KR20200121530A - Magnesium alloy processing method for improving strength, ductility and yield symmetry, and a magnesium alloy rod produced thereby - Google Patents

Abstract

The present invention relates to a magnesium alloy processing method for improving strength, ductility and yield asymmetry for use in biomaterials, and a magnesium alloy bar manufactured thereby. The magnesium alloy processing method for improving strength, ductility and yield asymmetry includes: a step of heating the magnesium alloy; and a step of injecting the heated magnesium alloy into a multi-hole rolling mill to perform multi-hole rolling.

Description

Magnesium alloy processing method for improving strength, ductility and yield isotropy, and magnesium alloy rod produced thereby}

The present invention relates to a magnesium alloy processing method for improving strength, ductility and yield isotropy, and to a magnesium alloy bar manufactured thereby, and more specifically, to a magnesium alloy with high strength, high ductility, and excellent yield isotropy to provide very complex stress conditions. It relates to a magnesium alloy processing method and a magnesium alloy rod manufactured thereby to be used in a biomaterial that suffers from.

Mg-6Zn-0.5Zr (hereinafter referred to as'ZK60') magnesium alloy is attracting attention as a biomaterial because it does not contain alloying elements that cause cytotoxicity with adequate mechanical performance.

In order to further enhance the mechanical performance of this material, it is necessary to induce a so-called ultrafine-grained (``UFG'') structure that makes the average grain size inside the material less than 1 μm.

It is difficult to make a UFG structure with an extrusion method commonly used in manufacturing ZK60 rods, and accordingly, a method using a rigid plastic processing method such as equal-channel angular pressing or high-pressure torsion has been proposed.

However, the UFG ZK60 material manufactured by the rigid plastic processing method is limited in size to several centimeters, so industrial application is close to impossible.

On the other hand, many magnesium alloys, including ZK60, show yield anisotropy in which the tensile and compressive yield stresses do not match. In an actual product environment where various and complex stresses are applied, such yield anisotropy is a problem in that it can cause structural instability.

An object of the present invention is to provide a magnesium alloy processing method that can be used for biomaterials undergoing very complex stress conditions by imparting high strength, high ductility and excellent yield isotropy to the magnesium alloy.

Another object of the present invention is to provide a magnesium alloy rod having high strength, high ductility, and excellent yield isotropy.

A magnesium alloy processing method for improving strength, ductility and yield isotropy for one object of the present invention includes heating the magnesium alloy and introducing the heated magnesium alloy into a multi-hole rolling mill to perform multi-hole rolling.

In this case, the heating step is preferably performed at 350°C to 450°C for 30 minutes to 2 hours.

In addition, the magnesium alloy includes Zn and Zr as alloying elements, and may contain 90% by weight or more of Mg, more preferably Zn: 4.8 to 6.2% by weight, Zr: 0.3 to 0.8% by weight, Mg: 90 It may contain at least% by weight.

On the other hand, the multi-hole rolling is preferably performed at 250 ℃ to 450 ℃.

In addition, in the multi-hole rolling step, the heated magnesium alloy may be rotated at a predetermined angle and introduced into the multi-hole rolling mill.

In addition, the multi-hole rolling step is preferably performed at a cross-sectional reduction rate of 60% or more.

In an embodiment of the present invention, the magnesium alloy obtained after the multi-hole rolling step may have an average grain size of 1 μm or less, a tensile yield strength and a compressive yield strength of 330 MPa or more, and a maximum elongation of 18% or more, The product of the yield strength and the maximum elongation may have a range of 6000 to 7000 MPa%.

In addition, it is preferable that the compressive yield strength/tensile yield strength ratio of the magnesium alloy obtained after the multi-hole rolling step is 0.90 or more.

On the other hand, the magnesium alloy rod for another object of the present invention is manufactured according to the magnesium alloy processing method of the present invention, characterized in that the compressive yield strength / tensile yield strength ratio is 0.9 or more.

