WO2021215666A1 - High-quality magnesium alloy processed material and manufacturing method therefor - Google Patents

Abstract

The present invention relates to a magnesium alloy extrudate comprising 5.0-8.0 wt% of bismuth (Bi), 2.0-7.0 wt% of tin (Sn), the balance of magnesium (Mg) and inevitable impurities, and the magnesium alloy extrudate according to the present invention further comprises tin (Sn) in an Mg-Bi binary alloy so as to have a microstructure composed of uniform and fine grains, which are obtained by the acceleration of dynamic recrystallization during extrusion, and fine secondary phase (Mg₃Bi₂ or Mg₂Sn) particles precipitated during extrusion, and thus exhibits, even without containing rare earth metals, strength and elongation greatly superior to that of a conventional magnesium extrudate.

Description

High-physical magnesium alloy processing material and manufacturing method thereof

The present invention is a national research and development project implemented by the Ministry of Science and ICT (task unique number: 1711099226, (organization) detailed project number: 2019R1A2C1085272, project name: personal basic research (middle-level researcher support project), research project name: low cost for next-generation eco-friendly vehicles Development of full-cycle element technology for manufacturing high-performance lightweight magnesium, specialized research and management institution: National Research Foundation of Korea, organized institution: Kyungpook National University Industry-University Cooperation Foundation, research period: 2019. 09. 01 ~ 2024. 02. 29) supported by Kyungpook National University The present invention relates to a high-physical magnesium alloy processed material that does not contain expensive rare earth metals and overcomes low mechanical properties, which is a disadvantage of conventional commercially available magnesium extruded materials, and a method for manufacturing the same.

As international environmental regulations become increasingly stringent, lightweight vehicles with low carbon dioxide emissions and excellent fuel efficiency have become a major focus of the automotive industry.

Accordingly, magnesium based on magnesium (Mg: 1.738 g/cm 3 , Fe: 7.874 g/cm 3 , Ti: 4.506 g/cm 3 and Al: 2.70 g/cm 3 ), which has a significantly lower density than other metals for structural materials. Alloys are attracting considerable interest in the industry as a material for reducing the weight of automobiles.

On the other hand, existing research on magnesium alloys has focused on magnesium alloys for casting to be applied to automobile engines and gear parts based on the excellent castability of magnesium. As it has casting defects such as holes and shrinkage cavities, studies on magnesium alloy workpieces obtained through plastic working processes such as extrusion, rolling, and forging to obtain better mechanical properties are being actively conducted.

In particular, magnesium alloy extruded material exhibits superior mechanical properties compared to cast material and can be manufactured in various shapes, so it is suitable for use in automobile body and chassis components such as bumper beams, radiator supports, engine cradles and subframes.

However, the low strength and high price of the magnesium alloy extruded material compared to the aluminum alloy extruded material is still a great obstacle to the wide application of the magnesium alloy extruded material in the automobile industry and the like.

The technical problem to be solved by the present invention is significantly improved mechanical properties compared to the conventional magnesium extruded material, which did not contain expensive rare earth elements as alloy elements and had difficulty in expanding industrial applications due to conventional low mechanical properties (strength, elongation) It is to provide a novel magnesium alloy having a and a method for manufacturing the same.

In order to achieve the above technical object, the present invention provides 5.0 to 8.0 wt% of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; And it provides a magnesium alloy extruded material containing unavoidable impurities.

In addition, the magnesium alloy extruded material is more than 6.0% by weight and 8.0% by weight or less of bismuth (Bi); greater than 4.0% by weight 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and unavoidable impurities.

In addition, the magnesium alloy extruded material is characterized in that it does not contain a rare-earth metal as an alloying element.

In addition, the magnesium alloy extruded material is a secondary phase (second phase) Mg ₃ Bi ₂ or Mg ₂ Sn It is characterized in that it contains precipitated particles.

