Classifications

• • 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
• • 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
  • B22D17/00—Pressure die casting or injection die casting, i.e. casting in which the metal is forced into a mould under high pressure
  • B22D17/08—Cold chamber machines, i.e. with unheated press chamber into which molten metal is ladled
• • 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
  • B22D21/02—Casting exceedingly oxidisable non-ferrous metals, e.g. in inert atmosphere
  • B22D21/04—Casting aluminium or 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
  • C22C23/02—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
  • C22C23/04—Alloys based on magnesium with zinc or cadmium as the next major constituent

Definitions

• the present invention relates to a process for casting a magnesium alloy consisting of aluminium, zinc and manganese, and the balance being magnesium and unavoidable impurities, the total impurity level being below given % by weight.
• Magnesium-based alloys are widely used as cast parts in automotive industries, and with increasing importance in 3C components (3C: computers, cameras and communications). Magnesium-based alloy cast parts can be produced by conventional casting methods, which include die-casting, sand casting, permanent and semi-permanent mould casting, plaster-mould casting and investment casting.
• Mg-based alloys demonstrate a number of particularly advantageous properties that have prompted an increased demand for magnesium-based alloy cast parts in the automotive industry. These properties include low density, high strength- to-weight ratio, good castability, easy machinability and good damping characteristics.
• Most common magnesium die-casting alloys are such as Mg-Al- alloys or Mg-AI-Zn-alloys with ⁇ 0.5% Mn, mainly Mg-9%AI-1%Zn (designated AZ91 ), Mg-6%AI (AM60) and Mg-5%AI (AM50).
• WO 2006/000022 A1 describes a magnesium-based alloy containing zinc, aluminium, calcium and/or beryllium or optionally manganese by which is provided an attempt to improve the surface finish of cast magnesium components.
• the WO reference is, however, not particularly concerned with the castability of the alloy.
• With the present invention is provided to provide relatively low cost magnesium- based alloy with improved surface finish and improved castability.
• the invention is characterized by an alloy containing 10,00 - 13.00 % by weight of aluminium,
• the alloy is cast in a die the temperature of which is controlled in the range of
• the die is filled in a time which expressed in milliseconds is equal to the product of a number between 2 and 300 multiplied by the average part thickness expressed in millimeter,
• products may be made having excellent surface finish, reasonable ductility and acceptable mechanical properties as well as corrosion properties.
• the aluminium content is between 5.00 and 13.00 % by weight. If less than 10.00% Al is present the Zn content is restricted to 10.00 - 22.00 % by weight. Lower Zn contents give poorer combination of castablity and surface finish. If more than 10.00% Al is present, the range of Zn can be extended to 0.00 - 22.00% still giving satisfactory castability and surface finish.
• composition of the alloy is selected in such a way that the aluminium content is between 10.00 and 12.00
• Alloys with equivalent castability and surface finish can be prepared if the composition of the alloy is such that the aluminium content is between 6.00 and 12.00 % by weight and the Zn-content is between 10.00 and 22.00 % by weight. These alloys offer the advantages of lower casting temperature.
• Figs. 1 A, B each shows schematically cold chamber and hot chamber die casting machines, respectively
• Fig. 2 is a diagram showing the relationship between the solidification rate and the microstructure (grain size and secondary dendrite arm spacing) of cast Mg alloys,
• Fig. 3 is a diagram showing the grain size vs. ductility of Mg alloys
• Fig. 4 is a diagram showing the grain size vs. tensile yield strength of Mg alloys
• Fig. 5 shows a chart from a prior art reference, G. S Foerster; "New developments in magnesium die casting", IMA proceedings 1976 p. 35-39, who split the composition range into a castable -, a brittle - and a hot cracking region
• Fig. 6 shows the Mg-rich corner of the Mg-Al-Zn phase diagram with lines of constant liquidus temperature
• Fig. 7 shows a diagram with the fraction solid (expressed in % by weight) on the horizontal axis versus the temperature ( 0 C) on the vertical axis for three different Mg alloys
• Figs. 8 - 10 show three different Mg alloy components being cast with three different dies
• Fig. 11 is a diagram showing casting defects, average number of cracks and defect ribs on the box die, Fig. 8, plotted as lines of equal number of defects in a diagram, where the Zn content is plotted along the x-axis and the Al content along the y-axis,