In addition, the magnesium alloy bar may have an average grain size of 1 μm or less, a tensile yield strength and a compressive yield strength of 330 MPa or more, a maximum elongation of 18% or more, and a product of a yield strength and a maximum elongation of 6000 to 7000 MPa%. It is preferred to have a range.

According to the present invention, the UFG structure can be successfully induced by applying a multi-pass caliber-rolling process to the magnesium alloy, thereby securing high strength and high ductility, as well as yield anisotropy of the magnesium alloy. It is possible to realize excellent yield isotropy by almost removing.

In addition, the magnesium alloy rod of the present invention not only secures high strength and high ductility, but also has excellent yield isotropy, so that the difference between tensile yield strength and compressive yield strength is not large, so that practical strength is increased, and very complex stress state It can also be used for biomaterials that suffer from ).

In addition, as the magnesium alloy rod of the present invention is processed and manufactured through a multi-hole rolling process, large-scale production is possible.

1 is a view showing the shape of a magnesium alloy before and after processing according to an embodiment of the present invention.
2A is a view showing an image of a cross-sectional microstructure observed by SEM before and after processing of a magnesium alloy according to an embodiment of the present invention.
FIG. 2B is a view showing an image of a cross-sectional microstructure after processing of a magnesium alloy according to an embodiment of the present invention observed with an SEM.
3 is a view showing a surface image of a magnesium alloy according to Comparative Example 1 of the present invention.
FIG. 4 is a diagram showing an image of a cross-sectional microstructure of a magnesium alloy according to Comparative Example 2 of the present invention observed after processing with an SEM.
5 is a graph showing the compressive yield strength and compression/tensile yield strength ratio of a magnesium alloy bar and a commercial extruded material according to an embodiment of the present invention.
6 is a graph showing the product of yield strength and maximum elongation of a magnesium alloy bar, a commercial extruded material, and an ECAP rigid-plastic processed material according to an embodiment of the present invention.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings. In the present invention, various modifications may be made and various forms may be applied, and specific embodiments will be illustrated in the drawings and described in detail in the text. However, this is not intended to limit the present invention to a specific form of disclosure, it is to be understood as including all changes, equivalents, or substitutes included in the spirit and scope of the present invention. In describing each drawing, similar reference numerals have been used for similar elements.

The terms used in the present application are only used to describe specific embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present application, terms such as "comprise" or "have" are intended to designate the existence of features, steps, actions, components, parts, or a combination thereof described in the specification, but one or more other features or steps It is to be understood that it does not preclude the possibility of addition or presence of, operations, components, parts, or combinations thereof.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meaning as commonly understood by one of ordinary skill in the art to which the present invention belongs. Terms as defined in a commonly used dictionary should be interpreted as having a meaning consistent with the meaning in the context of the related technology, and should not be interpreted as an ideal or excessively formal meaning unless explicitly defined in this application. Does not.

The magnesium alloy processing method for improving strength, ductility, and yield isotropy of the present invention includes heating the magnesium alloy and introducing the heated magnesium alloy into a multi-hole rolling mill to perform multi-hole rolling.

Here, the multi-hole rolling refers to a method of performing rolling by digging a caliber of various sizes between two rolling rolls, gradually reducing the size from the largest cross-sectional area, and inserting the heated magnesium alloy between each hole.

Specifically, in the present invention, the step of heating the magnesium alloy first proceeds.

The heating step is preferably performed for 30 minutes to 2 hours at 350 ℃ ~ 450 ℃. When the heating temperature is less than 350°C, the workability of multi-hole rolling is greatly reduced, and when the heating temperature exceeds 450°C, dynamic precipitation occurs during multi-hole rolling, resulting in mechanical properties of the magnesium alloy being finally processed. It can be degraded. In addition, when the heating time is less than 30 minutes, uniform heating may not be possible, and when the heating time exceeds 2 hours, economic loss is large.

Therefore, it is most preferable to perform the heating step at 400° C. for 1 hour.

