Patent Application
20150315690
1. Field of the Invention
The present disclosure relates to a method of preparing a magnesium alloy extrudate and a magnesium alloy extrudate prepared thereby, and more particularly, to a method of preparing a magnesium alloy extrudate having more improved strength and elongation than those obtained from a typical extrusion method by applying a predetermined amount of compressive deformation to a homogenized billet.
2. Description of the Related Art
Research into weight reduction of products and developing new materials has been actively conducted worldwide to increase the efficiency of transportation equipment and cope with environmental pollution caused by exhaust gas and various environmental regulations. Light metals, such as aluminum and magnesium, and alloys thereof have emerged as main target materials, and the use thereof as a new material for structural components of transportation industries requiring high specific strength has been significantly increased.
Magnesium alloys have been on the spotlight due to their excellent properties, such as machinability, electromagnetic shielding properties, and vibration absorption capacity, as well as high specific strength, while being a lightweight metallic material having the lowest density among all structural materials. Also, since magnesium alloys may be used for special purposes by controlling an alloy composition according to each application, research into utilizing the magnesium alloys in various fields, such as automotive parts, aircraft parts, and portable electronic devices, has been actively conducted.
There are generally two types of magnesium alloy product. One is cast products manufactured by casting and the other one is wrought products manufactured by processing a cast billet or slab by rolling, extrusion, or forging. With respect to the casting method, magnesium alloy parts may be manufactured by various casting processes such as sand casting, gravity casting, precision casting, die casting, and a semi-solid forming technique, and most of the magnesium alloy parts are currently manufactured by die casting. However, in a case where a magnesium alloy is manufactured by casting, a post-treatment process is required because the surface thereof is very rough after the casting, and it is not suitable for manufacturing a high quality alloy product requiring high strength due to casting defects such as inclusions, interdendritic shrinkage voids, porosity, and cavity. Therefore, a technique of manufacturing a wrought material, such as a rolled material, an extrudate, and a forged material, having superior mechanical properties than cast materials, is essential in order to expand the use and application of magnesium alloys.
Although wrought magnesium alloys have better strength and ductility than cast materials, the wrought magnesium alloys have lower mechanical properties than commercial wrought aluminum alloys. Thus, many efforts to improve the strength and ductility of the wrought magnesium alloys through various methods, such as the addition of alloying elements, the application of powder metallurgy, and the control of process conditions, are underway.
For example, as a method of improving the strength of a typical magnesium alloy, Korean Patent Application Laid-Open Publication No. 10-2008-0085662 (publication date: 2008 Sep. 24) discloses a manufacturing method, in which the composition of a magnesium alloy is controlled and die casting and plastic deformation are combined, as a method of improving mechanical strength of a magnesium alloy. Also, Korean Patent Application Laid-Open Publication No. 10-2012-0095184 (publication date: 2012 Aug. 28) discloses a method of controlling the composition of a magnesium alloy as a method of improving mechanical strength and ductility of a magnesium alloy extrudate. However, as in the present invention, a method of simultaneously improving the strength and ductility of a magnesium alloy extrudate by applying a predetermined amount of compressive deformation to a magnesium alloy billet homogenized before extrusion has not been disclosed yet.
Accordingly, during research into a method of preparing a magnesium alloy extrudate having excellent strength and ductility, the present inventors developed a method of preparing a magnesium alloy extrudate by extrusion after a predetermined amount of compressive deformation is applied to a homogenized magnesium alloy billet. The present inventors recognized that a magnesium alloy extrudate having improved mechanical strength as well as improved ductility may be prepared by adding a simple compressive deformation process to a typical extrusion method as in the above method, thereby leading to the completion of the present invention.
It is an object of the present invention to provide a method of preparing a magnesium alloy extrudate.
It is another object of the present invention to provide a magnesium alloy extrudate prepared according to the above method.
To achieve the above objects, the present invention provides a method of preparing a magnesium alloy extrudate including: melting a magnesium alloy raw material (step 1); casting the magnesium alloy raw material melted in step 1 to prepare a magnesium alloy billet (step 2); homogenizing the magnesium alloy billet prepared in step 2 (step 3); applying 3% to 20% of compressive deformation to the homogenized magnesium alloy billet of step 3 (step 4); and extruding the compressive deformed magnesium alloy billet of step 4 (step 5).
The present invention also provides a magnesium alloy extrudate prepared by the above method.
