KR102494830B1 - Fabrication Method of Al-Li Alloy Using Multi-Stage Aging Treatment - Google Patents

Abstract

A multi-stage heat treatment method for an aluminum-lithium alloy according to the present invention comprises the steps of: a) performing solution treatment on an aluminum-lithium alloy containing 1.9-2.6 wt% of Li, 2.4-3.0 wt% of Cu, 0.08-0.15 wt% of Zr, and the remaining aluminum and other unavoidable impurities; b) stretching the solution treated alloy; and c) performing primary aging on the stretched alloy at a first heat treatment temperature and then performing secondary aging at a second heat treatment temperature higher than the first heat treatment temperature. Accordingly, an aluminum-lithium alloy having improved mechanical properties can be produced.

Description

Manufacturing method of Al-Li alloy using multi-stage aging treatment {Fabrication Method of Al-Li Alloy Using Multi-Stage Aging Treatment}

The present invention relates to a method for producing an Al-Li alloy, and more particularly, to a method for producing an Al-Li alloy having improved mechanical properties.

Aluminum-lithium alloys have low density and high strength and modulus of elasticity, making them suitable for aircraft. It is attracting attention as an ultra-light structural material for missiles, lightweight armor, and various military supplies.

However, aluminum-lithium alloys have inherent problems in that ductility and toughness are very low compared to conventional high-strength aluminum alloys. It is well known that the problem of low ductility and toughness of aluminum-lithium alloys is closely related to the precipitation of the quasi-equilibrium phase δ' and the resulting strengthening mechanism. Therefore, like 2090 aluminum alloy, Cu, Mg, etc. are added to the aluminum-lithium alloy to suppress the δ' phase during heat treatment and the third precipitation phase (S' phase, θ' phase, T1 phase, etc.) to the base A technique has been developed to reduce the damage of the δ' phase by finely dispersing and to increase the strength by the third precipitated phase.

S' phase, θ' phase, T1 phase, etc. are not uniformly precipitated in the substrate in the T6 treatment that performs artificial aging after solution heat treatment, but is coarsely precipitated at defect sites such as dislocations or stacking faults. It is known. Accordingly, a T8 treatment is performed to uniformly and finely form a precipitate phase in the matrix by applying strain after solution heat treatment and then aging treatment.

However, even in the T8 treatment, a considerable amount of δ' phase is present, which reduces the mechanical properties of the alloy, and also requires a very long aging treatment to increase the fraction of the third precipitated phase, which deteriorates productivity.

Korean Patent Publication No. 2017-0077886 A

An object of the present invention is to provide a method capable of producing an aluminum-lithium alloy having improved mechanical properties.

Another object of the present invention is to provide a method for producing an aluminum-lithium alloy having improved mechanical properties through heat treatment at a low temperature for a short period of time.

The multi-stage aging treatment method for an aluminum-lithium alloy according to the present invention is a) aluminum-lithium containing 1.9-2.6% Li, 2.4-3.0% Cu, 0.08-0.15 Zr, balance aluminum and other unavoidable impurities, by weight. solution heat treating the alloy; b) stretching the solution heat treated alloy; and c) first aging the elongated alloy at a first heat treatment temperature and then second aging at a second heat treatment temperature higher than the first heat treatment temperature.

In the multi-stage aging treatment method according to one embodiment, the first heat treatment temperature T1 may be 130 to 145 °C.

In the multi-stage aging treatment method according to one embodiment, during the first aging treatment, the temperature range from 110 ° C. to T1 may be slowly raised to 0.5 ° C./min or less.

In the multi-stage aging treatment method according to one embodiment, the second heat treatment temperature may be 155 to 190 °C.

In the multi-stage aging treatment method according to one embodiment, step b) may include cold working to have a permanent strain of 3 to 6%.

In the multi-stage aging treatment method according to one embodiment, step c) may further include stretching the primary aging alloy between the primary aging treatment and the secondary aging treatment.

