KR101937358B1 - Forming method for Mg-Li based alloy - Google Patents

Abstract

The present invention relates to an Mg-Li-based alloy having at least two phases with different strain rates, wherein a method of forming the Mg-Li based alloy comprises: a first step of molding at a predetermined strain rate; and a second step of forming at a strain rate relatively lower than that of the first step, and a range of linear strains in the first step is greater than 2.0% and less than 10.0%.

Description

Method for Forming Mg-Li Based Alloy {Forming method for Mg-Li based alloy}

The present invention relates to a method of forming a Mg alloy, and more particularly, to a method of forming an Mg-Li-based alloy having at least two phases having different strain rate sensitivities.

A variety of sheet material forming methods such as press working are used for forming sheet materials in automobile, aircraft and other industrial fields. During the sheet material forming process using these materials, the material may be stretched locally or wholly, bending, A complicated deformation consisting of deformation such as flanging, deep drawing or the like or a combination thereof occurs.

Therefore, in forming the plate, the formability of the plate, which means the degree to which the plate can be plastic deformed without causing fracture, is more important than any physical property.

On the other hand, Mg alloys have attracted attention as an alloy for structural materials showing low specific gravity, excellent specific strength and rigidity, but they have a crystal structure of hexagonal close packed (HCP) and have a sufficient slip system at room temperature In a basal texture formed on a Mg alloy by hot rolling or extrusion, this slip system has a Schmid factor (Schmid factor) close to zero for strain along the c axis, ), The moldability at room temperature is lowered and the use thereof is not widespread.

In recent years, in order to overcome the above problems and further reduce the weight, a very low density Li of 0.534 g / cm 3 is added to the Mg alloy, or the crystal grain size is made extremely precise. One of the methods is to add various elements such as Zn, Al and Mn to the Mg-Li alloy to contribute to the improvement of strength and corrosion resistance of the magnesium alloy. However, the alloys are excellent in strength and corrosion resistance, but have a drawback that the elongation rate is low and molding is not easy.

Therefore, in order to use Mg-Li-based alloy sheet in various fields than today, it is urgently required to develop Mg-Li-based alloy sheet and molding method having excellent moldability at room temperature and further having improved yield strength and tensile strength .

(Prior art) Jian-Yih Wang and three others. Effect of Al and Mn on the Mechanical Properties of Various ECAE Processed Mg-Li-Zn Alloys. Materials Transactions, Vol. 47, No. 4 (2006)

It is an object of the present invention to provide a method for forming a Mg-Li-based alloy which is excellent in strength and ductility without deteriorating moldability at room temperature. However, these problems are exemplary and do not limit the scope of the present invention.

According to one aspect of the present invention, there is provided a method for forming a Mg-Li-based alloy. The method for forming a Mg-Li-based alloy according to claim 1, wherein the Mg-Li-based alloy has at least two phases different in strain rate sensitivity from each other. And a second step of forming at a strain rate relatively lower than that of the first step, wherein a range of linear strains in the first step is greater than 2.0% and less than 10.0%.

The Mg-Li alloy includes a first phase having a relatively low strain rate sensitivity index and a second phase having a relatively high strain rate sensitivity index, Wherein the deformation is concentrated on the first phase because the strength of the second phase is higher than that of the first phase, and the strength of the first phase is higher in the second phase than in the second phase, Lt; / RTI >

Wherein the first phase is an alpha phase having a dense hexagonal lattice structure (HCP) and the second phase is a beta phase having a body-centered cubic structure (BCC) ).

In the method for forming an Mg-Li-based alloy, the Mg-Li-based alloy may include at least one of Zn, Al, and Mn.

In the method of forming the Mg-Li-based alloy, the strain rate in the first step is And the strain rate in the second step may be 0.00001 / sec or more and less than 0.005 / sec.

According to one embodiment of the present invention as described above, it is possible to realize a Mg-Li-based alloy molding method which is simple in molding process, has excellent elongation and strength, and is excellent in moldability even at room temperature. Of course, the scope of the present invention is not limited by these effects.

Figure 1 is a graph of the nominal stress-nominal strain (Eng.Stress-Eng.Strain) of the experimental and comparative examples.
Fig. 2 shows elongation data according to the linear strain rates of the respective experimental examples.
FIG. 3 is a state diagram of a Mg-Li binary system applied to a method of forming an Mg-Li-based alloy according to an embodiment of the present invention.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be understood, however, that the invention is not limited to the disclosed embodiments, but may be embodied in many different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, Is provided to fully inform the user. Also, for convenience of explanation, the components may be exaggerated or reduced in size.

The method for forming an alloy according to the present invention is a method for forming a Mg-Li-based alloy having at least two phases having different strain rate sensitivities, A first step and a second step of molding at a strain rate relatively lower than that of the first step.

