MXPA06015208A - Die cast magnesium alloy. - Google Patents

Abstract

A magnesium based alloy contains zinc, aluminium, calcium and/or beryllium, optionally manganese, and the balance magnesium except for incidental impurities. The zinc and aluminium contents fall within a quadrangle defined by lines AB,BC,CD and DA and the calcium and beryllium contents fall within a quadrangle defined by lines EF, FG, GH and HE wherein: A is 10% Zn - 2.5% Al, B is 10% Zn - 5% Al, C is 13% Zn - 6.4% Al, D is 19% Zn - 2.5 % Al, E is 0.01% Ca - 0% Be, F is 1% Ca - 0% Be, G is 0% Ca - 0.0025% Be, and H is 0% Ca - 0.0001% Be.

Description

MAGNESIUM MIXED PRESSURE MAGNESIUM FIELD OF THE INVENTION The present invention relates to magnesium / zinc / aluminum alloys (Mg-Zn-Al) which contain small amounts of calcium or beryllium.

BACKGROUND OF THE INVENTION Due to the excellent proportions of tenacity with respect to weight, magnesium alloys are well recognized as commercially desirable materials. The most commonly used magnesium alloy is AZ91, which contains approximately 90% magnesium, 9% aluminum and 1% zinc. On a weight basis, zinc is about 65% of the magnesium price and therefore magnesium alloys with an increased zinc content may be desirable provided they show commercially satisfactory properties. A serious disadvantage of using magnesium alloys is the danger of ignition of the molten alloy. Magnesium alloys which are sufficiently resistant to oxidation to eliminate the need for a cover of protective gases or the like, when the molten alloy is exposed to air, may be advantageous.