Furthermore, the present invention provides a part for transportation equipment manufactured using the above magnesium alloy extrudate.
A method of preparing a magnesium alloy extrudate according to the present invention may provide a magnesium alloy extrudate having simultaneously improved mechanical strength and ductility using a simple method by adding a simple compressive deformation process to a typical extrusion method, wherein a magnesium alloy extrudate is prepared by applying a predetermined amount of compressive deformation to a homogenized magnesium alloy billet and then extruding the magnesium alloy billet.
Hereinafter, the present invention will be described in detail.
As illustrated in
melting a magnesium alloy raw material (step 1);
casting the magnesium alloy raw material melted in step 1 to prepare a magnesium alloy billet (step 2);
homogenizing the magnesium alloy billet prepared in step 2 (step 3);
applying 3% to 20% of compressive deformation to the homogenized magnesium alloy billet of step 3 (step 4); and extruding the compressive deformed magnesium alloy billet of step 4 (step 5).
Hereinafter, the method of preparing a magnesium alloy extrudate according to the present invention will be described in detail for each step.
In the method of preparing a magnesium alloy extrudate according to the present invention, step 1 is melting a magnesium alloy raw material.
Any magnesium alloy raw material may be used without limitation as long as it is a commercial magnesium alloy. Since the present invention uses a technique using a concept in which a twin is easily generated in magnesium or magnesium alloys during room temperature deformation due to insufficient slip systems, the present invention may be applied to all magnesium alloys regardless of their compositions. That is, magnesium pure metal or magnesium alloys may be used as the magnesium alloy raw material, and any composition of the magnesium alloy raw material may be used without limitation.
Also, in addition to the magnesium alloys, the present invention may be equally applied to titanium (Ti), zinc (Zn), and cobalt (Co) alloys having a hexagonal close-packed (HCP) structure in which a twin is easily generated by deformation.
Next, in the method of preparing a magnesium alloy extrudate according to the present invention, step 2 is casting the magnesium alloy raw material melted in step 1 to prepare a magnesium alloy billet.
In step 2, the magnesium alloy raw material melted in step 1 (hereinafter, referred to as “magnesium alloy melt”) may be cast in a temperature range of 650° C. to 750° C. In the case that the magnesium alloy melt is cast at a temperature less than 650° C., since the fluidity of the magnesium alloy melt may decrease, casting may be difficult. In the case in which the magnesium alloy melt is cast at a temperature greater than 750° C., since the magnesium alloy melt may be rapidly oxidized, an impurity may be mixed during the casting. Thus, the purity and quality of the magnesium alloy billet thus prepared may be decreased.
In this case, a method of casting the magnesium alloy melt is not particularly limited as long as it is typically used in the art, and for example, gravity casting, continuous casting, sand casting, and pressure casting may be used.
Next, in the method of preparing a magnesium alloy extrudate according to the present invention, step 3 is homogenizing the magnesium alloy billet prepared in step 2.
The homogenization may make more uniform structure by dissolving the segregation of alloying elements and/or second phases which occurs in a process of casting the magnesium alloy melt and may improve high-temperature processability and mechanical properties of a magnesium alloy billet. The homogenization of the magnesium alloy billet may be performed by performing a heat treatment process in a temperature range of 300° C. to 550° C. for 0.5 hours to 96 hours, and then performing a cooling process. However, the temperature range of the homogenization treatment may be appropriately selected by the person skilled in the art according to the type of elements constituting the magnesium alloy billet.
For example, with respect to a magnesium (Mg)-tin (Sn)-based alloy, the homogenization treatment may be performed in a temperature range of 400° C. to 550° C. In the case that the homogenization treatment is performed at a temperature less than 400° C., since an amount of tin dissolved in a magnesium matrix is decreased, the strengthening effect of the alloy caused by dynamic precipitation which occurs during plastic deformation, such as extrusion, rolling, and forging, at warm or high temperatures may be not significant. Also, since a coarse Mg2Sn phase formed during the casting process may not be sufficiently removed, ductility of the magnesium alloy may be reduced.
In the case in which the magnesium alloy billet is homogenized at a temperature greater than 550° C., since the homogenization treatment temperature is higher than a solidus temperature of the magnesium alloy, local melting of the magnesium alloy billet may occur. Thus, physical properties may be reduced.
