KR100570905B1 - A method for increasing the ductility of magnesium alloy through texture control - Google Patents

Abstract

The present invention relates to a method for increasing the ductility of a magnesium alloy having low ductility due to brittleness resulting from a hexagonal close lattice structure and poor workability at room temperature.

The present invention provides a method of increasing the ductility of a magnesium alloy, the method comprising heating a magnesium alloy material and a die, step II of performing an ECAP process on a preheated magnesium alloy material, and an ECAP processed magnesium alloy material for at least 20 minutes. Comprising step III maintaining in a temperature range of < RTI ID = 0.0 >

The step I and the step II is characterized in that it consists of a process of performing step III after repeating one or more times in sequence.

According to the present invention, magnesium alloys suitable for processing into complicated parts and the like can be produced by having ductility superior to aluminum alloys that are usually used as lightweight materials.

Magnesium alloy, ECAP processing, annealing, ductility, texture

Description

A METHOD FOR INCREASING THE DUCTILITY OF MAGNESIUM ALLOY THROUGH TEXTURE CONTROL}

1 is a side view of an apparatus for ECAP processing magnesium alloy.

2 is a plan view of an apparatus for ECAP processing magnesium alloy.

3 is a front view of an apparatus for ECAP processing magnesium alloy.

Figure 4 is a graph showing the result of comparing the uniform elongation, maximum elongation, yield strength and the like when the treatment according to the present invention with the comparative materials.

5 is a graph showing the strain behavior during the tensile test of each material.

Figure 6 is a graph comparing the strain behavior during the tensile test according to the number of treatment when treated according to the present invention.

Figure 7 is a graph showing the strain behavior during the tensile test of the aluminum material to compare the yield strength and ductility of the magnesium alloy prepared by the present invention.

8 is a graph showing a result of comparing the degree of ductility increase after processing the magnesium alloy sheet according to claim 5 with the raw material.

(Explanation of the sign)

1: entrance 2: arc angle (curvature angle)

3: square angle 4: sample

5: pressurized piston 6: outlet

The present invention relates to a method for increasing the ductility of a highly brittle magnesium alloy at room temperature, and more particularly, to develop a magnesium alloy aggregate structure by using an ECAP (Equal Channel Angular Process) processing method. It is about how to increase.

Magnesium alloy is a lightweight material of plastic level, has the effect of blocking electromagnetic waves and can be used as an environmentally friendly material because it is easier to recycle than conventional plastic materials, and its utilization value is high.

However, pure magnesium or a magnesium alloy to which a small amount of solute element is added has a brittle characteristic at room temperature as a hexagonal close lattice (HCP). Therefore, processing of magnesium alloy is difficult because of low ductility due to the crystal structural cause of magnesium alloy.

Therefore, in order to manufacture a magnesium alloy in a desired shape, a die casting method, which is one of casting methods, has been conventionally used. Die casting is also called die casting. It is a precision casting method in which molten metal is injected into a mold made of steel machined to be exactly matched to the required casting shape to obtain the same casting as the mold. It is called a fetish.

By the way, when magnesium alloy is manufactured by the die casting method, it is difficult to produce a material having a thickness of 0.6 mm or less, and the surface is not beautiful. Moreover, there is a problem that it is difficult to manufacture a component having a complicated shape. Therefore, there is a need for a method of plastically forming magnesium alloys by increasing ductility.

Conventionally, in order to increase the ductility of the magnesium alloy, when the magnesium alloy is processed at high temperature using extrusion or rolling heat treatment, etc., the grain size of the magnesium alloy may be finely obtained, thereby obtaining a magnesium alloy with increased ductility.

However, the effect of grain refinement alone is difficult to replace a material such as aluminum because the ductility of the magnesium alloy material is not sufficient when performing a high degree of deformation.

An object of the present invention is to provide a method for processing a magnesium alloy for producing a magnesium alloy excellent in ductility by developing an aggregate structure in the magnesium alloy.

It is still another object of the present invention to provide a method for producing a magnesium alloy having excellent ductility through minimal processing even for a material having a coarse grain size, such as a magnesium alloy ingot.

The present invention, as described in claim 1,
As a method of increasing the ductility of magnesium alloys,

Heating the magnesium alloy material and the die I,

Step II of carrying out the ECAP process on the preheated magnesium alloy material and

Holding ECAP processed magnesium alloy material at 280 ° C. to 350 ° C. for at least 20 minutes;

And repeating step I and step II one or more times in succession in sequence, and performing step III.

