KR101639590B1

KR101639590B1 - Grain refiner for magnesium alloy and grain refinement method for magnesium alloy

Info

Publication number: KR101639590B1
Application number: KR1020150134266A
Authority: KR
Inventors: 배준호; 김영민; 유봉선; 임창동; 문병기; 김하식
Original assignee: 한국기계연구원
Priority date: 2015-01-15
Filing date: 2015-09-23
Publication date: 2016-07-15

Abstract

The present invention relates to a cast grain refinement method of magnesium alloy, using aluminum composite material containing SiC. The fine grain in the magnesium alloy comprises a secondary phase of polygonal form comprising Al-Mn-Si-C. According to the present invention, the magnesium alloy prepared by the composite material is characterized in a reduced process cost and the superior refinement efficiency, compared to the conventional methods.

Description

TECHNICAL FIELD [0001] The present invention relates to a method of refining a crystal grain of a magnesium alloy and a method of refining a grain of a magnesium alloy,

The present invention relates to a method for refining the grain of magnesium which can be carried out at a low cost and with a high efficiency at the time of casting a magnesium alloy containing aluminum, such as commercial AZ80 and AZ91.

The grain refiners added during the casting process of magnesium alloys exhibit various advantages such as improvement of mechanical properties, reduction of casting defects, segregation inhibition, improvement of formability, and improvement of surface characteristics as in general metals.

There are various methods for refining the casting crystal grains of the magnesium alloy. The zirconium addition method, which is known as the most effective method, is widely used as a method of refining magnesium grains by adding zirconium in an amount of 0.5 wt% to 1.0 wt%. However, in a magnesium alloy containing aluminum and manganese alloying elements, There is a problem in that it is difficult to use because the refinement effect disappears due to the reaction, and the commercial magnesium alloy contains many of these elements, which is difficult to put into practical use.

In the magnesium alloy containing aluminum, the carbon addition method shows an excellent effect. The method is divided into a method of directly injecting a fine carbon powder into a molten metal and a method of injecting an inorganic compound containing carbon. The carbon addition method is known to be the most important finishing method in magnesium (Mg) -Aluminum (Al) based alloys because it does not need to heat the molten metal to a high temperature as compared with the superheat treatment method and is excellent in economy.

However, the method of directly injecting the carbon powder in the above-described carbon addition method is a method of directly injecting carbon-containing fine carbon powder, carbon black, etc. into the molten metal as a method in which the carbon powder is not uniformly dispersed during charging, There is a disadvantage that the refining efficiency is lowered.

As a result, the method of adding carbon into a molten metal as a general inorganic compound has been widely used. However, techniques for refining casting grains using various inorganic compounds have been developed. However, , High cost, complicated process or work hazard.

On the other hand, as disclosed in Patent Documents 1 and 2, there is a grain refinement method using SiC powder among carbon / inorganic compound microfabrication agents, but the refinement efficiency is greatly influenced by the particle size and shape of SiC. Particularly, This is not so simple and greatly reduces the refinement efficiency. The present invention greatly improves the refinement efficiency of the SiC inorganic compound and more stable and simple processing method.

Korean Patent Publication No. 10-1214939 Korean Patent Publication No. 10-2014-0131456

Disclosure of the Invention An object of the present invention is to provide a method for refining magnesium casting crystal grains containing aluminum, which is simpler in process than the conventional SiC addition method and has an excellent refining efficiency.

In order to solve the above problems, the present invention provides a method for grain refinement of a magnesium alloy, which comprises adding a grain refinement agent composed of an aluminum alloy composite material containing SiC to a magnesium alloy containing Al as an alloy element .

Since the microfabrication agent according to the present invention is made of a composite material with an aluminum alloy, the microfabrication agent according to the present invention can be easily put into the magnesium molten metal as compared with the conventional microcrystallizing agent in the form of SiC particles, The crystal grain refinement effect can be improved.

In addition, the SiC particle addition method using the conventional method requires an additional injection device for use in a large-capacity casting facility as in an industrial field, which causes an increase in the process cost and a large variation in the refinement efficiency. On the other hand, The addition method is excellent in reproducibility and can be added by conventional alloying process even in large capacity and it can be widely applied to various processes such as continuous casting, DC (direct chill) casting, die casting, low pressure die casting, .

