WO2010041791A1

WO2010041791A1 - Magnesium alloy panel having high formability and method of manufacturing the same

Info

Publication number: WO2010041791A1
Application number: PCT/KR2008/007169
Authority: WO
Inventors: Kyung Hun Kim; Jun Ho Bae; Jung Gu Lee; Nack Joon Kim; Dae Hoon Kang; Geun Tae Bae; Dong Wook Kim
Original assignee: Postech Academy-Industry Foundation; Samchully Pahrm. Co., Ltd.
Priority date: 2008-10-06
Filing date: 2008-12-04
Publication date: 2010-04-15
Also published as: KR20100038809A

Abstract

Provided is a magnesium alloy panel having high strength, high elongation, and low anisotropy, and thus having much higher press formability than the typical magnesium alloy panel. This is realized by adding yttrium (Y) in Mg-Zn based alloy, which is a precipitation-hardenable alloy, and performing twin roll strip casting and subsequent thermomechaincal treatment thereon to refine the microstructure and to control the behavior of the dispersed phase.

Description

MAGNESIUM ALLOY PANEL HAVING HIGH FORMABILITY AND METHOD OF MANUFACTURING THE SAME

Technical Field

[1] The present disclosure relates to a magnesium alloy panel with high formability, and a method for manufacturing the same, and more particularly, to a method for manufacturing a magnesium alloy panel, including controlling composition of a magnesium alloy, twin roll casting the magnesium alloy to produce the magnesium alloy panel, and performing subsequent thermomechanical treatment on the magnesium alloy panel to improve formability, particularly press formability, as well as productivity, and the magnesium alloy panel manufactured by this manufacturing method. Background Art

[2] A magnesium alloy is a structural alloy with low specific gravity, high specific strength and high rigidity. Recently, there is an increasing demand for the magnesium alloy as a material for portable electronic devices such as mobile phones and notebook computers requiring light weight, and as a material for automobiles with improved fuel efficiency.

[3] However, studies on the magnesium alloy have been focused on casting parts, and particularly, on the improvement of high temperature properties for automobile engines or gear parts. On the other hand, studies have not been sufficiently conducted on the wrought magnesium alloy that can be applied to more various fields such as a panel.

[4] Recently, demands on magnesium alloy panel products continue to increase in order to apply the magnesium alloy to more various fields. However, this requires development of a magnesium alloy panel having high ductility and high formability, so that the magnesium alloy panel can be formed into parts of various shapes.

[5] One of the most well-known method for manufacturing the magnesium alloy panel is to prepare a casting through a semi-continuous casting process such as a typical die casting type casting process (hereinafter, referred to as "DC casting"), and then to produce a panel of a target thickness from the casting through hot extrusion followed by rolling.

[6] However, the casting prepared by the above-described semi-continuous casting process has too large a grain size to be directly used to form a product through pressing, forging or the like. Accordingly, a further process such as hot extrusion is required to refine the grain. In addition, the hot extrusion of the magnesium alloy, which is an active metal, should be performed slowly to ensure sufficient cooling of the product. Otherwise, deformation heat generated during the hot extrusion may cause surface blackening or burning. Accordingly, this imposes a limit on the extrusion speed.

[7] Therefore, the typical method of manufacturing the magnesium alloy panel has the drawbacks of very low productivity and high production cost. In addition, because the casting has a large grain size and a great thickness after the hot extrusion only, it is necessary to perform a rolling process with a great reduction ratio for several times. This may develop a texture, and thus increase anisotropy in the panel. Consequently, the magnesium alloy panel manufactured in accordance with the typical method has poor formability, particularly poor press workability, and thus it is difficult to form the magnesium alloy panel into a complex shape.

[8]

Disclosure of Invention Technical Problem

[9] The present invention addresses the above-identified, and other problems associated with the typical magnesium alloy panel. The present disclosure provides a magnesium alloy panel having high strength, high elongation, and low anisotropy, and thus having much higher formability than the typical magnesium alloy panel, which is realized by adding yttrium (Y) in Mg-Zn based alloy, which is a precipitation-hardenable alloy, and performing twin roll casting and subsequent thermomechanical treatment thereon to refine the microstructure and to control the behavior of dispersed phases.

[10] The present disclosure also provides a method of manufacturing a magnesium alloy panel having high strength and high formability, at low cost. Technical Solution

[11] In accordance with an exemplary embodiment, a magnesium alloy panel having high formability includes 1 wt.% to 10 wt.% zinc (Zn), 0.2 wt.% to 5 wt.% yttrium (Y), and a balance of magnesium (Mg), with unavoidable impurities, and has an average grain diameter of 20 μm or smaller.