In addition, the magnesium alloy contains Zn and Zr as alloying elements, and preferably contains at least 90% by weight of Mg, and Zn: 4.8 to 6.2% by weight, Zr: 0.3 to 0.8% by weight, Mg: 90% by weight or more It is more preferable to include, but is not limited thereto, for example, Mg-6Zn-0.5Zr (hereinafter'ZK60') magnesium alloy may be used.

The magnesium alloy is preferably subjected to homogenization heat treatment, in order to prevent precipitation of carbides that may adversely affect the mechanical properties of the finally processed magnesium alloy.

In addition, the magnesium alloy is preferably made of a rod shape, but is not limited thereto.

Next, the step of multi-hole rolling by introducing the heated magnesium alloy into a multi-hole rolling mill is performed.

Specifically, the heated magnesium alloy is repeatedly added to the multi-hole rolling mill, and the multi-hole rolling is preferably performed at 250°C to 450°C. When the heating temperature is less than 250° C., the dynamic recrystallization region is too narrow to make a wide UFG region as shown in FIG. 2, and thus it is difficult to secure high strength and high ductility characteristics. In addition, when the heating temperature exceeds 450°C, it is too close to the melting point of the magnesium alloy, which causes safety problems when performing the processing process, and it becomes difficult to create the UFG region due to the strong grain growth effect that occurs at high temperatures.

In addition, it is preferable that the heated magnesium alloy is rotated at a predetermined angle, preferably at an angle of 90°C, and introduced into the multi-hole rolling mill at each repeated injection. At this time, during repeated injection, the heated magnesium alloy is not reheated.

On the other hand, the step of multi-hole rolling is preferably performed at a cross-sectional reduction ratio of 60% or more, and when applying a cross-sectional reduction ratio of less than 60%, sufficient plastic deformation is not applied, making it difficult to create an ultrafine-grained (UFG) area. There is a problem.

That is, the cross-section of the magnesium alloy obtained after processing is reduced by more than 60% compared to the cross-section before the magnesium alloy processing, so that sufficient plastic deformation is applied to successfully induce the UFG structure of the magnesium alloy.

The magnesium alloy obtained through the above-described processing method produces a UFG structure having an average grain size of 1 μm or less, and thus the tensile yield strength and compressive yield strength are 330 MPa or more, the maximum elongation is 18% or more, and yield. The product of the strength and the maximum elongation may exhibit high strength and high ductility properties having a range of 6000 to 7000 MPa%.

In addition, the yield anisotropy of the magnesium alloy before processing is almost removed, and the compressive yield strength/tensile yield strength ratio shows a high value of 0.90 or more.

Here, the yield anisotropy is a characteristic that the tensile yield stress and the compressive yield stress do not coincide. In general, the yield anisotropy of a material is expressed as the compressive yield strength/tensile yield strength ratio, and the closer the value to 1 is recognized as having yield isotropy. do.

That is, in the case of the obtained magnesium alloy, it may exhibit excellent properties in yield isotropy compared to commercial extruded materials.

On the other hand, in another embodiment of the present invention, it is possible to provide a magnesium alloy rod manufactured by the processing method of the present invention.

The magnesium alloy bar has a compressive yield strength/tensile yield strength ratio of 0.90 or more, and exhibits excellent yield isotropic properties.

In addition, the magnesium alloy bar includes a UFG structure having an average grain size of 1 μm or less, and the tensile yield strength and compressive yield strength are 330 MPa or more, the maximum elongation is 18% or more, and the product of the yield strength and the maximum elongation is 6000. To 7000 MPa%.

That is, the magnesium alloy rod of the present invention not only secures high strength and high ductility, but also has excellent yield isotropy, so that the difference between tensile yield strength and compressive yield strength is not large, so that practical strength is increased, and a very complex stress state It can also be used for bio-materials that suffer from problems.

In addition, as the magnesium alloy rod of the present invention is processed and manufactured through a multi-hole rolling process, large-scale production is possible.

Hereinafter, various embodiments and experimental examples of the present invention will be described in detail. However, the following examples are only some examples of the present invention, and the present invention should not be construed as being limited to the following examples.