The strain rate sensitivity index is an index indicating the degree to which the material under strain varies in strength with the strain rate. The large strain rate sensitivity index of a specific material means that the material is characterized by an increase in strength when the material is rapidly deformed.

Here, the strain at the first step may be referred to as a pre-strain. According to an embodiment of the present invention, the range of the pre-strain may be greater than 2.0% and less than 10.0%.

At least two phases having different strains may include a dense hexagonal lattice structure (HCP) and a body-centered cubic structure (BCC). Hereinafter, the two phase structures of the Mg-Li-based alloy to be molded in the present invention will be described later with reference to Fig.

FIG. 3 is a state diagram of a Mg-Li binary system applied to a method of forming an Mg-Li-based alloy according to an embodiment of the present invention.

Referring to FIG. 3, according to the Mg-Li binary phase diagram, when the content of Li is between about 5.0 wt% and 11.0 wt%, the β-phase of Li solid solution having a body centered cubic (BCC) And coexist with the α-phase, which is hexagonal close packed (HCP).

Mg-Li-based alloys having at least two phases different from each other are added with various elements such as Zn, Al and Mn in addition to Li in order to further improve strength and corrosion resistance. However, the alloy has excellent strength and corrosion resistance, but has a problem that the elongation is low and the molding is not easy.

In order to solve this problem, in the present invention, the strain of the alpha phase (alpha phase) of the beta phase (beta phase) and the dense hexagonal lattice (HCP) of the body center cubic (BCC) structure in which the strain rate sensitivity index is different in Mg- Behavior characteristics. That is, the elongation of the Mg-Li alloy can be increased by performing a step of deforming at a different strain rate in the state where the α-phase and the β-phase having different strain rate sensitivity indices are different from each other.

In the section which is deformed at different strain rates, the phases in which plastic deformation occurs are different from each other. For example, in the first stage where the strain rate is high, strain is concentrated on the α phase, which is relatively lower than the β phase where the strain rate sensitivity index is relatively large, so that the strain of the β phase is minimized. The deformation is concentrated on the β-phase with a high strain-rate sensitivity index at the strain rate, thereby causing all deformation within the residual strain of β-phase.

A predetermined strain rate may be selected to distinguish between the first and second steps. In one embodiment of the present invention, the strain rate for distinguishing the first step and the second step may be 0.005 / sec. That is, the first section may have a strain rate of 0.005 / sec or more, and the second section may have a strain rate of less than 0.005 / sec.

In addition, the strain value (i.e., line strain) in the first step may have a specific range, and the maximum value of the elongation may appear within the range of the line strain.

Hereinafter, the deformation behavior of the Li-Mg alloy according to the strain rate will be described in more detail.

It is known that the β phase of the body-centered cubic (BCC) structure in the Mg-Li alloy having α phase and β phase has a high strain rate sensitivity at room temperature. This means that as the strain rate increases, the strength increases. On the other hand, it is known that the α phase of the dense hexagonal lattice (HCP) is relatively small compared to the β phase. This means that even if the strain rate increases, there is little change in the strength.

When strain rate is small, strain rate sensitivity index hardly shows any effect. In this environment, the higher the Li content, the lower the intensity of the β phase than α. Therefore, when the load is applied in an environment with low strain rate, deformation concentrates on the β phase with relatively low strength.

On the contrary, when the strain rate is large, the influence of the strain rate sensitivity index is large. That is, in the α phase with a small strain rate sensitivity index, a large change in the strength does not appear. However, in the case of β having a high strain rate sensitivity index, the strength rapidly increases and the α phase and the strength become almost equal or exceed. Therefore, when a load is applied in an environment with a high strain rate, the deformation amount of the α phase is increased as compared with the case where the strain rate is low. In particular, when the strength of the β phase exceeds the strength of the α phase,

The strain of the Mg-Li-based alloy is divided into two stages according to the strain rate. In the first stage, the strain rate is increased and then the strain rate is decreased. When the strain is changed to the second stage, A higher elongation can be obtained. That is, as the strain is rapidly changed in the first step, the strain concentrates on the α phase having a relatively low intensity, and therefore, the strain is shifted to the second phase in a state where the β phase is hardly deformed. This means that the deformation step has passed to the second step in a state in which there is hardly any decrease in the acceptable deformation amount of? Phase.

In the second phase where the subsequent strain rate is low, the strain is concentrated on the β phase, so that the β phase, which is hardly deformed in the previous step, participates in the deformation all within the acceptable strain amount.

In order to obtain the effect of improving the elongation according to the above-described strain behavior in the present invention, a strain rate of at least 0.005 / sec or more is required in the first step.

If the strain rate in the first step is less than 0.005 / sec, the strain rate sensitivity index exhibits a relatively insensitive deformation behavior. Therefore, if the strain rate in the first step is slower than 0.005 / sec, deformation occurs not only in the α phase of the dense hexagonal lattice (HCP) but also in the β phase of the body center cubic (BCC) The effect of improving the elongation is not obtained even if the speed is controlled to be slower than that.