The document of E.U.A. 2380200 (Stroup et al), which was issued in 1945, relates to magnesium-based alloys and methods to avoid the oxidation of magnesium and magnesium-based alloys. The patent states that: "A general objective is the provision of improvements which mitigate the difficulties arising from the propensity of magnesium to oxidize when in contact with air, moisture or other oxygen-containing media. A metal such as magnesium depends not only on its essential properties or those which can be imparted by it by alloying with smaller amounts of other metals but also by its ease with which the metal, or its alloys, can be returned to melt, pressure melt, work or otherwise conform to the various conditions and forms necessary for end use Magnesium's susceptibility to destructive oxidation when it is in the molten state is great Under many conditions, normal to handling of other molten metals, the molten magnesium is burned or otherwise reverted to the oxide in a very substantial part. When solid, the magnesium base alloys are oxidized, under certain conditions to a comparatively severe degree. Since the extensive handling of magnesium and magnesium-based alloys in the molten condition is necessary preliminary to operations designed to shape or work the metal, the difficulties that arise from this pronounced tendency to rust are found in almost any case and are universal. to the magnesium industry. "" Faced with this problem, the industry has devised methods and devices by which to protect molten magnesium and magnesium base alloys from contact with air and moisture or other detrimental means during operations. or manufacturing One such method is to wrap the metal in a protective gas. Another is to constantly protect your exposed surfaces with a flow of salt. Other more elaborate methods and devices are frequently needed. Other means have also been sought to minimize the tendency of magnesium and magnesium-based alloys to be oxidized and thus reduce the need for costly protective measures mentioned above. Calcium has been mixed with magnesium for this purpose and although magnesium or a magnesium-based alloy combined in this way does not oxidize as badly as before, the total effect is not enough to do more than supplement the usual protective measures. Better results have been obtained when beryllium has been added to magnesium or magnesium-based alloys, it has been found that the effect of beryllium in minimizing the oxidation of magnesium is much greater than that of the corresponding amount of calcium. "The US patent No. 4543234 (Foerster) is related to Mg-Al-Zn-Si-Mn alloys containing 0.0025 - 0.0125% dissolved beryllium "to inhibit burning, where the amount of beryllium increases as the oxygen content of the atmosphere increases. "US 4543234 also notes that" a beryllium content of the order of 0.001 percent is considered inadequate for the purpose of inhibiting excessive oxidation. of molten magnesium. "A document entitled" Characterization of the oxidation surface layer on non-fuel Ca-bearing Mg melts "by M. Sakamoto, S. Akiyama and K. Ogi, presented at the 4th Asian Foundry Congress, 27th - 31st October 1996, reports that the ignition temperatures of calcium-containing magnesium-based alloys (see Figure 2 in that document) .The measured ignition temperatures vary considerably when the same alloy composition is repeated. these repeated sequences, alloys with 0.5% or more of calcium do not ignite until the melting point of the alloy is exceeded; however, cases are shown where ignition occurs below the melting point for calcium concentrations as high as 4%. The patent of E.U.A. No. 5855697 (Luo et al) relates to a magnesium alloy having properties superior to elevated temperature and is not related to oxidation suppression. The document of E.U.A. 5855697 indicates that it is known that the addition of calcium improves the resistance at high temperature and the resistance to permanent deformation by fatigue and the content of calcium of 0.2% by weight and greater are desirable. It is further noted that said calcium additions severely deteriorate the susceptibility to die casting which renders the alloy unacceptable to be subjected to die casting by a conventional die casting process. The document of E.U.A. 5855697 describes that the susceptibility to die casting of a magnesium-aluminum-calcium alloy can be restored by zinc inclusion. A zinc content of from about 6 to about 12% by weight, more preferably from about 6 to about 10% by weight, is described as the "upper limit of the zinc range which is set at about 12% by weight, of more preferably about 10% by weight, so that the density of the alloy remains low ". It is claimed that the presence of zinc to enable calcium to be "added in amounts of up to 2% by weight, preferably up to 1.5% by weight, in order that the alloy obtains the maximum resistance to fatigue permanent deformation. and that at the same time maintains a good susceptibility for die-casting. " The document of E.U.A. 5855697 exemplifies the alloys that are included in the following. Accordingly, the document of E.U.A. 5855697 does not exemplify an alloy containing more than 8.15% Zn. • Mg - 5% Al - 8% Zn with Ca contents varying between 0 and 2% (see figures 2 and 3 of US document 5855697) • Mg - 5% Al - 1% Zn with Ca contents varying between 0 and 2% (see figures 2 and 3 of US 5855697) • Mg - 4.57% Al - 8.15% Zn - 0.23% Ca - 0.25% Mn (see table 1 of US document 5855697) • Mg - 4.74% Al - 8.12% Zn - 0.59% Ca - 0.25% Mn (see table 1 of document US 5855697) • Mg - 4.67% Al - 8.12% Zn - 1.17% Ca - 0.27% Mn (see table 1 of the document of USA 5855697) BRIEF DESCRIPTION OF THE INVENTION The present invention provides an alloy consisting of: zinc (Zn) and aluminum (Al) in quantities which are within a square defined by lines AB, BC, CD and DA where: A is 10% Zn - 2.5% Al, B is 10% Zn - 5% Al, C is 13% Zn - 6.4% Al, and D is 19% Zn - 2.5% Al; calcium (Ca) or beryllium (Be) in quantities which are within a square defined by the lines EF, FG, GH and HE, where: E is 0.01% Ca - 0% Be, F is 1% of Ca - 0% of Be, G is 0% of Ca - 0.0025% of Be, and H is 0% of Ca - 0.0001% of Be optionally Mn; and the rest is Mg, except for incidental impurities. Unless stated otherwise, all percentages in this document are% by weight. The square defined by lines AB, BC, CD and DA is illustrated in Figure 1, which is a graph of aluminum content versus zinc. The square defined by the lines EF, FG, GH and HE are illustrated in figure 2, which is a graph of the content of beryllium versus calcium. All alloys of the present invention contain a minimum of 10% zinc, preferably more than 11% zinc, more preferably more than 12% zinc, much more preferably approximately 12-14% zinc and much more preferably about 12-13% zinc. More surprisingly, the present inventor has determined that said zinc additions suppress the ignition of the alloy in the molten state in the absence of alkaline earth elements such as beryllium or calcium. Without wishing to be bound by any theory, it is considered that the suppression of ignition is a consequence of the magnesium and zinc vapor pressures and the amount of zinc present in the alloys. The vapor pressures of zinc and magnesium above the molten alloy can be calculated using information from the document entitled "Vapor Composition and Activities in Mg-Zn Liquid Alloy at 293K" by KT Jacob, S. Srikanth and Y. Waseda in Thermochimica Acta , 1988, vol 130, pages 193-203. The ratio of the vapor pressure of the zinc to the vapor pressure of the magnesium increases rapidly as the amount of zinc in the molten alloy increases. It is estimated that a molten alloy containing 10% by weight of zinc and 90% by weight of magnesium produces a vapor containing 22% by weight of zinc and 78% by weight of magnesium. Without wishing to join any theory, it is considered that zinc vapor interferes with the ignition of magnesium vapor. Although molten alloys containing more than 10% zinc resist ignition, they tend to form a black end layer on the surface of a solidified sample. The addition of a small amount of calcium or a small amount of beryllium has been found to be sufficient to result in a surface with a bright appearance when solidified. It has been found that an amount as small as 0.01% calcium or as small as 0.0001% beryllium in combination with the zinc and aluminum contents, according to the present invention, is sufficient to obtain this effect. Without wishing to join any theory, it is considered that the appearance of a shiny surface is a consequence of an enrichment in the calcium or beryllium content of the - layer of oxide that forms on the surface of the melt. When present, the calcium content is preferably 0.01-0.5%, more preferably 0. 01-0.3%, more preferably 0.02-0.3%, more preferably 0.05-0.3%, more preferably 0.05-0.2%, more preferably 0.05-0.15% and much more preferably approximately 0.1%. Calcium contents exceeding 1% are undesirable because they have been found to decrease the mechanical properties of the alloys and cause melting of cast iron when subjected to pressure casting. When present, the beryllium content is preferably 0.0002-0.0025%, more preferably 0.0002-0.002%, more preferably 0.0005-0.002%, more preferably 0.0005-0.015%, more preferably 0.0005-0.001% and much more preferably 0.0008%. Beryllium contents exceeding 0.0025% are unnecessary in order to obtain the desired effect. In view of the toxicity of beryllium it is therefore desirable to minimize its use by keeping the beryllium content below this concentration. Manganese (Mn) is an optional component of the alloys which can be included if there is a requirement for iron (Fe) removal. When the Mn is a component, it is preferably present in amounts less than 1%, more preferably less than 0.75%, more preferably 0.1-0.5%, more preferably 0.2-0.4% and much more preferably about 0.3% . Other elements may also form optional components of the alloys with the proviso that they do not impair the commercially significant properties of the alloys. The presence of iron reduces the resistance to corrosion. Preferably, the alloys of the present invention contain less than 100 ppm iron, more preferably less than 40 ppm iron and much more preferably substantially no iron. The present inventor has determined that the corrosion resistance decreases as the aluminum content decreases. All the alloys of the present invention contain a minimum of 2.5% aluminum. Preferably, the alloys of the present invention contain 2.5-5% aluminum, more preferably about 3-4.5% aluminum and much more preferably about 3.5-4% aluminum. The present inventor has also determined that the brittleness increases with the zinc rich and aluminum rich side of the CD line. The presence of nickel (Ni) reduces the resistance to corrosion. Preferably, the alloys of the present invention contain less than 25 ppm of nickel, more preferably less than 10 ppm of nickel and much more preferably substantially free of nickel. The presence of silicon (Si) reduces the resistance to corrosion and mechanical properties. Preferably, the alloys of the present invention contain less than 0.1% silicon, more preferably less than 0.08% silicon, and much more preferably substantially free of silicon. In addition to the ignition resistance when melted, various preferred embodiments of the present invention exhibit one or more additional commercially desirable properties such as recyclability, smelting susceptibility, hot fracture resistance, corrosion resistance, resistance to permanent deformation. by fatigue, low sound damping coefficients and a good surface finish. A significant trade impediment to the use of magnesium alloys is the waste resulting from the difficulty of recycling what are called "leftovers" which include sprues, biscuits, etc., from die casting. Typically, 30-70% of die casting consists of feeders and biscuits that need to be recycled. The difficulties in recycling magnesium alloys are generally attributed to a significant amount of surface oxide resulting in high losses of molten material in the form of slag and debris. Generally, the recycling is carried out in a separate operation in order to allow the separation of the oxides without being entrained in the molten material and to be included in subsequent pressure castings. Surprisingly, the present inventor has determined that at least the preferred embodiments of the alloys of the present invention have an improved susceptibility to recycling. The lugs and other scrapings from die-castings to alloys of the present invention have been successfully returned directly to the melts without any refinement or purification. Without wishing to join any theory, it is considered that the susceptibility has recycled is closely related to the modification of the oxidation behavior which generates the suppression of ignition of the molten alloys.

