WO2009096622A1 - Magnesium alloy panel having high strength and manufacturing method thereof - Google Patents

Magnesium alloy panel having high strength and manufacturing method thereof Download PDF

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Publication number
WO2009096622A1
WO2009096622A1 PCT/KR2008/000743 KR2008000743W WO2009096622A1 WO 2009096622 A1 WO2009096622 A1 WO 2009096622A1 KR 2008000743 W KR2008000743 W KR 2008000743W WO 2009096622 A1 WO2009096622 A1 WO 2009096622A1
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Prior art keywords
magnesium alloy
panel
high strength
manufacturing
less
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PCT/KR2008/000743
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French (fr)
Inventor
Geun Tae Bae
Sung Soo Park
Nack Joon Kim
Dae Hoon Kang
Kwang Seon Shin
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Postech Academy-Industry Foundation
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Publication of WO2009096622A1 publication Critical patent/WO2009096622A1/en

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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C23/00—Alloys based on magnesium
    • C22C23/04—Alloys based on magnesium with zinc or cadmium as the next major constituent
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L21/00—Processes or apparatus adapted for the manufacture or treatment of semiconductor or solid state devices or of parts thereof
    • H01L21/67—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere
    • H01L21/683—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping
    • H01L21/687—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping using mechanical means, e.g. chucks, clamps or pinches
    • H01L21/68714—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping using mechanical means, e.g. chucks, clamps or pinches the wafers being placed on a susceptor, stage or support
    • H01L21/68728—Apparatus specially adapted for handling semiconductor or electric solid state devices during manufacture or treatment thereof; Apparatus specially adapted for handling wafers during manufacture or treatment of semiconductor or electric solid state devices or components ; Apparatus not specifically provided for elsewhere for supporting or gripping using mechanical means, e.g. chucks, clamps or pinches the wafers being placed on a susceptor, stage or support characterised by a plurality of separate clamping members, e.g. clamping fingers
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B22—CASTING; POWDER METALLURGY
    • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
    • B22D11/00—Continuous casting of metals, i.e. casting in indefinite lengths
    • B22D11/06—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars
    • B22D11/0622—Continuous casting of metals, i.e. casting in indefinite lengths into moulds with travelling walls, e.g. with rolls, plates, belts, caterpillars formed by two casting wheels
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10—TECHNICAL SUBJECTS COVERED BY FORMER USPC
    • Y10S—TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y10S414/00—Material or article handling
    • Y10S414/135—Associated with semiconductor wafer handling
    • Y10S414/141—Associated with semiconductor wafer handling includes means for gripping wafer

Definitions

  • the present invention relates to a magnesium alloy panel having high strength and a method for manufacturing the same, and particularly, to a method for manufacturing a magnesium alloy panel which can realize high strength as well as high productivity through composition control of the magnesium alloy and microstructure control of the magnesium alloy panel by the use of strip casting and subsequent heat treatment, and a magnesium alloy panel manufactured by the same.
  • Magnesium alloy has the lowest specific gravity among practical structural materials, and has excellent specific strength and rigidity. Accordingly, there are increasing demands for the magnesium alloy as a material for cases for portable electronic devices and automobiles requiring light weight .
  • the most widely used method for manufacturing a magnesium alloy which is now put to practical use is a method for casting the magnesium alloy by using injection molding, such as diecasting and thixomolding.
  • injection molding such as diecasting and thixomolding.
  • subsequent processes for removing the casting defects are necessarily required. This significantly decreases the productivity, increases manufacturing cost, and makes it difficult to enhance the strength of the magnesium alloy through the casting method.
  • magnesium alloy manufactured by the casting process is great in grain size, it is difficult to directly process the magnesium alloy by the use of pressing and the like, or by the use of forging and the like. Accordingly, when manufacturing a panel for pressing and a material for forging by using the semi-continuous casting, such as the DC casting, it is necessary to reheating and hot extruding the material obtained from the casting to refine the grain.
  • magnesium alloy is an active metal, it is necessary to perform the extrusion with a low speed enough to sufficiently cool the extruded material to prevent surface blackening or burning caused by deformation heat generation during the hot extrusion. Accordingly, this imposes a limit on the extrusion speed.
  • the method using the casting as a material for pressing or forging has the problems of significant decrease in the productivity and increase in the product price. Furthermore, a material for hot extrusion is insufficient in grain refinement when being formed to a complex shape, and thus it is difficult to process it to a complex shape.
  • the commercial magnesium alloy panels have a yield strength of about 200 MPa, and a tensile strength of about 300 MPa or lower, which are insufficient to be employed in the applications requiring high strength. They have an elongation of about 10 %, which is insufficient to form various shapes. Furthermore, because of the above described problems of the method for manufacturing magnesium alloy panel, the typical magnesium alloy panels have the problem of high manufacturing cost. [Disclosure] [Technical Problem]
  • An objective of the present invention is to provide a high strength magnesium alloy panel having high strength and high elongation and thus having improved formability, which is realized by adding Mn or Mn and Al to Mg-Zn based alloy, which is a precipitation hardening alloy, and performing strip casting and subsequent heat treatment to refine the microstructure and to control the behavior of dispersoid phases and precipitate phases.
  • Another objective of the present invention is to provide a method for manufacturing a magnesium alloy panel, capable of manufacturing a high strength magnesium alloy panel excellent in strength and elongation, at low cost.
  • an embodiment of the present invention provides a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 ⁇ m or less.
  • a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 ⁇ m or less.
  • a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 ⁇ m or less.
  • a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 ⁇ m or less.
  • a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 ⁇ m or less.
  • the Zn content ranges from 5.0 wt.% to 7.0 wt.%
  • the Mn content ranges from 0.75 wt.% to 2.0 wt.%
  • the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
  • the average grain size of the crystal grains may be 10 ⁇ m or less.
  • an average grain size of dispersoids in a microstructure of the panel is 5 ⁇ m or less.
  • a volume fraction of the dispersoids is 5 % or less.
  • an average grain size of precipitate particles which are precipitated in a microstructure of the panel by an aging treatment is 200 run or less, and an aspect ratio of the precipitate particles is 20 or less, more preferably, 10 or less.
  • the magnesium alloy has a tensile strength of 300 MPa or more and an elongation of 15 % or more.
  • the present invention provides a method for manufacturing a high strength magnesium alloy panel, including: (a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn and the balance magnesium and incidental impurities; (b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool the
  • the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn.
  • the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn and 1 wt.% to 6 wt.% Al.
  • the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn and 0.05 wt.% to 1 wt.% Cu.
  • the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, and 0.05 wt.% to 1 wt.% Cu.
  • the Zn content ranges from 5.0 wt.% to 7.0 wt.%
  • the Mn content ranges from 0.75 wt.% to 2.0 wt.%
  • the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
  • the method may further include, after the step of (c), a step of performing a two step aging treatment (T6 heat treatment) where an aging treatment is performed on the solution treated panel at 70 °C to 150 °C for 24 hours to 96 hours, and then at 150 °C to 250 °C for 24 hours or more.
  • T6 heat treatment a two step aging treatment
  • the method may further include, after the step of (c), a step of rolling the solution treated panel, which is preheated to a temperature ranging from 200 °C to 400 °C , with a reduction ratio of 20 % or less for each pass to a total reduction of 70 % or less, and a step of performing a solution treatment on the rolled panel at 200 °C to 400 °C for 0.5 hour to 10 hours, and the method may further include a step of performing a two step ageing treatment (T6 heat treatment) where an aging treatment is performed on the solution treated panel at 70 °C to 150 ° C (preferably, at 70 °C to 100 °C) for 24 hours to 96 hours, and then at 150 °C to 250 °C (preferably, at 150 ° C to 180 °C) for 24 hours or more.