In addition, the cast magnesium alloy which has been subjected to the fineness treatment according to the present invention can have a grain size of 50 to 100 mu m which is significantly finer than the grain size before the fineness treatment, and can exhibit improved mechanical properties and processability, Transportation equipment, electronic products, and sports and leisure goods.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a microstructure photograph showing the addition of 1 wt% of an aluminum composite material to which 23.5 wt% SiC is added to an AZ80 alloy. Fig.
Fig. 2 is a microstructure photograph showing the addition of 0.7 wt% of an aluminum composite material to which 23.5 wt% SiC is added to an AZ80 alloy.
3 is a microstructure photograph showing the addition of 0.5 wt% of an aluminum composite material to which 23.5 wt% SiC is added to an AZ80 alloy.
Fig. 4 is a microstructure photograph of a commercially available AZ80 alloy cast without adding a microfine agent.
5 is a photograph of microstructure obtained by adding SiC powder as a microfilizer to commercial AZ91 alloy.
6 shows the result of analyzing the composition of the secondary phase formed in the microstructure of the magnesium alloy cast when 0.7 wt% of the aluminum composite material to which 23.5 wt% SiC was added to the AZ80 alloy was added.

Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, the following embodiments of the present invention may be modified into various other forms, and the scope of the present invention is not limited to the embodiments described below. Embodiments of the present invention are provided to more fully describe the present invention to those skilled in the art.

The method for refining the grain size of a magnesium alloy according to the present invention can be applied to the refinement of a magnesium alloy containing Al as an alloy element and is characterized in that the grain refinement agent made of an aluminum composite material containing SiC is added in an amount of 0.5 to 1 weight %.

When the Al content is less than 5% by weight, the magnesium alloy contains Al. In this case, as the casting alloy, the rigidity of the product due to solidification of the alloy is lowered and the casting composition is significantly lowered. The mechanical properties are deteriorated due to the coarse Mg ₁₇ Al ₁₂ process phase formed in the grain boundary, so that the content of Al is preferably 5 to 10 wt%.

If the content of Zn is less than 0.1 wt%, the solid solution strengthening effect is greatly deteriorated. If the Zn content is more than 2 wt%, a low melting point intermetallic compound is formed in the segregation zone in the alloy , Which may cause defects in the heat treatment, and therefore may preferably contain 0.1 to 2% by weight.

The magnesium alloy may be any casting alloy containing Al without any particular limitation, but a commercially available magnesium alloy such as AM60 alloy, AZ80 alloy and AZ91 alloy may be preferably used.

When the average size of the SiC particles contained in the composite material is less than 5 탆, it is difficult to uniformly disperse in the composite material, resulting in an increase in cost due to an increase in material and manufacturing process cost, It is preferable to maintain the thickness at 5 to 15 占 퐉.

If the content of SiC contained in the composite material exceeds 25 wt%, it is difficult to uniformly disperse the composite material and segregation may occur due to intergranular bonding. The lower limit is not limited, but it is preferable to add the content in the content close to the upper limit value in order to effectively maximize the refinement efficiency.

When aluminum alloy is used, the aluminum alloy component should be considered in consideration of the relationship between the added alloy component and the magnesium alloy component, and Al 359 Commercial alloys such as alloys may be used.
The amount of the aluminum composite material added is preferably such that the content of Si in the cast magnesium alloy does not exceed 0.2 wt%.

Further, when a grain refinement agent made of an aluminum alloy composite material containing SiC is added, a secondary phase including Al, Mn, Si and C may be formed in the magnesium alloy microstructure, When the secondary phase is formed, grain refinement can be improved as compared with the prior art.

If Mn is less than 0.1% by weight, the corrosion resistance due to Fe impurities of the alloy is greatly deteriorated. If the content of Mn is less than 0.1% by weight, It is preferable that the magnesium alloy is controlled to be in the range of 0.1 to 0.5 wt.% In the cast magnesium alloy since the ductility is weakened due to the coarse Al-Mn intermetallic compound.