In the multi-stage aging treatment method according to one embodiment, during the secondary aging treatment of the elongated alloy in step c), the temperature range from 110 ° C to the first heat treatment temperature T1 is 0.5 ° C / min or less. And, a temperature range from T1 to T2, which is the second heat treatment temperature, may have a temperature elevation profile in which the temperature is raised at a high rate of 50° C./min or more.

In the multi-stage aging treatment method according to one embodiment, the elongation in step c) may be performed by cold working or hot working at a permanent set of 1 to 4%.

In the multi-stage aging treatment method according to one embodiment, the heat treatment time at the first heat treatment temperature may be 1 to 3 hours, and the heat treatment time at the second heat treatment temperature may be 6 to 25 hours.

The present invention includes an aluminum-lithium alloy treated by the above-described multi-stage aging treatment method for an aluminum-lithium alloy.

In the multi-stage aging treatment method of an aluminum-lithium alloy according to the present invention, after forming nucleation sites in a matrix by elongation and then forming a large amount of fine nuclei of a precipitate phase including a T1 phase by low-temperature primary aging treatment, By growing the nuclei of the precipitation phase through the secondary aging treatment performed at a high temperature, there is an advantage in that the aluminum-lithium alloy with improved mechanical properties can be manufactured with a short aging time by increasing the density of the T1 phase.

Hereinafter, the multi-stage aging treatment method of the present invention will be described in detail. At this time, if there is no other definition in the technical terms and scientific terms used, they have meanings commonly understood by those of ordinary skill in the art to which this invention belongs, and will unnecessarily obscure the gist of the present invention in the following description. Descriptions of possible known functions and configurations are omitted.

Also, the singular forms used in the specification and appended claims may be intended to include the plural forms as well, unless the context dictates otherwise.

In this specification and the appended claims, terms such as first and second are used for the purpose of distinguishing one element from another, not in a limiting sense.

In this specification and the appended claims, terms such as include or have mean that features or elements described in the specification exist, and unless specifically limited, one or more other features or elements may be added. It does not preclude the possibility that it will happen.

In this specification and the appended claims, when a part of a film (layer), region, component, etc. is on or on another part, not only when it is in contact with and directly on top of another part, but also when another film ( layer), other areas, other components, etc. are interposed.

In this specification and appended claims, unless specifically limited, % or ratio means weight % or weight ratio.

As is well known, as an aluminum-lithium alloy that precipitates mismatched precipitates other than δ' phase, 2090 aluminum alloy may be mentioned. In this 2090 aluminum alloy, copper (Cu) and zirconium (Zr) It is an added alloy. Copper forms a plate-like T1 phase (Al ₂ CuLi) and precipitates in misalignment with the matrix to strengthen the alloy, while zirconium forms a spherical β phase (Al ₃ Zr) with microstructure and similarly strengthens the alloy and suppresses recrystallization do. These Al-Li-Cu-Zr-based 2090 aluminum alloys have different precipitation behaviors depending on their spherical composition, so that when the spherical composition is different, heat treatment for enhancing physical properties must be different.

In order to improve strength and prevent ductility degradation of aluminum-lithium alloys containing 1.9-2.6% Li, 2.4-3.0% Cu, 0.08-0.15 Zr, balance aluminum and other unavoidable impurities on a weight percent basis, the applicant In the process of conducting research on the aging process that can maximize the precipitation of fine T1 phase (Al ₂ CuLi), it was found that the precipitation behavior of the T1 phase in the alloy of the corresponding composition was sensitively affected by strain and temperature. As a result of intensifying research based on these findings, an aging treatment method capable of producing an aluminum-lithium alloy with improved strength and prevention of ductility degradation at a low temperature in a much shorter time than before was developed and filed for the present invention. reached

The multi-stage aging treatment method according to the present invention is a) solution treatment of an aluminum-lithium alloy containing 1.9-2.6% Li, 2.4-3.0% Cu, 0.08-0.15 Zr, balance aluminum and other unavoidable impurities on a weight percent basis. doing; b) stretching the solution heat treated alloy; and c) first aging the elongated alloy at a first heat treatment temperature and then second aging at a second heat treatment temperature higher than the first heat treatment temperature.

