NO860423L - PRESS RELEASE AND PROCEDURE OF MANUFACTURING THEREOF. - Google Patents

Description

The present invention generally relates to magnesium alloys containing beryllium which are sufficiently resistant to oxidation in the molten state to avoid the need for the use of a protective slag coating to prevent unreasonable melt oxidation or burning when exposed to oxygen-containing atmospheres. Beryllium causes a reduction in the tendency of molten magnesium alloys to oxidize when exposed to oxygen-containing atmospheres, such as e.g. air.

Eliminating the need to apply a protective slag coating to molten magnesium alloys is advantageous in several respects. First and foremost, the elimination of slag covers results in a significant cost reduction. In addition, the absence of slag covers means that slag particles cannot be mixed into the resulting casting in the form of slag inclusions. Absence of slag cover also results in increased magnesium yield because the absence of inclusions and the subsequent loss of molten magnesium in the slag cover is avoided.

It is known from the prior art to add beryllium to magnesium-based alloys for various purposes. US Patent Nos. 2,380,200, 2,380,201, 2,383,281, 2,461,229 and 3,947,268 as well as an article by F. L. Burkett entitled "Beryllium in Magnesium Die Casting Alloys" from AFS Transactions, Volume 62, Pages 2-4 ( 1954) describes the addition of beryllium to magnesium-based alloys. From this literature one can learn from US Patent Nos. 2,380,200 and 2,380,201 as well as the article by Burkett that beryllium reduces the tendency of molten magnesium alloy to oxidize. These previous efforts to reduce oxidation do not include beryllium additions in the quantities covered by the present invention and do not seem to include the burden of limiting the manganese content to allow an increased beryllium solubility in the magnesium alloy. Moreover, Burkett's article suggests that higher be ry 11 ium limits must be avoided.

The magnesium alloys according to the present invention comprise

up to approx. 12% aluminium, up to approx. 30% zinc, up to approx.

1.5% silicon, up to approx. 0.18% manganese and from approx. 0.0025% to 0.0125% beryllium and the rest essentially magnesium. All percent-

ter of the content is given in % by weight. It is preferred to limit

the manganese content to a maximum of approx. 0.05% if the bery11 ium content is within the range from approx. 0.011% to 0.0125% to increase the solubility of beryllium in molten magnesium to a limit sufficient to allow the above-mentioned amounts of beryllium to be dissolved in the magnesium. E.g. will approx. 0.15% manganese allow dissolution of approx. 0.007% beryllium in molten magnesium in um.

It is preferred to keep the manganese content from approx. 0.04% to approx. 0.15% and the beryllium content from approx. 0.005% to approx. 0.0125% in the magnesium alloys according to the present invention to increase the alloy's corrosion resistance. It is further preferred to limit the manganese content from approx. 0.08% to 0.15% and the beryllium content from approx. 0.006% to 0.01% to further increase the magnesium alloy's corrosion resistance.

Present invention principle can be easily adapted for use

in the production of die-cast magnesium alloys. flagnesium die casting alloys may contain from 1% to 12% aluminum, up to 30% zinc, odp to 1.5% silicon, from 0.2% to 1.0% manganese and the remainder substantially magnesium.

Zinc content in magnesium alloys has generally been limited to a maximum of 1.5% zinc. Zinc in amounts opD up to 1.5% in magnesium alloys improves the alloy's mechanical properties and corrosion resistance while retaining very good die casting properties. Alloys with more than 1.5% zinc show a noticeable increase in red brittleness and cracking during casting. The fact is that support of magnesium alloys is a problem when the zinc content is above 1.5% and below 12%. This is due to an extension of the freezing interval. These problems have been limited by regulating the aluminum and zinc content in the magnesium alloy. In contrast, magnesium alloys show better casting properties when the zinc content in the alloy is between approx. 12% and approx. 30% than between 1.5% and 12%.

Magnesium alloys with a zinc content greater than 1.5% have advantages and disadvantages. The advantages of these alloys include a lower melting point and greater fluidity. These advantages combine, depending on the zinc content, the fact that the casting can take place at temperatures 10°C to 38°C lower than usual when casting magnesium alloys with a low zinc content, while maintaining good fluidity. The low melting point further increases the magnesium alloys' resistance to oxidation during casting. However, the high zinc content in the alloys can cause problems with castability, density, formability and increased costs.

The higher the zinc content in the magnesium alloy, the higher the density, price and brittleness. The problems with the high zinc-containing alloys are outweighed by the benefits gained when used for special application purposes. Therefore, care must be taken in recommending the correct high-zinc alloy for appropriate use.