[Example]

Mg-6Zn-0.5Zr consisting of Zn: 6% by weight, Zr: 0.5% by weight, Mg: 93% by weight and the remaining inevitable impurities was processed into a bar having a diameter of 26 mm and a length of 50 mm. The bar thus processed was heated at 400° C. for 1 hour using an electric furnace.

Next, the heated bar was taken out from the electric furnace and then put into a multi-hole type rolling mill to perform multi-hole rolling at a temperature of 300°C. Each rolling pass rotates the bar by 90°C and inserts it into the multi-hole rolling mill repeatedly until 6 passes (reheating between passes is not performed), and finally, the total sectional reduction rate is 84%, with a length of about 2 m magnesium. The alloy bar was fabricated (see Fig. 1).

[Comparative Example 1]

A magnesium alloy rod was fabricated and manufactured in the same manner as in Example, except that multi-hole rolling was performed at a temperature of 150°C.

[Comparative Example 2]

A magnesium alloy rod was fabricated and manufactured in the same manner as in the Example, except that multi-hole rolling was performed at a cross-sectional reduction rate of 40%.

Microstructure

Using SEM, the microstructure of the magnesium alloy bar according to Examples and Comparative Example 2 was observed, and images of observing the surface of the magnesium alloy bar according to Comparative Example 1 are shown in FIGS. 2 to 4.

In the case of the Example, it was confirmed that the UFG structure having an average grain size of 0.6 μm was generated as shown in FIGS. 2A and 2B. This UFG structure was confirmed over 88% of the observation area, and due to this, it was found that a homogeneous UFG structure was successfully secured in most areas of the magnesium alloy rod according to the example.

On the other hand, referring to FIG. 3, in the case of Comparative Example 1 in which multi-hole rolling was performed at a temperature of 150° C., the temperature at the time of multi-hole rolling was too low, and it was found that the surface of the material was broken from the beginning of rolling. It is believed that it is difficult to secure high strength and high ductility because it is difficult to secure a UFG area.

On the other hand, in the case of Comparative Example 2 in which multi-hole rolling was performed at a cross-sectional reduction rate of 40%, as shown in FIG. 4, the average grain size was found to be about 10 μm, and thus, sufficient plastic deformation at a cross-sectional reduction rate of 40% It was confirmed that the UFG (<1㎛) structure could not be made because of this.

Mechanical properties

In order to compare the mechanical properties of the magnesium alloy rod and commercial extruded material (Magnesium Alloy AZ31 Rod, GFM) according to the embodiment, tensile stress and compressive stress tests were performed using a conventional tensile and compression tester.

Specific test results are shown in Table 1 and FIG. 5, respectively.

Mechanical performance (unit) Example Commercial extruded material Tensile yield strength (MPa) 364 266 Tensile strength (MPa) 389 321 Elongation (%) 18 15 Compressive yield strength (MPa) 337 143 Compression/tensile yield strength ratio 0.93 0.54

Referring to Table 1, the magnesium alloy bar according to the embodiment had higher tensile yield strength, tensile maximum strength, maximum elongation, compressive yield strength, and compression/tensile yield strength ratio values compared to commercial extruded materials. It was confirmed that both strength and ductility were improved compared to commercial extruded materials.

In particular, in the case of the compressive yield strength (see Fig. 5), it was found that the improvement was more than twice (235%) compared to the commercial extruded material, and this improvement in the compressive yield strength led to securing yield isotropy.

That is, the magnesium alloy rod according to the embodiment has improved compressive yield strength, exhibiting a high compressive yield strength/tensile yield strength ratio of 0.93, and has a value much closer to 1 compared to the commercial extruded material showing a value of 0.54, yielding It exhibited excellent isotropic properties.

Meanwhile, since the strength and ductility of materials are generally inversely proportional, comprehensive mechanical properties can be compared by comparing the product of the two properties (yield strength X maximum elongation).

6 is a graph showing the product of the yield strength and the maximum elongation of a magnesium alloy bar, a commercial extruded material, and an ECAP (Equal channel angular pressing) rigid plastic processed material according to an embodiment.