  • T6 heat treatment an aging treatment is performed on the solution treated panel at 70 °C to 150 ° C (preferably, at 70 °C to 100 °C) for 24 hours to 96 hours,
  • the Mn in the step (a) is added in the alloy melt by adding Mg-Mn alloy in the alloy melt .
  • the distance between the two cooling rolls in the step (b) is kept at 10 mm or less, and the rotation speed of the cooling rolls is kept at 10 m/min or less.
  • twin roll strip casting method performs a casting process and a hot rolling process at the same time in a single process, and thus is very expensive in comparison to the conventional ingot casting method.
  • twin roll strip casting method performs a casting process and a hot rolling process at the same time in a single process, and thus is very expensive in comparison to the conventional ingot casting method.
  • Zinc (Zn) 1 wt.% to 10 wt.%
  • Zinc (Zn) has a solubility of 6.2 wt.% in a magnesium matrix at 340 "C .
  • Zn When Zn is added in the magnesium matrix in an amount 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 precipitate 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 age hardening effect.
  • Manganese (Mn) refines precipitate phases, providing the effect of improving the strength and the elongation, and improving the corrosion resistance.
  • the effects can be obtained when Mn is added in an amount of 0.25 wt.% or more.
  • Mn Above 3.0 wt.% Mn, it is difficult to add Mn by a general melting process, and most of Mn above 3.0 wt.% exist in a form of ⁇ -Mn in the matrix, thus having no effects on the improvement of the strength and the elongation, which is not preferable in view of manufacturing cost. Accordingly, it is preferable that Mn is added in an amount ranging from 0.75 wt.% to 2.0 wt.%.
  • Al forms an Al-Mn based precipitate phase which is a thermally stable dispersoid phase, thus providing a further strengthening effect and an effect of reforming the Mg-Zn related acicular precipitation phases.
  • the solubility of Al in the Mg matrix is approximately 1 wt.%. Accordingly, below 1.0 wt.% Al, it is difficult to expect the above mentioned effect of the Al addition.
  • MgI 7 Al 12 phase may precipitate, which would precipitate coarsely along the grain boundary to decrease the strength and the fracture strength, and thus it is preferable that Al is added in an amount of 6.0 wt.% or less. Furthermore, even if Al is added in an amount of 6.0 wt.% or less, a Mg-Al based precipitate phase can be formed if only the Al content is larger than that of Zn. Accordingly, it is more preferable that the Al content is kept below the Zn content.
  • Copper (Cu) deteriorates the corrosion resistance of a generally used Mg-Al based alloy, and thus has a limitation in its use.
  • Cu finely disperses Cu-Mg-Zn phases in a Mg-Zn-Mn based alloy or a Mg-Zn-Mn-Al based alloy, to suppress segregation caused by a Mg-Zn phase or a Mg-Al-Zn phase, thus making uniform the microstructure of the material. Accordingly, the addition of Cu may improve the elongation of the material.
  • Cu is added in an amount less than 0.05 wt.%, it is difficult to expect the above mentioned effect, and when Cu is added in an amount more than 1 wt.%, the corrosion resistance of the alloy may be decreased. Therefore, it is preferable that Cu is added in an amount less than 1.0 wt.%, and it is more preferable that Cu is added in an amount less than 0.5 wt.% in order to form the effective Cu-Mg-Mn phase to prevent the deterioration of the corrosion resistance.
  • an average grain diameter (size) of the crystal grains is 20 ⁇ m or less. This is because if the average size of the crystal grain is larger than 20 / mi, the strength of the material decreases. It is more preferable that the average size of the crystal grain is 10 (M.
  • an average grain size of the dispersoids is 5 ⁇ m or less.
  • a volume fraction of the dispersoids is 5 % or less. This is because if the volume fraction of the dispersoids is higher than 5 %, although the strength increases, the dispersoids may impede the movement of the dislocations to reduce the ductility of the magnesium alloy panel.
  • the particles precipitated by the aging treatment have an average grain size of 200 nm or less. This is because if the average grain size is larger than 200 nm, the strength may be decreased.
  • an aspect ratio of the precipitate particles is 20 or less. This is because the anisotropy in the crystal grain increases if the aspect ratio is larger than 20. It is more preferable that the aspect ratio of the precipitate particles is 10 or less.
  • the magnesium alloy panel can have a tensile strength of 300 MPa or more and an elongation of 15 % or more through the above mentioned alloy design, the microstructure refinement, and the control of the dispersoid phase. That is, it is possible to obtain a high strength magnesium alloy having an excellent formability, which could not obtained in the conventional commercial magnesium alloy panel.
  • the alloy melt is kept at a temperature ranging from 650 °C to 750 ° C . This is because, below 650 °C , the alloy melt is solidified before contacting the rolls, and thus cannot pass between the rolls. In addition, above 750 °C , a liquid phase still remains even after passing between the rolls, thus generating solidification defects or cracks in the surface.
  • a cooling rate of the alloy melt is kept at 10 K/s to 10
  • the distance between the two cooling rolls is kept at 10 mm or less, and the rotation speed of the rolls is kept at 10 m/min to obtain the above mentioned cooling rate.
  • the temperature and the time for the solution treatment is set as follows based on the diffusivity of zinc, which is a major alloying element, the secondary dendrite arm spacing (SDAS), and the fact whether or not incipient melting occurs, which is determined by differential thermal analysis (DTA) / differential scanning calorimetry (DSC). That is, the solution treatment is set to be performed at 250 ° C to 450 ° C for 0.5 hour to 24 hours, and more preferably, at 300 ° C to 330 °C for 1 hour to 5 hours.
  • DTA differential thermal analysis
  • DSC differential scanning calorimetry
  • a double aging treatment may be selectively performed on the panel manufactured as described above, where a primary aging treatment is performed at 70 °C to 150 °C (more preferably, at 70 °C to 100 ° C) for 24 hours to 96 hours, and then a secondary aging treatment is performed at 150 °C to 250 °C (more preferably, at 150 °C to 180 °C) for 24 hours or more.
  • a primary aging treatment is performed at 70 °C to 150 °C (more preferably, at 70 °C to 100 ° C) for 24 hours to 96 hours
  • a secondary aging treatment is performed at 150 °C to 250 °C (more preferably, at 150 °C to 180 °C) for 24 hours or more.
  • Such a double aging is intended for performing the primary aging at a temperature below the G.P.
  • the primary aging temperature is restricted to a range from 70 ° C to 150 °C , which is somewhat lower than the generally known G.P. zone solvus temperature of ⁇ l' phase.
  • the aging time is set to a range that is sufficient to expect some degree of improvement in the hardness through the formation of the G.P. zone, by measuring the hardness.
  • the secondary aging temperature is set to 150 °C to 250 ° C . This is because, below 150 °C , a great deal of time will be spent in reaching the maximum hardness, causing problems in the process. In addition, above 250 ° C , the maximum hardness decreases although it can be obtained in a short time.
  • the method for manufacturing the magnesium panel in accordance with an embodiment of the present invention may further include the step (thermomechanical treatment step) of rolling the solution treated panel, which is preheated to 200 °C to 400 °C , with a reduction ratio of 20 % or less for each pass to a total reduction of 70 % or less.