The characteristic part of the structure of this invention is demonstrated below.

ECAP (Equal Channel Angular Pressing) processing method has an inlet (1), outlet (6) and the bent portion, as shown in Figures 1-3, at the inlet (1) and outlet (2) and any point The cross-sectional shape perpendicular to the axis to be processed is the same as the cross-sectional shape of the material to be processed, and means a method of passing the material through a channel having a curved portion in the middle. ECAP processing causes the material to be subjected to extreme shear deformation at the bend.

First, when the ECAP processing method is applied to a conventional material such as aluminum, the strength is increased due to the grain refinement effect, but the ductility is significantly reduced, and thus it was not used to increase the ductility.

However, the inventors of the present invention have found that, in the case of magnesium alloy, the strength is not increased or decreased, but the ductility is significantly increased. Therefore, when the ECAP processing method is applied to magnesium alloy, the ductility increase effect can be obtained. The reason is that plastic deformation of magnesium alloy is caused by basal plane or prism face slip. When ECAP is applied to rolled or extruded materials, these surfaces rotate during pure shear deformation process as shown in Table 1. This is because texture conversion to a state where slip is likely to occur in the slip surface greatly increases the work hardening performance. As shown in FIG. 4, when the ECAP process is performed about eight times, the tensile ductility of the magnesium alloy is increased to be similar to that of a general aluminum alloy, for example, an alloy called Al 6061 alloy of FIG. 7.

ECAP machining count Organization Schmidt Factor Basal plane Prism face 0 (Processing voltage output direction) [10-10] 0; 0; 0 0; 0.43; 0.43 2 (10-14) [-2021] 0; 0.33; 0.33 0; 0.36; 0.36 3 (10-12) [-2201] + (10-11) [-12-10) 0; 0.33; 0.33 0; 0; 0 0; 0.36; 0.36 0; 0.43; 0.43 4 (10-11) [0-221] + (10-12) [0-221] 0; 0.33; 0.33 0; 0.33; 0.33 0; 0.36; 0.36 0; 0.36; 0.36 8 (10-11) [0-111] + (10-12) [-12-10] 0; 0.44; 0.44 0; 0; 0 0; 0.23; 0.23 0; 0.43; 0.43

However, as mentioned above, unlike conventional materials, magnesium alloys are susceptible to breakage due to their fragile properties when processed at room temperature, and thus preheating steps like step I are required before each ECAP process to prevent them.

The ECAP process alone can increase the ductility of the magnesium alloy, but in the case of a process requiring severe deformation, additional ductility needs to be increased. Therefore, step III is an essential process for further ductility of the magnesium alloy.

If additional ductility increases are required for ECAP-processed magnesium alloys, a process of increasing the size of the grains of magnesium alloys by maintaining the magnesium alloy for a certain time within a certain temperature range is effective. As can be seen in Figure 5, the inventors of the present invention, the metal material is usually a fine grain grain ductility is increased, but the magnesium alloy subjected to pure shear processing by ECAP processing, such as in the case of the present invention will be coarse grains Rather, it was found that ductility increases. The reason is that, as shown in Table 1, the magnesium alloy subjected to the pure shear processing is developed as the internal grains become finer, and when the magnesium alloy is maintained at a temperature at which recrystallization does not occur for a long time, the grain structure remains as it is, This is because the coarsening decreases the dislocation density inside the grain and the stress state accumulated during the sintering process is alleviated, thereby making the inner slip easier and changing the work hardening.

In addition, the present invention can be applied to a plate, in which case ECAR (Equal Channel Angular Rolling) processing method and two-speed rolling method can be applied. In the case of ECAR, unlike the ECAP processing method, it is fundamentally different in that it is applied to a plate rather than a bar or a bar, so the description thereof will be omitted. In the case of using the second rolling, the present invention at this time

I. preheating the magnesium alloy ingot,

II. Second-rolling the preheated magnesium alloy ingot,

III. And maintaining the rolled magnesium alloy material at 280 ° C to 350 ° C for at least 20 minutes.

Since other processes are the same as those of the invention provided in claim 1, the description thereof will be omitted and only the second rolling of step II will be described.

