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
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D17/00—Pressure die casting or injection die casting, i.e. casting in which the metal is forced into a mould under high pressure
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B22—CASTING; POWDER METALLURGY
  • B22D—CASTING OF METALS; CASTING OF OTHER SUBSTANCES BY THE SAME PROCESSES OR DEVICES
  • B22D21/00—Casting non-ferrous metals or metallic compounds so far as their metallurgical properties are of importance for the casting procedure; Selection of compositions therefor
  • B22D21/002—Castings of light metals
  • B22D21/007—Castings of light metals with low melting point, e.g. Al 659 degrees C, Mg 650 degrees C

Definitions

• the present invention relates to a method of preparing a heat-resistant magnesium alloy member having an excellent molding property and an excellent elongation property while keeping creep resistance property.
• Magnesium alloy is the most low density one of the metal materials which are in practically use at present, and is strongly expected as a lightweight material for automobiles in future.
• the magnesium alloy which is most popularly used at present is Mg-Al-Zn-Mn alloy (e.g., AZ91D alloy), and as it has a high strength at a room temperature and a good corrosion resistance, it is applied to transmission cases for an automobile, cylinder head covers, and the like.
• Mg-Al-Zn-Mn alloy e.g., AZ91D alloy
• Mg-Al-Zn-Mn alloy e.g., AZ91D alloy
• it has such defects that, at a temperature range exceeding 120°C, it begins to show loss of strength characteristics, and especially becomes inferior in creep resistance, leading to a problem of yielding of bearing surface of the screw tightening part on the level of the packaged product.
• Mg-Al-Si AS41 magnesium alloy As an aluminum alloy having an improved heat-resistance, there is used Mg-Al-Si AS41 magnesium alloy. However, though said alloy shows better creep resistance than the above AZ91D, it shows insufficient characteristics in the neighborhood of 150°C of the use temperature, and moreover, as it shows low tensile strength characteristics at both room temperature and high temperature, it is required to be of thick wall to secure the required strength, thereby providing a problem of lowering the weight lightening effect due to magnesium materials.
• alloys such as QE22 with addition of silver or rare earth metals to improve a heat resistance thereof, but they have defects of being expensive and not suited to die-cast due to a poor casting property.
• JP-A-8 041 576 discloses a high strength magnesium alloy excellent in tensile strength and creep resistance having a composition consisting of 1.0 - 4.0 wt.-% Al, 1.0-8.0 wt.-% rare earth element, 0.3 - 1.3 wt.-% Ca, 0.1 - 2.0 wt.-% Mn, the balance of magnesium and inevitable impurities.
• JP-A-7 331 375 discloses a heat resistant magnesium alloy for casting having a composition consisting of 1-3.5 wt.-% Al, 0.25 - 3.5 wt.-% Zn, 0.5 - 4.0 wt.-% rare earth component, 0.1-1.0 wt.-% Mn, 0.1 - 1.0 wt.-% Ca, the balance of Mg and inevitable impurities.
• JP-A-7 278 717 discloses a member of a magnesium alloy excellent in creep resistance in the pressurized part having a composition including 1.5 to 10.0 wt.-% aluminum, less than 2.5 wt.-% rare earth (RE) component(s), 0.2 to 5.5 wt.-% calcium and the balance of magnesium with impurities.
• JP-A-7 118 785 discloses a magnesium alloy for casting and non porosity magnesium alloy castings therefrom containing 0.5 - 10 wt.-% calcium.
• an object of the present invention is to provide a molding method for preparing a heat-resistant magnesium alloy member having excellent molding property and elongation while maintaining the physical properties, especially creep resistance, suited to the engine parts of automobiles and the like, in place of conventional die-cast methods.
• the present inventors have found out that, in the Al-Ca magnesium alloy, when a semi-solid molding method of injection molding under the state of solid phase and liquid phase being present in mixture is applied in place of the die-cast method, the seizure of metal mold can be prevented, and also an excellent strength can be imparted to the molded member.
• the addition amount of aluminum in order to maintain the state of presence in mixture of solid phase and liquid phase, it is necessary to increase the addition amount of aluminum as large as possible.