In addition, the magnesium alloy extruded material is 7.0% by weight of bismuth (Bi); 6.0% by weight of tin (Sn); Magnesium (Mg) balance; and unavoidable impurities, wherein the ultimate tensile strength (UTS) × elongation value is 3689 MPa·%.

And, in another aspect of the present invention, as a method for manufacturing the magnesium alloy extruded material, (a) 5.0 to 8.0 wt% of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and casting a molten magnesium alloy containing unavoidable impurities to prepare a magnesium alloy billet; (b) homogenizing heat treatment and cooling the magnesium alloy billet prepared in step (a); And (c) extruding the homogenized heat-treated magnesium alloy billet in step (b); provides a method of manufacturing a magnesium alloy extruded material comprising a.

In this case, 5.0 to 8.0 wt% of bismuth (Bi) in step (a); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and maintaining a molten magnesium alloy containing unavoidable impurities at 670 to 770 ^o C for 20 minutes and then casting to prepare a magnesium alloy billet.

In addition, in step (b), the magnesium alloy billet is ^{subjected to a homogenization heat treatment at 350 to 550 o} C for 0.5 to 72 hours, followed by water cooling.

In addition, it is characterized in that the extruded after preheating the billet homogenized heat treatment in step (c) at 200 ~ 450 ^{o C.}

Magnesium alloy according to the invention extruded material is Mg-Bi 2 alloy by further comprising a tin (Sn) in the alloy is the dynamic recrystallization to promote the extrusion obtained even while the secondary precipitated in the microstructure and an extrusion consisting of a fine crystal grain phase (Mg ₃ Bi ₂ or Mg ₂ Sn) particles, and exhibits significantly improved strength and elongation compared to conventional magnesium extruded materials without including rare earth metals.

1 is an inverse pole figure map and grain size distribution measurement results for each of the magnesium alloy extruded materials prepared in Comparative Examples 2 and 3;

2 is a scanning electron microscope (SEM) photograph of each of the magnesium alloy extruded materials prepared in Comparative Examples 2 and 3;

3 is a graph showing ultimate tensile strength (UTS) × elongation values of each of the magnesium alloy extruded materials prepared in Comparative Example 2 and Examples 1 to 3;

4 is a scanning electron microscope (SEM) photograph showing a fracture surface after a tensile test of the magnesium alloy extruded tensile specimen prepared in Comparative Examples 2 and 3, respectively.

In describing the present invention, if it is determined that a detailed description of a related well-known function or configuration may unnecessarily obscure the gist of the present invention, the detailed description thereof will be omitted.

Since the embodiment according to the concept of the present invention may have various changes and may have various forms, specific embodiments will be illustrated in the drawings and described in detail in the present specification or application. However, this is not intended to limit the embodiment according to the concept of the present invention with respect to a specific disclosed form, and should be understood to include all changes, equivalents or substitutes included in the spirit and scope of the present invention.

The terms used herein are used only to describe specific embodiments, and are not intended to limit the present invention. The singular expression includes the plural expression unless the context clearly dictates otherwise. In this specification, terms such as "comprises" or "have" are intended to designate that the described features, numbers, steps, operations, components, parts, or combinations thereof exist, and include one or more other features or numbers. , it is to be understood that it does not preclude the possibility of the presence or addition of steps, operations, components, parts, or combinations thereof.

Hereinafter, the present invention will be described in detail.

The magnesium alloy extruded material according to the present invention includes 5.0 to 8.0 wt% of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; And it is manufactured by extruding a magnesium alloy containing unavoidable impurities.

The reason for limiting the alloy composition as described above in the magnesium alloy extruded material according to the present invention is as follows.

Bismuth (Bi)

Bismuth (Bi) may be added to the magnesium alloy to have favorable conditions for high-temperature extrusion and precipitation strengthening. The maximum solid solution limit of Bi that can be dissolved in magnesium is as high as 9.0 wt% at 551 °C, and _{the melting point of the secondary phase Mg 3} Bi ₂ formed by adding Bi is 823 °C. The structure and mechanical properties of the final extruded material can be improved.