  • the above mentioned preheating temperature range is sufficient to obtain a normal panel. Accordingly, the processing of the panel is preferably performed in the above mentioned temperature range, and more preferably, in a temperature range from 250 "C to 330 °C .
  • a solution treatment may be performed again at 200 ° C to 400 °C (more preferably, at 300 °C to 330 °C) for 0.5 hour to 10 hours (more preferably, for 1 hour to 5 hours) after the thermomechanical treatment step.
  • Mg-Mn master alloy In the method for manufacturing a magnesium panel in accordance with an embodiment of the present invention, it is preferable to use a Mg-Mn master alloy to add Mn in the magnesium alloy melt. This is because the solubility of Mn in Mg is low and the diffusivity thereof is also low, and thus it is difficult to add a desired amount of Mn during the casting.
  • the Mg-Mn master alloy preferably contains 1.5 wt.% to 4 wt.% Mn.
  • Fig. 1 is a schematic view of a strip caster used in an embodiment of the present invention.
  • Fig. 2 is a micrograph illustrating a cast microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 3 is a micrograph illustrating a cast microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 4 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method and subjected to a T4 heat treatment in accordance with an embodiment of the present invention.
  • Mg-6Zn-lMn magnesium alloy
  • Fig. 5 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl ) panel manufactured by a strip casting method and subjected to a T4 heat treatment in accordance with an embodiment of the present invention.
  • Mg-6Zn-IMn-IAl magnesium alloy
  • Fig. 6 is a micrograph illustrating fine dispersoids after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 7 is a micrograph illustrating fine dispersoids after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 8 is a micrograph illustrating local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 9 is a micrograph illustrating local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 10 is a micrograph illustrating secondary phases which are uniformed distributed without local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-O.3Cu) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Mg-6Zn-IMn-O.3Cu magnesium alloy
  • Fig. 11 is a micrograph illustrating secondary phases which are uniformed distributed without local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn-lAl-0.3Cu) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Mg-6Zn-lMn-lAl-0.3Cu magnesium alloy
  • Fig. 12 is a micrograph illustrating fine dispersoids and precipitate particles after a T6 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 13 is a micrograph illustrating fine dispersoids and precipitate particles after a T6 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
  • Fig. 14 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting and subjected to a thermomechanical treatment and then a T4 heat treatment in accordance with an embodiment of the present invention.
  • Fig. 15 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting and subjected to a thermomechanical treatment and then a T4 heat treatment in accordance with an embodiment of the present invention.
  • Mg-6Zn-IMn-IAl magnesium alloy
  • the alloy melt having the above composition was kept at a temperature of 700 °C , and transported to a tundish as shown in Fig. 1. Then, the alloy melt was injected between two water-cooled cooling rolls provided to a twin roll strip caster. Here, the tundish was kept at the same temperature as that of the alloy melt in the melting furnace.
  • the distance 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 alloy melt may be cast with a cooling rate of 100 K/s to 400 K/s.
  • ResultantIy a magnesium alloy sheet of 5 m length, 70 mm width, and 2 mm thickness was obtained.
  • the cast panel was subjected to the following three heat treatments.
  • the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours.
  • the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours, and then subjected to a T6 heat treatment where a primary aging treatment was performed at 70 °C for 24 hours, and directly thereafter, a secondary aging treatment was performed at 150 °C for 24 hours.
  • a T4 heat treatment or solution treatment
  • T6 heat treatment where a primary aging treatment was performed at 70 °C for 24 hours, and directly thereafter, a secondary aging treatment was performed at 150 °C for 24 hours.
  • a portion of the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours.
  • the T4 heat treated panel was preheated to 300 °C again, and then rolled by rolls, which was heated to 150 °C , with a reduction ratio of 10 % for each pass to a total reduction of 50 %.
  • the rolled panel was subjected again to a T4 heat treatment at 330 ° C for 30 minutes, and then subjected to a T6 heat treatment where a primary aging treatment was performed at 70 °C for 24 hours and then a secondary aging treatment was performed at 150 °C for 24 hours.
  • the microstructure of the panel manufactured as described above was analyzed using a scanning electron microscope and a transmission electron microscope.
  • Figs. 2 and 3 are micrographs illustrating cast microstructures of panels obtained by casting Mg-6Zn-lMn alloy (No. 2) and Mg-6Zn-IMn-IAl (No. 5) using a twin roll strip caster, respectively. As shown in Figs. 2 and 3, both alloys showed a solidification structure of equiaxed crystals in the thickness direction of the panel, and the distance between the solidification cells was 5 / an to 10 ⁇ m in Mg-6Zn-IMn-IAl alloy, which was finer than that of Mg-6Zn-lMn alloy in which Al was not added.
  • Figs. 4 and 5 are micrographs illustrating microstructures of Mg-6Zn- IMn alloy (No. 2) and Mg-6Zn-IMn-IAl (No. 5), respectively, taken after the casting and the T4 heat treatment. It was observed - that the crystal grain size of Mg-6Zn-IMn-IAl alloy was somewhat smaller than that of Mg-6Zn-lMn alloy, and the former was 12 ⁇ m and the latter was 15 ⁇ m.
  • the panel manufactured by strip casting and subjected to a T4 heat treatment was observed by using a transmission electron microscope (TEM).
  • TEM transmission electron microscope
  • the particles had a size of about 50 nm to about 500 nm (average size of 200 nm or less).
  • Mg-6Zn-IMn-IAl alloy much finer particles having a size of about 10 nm to 50 nm are uniformly distributed in the crystal grains.
  • the dispersed particles in both alloys all have the same volume fraction of about 2 %, and were estimated to play an important role in improving the strength of the alloy.
  • Mg-Zn based phase might form coarsely in the Mg-6Zn-lMn alloy
  • Mg-Al-Zn based phase which are known as a phi phase might form coarsely in the Mg-6Zn-IMn-IAl alloy.
  • the precipitate phase has a parallel orientation relationship with Mg ⁇ 0001> matrix plane in a [2-1-10] zone axis.
  • the aspect ratio of the precipitate particles in the Mg-6Zn-lMn alloy was about 15.2, and the aspect ratio of the precipitate particles in the Mg-6Zn-IMn-IAl alloy was about 4.9, which was smaller than that of the Mg-6Zn-lMn alloy.
  • the size, the distribution, and the orientation relationship with the matrix of the precipitate phase in the crystal grain was similar to those subjected to the T6 heat treatment after the casting.
  • a tensile specimen of 6 mm gauge length, 5 mm gage width and 1 mm thickness was prepared and subjected to a tensile test with a strain rate of
  • Nos. 1 to 7 alloy panels had the strength or the elongation equal to or superior than that of the conventional commercial magnesium alloy panel.
  • the manufacturing method in accordance with the embodiment of the present invention has a significant advantage in manufacturing cost and productivity.
  • Nos. 8 to 11 alloy panels added with Cu had significantly improved elongation and equivalent or higher strength in comparison to the conventional commercial magnesium alloy. Accordingly, they have an advantage in manufacturing cost and productivity, and they also have the advantage that they can be applied to applications requiring formability or toughness in view of physical property.
  • the alloy melts having the compositions of Nos. 1, 2, 5 and 7 of Table 1 were strip cast, and then subjected to the solution treatment (T4 heat treatment) and the above mentioned thermomechanical treatment (TMT treatment), and again to the T4 heat treatment or the T6 heat treatment. Thereafter, they were subjected to the tensile test to evaluate their tensile properties.