The manganese content of the alloys according to the present invention

se is important because of its influence on the solubility and ease of alloying beryllium in molten magnesium. For-

since this influence has not previously been recognized, AZ91B, a die-casting alloy that has a wide application, with a nominal content of 9% aluminum, 0.7% zinc, 0.2% manganese, maximum 0.5% silicon, maximum 0, 3% copper, maximum 0.03% nickel and the rest essentially magnesium, have contained less than 0.001% beryllium. It has been discovered that beryllium is soluble in AZ91B magnesium alloys to a greater extent than previously thought. In any event, a beryllium level on the order of 0.001% is considered insufficient to achieve good protection of the molten magnesium. It is on the contrary Dåvist that approx. 0.0025% to approx. 0.015% beryllium must be dissolved in molten magnesium or its alloys to prevent burning, and that the amount of beryllium must be increased with increasing oxygen content in the atmosphere. Therefore, the manganese content must not exceed more than approx. 0.18%, preferably no more than approx. 0.15%. If nitrogen atmospheres or short exposure times come into play, an addition of from approx. 0.0025% to 0.005% beryllium sufficient to provide the necessary protection of molten magnesium. However, if longer exposure times or significant air leaks into the nitrogen atmosphere occur, it is recommended

beryllium content in the order of approx. 0.005% to 0.001%

On the other hand, should it be desirable to prevent burning of molten magnesium or magnesium alloys exposed to air, a beryllium content of approx. 0.011% to 0.0125% preferred. Such beryllium contents require that the manganese content be limited to no more than approx. 0.05%.

The beryllium level to be used depends on the amount of oxygen in the atmosphere above the smelting. E.g. if the molten magnesium is exposed to air without a cover, the oxygen content of the atmosphere above it will remain approximately 20% and therefore a high beryllium level of the order of 0.01% to 0.015% will be necessary to avoid unreasonable oxidation or burning. If the molten magnesium is to be exposed for longer periods, it may be desirable from time to time to add beryllium to compensate for the beryllium that is oxidized or to add larger amounts of beryllium, i.e. 0.02%, so that the excess above the solubility limit is gradually dissolved to compensate for oxidation loss and thereby maintain the amount of beryllium at, or close to, the saturation limit in the molten magnesium in µm.

In order to reduce the beryllium level, which is necessary to provide a good melt protection, it is necessary and desirable to keep the oxygen level as low as possible. Placing a cover or hood over the molten magnesium helps in this regard. The reaction of the molten metal with oxygen in the closed air space will lower the oxygen content in the atmosphere. If the system is very dense and the resulting oxygen content becomes very low, bery11 iumni values as low as 0.0025% will provide adequate protection. If the system is not tight, or from time to time is opened for short periods, e.g. to scoop up, it may be desirable to introduce sufficient nitrogen or other inert gases to maintain a low oxygen content.

In such cases, a medium beryllium nickel level can be used, i.e. 0.005% to 0.01%. Other protective gases such as e.g. SF2^

SO2 and various inert gases can also be used although nitrogen is preferred because it is readily available.

Pollution such as e.g. iron leads to the formation of insoluble

equal intermetallic compounds with beryllium and should therefore be limited to a minimum. Because manganese in the presence of aluminum quantities of the order of 1% to 12% forms a relatively insoluble phase with iron which settles in the smelting, small amounts of man-"

gan, as e.g. 0.1%, is added to die casting alloys

to clean these. However, the manganese level must not be high enough to precipitate beryllium. Typically, the manganese content should be lowered from 0.18% to 0.05%. if the beryllium level is increased from 0.0025% to 0.015% in magnesium alloys containing approx. 9% aluminum.

The following experimental results illustrate some of the principles of the present invention: A magnesium sample alloy containing approx. 9% aluminium, approx. 0.7% zinc and approx. 0.0025% beryllium was kept under a hood-

for 8 hours without burning or unreasonable oxidation.

A 59 kg batch of an alloy containing 7.1% aluminium, 0.71% zinc, 0.05% manganese and the balance magnesium was melted, covered with a flux and held under a hood at 677°C. After the flux was removed by foaming began

the alloy to burn after 1 min. The burning was then extinguished by the creation of a slag cover. The hood was closed and nitrogen was introduced over the surface of the slag-covered melting bath at a rate of 849 l/h for approx. 5 min. The hood was closed and the slag cover removed and the nitrogen flow continued at a rate of 849 l/h. After 30 min. blooms (limited areas of high oxidation) began to form and increased in size. After 51 min. the roses began to burn slowly, emitting a clear light. The hood door was then briefly opened periodically to "/allow pouring out and casting of sample ingots. The burning became stronger after 5 min. of casting and very intense after 15 min..