Furthermore, in the case that the homogenization treatment of the magnesium alloy billet is performed in the above temperature range for less than 0.5 hours, the diffusion of the alloying elements insufficiently occurs so that the effect of the homogenization treatment may not be obtained. In the case in which the homogenization treatment of the magnesium alloy billet is performed for greater than 96 hours, an increase in the effect of the homogenization treatment over time may not be large, and thus, it may not be economical.
Next, in the method of preparing a magnesium alloy extrudate according to the present invention, step 4 is applying 3% to 20% of compressive deformation to the homogenized magnesium alloy billet of step 3.
The compressive deformation may form twins in the homogenized magnesium alloy. Twinning in the magnesium alloy having a hexagonal close-packed structure may act as an important deformation mechanism at room temperature. In this case, since twins formed by the compressive deformation may act as nucleation sites for recrystallization during the subsequent extrusion, a more uniform and finer structure of the extrudate may be obtained due to an increase in a fraction of recrystallized region. Thus, the strength of the magnesium alloy extrudate may be improved. Also, since a fraction of large unrecrystallized grains, in which cracks are easily formed during tensile deformation to reduce ductility, is significantly decreased when the extrusion is performed after the compressive deformation is applied, elongation as well as strength may be simultaneously improved.
20% to 30% of compressive deformation may be applied to the magnesium alloy billet homogenized in step 3. In the case that less than 3% of compressive deformation is applied to the homogenized magnesium alloy billet, a very small number of twins may be formed in the magnesium alloy billet, and thus, the strength and elongation of the magnesium alloy extrudate thus prepared may not be improved. In the case in which greater than 20% of compressive deformation is applied to the homogenized magnesium alloy billet, defects, such as cracks and fracture, may occur in the magnesium alloy billet. The amount of the compressive deformation may be appropriately selected by the person skilled in the art according to the composition of the magnesium alloy. The compressive deformation may be applied in any direction to the magnesium alloy billet, and the direction of the compressive deformation may be appropriately selected by the person skilled in the art according to the shape of the billet and the conditions of the extrusion.
The compressive deformation in step 4 may be applied in a temperature range of room temperature to 250° C. In the case that the compressive deformation is applied at a temperature lower than room temperature, since the material may be hardened, defects, such as cracks and fracture, may occur in the magnesium alloy billet during the application of the compressive deformation. In the case in which the compressive deformation is applied at a temperature greater than 250° C., twins are difficult to be formed due to the activation of non-basal plane slip, and accordingly, the improvement of the strength and elongation of the magnesium alloy extrudate thus prepared may not be expected.
In this case, “room temperature” described as a temperature, at which the compressive deformation may be applied, denotes a normal temperature at which heating is not particularly performed, wherein the room temperature may be defined as a temperature range of about 0° C. to about 50° C., and for example, may be a temperature of about 20±5° C.
The method of preparing a magnesium alloy extrudate according to the present invention does not require new device and equipment except a device for applying compressive deformation. Therefore, the method may be immediately applied to a process of preparing a magnesium alloy extrudate using a typical extrusion method.
Next, in the method of preparing a magnesium alloy extrudate according to the present invention, step 5 is extruding the compressive deformed magnesium alloy billet of step 4.
In the present invention, the extrusion may be performed after preheating in a temperature range of 150° C. to 450° C. in order to smoothly perform the extrusion of the compressive deformed magnesium alloy billet of step 4. In the case that the preheating temperature of the compressive deformed magnesium alloy billet is less than 150° C., excessive extrusion pressure may be required during the extrusion of the compressive deformed magnesium alloy billet. In the case in which the preheating temperature of the compressive deformed magnesium alloy billet is greater than 450° C., grains in the magnesium alloy may grow and become coarse, which reduces the strength of the magnesium alloy extrudate thus prepared. And, surface defects may occur because localized melting due to high extrusion temperature may occur in some alloys according to the composition of the alloy.
In this case, the extrusion may be conducted by direct extrusion, indirect extrusion, and continuous extrusion. However, the extrusion is not limited thereto, and may be appropriately selected according to the use or the aim of the person skilled in the art.
Also, the method of preparing a magnesium alloy extrudate according to the present invention may further include processing the magnesium alloy billet into an appropriate shape suitable for compressive deformation and extrusion before and after step 4.
