mm thick hot blanks
Solution treated at 540°C for 1 hour, air cooled, and
Overaging treatment was performed at 350°C for 16 hours. Next, the blank is cold rolled to a thickness range of 4 mm ~
Sheets of 0.8 mm and size 2 m x 1 m were produced with the necessary intermediate annealing. The rolled sheets were solution treated at 540°C for 20 minutes, cold water quenched, and aged at 170°C. Table 1 shows 1.6mm thickness sheet T6
Changes in tensile properties with aging time in (no stretching) temper and T8 (2% stretching before aging) temper are shown, and the tensile properties were measured in the longitudinal and transverse directions. Similar property levels were achieved in sheet materials in the 4.0 mm to 0.8 mm thickness range.

[Table] 0.2%PS = 0.2% stress resistance TS = tensile stress E1 = elongation MPa = megapascal During aging under T6 conditions, the peak value of alloy strength is 0.2%PS of 440MPa, tensile strength of 520MPa and 6-7.5% growth can be achieved.
These characteristics are the most widely used high strength 2000
Notably higher than that of the series alloy (2014-T6 0.2% - proof stress = 380 MPa, tensile strength = 440 MPa, elongation = 7%, minimum specified properties in sheet). This material also exceeds the minimum property requirements of 7075 sheet at T73 temper. At the aging of T8 condition, the peak value of alloy strength is
A 0.2% proof stress of 475 MPa and a tensile strength of 535 MPa can be achieved, which values are approximately comparable to the fully heat treated minimum sheet specifications for 7075 alloy (T6 temper). Figure 3 shows the tensile properties after aging under T8 conditions.
Figure 3 shows the 25 mm plate of the alloy of Example 1.
Changes in longitudinal tensile properties with aging time at 170°C are shown in comparison with 25mm plates specified by 2014-T651 and 7010-T7651. In the drawings, TS = tensile strength PS = yield strength EL = elongation DTD5120E and BS2L93 are the relevant prescribed standards for the two comparative alloys. Figure 3 shows a 508mm x 178mm cast ingot within the specific composition limits of this application.
Figure 2 shows statistical changes in 0.2% yield strength and tensile stress for sheets produced in the 5.0 mm to 0.8 mm thickness range. These results indicate that most of the manufactured sheets have a 0.2% yield strength greater than the 0.2% stress resistance minimum specified value for 7075-T6 and a tensile strength approximately 50% greater than the minimum tensile strength specified level for 7075-T6. Recognize. Considering the reduced density of the alloy (8-10% compared to 7075), the specific strength level of the alloy is
Notably larger than the 7075−T6 material. Example 3 Alloy composition (wt%) Lithium 2.39 Magnesium 0.70 Copper 1.81 Zirconium 0.16 Titanium 0.014 Aluminum Balance (includes incidental impurities) The alloy was cast as an ingot with a diameter of 216 mm by a direct quench casting method. Next, get 540 ingots.
Homogenization was carried out for 16 hours at ℃, and surface defects were removed by peeling. The ingot was divided into two pieces with a diameter of 185 mm and a length of 600 mm. These were preheated to 440°C and extruded using a 212mm diameter chamber. One piece is extruded at a speed of 5 m/min through a die with a cross section of 95 mm x 20 mm, and the other piece is extruded at a speed of 5 m/min through a die with a cross section of 54 mm in diameter.
It was extruded at min. The extrudates were solution treated at 535°C for 1 hour and quenched in cold water. The material was controlled 2.5% stretched and aged at 190°C for 16 hours. Tensile specimens were taken from the front and back of the extrusion. The results of the tensile test are as follows.

[Table] These results show that the alloy is in extruded form and is 7075−T6.
It shows that the strength level can be achieved. Example 4 Alloy composition (wt%) Lithium 2.56 Magnesium 0.66 Copper 1.98 Zirconium 0.12 Aluminum Balance (includes incidental impurities) The alloy was cast as an ingot with a diameter of 216 mm by a direct quench casting method. Next, get 540 ingots.
Homogenization was carried out for 16 hours at ℃, and surface defects were removed by peeling. Preheat the ingot to 480℃, and
It was cast into a 100mm rectangular bar. The bar was solution heated at 540℃ for 2 hours, quenched with cold water, and then heated to 190℃.
The statute of limitations expired for 16 hours. The tensile properties of the cast bars were as follows. Longitudinal direction 0.2% PS = 459 MPa TS = 546 MPa EL = 6% Lateral direction 0.2% PS = 401 MPa TS = 468 MPa EL = 3% These results indicate that the alloy is 7075-T73 in cast form.
It shows that the characteristics can be achieved. All as-heat-treated alloys offer an 8-10% density savings and a modulus of 10% compared to all existing specified aerospace aluminum alloys.
Increased by ~12%. The fracture toughness and fatigue life of the sheet materials were measured. The longitudinal-lateral (L-T) fracture toughness (Kc) of the 1.6 mm sheet was measured to be 68.5 MPa√ with a proof stress value of 425 MPa. The average L-T fatigue life at a yield value of 425 MPa was determined to be 3.14 x 10 ⁵ cycles (average of three samples) at a maximum test stress of 140 MPa.
Tests were performed on unnotched samples (Kt = 2.5) and tested in uniaxial tension at a stress ratio of +0.1. The alloy was found to exhibit superplastic behavior with an elongation of 400-700% in cold rolled 1.6 mm sheets heat treated in a hot blank prior to cold rolling in accordance with the above-described aspect of the invention. We have found that when the alloy according to the invention is cast in round billet form and extruded, the resulting tensile properties are 10-15% higher than those obtained for sheet material at the same heat treatment conditions. was also shown. The alloy according to the invention has acceptable casting properties.

[Brief explanation of the drawing]

FIG. 1 is a diagram showing differential scanning calorimetry curves for three types of alloy compositions, and FIGS. 2 and 3 are diagrams showing changes in tensile properties with aging time and statistical data about the tensile properties. be.

Claims (1)

[Claims] Composition in the following range in 1% by weight: Lithium 2.3-2.9%; Magnesium 0.5-1.0%; Copper 1.6-2.4%; Zirconium 0.05-0.25%; and the balance of aluminum and unavoidable impurities. However, the peak value of tensile strength after aging treatment is
A high-strength aluminum alloy with good formability that has a strength of 480 MPa or more and a density that is less than 95% of pure aluminum. 2. An alloy according to claim 1, wherein the copper to magnesium ratio is between 1.6:1 and 4.8:1, preferably 3:1. 3 composition in the following ranges by weight: lithium 2.3-2.9%; magnesium 0.5-1.0%; copper 1.6-2.4%; zirconium 0.05-0.25%; titanium up to 0.5%, manganese up to 0.5%,
At least one of nickel of 0.5% or less and chromium of 0.5% or less; and the remainder of aluminum and unavoidable impurities; and the peak value of the tensile strength after aging treatment is
A high-strength aluminum alloy with good formability that has a strength of 480 MPa or more and a density that is less than 95% of pure aluminum. 4. An alloy according to claim 3, wherein the copper to magnesium ratio is between 1.6:1 and 4.8:1, preferably 3:1.