• a method of molding a magnesium alloy molding member comprising 2 to 6% by weight of aluminum and 0.5 to 4% by weight of calcium and contains no more than 0.15 % by weight of Sr, and the balance of magnesium and unavoidable impurities which may contain no more than 2 % by weight of at least one element selected from the group consisting of zinc, manganese, zirconium and silicon, and/or no more than 4 % by weight of rare earth metals, wherein a Ca/Al ratio is 0.49 to 0.8, to have an excellent anti-creep property, molding property, and elongation.
• the magnesium alloy in order to obtain solid phase dissolution in magnesium, to exhibit age-hardening, and to elevate mechanical strength, it has been understood to be preferable to add 2 - 10% by weight of aluminum. While it is necessary in the present invention to add more than 2% by weight of aluminum, when the amount of addition exceeds 6% by weight, it has been found that the elongation is lowered even if the semi-solid injection molding would be carried out. Accordingly, in order to obtain the designed effect while carrying out the semi-solid injection molding, it has been found that the addition amount should be limited to no more than 6% by weight.
• Strontium is used as a micronizing agent in the casting of magnesium, and as it can display the micronizing effect in solid phase in the semi-solid injection molding of the present invention, it is preferably added.
• the suitable addition amount is no more than 0.15% by weight.
• the above molding member shows the crystal particle size of no more than 30 ⁇ m with the tensile strength of 180 Mpa (298°K; ref. Fig. 9) or more, and excellent creep resistance of the minimum creep rate of no more than 4 x 10 -10 /S under the test temperature of 150°C and the test load of 50 MPa (according to JIS Z 2271 "method of tensile creep test of metal material"). Accordingly, it is suitable for the transmission part or engine part for automobiles. Especially, when the Ca/Al ratio is 0.49 to 0.6, the molding member shows a more excellent creep resistance.
• the alloy material in case of molding by a semi-solid injection molding method, it has been found that the material in the form of metal particles or pellets into which internal strain is introduced is effective for micronizing the crystals (ref. Fig. 10).
• a cutting method is advantageous costwise.
• the present invention is to provide a method for molding a heat-resistant magnesium alloy member characterized by carrying out a semi-solid injection molding, while maintaining an excellent creep resistance property with having an excellent molding property and elongation.
• the die-cast method is in general to make injection into the metal mold at a temperature of 30 - 50°C above a melting temperature
• injection can be made at a temperature higher than the solidus temperature of the alloy and lower than the liquidus temperature, and accordingly the injection temperature is lowered by at least 30 - 60°C, so that the seizure to the metal mold can be prevented.
• the solid phase ratio in the semi-solid state is preferably no more than 30%.
• the solid phase ratio in the semi-solid state is preferably no more than 30%.
• the above magnesium alloy may further contain no more than 2% by weight of at least one element selected from the group consisting of zinc, manganese, zirconium, and silicon, and/or no more than 4% by weight of a rare earth metal (e.g., yttrium, neodymium, lanthanum, cerium, misch metal). These are to improve the strength or high temperature strength of the above magnesium alloy effectively in the range no more than the upper limit thereof.
• a rare earth metal e.g., yttrium, neodymium, lanthanum, cerium, misch metal.
• Fig. 1 is a schematic diagram showing the constitution of the molding machine to be used for the semi-solid molding process and injection molding process according to the present invention.
• Fig. 2 is a graph for making comparison of the creep characteristics of various magnesium alloy molding members.
• Fig. 3 is a graph to show the relations between the Ca/Al ratio and the elongation at room temperature in various magnesium alloy molding members.
• Fig. 4 is a schematic diagram showing a metal mold for testing casting cracks.
• Fig. 5 is a graph showing the relation between the solid phase diameter and the staying time.
• Fig. 6 is a graph showing the minimum creep strain rates of various magnesium alloy molding members.