Furthermore, the method of preparing a magnesium alloy extrudate according to the present invention may further include performing an aging treatment after step 5. However the aging treatment in the present invention is only optional, and a magnesium alloy extrudate having improved strength and ductility may be prepared even in the case in which the aging treatment is not performed.
Since alloying elements other than magnesium that are included in the magnesium alloy may precipitate at grain boundaries or in grains by the aging treatment, the strength of the magnesium alloy extrudate may be further improved by the resulting precipitation hardening effect.
For example, the aging treatment may be performed in a temperature range of 150° C. to 250° C. for 1 hour to 360 hours. In the case that the aging treatment is performed at a temperature less than 150° C., a time for the magnesium alloy to reach the maximum strength may be increased, and thus, it may not be economical. In the case in which the aging treatment is performed at a temperature greater than 250° C., the time for the magnesium alloy to reach the maximum strength may be reduced, but the size of a precipitation phase may be increased due to high temperature. Thus, the strength of the magnesium alloy may be decreased.
Also, the present invention may provide a magnesium alloy extrudate that is prepared by the above preparation method.
In general, physical properties of a magnesium alloy are estimated by the product of tensile strength and total elongation (TS×EL). In general, a metallic material has a tendency that elongation decreases when tensile strength increases, and the tensile strength decreases when the elongation increases. Thus, a magnesium alloy may be estimated in two views of strength and ductility using a TS×EL value of the magnesium alloy. In this case, it may be estimated that a magnesium alloy having a large TS×EL value has excellent tensile properties. Since the above TS×EL value is proportional to the amount of energy which may be absorbed by the metallic material during fracture, it may be estimated that toughness is also excellent.
Also, the present invention may provide a part for transportation equipment that is manufactured using the magnesium alloy extrudate.
A TS×EL value of the magnesium alloy extrudate, which is prepared by applying a predetermined amount of compressive deformation to the homogenized magnesium alloy billet according to the present invention and then extruding the magnesium alloy billet, is increased by about 3% to about 32%. This indicates that only the strength is not simply improved but overall tensile properties are improved. Thus, it is expected that the magnesium alloy extrudate of the present invention may be widely used in various industrial sectors including transportation equipment industry, such as aircraft industry, and electronic component industry.
In particular, since the magnesium alloy extrudate may exhibit excellent properties, such as machinability, electromagnetic shielding properties, and vibration absorption capacity, as well as high specific strength, the magnesium alloy extrudate may be manufactured as parts for aircraft requiring high specific strength and precision processing.
Hereinafter, the present invention will be described in more detail according to examples. However, the following examples are provided for illustrative purposes only, and the scope of the present invention should not be limited thereto in any manner.
Step 1: Melting Magnesium Alloy Raw Material
Pure magnesium (Mg, 99.9 wt %), pure tin (Sn, 99.9 wt %), pure aluminum (Al, 99.9 wt %), pure zinc (Zn, 99.995 wt %), pure zirconia (Zr, 99.99 wt %), and pure copper (Cu, 99.997 wt %) were used, and magnesium alloys having compositions of the following Table 1 were melted in a graphite crucible using a high frequency induction melting furnace. An upper portion of the melted magnesium alloy (magnesium alloy melt) was covered with a mixed gas of SF6 and CO2 to block the contact with air and thus, oxidation was prevented.
Step 2: Preparing Magnesium Alloy Billet
The magnesium alloy melt of step 1 was maintained at 700° C. for 10 minutes, and a magnesium alloy billet having a diameter of 80 mm and a length of 200 mm was then prepared by pouring the melt to a steel mold preheated at 200° C.
Step 3: Homogenizing Magnesium Alloy Billet
The magnesium alloy billet prepared in step 2 was homogenized by heating at a rate of 1° C./min in an inert atmosphere and heat treating in a temperature range of 400° C. to 490° C. for 12 hours to 15 hours. Also, in order to suppress the formation of a coarse precipitation phase which may occur during a cooling process of the billet, the billet was cooled with water at room temperature. Homogenization treatment conditions for each example are presented in Table 2 below.
Step 4: Applying Compressive Deformation
10% of compressive deformation was applied to the magnesium alloy billet homogenized in step 3 at a strain rate of about 0.1/s in a longitudinal direction at room temperature using a 150 ton hydraulic press.
