GB2085471A - Oxidation resistant magnesium alloy - Google Patents

Abstract

Magnesium alloys containing from up to 12% aluminium, between 1.5% and 30% zinc, up to 1.5% silicon, up to 0.18% manganese, 0.0025% to 0.015% beryllium are die cast without need for protective flux coverings to produce die cast products that do not contain harmful flux inclusions.

Description

SPECIFICATION Oxidation resistant magnesium alloy The invention generally relates to magnesium alloys that contain beryllium and are sufficiently resistant to oxidation in the molten condition to obviate the need for the use of protective flux covers to prevent excessive melt oxidation or burning when exposed to oxygen-containg atmospheres. Beryllium functions to reduce the propensity of molten magnesium alloys to oxidize when exposed to oxygencontaining atmospheres such as air.

The elimination of the need to employ a protective flux cover for molten magnesium alloys is advantageous from at least several respects. First of ali, the elimination of flux covers results in a significant cost reduction. In addition, the absence of flux covers means that flux particles cannot become mixed into the molten magnesium metal and then become trapped in the resultant casting in the form of flux inclusions. The lack of flux covers also results in increased magnesium yields because of the lack of entrapment and subsequent loss of molten magnesium in the flux covering.

It is known in the art to add beryllium to magnesium base alloys for various purposes. United States Patent Numbers 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" which appeared in AFS Transactions, Volume 62, pages 2-4 (1954) disclose the addition of beryllium to magnesium base alloys. Of the above cited information, United States Patent Numbers 2,380,200 and 2,380,201 and the Burkett article teach that beryllium reduces the propensity for molten magnesium alloys to oxidize.

These prior efforts to reduce oxidation do not involve beryllium additives at the levels of the invention and do not appear to involve the imposition of a restriction of manganese content to permit increased beryllium solubility in the magnesium alloy. Moreover, the Burkett article suggests that higher beryllium levesl must be avoided.

Our co-pending GB Application No. 8016671, published as Specification No. 20511 29A, relates to magnesium alloys containing beryllium and, optionally, aluminium, manganese and zinc with a maximum amount of zinc of 1.5%. The present invention relates to alloys which differ principally in containing larger amounts of zinc up to 30%.

The present invention accordingly provides a magnesium alloy consisting essentially of up to 12% aluminium, between 1.5 and 30% zinc, up to 1.5% silicon, up to 0.18% manganese, from 0.0025% to 0.0125% beryllium, balance essentially magnesium. All compositional percentages are stated in weight percent.

The alloys of the present invention have the advantage that they have good resistance to oxidation in the molten state, good erosion resistance and good ductility and tensile strength. The good resistance to oxidation in the molten state gives rise to the further advantage that the solid alloy, in particular die castings, can be made essentially free of flux or oxide inclusions.

In the alloys of the invention it is preferred to restrict the manganese content to a maximum of 0.05% when the beryllium content in between 0.011% and 0.0125% to increase the solubility of beryllium in molten magnesium to an extent sufficient to enable the above mentioned amount of beryllium to be dissolved in the magnesium. For example, about 0.15% manganese will permit the dissolution of from about 0.007% beryllium in molten magnesium.

It is preferred to maintain manganese from 0.04% to 0.15% and beryllium from 0.005% to 0.0125% in the magnesium alloys of the invention to enhance corrosion resistance of the alloy. It is further preferred to restrict manganese from 0.08% to 0.15% and beryllium from 0.006% to 0.01% to further enhance corrosion resistance of the magnesium alloys.

The principles of the invention are readily adaptable for use in the production of magnesium alloy die casting. Magnesium die casting alloys according to this invention may contain from 1% to 12% aluminium, between 1.5 and 30% zinc, up to 1.5% silicon, from 0.2% to 1.0% manganese, balance essentially magnesium.

The zinc content of magnesium alloys has generally previously been limited to a maximum of 1.5% zinc. Zinc at levels up to 1.5% in the magnesium alloy improves the mechanical properties and corrosion .resistance of the alloy while maintaining very good die casting properties. Alloys having a zinc content above 1.5% show a marked increase in hot shortness or cracking during casting. In fact, casting of magnesium alloys presents problems when the zinc content is above 1.5% and below 12%. This is due to the broadening of the solidification temperature range. These problems have been limited previously by controlling the aluminium and zinc content present in the magnesium alloy. In contrast, when the zinc content in the magnesium alloy is between 12% and 30%, the alloys display better casting properties than those exhibited between 1.5% and 12%.