• Fig. 7 is a schematic diagram showing the metal mold for evaluating the flowing properties of various magnesium alloys.
• Fig. 8 is a graph showing the relations between the solid phase ratio and the flowing length in the alloy composition in Example 2 measured by using a metal mold of Fig. 7.
• Fig. 9 is a graph showing the relations between the average crystal particle size and the tensile strength of the member molded from the alloy composition of Example 3.
• Fig. 10 is a schematic diagram showing the solid phase growth stages in the cases of using the metal particles having no work strain and those having the work strain.
• Fig. 1 there is shown the whole constitution of the molding machine 1 to be used for the semi-solid molding method according to the present invention.
• the material 3 of magnesium alloy metal particles or pellets (more than 3 mm in diameter) manufactured by the method of cutting or the like is charged into the hopper 8 in the drawing.
• the material 3 is supplied to the cylinder 4 from the hopper 8 through the inlet 7 of argon atmosphere.
• the material 3 is heated while being sent forward by the screw 2.
• This heating zone is shown by the mark 10.
• the magnesium alloy material 3 shows a molten state, but at a level lower than the liquidus temperature the material becomes semi-solid condition in which the solid phase and the liquid phase are present in mixture, as illustrated.
• the magnesium alloy which is in a semi-solid condition its shearing force acts to separate the solid phase finely as illustrated by agitation by the screw rotation.
• the screw 2 is pushed forward with the rear high speed injection mechanism 5
• the molten material in which the solid phase has been finely cut under the semi-solid state is injected at high speed from the nozzle 9 as illustrated and filled in the metal mold 6.
• the contents in the metal mold are held under pressure until solidification, and thereafter the metal mold is opened to take out the molding product.
• the semi-solid molding temperature was varied in the metal mold for evaluating flowing property as shown in Fig. 7, the molten material was introduced in the illustrated direction, and its flowing property was evaluated. The results are shown in Fig. 8. From the results it can be seen that, when the solid phase rate exceeds 30%, the flow length is sharply lowered, and as this flow gives effect on the particle size of the texture crystals of the molding material, desirably the molding is made under the solid phase condition of no more than 30% in the semi-solid molding method.
• the magnesium alloy material is used in the form of the metal particles or pellets.
• the metal particles form the nuclei of recrystallization shortly after the heating, and increase the solid phase diameter. Therefore, when comparison is made between the case of using the metal particles having no work strain and that of using the metal particles having work strain, it can be understood that the growth rates of the solid phase are different as shown in Fig. 10, and the latter is superior to the former in the point of micronization of the crystal particle size of the molding member.
• the present invention by carrying out semi-solid molding at a temperature lower than the liquidus level, the problems of hot crack and seizure to the metal mold which had been remarkable in the conventional die-cast process are dissolved, and on the other hand, the strength at room temperature and high temperature along with elongation equivalent to or higher than those of the conventional process can be retained.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Manufacture Of Alloys Or Alloy Compounds (AREA)
• Forging (AREA)
• Injection Moulding Of Plastics Or The Like (AREA)

Applications Claiming Priority (3)

Publications (2)

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• 1996
  • 1996-04-04 JP JP08283296A patent/JP3415987B2/ja not_active Expired - Fee Related
• 1997
  • 1997-04-04 EP EP97105641A patent/EP0799901B1/en not_active Expired - Lifetime
  • 1997-04-04 CN CN971050007A patent/CN1065003C/zh not_active Expired - Fee Related
  • 1997-04-04 DE DE69706737T patent/DE69706737T2/de not_active Expired - Lifetime
  • 1997-04-04 KR KR1019970012613A patent/KR970070224A/ko not_active Application Discontinuation
• 2001
  • 2001-09-12 US US09/949,621 patent/US20020020475A1/en not_active Abandoned

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