  • T4 heat treatment solution treatment
  • T6 heat treatment thermomechanical treatment
  • the results are presented in the following Table 4.
  • Nos. 1 to 3 specimens subjected to the T6 heat treatment had high strength of 250 MPa or more of yield strength and 300 MPa or more of tensile strength as well as high elongation.
  • No. 5 specimen had a high elongation (16.2 %) in comparison to the conventional commercial magnesium alloy panel while having a high yield strength and a high tensile strength (307 MPa and 330 MPa, respectively). Accordingly, it can be concluded that the No. 5 specimen can be applied to a field requiring high strength and high toughness.
  • No. 7 specimen had very high yield strength and tensile strength. However, it had somewhat low elongation. Accordingly, it can be concluded that the No. 7 specimen is suitable for the application requiring high strength rather than elongation.

Abstract

Provided is a high strength magnesium alloy panel that is improved in mechanical properties, toughness and formability, and a method for manufacturing the same. This is realized by adding Mn or Mn and Al to Mg-Zn based alloy, which is a precipitation hardening alloy, refining the structure, and controlling the behavior of the dispersoid phases and the precipitate phases.

Description

[DESCRIPTION] [Invention Title]
MAGNESIUM ALLOY PANEL HAVING HIGH STRENGTH AND MANUFACTURING METHOD THEREOF [Technical Field]
The present invention relates to a magnesium alloy panel having high strength and a method for manufacturing the same, and particularly, to a method for manufacturing a magnesium alloy panel which can realize high strength as well as high productivity through composition control of the magnesium alloy and microstructure control of the magnesium alloy panel by the use of strip casting and subsequent heat treatment, and a magnesium alloy panel manufactured by the same. [Background Art]
Magnesium alloy has the lowest specific gravity among practical structural materials, and has excellent specific strength and rigidity. Accordingly, there are increasing demands for the magnesium alloy as a material for cases for portable electronic devices and automobiles requiring light weight .
However, researches on the magnesium alloy have been focused on the high temperature properties thereof to employ it in automobile engines and gear parts. On the other hand, researches have not been sufficiently conducted on the wrought magnesium alloy for use in various fields, such as panels, requiring light weight.
The most widely used method for manufacturing a magnesium alloy, which is now put to practical use is a method for casting the magnesium alloy by using injection molding, such as diecasting and thixomolding. However, because magnesium has a low latent heat per unit volume, and thus is ready to produce casting defects, subsequent processes for removing the casting defects are necessarily required. This significantly decreases the productivity, increases manufacturing cost, and makes it difficult to enhance the strength of the magnesium alloy through the casting method. There have been proposed a method for manufacturing a product by performing hot extrusion of a casting obtained by a semi-continuous casting process, such as a diecasting type casting (hereinafter, referred to as "DC casting"), performing rolling and the like on the extruded material to a thinner panel and then forming a final product by using pressing and the like, or directly forging the extruded material to form the final product.
However, because magnesium alloy manufactured by the casting process is great in grain size, it is difficult to directly process the magnesium alloy by the use of pressing and the like, or by the use of forging and the like. Accordingly, when manufacturing a panel for pressing and a material for forging by using the semi-continuous casting, such as the DC casting, it is necessary to reheating and hot extruding the material obtained from the casting to refine the grain. In addition, because magnesium alloy is an active metal, it is necessary to perform the extrusion with a low speed enough to sufficiently cool the extruded material to prevent surface blackening or burning caused by deformation heat generation during the hot extrusion. Accordingly, this imposes a limit on the extrusion speed.
Therefore, the method using the casting as a material for pressing or forging has the problems of significant decrease in the productivity and increase in the product price. Furthermore, a material for hot extrusion is insufficient in grain refinement when being formed to a complex shape, and thus it is difficult to process it to a complex shape.
Compositions and tensile properties of commercial magnesium alloy panels which are now put to commercial use are presented in Table 1. [Table 1]
Compositions and tensile properties of commercial magnesium alloy panels
Figure imgf000004_0001
As shown in Table 1, the commercial magnesium alloy panels have a yield strength of about 200 MPa, and a tensile strength of about 300 MPa or lower, which are insufficient to be employed in the applications requiring high strength. They have an elongation of about 10 %, which is insufficient to form various shapes. Furthermore, because of the above described problems of the method for manufacturing magnesium alloy panel, the typical magnesium alloy panels have the problem of high manufacturing cost. [Disclosure] [Technical Problem]
The present invention addresses the above identified problems of the commercial magnesium alloy panels. An objective of the present invention is to provide a high strength magnesium alloy panel having high strength and high elongation and thus having improved formability, which is realized by adding Mn or Mn and Al to Mg-Zn based alloy, which is a precipitation hardening alloy, and performing strip casting and subsequent heat treatment to refine the microstructure and to control the behavior of dispersoid phases and precipitate phases.
Another objective of the present invention is to provide a method for manufacturing a magnesium alloy panel, capable of manufacturing a high strength magnesium alloy panel excellent in strength and elongation, at low cost. [Technical Solution]
In order to achieve the above mentioned objective, an embodiment of the present invention provides a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less. In accordance with an embodiment of the present invention, there is provided a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
In accordance with an embodiment of the present invention, there is provided a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
In accordance with an embodiment of the present invention, there is provided a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
In accordance with an embodiment of the present invention, there is provided a magnesium alloy panel including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
In accordance with an embodiment of the present invention, it is preferable that the Zn content ranges from 5.0 wt.% to 7.0 wt.%, the Mn content ranges from 0.75 wt.% to 2.0 wt.%, and the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
In accordance with an embodiment of the present invention, wherein the average grain size of the crystal grains may be 10 μm or less.
In accordance with an embodiment of the present invention, an average grain size of dispersoids in a microstructure of the panel is 5 μm or less.
In accordance with an embodiment of the present invention, a volume fraction of the dispersoids is 5 % or less.
In accordance with an embodiment of the present invention, an average grain size of precipitate particles which are precipitated in a microstructure of the panel by an aging treatment is 200 run or less, and an aspect ratio of the precipitate particles is 20 or less, more preferably, 10 or less.
In accordance with an embodiment of the present invention, the magnesium alloy has a tensile strength of 300 MPa or more and an elongation of 15 % or more.
In order to achieve the above mentioned another objective, the present invention provides a method for manufacturing a high strength magnesium alloy panel, including: (a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn and the balance magnesium and incidental impurities; (b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool the
2 3 magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s; and (c) a step of performing a solution treatment on the produced panel at 250 °C to 450 0C for 0.5 hour to 24 hours.
In accordance with an embodiment of the present invention, the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn.
In accordance with an embodiment of the present invention, the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn and 1 wt.% to 6 wt.% Al.
In accordance with an embodiment of the present invention, the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn and 0.05 wt.% to 1 wt.% Cu.
In accordance with an embodiment of the present invention, the magnesium alloy melt may further include 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, and 0.05 wt.% to 1 wt.% Cu.
In accordance with an embodiment of the present invention, it is preferable that the Zn content ranges from 5.0 wt.% to 7.0 wt.%, the Mn content ranges from 0.75 wt.% to 2.0 wt.%, and the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
In accordance with an embodiment of the present invention, the method may further include, after the step of (c), a step of performing a two step aging treatment (T6 heat treatment) where an aging treatment is performed on the solution treated panel at 70 °C to 150 °C for 24 hours to 96 hours, and then at 150 °C to 250 °C for 24 hours or more.