Further tests were carried out by adding several large amounts of beryllium to the molten magnesium test alloy, as described in the previous section. In general, the tests indicate that bery1li umti Isetner reduces the molten alloy's tendency to burn. When beryllium in an amount of 0.008% was added, the alloy was held satisfactorily under an 849 l/h nitrogen flow and the subsequent die casting of test ingots. This alloy was also kept in air without burning for approx. 15 min. As the beryllium content was increased during the various tests, it was noted that the oxidation resistance of the molten magnesium alloy increased and that reduced amounts of nitrogen flow were required for satisfactory operation. When approx. 0.011% to 0.013% beryllium was added to the molten alloy, the surface of the alloy became silvery in appearance and held up satisfactorily upon exposure to air and subsequent die casting. When the silvery protective surface film was intentionally destroyed, a new film immediately formed, showing that the protective function of beryllium was still active. After exposure to air for approx. After 1 hour, however, oxidation flowers began to form, and these grew slowly.

When 0.0025% beryllium was alloyed into magnesium in the untested alloy, the smelting was held satisfactorily under a nitrogen flow of 849 l/h with the door closed and the subsequent casting into test ingots. After 15 min. the molten magnesium alloy bloomed strongly and began to burn. When 0.007% to 0.01% beryllium was alloyed, the casting was successfully carried out without blooms occurring with 1700 l/h nitrogen. The door in the hood was then kept open for 15 min. without flowers forming. The nitrogen flow was then stopped and the molten alloy was held for a further 15 min. without flowers forming. After the alloy was saturated with approx. 120 - 130 ppm. beryllium at B49°C - 704°C it was kept in air with the 'door open for 30 min. without flowers forming, and was then successfully cast without a nitrogen atmosphere. Extensive exposure eventually led to the formation of flowers.

To determine the compatibility of manganese and beryllium in magnesium alloys, two AZ91B ingots, containing approx. 0.2% manganese added to the smelt. This addition reduced the beryllium content to approx. 0.008% and increased the manganese content to 0.12%. The molten alloy was successfully cast with a current applied

1700 l/h nitrogen and with the door in the hood opened only when necessary. Part of the melt was poured into air in a large mould. No discoloration was noted on the metal surface as it solidified slowly. Another AZ91B ingot was added to the molten alloy with the result that the beryllium content was lowered to approx. 0.007% and the manganese content increased to approx. 0.15%. Sample ingots were again cast under 1700 l/h nitrogen. Several flowers had formed by the end of the experiment.

The variations in manganese and berylium levels had no noticeable effect on the castability of the magnesium core alloy. Some for-

and

improvement in flowability and surface appearance seems to be the result of an increased beryllium content because less oxidation of the molten material is obtained.

Five die casting ingots of each alloy were tension knurled

to determine the effect of beryllium and manganese. The results shown in Table I indicate that lower manganese content and higher beryllium content act to increase both formability and tensile strength in the magnesium sample alloy.

Sandblasted test ingots of each alloy were also immersed in salt water (3% NaCl] for 3 days to determine corrosion resistance. The ingots were sandblasted to remove the casting surface. The results in Table II show that bery 1li umt in I deposits reduces the salt-water corrosion rate by magnesia cures the rings to the same low level achieved with manganese additions. Small amounts of manganese, ie 0.12%, reduce the amount of beryllium needed for good corrosion resistance. The increased effect of beryllium can be attributed to a reduction in the iron content.

Claims (7)

1. Die casting, characterized by being substantially free of slag (flux) inclusions, having good corrosion resistance, good formability and tensile strength, and consisting essentially of from 1% to 12% aluminium, up to 30% zinc, up to 1.5% silicon, from 0.04% to 0.15% manganese, from 0.005% to 0.0125% beryllium and the rest magnesium. 2. Die casting according to claim 1, characterized in that the alloy contains manganese from 0.08% to 0.15% and beryllium from 0.006% to 0.01%. 3. Die casting according to claim 1, characterized in that the die casting contains a maximum of 0.05% manganese and from 0.011% to 0.0125% beryllium. 4. Process for producing a die-cast magnesium alloy, characterized by) providing a molten pool of a magnesium alloy consisting essentially of 1% to 12% aluminum, up to 30% zinc, up to 1.5% silicon, up to 0, 18% manganese, from 0.0025% to 0.0125% beryllium and the remainder substantially magnesium, and wherein the pond is exposed to an oxygen-containing atmosphere, and b) die casting the molten magnesium alloy to form a die T) characterized by being practically free of slag (flux) inclusions. 5. Method according to claim 4, characterized in that the melt is exposed to an atmosphere which contains a greater amount of nitrogen than what is contained in air. 6. Method according to claim 4, characterized in that the magnesium alloy contains from 0.005% to 0.01% beryllium. 7. Method according to claim 4, characterized in that the magnesium alloy contains from 0.01% to 0.015% beryllium and up to 0.05% manganese and that the smelting is exposed to air.