EXAMPLES Example 1 Magnesium alloys without addition of beryllium and with varying amounts of aluminum, zinc and calcium are melted at 700 ° C under a protective atmosphere containing sulfur hexafluoride (SF6), then poured into a mold in air . The upper surface of the resulting melt is left exposed to the air. Four different types of behavior were observed, based on the composition. Behavior 1 - the surface of the foundry initially turned black and then fire, as illustrated in Figure 3. Behavior 2 - the surface turned black but did not catch fire, as illustrated in Figure 4. Behavior 3 - the The surface was initially bright and then burned, as illustrated in Figure 5. Behavior 4 - the surface remained bright without ignition, as illustrated in Figure 6. Table 1 shows the list of observed behavior for a range of different alloys . The addition of more than 10% zinc is sufficient to prevent ignition and results in a blackened surface. Additions of calcium without zinc produce a shiny surface, but 0.8% calcium is required to prevent ignition. The addition of calcium to alloys with sufficient zinc prevents fire by converting the surface to a shiny appearance with as little as 0.05% calcium which produces a partially glossy surface. Increases in calcium content generate a progressive decrease in the amount of blackening. At 0.4% calcium, no blackening is observed. Alloys containing 10% zinc (see Table 1) turned black and then burned, while alloys with higher zinc contents did not ignite. The alloys were deliberately poured at high temperature (700 ° C) to remove the low temperature as a possible reason for the absence of ignition. It was anticipated that commercial smelting may occur at a temperature in the order of 30-40 ° C lower with a consequent decrease in susceptibility to catch fire.