• Fig. 12 is a diagram showing surface finish represented as a rating from 1 to 5 on the box die, Fig. 8, plotted as lines of equal rating in a diagram, where the Zn content is plotted along the x-axis and the Al content along the y-axis,
• Fig. 13 is a diagram showing where the z-axis is representing the tensile strength expressed in MPa, while the x and y-axes are representing the Al and Zn contents, respectively, and where the ductility is represented as lines of equal % elongation in the same diagram,
• Fig. 14 is a diagram showing corrosion rates in terms of weight loss being represented as lines of equal corrosion rates (mg/cm 2 /day), where the Zn content is plotted along the y-axis and the Al content along the x-axis.
• Figs. 1A and 1 B there are schematically shown cold chamber and hot chamber die castings machines respectively, each machine has a die 10, 20 provided with a hydraulic clamping system 11 , 21 , respectively.
• Molten metal is introduced into the die by means of a shot cylinder 12, 22 provided with a piston 13, 23, respectively.
• a shot cylinder 12, 22 provided with a piston 13, 23, respectively.
• an auxiliary system for metering of the metal to the horizontal shot cylinder is required.
• the hot chamber machine shown in Fig. 1 B, uses a vertical piston system 12, 23 directly in the molten alloy.
• the steel die 10, 20 is equipped with an oil (or water) cooling system controlling the die temperature in the range of 200- 300 0 C.
• a prerequisite for good quality is a short die filling time to avoid solidification of metal during filling.
• a die filling time in the order of 10 "2 s x average part thickness (mm) is recommended. This is obtained by forcing the alloy through a gate with high speeds typically in the range 30-300 m/s. Plunger velocities up to 10 m/s with sufficiently large diameters are being used to obtain the desired volume flows in the shot cylinder for the short filling times needed.
• Fig. 2 there is shown the relationship between the solidification range and the microstructure of a cast alloy.
• the solidification rate expressed as °C/s and on the left hand vertical scale the secondary dendrite arm spacing expressed in ⁇ m is shown, whereas on the right hand vertical scale the grain diameter expressed in ⁇ m is shown.
• Line 30 indicates the grain size obtained, whereas line 31 is the obtained value for the secondary dendrite arm spacing.
• cooling rate With die casting grain refining is obtained by the cooling rate. As mentioned above cooling rates in the range of 10-1000°C/s are normally achieved. This typically results in grain sizes in the range of 5-100 ⁇ m.
• the castability term describes the ability of an alloy to be cast into a final product with required functionalities and properties. It generally contains 3 categories; (1) the ability to form a part with all desired geometry features and dimensions, (2) the ability to produce a dense part with desired properties, and (3) the effects on die cast tooling, foundry equipment and die casting process efficiency.
• 3C industry extremely thin-walled components for e.g. lap-top and cell phone housings, often less than 0.5 mm, are cast. This puts strong requirements on the ability of the alloy to fill the mould and at the same time provide a smooth and shiny surface.
• AZ91 is the most common alloy for these applications, mainly due to the better castability compared to AM50 and AM60. However, the surfaces of thin walled components of AZ91 are often not satisfactory. Usually, a conversion coating is applied to these components. With a less shiny surface sometimes including areas with segregation of elements, multiple layers of coating has to be used. Generally, the better surface quality, the less coating is needed.
• the Mg-Al-Zn alloys with the Al and Zn content as specified in the present invention will start to solidify around 600 0 C, depending on the Al and Zn content. This is indicated in Fig. 6 where lines of constant liquidus temperature in the Mg-comer of the Mg-Al-Zn phase diagram are shown. As a result, the casting temperature, typically 7O 0 C above the liquidus, can be significantly lower than for the conventional AM50, AM60 and AZ91 alloys. Due to the fact that the eutectic Mg 17 AI 12 phase melts at around 420 0 C, the conventional Mg-Al alloys like AM50, AM60 and AZ91 will have a solidification range of nearly 200 0 C as shown in the annexed Fig.
• the alloy will solidify completely at temperatures significantly lower than 42O 0 C as is the case for the conventional alloys AM50, AM60 and AZ91.
• Mg-Al die casting alloys improves the die castability. This is due to the fact that Mg-Al alloys have a wide solidification range, which makes them inherently difficult to cast unless a sufficiently large amount of eutectic is present at the end of solidification. This can explain the good castability of AZ91 D consistent with the cooling curves shown in Fig. 7. With the large amount of Zn in addition to Al in the present alloys there is an even larger amount of (modified) eutectic present at the end of solidification, explaining the improved castability of the invented Mg-Al-Zn alloys.