In the method according to the present invention, after solution heat treatment, the alloy is stretched to form a nucleation site of a precipitate phase including a T1 phase (Al ₂ CuLi), and then, through low-temperature aging, primary aging treatment, uniformly in the base. By forming fine precipitated nuclei and directly growing the previously generated nuclei without an incubation period for nucleation through secondary aging treatment, which is relatively high temperature aging, to increase the density of the T1 phase (Al ₂ CuLi), reducing the ductility of the alloy. prevent and increase strength.

The aluminum-lithium alloy (hereinafter also referred to as an alloy) may contain 1.9-2.6% Li, 2.4-3.0% Cu, 0.08-0.15 Zr, the balance aluminum and other unavoidable impurities on a weight percent basis, and more specifically , the alloy may contain 1.9-2.3% Li, 2.6-3.0% Cu, 0.10-0.14 Zr, balance aluminum and other unavoidable impurities. Examples of other unavoidable impurities include, but are not limited to, 0.3% or less iron, 0.06% or less silicon, 0.05% or less titanium, 0.01% or less magnesium, and 0.05% or less manganese.

The solution heat treatment in step a) is sufficient as long as it is performed under conditions commonly used to dissolve solute elements (additional elements) in aluminum-lithium alloys including 2090 aluminum alloys and make them supersaturated through quenching. For example, the solution treatment may be performed in a salt bath at 520 to 540 ° C. for 0.5 to 2 hours, and then quenched in ice water, but the present invention is not limited by the specific conditions of the solution treatment.

After the solution heat treatment, stretching for permanent deformation accompanied by creation of defects may be performed. Specifically, the elongation in step b) may be performed by cold working to have a permanent set of 3 to 6%, preferably 4 to 6%. At this time, the cold working may be cold rolling by a roller, but it is sufficient if it is performed using a conventional method and apparatus used for cold working for elongation of an aluminum alloy.

In order to homogeneously form fine nuclei of the precipitated phase including the T1 phase at the nucleation site provided by stretching, the first heat treatment temperature, T1, is preferably a low temperature of 130 to 145 ° C., more preferably 135 to 145 ° C. can The heat treatment time during the primary aging, that is, the time during which the alloy is maintained at the temperature of T1 may be 1 to 6 hours, 1 to 4 hours, 1 to 3 hours, 1 to 2 hours, or 0.5 to 1.5 hours. The primary aging treatment performed for a short time of 0.5 to 1.5 hours is more advantageous than when an additional stretching step is performed between the primary aging treatment and the secondary aging treatment as will be described later.

After forming a large amount of fine precipitate nuclei in the matrix through primary aging, secondary aging may be performed to grow the nuclei. The temperature of the secondary aging (second heat treatment temperature, T2) is 155 to 190 ° C., specifically, so that sufficient solute is supplied around the nucleus by diffusion of the solute and at the same time, the unnecessary coarsening of the precipitated phase can be suppressed. It may be performed at 155 to 180 ° C, more specifically at 155 to 165 ° C. At this time, the heat treatment time during the secondary aging, that is, the time during which the alloy is maintained at the temperature of T2 may be 6 to 25 hours.

In one advantageous example, during the first aging, the temperature range in which the nucleation of the precipitate phase including the T1 phase can occur is slowly raised and the first aging can be performed. Specifically, with the aid of stretching, the temperature range in which nucleation of the T1 phase can occur may be 110° C. or higher, and thus, the temperature range from 110° C. to T1 may be slowly raised.

When the temperature is raised at a low rate, the temperature increase rate may be 0.5°C/min or less, specifically 0.1 to 0.5°C/min, more specifically 0.1 to 0.4°C/min, and more specifically 0.1 to 0.3°C/min. When the temperature is raised at a low rate of 0.5° C./min or less in the above-described temperature range and the first aging is performed, nuclei of the precipitate phase including the T1 phase may be formed more homogeneously and finely in the matrix. In addition, when the slow heating process is performed, the time (primary aging treatment time) maintained at the fixed temperature of T1 after the temperature rises to T1 may be 1 to 3 hours, 1 to 2 hours, or 0.5 to 1.5 hours.