In accordance with an embodiment of the present invention, the method may further include, after the step of (c), a step of rolling the solution treated panel, which is preheated to a temperature ranging from 200 °C to 400 °C , with a reduction ratio of 20 % or less for each pass to a total reduction of 70 % or less, and a step of performing a solution treatment on the rolled panel at 200 °C to 400 °C for 0.5 hour to 10 hours, and the method may further include a step of performing a two step ageing treatment (T6 heat treatment) where an aging treatment is performed on the solution treated panel at 70 °C to 150 °C (preferably, at 70 °C to 100 °C) for 24 hours to 96 hours, and then at 150 °C to 250 °C (preferably, at 150 °C to 180 °C) for 24 hours or more.
In accordance with an embodiment of the present invention, the Mn in the step (a) is added in the alloy melt by adding Mg-Mn alloy in the alloy melt .
In accordance with an embodiment of the present invention, the distance between the two cooling rolls in the step (b) is kept at 10 mm or less, and the rotation speed of the cooling rolls is kept at 10 m/min or less.
One of the significances of the method for manufacturing the magnesium alloy panel in accordance with the embodiments of the present invention is to cast the magnesium alloy using a twin roll strip casting method. The twin roll strip casting method performs a casting process and a hot rolling process at the same time in a single process, and thus is very expensive in comparison to the conventional ingot casting method. In addition, it is
2 3 possible to provide a cooling speed of 10 K/s to 10 K/s, which is much faster than that of the ingot casting method. Such a rapid cooling rate results in microstructure refinement and decreased segregation. In addition, while the slow cooling rate of the general ingot casting method may form a coarse intermetallic compounds, which could deteriorate the tensile properties, the rapid cooling rate of the strip casting method may finely disperse the intermetallic compounds, which is favorable.
The reasons for the above mentioned restrictions on the alloy composition and the related values are as follow.
Zinc (Zn): 1 wt.% to 10 wt.%
Zinc (Zn) has a solubility of 6.2 wt.% in a magnesium matrix at 340 "C . When Zn is added in the magnesium matrix in an amount 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 precipitate 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 age hardening effect.
Manganese (Mn): Q.25 wt.% to 3.0 wt.%
Manganese (Mn) refines precipitate phases, providing the effect of improving the strength and the elongation, and improving the corrosion resistance. The effects can be obtained when Mn is added in an amount of 0.25 wt.% or more. Above 3.0 wt.% Mn, it is difficult to add Mn by a general melting process, and most of Mn above 3.0 wt.% exist in a form of α-Mn in the matrix, thus having no effects on the improvement of the strength and the elongation, which is not preferable in view of manufacturing cost. Accordingly, it is preferable that Mn is added in an amount ranging from 0.75 wt.% to 2.0 wt.%.
Aluminum (Al): l.Q wt.% to 6.0 wt%
In Mg-Zn-Mn ternary alloy, aluminum (Al) forms an Al-Mn based precipitate phase which is a thermally stable dispersoid phase, thus providing a further strengthening effect and an effect of reforming the Mg-Zn related acicular precipitation phases. In the aging temperature range in accordance with an embodiment of the present invention, the solubility of Al in the Mg matrix is approximately 1 wt.%. Accordingly, below 1.0 wt.% Al, it is difficult to expect the above mentioned effect of the Al addition. In addition, above 6.0 wt.% Al, MgI7Al12 phase may precipitate, which would precipitate coarsely along the grain boundary to decrease the strength and the fracture strength, and thus it is preferable that Al is added in an amount of 6.0 wt.% or less. Furthermore, even if Al is added in an amount of 6.0 wt.% or less, a Mg-Al based precipitate phase can be formed if only the Al content is larger than that of Zn. Accordingly, it is more preferable that the Al content is kept below the Zn content.
Copper (Cu): Q.05 wt.% to 1.0 wt.%
Copper (Cu) deteriorates the corrosion resistance of a generally used Mg-Al based alloy, and thus has a limitation in its use. However, Cu finely disperses Cu-Mg-Zn phases in a Mg-Zn-Mn based alloy or a Mg-Zn-Mn-Al based alloy, to suppress segregation caused by a Mg-Zn phase or a Mg-Al-Zn phase, thus making uniform the microstructure of the material. Accordingly, the addition of Cu may improve the elongation of the material. However, when Cu is added in an amount less than 0.05 wt.%, it is difficult to expect the above mentioned effect, and when Cu is added in an amount more than 1 wt.%, the corrosion resistance of the alloy may be decreased. Therefore, it is preferable that Cu is added in an amount less than 1.0 wt.%, and it is more preferable that Cu is added in an amount less than 0.5 wt.% in order to form the effective Cu-Mg-Mn phase to prevent the deterioration of the corrosion resistance.
In the magnesium alloy panel in accordance with an embodiment of the present invention, it is preferable that an average grain diameter (size) of the crystal grains is 20 μm or less. This is because if the average size of the crystal grain is larger than 20 /mi, the strength of the material decreases. It is more preferable that the average size of the crystal grain is 10 (M.
In the microstructure of the magnesium alloy panel in accordance with an embodiment of the present invention, it is preferable for the strength that an average grain size of the dispersoids is 5 μm or less. Also, it is preferable that a volume fraction of the dispersoids is 5 % or less. This is because if the volume fraction of the dispersoids is higher than 5 %, although the strength increases, the dispersoids may impede the movement of the dislocations to reduce the ductility of the magnesium alloy panel.
In the microstructure of the magnesium alloy panel in accordance with an embodiment of the present invention, it is preferable that among the dispersed particles, the particles precipitated by the aging treatment have an average grain size of 200 nm or less. This is because if the average grain size is larger than 200 nm, the strength may be decreased.
In accordance with an embodiment of the present invention, it is preferable that an aspect ratio of the precipitate particles is 20 or less. This is because the anisotropy in the crystal grain increases if the aspect ratio is larger than 20. It is more preferable that the aspect ratio of the precipitate particles is 10 or less.
In accordance with an embodiment of the present invention, the magnesium alloy panel can have a tensile strength of 300 MPa or more and an elongation of 15 % or more through the above mentioned alloy design, the microstructure refinement, and the control of the dispersoid phase. That is, it is possible to obtain a high strength magnesium alloy having an excellent formability, which could not obtained in the conventional commercial magnesium alloy panel.
In accordance with an embodiment of the present invention, it is preferable that the alloy melt is kept at a temperature ranging from 650 °C to 750 °C . This is because, below 650 °C , the alloy melt is solidified before contacting the rolls, and thus cannot pass between the rolls. In addition, above 750 °C , a liquid phase still remains even after passing between the rolls, thus generating solidification defects or cracks in the surface.
In accordance with an embodiment of the present invention, it is 2 3 preferable that a cooling rate of the alloy melt is kept at 10 K/s to 10
K/s. This is because, if the cooling rate is lower than 10 K/s, the microstructure is not quite different from that of the typical mold casting
3 method, and, above 10 K/s, the process is difficult to perform commercially except the rapid solidification method providing a very thin ribbon shape.
In accordance with an embodiment of the present invention, it is preferable that the distance between the two cooling rolls is kept at 10 mm or less, and the rotation speed of the rolls is kept at 10 m/min to obtain the above mentioned cooling rate.