Second-speed rolling refers to a method of imparting shear stress to a plate by varying the speed of the upper and lower rolling rolls that provide stress to the plate to roll the plate. The plate subjected to the second-speed rolling has a shear stress because the processing speed of the upper and lower sides is different, and thus, the aggregate structure is developed.

(Supplementary Example)

In the present invention, the process of forming the texture and maintaining the material at high temperature (annealing process) are organically combined, and each process is an essential process for increasing the ductility of the magnesium alloy.

First of all, the following supplementary experiment was conducted to investigate the effect of aggregated tissue forming process in which step I and step II were performed successively.

In this example, an AZ61 alloy containing aluminum, zinc and magnesium in a weight ratio of 6: 1: 93 was used.

In this embodiment, the preheating temperature of the magnesium alloy and the die was 280 ° C. in order to reduce the low temperature embrittlement of the magnesium alloy. The preheating temperature range is suitably 200 ° C. to 350 ° C. for the magnesium alloy and the die, respectively. At a temperature below the lower limit of the temperature range, as described above, fracture of the material occurs due to brittleness of the magnesium alloy, and at a temperature exceeding the upper limit of the preheating temperature range, recrystallization occurs to coarsen grains of the magnesium alloy, Aggregation is difficult to develop.

The narrower the angle between the axis passing through the center of the inlet 1 of the die (hereinafter referred to as the inlet axis) and the axis passing through the center of the outlet 6 (hereinafter referred to as the exit axis), i. It is effective processing because the shear stress acting becomes large, but when too narrow, it is difficult to pass the material (4) to the die, especially in the case of the magnesium alloy of the present invention, the material damage due to the brittleness of magnesium alloy There is a risk of happening. The inventors of the present invention found that the angle is about 90 ° to 135 °, and accordingly, in this embodiment, the experiment was performed by setting the angle to 90 °.

In addition, in the drawing, the arc angle (curvature angle) 2 of the curved portion of the outer portion of the channel (left side in the drawing) is not significantly limited, but is usually less than or equal to 90 ° to smoothly escape the material. The experiment was conducted by setting this to 90 °.

The number of times to perform step I and step II (hereinafter, referred to as "assembly processing") is appropriate for at least four times in order to develop an easy-slip aggregate inside and to refine the microstructure. Was carried out. In addition, the temperature required to coarse the internal grains in the state of maintaining the magnesium alloy texture is 200 ℃ to 350 ℃ in this example, the experiment was carried out at 300 ℃.

After the final treatment of the magnesium alloy, ECAP processed according to this embodiment, magnesium alloy of the same composition without ECAP processing (hereinafter referred to as Comparative Material 1) and ECAP processed aluminum (hereinafter referred to as Comparative Material 2) used for similar purposes as magnesium alloy Tensile specimens were taken and subjected to a tensile test.

6 and 7 show the results of comparing the magnesium alloy processed according to the present example with Comparative Material 1 and Comparative Material 2.

As shown in Fig. 6, the maximum elongation of the magnesium alloy processed according to the present embodiment was obtained after eight times of texture formation processing, compared to 1.7 (not shown) when the magnesium alloy was not processed. It is about twice as high, and the value is similar when compared with the ECAP-free aluminum material (when the number of treatments of FIG. 7 is 0). Figure 6 also shows that the maximum elongation is comparable compared to aluminum material, but the uniform elongation, i.e., strain until the necking occurs (strain up to the point where stress is shown in the stress-strain curve) is at least 1.5 times that of the magnesium alloy treated with ECAP. It is larger. In production processing using plastic deformation, uniform elongation is more important than maximum elongation, because the thickness of the product must be uniform. For this reason, it can be expected that the magnesium alloy processed by ECAP will exhibit better plastic deformation behavior than aluminum materials.

However, as shown in FIG. 5, the yield strength of each material is compared. As a result, the magnesium alloy processed according to the present embodiment shows a yield strength as low as 26% compared to the unprocessed magnesium alloy, but is widely used as a structural material. Since it shows a higher value than the yield strength of aluminum, it can be used as a structural material. In addition, since the work hardening ability is so large that the strength is greatly increased after molding, the strength after molding can be increased to about 300 MPa.

(Example 1)

In order to further increase the ductility of the ECAP-treated magnesium alloy, the following experiment was conducted.