TABLE 1 Behavior of molten magnesium alloys exposed to air EXAMPLE 2 Additional melts are prepared and poured into a mold in the same manner as described above, in Example 1. A metal scraper is then applied to the metal surface after pouring but while the metal is l molten. Figure 7 illustrates the behavior of pure magnesium which oxidizes so rapidly that it is not possible to expose the bright metal. Figure 8 illustrates the behavior of an alloy of Mg-5% Zn which also oxidizes rapidly. The shiny metal can be exposed but only for a small fraction of a second. Figure 9 illustrates the behavior of an Mg-10% Zn alloy. The tendency to oxidation is greatly reduced as indicated by the absence of "cauliflower-like" growths around the perimeter and the increase in exposed bright metal.

Figures 10 and 11 illustrate the behavior of the alloys of Mg-15% Zn and Mg-20% Zn, respectively. In both cases, it is relatively easy to expose the shiny metal which requires a few seconds for it to oxidize again. None formed the "cauliflower-like" growths. An additional series of alloys is produced, all with 0.1% calcium and varying amounts of zinc. Figures 12, 13 and 14 show the appearance of the alloys immediately after pouring (figures 12a, 13a, and 14a), after a short period (approximately 1 minute) afterwards (figures 12b, 13b and 14b). Figures 12a and 12b show the behavior of a zinc-free alloy. After initially exhibiting a bright appearance, this alloy develops "cauliflower-like" growths and then ignites. Figures 13a and 13b show the behavior of an alloy containing 5% zinc. This alloy also develops "cauliflower-like" growths and ignites but at a lower rate compared to the zinc-free alloy of Figure 12. Figures 14a and 14b show the behavior of an alloy with 10% zinc. In this alloy, both "cauliflower-like" growths and ignition are suppressed. The final appearance after the sample is allowed to cool in air to room temperature does not change from what is obtained in Figure 14b.