Magnesium alloys with zinc contents greater than 1.5% have advantages and disadvantages. The advantages of these alloys include lower melting points and greater fluidity. These advantages combine, depending on the zinc content, to alloy casting at a temperature of 500 to 1 0O0F lower than that generally employed in casting low zinc magnesium alloys, while still maintaining good fluidity. The low melting point additionally increases oxidation resistance of the magnesium alloys during casting.

However, the high zinc alloys may have problems with castability, density, ductility, and increased cost.

The greater the zinc content in the magnesium alloy, the greater the density, cost and brittleness.

The problems with the high zinc alloys are offset by the benefits received in their use for certain applications. Therefore, care must be taken in recommending the appropriate high zinc alloy for any intended use.

The manganese content of the alloys of the invention is important because of its influence upon the solubility and ease of alloying of beryllium in molten magnesium.

Generally the greater the manganese content the lower the beryllium solubility. This effect and the benefit of including beryllium in magnesium alloys of the type according to this invention has not heretofore been recognized. Thus, typical prior art die casting alloys would contain about 0.001% beryllium and this is insufficient to confer good oxidation resistance to the molten alloy. We have found that from 0.0025% to 0.015% beryllium should be dissolved in molten magnesium or its alloys to inhibit burning, with the amount of beryllium being increased with increasing oxygen content of the atmosphere. Accordingly, the manganese content should not exceed more than about 0.1 8to, preferably no more than about 0.15%.When nitrogen atmospheres and short exposure times are involved, additions of from about 0.0025% to 0.005% beryllium are sufficient to provide protection of molten magnesium. However, when longer exposure times or significant air leakage into the nitrogen atmosphere occurs, beryllium contents on the order of from about 0.005% to 0.0196 are recommended.

On the other hand, should it be desired to inhibit the burning of molten magnesium or magnesium alloys held in air, a beryllium content of from 0.01 % to 0.015% particularly from 0.0111 to 0.01251 %. Such beryllium contents require manganese to be restricted to no more than about 0.05%.

The beryllium level used depends upon the amount of oxygen in the atmosphere over the melt. For example, if the molten magnesium is exposed to air without a cover, the oxygen content of the atmosphere will remain at about 20%, and, accordingly, high beryllium levels, on the order of 0.01 > ó to 0.015%, will be needed to avoid excessive oxidation or burning. Should the molten magnesium be exposed for prolonged periods, it may be desirable to periodically add beryllium to compensate for beryllium that is oxidized, e.g., 0.02%, in order that the excess above the solubility limit will gradually dissolve to compensate for oxidation losses and thereby maintain the beryllium at or close to the saturation level in the molten magnesium.

To reduce the beryllium level required for good melt protection it is desirable to keep the oxygen level as low as practical. Placement of a lid or hood over the molten magnesium is helpful in this regard.

Reaction of the molten metal with oxygen in the enclosed air will lower the oxygen content of the atmosphere. If the system is very tight and the resultant oxygen content becomes very low, beryllium levels as low as 0.0025% will provide adequate protection. If the system is not tight or is periodically opened for brief periods for operations such as ladling, it may be desirable to introduce sufficient nitrogen or other inert gases to maintain the low oxygen contents. In such situations an intermediate beryllium level, e.g., 0.005% to 0.01%, may be used. Other protective gases such as SF2, SO2, and various inert gases may also be used, although nitrogen is preferred due to its relative availability.

Impurities such as iron tend to form insoluble intermetallic compounds with beryllium and accordingly should be minimized. Because manganese, when in the presence of aluminium contents on the order of 1% to 12%, forms a relatively insoluble phase with iron which then settles to the bottom of the melt, small quantities of manganese such as 0.1% may be included in die casting alloys for purification purposes. However, the manganese level should not be high enough to precipitate beryllium. Typically, manganese contents should be decreased from 0.18% to 0.05% as the beryllium level increases from 0.0025% to 0.015% in magnesium alloys containing about 9% aluminium.

The following experimental results illustrate certain of the principles of the invention.