Step 5: Extruding
The magnesium alloy billet compressive deformed in step 4 was machined into a rod shape with a diameter of 51 mm, and a magnesium alloy extrudate was then prepared by extruding the rod-shaped billet into a rod shape with a diameter of 16 mm using an indirect extruder (maximum extrusion pressure: 500 tonf) (extrusion temperature: 200° C., extrusion ratio: 20:1, ram speed: 0.1 mm/s).
A magnesium alloy extrudate was prepared in the same manner as in Example 4 except that 5% of compressive deformation was applied in step 4 of Example 4.
A magnesium alloy extrudate was prepared in the same manner as in Example 4 except that 15% of compressive deformation was applied in step 4 of Example 4.
Magnesium alloy extrudates were prepared in the same manner as in Examples 1 to 6 except that compressive deformation of step 4 in Examples 1 to 6 was not applied.
A magnesium alloy extrudate was prepared in the same manner as in Example 4 except that 2% of compressive deformation was applied in step 4 of Example 4.
Analysis
1. Microstructure Analysis of Magnesium Alloy Billet According to Compressive Deformation
In order to analyze microstructures of the magnesium alloy billets according to compressive deformation in the present invention, microstructure analysis was performed using an optical microscope before and after the compressive deformation, and the results thereof are presented in
Referring to
In order to investigate the effect of the compressive deformation in the magnesium alloy extrudate according to the present invention, the magnesium alloy extrudates of Examples 1 to 6 and Comparative Examples 1 to 6 were analyzed using an optical microscope and electron back scattered diffraction. The results thereof are presented in
Referring to
Thus, it may be considered that the method of preparing a magnesium alloy extrudate by applying a predetermined amount of compressive deformation to the homogenized magnesium alloy billet and extruding the magnesium alloy billet according to the present invention may form twins in the magnesium alloy to simultaneously improve the strength and ductility of the magnesium alloy extrudate.
In order to evaluate mechanical properties of the magnesium alloys according to the present invention, rod-shaped specimens having a gauge length of 25 mm and a gauge diameter of 6 mm were prepared using the magnesium alloy extrudates of Examples 1 to 8 and Comparative Examples 1 to 7, and tensile tests were performed on the rod-shaped specimens at a strain rate of 1×10−3 S−1 using a room-temperature tensile tester (INSTRON 4206). The results thereof are presented in Table 3 below. The results of Example 4, Example 7, Example 8, Comparative Example 4, and Comparative Example 7, which were prepared from the same TAZ811 alloy, were extracted from the following Table 3 to be presented in Table 4 below.
Referring to Table 3, it may be understood that yield strengths, tensile strengths, elongations, and TS×EL values of the magnesium alloy extrudates of Examples 1 to 8 according to the present invention were improved in comparison to those of the magnesium alloy extrudates of Comparative Examples 1 to 7 which were not subjected to the compressive deformation after the homogenization treatment.
In particular, it may be understood that, with respect to Example 8 using a TAZ811 alloy, the yield strength and tensile strength were respectively increased by 46 MPa and 45 MPa in comparison to Comparative Example 4, and with respect to Example 6 using a ZK60-1Cu alloy, the elongation was improved by 22% in comparison to Comparative Example 6. It may also be understood that, with respect to Example 8 using a TAZ811 alloy, the TS×EL value was improved by 39% in comparison to Comparative Example 7.
Referring to Table 4, it may be confirmed that the yield strength, tensile strength, and elongation were improved as the compressive deformation amount increased, and it may be confirmed that, with respect to Example 4, Example 7, and Example 8 having a compressive deformation amount of 5% to 15%, the yield strength, tensile strength, and elongation were significantly improved in comparison to those of Comparative Example 4 which was not subjected to the compressive deformation. In particular, with respect to Example 8, it may be confirmed that 15% of compressive deformation results in a considerable improvement of the yield strength, tensile strength, and elongation by 46 MPa, 45 MPa, and 2.2%, respectively.
Therefore, it may be understood that the method of preparing a magnesium alloy extrudate according to the present invention may improve the strength and ductility of the magnesium alloy extrudate by applying a predetermined amount of compressive deformation to the homogenized magnesium alloy billet and extruding the magnesium alloy billet, and it may also be understood that an extrudate having excellent physical properties may be prepared by increasing the amount of compressive deformation.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2013-0001125 | Jan 2013 | KR | national |
| 10-2013-0083597 | Jul 2013 | KR | national |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/KR2013/009795 | Oct 2013 | US |
| Child | 14712609 | US |
