WO1993017136A1

WO1993017136A1 - Fluxless melting and refining of magnesium and/or magnesium alloys

Info

Publication number: WO1993017136A1
Application number: PCT/US1992/001401
Authority: WO
Inventors: Vladimir Petrovich; William E. Ii Mercer; William G. Green
Original assignee: The Dow Chemical Company
Priority date: 1992-02-21
Filing date: 1992-02-21
Publication date: 1993-09-02

Abstract

A process for fluxless melting and refining magnesium or magnesium alloys including sparging an inert gas, such as argon, into a bath of molten magnesium or magnesium alloy sufficient to purify and remove nonmetallic inclusions present in the molten magnesium or magnesium alloy.

Description

FLUXLESS MELTING AND REFINING OF MAGNESIUM AND/OR

MAGNESIUM ALLOYS

Background of The Invention

This invention relates to melting and refining of magnesium and/or magnesium alloys and more particularly, this invention relates to melting and refining of scrap magnesium and/or magnesium alloys.

There is an increasing demand for improvement of quality of cast and fabricated products of magnesium. Equally important is an increasing demand for the recovery of magnesium scrap from casting and fabrication processes. The processes of refining of magnesium or remelting scrap magnesium require removal of the impurities from the magnesium.

In the magnesium industry it is known to use fluxing agents for purifying or refining of magnesium, i.e., the fluxing agents are a mixture of various chloride salts used for the protection of molten metal and of the removal of oxides and other non-metallic inclusions (NMI). For example^", conventional commercial processes settle oxides by mixing flux of various compositions into a crucible containing a molten body of the magnesium. The flux wets the oxides which settle to the bottom of theerucible to produce a sludge containing the oxides, certain metals and salts intimately mixed. The sludge, being more dense than the melt, settles to the bottom of the crucible thereby enabling the now refined πielt to be separated from the sludge. The separated, refined magnesium can then be cast directly into ingots. One flux which has been commercially used to refine magnesium and its alloys contains approximately 40 weight percent (wt. % ) magnesium chloride, 55 wt. % potassium chloride and 5 wt. % calcium fluoride and will be referred to hereunder as M-130 flux.

The use of the fluxes has its disadvantages however. For example, some of the flux can remain in the cast metal causing flux inclusions and adversely affecting corrosion resistant properties of magnesium and its alloys. Also, the sludge produced by fluxing creates a disposal problem and entraps a quantity of metal resulting in an accumulated melt loss. The flux also creates an environmental problem by generating HCl, which can further corrode the equipment. In addition, the use of flux adds cost to the process of refining.

Because of the disadvantages of using flux, a fluxless operation would be more desirable. It is therefore, desired to provide a fluxless process for melting and refining of magnesium and magnesium alloys. In particular, it is desired to provide a process for fluxless melting and refining of die cast scrap magnesium and magnesium alloys; Summary of The Invention

The present invention is directed to a process for fluxless melting and refining of magnesium or ,- magnesium alloys including contacting a bath of molten magnesium or magnesium alloy with an inert gas, such as argon, sufficient to purify and remove nonmetallic inclusions present in the molten magnesium or magnesium alloy.

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Brief Description of The Drawings

Figure 1 is a schematic view showing a molten metal bath and a sparging apparatus for the process of M r the present invention.

Figure 2 is a graph showing the effect of argon on the number of NMI in a bath of molten magnesium alloy.

20 Detailed Description of The Preferred Embodiments

The process of the present invention is used for the refinement of magnesium or magnesium alloy melts for removing or decreasing the number of non-metallic

25 inclusions in the magnesium or magnesium alloys. Non¬ metallic inclusions (NMI) include oxides present in the melt. NMI's in the melt will increase in direct proportion to the surface area of the ingot or parts

30 being melted. NMI's also increase because of improper melting and casting practices. NMI vary in size and quantity depending on the source of magnesium or its alloy and its production. For example, the size of NMI can range from about 0.25 inch to about less than 0.001 35 inch. For a magnesium alloy such as AZ91- for example, the number of NMI in the melt typically can be from about 100 to about 10.000. The internal quality of ingots produced from melts particularly those ingots with a high NMI count and/or fabricated parts from melts or ingots is improved using the process of the present invention.

In its broadest scope, the process of the present invention involves fluxless melting of magnesium or its alloy whereby floating of impurities to the top 10 of such melt is accomplished by inert gas sparging and skimming dross formed on the top of the melt. Optionally the melt may be filtered to further reduce the NMI.

