JPH0647710B2 - Heat treatment method for aluminum-lithium alloy - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a heat treatment method for improving the draw ratio of an aluminum-lithium alloy.

[0002]

2. Description of the Related Art The specific gravity of metallic lithium (Li) is 0.53.
It is the lightest of all metals at g / cm ³ and has excellent ductility, but it is chemically active and is not very useful when used alone with lithium metal.
When it becomes a lithium alloy, it not only greatly improves the strength of aluminum, but also serves to considerably reduce the weight of the aluminum alloy itself.

In particular, aluminum-lithium alloy has a low density but has a high strength and a high elastic modulus.
It is expected to be used not only as an ultra-light structural material in the fields of aviation and space, but also in various industrial fields that require the above characteristics. Explaining this more specifically, when the aluminum-lithium alloy is used as a substitute for the high-strength aluminum alloy for aircraft structures that is currently generally used, it is possible to reduce the weight by about 7 to 9%. In addition to increasing the speed and cruising range, it is possible to improve the transportation capacity, and to provide the same level of protection as the existing aluminum armored material that requires high strength and hardness,
It is possible to manufacture a lightweight structure of about 10%, and it is expected to be useful as a material suitable for armor plate materials and missile fields.

By the way, the aluminum-lithium alloy has a problem in terms of material properties that the draw ratio and the fracture toughness are very low as compared with the existing 2xxx and 7xxx high strength aluminum alloys. Thus, aluminum
The causes of low elongation and fracture toughness in lithium alloys are mainly the occurrence of planar slip due to the δ ′ phase (Al ₃ Li), which is the main precipitation phase of aluminum-lithium alloys, and the reason for this. Grain boundary precipitation zone
It is known to be caused by grain boundary embrittlement due to preferential deformation in the itate free zone.

Therefore, as a method for increasing the draw ratio and fracture toughness of an aluminum-lithium alloy, in order to suppress the plane slip of dislocations due to the δ'phase, other inconsistencies other than the δ'phase are present in the matrix. It is considered to prevent grain boundary embrittlement by depositing precipitates and dispersing plane slips of dislocations. Thus, in the aluminum-lithium alloy, δ ′
As an alloy in the form of precipitating incommensurate precipitates other than the phases 20
90 aluminum alloy can be mentioned, but this 2090 aluminum alloy is aluminum-lithium binary alloy with copper
It is an alloy to which (Cu) and zirconium (Zr) are added.

[0006] Here, copper is added as a strengthening element to form a plate-like T ₁ phase (Al ₂ CuLi) which is inconsistent with the matrix and is precipitated to effectively stop the plane slip of dislocations, thereby forming an alloy. Contribute to strengthening. Then, zirconium refines the cast structure while forming a spherical β phase (Al ₃ Zr), which stops the grain boundary migration and suppresses recrystallization. However, the currently known Al-Li-Cu-based 2090 alloy is reported to have different precipitation behavior depending on the content ratio of copper and lithium, and therefore, any change in the content of copper and lithium in the 2090 alloy range. It is difficult to apply the same heat treatment to the conditions,
Moreover, until now, specific heat treatment conditions for each composition range have not been clearly standardized.

As an example of a method generally used as a heat treatment method for such an Al--Li--Cu type 2090 alloy,
After solution treatment to obtain the highest strength, a method of aging at 190 ℃ until reaching the highest strength condition, and 1 to 5 after solution treatment
% Stretching (stretchi-ng) or cold rolling (cold
After rolling, a method of aging at 190 ° C can be mentioned.

[0008]

However, there is a limit to the improvement of the draw ratio by such a conventional heat treatment method, and in particular, it is 2090 in the range of Al-2.3 to 2.5 Li-2.7 to 3.0 Cu-0.08 to 0.15Zr. The alloy contains a relatively large amount of lithium and tends to have a lower draw ratio. As a result of an experiment performed on a sample obtained by atmospheric melting, as shown in Table 1 below, when stretching was performed after solution treatment, stretching was performed. It can even be seen that it shows a very low draw ratio of less than 1%.

[0009]

[Table 1]

Therefore, development of a new heat treatment method is required to improve the elongation of the Al-Li-Cu system 2090 alloy. The present invention has been made in view of the above circumstances, and
It is an object to provide a new heat treatment method for improving the draw ratio of an aluminum-lithium alloy.

[0011]

[Means and Actions for Solving the Problems] In order to achieve such an object, the heat treatment method of the present invention is, as shown in the flow chart of FIG. 1, Al-1.9 to 2.6 Li-2.4 to 3.0 Cu-0.08 to 0.
After solution-treating an aluminum-lithium alloy having a composition range of 15 Zr, after aging at room temperature, cold rolling or stretching is performed, and further one-step low temperature aging is performed, followed by re-dissolution of the δ ′ phase. After performing reverting processing,
A two-step aging treatment was performed to reprecipitate the δ'phase.

That is, 2090 aluminum-lithium alloy
Strengthening effect is δ '(Al₃Li) Phase and T₁(Al ₂CuLi) phase two analysis
Of these, the δ ′ phase is the plane slip of dislocation.
And induces grain boundary destruction, but T₁Phase is dislocation shift
In consideration of the fact that the motion is stopped and dispersed, the precipitation rate is slow.
I T₁Select the δ'phase that is overaged while precipitating the phase sufficiently.
Selectively re-dissolve and then re-precipitate heat treatment method.
To go.