However, in the first section, if the strain rate is excessively high, for example, at a strain rate exceeding 10 / sec, the fracture of the Mg-Li-based alloy may occur before the elongation increases. Therefore, the strain rate in the first step should be appropriately controlled in the range of 0.005 / sec to 10 / sec.

On the contrary, if the strain rate is too low in the second step, the time required for the total strain can be unnecessarily increased. Therefore, the strain rate in the second step should be suitably controlled in the range of 0.00001 / sec to less than 0.005 / sec.

Hereinafter, experimental examples for grasping the relation of the elongation of the Mg-Li-based alloy with the change of the strain rate will be described. It should be understood, however, that the present invention is not limited to the following examples.

The Mg-Li based alloy according to Experimental Example of the present invention was subjected to a tensile test using a Mg-Li alloy containing 9.0% by weight of Li, 1.0% by weight of Zn, and inevitable impurities. In the tensile test, the strain rate was set to 0.01 / sec at the first stage before and after the point at which the strain was 3.0%, 5.0%, 7.0%, 10.0%, 15.0% and 20.0% / sec and the elongation was measured. Table 1 shows the conditions of the specimen used in the experiment of the present invention.

On the other hand, in order to comparatively compare the elongation, a tensile test was conducted using the same Mg-Li alloy as used in the above Experimental Examples. In Comparative Example 1, the strain rate was maintained at 0.01 / sec and the Comparative Example 2 was maintained at 0.001 / sec without a section in which the strain rate was changed during the tensile test.

FIG. 1 shows a nominal stress-nominal strain (Eng.Stress-Eng.Strain) graph of the experimental examples and comparative examples, and elongation data according to the linear strain of each of the experimental examples is shown in FIG. In FIG. 2, broken lines indicate engineering strains in Comparative Example 1 and Comparative Example 2.

Referring to FIGS. 1 and 2, in the case of Comparative Example 1 in which the strain rate is 0.01 / sec, the fracture occurs in a state where the linear strain does not progress a little. In the case of Comparative Example 2, which has a relatively slow strain rate of 0.001 / sec, the elongation is about 0.4 or more.

On the other hand, in the case of the experimental example in which the strain rate is divided into two different stages, it can be seen that the elongation rate varies according to the change of the line strain in the first stage.

2 (Experimental Examples 1 to 3) (Experimental Examples 4 to 6, Comparative Examples 1 and 2) in which the linear strain value is greater than 2% and smaller than 10% ), And it was confirmed that the elongation ratio was higher than that of the comparative example. This means that there is a critical point in the line strain variation period that divides the strain rate into the first stage and the second stage.

If the linear strain at the first step is less than 2%, the deformation amount is too small because an elongation improving mechanism is developed according to the difference of the strain rate sensitivity index. On the other hand, when the line strain is large, the strain is higher than 10%, the α-phase increases in strength due to work hardening even though the α-phase has lower strength than the β-phase, So that deformation also occurs. Accordingly, as the strain that the β phase can accommodate in the second step after the first step is reduced, the final elongation rate is rather reduced. Therefore, in order to obtain a high elongation, it is necessary to appropriately control the linear strain in the first step in a range of greater than 2.0% and less than 10.0%.

While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, but, on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the invention. Accordingly, the true scope of the present invention should be determined by the technical idea of the appended claims.

Claims (5)

A method of forming a Mg-Li-based alloy having at least two phases having different strain rate sensitivities,
A first step of molding at a predetermined strain rate; And
And a second step of forming at a strain rate relatively lower than the first step,
Wherein the range of the linear strain in the first step is larger than 2.0% and smaller than 10.0%
Mg-Li-based alloy. The method according to claim 1,
The Mg-Li-based alloy includes a first phase having a relatively low strain rate sensitivity index and a second phase having a relatively high strain rate sensitivity index,
In the first step, the strength of the second phase is higher than that of the first phase so that deformation is concentrated on the first phase,
Wherein in the second step, the deformation is concentrated on the second phase because the strength of the first phase is higher than that of the second phase,
Mg-Li-based alloy. 3. The method of claim 2,
The first phase is an alpha phase (H phase) having a dense hexagonal lattice structure (HCP)
Wherein the second phase is a beta phase (beta phase) having a body-centered cubic structure (BCC)
Mg-Li-based alloy. 3. The method of claim 2,
Wherein the Mg-Li-based alloy comprises at least one of Zn, Al and Mn.
Mg-Li-based alloy. 3. The method of claim 2,
The strain rate in the first step ranges from 0.005 / sec to 10 / sec,
Wherein the strain rate in the second step is from 0.00001 / sec to less than 0.005 / sec,
Mg-Li-based alloy.

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