15 Any magnesium or magnesium base alloy may be used in the present invention. For example, those containing various amounts of Al, Zn, Mn, rare earth metals, Zr, Ag, I, Th, and the like can be used. Alloys of magnesium such as AZ91, EZ33, ZK60, AM 60 and other

20 alloys listed in American Society for Testing and Materials (ASTM) BδO-1987, page 34 are useful for processing in accordance with the present invention. Herein the process will be described with regard to AZ91

-_t- magnesium alloy and more specifically with AZ91 die casting scrap metal. The present invention is advantageously used in obtaining a scrap-derived ingot product because a greater number of NMI are usually found in melting of scrap. For example, die cast scrap

30 resulting from die casting operations may be the source of scrap metal. This scrap is usually dirty and oily which adds to the problem of separating the oxides from the melt. One advantage in using fluxless melting of die cast scrap is that such process provides remelting

35 of large quantities of scrap and recovering some costs of a die casting process.

Any gas inert or slightly reactive to magnesium and magnesium base alloys may be used in the present invention such as Ar, air, N₂, He, C0₂, Cl₂, SF₆ BF₃ and HCl. Preferably, argon gas is used in the present invention to minimize melt loss.

The process of the present invention may be carried out in any melting and casting operation using conventional equipment with one or more pots or crucibles.

With reference to Figure 1, there is shown crucible 10 containing a molten magnesium 11. A sparger tube 12 with holes 12a is immersed into the bath of melt 11. A container 13 supplies a gas through flow meter 14 and the flow is controlled with valves 15 and 16. The gas is bubbled through the bath of melt and the bubbles 17 carrying the impurities float to the top surface of the melt forming a layer of dross 20. The gas thereafter escapes into the atmosphere. The resultant dross formed at the top of the melt can then be skimmed off in familiar fashion.

While in Figure 1 there is shown a sparger tube 12 for bubbling the gas through the melt, it is understood that any other conventional method of bubbling the gas through the melt can be used, for example, centrifugal pumps or other diffussion devices or dispersion tubes and pumps. As aforementioned argon gas is the preferred gas used in the present invention for bubbling through the melt. The bubbling of argon gas through the melt is herein referred to as "argonization." The present invention will herein be described with the use of argon gas.

Figure 2 is a graphical illustration of one embodiment of a scrap melting process and the effect of

10 argonization on the number of NMI in the scrap melting process. Figure 2 shows that as the p.ounds of scrap added to a crucible Increases the number of NMI also increases. However, when bubbling of argon through the melt begins the number of NMI drasticallv decreases. 15

In carrying out one embodiment of the process of the present invention, a magnesium or magnesium alloy scrap is preferably degreased by any environmentally accepted method, for example, by washing in a solvent

20 such as CHLOROTHENE^® (Trademark of The Dow Chemical

Company). Degreasing helps keep the subsequent melt cleaner of non-metallic inclusions and dissolved gasses and reduces smoke and soot within the melting

-_cr facilities. Non-degreased "oily" scrap produces copious quantities of black, sooty smoke during meltdown and adds large amounts of dissolved gas (which is presumed to be hydrogen) in the melt. Washing or degreasing the scrap, for example, in CHLOROTHENE^® prior to melting

30 also minimizes the "dissolved gas" problem.

The scrap is then preheated thoroughly prior to melting to as high as practical above the boiling point of water. It is desired to assure that all moisture on 35 the scrap is driven off from the scrap because of the well known violent reaction of magnesium with water. Preferably, the scrap is preheated to a temperature of at least 150 degrees centigrade (° C) and more preferably to a temperature of from about 200° C to about 300° C.

After preheating the scrap, scrap is charged into a crucible for melting. The scrap can be charged directly into a pot opened to the atmosphere or the scrap can be charged into the pot through a chamber positioned above a covered pot. The charging of the scrap could be carried out by conventional mechanical means. The chamber prevents the contact of the pot atmosphere directly with air above the pot during changing operation so as to minimize protective atmosphere losses. It is advantageous to minimize protective atmosphere losses to minimize the cost of the scrap melting process. The protective atmosphere used in the pot may be conventional protective atmosphere known in the art to prevent burning of the molten metal. For example, one of such protective atmospheres in the pot preferably is kept to a 0.4% to 0.5% SF₆ concentration. The atmosphere may contain any concentration of C0₂ to air ratio. If AZ91 is used, for example, and AZ91 contains at least about 6 ppm of beryllium, there will be no burning problem during melting and holding of the scrap in the pot at a pot temperature of up to about 800° C.