In the strip cast alloy panel, segregation of alloying elements may occur during the casting, and thus the uniformity of the properties of the products may be deteriorated. Accordingly, a solution treatment (T4 heat treatment) is required after the strip casting. In accordance with an embodiment of the present invention, the temperature and the time for the solution treatment is set as follows based on the diffusivity of zinc, which is a major alloying element, the secondary dendrite arm spacing (SDAS), and the fact whether or not incipient melting occurs, which is determined by differential thermal analysis (DTA) / differential scanning calorimetry (DSC). That is, the solution treatment is set to be performed at 250 °C to 450 °C for 0.5 hour to 24 hours, and more preferably, at 300 °C to 330 °C for 1 hour to 5 hours.
A double aging treatment (T6 heat treatment) may be selectively performed on the panel manufactured as described above, where a primary aging treatment is performed at 70 °C to 150 °C (more preferably, at 70 °C to 100 °C) for 24 hours to 96 hours, and then a secondary aging treatment is performed at 150 °C to 250 °C (more preferably, at 150 °C to 180 °C) for 24 hours or more. Such a double aging is intended for performing the primary aging at a temperature below the G.P. zone solvus temperature of βl' phase, which is a main precipitate phase in the Mg-Zn based alloy, and then by performing the secondary aging at a temperature thereabove to maximize the effect of the precipitate phase advantageous for the strengthening. Therefore, the primary aging temperature is restricted to a range from 70 °C to 150 °C , which is somewhat lower than the generally known G.P. zone solvus temperature of βl' phase. In addition, the aging time is set to a range that is sufficient to expect some degree of improvement in the hardness through the formation of the G.P. zone, by measuring the hardness. Meanwhile, the secondary aging temperature is set to 150 °C to 250 °C . This is because, below 150 °C , a great deal of time will be spent in reaching the maximum hardness, causing problems in the process. In addition, above 250 °C , the maximum hardness decreases although it can be obtained in a short time.
The method for manufacturing the magnesium panel in accordance with an embodiment of the present invention may further include the step (thermomechanical treatment step) of rolling the solution treated panel, which is preheated to 200 °C to 400 °C , with a reduction ratio of 20 % or less for each pass to a total reduction of 70 % or less. The above mentioned preheating temperature range (processing temperature range) is sufficient to obtain a normal panel. Accordingly, the processing of the panel is preferably performed in the above mentioned temperature range, and more preferably, in a temperature range from 250 "C to 330 °C .
In the method for manufacturing the magnesium panel in accordance with an embodiment of the present invention, a solution treatment may be performed again at 200 °C to 400 °C (more preferably, at 300 °C to 330 °C) for 0.5 hour to 10 hours (more preferably, for 1 hour to 5 hours) after the thermomechanical treatment step.
In the method for manufacturing a magnesium panel in accordance with an embodiment of the present invention, it is preferable to use a Mg-Mn master alloy to add Mn in the magnesium alloy melt. This is because the solubility of Mn in Mg is low and the diffusivity thereof is also low, and thus it is difficult to add a desired amount of Mn during the casting. The Mg-Mn master alloy preferably contains 1.5 wt.% to 4 wt.% Mn.
In accordance with the embodiments of the present invention, it is possible to obtain a high strength magnesium panel with a reduced number of manufacturing steps in comparison to the related art.
[Advantageous Effects]
In accordance with the embodiments of the present invention, contrary to the conventional commercial method for manufacturing a magnesium alloy panel, it is possible to refine the crystal grains, and control the size, shape and volume fraction of the dispersoids through the casting performed by a twin roll strip casting method, the alloy design suitable for the strip casting, and the subsequent treatment (heat treatment or thermomechanical treatment). As a result, it is possible to provide a high strength and high toughness magnesium alloy panel that is improved in the hardness, the strength and the elongation in comparison to the conventional commercial magnesium alloy panel, and thus can be practically applied to the automobile and electronic industries.
In accordance with the embodiments of the present invention, it is possible to reduce the number of manufacturing steps in comparison to the conventional commercial method for manufacturing a magnesium alloy panel, and thus it is possible to manufacture a high strength magnesium alloy panel at low cost in comparison to the conventional commercial magnesium alloy panel. [Description of Drawings]
Fig. 1 is a schematic view of a strip caster used in an embodiment of the present invention.
Fig. 2 is a micrograph illustrating a cast microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 3 is a micrograph illustrating a cast microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 4 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method and subjected to a T4 heat treatment in accordance with an embodiment of the present invention.
Fig. 5 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl ) panel manufactured by a strip casting method and subjected to a T4 heat treatment in accordance with an embodiment of the present invention.
Fig. 6 is a micrograph illustrating fine dispersoids after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 7 is a micrograph illustrating fine dispersoids after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 8 is a micrograph illustrating local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 9 is a micrograph illustrating local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 10 is a micrograph illustrating secondary phases which are uniformed distributed without local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-IMn-O.3Cu) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 11 is a micrograph illustrating secondary phases which are uniformed distributed without local segregation after a T4 heat treatment of a magnesium alloy (Mg-6Zn-lMn-lAl-0.3Cu) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 12 is a micrograph illustrating fine dispersoids and precipitate particles after a T6 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention.
Fig. 13 is a micrograph illustrating fine dispersoids and precipitate particles after a T6 heat treatment of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting method in accordance with an embodiment of the present invention. Fig. 14 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-lMn) panel manufactured by a strip casting and subjected to a thermomechanical treatment and then a T4 heat treatment in accordance with an embodiment of the present invention.
Fig. 15 is a micrograph illustrating a microstructure of a magnesium alloy (Mg-6Zn-IMn-IAl) panel manufactured by a strip casting and subjected to a thermomechanical treatment and then a T4 heat treatment in accordance with an embodiment of the present invention. [Best Mode]
The present invention will now be described in detail with reference to specific embodiments. The invention may, however, be embodied in many different forms by those skilled in the art and should not be construed as being limited to the embodiments set forth herein.
Manufacture of magnesium alloy panel
First, pure Mg (99.9 %), Mg-2.5 wt.% Mn master alloy, pure Zn (99.995 %), and pure Al (99.99 %) were melted under a gas mixture of CO2 and SF6 in an induction melting furnace to prepare Mg alloy melt having the composition as listed in Table 2. [Table 2]
Compositions of magnesium alloys in accordance with an embodiment of the present invention
Figure imgf000016_0001
The alloy melt having the above composition was kept at a temperature of 700 °C , and transported to a tundish as shown in Fig. 1. Then, the alloy melt was injected between two water-cooled cooling rolls provided to a twin roll strip caster. Here, the tundish was kept at the same temperature as that of the alloy melt in the melting furnace.
The distance 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 alloy melt may be cast with a cooling rate of 100 K/s to 400 K/s. ResultantIy, a magnesium alloy sheet of 5 m length, 70 mm width, and 2 mm thickness was obtained.
The cast panel was subjected to the following three heat treatments.
First, the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours.
Next, the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours, and then subjected to a T6 heat treatment where a primary aging treatment was performed at 70 °C for 24 hours, and directly thereafter, a secondary aging treatment was performed at 150 °C for 24 hours.
Finally, a portion of the cast panel was subjected to a T4 heat treatment (or solution treatment) at 330 °C for 2 hours. The T4 heat treated panel was preheated to 300 °C again, and then rolled by rolls, which was heated to 150 °C , with a reduction ratio of 10 % for each pass to a total reduction of 50 %. The rolled panel was subjected again to a T4 heat treatment at 330 °C for 30 minutes, and then subjected to a T6 heat treatment where a primary aging treatment was performed at 70 °C for 24 hours and then a secondary aging treatment was performed at 150 °C for 24 hours.