It is preferable that the secondary phase is formed such that the composition ratio of Al / Mn is 1.3 to 1.6 and the average particle size is less than 10 탆. If the secondary phase is out of the above composition range and particle size, .

In addition, the shape of the secondary phase is preferably polygonal to achieve grain refinement of the magnesium cast structure.

When the magnesium alloy is cast, it is preferable that the temperature of the molten metal is maintained at 600 to 780 ° C. If the molten metal temperature is lower than 600 ° C, the molten state can not be maintained. If the molten metal temperature exceeds 780 ° C, And the reactivity of the magnesium alloy increases, so that magnesium may be oxidized or impurities may be contained.

The magnesium molten metal to which the aluminum-manganese refinement alloy is added can be finally made of magnesium alloy through various casting methods such as die casting, casting by low-pressure casting, continuous casting, thin plate casting, precision casting and die casting.

Hereinafter, the present invention will be described in more detail based on preferred embodiments of the present invention and comparative examples thereof.

[Example 1]

1% by weight of an aluminum composite material to which 23.5% by weight SiC was added to a molten commercial AZ80 alloy melted at 750 DEG C was maintained for about 5 to 10 minutes and cast into a billet having a diameter of 80 mm.

In order to confirm the effect of micronization of the added micronization agent, the upper portion of the lower part of the billet was cut to have a surface finish, and then microstructure was observed to evaluate grain refinement.

[Example 2]

0.7% by weight of an aluminum composite material to which 23.5% by weight SiC had been added to a molten commercial AZ80 alloy melted at 750 DEG C was maintained for about 5 to 10 minutes and cast into a billet having a diameter of 80 mm.

[Example 3]

0.5% by weight of an aluminum composite material to which 23.5% by weight SiC had been added to a molten commercial AZ80 alloy melted at 750 DEG C was maintained for about 5 to 10 minutes and cast into a billet having a diameter of 80 mm.

[Example 4]

1% by weight of an aluminum composite material to which 23.5% by weight SiC had been added was cast into a billet having a diameter of 80 mm after being maintained for about 5 to 10 minutes in a molten commercial AZ91 alloy melted at 750 ° C.

[Example 5]

0.7% by weight of an aluminum composite material to which 23.5% by weight SiC had been added to a molten commercial AZ91 alloy melted at 750 DEG C was maintained for about 5 to 10 minutes and cast into a billet having a diameter of 80 mm.

[Example 6]

0.5% by weight of an aluminum composite material to which 23.5% by weight of SiC had been added was cast in a form of a billet having a diameter of 80 mm and maintained for about 5 to 10 minutes in a molten commercial AZ91 alloy melted at 750 ° C.

[Comparative Example 1]

A commercially available AZ80 alloy molten at 730 DEG C was cast into a billet having a diameter of 80 mm without adding a finishing agent.

In order to check the state of the casting structure, the upper portion of the lower portion of the billet was cut to have a surface, and then the microstructure was observed.

[Comparative Example 2]

A commercially available AZ91 alloy molten at 730 DEG C was cast in the form of a billet having a diameter of 80 mm without adding a finishing agent.

[Comparative Example 3]

1% by weight of SiC powder having an average particle size of 10 탆 was added as a refining agent to a molten commercial AZ80 alloy melted at 730 캜 and held for about 5 to 10 minutes, followed by casting into a billet having a diameter of 80 mm.

Microstructure

Table 1 below summarizes the results of measuring the average crystal grain sizes of the magnesium alloys produced according to the above-described Examples and Comparative Examples.

object
alloy additive Parent alloy addition amount Si content in magnesium alloy Average grain
Size (㎛) Example 1 AZ80 Al-23.5 wt% SiC 1 wt% 0.16 wt% 92 Example 2 0.7 wt% 0.11 wt% 92 Example 3 0.5 wt% 0.08 wt% 182 Comparative Example 1 - - - 300 Example 4 AZ91 Al-23.5 wt% SiC 1 wt% 0.16 wt% 90 Example 5 0.7 wt% 0.11 wt% 89 Example 6 0.5 wt% 0.08 wt% 95 Comparative Example 2 - - - 400 to 500 Comparative Example 3 AZ91 SiC powder 1 wt% 0.7 wt% 130 ~ 250

Figs. 1 to 3 are photographs of microstructure of AZ80 alloy subjected to refinement treatment according to Examples 1 to 3, Fig. 4 is photographs of microstructure of AZ80 alloy without refinement treatment, Fig. 5 is a microstructure photograph Of the AZ80 alloy.