[12] The above composition of the magnesium alloy is selected for the following reasons.

[13] Zinc (Zn) has a solubility of 6.2 wt.% in a magnesium matrix at 340 ⁰C. When Zn is added in the magnesium matrix above 1.0 wt.%, acicular precipitation phases form after a heat treatment, showing age-hardening behavior. Below 1.0 wt.% Zn, it is difficult to expect the age -hardening effect. Above 10.0 wt.% Zn, equilibrium phases are precipitated along the grain boundary, deteriorating the mechanical properties of the magnesium alloy. Therefore, it is preferable that the content of zinc in the magnesium alloy ranges from 1 wt.% to 10 wt.%. It is more preferable that the content of zinc in the magnesium alloy ranges from 5.0 wt.% to 7.0 wt.% to maximize the effect.

[14] Yttrium (Y) forms Mg-Zn-Y based dispersed phase in Mg-Zn-Y ternary alloy system. Particularly, when the weight ratio of Y/Zn is 0.7 or lower, Mg-Zn-Y qua- sicrystal phase may form, which is thermally stable and can increase strength and elongation of the alloy. Twin roll casting process with high cooling rate can generate more uniformly dispersed second phase and increase the volume fraction of the qua- sicrystal phase considerably. Below 0.2 % Y, only a low volume fraction of qua- sicrystal phase forms, making it difficult to increase strength and formability of the alloy. On the other hand, above 7 wt.% Y, the quasicrystal phase does not form. Therefore, it is preferable that the content of yttrium in the magnesium alloy ragnes from 0.2 wt.% to 7 wt.%. It is more preferable that the content of yttrium in the magnesium alloy ranges from 1.0 wt.% to 5.0 wt.% to maximize the effect.

[15] Meanwhile, when the weight ratio of Y/Zn is 0.2 or lower, the effect of formability improvement due to the formation of the quasicrystal phase is not sufficient. On the other hand, when the weight ratio of Y/Zn is higher than 0.7, it is difficult to form the quasicrystal phase. Therefore, the weight ratio of Y/Zn ranges preferably from 0.2 to 0.7, and more preferably from 0.22 to 0.7.

[16] It is preferable that the average grain diameter (size) is 20 μm or smaller. This is because when the average grain diameter is greater than 20 μm, the strength of the material decreases. In consideration of formability and strength, the average grain diameter is preferably 10 μm or smaller, and more preferably 5 μm or smaller.

[17] Also, the magnesium alloy panel in accordance with the exemplary embodiment has a limit dome height (LDH) of 5 mm or greater. The 'limit dome height' as used herein is measured as follows. A disc-shaped specimen of 50 mm diameter and 0.7 mm thickness is prepared, and an outer periphery of the disc-shaped specimen is fixed with a force of 5 kN. Thereafter, deformation is imposed on the specimen at a rate of 0.1 mm/sec using a spherical punch of 20 mm diameter. Then, the deformed height of the disc-shaped specimen up to the fracture is measured, which is referred to as the 'limit dome height.'

[18] In view of strength, dispersed particles in the microstructure of the magnesium alloy preferably have an average diameter of 5 μm or smaller, and more preferably, 2 μm or smaller. Further, it is preferable that the volume fraction of the dispersed particles is 5 % or lower. This is because when the volume fraction of the dispersed particles is higher than 5 %, strength increases, but the dispersed particles may impede the movement of the dislocations to reduce the ductility of the magnesium alloy panel.

[19] In accordance with another exemplary embodiment, a method of manufacturing a magnesium alloy panel with high formability includes the steps of: (a) preparing a molten magnesium alloy including 1 wt.% to 10 wt.% zinc (Zn), 0.2 wt.% to 7 wt.% yttrium (Y), and a balance of magnesium (Mg), with unavoidable impurities; (b) preparing a panel by injecting the molten magnesium alloy kept at a temperature ranging from 650 ⁰C to 750 ⁰C between two rotating cooling rolls to cool the molten magnesium alloy at a cooling rate of 10² K/s to 10³ K/s; (c) performing solution treatment (T4 heat treatment) on the panel for 1 hour to 24 hours at a temperature ranging from 350 ⁰C to 450 ⁰C; (d) rolling the solution-treated panel, after preheating it to a temperature ranging from 350 ⁰C to 450 ⁰C, with a reduction ratio of 40 % or lower per pass to achieve a final reduction ratio of 80 % or lower; and (e) performing heat treatment on the rolled panel for 5 minutes to 24 hours at a temperature ranging from 250 ⁰C to 450 ⁰C.