Analysis of microstructure
The microstructure of the panel manufactured as described above was analyzed using a scanning electron microscope and a transmission electron microscope.
Figs. 2 and 3 are micrographs illustrating cast microstructures of panels obtained by casting Mg-6Zn-lMn alloy (No. 2) and Mg-6Zn-IMn-IAl (No. 5) using a twin roll strip caster, respectively. As shown in Figs. 2 and 3, both alloys showed a solidification structure of equiaxed crystals in the thickness direction of the panel, and the distance between the solidification cells was 5 /an to 10 μm in Mg-6Zn-IMn-IAl alloy, which was finer than that of Mg-6Zn-lMn alloy in which Al was not added.
Figs. 4 and 5 are micrographs illustrating microstructures of Mg-6Zn- IMn alloy (No. 2) and Mg-6Zn-IMn-IAl (No. 5), respectively, taken after the casting and the T4 heat treatment. It was observed - that the crystal grain size of Mg-6Zn-IMn-IAl alloy was somewhat smaller than that of Mg-6Zn-lMn alloy, and the former was 12 μm and the latter was 15 μm.
The panel manufactured by strip casting and subjected to a T4 heat treatment was observed by using a transmission electron microscope (TEM). As a result, as shown in Figs. 6 and 7, it was observed that a lot of fine particles were distributed in the crystal grains. It was also observed that in Mg-6Zn-lMn alloy, the particles had a size of about 50 nm to about 500 nm (average size of 200 nm or less). In Mg-6Zn-IMn-IAl alloy, much finer particles having a size of about 10 nm to 50 nm are uniformly distributed in the crystal grains. The dispersed particles in both alloys all have the same volume fraction of about 2 %, and were estimated to play an important role in improving the strength of the alloy.
When the T4 heat treatment was performed after the casting, the local segregation as shown in Figs. 8 and 9 might deteriorate the elongation.
Particularly, Mg-Zn based phase might form coarsely in the Mg-6Zn-lMn alloy, and Mg-Al-Zn based phase which are known as a phi phase might form coarsely in the Mg-6Zn-IMn-IAl alloy.
However, as shown in Figs. 10 and 11, when Cu was added therein, the Mg-Zn-Cu based phases formed uniformly in the microstructure, and the coarse segregation was suppressed. As a result, a uniform microstructure was induced, which might improve the elongation of the panel.
As shown in Figs. 12 and 13, when the T6 heat treatment was performed after the casting, in the Mg-6Zn-lMn alloy and the Mg-6Zn-IMn-IAl alloy, the precipitate phase has a parallel orientation relationship with Mg <0001> matrix plane in a [2-1-10] zone axis. The aspect ratio of the precipitate particles in the Mg-6Zn-lMn alloy was about 15.2, and the aspect ratio of the precipitate particles in the Mg-6Zn-IMn-IAl alloy was about 4.9, which was smaller than that of the Mg-6Zn-lMn alloy.
As shown in Figs. 14 and 15, when the T4 heat treatment was performed after the casting, and the thermomechanical treatment and then the T4 heat treatment were performed, the crystal grain size was reduced to 7 μm in the Mg-6Zn-lMn alloy, and to 4 μm in the Mg-6Zn-IMn-IAl alloy. As a result, it could be seen that the amount of reduction was large in the alloy added with Al than in the alloy without the addition of Al.
Meanwhile, when the T4 heat treatment was performed after the casting, and the thermomechanical treatment and then the T6 heat treatment were performed, the size, the distribution, and the orientation relationship with the matrix of the precipitate phase in the crystal grain was similar to those subjected to the T6 heat treatment after the casting.
Tensile property test
A tensile specimen of 6 mm gauge length, 5 mm gage width and 1 mm thickness was prepared and subjected to a tensile test with a strain rate of
-4 -1
6.4 x 10 s to estimate the tensile property of the magnesium alloy panel manufactured as described above.
First, the specimen having the composition of Nos. 1 to 11 of Table 1 was subjected to T4 heat treatment or T6 heat treatment, and then subjected to a tensile test to estimate the tensile property thereof. The test results are presented in the following Table 3. [Table 3]
Tensile properties of specimens subjected to the T4 heat treatment or the T6 heat treatment after the strip casting
Figure imgf000020_0001
As can be seen from Table 3, Nos. 1 to 7 alloy panels had the strength or the elongation equal to or superior than that of the conventional commercial magnesium alloy panel. However, because the Nos. 1 to 7 alloy panels in accordance with the embodiment of the present invention were manufactured by a simpler process than the conventional commercial magnesium alloy panel, the manufacturing method in accordance with the embodiment of the present invention has a significant advantage in manufacturing cost and productivity.
Also, Nos. 8 to 11 alloy panels added with Cu had significantly improved elongation and equivalent or higher strength in comparison to the conventional commercial magnesium alloy. Accordingly, they have an advantage in manufacturing cost and productivity, and they also have the advantage that they can be applied to applications requiring formability or toughness in view of physical property.
In addition, the alloy melts having the compositions of Nos. 1, 2, 5 and 7 of Table 1 were strip cast, and then subjected to the solution treatment (T4 heat treatment) and the above mentioned thermomechanical treatment (TMT treatment), and again to the T4 heat treatment or the T6 heat treatment. Thereafter, they were subjected to the tensile test to evaluate their tensile properties. The results are presented in the following Table 4.
[Table 4]
Tensile properties of specimens subjected to the thermomechanical treatment followed by the T4 heat treatment or the T6 heat treatment
Figure imgf000021_0001
As can be seen in Table 4, for the specimens subjected to the T4 heat treatment, the tensile strengths of Nos. 1, 2 and 5 were the same as or higher than that of the conventional magnesium alloy panel of Table 1, and the elongations thereof were significantly improved in comparison to the conventional magnesium alloy panel of Table 1. Meanwhile, in No. 7, the elongation was somewhat lowered, however, the yield strength and the tensile strength were significantly improved. Accordingly, it can be concluded that No. 7 can be applied to a filed requiring strength rather than elongation.
Meanwhile, it can be seen that Nos. 1 to 3 specimens subjected to the T6 heat treatment had high strength of 250 MPa or more of yield strength and 300 MPa or more of tensile strength as well as high elongation.
Particularly, No. 5 specimen had a high elongation (16.2 %) in comparison to the conventional commercial magnesium alloy panel while having a high yield strength and a high tensile strength (307 MPa and 330 MPa, respectively). Accordingly, it can be concluded that the No. 5 specimen can be applied to a field requiring high strength and high toughness.
Meanwhile, No. 7 specimen had very high yield strength and tensile strength. However, it had somewhat low elongation. Accordingly, it can be concluded that the No. 7 specimen is suitable for the application requiring high strength rather than elongation.

Claims

[CLAIMS] [Claim 1]
A high strength magnesium alloy panel comprising 1 wt.% to 10 wt.% Zn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
[Claim 2]
A high strength magnesium alloy panel comprising 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
[Claim 3]
A high strength magnesium alloy panel comprising 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
[Claim 4]
A high strength magnesium alloy panel comprising 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
[Claim 5]
A high strength magnesium alloy panel comprising 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, 0.05 wt.% to 1 wt.% Cu, and the balance magnesium and incidental impurities, wherein an average grain size of crystal grains thereof is 20 μm or less.
[Claim 6]
The high strength magnesium alloy panel of any one of claims 1 to 5, wherein the Zn content ranges from 5.0 wt.% to 7.0 wt.%.
