Classifications

• • 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
  • C22C23/02—Alloys based on magnesium with aluminium as the next major constituent
• • 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
• • 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
  • C22C23/06—Alloys based on magnesium with a rare earth metal as the next major constituent

Definitions

• the present invention relates to a wrought magnesium alloy, which contains a second phase consisting of an intermetallic compound, thereby having excellent strength, formability, and corrosion resistance. More particularly, the present invention pertains to a wrought magnesium alloy, which comprises 0.1 - 1.5 at% group ⁇ ia, 1.0 - 4.0 at% group nib, 0.35 at% or less of one selected from the group consisting of groups Ha, IVa, Vila, IVb, and a mixture thereof, 1.0 at% or less group lib, and a balance of Mg and unavoidable impurities and which thus has a second phase composite microstructure consisting of an intermetallic compound, and a method of producing the same.
• Background Art
• magnesium alloys have been developed as light structural materials for airplanes and automobiles.
• HCP hexagonal close packed
• formability is very low, and thus, the application is limited to a field in which the forming is achieved using a casting process.
• its practical use is limited due to problems, such as severe oxidation of melts, reduction of strength at high temperatures, and low corrosion resistance.
• Effort has been made to avoid the above disadvantages, thus enabling stable dissolution in atmospheric air, while employing a sulfur hexafluoride (SF 6 ) gas, a carbon dioxide gas, an argon gas and the like, and production of a plate by Direct Chilled casting process.
• SF 6 sulfur hexafluoride
• a Mg-Zn alloy shows an excellent age hardening behavior, and is advantageous in that since a microstructure is refined through heat treatments, strength and ductility significantly increase and it is easy to work and weld.
• it is disadvantageous in that since micropores are formed as a casting process due to the addition of Zn, it is difficult to apply the Mg-Zn alloy to a casting process, such as die-casting. Additionally, it is difficult to desirably improve strength because it is grown as the coarse grain.
• studies have been made to improve formability using a grain boundary slip, in which some alloy elements are added to Mg-Zn binary alloys to refine grains. With respect to this, J. P. Doan and G.
• Ansel suggest a method of improving the strength of an alloy, in which Zr is added to refine grains constituting a Mg-Zn alloy (J. P. Doan and G. Ansel, Trans, AIME, vol. 171 (1947), pp. 286-295).
• Zr has a high melting point and a low solubility to Mg at room temperature, it mostly exists at a grain boundary, thus acting as a fracture initiation site when external stress is applied.
• Laid-Open Publication No.2003-0048412 discloses an alloy, which contains 3.0 - 10.0 wt% Zn, 0.25 - 3.0wt% Mn, Al, Si, and Ca.
• the alloy containing Zn in an amount of 2 % or more has high strength, it has a disadvantage in that free zinc (Zn) readily forms a low melting point eutectic phase.
• Zn free zinc
• Mg Zn having a low melting point that is less than 350°C
• corrosion resistance is low.
• the plate is easily cracked at both sides during a rough rolling process for breaking the coarse dendrite structure, so draw-ability is poor because of high anisotropy.
• 2002-0078936 (U.S. Pat. No. 6471797) discloses a method of improving strength and formability using a Mg-Zn-Y eutectic ternary alloy quasi-crystalline phase which contains 1 - 10 at% Zn and 0.1 - 3 at% Y.
• this method is disadvantageous in that the amount of Zn must be enough so as to desirably assure a quasi-crystalline phase effect.
• the composition of a cast product is not uniform because a specific gravity between zinc and magnesium is significantly different.
• the micro-pores at the grain boundary reduce corrosion resistance, and tears form at sides of a plate during the hot rolling process.
• H07-54026 entitled 'magnesium alloy having high strength and method of producing the same'
• U.S. Pat. 4675157, 4765954, 4853035, 4857109, 4938809, 5071474, 5078806, 5078807, 5087304, 5129960, and 5316598, EP No. 0,361,136Al, and French Pat. No. 2,688,233 disclose the formation of an amorphous structure through a Rapid Solidification process. Since the cooling rate must be conducted at 10 - 10 °C /s to form the amorphous structure, the patents are useful to produce powder or a thin strip, but not to produce a common plate shape. Accordingly, an ingot, which is produced by compacting amorphous powder under the recrystallization temperature, is employed in order to conduct a rolling or a press forming.
• 5693158, 5800640 and 6395224 disclose a method of producing goods having low crack sensitivity, in which Sr, Li or B is employed and heat treatments are conducted to refine a particle size of crystals of a cast product.
• these patents are useful to cast products, but cannot be directly applied to wrought products.
• Japanese Pat. Laid-Open Publication No. H10-147830A discloses the use of 6 - 12 wt% Y and 1 - 6 wt% Gd, and hot forging and subsequent aging processes to improve creep resistance to be applied to engine parts.