As shown in Figure 6, the product of the yield strength and the maximum elongation of the magnesium alloy bar according to the embodiment is in the range of 6000 to 7000 MPa%, whereas the commercial extruded material was 3000 to 5000 MPa% lower than that of the magnesium alloy bar, and ECAP rigid plastic processed material also 3000 to 6000 MPa%. Through this, it was confirmed that the magnesium alloy rod according to the embodiment of the present invention has excellent mechanical performance compared to the commercial extruded material and the ECAP rigid plastic processed material.

In the case of very few ECAP rigid-plastic processed materials, as they have a high elongation of 30% or more, they exhibited a higher yield strength and maximum elongation product than the magnesium alloy bar of the present invention, but these materials are only a few cm long, so industrial mass production is not possible. Difficult disadvantages exist and cannot replace the advantages of the magnesium alloy rod of the present invention capable of large-scale and large-scale production.

Although the above has been described with reference to preferred embodiments of the present invention, those skilled in the art will be able to variously modify and change the present invention without departing from the spirit and scope of the present invention described in the following claims. You will understand that you can.

Claims (13)

Heating the magnesium alloy; And
Incorporating the heated magnesium alloy into a multi-hole type rolling mill and multi-hole rolling; containing, a magnesium alloy processing method for improving strength, ductility and yield isotropy.
The method of claim 1,
The heating step is characterized in that carried out for 30 minutes to 2 hours at 350 ℃ ~ 450 ℃, the magnesium alloy processing method for improving strength, ductility and yield isotropy.
The method of claim 1,
The magnesium alloy includes Zn and Zr as alloying elements, and magnesium alloy processing method for improving strength, ductility and yield isotropy, characterized in that it contains 90% by weight or more of Mg.
The method of claim 3,
The magnesium alloy is Zn: 4.8 to 6.2% by weight, Zr: 0.3 to 0.8% by weight, Mg: magnesium alloy processing method for improving strength, ductility and yield isotropy, characterized in that it comprises 90% by weight or more.
The method of claim 3,
The multi-hole rolling step is characterized in that performed at 250 ℃ to 450 ℃, the magnesium alloy processing method for improving strength, ductility and yield isotropy.
The method of claim 5,
In the step of multi-hole rolling, the heated magnesium alloy is rotated at a predetermined angle and introduced into a multi-hole rolling mill. A method of processing a magnesium alloy for improving strength, ductility and yield isotropy.
The method of claim 6,
The multi-hole rolling step is characterized in that carried out at a cross-sectional reduction rate of 60% or more, strength, ductility, and magnesium alloy processing method for improving yield isotropy.
The method of claim 7,
The magnesium alloy obtained after the multi-hole rolling step is characterized in that the average grain size is less than 1㎛, the magnesium alloy processing method for improving strength, ductility and yield isotropy.
The method of claim 7,
The tensile yield strength and compressive yield strength of the magnesium alloy obtained after the multi-hole rolling step is 330 MPa or more, the maximum elongation is 18% or more, and the product of the yield strength and the maximum elongation is in the range of 6000 to 7000 MPa%. To improve the strength, ductility and yield isotropy of magnesium alloy processing method.
The method of claim 7,
The compressive yield strength/tensile yield strength ratio of the magnesium alloy obtained after the multi-hole rolling step is 0.90 or more, wherein the magnesium alloy processing method for improving strength, ductility and yield isotropy.
It is manufactured by the processing method according to claim 7,
Compressive yield strength / tensile yield strength ratio, characterized in that 0.90 or more, magnesium alloy bar.
The method of claim 11,
The magnesium alloy bar, characterized in that the average grain size is less than 1㎛, magnesium alloy bar.
The method of claim 11,
The tensile yield strength and compressive yield strength of the magnesium alloy bar is 330 MPa or more, the maximum elongation is 18% or more, and the product of the yield strength and the maximum elongation is in the range of 6000 to 7000 MPa%, magnesium alloy bar.

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