[20] In the preparing of the molten magnesium alloy, it is preferable that the content of yttrium is controlled by adding a magnesium- yttrium master alloy. This is because yttrium has a high melting point, and thus it is difficult to add a desired amount of yttrium during the casting. The magnesium- yttrium master alloy preferably contains 1.5 wt. % to 4 wt. % Y.

[21] It is preferable that the molten magnesium alloy is kept at a temperature ranging from

650 ⁰C to 750 ⁰C. This is because, below 650 ⁰C, the magnesium alloy melt is solidified before contacting the rolls and thus cannot pass between the rolls, and, above 750 ⁰C, a liquid phase still remains after passing between the rolls, causing solidification defects or cracks in the surface.

[22] It is preferable that the cooling rate of the magnesium alloy melt ranges from 10² K/s to 10³ K/s. This is because, below 10² K/s, the inventive method is not quite different in microstructure from the typical mold casting, and, above 10³ K/s, the process is difficult to perform commercially except a rapid solidification technique providing a very thin ribbon shape.

[23] In accordance with the embodiment, a twin roll strip casting process is used. The twin roll strip casting process performs casting and hot rolling simultaneously. Accordingly, the twin roll strip casting is much more economical than the typical ingot casting, and can realize a cooling rate of 10² K/s to 10³ K/s, which is much more rapid than that of the typical ingot casting. Such a rapid cooling rate makes it possible to further refine cast structure, reduce segregation, and finely disperse intermetallic compounds that could decrease tensile properties if the cooling rate were slow. In addition, because the casting step can provide a relatively thin panel in comparison to other casting methods, it is possible to reduce the required reduction ratio and the required number of rolling passes in the rolling step. As a result, it is possible to minimize the texture, thereby reducing the anisotropy that decrease the press formability.

[24] For the rapid cooling rate, it is preferable that the gap between the two cooling rolls is 10 mm or smaller, and the rotation speed of the rolls is 10 m/min or slower.

[25] During the twin roll strip casting, segregation of alloying elements may occur in the alloy panel, to decrease the uniformity of the properties of the products. Accordingly, solution treatment is required after the strip casting. The solution treatment is performed for 1 hour to 24 hours at 350 ⁰C to 450 ⁰C. The process time and the process temperature is established based on the diffusivity of zinc, which is a major alloying element, the secondary dendrite arm spacing (SDAS), the fact whether or not incipient melting occurs, which can be detected by differential thermal analysis (DTA) / differential scanning calorimetry (DSC), and the oxidation level.

[26] The method may further include, after the step (c), preheating the solution-treated panel to a temperature ranging from 350 ⁰C to 450 ⁰C, and then rolling it with a reduction ratio of 40 % or lower per pass to achieve a final reduction ratio of 80 % or lower. The preheating temperature range (processing temperature range) is selected because it is possible to obtain a satisfactory panel in the above temperature range. It is preferable that the final reduction ratio is 80 % or lower because the increase in the final reduction ratio develops a texture to deteriorate formability of the magnesium alloy.

Advantageous Effects

[27] Contrary to the typical method for manufacturing the commercial magnesium alloy panel, the method in accordance with the exemplary embodiments includes performing twin roll strip casting, designing an alloy composition suitable for the strip casting, and performing subsequent treatment (heat treatment or thermomechanical treatment). As such, it is possible to refine grains, and to control size, shape and volume fraction of a dispersed phase. Consequently, it is possible to obtain a magnesium alloy panel with improved elongation and improved formability, so that it can be widely applied to automobiles, electronic devices, and the like.

[28] In accordance with the exemplary embodiments, it is possible to produce a magnesium alloy panel containing less alloying elements, but more second phases, in comparison to the typical Mg-Zn-Y alloy. As such, it is possible, by distributing finely dispersed phases more uniformly, to obtain a magnesium alloy panel with high strength and high formability at lower cost than the typical alloy.

[29] In accordance with the exemplary embodiments, it is possible to reduce the number of manufacturing steps in comparison to the typical method. As such, it is possible to produce a magnesium alloy panel at lower cost than the typical commercial magnesium alloy panel. In addition, it is possible to reduce the required final reduction ratio considerably. As such, it is possible, by minimizing texture formation, to improve press formability. Brief Description of Drawings

[30] FIG. 1 is a schematic view of a strip caster for manufacturing a magnesium alloy panel in accordance with an exemplary embodiment.

[31] FIG. 2 is a schematic view illustrating a method for evaluating limit dome height of a magnesium alloy panel in accordance with an exemplary embodiment.