EXAMPLE 3 Additional melts are prepared and poured into a mold, in the same manner as described above in Example 1. The melts contain 13% zinc, 3.6% aluminum and varying amounts of beryllium and calcium. The calcium and beryllium contents of these alloys are given in Table 2. Alloys 1 and 6 have no calcium and the alloys 1-4 do not have beryllium. The final appearance of the melts is shown in Figure 15. All of the alloys containing certain calcium or beryllium solidify with a bright layer. Alloy 1, which is free of both calcium and beryllium, solidifies with a blackened layer.

TABLE 2 Magnesium Alloy Compositions It is clearly understood that although the prior art publications refer to the present, these references do not constitute the admission that some of these documents form part of the general knowledge common in the art in Australia or in any other country.

Claims (16)

1. Alloy consisting of zinc (Zn) and aluminum (Al) in quantities which are within a square defined by lines AB, BC, CD and DA where: A is 10% Zn - 2.5% Al, B it is 10% Zn - 5% Al, C is 13% Zn - 6.4% Al, and D is 19% Zn - 2.5% Al; calcium (Ca) or beryllium (Be) in quantities which are within a square defined by the lines EF, FG, GH and HE, where: E is 0.01% Ca - 0% Be, F is 1% of Ca - 0% of Be, G is 0% of Ca - 0.0025% of Be, and Hes 0% of Ca - 0.0001% of Be optionally manganese; and the rest is magnesium, except for incidental impurities.

2. Alloy as described in claim 1, which contains more than 11% zinc.

3. Alloy as described in claim 2, which contains 12-14% zinc.

4. Alloy as described in any of the preceding claims, which contains 2.5-5% aluminum.

5. Alloy as described in claim 4, which contains 3-4.5% aluminum.

6. Alloy as described in any of the preceding claims which contains 0.01-0.5% calcium.

7. Alloy as described in claim 6, which contains 0.05-0.2% calcium.

8. Alloy as described in any of the preceding claims that contains 0.0002-0.002% beryllium.

9. Alloy as described in claim 8, which contains 0.0005-0.001% beryllium.

10. Alloy as described in any of the preceding claims which contains manganese in an amount less than 1%.

11. Alloy as described in claim 10, which contains 0.1-0.5% manganese.

12. Magnesium-based alloy consisting of: 11 - 13.5% zinc, 3 - 4.5% aluminum, 0.05 - 0.15% calcium, 0.0005 - 0.001% beryllium, optionally manganese in an amount less than 0.5% and the rest It is magnesium except for incidental impurities.

13. Magnesium-based alloy consisting of: 11.5 - 13.5% zinc, 3 - 4.5% aluminum, optionally manganese in an amount less than 0.5% either 0.05 - 0.15% calcium, or 0.0005 - 0.001% beryllium, and the rest is magnesium except for incidental impurities.

14. Magnesium-based alloy consisting of: 11.5 - 13.5% zinc, 3 - 4.5% aluminum, 0.2 - 0.4% manganese, 0.05 - 0.15% calcium, 0.0005 - 0.001% beryllium, and ^ the rest is magnesium except for incidental impurities.

15. Magnesium based alloy consisting of: 11.5 - 13.5% zinc, 3 - 4.5% aluminum, 0.2 - 0.4% manganese, either 0.05 - 0.15% calcium, 0.0005 - 0.001% beryllium, and the rest It is magnesium except for incidental impurities.

16. Alloy as described in any of claims 12-15 containing 12-13% zinc, 3.5-4% aluminum, less than 0.08% silicon, less than 40 ppm iron and less than 10 ppm of nickel.