The test results specifically set out below relate to alloys having zinc contents of about 0.7% and which are, thus, outside the present invention. However, the results correspond mutatis mutandis to those obtained for alloys having higher zinc contents according to the present invention.

A magnesium test alloy containing about 9% aluminium 0.71% zinc, 0.0596 manganese, balance magnesium was melted, covered with a flux and held under a hood at 12500 F. Following removal of the flux by skimming, burning of the molten alloy occurred after 1 minute. The burning was then extinguished with the establishment of a flux cover. The hood was closed and nitrogen was flooded over the surface of the flux-covered molten bath at a rate 9f 30 cfh for about 5 minutes. The hood was closed, the flux cover removed, and nitrogen flow was continued at a rate of 30 cfh. After 30 minutes, blooms (localized areas of high oxidation) began to form and increase in size. After 51 minutes the blooms began to burn slowly and emit a bright light.The hood door was then briefly opened periodically to permit ladling and casting of test bars. Burning became more vigorous after 5 minutes of casting and very intense after 1 5 minutes.

Additional tests were conducted by adding various amounts of beryllium to the molten magnesium test alloy described in the preceding paragraph. In general, the tests indicated that beryllium additions decrease the tendency of the molten alloy to burn. When on the order of 0.008% beryllium was incorporated,lhe alloy was held satisfactorily under a 30 cfh nitrogen flow and then die cast into test bars. This alloy was also held in air without burning for approximately 1 5 minutes. 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 lessened rates of nitrogen flow were required for satisfactory operations.When about 0.011% to 0.013% beryllium was incorporated into the molten alloy, the surface of the alloy became silvery in appearance and was satisfactorily held under exposure to air and then die cast. When the silvery protective surface film was deliberately disrupted, a new film formed instantly, indicating that the protective function of beryllium was still operative. Following exposure to air for about 1 hour, however, oxide blooms began to form and grow slowly.

When 0.0025% beryllium was alloyed into the magnesium test alloy, the melt was satisfactorily held under a nitrogen glow of 30 cfh with door closed and then was cast into test bars. Following 1 5 minutes, the molten magnesium alloy was heavily bloomed and commenced burning. When 0.007% to 0.01% beryllium was alloyed, the casting run was successfully completed without the occurrence of blooming with 60 cfh nitrogen. The door of the hood was then held open for 15 minutes without bloom formation. Nitrogen flow was then stopped and the molten alloy was held for an additional 1 5 minutes without bloom formation.After the alloy was saturated with about 120-1 30 ppm beryllium at 12000--13000F, it was held in air with the door open for over 30 minutes without bloom formation and was then successfully cast without a nitrogen atmosphere. Extended holding, however, finally led to bloom formation.

To determine the compatibility of manganese and beryllium in magnesium alloys, two AZ91 B ingots containing about 0.2% manganese were added to the melt. This addition reduced the beryllium content to about 0.008% and increased the manganese content to 0.12%. The molten alloy was successfully die cast with a flow of 60 cfh nitrogen and the hood door opened only as required. A portion of the melt was poured in air into a large ingot mold. No discoloration was noted on the metal surface as it slowly solidified.

Another AZ91 B ingot was added to the molten alloy with a resultant lowering of the beryllium content to about 0.007% and an increase in the manganese level to about 0.1 5%. Test bars were again cast under 60 cfh of nitrogen. Several blooms had formed at the end of the run.

The variations in manganese and beryllium level had no apparent effect upon the castability of the magnesium test alloy. Some improvement in fluidity and surface appearance appears to result from increasing beryllium content because of less oxidation of the molten material.

Five die cast bars of each alloy were tested in tension to determine the effect of beryllium and manganese. The results set forth in Table I indicate that lower manganese and higher beryllium function to increase both ductility and tensile strength of the magnesium test alloy.

Sanded test bars of each alloy were also immersed in salt water (3% NaCI) for 3 days to determine corrosion resistance. The bars were sanded to remove the cast surface. The results in Table II indicate that beryllium additions reduce the salt water corrosion rate of the magnesium test alloy to the same low level obtained by manganese additions. Small amounts of manganese, e.g., 0.12% reduce the amount of beryllium required for good corrosion resistance. The improvement effected by beryllium can be attributed to a reduction in iron content.