When the content of bismuth contained in the magnesium alloy extruded material according to the present invention is less than 5.0 wt%, the precipitation strengthening effect cannot be effectively exhibited after extrusion due to the lack of dissolved Bi solute atoms, so the strength of the final extruded material is low and exceeds 8.0 wt% In this case, the coarse Mg ₃ Bi ₂ dispersed phase remaining after the homogenization heat treatment may remain in the final extruded material and may cause premature failure during the tensile test, which is not preferable.

Therefore, the magnesium alloy extruded material according to the present invention preferably contains Bi in the range of 5.0 to 8.0 wt%, and more preferably, Bi in excess of 6.0 wt% in order to further improve the mechanical properties of the extruded material. can

Tin (Sn)

Tin (Sn) exhibits an aging strengthening behavior by forming a _{fine Mg 2} Sn precipitated phase through heat treatment when the maximum solid solution amount that can be dissolved in magnesium is 14.5 wt % at 561 ° C., and when 1.0 wt % or more is added.

When Sn is added to the magnesium alloy in an amount of less than 2.0 wt %, the amount of Sn is not sufficient to promote dynamic recrystallization, so the size of the grains after extrusion is not uniform. , when added in excess of 7.0 wt%, _{the fraction of the coarse Mg 2} Sn phase formed during casting is excessive, so that it cannot be completely dissolved during homogenization heat treatment, and exists in the final material after extrusion, which may lead to deterioration of mechanical properties of the alloy.

Accordingly, the magnesium alloy extruded material according to the present invention preferably contains Sn in the range of 2.0 to 7.0 wt%, and more preferably, in order to further improve the mechanical properties of the extruded material, it may contain Sn exceeding 4.0 wt%. can

Other unavoidable impurities

The magnesium extruded material according to the present invention may contain impurities that are unavoidably mixed in the raw material of the alloy or in the manufacturing process, and among these impurities, Fe, Cu, and Ni are components that play a role in worsening the corrosion resistance of the magnesium alloy. Accordingly, it is preferable to maintain the Fe content of 0.004 wt% or less, the Cu content of 0.005 wt% or less, and the Ni content of 0.001 wt% or less.

Furthermore, the magnesium extruded material according to the present invention may further include one or more alloying elements as necessary in addition to the aforementioned Bi and Sn, and such additional alloying elements are typically aluminum (Al), zinc (Zn) as shown below. , manganese (Mn), calcium (Ca), and the like, but is not necessarily limited thereto.

Aluminum (Al)

Aluminum (Al) is an element added to the Mg-Bi alloy to improve the physical properties of the magnesium alloy.

When aluminum is added in an amount of less than 1.0% by weight, the amount of aluminum is not sufficient to promote dynamic recrystallization, so the size of the grains after extrusion is not uniform, and it is difficult to expect an effect of increasing strength and ductility due to coarse initial grains. On the other hand, if it exceeds 9.0 wt %, the coarse Mg ₁₇ Al ₁₂ phase formed during the solidification process during casting is not completely dissolved in the homogenization heat treatment and is present in the final material after extrusion, and this coarse phase is the risk of premature fracture during the tensile test. It can be a cause and is not recommended.

Accordingly, the magnesium alloy extruded material according to the present invention preferably contains Al in an amount of 1.0 to 9.0 wt%.

Zinc (Zn)

Like aluminum, zinc (Zn) plays a role in increasing the strength of magnesium alloys through solid solution strengthening and precipitation strengthening.

When zinc is added in an amount of less than 0.1% by weight, an effect of increasing strength cannot be expected, and when added in an amount exceeding 3.5% by weight, microgalvanic corrosion may be promoted, and corrosion resistance of the extruded material may be reduced.

Therefore, when the magnesium alloy extruded material according to the present invention contains Zn, it is preferably included in the range of 0.1 to 3.5 wt%.