• the patent cannot be applied to wrought products because the product cost significantly increases due to the use of a lot expensive elements, and the coarse intermetallic compounds are incoherent to the matrix.
• an object of the present invention is to provide a wrought magnesium alloy which contains the intermetallic compound coherent to a matrix microstructure and which has a second phase composite microstructure, thereby improving elongation and anisotropy to assure excellent formability and corrosion resistance.
• an alloy consisting of three or more elements is used to activate a slip plane.
• Ilia and nib groups are added together to reduce stacking fault energy and to improve corrosion resistance of the matrix microstructure.
• fine intermetallic compound particles dispersed during extrusion and rolling processes are employed to improve strain hardening ability and formability.
• the present invention provides a wrought magnesium alloy having excellent formability and plating properties, which comprises 0.1 - 1.5 at% group Ilia, 1.0 - 4.0 at% group HIb, 0.35 at% or less of one selected from the group consisting of groups Ha, IVa, Vila, IVb, and a mixture thereof, 1.0 at% or less of group lib, and a balance of Mg and impurities and which thus has a second phase intermetallic compound.
• FIG. 1 illustrates a box sample formed using a wrought magnesium alloy sheet, according to the present invention
• FIG. 2 illustrates a cup-shaped sample formed using the wrought magnesium alloy sheet, according to the present invention
• FIG. 3 illustrates the box sample formed using AZ31 sheet
• FlG. 4 illustrates a microstructure of a material of No. 1 in Table 1, which is cast and then diffusion annealed at 400 °C for 5 hours;
• FlG. 5 illustrates a microstructure of an extruded material according to the present invention, which is annealed
• FlG. 6 illustrates a microstructure of a rolled sheet according to the present invention. Best Mode for Carrying Out the Invention
• the present invention is characterized in that the fine 2nd phase precipitates, which is coherent to matrix microstructure, is formed in the solid solution microstructure having excellent ductility, thereby making grains fine and improving formability.
• the strength of most materials increases. The reason is that a dislocation moves along the specific slip plane in the course of plastic deformation of the metal in such a way that the dislocation does not directly move from one grain to another grain. But direction of dislocation changes its route because of the grain boundary barrier effect. Accordingly, since the grain boundaries act as barriers in the movement of the dislocation, dislocations are pile up at a grain boundary, thereby preventing deformation.
• the high temperature stable phase must be capable of being formed in order to make the grain fine, and desired solid solubility must be assured at high temperatures in order to be coherent to a matrix microstructure. Furthermore, a size difference between elements of a matrix metal and atoms must be about 15 % so as to assure a desirable matrix reinforcement effect.
• Many studies have been made of the effect of an intermetallic compound on a solid solution. Particularly, a matrix reinforcement effect caused by the dispersion of fine intermetallic compound particles is well known in the metallurgy engineering (Mechanical Metallurgy, 2nd ed., George E. Dieter, McGraw-Hill, 1981, pp. 221-227).
• the intermetallic compound has a high melting point and strong bonding strength, thus having high hardness and thermally stable. Because of the finely dispersed second phase particles, these alloys are much more resistant to recrystallization and grain growth than single-phase alloys. However, if the intermetallic compound has a microstructure that is incoherent to the matrix microstructure, it acts as a fracture initiation site and thus has increased strength, but elongation or total ductility is reduced even though the matrix microstructure has ductility.
• the second phase of conventional magnesium alloy is not a high melting point phase in the matrix microstructure.
• the 2 nd phase is low melting point eutectic phase during the solidification instead of the precipitates. Therefore, the eutectic phase is mostly incoherent to the matrix microstructure. It is scarcely an atomic match for the matrix microstructure, thus effectively preventing grain growth or over-aging.
• it reduces the formability of a material or acts as a fracture initiation site. And thus, these type alloys are unsuitable for wrought magnesium alloy. Even though a duplex microstructure is formed, the movement of the dislocation is ineffectively prevented if the second phase is not strong, resulting in undesirably improved anisotropy or strength.
• group Ilia elements employed in the present invention readily form intermetallic compounds having a cubic lattice and thus have high a matrix reinforcement effect and ductility.
• the stacking faults are formed because a stacking order of a closely packed side is changed unlike a normal stacking order, and it is known that they are mostly formed due to plastic deformation. It is difficult to form stacking faults if stacking fault energy is high, and thus, strain hardening required as a press material is not high. Accordingly, since pure aluminum or copper has high stacking fault energy, energy supplied during a room temperature process is mostly converted into heat. Thus, it is difficult to accumulate internal deformations, and a driving force for nucleation is reduced during recrystallization.