In the present invention, in order to prevent the generation of coarse grain boundary precipitates as the temperature of heat treatment increases, the solution treatment is followed by room temperature aging, followed by stretching or cold rolling and one-step low temperature aging. Is carried out so as to induce uniform precipitation of the T ₁ phase. The 2090 aluminum-lithium alloy obtained through such a heat treatment method of the present invention is
The draw ratio is 5 while maintaining the original strength of the alloy as it is.
It will be possible to improve up to ~ 6%.

The reverting treatment is carried out at 230 ° C. to 30 ° C.
Perform 2 minutes aging at 200 ℃ for 1 minute at 0 ℃
I try to do it for 20 minutes at ~ 250 ℃. This is Al-Li
In binary alloys, discontinuous precipitation (DP) at grain boundaries
It is known that preci-pitation) occurs, but if discontinuous precipitation occurs at grain boundaries in this way, the grain boundaries become fragile and act on factors of brittle fracture (see:
DB Williams and JW Edington, ACTA METAL., Vol.24,
P323, 1976).

Therefore, the Al-Li-Cu system and Al-Li-Cu-
There are almost no examples in which discontinuous precipitation was observed in Mg-based practical alloys, but the present invention eliminates the discontinuous precipitation acting as a factor of the brittle fracture described above, so that the δ, δ′-shaped sol of FIG. The heat treatment is performed in the above temperature range so as to avoid the discontinuous precipitation range observed in the phase diagram showing the bath line.

[0016]

EXAMPLES Examples of the present invention will be described below. Example An aluminum-lithium alloy having an Al-2.33Li-2.54Cu-0.09Zr composition was manufactured through a method for manufacturing an aluminum-lithium alloy by melting in the atmosphere of Korean Patent Application No. 90-8873 filed earlier by the applicant. After that, the sample was homogenized, extruded, and hot-rolled at 450 ° C. to produce a 2.2 mm-thick plate material.

A sample of such a plate material was subjected to solution treatment at 570 ° C. for 1 hour as shown in the heat treatment process explanatory diagram of FIG.
After aging at room temperature for 5 hours, cold rolling at 5% and 24 at 120 ° C
After aging at a low temperature for a long time, a reverting treatment was performed at 270 ° C for 1 minute to re-dissolve the precipitated δ'phase.
Specimen 1 was produced by aging treatment at ℃ for 20 minutes and reprecipitating the δ'phase.

Next, the same heat treatment process as that of the sample 1 was performed, except that the sample 2 was manufactured by stretching 5% instead of cold rolling 5%. Also, the same heat treatment process as in the above-mentioned sample 1 is performed, but the reverting treatment temperature is changed to 250 ° C., and the two-step aging treatment for reprecipitation of δ ′ is changed to 250 ° C. I made it. Table 2 below shows the measurement results of the strength and the elongation of each sample obtained through the heat treatment method of this example.

[0019]

[Table 2]

As can be seen from Table 2 above, each of the test pieces obtained by the heat treatment method of the present embodiment has a higher thermal conductivity than the material produced by the conventional heat treatment method of 3% stretching shown in Table 1. It can be seen that the strength is somewhat inferior, but the drawing rate is remarkably increased. As described above, the 2090 aluminum-lithium alloy obtained by the heat treatment method can improve the stretch ratio to 5 to 6% while maintaining the original strength of the conventional alloy.

In the heat treatment method of the present invention, as the lithium composition changes, the reverting temperature is set to 23 from the conditions for the δ'-phase and δ-phase solvus lines in FIG.
The temperature can be changed from 0 ° C to 300 ° C, and the two-step aging treatment for reprecipitation treatment of the δ'phase can also be changed within the range of 200 ° C to 250 ° C.

[0022]

As described above, according to the heat treatment method of the present invention, the draw ratio can be greatly improved without lowering the strength of the aluminum-lithium alloy.
Further, when the present invention is applied to an aluminum-lithium alloy having another composition range, it can be expected to have a relatively high strength and an improved draw ratio.

[Brief description of drawings]

FIG. 1 is a flow chart schematically showing a heat treatment method of the present invention.

FIG. 2 is a state diagram showing solvus lines of δ and δ ′ phases in an Al-Li alloy.

FIG. 3 is an explanatory view showing a heat treatment process of one embodiment of the heat treatment method of the present invention.

Claims (3)

[Claims] 1. Al-1.9 to 2.6 Li-2.4 to 3.0 Cu-0.08 to
After solution heat treatment of aluminum-lithium alloy of 0.15Zr composition, normal temperature aging, cold rolling or stretching, 1 step low temperature aging, reverting for re-dissolution of δ'phase A heat treatment method for an aluminum-lithium alloy, which comprises performing a two-step aging treatment after the treatment. 2. The heat treatment method for an aluminum-lithium alloy according to claim 1, wherein the reverting treatment is performed at 230 ° C. to 300 ° C. for 1 minute. 3. The aluminum according to claim 1, wherein the two-step aging treatment is carried out at 200 ° C. to 250 ° C. for 20 minutes.
Method for heat treatment of lithium alloy.