Manganese (Mn)

Manganese (Mn) not only strengthens the solid solution but also forms various dispersed particles by combining with aluminum (Al), thereby contributing to an increase in the strength of the alloy and improving the corrosion resistance of the alloy.

When manganese (Mn) is added to the magnesium alloy in an amount of less than 0.05 wt%, it is difficult to expect this effect, and when it is added in excess of 1.5 wt%, coarse manganese (Mn) particles are formed in the molten metal at a temperature of 750 ° C or less. This results in deterioration of the mechanical properties of the alloy.

Therefore, when the magnesium alloy extruded material according to the present invention contains Mn, it is preferably included in the range of 0.0 5 to 1.5 wt %.

Calcium (Ca)

Calcium (Ca) forms a Mg-Al-Ca intermetallic compound in the magnesium alloy containing aluminum to improve strength and heat resistance, and forms a thin and dense CaO oxide layer on the surface of the molten metal to inhibit oxidation of the magnesium alloy. improve the fire resistance of

When calcium is added in an amount of less than 0.05% by weight, the effect of improving the ignition resistance is not great. When it exceeds 2.0% by weight, the castability of the molten metal is deteriorated, hot cracking occurs, and the die sticking with the mold is reduced. As it increases, there is a problem such as a large decrease in elongation, and in the case of an extrusion process, there is a problem in that the extrusion load is greatly increased and surface cracks occur.

Therefore, when the magnesium alloy extruded material according to the present invention includes Ca, it is preferably included in the range of 0.05 to 2.0 wt%.

In order to prepare the extruded material of high physical properties magnesium alloy as described above, in the present invention, (a) 5.0 to 8.0 wt% of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and casting a molten magnesium alloy containing unavoidable impurities to prepare a magnesium alloy billet; (b) homogenizing heat treatment and cooling the magnesium alloy billet prepared in step (a); And (c) extruding the homogenized heat-treated magnesium alloy billet in step (b); provides a method of manufacturing a magnesium alloy extruded material comprising a.

In step (a), 5.0 to 8.0% by weight of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; And after preparing a molten metal containing unavoidable impurities, it may be injected into a preheated metal mold to cast a billet.

In the casting process in this step, it is preferable to cast ^{the magnesium alloy molten metal after maintaining it at 670 to 770 o C for 20 minutes.} When the magnesium alloy molten metal is cast at less than 670° C., the fluidity of the molten magnesium alloy is low and casting is difficult. In addition, when the molten magnesium alloy is cast at a temperature exceeding 770° C., the molten magnesium alloy is rapidly oxidized and impurities may be mixed during casting, thereby reducing the purity of the magnesium alloy billet manufactured therefrom.

In addition, the molten magnesium alloy may be prepared by melting the raw material of the magnesium alloy, and the method for preparing the molten magnesium alloy is not limited thereto as long as it is a method commonly used in the art. For example, gravity casting, continuous casting , sand casting or pressure casting can be used.

Next, the step (b) is a step of cooling after homogenization heat treatment of the manufactured magnesium alloy billet, and the homogenization treatment improves the heterogeneous structure due to segregation of alloy elements generated in the process of casting the magnesium alloy molten metal, and , it is possible to form equiaxed a-Mg particles to improve the high-temperature workability and mechanical properties of the magnesium alloy.

The range of the homogenization treatment temperature can be appropriately selected by those skilled in the art according to the type of constituent elements constituting the magnesium alloy billet, and the homogenization treatment of the magnesium alloy billet is preferably performed at 350 to 550° C. for 0.5 to 72 hours. When the homogenization treatment temperature is less than 350 ℃, the temperature is low, so that the homogenization of the segregation of alloy elements and solid solution into the matrix of the secondary phase formed during solidification is not sufficiently achieved. There is a problem in that the dissolution of phosphorus may occur and the physical properties may be deteriorated.

In addition, when the homogenization treatment time is less than 0.5 hours, the diffusion of the alloy elements of the magnesium alloy billet does not occur sufficiently, so the effect of the homogenization treatment may not appear. It is not economical because the rise is not large.