The scrap is stirred in the pot with conventional mechanical stirring means to the point of appearance of a "gentle" vortex in the melt until all the scrap is molten. The pot temperature is controlled to a temperature of from about 630° C to about 750° C.

The stirring action in the pot helps to even the temperature gradients from top to bottom within the pot and helps to increase the melting rate of the scrap.

After the scrap is molten, about 1 to about 1.5 pounds (lb.) of argon is sparged through the molten metal at from about 2 standard cubic feet per hour (SCFH) to about 35 SCFH for about 5 minutes to about 25 minutes at about 630° C to about 750° C. A sponge-like froth forms on the top of the melt which may be removed by skimming. The froth or dross formed on the melt should be removed at least once during.the time of sparging to maximize bubbling rates. The dross should be removed from the melt at the end of argon bubbling.

The dross which forms on the top of the melt is removed from the top by skimming with any conventional mechanical means such as a surface skimmer.

Once the dross is removed from the melt, manganese may be alloyed, by conventional methods, into the melt, if desired, at about 630° C to about 750° C. to decrease the iron content of the melt and produce a high purity magnesium alloy.

The melt can be transferred to a casting pot or a transfer ladle by pumping through a transfer line or tilting or any other means commonly used in foundry operations.

Optionally, the melt may be pumped through a suitable filter to remove non-metallic inclusions from the melt. For example, any conventional filter suited for a particular pot geometry and capacity may be used such as a Johnson screen filter with 0.045 inch openings and 31% open area. The filter further removes NMI and may be used before the argonization step or after the argonization step. Preferably, an ample screen surface area to pot volume ratio is used. For example, at 31% open area, the static screen to weight ratio is 0.07 in^/lb. or for an estimated throughput of 1.5 lb/sec. (5400 lbs/hr) the casting capacity ratio is 0.04 in²/lb.

Then the melt is pumped into molds or die cast machines to form ingots or castings. For example, the molten material may be cast into ingots such as DOWLOK^® 10 (trademark of The Dow Chemical Company) or any other desired casting shape.

Determination of NMI's

,,- For establishing a standard, a selected number of ingots are fractured using an air-hydraulically operated ram. For example, 112 ingots (1800 lbs.) is selected and fractured. The ingots may be fractured, for example, in half. The fresh fractures are visually 0 examined for non-metallic inclusions (NMI) using a stereo microscope at 10X and 45X magnification. The number of visually detectable particles, i.e. NMI, are counted for the total fracture surface area. A value expressed in number of NMI per square inch of total 5 surface area of fracture is obtained for each ingot and averaged.

The counted non-metallic inclusions are sorted into the following three groups:

30

1. large size - greater than 0.02 inch;

2. intermediate size-- 0.01 - 0.02 inch: and

3. small size - 0.001 - 0.01 inch.

35 The procedure for 45X magnification is to count the number of NMI in the field of view of the stereo microscope in 5 random passes from the left edge to the right edge. For example, a Bausch and Lomb stereo binocular microscope with fluorescent flat incident light may be used. The field of view at 45X is 0.01 square inch. The numbers are averaged per pass and multiplied by 100 to express it as the number of NMI per square Inch. At 10X magnification the same procedure for the 45X magnification is carried out except that the field of view is 0.2 square inch and the multiplying factor is 5. For particle size determination a standard grading in the ocular is used. No filters are used on the lenses.

Example

Figure 1 illustrates a system similar to that used to carry out the present example. With reference to Figure 1, a 3/4 inch steel tube sparger was connected to an argon cylinder and placed in a molten bath of AZ91 magnesium alloy with one end of the sparger placed to the bottom of a stainless steel pot about 56 inches In height and 34 inches inside diameter.

The sparging time was about 25 minutes and the flow rate was about 25 standard cubic feet per hour (SCFH) at 5 pounds per square inch (psi) line pressure. The sparger submerged in the melt in the pot was moved in the pot to about four general areas of the pot during argon bubbling ("argonization"). The argon consumption during the 25 minute "argonization" at 25 SCFH was 1.5 lb for the pot which contained_,about 3000 lbs of AZ91. The number of NMI/in² was lowered as shown in the Table below. The argonization temperature was approximately 700° C plus or minus 10° C. Table

Note A: Sample 2 was cast from the same melt that Sample 1 was obtained, but after argonization. Sample 3 was cast from the melt from which Sample 2 was obtained.