In this example, a magnesium alloy called AZ31 containing aluminum, zinc and magnesium in a weight ratio of 3: 1: 96 was used. Similar to the supplemental example above, the die and material were heated to 250 ° C. and then ECAP processed to 6 passes, followed by annealing at 300 ° C. for 20 minutes and 80 minutes.

The results are shown in FIG. As can be seen in Figure 5, the yield strength decreases as the annealing time increases, but it can be seen that the tensile elongation increases. The yield strength decreases with annealing time due to the increase in grain size because there is no change in the texture at the annealing temperature, and the reason for the increase in ductility is that the dislocation density inside the grain is lowered and the accumulated stress state inside the sintering process is alleviated. This is because the inner slip becomes easier and the work hardening changes. Such annealing temperature is suitably in the range of 280 ° C to 350 ° C. Yield strength was lowered to 90 MPa level after annealing at 300 ° C. for 80 minutes, but the tensile elongation and uniform elongation were improved by 50% compared to the general aluminum material. The low yield strength of 90 MPa may be a problem as a structural material. However, in the case of magnesium alloy, the work hardening is very large, and when the magnesium alloy manufactured according to the present embodiment is processed into a desired shape later than 220 MPa after molding The strength in the final product can be said to be sufficient. The low initial yield strength may also be an advantage in that less force is required for processing, which can contribute to productivity gains.

(Example 2)

An embodiment of a method of processing a magnesium alloy sheet material which is another configuration of the present invention will be described.

As described above, the second embodiment has the same preheating step (step I) and high temperature holding step (step III) as in the first embodiment, and thus, only the step of performing the second speed rolling will be described in detail except for the description thereof. . In this example, an AZ31 alloy containing aluminum, zinc and magnesium in a weight ratio of 3: 1: 96 was used.

In the second rolling, the circumferential speed of the upper rolling roll was set to 8 m / min, and the circumferential speed of the lower rolling roll was set to 1 m / min. Rolling was performed twice at 300 ° C. until the thickness was reduced to 0.8 mm.

The treated material was subjected to a tensile test in the same manner as in Example 1 by taking a specimen from the central portion. In FIG. 8, the tensile test results of the specimens of this example were compared. As shown in Figure 8, it can be seen that the specimen of this embodiment shows a higher tensile elongation than the magnesium alloy of the same composition without the ECAP process. Increasing the speed difference between the upper and lower rolls of the second speed rolling and the amount of rolling deformation is expected to achieve higher tensile elongation as in the ECAP process.

In addition, as shown in FIG. 8, the yield strength of each material was compared. As a result, the magnesium alloy which had undergone treatment according to the present embodiment had a lower value than that of Comparative Material 1, which was not treated, but is normally used as a structural material. (FIG. 7) It can be seen that the comparative material 2 level is shown.

Therefore, it was confirmed that the magnesium alloy treated by the present invention can be suitably used as a lightweight material having good workability.

The present invention relates to a method for producing a magnesium alloy having superior ductility than an aluminum alloy commonly used as a lightweight structural material by dramatically improving the ductility of a magnesium alloy that is light in weight but has high use, but has poor workability. When the method according to the present invention is used, it can be effectively used in fields such as structural materials or exterior materials, which are common uses, and also can be effectively processed using magnesium alloy even for parts having a complicated shape.

Claims (7)

As a method of increasing the ductility of magnesium alloys, Heating the magnesium alloy material and the die I, Step II of ECAP Processing on Preheated Magnesium Alloy Material A step III of maintaining the ECAP processed magnesium alloy material in a temperature range of 280 ° C to 350 ° C for at least 20 minutes; And repeating step (I) and step (II) one or more times in sequence and performing step III. The method of increasing the ductility of a magnesium alloy according to claim 1, wherein the temperature for heating the magnesium alloy material and the die is in the range of 200 ° C to 350 ° C, respectively. The angle between the axis passing through the center of the inlet of the die and the axis passing through the center of the exit during ECAP processing is between 90 ° and 135 ° and the angle of curvature is between 0 ° and 90 °. To increase the ductility of magnesium alloy delete As a method of increasing the ductility of a magnesium alloy having poor ductility, I. preheating the magnesium alloy ingot, II. Second-rolling the preheated magnesium alloy ingot, III. Maintaining the rolled magnesium alloy material at a temperature range of 280 ° C. to 350 for at least 20 minutes. 6. The method of claim 5 wherein the heating temperature in step I ranges from 200 ° C to 350 ° C. delete

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