As shown in Fig. 4, the grain size of the cast structure of the AZ80 alloy not subjected to the micronization treatment was observed at a level of approximately 300 mu m.

1 to 3, the microstructure of the AZ80 alloy produced according to Examples 1 to 3 of the present invention shows a remarkably small grain size as compared with Comparative Example 1 in which the micronization treatment was not performed, When the amount of the fine refining agent made of the aluminum composite material to which SiC is added is more than 0.5 weight%, the grain refinement efficiency is more excellent than the method of directly adding SiC.

However, if the aluminum composite material containing 23.5 wt% SiC is added in an amount exceeding 1.25 wt%, it is expected that the refinement efficiency will be further improved. However, since the intermetallic compound produced by increasing the fraction of the magnesium- The mechanical properties of the cast magnesium alloy may be rather deteriorated. Therefore, it is preferable to add the amount of the fineness agent made of the aluminum composite material to which 23.5 wt% SiC is added in an amount of 1.25 wt% or less.

FIG. 5 shows that 1% by weight of SiC powder of 10 탆 was added according to Comparative Example 2. Although the addition amount of SiC itself was larger than that of the aluminum composite material containing 23.5% by weight SiC according to the present invention, Is about 120 to 250 mu m, which is lower than that of the first embodiment of the present invention, and the deviation of the micronization efficiency according to the experiment is remarkably different. On the other hand, when the SiC powder is added directly, the dispersion in the molten metal is not easy and the work risk is increased. Therefore, the refinement treatment according to the comparative example 2 not only lowers the refinement effect but also is a disadvantage in terms of productivity can do.

6 shows the result of analysis of the secondary phase observed in the microstructure of the AZ80 alloy produced according to Example 3 of the present invention.

As shown in Fig. 6, the secondary phase observed in the casting structure of Example 2 is composed of Al-Mn-Si-C, polygonal having a size within 5 to 10 탆, and the secondary phase is not reported in the magnesium casting structure. It is presumed that the generation of the secondary phase influences the finer effect than the conventional microfabrication agent and the SiC addition method.

The aluminum composite material to which 23.5 wt% SiC is added as the microfabrication agent may be slightly changed in grain refinement efficiency depending on the kind of elements and the content of SiC in the Al alloy. However, in the refined magnesium alloy, Al-Mn-Si-C If the second phase can be formed, the range is not significantly affected. However, it is considered that the method of micronization through the alloy of the composition specified in the present invention is preferable from the viewpoint of the price, the process method and the refining efficiency.

Claims

A process for producing a magnesium alloy comprising the steps of dissolving a magnesium alloy containing Al and Mn in a solution of a grain refining agent comprising an aluminum composite material containing SiC,
Mn, Si, and C in the microstructure of the cast magnesium alloy by adding and casting the composite material in an amount of 0.5 wt% to 1.25 wt% based on the aluminum composite material to which SiC is added in an amount of 23.5 wt% phase of the magnesium alloy is formed.

The method according to claim 1,
Wherein the aluminum composite material has a SiC content of more than 0 wt% and less than 25 wt%.

The method according to claim 1,
Wherein the magnesium alloy contains 5 to 10 wt% of Al and 0.1 to 2 wt% of Zn.

The method according to claim 1,
Wherein the secondary phase has a weight ratio of Al / Mn of 1.3 to 1.6.

The method according to claim 1,
Wherein the shape of the secondary phase is a polygonal shape.

The method according to claim 1,
Wherein the amount of the aluminum composite material added is such that the content of Si in the cast magnesium alloy is not more than 0.2 wt%.