In an advantageous example, a step of stretching the primary aged alloy may further be performed between the primary aging treatment and the secondary aging treatment. That is, step c) may include: c1) primary aging treatment of the alloy elongated in step b) at a first heat treatment temperature; c2) stretching (additional stretching) the alloy subjected to primary aging treatment; and c3) subjecting the additionally elongated alloy to secondary aging treatment at a second heat treatment temperature higher than the first heat treatment temperature.

In a more advantageous embodiment, step c) is performed by first aging the alloy elongated in step b) at T1, which is a temperature of 130 to 145 ° C, by slowly raising the temperature from 110 ° C to T1 at a temperature of T1. Primary aging treatment for 0.5 to 1.5 hours; c2) stretching the primary aged alloy (hereinafter, additional stretching); and c3) subjecting the additionally elongated alloy to secondary aging treatment at a second heat treatment temperature higher than the first heat treatment temperature.

The slow temperature increase in step c1) provides a driving force for nucleation in a wide range through the temperature increase process, enabling the generation of a large amount of fine nuclei compared to heat treatment performed at the same temperature with a normal temperature increase (5 ~ 30 ° C / min) , The short heat treatment in step c1) partially latents the nucleation driving force in the alloy even after step c1) is performed, enabling additional nucleation in step c3) again by the aid of elongation.

The additional stretching in step c2) may be performed to have a permanent strain of 1 to 4%, preferably 2 to 4%, and may be performed by cold working or hot working. Cold working may mean cold rolling by rollers, and hot working may mean hot rolling by heated rollers, but is not limited thereto. When hot working is performed, the temperature of hot working may be substantially equal to or similar to T1.

In one advantageous example, the further stretching in step c2) may be cold working. At this time, the alloy subjected to heat treatment at the temperature of T1 for a desired time may be cooled through air cooling or quenching, and then cold working may be performed.

By the short heat treatment in step c1) and additional elongation by cold working in step c2), the alloy may have the potential to additionally form precipitate nuclei that cannot be produced by primary aging.

For this additional nucleation, in the secondary aging treatment of the elongated alloy, the temperature range from 110 ° C to the first heat treatment temperature T1 is raised at a low rate of 0.5 ° C / min or less, and from T1 to the second heat treatment temperature The temperature range up to T2 may have a temperature elevation profile in which the temperature is raised at a high rate of 50° C./min or more, and heat treatment may be performed for a predetermined time at a temperature of T2 reached by the temperature elevation profile.

Due to the slow temperature increase during the secondary aging, additional precipitate fine nuclei may be formed in the alloy. The temperature increase rate during the slow temperature increase during the second aging is 0.5°C/min or less, specifically 0.1 to 0.5°C/min, more specifically 0.1 to 0.4°C/min, independently of the low rate temperature increase during the first aging. Specifically, it may be 0.1 to 0.3 °C / min.

After passing through the T1 temperature range at 110℃ through slow temperature increase during the secondary aging, a large amount of fine precipitated nuclei are formed in the base of the alloy, so if the diffusion of solute is limited, the precipitated nuclei are consumed for the growth of other nuclei There is a risk of disappearance or loss of size uniformity between precipitated phases.

In order to prevent this, it is preferable that the heat treatment is performed at a constant temperature of T2 by passing the temperature range from T1 to T2, which is the second heat treatment temperature, as quickly as possible, which affects the diffusion rate of the solute. Specifically, it is advantageous that the temperature range from T1 to T2 after the low-speed heating is rapidly raised at a rate of 50° C./min or more, and heat treatment is performed for a certain period of time at the temperature T2 reached by the high-speed heating. Substantially, at high-speed heating, the heating rate may be 50 to 100 °C/min, more substantially, 60 to 80 °C/min.