[Claim 7]
The high strength magnesium alloy panel of any one of claims 2 to 5, wherein the Mn content ranges from 0.75 wt.% to 2.0 wt.%.
[Claim 8] The high strength magnesium alloy panel of claim 4 or 5, wherein the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
[Claim 9]
The high strength magnesium alloy panel of claim 3 or 5, wherein the Al content is equal to or lower than the Zn content.
[Claim 10]
The high strength magnesium alloy panel of any one of claims 1 to 5, wherein the average grain size of the crystal grains is 10 μm or less.
[Claim 11]
The high strength magnesium alloy panel of any one of claims 1 to 5, wherein an average grain size of dispersoids in a microstructure of the panel is 5 μm or less.
[Claim 12]
The high strength magnesium alloy panel of claim 11, wherein a volume fraction of the dispersoids is 5 % or less.
[Claim 13]
The high strength magnesium alloy panel of any one of claims 1 to 5, wherein an average grain size of precipitate particles in a microstructure of the panel is 200 nm or less.
[Claim 14]
The high strength magnesium alloy panel of claim 13, wherein an aspect ratio of the precipitate particles is 20 or less.
[Claim 15]
The high strength magnesium alloy panel of claim 13, wherein an aspect ratio of the precipitate particles is 10 or less.
[Claim 16]
The high strength magnesium alloy panel of any one of claims 1 to 5, wherein the magnesium alloy has a tensile strength of 300 MPa or more and an elongation of 15 % or more.
[Claim 17]
A method for manufacturing a high strength magnesium alloy panel, comprising:
(a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn and the balance magnesium and incidental impurities!
(b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 0C to 750 °C , between two rotating cooling rolls to cool
2 3 the magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s; and
(c) a step of performing a solution treatment on the produced panel at 250 °C to 450 °C for 0.5 hour to 24 hours.
[Claim 18]
A method for manufacturing a high strength magnesium alloy panel, comprising:
(a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn and the balance magnesium and incidental impurities;
(b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool
2 3 the magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s! and
(c) a step of performing a solution treatment on the produced panel at 250 °C to 450 °C for 0.5 hour to 24 hours.
[Claim 19]
A method for manufacturing a high strength magnesium alloy panel, comprising:
(a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al and the balance magnesium and incidental impurities;
(b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool
2 3 the magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s; and
(c) a step of performing a solution treatment on the produced panel at 2500C to 4500C for 0.5 hour to 24 hours.
[Claim 20]
A method for manufacturing a high strength magnesium alloy panel, comprising:
(a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 0.05 wt.% to 1 wt.% Cu and the balance magnesium and incidental impurities;
(b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool
2 3 the magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s; and
(c) a step of performing a solution treatment on the produced panel at 250 °C to 450 °C for 0.5 hour to 24 hours.
[Claim 21]
A method for manufacturing a high strength magnesium alloy panel, comprising:
(a) a step of producing a magnesium alloy melt including 1 wt.% to 10 wt.% Zn, 0.25 wt.% to 3 wt.% Mn, 1 wt.% to 6 wt.% Al, 0.05 wt.% to 1 wt.% Cu and the balance magnesium and incidental impurities;
(b) a step of producing a panel by injecting the magnesium alloy melt, which is kept at 650 °C to 750 °C , between two rotating cooling rolls to cool
2 3 the magnesium alloy melt with a cooling rate of 10 K/s to 10 K/s; and
(c) a step of performing a solution treatment on the produced panel at 250 °C to 450 °C for 0.5 hour to 24 hours.
[Claim 22]
The method for manufacturing a high strength magnesium alloy panel of any one of claims 17 to 21, wherein the Zn content ranges from 5.0 wt.% to 7.0 wt.%.
[Claim 23]
The method for manufacturing a high strength magnesium alloy panel of any one of claims 18 to 21, wherein the Mn content ranges from 0.75 wt.% to 2.0 wt.%.
[Claim 24]
The method for manufacturing a high strength magnesium alloy panel of claim 20 or 21, wherein the Cu content ranges from 0.05 wt.% to 0.5 wt.%.
[Claim 25]
The method for manufacturing a high strength magnesium alloy panel of claim 19 or 21, wherein the Al content is equal to or lower than the Zn content.
[Claim 26]
The method for manufacturing a high strength magnesium alloy panel of any one of claims 17 to 21, the method further comprising, after the step of (c), a step of performing an aging treatment on the solution treated panel at 70 °C to 150 °C for 24 hours to 96 hours, and then at 150 °C to 250 °C for 24 hours or more.
[Claim 27]
The method for manufacturing a high strength magnesium alloy panel of any one of claims 17 to 21, the method further comprising, after the step of (c), a step of rolling the solution treated panel, which is preheated to a temperature ranging from 200 °C to 400 °C , with a reduction ratio of 20 % or less for each pass to a total reduction of 70 % or less, and a step of performing a solution treatment on the rolled panel at 200 °C to 400 °C for 0.5 hour to 10 hours.
[Claim 28]
The method for manufacturing a high strength magnesium alloy panel of claim 27, the method further comprising a step of performing an aging treatment on the solution treated panel at 70 "C to 150 °C for 24 hours to 96 hours, and then at 150 °C to 250 °C for 24 hours or more.
[Claim 29]
The method for manufacturing a high strength magnesium alloy panel of any one of claims 17 to 21, wherein the Mn in the step (a) is added in the alloy melt by adding Mg-Mn alloy in the alloy melt.
[Claim 30] The method for manufacturing a high strength magnesium alloy panel of any one of claims 17 to 21, the distance between the two cooling rolls in the step (b) is kept at 10 mm or less, and the rotation speed of the cooling rolls is kept at 10 m/min or less.
PCT/KR2008/000743 2008-01-30 2008-02-05 Magnesium alloy panel having high strength and manufacturing method thereof WO2009096622A1 (en)

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CN102978497A (en) * 2012-11-21 2013-03-20 中国科学院金属研究所 Casting magnesium alloy with high strength and toughness and preparation method thereof
DE102012108648A1 (en) * 2012-09-14 2014-03-20 Mgf Magnesium Flachprodukte Gmbh Producing cast-rolled sheet/tape of magnesium alloy comprises casting magnesium alloy into casting tape, heat treating casting tape, and cooling casting tube, where casting tape directly is rolled to final thickness using roll caster
CN105385920A (en) * 2015-12-17 2016-03-09 山东省科学院新材料研究所 Heated and reinforced high-strength cast magnesium alloy and preparation method thereof
CN106715736A (en) * 2014-10-15 2017-05-24 住友电气工业株式会社 Magnesium alloy, magnesium alloy plate, magnesium alloy member, and method for producing magnesium alloy
CN107326235A (en) * 2017-07-20 2017-11-07 重庆大学 A kind of high-strength Mg Zn Al series deformation magnesium alloys containing Cu and preparation method thereof
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CN108480577A (en) * 2018-03-13 2018-09-04 闻喜县远华冶金材料有限公司 The production method of casting magnesium alloy profiles
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EP2567760A4 (en) * 2010-04-27 2013-11-20 Sun Xing Chemical & Metallurg Materials Shenzhen Co Ltd High-efficiency continuous casting-rolling method for forming magnesium alloy plate
DE102012108648A1 (en) * 2012-09-14 2014-03-20 Mgf Magnesium Flachprodukte Gmbh Producing cast-rolled sheet/tape of magnesium alloy comprises casting magnesium alloy into casting tape, heat treating casting tape, and cooling casting tube, where casting tape directly is rolled to final thickness using roll caster
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