• the group HIb and ⁇ ia elements are alloyed with magnesium acting as a matrix element, thereby reducing the stacking fault energy of the intermetallic compound to provide ductility. Additionally, fine second phases promote nucleation during a reheating process to make fine grains. Intermetallic compound particles prevent grain growth at a recrystallization temperature or higher.
• the present inventor came to a conclusion that when a group Ilia is alloyed with magnesium to form a solid solution having low stacking fault energy, when a group IIIb is added to the solid solution to increase a solid-solution strengthening effect, and when a group lib and other miniaturized elements are added to form a structure which contains an intermetallic compound coherent thereto, it is possible to create a material having excellent strain hardening ability, fineness through recrystallization by heat treatment, and improved anisotropy.
• the group Ilia that is, an essential element in the present invention, includes Sc, Y, lanthanides, and actinides.
• Sc, Y, or lanthanides be employed alone or in combination instead of actinides radiating radioactive rays. They are solid-solved in Mg, thus reducing a c/a ratio to increase ductility and reducing the stacking fault energy to increase the driving force for nucleation by recrystallization.
• particles which exist in a form of Mg RE at high temperatures during a solidification process, form prism-shaped plate particles having a HCP structure, that is, a DO lattice structure, such as Mg RE or Mg RE , at about 550 °C through a peritectic transformation.
• RE is an abbreviated form of rare-earth elements belonging to the group ⁇ ia
• the particles have a high reinforcement effect and are coherent to the matrix, and consequently, they do not act as the fracture initiation site.
• the particles may be compacted into a rod, a sphere, or a cube.
• a eutectic phase which is not solid-solved after a diffusion heat-treatment, is finely dispersed during extrusion and rolling processes, thereby preventing grain growth during heat-treatment and acting as a site for nucleation by re- crystallization.
• the amount of the group Ilia is less than 0.1 %, the second phase is formed in an insufficient amount.
• the amount is more than 1.5 %, a fineness effect is saturated, and consequently, elongation is reduced and a production cost increases. This is the reason why that amount is limited.
• the group HIb includes B, Al, Ga, In, and Tl. Since Ga, In, and Tl having a low melting point form a low melting point eutectic phase, it is preferable to employ only Al or a mixture of B and Al. The group nib forms a fine deposit and thus contributes to reinforcement of the matrix. Al is used as a main alloy element. Since B has a low solid solubility to magnesium and forms a high melting point compound, such as B Y, B Y , or B Y , it is employed in conjunction with Al in an amount of 0.010 % or less so as to make the fine grains.
• Al of the group nib is solid-solved in Mg to increase corrosion resistance and to prevent the growth of a dendrite microstructure, thereby making a cast microstructure fine. Furthermore, since Al forms fine cubes, such as Al RE or Al RE during the solidification process and increases ductility of the matrix microstructure, it is possible to produce goods having high strength and excellent ductility.
• the amount of Al is less than 1.0 %, it is difficult to assure the desirable reinforcement effect.
• the amount is more than 4.0 %, since an unstable rod- or plate-shaped Al Mg or Al Mg phase is enlarged in a grain boundary, even though room temperature strength is high, high temperature strength and corrosion resistance are reduced. This is the reason why that amount is limited.
• group Ha % or less of group Ha, group IVa, group Vila, or group IVb is selectively employed alone or in combination, and 1.0 % or less of group lib is employed alone or in combination.
• the group Ha, group IVa, and group Vila are used as a supplemental agent of the group ⁇ ia and group i ⁇ b.
• group Ha it is preferable to use Ca and Sr. Since Be, Ba, and Ra make toxic gases, they can be used only if a special ventilation device is adopted. Ca and Sr are particularly useful to make a fine cast structure in the casting a billet having a diameter of 200 mm or more in the present invention, and form disk- shaped particles, such as (Mg, Al) Ca, thereby improving a reinforcement effect.
• Mn of the group Vila is a cheap alloy element, prevents the formation of Al Mg and Al Mg phases, and promotes the formation of high temperature cubic Al Y to contribute to the fining of the grains and the improvement of corrosion resistance.
• Tc and Re of the group Vila are costly and thus are used in unavoidable cases.
• the group Ha, group IVa, group Vila, and group IVb elements have low solid solubility to magnesium, and thus, if they are excessively added, segregation occurs or coarse particles having high brittleness are formed when a cooling rate is low after a casting process. Accordingly, the amount is limited to 0.35 % or less.
• the group lib includes Zn, Cd, and Hg. Since Hg is toxic to humans when breathing, use of Hg is limited, and it is used in conjunction with an additional protection device.