TABLE I % Be % Mn %E TYS* TS* 0 0.05 6 21,500 36,300 0.0025 0.05 7 22,900 38,900 0.0086 0.05 6 22,700 36,800 0.0113 0.04 7 21,000 38,200 0.0125 0.04 5 22,000 37,800 0.0081 0.12 6 22,700 39,000 0.0071 0.15 8 21,900 40,500 0.0006** 0.2 4 21,700 34,600 * Pounds per square inch ** (AZ91 B) TABLE II Corrosion % Be % Mn % Fe Rate-lPY* - 0.05 > 0.15 1.30 0.0025 0.05 0.15 0.95 0.0086 0.05 0.008 0.17 0.0113 0.04 0.005 0.03 0.0125 0.04 0.005 0.03 0.0081 0.12 0.006 0.03 0.0071 0.15 0.007 0.03 0.0006** 0.2 0.003 0.03 * Inches per year ** (AZ91 B)

Claims (22)

1. A magnesium alloy consisting essentially of up to 12% aluminium, between 1.5% and 30% zinc, up to 1.5% silicon, up to 0.18% manganese, from 0.0025% to 0.01 5% beryllium, balance essentially magnesium.

2. An alloy as claimed in Claim 1 containing from 0.0025% to 0.005% beryllium.

3. An alloy as claimed in Claim 1, containing from 0.005% to 0.01% beryllium.

4. An alloy as claimed in Claim 1 containing from 0.04% to 0.15% manganese and from 0.005% to 0.0125% beryllium.

5. An alloy as claimed in Claim 4 comprising from 0.08% to 0.15% manganese and from 0.006% to 0.01% beryllium.

6. An alloy as claimed in Claim 1 comprising not more than 0.05% manganese and from 0.011% to 0.0125% beryllium.

7. An alloy as claimed in any one of Claims 1 to 6 containing from 1% to 12% aluminium.

8. An alloy as claimed in Claim 7 in the form of a die casting.

9. An alloy as claimed in any one of Claims 1 to 8 containing from 12% to 30% zinc.

10. A die casting being essentially free of flux inclusions, consisting essentially from 1 % to 12% aluminium, between 1.5% and 30% zinc, up to 1.5% silicon, up to 0.18% manganese, from 0.0025% to 0.015% beryllium, balance essentially magnesium.

11. A die casting as claimed in Claim 10 wherein the alloy contains from 0.0025% to 0.005% beryllium.

12. A die casting as claimed in Claim 10 wherein the alloy contains from 0.005% to 0.01 % beryllium.

13. A die casting as claimed in Claim 10 wherein the alloy contains from 0.04% to 0.15% manganese and from 0.05% to 0.0125% beryllium.

14. The die casting as claimed in Claim 10, wherein the alloy contains manganese from 0.08% to 0.15% and beryllium from 0.006% to 0.01%.

1 5. A die casting as claimed in Claim 10 wherein the alloy contains a maximum of 0.05% manganese and from 0.011% to 0.0125% beryllium.

16. A method of making a magnesium alloy' die casting, comprising: a. providing a molten pool of a magnesium alloy consisting essentially of 1% to 12% aluminium, between 1.5% and 30% zinc, up to 1.5% silicon, up to 0.18% manganese, from 0.0025% to 0.015% beryllium, balance essentially magnesium.

b. die casting the molten magnesium alloy.

1 7. A method as claimed in Claim 1 6 wherein the molten pool is exposed to an oxygen-containing atmosphere.

18. A method as claimed in Claim 17, wherein the magnesium alloy contains from 0.0025% to 0.005% beryllium and the molten pool is exposed to an atmosphere containing a greater amount of nitrogen than that contained in air.

19. A method as claimed either in Claim 16 or Claim 17, wherein the magnesium alloy contains from about 0.005% to 0.01% beryllium.

20. A method as claimed in Claim 16, wherein the magnesium alloy contains from 0.01% to 0.015% beryllium and up to 0.05% manganese and the molten pool is exposed to air.

21. A method as claimed in any one of claims 16 to 20 wherein the alloy contains from 12 to 30% zinc.

22. Magnesium alloy die-castings whenever made by the method claimed in any one of claims 1 6 to 21.