• Magnesium alloys tend to ignite and oxidize (burn) in the molten state unless protected by cover gases such as SF 6 and dry air with or without CO 2 , or SO 2 and dry air. The oxidation aggravates with increasing temperature. Usually, small amounts of beryllium (10-15 ppm by weight) are also added to reduce the oxidation. Beryllium is known to form toxic substances and should be used with care. Especially the treatment of dross and sludge from the cleaning of crucibles requires considerable safety precautions due to an enrichment of Be- compounds in dross/sludge.
• One advantage of the present invention is that the alloy can be cast at temperatures significantly lower than for conventional alloys, thereby reducing the need for cover gases. For the same reason, beryllium additions can be kept at a minimum.
• the lower casting temperatures compared to conventional alloys also offer significant advantages as the lifetime of the metering system, the shot cylinder and the die will all be improved. With hot chamber die casting in particular, the lifetime of the gooseneck will be significantly extended.
• the alloys with lower casting temperature also have a potential for reducing the cycle time, thereby improving the productivity of the die casting operation.
• Test-bars of 6 mm diameter in accordance to ASTM B557M have been made, and the following test conditions have been used: • 10 kN lnstron test machine
• Example 3 Surface finish represented as a rating from 1 to 5 is plotted in Fig 12 as lines of equal rating in a diagram where the Zn content is plotted along the x-axis and the Al content along the y-axis. It is seen that the best regions in terms of surface finish rating are found with Al >11 % by weight and Zn ⁇ 3 % by weight; the lower Zn the better. Also, a region roughly defined by 8-12 % Al by weight and >10 % Zn by weight provides alloys with superior surface finish.
• Fig. 13 For a number of compositions the strength and elongation have been measured at room temperature. The results are shown in Fig. 13.
• the z-axis is representing the tensile strength expressed in MPa
• the x and y-axes are representing the Al and Zn contents, respectively.
• the ductility is represented as lines of equal elongation in the same diagram.
• tensile strength expressed in MPa increases with increasing content of alloying elements.
• the effect of increasing Al (% by weight) is significantly greater than the effect of Zn.
• Fig. 13 also indicates that the ductility in terms of % elongation decreases with increasing content of alloying elements.
• the line indicating 3 % elongation extends almost linearly from 12 % Al by weight and 0 % Zn to 0 % Al and 18 % Zn by weight.
• corrosion rates in terms of weight loss is represented as lines of equal corrosion rates (mg/cm 2 /day), in a diagram where the Zn content is plotted along the y-axis and the Al content along the x-axis. It is seen that for Zn contents lower than approximately 8 % by weight, the corrosion rates decrease with increasing Al content and are practically independent of the Zn content, whereas for Zn contents above approximately 12 % by weight the corrosion rates increases slightly with increasing Zn content and are practically independent of the Al content. The region defined by 8-12 % by weight of Zn represents a transition.
• the corrosion rate decreases from about 0.09 mg/cm 2 /day at 4 % Al by weight to approximately 0.03 mg/cm 2 /day at 9% Al by weight.
• the corrosion rate increases to 0.05 mg/cm 2 /day at 8% Zn by weight and 0.11 mg/cm 2 /day at 14% Zn by weight.

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• Engineering & Computer Science (AREA)
• Mechanical Engineering (AREA)
• Chemical & Material Sciences (AREA)
• Materials Engineering (AREA)
• Metallurgy (AREA)
• Organic Chemistry (AREA)
• Molds, Cores, And Manufacturing Methods Thereof (AREA)
• Continuous Casting (AREA)
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• Powder Metallurgy (AREA)

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• 2006
  • 2006-08-18 NO NO20063703A patent/NO20063703L/no not_active Application Discontinuation
• 2007
  • 2007-08-13 TW TW096129804A patent/TW200813237A/zh unknown
  • 2007-08-16 CN CN2007800307272A patent/CN101505891B/zh not_active Expired - Fee Related
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  • 2007-08-16 JP JP2009524572A patent/JP2010501721A/ja active Pending
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  • 2007-08-16 WO PCT/NO2007/000284 patent/WO2008020763A1/en active Application Filing
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• 2009
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