[32] FIG. 3 is an optical micrograph illustrating microstructure of a magnesium alloy panel manufactured in accordance with an exemplary embodiment.

[33] FIG. 4 is a scanning electron micrograph illustrating dispersed phases in a magnesium alloy panel manufactured in accordance with an exemplary embodiment. Best Mode for Carrying out the Invention

[34] The terms of a singular form as used in exemplary embodiments may include plural forms unless referred to the contrary. The meaning of "include," "comprise," "including," or "comprising" specifies a property, a region, a fixed number, a step, a process, an element and/or a component but does not exclude other properties, regions, fixed numbers, steps, processes, elements, components and/or groups.

[35] Though not defined specifically, all terms including technical and scientific terms as used herein have the meanings identical to what those skilled in the art generally understand by the terms. In addition, general terms defined in a dictionary are to be further interpreted as having the meanings corresponding to what are described in the present disclosure and related technical literatures, and are not to be interpreted as having ideal or formal meanings unless referred to the contrary.

[36] Hereinafter, specific exemplary embodiments will be described in detail with reference to the accompanying drawings. However, the present invention should not be construed as limited to the embodiments set forth herein. Accordingly, it will be understood by those skilled in the art that various modifications may be made thereto without departing from the spirit and scope of the invention.

[37] Manufacture of Magnesium Alloy Panel

[38] Molten magnesium alloy having the composition as listed in Table 1 was prepared by melting pure Mg (99.9%), Mg-2.5 wt.% Y master alloy and pure Zn (99.995 %) under a mixed gas of CO₂ and SF₆ in an induction melting furnace.

[39] The molten magnesium alloy kept at a temperature of 700 ⁰C was transported to a tundish 10 as shown in FIG. 1, and then injected between two water-cooled cooling rolls 20 of a twin roll strip caster. The tundish was kept at the same temperature as the molten magnesium alloy in the melting furnace.

[40] The gap between the two cooling rolls was kept at 2 mm and the rotation speed of the cooling rolls was kept at 4 m/min to 4.5 m/min, so that the molten magnesium alloy may be cast with a cooling rate of 100 K/s to 400 K/s. Resultantly, we obtained a magnesium alloy panel of 5 m length, 70 mm width, and 2 mm thickness.

[41] Then, the cast panel was heat treated as follows.

[42] The cast panel was subjected to T4 heat treatment (or solution treatment) for 2 hours at 330 ⁰C. Then, the solution-treated panel was preheated to 300 ⁰C and rolled between hot rolls heated to 150 ⁰C.

[43] The hot rolling was performed with a reduction ratio of 35 % per pass to obtain a panel having a final thickness of 1 mm to 0.7 mm. Thereafter, the panel was subjected to heat treatment as given in Table 1.

[44] Microstructure of Magnesium Alloy Panel

[45] Microstructure of the panel manufactured as described above was investigated. FIG.

3A is a micrograph illustrating microstructure of the magnesium alloy panel manufactured as described above. In FIG. 3A, it can be seen that grains of approximately 5 μm in size and finely dispersed phases are uniformly distributed. FIG. 3B is a scanning electron micrograph illustrating dispersed phases in the magnesium alloy panel. In FIG. 3B, it can be seen that finely dispersed phases of 1 μm or smaller in size are uniformly distributed. The volume fraction of the dispersed phase was 5 %. As a result, it can be concluded that, by using the method for manufacturing the magnesium alloy panel in accordance with the exemplary embodiment, it is possible to obtain fine grains and uniformly distributed dispersed phases by a simpler process.

[46] Evaluation of Formability of Magnesium Alloy Panel

[47] To evaluate tensile properties of a magnesium alloy panel manufactured in accordance with the exemplary embodiment, tensile specimens of 12.6 mm gauge length, 5 mm gauge width, and 1 mm thickness were prepared. The tensile specimens were subjected to a tensile test with a strain rate of 6.4 x 10⁴ s ¹ using a well known tensile tester.

[48] To evaluate press formability of the magnesium alloy panel, limit dome height

(LDH) test was performed. FIG. 2 is a schematic view illustrating a method for evaluating limit dome height (LDH) of the magnesium alloy panel in accordance with the exemplary embodiment. As shown in FIG. 2, a disc-shaped specimen of 50 mm diameter and 0.7 mm thickness was prepared, and inserted between an upper die and a lower die to be fixed with a force of 5 kN. As a lubricant, press oil was used. Then, the specimen was deformed at a rate of 0.1 mm/sec using a spherical punch of 20 mm diameter, and the deformed height of the disc- shaped specimen at the instant of fracture was measured.