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0

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Claims

WHAT IS CLAIMED IS:

1. A process for reducing the number of nonmetallic inclusions present in molten magnesium or magnesium base alloy die casting scrap, wherein the said alloy contains about 89 or more of magnesium, said process consisting essentially of the steps of: a. degreasing the scrap; b. preheating the scrap; c. melting the scrap at a temperature in the range of about 630°C to about 750°C to form a molten bath of metal without flux; 0 d. bubbling a gas through the bath of molten scrap near the bottom of the bath at a rate of from about 2 to about 25 standard cubic feet per hour for about 2 to about 35 minutes to purify and remove _C- nonmetallic inclusions present In the molten magnesium or magnesium alloy scrap; e. floating the nonmetallic inclusions present in the molten magnesium or magnesium alloy bath to the top of the bath to form a dross on the surface of said 0 bath; f. removing the dross from said bath, and g. adding an amount of manganese to the bath effective to decrease the amount of iron metal in the 5 magnesium or magnesium alloy. 2. The process of Claim 1 wherein the gas is selected from the group consisting of air, Ar, N2_> He, C02, Cl2, SF6, HCl, NH3, S02 and TiClij.

r 3- The process of Claim 1 where the gas is

SF6-

4. The process of Claim 1 wherein the gas is inert argon or helium.

10 5. The process of Claim 1 wherein the gas is inert argon.

6. The process of Claim 1 wherein the degreasing of the scrap is done using a solvent

15 comprising 1,1, 1-trichloroethane.

7. A process for reducing the number of nonmetallic inclusions present in molten magnesium or magnesium base alloy die casting scrap, wherein the si i ύ

20 alloy contains about 89% or more of magnesium, said process consisting essentially of the steps of: a. degreasing the scrap; b. preheating the scrap;

2_- at a temperature in the range of about 630°C to about 750°C to form a molten bath of metal without flux wherein the molten bath is gently stirred using mechanical stirring to aid in the melting; d. bubbling argon gas through the bath of 30 molten scrap near the bottom of the bath at a rate of from about 2 to about 25 standard cubic feet per hour for about 2 to about 35 minutes to purify and remove nonmetallic inclusions present in the molten magnesium or magnesium alloy scrap;

35 e. floating the nonmetallic inclusions present in the molten magnesium or magnesium alloy bath' to the top of the bath to form a dross on the surface of said bath; f. removing the dross from said bath, and g. adding an amount of manganese to the bath effective to decrease the amount of iron metal in the magnesium or magnesium alloy.

8. The process of Claim 1 comprising the additional step of making a casting using the purified molten magnesium or magnesium alloy.

9. A casting made using molten magnesium or magnesium alloy purified in accordance with the process of Claim 1.

10. A process for producing a casting from molten magnesium or magnesium base alloy die casting scrap, wherein the said alloy contains about 89% or more of magnesium, said process consisting essentially of the steps of: a. degreasing the scrap; b. preheating the scrap; c. melting the scrap at a temperature in the range of about 630°C to about 750°C to form a molten bath of metal without flux; d. bubbling a gas through the bath of molten scrap near the bottom of the bath at a rate of from about 2 to about 25 standard cubic feet per hour for about 2 to about 35 minutes to purify and remove nonmetallic inclusions present in the molten magnesium or magnesium alloy scrap; e. floating the nonmetallic inclusions present in the molten magnesium or magnesium alloy bath to the top of the bath to form a dross on the surface of said bath; f. removing dross formed on the surface of the magnesium or magnesium alloy bath; g. adding an amount of manganese to the bath effective to decrease the amount of iron metal in the magnesium or magnesium alloy, and h. casting an element from the purified molten magnesium or magnesium alloy scrap metal.

11. The process of Claim 10 wherein the gas is selected from the group consisting of air, Ar, N2_> He,

C02, Cl2, SF6, HCl, NH3, SO2 and TiCl4- .

12. The process of Claim 10 where the gas is SF6.

13. The process of Claim 10 wherein the gas is inert argon or helium.

14. The process of Claim 10 wherein the gas is inert argon.

15. The process of Claim 10 wherein the degreasing of the scrap is done using a solvent comprising 1,1, 1-trichloroethane.

16. The process of Claim 10 wherein the molten bath is gently stirred using mechanical stirring to aid in the melting.

17. The process of Claim 10 comprising the additional step of making a casting using the purified molten magnesium or magnesium alloy.