• Zn and Cd are added alone or in combination, a stacking fault structure is formed in a magnesium matrix microstructure to bring about strain hardening, and Zn and Cd are smoothly solid-solved with the group ⁇ ia and group IIIb elements to promote the formation of cubic particles, such as (Mg, Zn) 5 RE, Zn 6 Mg 2 RE, or (Mg, Zn) RE .
• the excessive amount of Zn and Cd increases gas solid solubility, thereby reducing corrosion resistance or plating workability and brining about the occurrence of hot tear and gravity separation phenomena.
• the amount is limited to 1.0 % or less, and preferably, 0.65 % or less.
• a slab for a magnesium alloy plate is produced through mold casting, Direct Chilled casting, continuous casting, or strip casting processes.
• a mold in which a cavity having a thickness of 30 mm, a width of 250 mm, and a height of 400 mm is formed, is preheated in a heating furnace heated to about 200°C.
• a molten magnesium alloy is poured into the mold at 710 - 760°C, and then machined so as to remove surface defects from a cast product.
• a surface temperature of a rolling roll must be maintained at 50 - 150°C so as to prevent the formation of fine surface cracks caused by the qu enched slab while the slab is in contact with the roll.
• the temperature of the rolling roll is more than 150°C, delamination, in which a portion of a rolling material clings to the rolling roll and then delaminates, occurs during the rolling process, thus roughening the surface of the slab. If the plate is not excessively cooled after the initial coarse rolling, it is possible to conduct the rolling process again without reheating.
• a second rolling process is repeatedly conducted in a reduction ratio of 50 % or less each time until the desired thickness is gained.
• the reduction ratio depends on the capacity of a motor of a rolling mill, a heat emitting state of a plate during a reduction process, elastic deformation of the rolling roll, and flatteness of the plate.
• a second process annealing be repeatedly conducted at 200 - 450°C each time while a duration time is maintained at 1 min/mm or more during the second rolling process.
• a rolled microstructure becomes fine, causing crack resistance.
• annealing is not necessarily conducted every rolling process.
• a duration time is maintained at 1 min/mm or more, which depends on the thickness, strength, and elongation of the plate.
• the annealing temperature is high and the time is long, elongation increases but strength is reduced.
• the annealing temperature is more than 350°C, undesirably, yield strength is significantly reduced.
• the heating temperature is more than 450°C, a free low melting point eutectic phase may be re-melted during the diffusion annealing and thus be separated from the material.
• the duration time and the heating temperature increase to improve workability.
• the diffusion-annealed material is reheated in a heating furnace at 250 - 400°C to be extruded.
• An extruder has an extrusion speed of a maximum of 20 m/min at an extrusion pressure of 850 MPa or more. If the extrusion is conducted at 500 MPa, the extrusion speed is significantly reduced to 3 - 4 m/min.
• a temperature of a container is 300 - 450°C. When the temperature is less than 300°C, many surface cracks are formed. When the temperature is more than 450°C, high temperature cracks or deformations are significantly formed during the extrusion process.
• the container is heated at about 350°C, and an extrusion ratio is typically 10 - 100. Additionally, in the present invention, the material may be wound in a coil form during the extrusion process, and thus, it is possible to conduct reciprocating rolling.
• the final annealing is conducted at 180 - 350°C while a duration time is maintained at 1 min/mm or more, which depends on a thickness, strength, and elongation of the plate.
• a duration time is maintained at 1 min/mm or more, which depends on a thickness, strength, and elongation of the plate.
• the annealing temperature is high and the time is long, elongation increases but strength is reduced.
• the annealing temperature is more than 350°C, undesirably, yield strength is significantly reduced.
• the heat treatment may be implemented using a rapidly heating device, such as a heater, employing a gas nozzle, or an induction heater, instead of the furnace.
• the annealing temperature may deviate from the above range, without departing from the scope and concept of the invention.
• O means that forming is achieved without cracks and local reduction of a thickness
• ⁇ means that cracks are not formed but a thickness deviation locally occurs
• x means that formability is very poor because of the formation of cracks.
• O means a state that plating thickness and adhesion of a plated surface are excellent.
• ⁇ means a state that adhesion is fair, pinhole is not observed, and plating thickness is ununiform.
• x means a state in which the pinholes are observed or plating layer comes off the surface somewhere in the specimen.
• a fine second phase intermetallic compound is dispersed so as to significantly improve the poor formability and corrosion resistance of a conventional magnesium plate.
• the magnesium plate has excellent properties as a structural material, and consequently, it is possible to apply the magnesium plate to structural materials used in portable electronic products, automobiles, or airplanes.

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• 2004
  • 2004-04-06 KR KR1020040023288A patent/KR100605741B1/ko active IP Right Grant
• 2005
  • 2005-03-11 WO PCT/KR2005/000697 patent/WO2006075814A1/en active Application Filing
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