In addition, in order to make the microstructure of the magnesium alloy billet into a super-solid solution state through the homogenization treatment, it is preferable to rapidly cool the magnesium alloy billet through water cooling or the like.

Finally, in step (c), the homogenization heat treatment is a step of extruding the magnesium alloy billet and processing it into an extruded material.

For example, a magnesium alloy extruded material can be manufactured by directly extruding or indirectly extruding a magnesium alloy. It is preferable to extrude after preheating to a temperature for 0.5 to 2 hours.

Hereinafter, the present invention will be described in more detail by way of examples.

Embodiments according to the present specification may be modified in various other forms, and the scope of the present specification is not to be construed as being limited to the embodiments described below. The embodiments of the present specification are provided to more completely explain the present specification to those of ordinary skill in the art.

<Examples 1 to 5>

In order to prepare a magnesium alloy extruded material according to the present invention, a magnesium alloy cast billet having the composition shown in Table 1 below was prepared as follows. The manufacturing process of the billet for extrusion is to dissolve a pure Mg ingot with a purity of 99.99% in a carbon crucible in a mixed gas atmosphere _{of CO 2} and SF _{6, add bismuth (Bi) and tin (Sn), and then stabilize at 720 ° C.} was maintained for 20 minutes, stirred sufficiently to make the molten metal uniform, and then tapped into a steel mold preheated to 210 °C. The chemical composition of the cast alloy was measured with an inductively coupled plasma spectrometer (PerkinElmer, Optima 7300DV).

Next, the cast billet was subjected to homogenization heat treatment in an inert gas atmosphere using an electric furnace at 490° C. for 24 hours, and then cooled with water to prevent static precipitation of Bi and Sn dissolved in the magnesium matrix during cooling.

After processing the homogenized heat-treated billet to a diameter of 68 mm and a length of 120 mm, it was preheated at 400° C. for 1 hour, and then directly extruded at a ram speed of 1 mm/s and an extrusion ratio of 25 to obtain a diameter of 13.6 mm. Eggplant was made of a rod-shaped extruded bar (bar).

<Table 1>

<Comparative Examples 1 to 3>

A magnesium alloy extruded material was prepared in the same manner as in Examples 1 to 5, except for having the alloy composition shown in Table 2 below.

<Table 2>

<Experimental example>

1 is an electron backscatter diffraction (EBSD) microstructure photograph of each of the magnesium alloy extruded materials prepared in Comparative Examples 2 and 3 and the results of measuring the area fraction according to the grain size.

Referring to FIG. 1 , the average grain size (59.6 μm) of the extruded material prepared according to Example 3 further containing tin (Sn) as an alloying element is the average grain size (91.3 μm) of the magnesium alloy extruded material prepared according to Comparative Example 2 μm), and the finer grain size increases the grain boundary reinforcing effect during tensile deformation to improve the strength of the extruded material.

FIG. 2 shows SEM photographs of the magnesium alloy (B5) extruded material prepared in Comparative Example 2 and the magnesium alloy (BT56) extruded material prepared in Example 3, respectively.

2, it can be seen that the extruded material prepared in Example 3 has more fine precipitates than the extruded material prepared in Comparative Example 2, and accordingly, the extruded material prepared in Example 3 has a smaller grain boundary due to the smaller grain size. Due to the reinforcing effect and the precipitation strengthening effect due to more precipitates, it has a higher strength than the extruded material prepared in Comparative Example 2 as will be described later.

In order to analyze the mechanical properties of the magnesium alloy extruded material, a tensile test specimen having a gauge diameter of 6 mm and a gauge length of 25 mm obtained by processing the magnesium alloy extruded material was subjected to an Instron 8516 tester of 1 x 10 ^-3 s ^{-1 at room temperature.} A tensile test was performed at a rate of strain, and the results are shown in Tables 1, 2, 3 and 4 above.