18. A casting made in accordance with Claim 10. AMENDED CLAIMS

[received by the International Bureau on 8 December 1992 (08.12.92); original claims 1-18 replaced by amended claims 1-7 (2 pages)]

1. A process for reducing the number of nonmetallic inclusions present in molten magnesium or magnesium base alloy die casting scrap, wherein the said alloy contains about 89 percent or more of magnesium, comprising the steps of: a. degreasing the scrap; b. preheating the scrap; c. melting the scrap at a temperature in the range of from 630°C to 750°C to form a molten bath of metal without flux; characterized by d. bubbling a gas through the bath of molten scrap near the bottom of the bath at a rate of from 2 to 25 standard cubic feet per hour (3.4 to 42.5 m3/hr) for 2 to 35 minutes to purify and remove nonmetallic inclusions present in the molten magnesium or magnesium alloy scrap; e. floating the nonmetallic inclusions present in the molten magnesium or magnesium alloy bath to the top of the bath to form a dross on the surface of said bath; f. removing the dross from said bath, and g. adding an amount of manganese to the bath effective to decrease the amount of iron in the magnesium or magnesium alloy.

2. The process of Claim 1, characterized in that the gas is selected from air, Ar, N2, He, C02, CI , SF6, HCl, NH3, SO2 and TiCli), or mixtures thereof.

3. The process of Claim 1, characterized in that the gas is selected from SF6, argon, helium, or mixtures thereof.

4. The process of Claim 1, characterized in that degreasing of the scrap is done using a solvent comprising 1,1, 1-trichloroethane.

5. The process of Claim 1, characterized by gently stirring the molten bath using mechanical stirring to aid in the melting.

6. The process of Claim 1, characterized by the step of making a casting using the purified molten magnesium or magnesium alloy.

7. A casting made by using molten magnesium or magnesium alloy purified in accordance with the process of Claim 1.

7136 STATEMENT UNDER ARTICLE19

The claims nave been amended to adapt them more closely to the requirements of the countries that have been designated for international filing. In this regard, Claim 1 has been amended by deleting the phrase "consisting essentially of; by deleting the word

"about", and by changing the gas rate from standard cubic feet per hour to cubic meters per hour.

The claims have been substantially reduced in number from a total of 18 claims to 7 claims. In this regard, independent Claims 7, 10, 15, 17 and 18 were cancelled. Claims 3, 4 and 5 were combined into a single dependent claim.

The present invention particularly relates to a fluxless melting and refining of magnesium or magnesium die casting scrap which contains about 89 percent or more magnesium to remove non-metallic inclusions and to purify the magnesium.

The only reference that is identified as being of particular relevance is U.S. Patent No. 4,203,581 (Pelton). Pelton discloses a particular type of apparatus which may be used for gas sparging of molten metal, with magnesium being among the several metals listed in column 1, lines 13 and 14. It is generally understood by practitioners of the relevant art that Mg and Mg based alloys are not synonymous or interchangeable with Al, Cu, Zn, etc. For instance, great care needs to be taken to prevent Mg particles and molten Mg from catching on fire by rapid oxidation, whereas molten Al does not present the same hazard. Mg is in Group IIA of the periodic table of the elements and has a valance of 2, whereas Al is in Group IIIA and has a valance of . Even though they are both often referred to as "light metals", it should be noted that Al witli a density of 2.70 g/cc is about 1.6 times that of Mg with a density of 1.74 g/cc. Tne point to be taken here into consideration is that there are significant differences between Mg and Al, especially in that Mg particles or molten Mg burns much more readily in air than does Al . Thus, there can be no foregone conclusion that whatever works with one metal will work with the other. For. example, Al or Al based alloys, are routinely used in making frying pans and cooking pots. One would be ill advised to cook over an open flame using pans or pots made of Mg.

The best that one can derive from the similarities of the properties of Mg and Al in the molten state is that an investigator might obviously be tempted to try one in place of the other in a purification procedure, though not being certain of success before hand. "Obvious to try" does not constitute obvious to

!SS" .

It is further submitted that Pelton does not provide a teaching of the specific steps of the method set forth in the claim of the present application, i.e. 1) degreasing the scrap; 2) preheating the scrap; 3) melting the scrap at the specified temperature to form a molten bath of metal without flux; 4) bubbling a gas through the molten metal at the specified rate for a specified pe.riod of time and, more importantly, adding manganese to the bath in an amount effective to decrease the amount of iron in the Mg or Mg alloy..

It is respectfully submitted that the cited reference does not teach the invention for which protection is desired in the present application.

Favorable consideration and allowance of the proposed new claims, as submitted to the national Patent Office is respectfully solicited.