[49] Deformed height of a commercially available magnesium alloy panel (AZ31 H24) was also measured to compare it with that of the magnesium alloy panel in accordance with the exemplary embodiment.

[50] Tensile properties and forming properties measured as described above are given in Table 1.

[51] Table 1 [Table 1] [Table ]

[52] As shown in Table 1, specimen Nos. 1 and 3 (experimental examples) were slightly lower in tensile strength, however, much higher in elongation closely related to formability, than AZ31 and H24 (comparative examples).

[53] In addition, in view of formability, specimen Nos. 2 to 6 (experimental examples) had LDH 0.25 to 1.5 times greater than AZ3 and H24 (comparative examples). Particularly, a magnesium alloy panel having LDH of 5.0 mm or higher can be used to form parts which are difficult to form by press forming the typical magnesium alloy panel. Accordingly, it is possible to more widely apply the magnesium alloy panel to various fields in comparison with the related art.

[54] In accordance with the exemplary embodiments, it is possible, by controlling the mi- crostructure and the reduction ratio, to manufacture a panel having high elongation and low anisotropy, and thus having high press formability. The microstructure may be controlled by controlling the alloy composition, particularly the weight ratio of Y/Zn, by refining the grains, and by controlling the dispersed phase.

Claims

[I] A magnesium alloy panel with high formability, comprising 1 wt.% to 10 wt.% Zn, 0.2 wt.% to 5 wt.% Y, and a balance of magnesium (Mg), with unavoidable impurities, and having an average grain size of 20 μm or lower.

[2] The magnesium alloy panel with high formability of claim 1, wherein the content of Zn ranges from 5.0 wt.% to 7.0 wt., and the content of Y ranges from 1.0 wt.% to 5 wt.%. [3] The magnesium alloy panel with high formability of claim 1 or 2, wherein the average grain size is 10 μm or smaller. [4] The magnesium alloy panel with high formability of claim 1 or 2, wherein the average grain size is 5 μm or smaller. [5] The magnesium alloy panel with high formability of claim 1 or 2, wherein a weight ratio of Y/Zn is from 0.2 to 0.7. [6] The magnesium alloy panel with high formability of claim 1 or 2, wherein weight ratio of Y/Zn is from 0.22 to 0. 7. [7] The magnesium alloy panel with high formability of claim 1 or 2, wherein an average size of a dispersed phase in microstructure of the magnesium alloy panel is 5 μm or smaller. [8] The magnesium alloy panel with high formability of claim 7, wherein the average size of the dispersed phase is 2 μm or smaller. [9] The magnesium alloy panel with high formability of claim 1 or 2, wherein a volume fraction of the dispersed phase is 5 % or lower. [10] The magnesium alloy panel with high formability of claim 1 or 2, wherein an elongation ratio of the magnesium alloy panel is 20 % or higher.

[I I] The magnesium alloy panel with high formability of claim 1 or 2, wherein a limit dome height (LDH) of the magnesium alloy panel is 5 mm or greater.

[12] A method for manufacturing a magnesium alloy panel with high formability, the method comprising the steps of:

(a) preparing a molten magnesium alloy including 1 wt.% to 10 wt.% zinc (Zn), 0.2 wt.% to 7 wt.% yttrium (Y), and a balance of magnesium (Mg), with unavoidable impurities;

(b) producing a panel by injecting the molten magnesium alloy kept at a temperature ranging from 650 ⁰C to 750 ⁰C between two rotating cooling rolls to cool the molten magnesium alloy at a cooling rate of 10² K/s to 10³ K/s;

(c) performing solution treatment (T4 heat treatment) on the panel for 1 hour to 24 hours at a temperature ranging from 350 ⁰C to 450 ⁰C;

(d) rolling the solution-treated panel, after preheating it to a temperature ranging from 350 ⁰C to 450 ⁰C, with a reduction ratio of 40 % or lower per pass to achieve a final reduction ratio of 80 % or lower; and

(e) performing heat treatment on the rolled panel for 5 minutes to 24 hours at a temperature ranging from 250 ⁰C to 450 ⁰C. [13] The method of claim 12, wherein the content of Zn ranges from 5.0 wt.% to 7.0 wt, and the content of Y ranges from 1.0 wt.% to 5 wt.%. [14] The method of claim 12 or 13, wherein a gap between the two cooling rolls is kept at 10 mm or smaller, and a rotation speed of the two cooling rolls is kept at

10 m/min or slower, during the step (b).