The alloy heated to the second heat treatment temperature (T2) for the secondary aging through the above-described temperature elevation profile, that is, 155 to 190 ° C, specifically 155 to 180 ° C, more specifically 155 to 165 ° C, is 6 to 25, specifically It may be heat treated at the same temperature (T2) for 10 to 20 hours, more specifically 10 to 16 hours, and then cooled through air cooling.

The heat treatment time required for the first aging and the second aging was greatly shortened compared to the conventional T8 treatment time of 78 hours at 160 ° C. It is possible by homogeneously generating a large amount of precipitate nuclei at low temperature through additional stretching.

A multi-stage aging treatment method of an aluminum-lithium alloy according to one embodiment includes, before step a), I) casting a slab by dissolving the alloy or raw metals constituting the alloy; II) homogenizing the slab; and III) hot-working the homogenized slab by forging or rolling and, if necessary, cold-working after hot-working. After step III), solution treatment of step a) may be performed. there is.

The melting of step I) may be performed using a melting method used to manufacture a conventional alloy slab, such as vacuum induction melting, and at this time, the thickness of the slab may be on the order of 30 to 50 mm. It goes without saying that after manufacturing a slab by casting, it is chamfered to a level of about 1 to 5 mm per side of the slab, and then homogenization treatment can be performed.

Homogenization in step II) is for removing residual stress during fine segregation and solidification, and is sufficient if performed under normal aluminum-lithium alloy homogenization treatment conditions. For example, the homogenization treatment may be performed at 510 to 550 ° C. for 18 to 30 hours under an inert atmosphere such as argon, but is not limited thereto.

During the hot working of step III), the homogenized slab may be hot rolled to a thickness of 1 to 10 mm. Hot rolling may be performed at a temperature of 440 to 480° C., and cold working to uniformly control grains after hot rolling may be performed at a level of about 15 to 30%, but the present invention is not limited thereto.

(Example 1)

An aluminum-lithium alloy having a composition according to Table 1 below was used.

(Table 1)

After melting and casting to a thickness of 40 mm using a vacuum induction melting furnace to prepare a slab, the surface was chamfered to 3 mm. Thereafter, homogenization was performed at 530° C. for 24 hours in an argon atmosphere. The homogenized slab was heated to 460 ° C, maintained for 30 minutes, hot-rolled to 7 mm for hardness test specimens, and hot-rolled to 1.3 mm for tensile test specimens or thermal analysis specimens, followed by 20% cold working. .

Thereafter, solution treatment was performed in a salt bath at 533° C. for 1 hour and then quenched in ice water at 0° C.

After cold working the solution heat treated alloy by 5%, the temperature was raised to 140℃ at a heating rate of 10℃/min, and the first aging treatment was performed at 140℃ for 3 hours, and then the second aging temperature was increased from 140℃ to 160℃. The temperature was raised at a heating rate of 10 °C/min, heat-treated at 160 °C for 18 hours, and then cooled in air.

(Example 2)

In the case of the first aging treatment, the temperature was raised at a heating rate of 10 °C/min up to 110 °C, and the temperature was raised at a heating rate of 0.3 °C/min from 110 °C to 140 °C, except for the first aging treatment at 140 °C for 2 hours. and carried out in the same manner as in Example 1.

(Example 3)

After the first aging treatment, the alloy is air-cooled without raising the temperature to the second aging treatment temperature, then cold worked (additional stretching) at 3%, then heated up to 110 ° C at a heating rate of 10 ° C / min, and then heated from 110 ° C to 140 ° C. It was carried out in the same manner as in Example 2, except that the temperature was raised at a heating rate of 0.3 ° C / min to ° C, heated at a heating rate of 60 ° C / min from 140 ° C to 160 ° C, and heat treatment was performed at 160 ° C for 14 hours.

(Comparative Example 1)

Performed in the same manner as in Example 1, but immediately aged the solution heat treated alloy without performing 5% cold working, but instead of the first aging treatment and the second aging treatment, at 190 ° C (heating rate of 10 ° C / min) The alloy was prepared by single aging for 25 hours.