According to Table 1, Table 2, and Figure 3, the tensile strength × elongation value (≥ 1405 MPa %) of the Mg-Bi-Sn ternary extruded material of Examples 1 to 5 in which tin (Sn) is further added as an alloying element is It was found to be more than twice the tensile strength × elongation value (≤ 638 MPa·%) of the Mg-Bi binary extruded material of Comparative Examples 1 to 3 not containing tin (Sn).

On the other hand, Figure 4 is a SEM photograph showing the fracture surface after the tensile test of the magnesium alloy (B5) extruded material prepared in Comparative Example 2 and the magnesium alloy (BT56) extruded tensile specimen prepared in Example 3, according to which in Comparative Example 2 The extruded material according to the extruded material has a coarse grain size, so twin crystals are easily formed during tensile deformation, and cleavage fracture occurs, thereby having a 2.2% lower elongation, whereas the extruded material according to Example 3 has a relatively small grain size. During tensile deformation, twin crystal formation is suppressed, resulting in ductile fracture, which results in a relatively high elongation of 7.7%.

As mentioned above, although embodiments of the present invention have been described with reference to the accompanying drawings, those of ordinary skill in the art to which the present invention pertains can implement the present invention in other specific forms without changing its technical spirit or essential features. You will understand that there is Therefore, it should be understood that the embodiments described above are illustrative in all respects and not restrictive.

The magnesium alloy extruded material according to the present invention does not contain rare earth metal and has significantly improved strength and elongation compared to the existing magnesium extruded material, so not only transportation equipment such as automobiles, aircraft, ships, etc. It can be usefully used as a material for

Claims (9)

1. 5.0 to 8.0% by weight of bismuth (Bi);

   2.0 to 7.0% by weight of tin (Sn);

   Magnesium (Mg) balance; And magnesium alloy extruded material containing unavoidable impurities.
2. According to claim 1,

   greater than 6.0% by weight and not more than 8.0% by weight of bismuth (Bi);

   greater than 4.0% by weight 7.0% by weight of tin (Sn);

   Magnesium (Mg) balance; And magnesium alloy extruded material containing unavoidable impurities.
3. According to claim 1,

   A magnesium alloy extruded material, characterized in that it does not contain a rare-earth metal as an alloying element.
4. According to claim 1,

   Magnesium alloy extruded material, characterized in that it _{comprises Mg 3} Bi ₂ or Mg ₂ Sn precipitated particles as a secondary phase (second phase).
5. According to claim 1,

   7.0% by weight of bismuth (Bi); 6.0% by weight of tin (Sn); Magnesium (Mg) balance; and unavoidable impurities;

   Magnesium alloy extruded material, characterized in that the ultimate tensile strength (ultimate tensile strength, UTS) × elongation (elongation) value is 3689 MPa · %.
6. (a) 5.0 to 8.0% by weight of bismuth (Bi); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and casting a molten magnesium alloy containing unavoidable impurities to prepare a magnesium alloy billet;

   (b) homogenizing heat treatment and cooling the magnesium alloy billet prepared in step (a); and

   (c) extruding the homogenized heat-treated magnesium alloy billet in step (b);
7. 7. The method of claim 6,

   5.0 to 8.0% by weight of bismuth (Bi) in step (a); 2.0 to 7.0% by weight of tin (Sn); Magnesium (Mg) balance; and maintaining a molten magnesium alloy containing unavoidable impurities at 670 to 770 ^o C for 20 minutes and then casting the magnesium alloy billet.
8. 7. The method of claim 6,

   Method for producing a magnesium alloy extruded material, characterized in that the magnesium alloy billet in step (b) is ^{subjected to a homogenization heat treatment at 350 to 550 o C for 0.5 to 72 hours, followed by water cooling.}
9. 7. The method of claim 6,

   Method for producing a magnesium alloy extruded material, characterized in that the extruded after preheating the billet homogenized heat treatment in step (c) at 200 ~ 450 ^{o C.}

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