(Comparative Example 2)

Performed in the same manner as in Example 1, but immediately aged the solution heat treated alloy without performing 5% cold working, but instead of the first aging treatment and the second aging treatment, at 160 ° C (heating rate of 10 ° C / min) The alloy was prepared by single aging for 78 hours.

(Comparative Example 3)

It is performed in the same manner as in Example 1, but the alloy subjected to 5% cold working after solution heat treatment is aged, but instead of the first aging treatment and the second aging treatment, 60 hours at 160 ° C (heating rate of 10 ° C / min) The alloy was prepared by single aging treatment during

Aging conditions were determined through previous experiments, and the aging treatment conditions in each Example and Comparative Example were conditions showing maximum hardness.

Table 2 below summarizes the volume fraction of the T1 phase shown in the thermal analysis curve for each sample. The volume fraction means the volume fraction actually produced compared to the maximum volume in which the T1 phase can be produced, and at this time, the maximum volume in which the T1 phase can be produced was calculated with the calorific value in the thermal analysis curve after solution heat treatment.

(Table 2)

A tensile test was performed on the alloys prepared in Comparative Example 3 and Examples 1 to 3, and the tensile strength, yield strength and elongation were measured and summarized in Table 3.

(Table 3)

As can be seen from Tables 2 and 3, about the heat treatment time required for maximum strength during single aging at 160 ° C through multi-stage heat treatment of 140 ° C for 3 hours and 160 ° C for 18 hours after 5% cold working It can be seen that an alloy having similar mechanical properties can be produced with a short heat treatment time of only 1/3 of the level.

In addition, it can be seen that a larger amount of T1-phase nucleation occurs due to the slow temperature increase, and the T1-phase fraction increases due to additional elongation and slow temperature increase, and it can be seen that the tensile strength, yield strength and elongation are greatly improved.

As described above, the present invention has been described by specific details and limited embodiments and drawings, but this is only provided to help a more general understanding of the present invention, the present invention is not limited to the above embodiments, and the present invention Those skilled in the art can make various modifications and variations from these descriptions.

Therefore, the spirit of the present invention should not be limited to the described embodiments, and it will be said that not only the claims to be described later, but also all modifications equivalent or equivalent to these claims belong to the scope of the present invention. .

Claims (10)

a) 1.9-2.6% Li, 2.4-3.0% Cu, 0.08-0.15 Zr, the balance being aluminum (Al) and other unavoidable impurities, by weight %, less than 0.3% iron (Fe), less than 0.06% silicon (Si) ), solution treatment of an aluminum-lithium alloy containing 0.05% or less of titanium (Ti), 0.01% or less of magnesium (Mg), and 0.05% or less of manganese (Mn);
b) first stretching the solution heat treated alloy; and
c) After the primary aging treatment of the primary elongated alloy at the first heat treatment temperature T1, and the secondary elongation of the primary aging treatment alloy, the secondary elongated alloy is subjected to a second heat treatment higher than the T1 Including; secondary aging treatment at a temperature of T2,
The T1 is 130 to 145 ° C, the T2 is 155 to 190 ° C,
During the primary aging treatment, the temperature range from 110 ° C to T1 is heated at a low rate of 0.1 ° C / min to 0.5 ° C / min and heat treated at the temperature of T1 for 1 to 3 hours,
During the secondary aging treatment, the temperature range from 110 ° C to T1 is slowly raised at a rate of 0.1 ° C./min to 0.5 ° C./min, and the temperature range from T1 to T2 is raised at a high rate at 50 ° C./min to 100 ° C./min. and heat treated at a temperature of T2 for 6 to 25 hours,
The first elongation of step b) is performed by cold working at a permanent strain of 3 to 6%, and the second elongation of step c) is performed by cold working or hot working at a permanent strain of 1 to 4%. -Multi-stage aging treatment method of lithium alloy.

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